Peat Foundation
Peat Foundation
Peat Foundation
ENGINEERING CONSIDERATIONS OF
PEAT
5.1. Initial considerations
The selection of a method for the construction or rehabilitation of a road over peat will normally
be based on environmental and economic considerations, as well as the performance
requirements expected of the new carriageway.
Most public roads, even relatively high speed roads, can stand fairly large settlements if they are
long and uniform, particularly if the ride quality is not affected. Short differential settlements
across the carriageway on the other hand can pose dangerous traffic hazards for fast moving
vehicles and need to be designed out if at all possible.
For this reason, main national strategic roads will usually be designed and constructed with safe,
proven, conservative methods of construction that will deliver the high speed carriageway
performance required. Lightly trafficked roads on the other hand, such as the low volume rural
roads that ROADEX deals with, will not normally need these high tolerances and will usually be
able to use the less expensive, less rigorous forms of construction, particularly where vehicle
speeds will be low.
Irrespective of the classification of the road however the overall structure will have to be
designed to meet the two main engineering criteria of stability and settlement, otherwise known
as “bearing capacity”.
5.2. Stability
All roads need to be designed to be stable and be constructed in such a fashion so as to produce a
sufficient factor of safety against foundation and sideslope failure. A typical road embankment
over peat can fail in various ways, eg:
a) By failure of the underlying peat along a slip surface, normally in the form of an arc:
Diagram showing failure of a cutting in peat along a slip surface in the form of an arc
Diagram showing failure of a cutting in peat along a slip surface in the form of an arc
Diagram showing failure of a cutting in peat along a slip surface in the form of an arc
b) By punching shear into the underlying peat:
Diagram of an embankment failing by punching shear into the underlying peat. Here the embankment
settlement is accompanied by heave of the adjacent peat bog alongside the embankment
Diagram of an embankment failing by punching shear into the underlying peat. Here the embankment
settlement is accompanied by heave of the adjacent peat bog alongside the embankment
Diagram of an embankment failing by punching shear into the underlying peat. Here the embankment
settlement is accompanied by heave of the adjacent peat bog alongside the embankment
c) By a tensile break outside the load:
Photograph of tension cracking during road excavations through a peat bog (C. Maciver, CNES)
Photograph of tension cracking during road excavations through a peat bog (C. Maciver, CNES)
Formation of tension cracks in peatland due to unstable cut slopes
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d) By peatslide along a lubricated surface below or within the peat:
Diagram illustrating a peatslide induced by rainfall and water penetrating down to the mineral base to
create a lubricated slip surface
Diagram illustrating a peatslide induced by rainfall and water penetrating down to the mineral base to
create a lubricated slip surface
Diagram illustrating a peatslide induced by rainfall and water penetrating down to the mineral base to
create a lubricated slip surface
The insitu stability of a peat slope can be assessed using the ‘infinite slope analysis’ model,
(Skempton and DeLory,1957), which assumes that the peat will slide as a block (translational
failure).
Appropriate geotechnical analyses should always therefore be carried out ahead of construction
to ensure that these failure conditions are avoided. Various forms of proprietary stability analyses
are available in the geotechnical market such as PLAXIS, OASYS, FLAC, SAGE, SLOPE,
SLOPEW, etc. The selection of the most suitable method of analysis (spreadsheet, general
analysis, finite difference/finite element analysis, 2 dimensional, 3 dimensional, etc) should be
left to an engineer experienced in the field. As part of the analysis it will be necessary to examine
the short term construction stability of the embankment, including the effects of the different
phases of the embankment construction, as well as the long term stability of the chosen method
of construction.
5.3. Settlement
The settlement of an embankment in peat is a long term process that never stops. It has two
distinct considerations; the magnitude of settlement and the rate of settlement. The rate of
settlement, and the time needed for the settlement to happen, is normally considered to the more
important parameter for a road construction project if post-construction maintenance is to be
minimised. Repairing a faulty road with settlements after it has been constructed usually requires
the road to be closed, incurring extra cost and delays to traffic. For this reason it is always best to
‘get it right first time’ in the original construction through good design.
b) The Janbu non linear theory method as used by the Icelandic Roads Administration
Whatever method is used however the performance of the works on site during construction must
be monitored to confirm that the actual settlements are proceeding as predicted.
The STA method consists of a series of settlement diagrams developed as a result of tests carried
out on 30 Swedish peat locations from 1979 to 1998. These are used to give an indication of the
primary settlement in peat in the absence of undisturbed samples of the insitu peat. The diagrams
bring together 4 of the main parameters governing settlement in peat: the thickness of peat, its
water content, the applied load and the time elapsed. The diagrams are based on experience from
tests on fibrous peat and medium-decayed peat. The diagrams assume that the peat is normally
consolidated. For a previously loaded peat a correction factor can be used. If the diagrams cannot
be used, appropriate data should be obtained from compression tests.
The method is based on the simple relationship between water content and deformation in a peat
as shown in the “Deformation v. Water Content” graph below
STA Diagram 1: Relationship of deformation v water content in peat for different loading levels.
STA Diagram 1: Relationship of deformation v water content in peat for different loading levels.
STA Diagram 1: Relationship of deformation v water content in peat for different loading levels.
The following example illustrates the process for the settlement of a 2.5m thick embankment on
4.5m of peat. The peat has been considered as 4 layers of 1.0m, 1.0m, 1.0m and 1.4m with layer
water contents of 1200%, 1200%, 1300% and 1000% respectively.
Diagram of the stratigraphy of the peat in the example.
Load v. Settlement for the example embankment. (black, red, yellow, white, blue,) This predicts a total
primary settlement in the peat of 0.98m at 10 kPa, up to 2.37m at 50 kPa (? = relative deformation, ? =
settlement component within the layer)
The “Deformation v water content” diagram being used to derive an embankment loading sequence for 10
kPa (black), 20 kPa (red), 30 kPa (yellow), 40 kPa (white) and 50 kPa (blue). These coloured points are
summarised in the loading sequence in the next table.
Load v. Settlement for the example embankment. (black, red, yellow, white, blue,) This predicts a total
primary settlement in the peat of 0.98m at 10 kPa, up to 2.37m at 50 kPa (? = relative deformation, ? =
settlement component within the layer)
The “Deformation v water content” diagram being used to derive an embankment loading sequence for 10
kPa (black), 20 kPa (red), 30 kPa (yellow), 40 kPa (white) and 50 kPa (blue). These coloured points are
summarised in the loading sequence in the next table.
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Abstracting the “?” settlement figures from the table produces a “load-settlement” curve for the
peat stratigraphy being considered.
Load v. settlement relationship for the example embankment. The coloured points refer to the “?” totals in
the “load v. settlement” table
Load v. settlement relationship for the example embankment. The coloured points refer to the “?” totals in
the “load v. settlement” table
Load v. settlement relationship for the example embankment. The coloured points refer to the “?” totals in
the “load v. settlement” table
This curve does not however take into account the buoyancy effects that will come into play as
the embankment settles into the water table. (The water table is assumed to be at the surface of
the peat in this example.)
Effect of buoyancy
Effect of buoyancy
Effect of buoyancy
This example uses the following densities:
In the case of the planned 2.5m thick embankment, this will be constructed in 2 stages: the first
layer placed at 1.2m thick (?q = 22.8 kPa) and the second layer at 1.3m thick. (1.2m + 1.3m =
2.5m, ?q = 47.5 kPa). The “load-settlement” curve for these loadings is shown here
Settlement v load relationship for the 2.5 m thick embankment corrected for buoyancy
Settlement v load relationship for the 2.5 m thick embankment corrected for buoyancy
Settlement v load relationship for the 2.5 m thick embankment corrected for buoyancy
These 2 embankment loading stages can be modelled in “Diagram 2” below.
The second layer is placed on the first layer when the underlying peat has consolidated
sufficiently to support the additional load (taken as when 70% of the primary consolidation of
the first layer has been reached.)
The stages are then output in a combined table where the predictions of the settlements from
“load-settlement” curve are presented alongside the expected time periods from Diagram 2 as
below.
Load
?q Time from “Diagram 2” Predicted final settlement
Layer (kPa) Consolidation (%) (Days) from curve (m) Settlement wit
70 19 1.22 0.85
Stage 22.8
1 70 28 2.01 1.41
47.5
Stage 80 44 1.61
2
85 55 1.71
90 71 1.81
95 99 1.91
99 163 1,99
The table only gives an indication of the settlement in a peat layer. If the peat layer is part of a
series of compressible layers the settlement in the other layers must be estimated also to arrive at
an overall prediction figure for settlement of the embankment.
a) control the rate of embankment construction in the short term to ensure that excess pore water
pressures have time to dissipate and that the underlying peat gains sufficient strength before
additional layers are placed.
b) predict the rate of post construction settlement over the longer term design life of the road.
ICERA road construction practice
Deep side ditches are dug on both sides of the new road line 15 metres off the proposed
centreline well in advance of the roadworks to establish a stable ground water regime for the
construction and maintenance of the new road.
Photograph of the side ditches dug in Iceland in advance of new roads to establish a stable ground water
regime.
Photograph of the side ditches dug in Iceland in advance of new roads to establish a stable ground water
regime.
Photograph of the side ditches dug in Iceland in advance of new roads to establish a stable ground water
regime.
Road embankments are normally placed in layers by ‘stage construction‘ to ensure stability
during the deposition of material. In practice this means that a maximum of 1m of fill is placed
on the bog surface as a first stage and thereafter left to settle by at least 50% of its predicted
settlement before further layers are placed. This stage settlement requirement is usually achieved
within 5 days of placing of the layer and as such does not normally constitute a delay for the
Contractor
Actual embankment settlements on site are checked at nominated locations (100-200m apart)
along the road centreline using a “Hydrostatic Profiler”.
Diagram of the ICERA method of measuring settlement below an embankment using a ''Hydrostatic
Profiler''.
Diagram of the ICERA method of measuring settlement below an embankment using a ''Hydrostatic
Profiler''.
Diagram of the ICERA method of measuring settlement below an embankment using a ''Hydrostatic
Profiler''.
For this a 50mm diameter plastic tube is placed on the bog surface transverse to the road line
prior to commencement of filling operations.
Referencing the tube on the peat surface for position and level.
Photograph of the 50mm plastic pipe laid on the bog surface ahead of filling operations
Referencing the tube on the peat surface for position and level.
Photograph of the 50mm plastic pipe laid on the bog surface ahead of filling operations
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As the layers of fill are placed on the peat the tube deflects as the embankment settles. A
pressure transducer can then be pulled through the tube to measure its deflected shape under the
embankment whenever required. The measurements obtained are presented as a cross section
through the embankment for use in measurement and earthworks control purposes..
Diagram of a settlement history of a 2.5m high embankment over peat constructed in stages. The top part
of this diagram shows the monitoring data of the top surface of the embankment during construction. The
bottom section of the diagram shows the monitoring data of the settlement into the peat. (The bottom line
on the top diagram and the bottom line on the bottom diagram are the same.)
Diagram of a settlement history of a 2.5m high embankment over peat constructed in stages. The top part
of this diagram shows the monitoring data of the top surface of the embankment during construction. The
bottom section of the diagram shows the monitoring data of the settlement into the peat. (The bottom line
on the top diagram and the bottom line on the bottom diagram are the same.)
Diagram of a settlement history of a 2.5m high embankment over peat constructed in stages. The top part
of this diagram shows the monitoring data of the top surface of the embankment during construction. The
bottom section of the diagram shows the monitoring data of the settlement into the peat. (The bottom line
on the top diagram and the bottom line on the bottom diagram are the same.)
The first layer of fill is designed to impose less than 20 kPa on the bog surface. Subsequent
layers are designed to add less than 30 kPa. Each layer is allowed to settle by 50% of its
predicted settlement before further layers are permitted to be placed. When these rules are
followed it is not normally necessary for additional measures to be employed to accelerate
consolidation. Counterbalance berms are occasionally used to increase overall stability.
Actual settlements are checked periodically at each measurement cross section during
construction and compared with the predicted settlement. If necessary, the measured settlements
are “back calculated” and updated to improve the modelling of future settlement. Opportunity is
also taken during these checks to ‘fine tune’ the requirements for overload and surcharge to
ensure that the desired amount of settlement occurs within the required timescale.
Long-section along Section 3 of the Bræðratunguvegur from Flúðir to Tungufljót summarising the planned
road earthworks design: depth of peat, amount of fill, and estimated settlement over the construction
period.
Long-section along Section 3 of the Bræðratunguvegur from Flúðir to Tungufljót summarising the planned
road earthworks design: depth of peat, amount of fill, and estimated settlement over the construction
period.
Long-section along Section 3 of the Bræðratunguvegur from Flúðir to Tungufljót summarising the planned
road earthworks design: depth of peat, amount of fill, and estimated settlement over the construction
period.
A final check is carried out at the completion of the earthworks stage, prior to laying the road
structural layers. At this point the embankment is normally given an “overlift” of material to
allow for any future settlement in the lifetime of the road, usually taken to be 20 years. This final
check is done at the finished embankment level so that any remedial work can be carried out
with fill material rather than with more expensive crushed sub-base.
Long-section along Bræðratunguvegur from Flúðir ? Tungufljót showing ''overlift'' for future settlement
Long-section along Bræðratunguvegur from Flúðir ? Tungufljót showing ''overlift'' for future settlement
In recognition of this ROADEX Partner countries tend to follow a geotechnical risk management
process for their road construction and improvement works, and particularly for those involving
peat, so that any geotechnical risks are identified ahead of the problems on site, and efforts made
to correctly manage them. Eurocode 7 recommends a geotechnical design and risk management
process for road construction and improvement projects as below:
The role of geotechnical risk management and the Geotechnical Risk Register (GRR) through a
project can be seen in the following chart.
?
Ground Investigation (Preliminary Geotechnical Report)
??
Recommended geotechnical design and risk managem
Sufficient information? road construction and improvement projects from Eur
? ? ?
Design ? Detailed design ? Update GRR
? ? ?
Construction ? Construction of Works ? Update GRR
? ? ?
Operations
? Maintanance ? Update GRR
the desk studies carried out, e.g. geological maps and records, aerial photographs, mines and
mineral workings, previous ground investigations, flood records, contaminated land, etc.
the site visits made, e.g. the initial walkover, geomorphological & geological mapping,
probing, trial pits, samples and testing, drainage/hydrology, etc.
the ground conditions – soils on the site and their engineering properties, significance of
geological formations, ground water conditions, etc.
a comparison of the options and risks
recommendations on instrumentation for site monitoring and the frequency of readings.
the Geotechnical Risk Register
Good communication between client, designer and contractor is essential for this process to
work. When all parties are working together openly on the project there is a better chance of any
risks being identified and considered early enough to offer solutions, or put contingency plans in
place.
Examples of pages from a risk register for a new thin embankment over peat are shown in the
following tables:
Table A. Typical Geotechnical Risk Register criteria for Probability (P), Impacts (I) and Risk (R). A risk
value of 1-4 is considered trivial, 5-8 tolerable, 9-12 significant, and above 12 unacceptable.
Table A. Typical Geotechnical Risk Register criteria for Probability (P), Impacts (I) and Risk (R). A risk
value of 1-4 is considered trivial, 5-8 tolerable, 9-12 significant, and above 12 unacceptable.
Table A. Typical Geotechnical Risk Register criteria for Probability (P), Impacts (I) and Risk (R). A risk
value of 1-4 is considered trivial, 5-8 tolerable, 9-12 significant, and above 12 unacceptable.
Ideally, only trivial risks (1-4) should be accepted, but in practice this will not always be
possible. Risks with a value above 9 should not be accepted. These should be reduced to less
than 9 by appropriate risk control measures, i.e. management and/or mitigation. An example of
this is shown in Table B below.
Table B. Simplified example of part of a risk register for a thin embankment over peat on a geotextile.
Risk rating (R) = Probability (P) x Impact (I)
Table B. Simplified example of part of a risk register for a thin embankment over peat on a geotextile.
Risk rating (R) = Probability (P) x Impact (I)
Table B. Simplified example of part of a risk register for a thin embankment over peat on a geotextile.
Risk rating (R) = Probability (P) x Impact (I)
An alternative method of presenting risk rating as a matrix is shown below in Table C
Table C. An alternative method of presenting Risk rating (R) = Probability (P) x Impact (I) as a matrix
Table C. An alternative method of presenting Risk rating (R) = Probability (P) x Impact (I) as a matrix
Table C. An alternative method of presenting Risk rating (R) = Probability (P) x Impact (I) as a matrix
In the above examples the geotechnical risk has been measured in terms of the potential delay to
the delivery of the Works. An alternative method for environmentally sensitive sites could be to
measure the “Impact” as the impact of the geotechnical hazard on the environment.
All site investigations should have an aim however, and only those investigations that are
actually necessary to provide the information for the design should be carried out. Too often hard
pressed engineers are tempted to omit site investigation on the grounds of economy and speed.
This is always a mistake. Roadworks involving peat must always be based on sound collected
data.
Further details regarding these techniques can be found in the ROADEX II project reports. The
desk study and site walkover are vital precursors to the main site investigation, and the
subsequent analysis of the site, but it will seldom be cost-effective to use all of the other
investigation methods listed.
The site investigation and survey methods for roads on peat generally follow the methods
outlined in the Permanent Deformation lesson with a few important additions:
a) Site visit and walkover
Photograph of a surveyor during a site walkover on an area of peatland.
Photograph of a surveyor during a site walkover on an area of peatland.
Photograph of a surveyor during a site walkover on an area of peatland.
The site visit and ’walkover’ is one of the most important elements of the site investigation. The
existing road, or the proposed line of road for a new alignment, is the “full-scale model” for the
project and provides the best information on the problems that are likely to arise and which must
be taken into account in the design. The site visit and walkover also gives the opportunity of
seeing the surface features of the peat first hand, e.g. ditches, watercourses, subsurface pipes,
surface topography, peat workings, waterlogged areas, areas of free water, etc, and making an
assessment how these can be accommodated in the works.
Isopachyte plot of peat depths based on a probing survey. (C Maciver, Western Isles Council)
Isopachyte plot of peat depths based on a probing survey. (C Maciver, Western Isles Council)
Isopachyte plot of peat depths based on a probing survey. (C Maciver, Western Isles Council)
Greater probing difficulty arises where there are soft compressible layers within the peat, such as
clay, gyttia or silt, gravel layers from old flooding episodes, or even chunks of woody materials.
Here probing alone cannot differentiate between the differing materials and a more sophisticated
weight probing or cone penetrometer will be necessary.
GPR is commonly used in investigations on existing roads. Depending on the antenna used a
GPR survey can give good information on the structural layers of the existing road, the overall
thickness of the road embankment, the depth of peat and any compressible layers within it.
Video and radargram of section of road constructed on peat, showing road construction layers and peat
depth.
Video and radargram of section of road constructed on peat, showing road construction layers and peat
depth.
Video and radargram of section of road constructed on peat, showing road construction layers and peat
depth.
GPR can also assist in the prediction of settlement on peat by giving information on the
settlement that has already occurred due to the existing weight. This can be used to calibrate
settlement analyses for new layers and/or widening. The use of GPR is discussed in Section 4.1.7
of the “Permanent Deformation” elearning lesson.
(d) Sampling
Some investigation of properties of the peat should always be carried out to get a notion of the
type of peat involved. This can be as simple as a Von Post classification of the peat, and an
estimation of water content if settlement calculations are to be attempted. Undisturbed peat
samples can be difficult to obtain however especially in peats with a high water content. A
simple and effective sampler has been produced by the Swedish Geotechnical Institute (SGI) to
overcome this.
Hand auger sampling of peat is also possible, and much better than no samples. A large range of
hand augers exist on the market that can recover a sample from depth to enanble a simple
classification of the peat type to be carried out.
An example of the range of sampling augers available on the market today with bayonet or screw fittings
that can recover a sample from depth for classification. (source: Geawelltech)
An example of the range of sampling augers available on the market today with bayonet or screw fittings
that can recover a sample from depth for classification. (source: Geawelltech)
An example of the range of sampling augers available on the market today with bayonet or screw fittings
that can recover a sample from depth for classification. (source: Geawelltech)
In road widening projects it is important that the consolidated peat below the existing road is
sampled as well as the peat beside the road to ensure that both old and new act together in the
finished structure.
Photograph of sample extracted by a “Russian peat corer” (C Maciver, Western Isles Council)
Photograph of sample extracted by a “Russian peat corer” (C Maciver, Western Isles Council)
Photograph of sample extracted by a “Russian peat corer” (C Maciver, Western Isles Council)
e) Shear strength
Getting an estimation of the insitu undrained shear strength of a peat area is not easy due to the
great variability of peat, both horizontally and vertically. For fibrous peats the absence of an
actual measured figure need not be a problem, but an estimate of the shear strength should be
obtained for a highly humified peat, particularly where the loadings on the peat will be
significantly increased by the construction of the new infrastructure.
An indication of shear strength may be obtained in the field by using a shear vane test but the
results obtained must be treated very cautiously.
Photograph of a Geotech field shear vane in action on a peatland (S Stapleton, ESBI)
Diagram showing the principles of the shear vane apparatus.The test involves a probe with four
orthogonal vanes being pushed into the peat to the test depth and rotated under a measured torque until
a failure is caused in the peat.
Photograph of a Geotech field shear vane in action on a peatland (S Stapleton, ESBI)
Diagram showing the principles of the shear vane apparatus.The test involves a probe with four
orthogonal vanes being pushed into the peat to the test depth and rotated under a measured torque until
a failure is caused in the peat.
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The shear vane test is relatively simple to perform and understand but research (Landva 1980)
has shown that the failure in peat does not necessarily occur on the edge of the vane but on a
failure surface 7mm to 10mm outside the vane due to the tearing effect of fibres.
Diagram of a cross-section through a shear vane at failure showing failure surface outside the periphery
of the vane due to the tearing effect of fibres. (Source Landva, Canadian Geotechnical Journal 1980)
Diagram of a cross-section through a shear vane at failure showing failure surface outside the periphery
of the vane due to the tearing effect of fibres. (Source Landva, Canadian Geotechnical Journal 1980)
Diagram of a cross-section through a shear vane at failure showing failure surface outside the periphery
of the vane due to the tearing effect of fibres. (Source Landva, Canadian Geotechnical Journal 1980)
Any shear strength results derived from a field vane test should therefore be treated very
carefully and an appropriate conservative correction factor used. It is not possible to give a hard
recommendation for this factor but the ROADEX view is that the vane shear strength obtained in
the field should be divided by at least 2, i.e. it should be halved, before it is used for design
purposes.
The ROADEX project recommends the following strategies as minimum site investigations for
low volume roads:
Bulk density and water content from undisturbed samples/trial pits if possible
GPR for thickness and construction of existing road (1.5GHz and 400MHz), depth of the peat
layer (275MHz).
In addition the following surveys have proven very useful when dealing with existing roads on
peat:
Digital video
These levels of investigation will enable to engineer to review of all of the data available,
conduct an initial site impact assessment, start the geotechnical risk register, and decide whether
any further intrusive ground investigations are necessary.
Testing can of course be carried out if deemed necessary. The Swedish Geotechnical Institute
use a direct shear apparatus and compressiometer which allows large deformations to be
produced in peat samples.
The direct shear apparatus which also allows large deformations. Sample mounted in the shear
apparatus (left), sample after shearing (right)
The direct shear apparatus which also allows large deformations. Sample mounted in the shear
apparatus (left), sample after shearing (right)
The direct shear apparatus which also allows large deformations. Sample mounted in the shear
apparatus (left), sample after shearing (right)
The Icelandic Roads Administration uses an oedometer to obtain the basic peat parameters for
their Janbu settlement analyses.
The use of the oedometer in laboratory testing in Iceland
The use of the oedometer in laboratory testing in Iceland
The use of the oedometer in laboratory testing in Iceland
However for projects on low volume roads the simpler ‘classification tests’ outlined in Section 1
are generally all that are used, together with empirical relationships to produce some guidance on
the likely behaviour of the peat insitu.
Classification and water content are the most commonly specified and these two tests can
probably be considered to be the minimum amount of testing required for best practice. With
knowledge of the water content it is possible to get a good approximate calculation of
settlements and consolidation with the STA method outlined in section 5.3.2.
7. TYPES OF CONSTRUCTION
7.1. Introduction
This section will summarise the most common construction methods for low volume roads on
peat. Further information on the methods is given in the ROADEX II report “Dealing with
Bearing Capacity Problems on Low Volume Roads Constructed on Peat”.
Road construction in peat can essentially be sub-divided into four broad classifications:
7.2 Avoidance
7.2. Avoidance
The simplest method of dealing with peat is to avoid it, go round it, do not cross it. This may
seem obvious at first glance but it is sometimes overlooked when planning a route corridor when
the design focus is on other things. But if circumstances permit (alignment, environment,
economics, etc) avoiding the peat is a sensible option.
Diagram of a long section through an embankment being constructed by the ''peat excavation and
replacement'' method.
Diagram of a long section through an embankment being constructed by the ''peat excavation and
replacement'' method.
Where it can be used the method is a dependable way of constructing an acceptable road across
peat with the minimum risk of settlement or shear failure, provided that all of the peat is
excavated down to the sound load bearing layer. In these circumstances the bearing capacity of
the new embankment will be dependant on the method and materials of construction. Most new
major roads in the ROADEX countries are built with the excavation method.
All ROADEX Partner organisations use the ''peat excavation and replacement'' method for the
construction of high speed roads.
All ROADEX Partner organisations use the ''peat excavation and replacement'' method for the
construction of high speed roads.
All ROADEX Partner organisations use the ''peat excavation and replacement'' method for the
construction of high speed roads.
Peat excavation and replacement is however only generally economic for the shallower depths of
peat where excavation quantities are likely to be small. Experience in the ROADEX countries
suggests that the economic limit for the excavation method normally lies somewhere between 3
and 4 metres of peat for public roads. The actual economic depth at a particular location will
depend on the local parameters, e.g. the type of peat, the area of peat, the cost of the backfill
material, availability of spoil areas, etc. (Note: Wind farm developers in Scotland currently
consider the economic depth to be 1.0m to 1.5m of peat.) What can be said with some certainty
is that after 4m of excavation it will become increasingly more difficult to keep the peat
excavation sides stable.
In deeper bogs local pockets of peat can be left unexcavated. These can produce bearing
capacity problems and settlements in the finished embankment where they are left in place;
If the peat has a low shear strength, the sideslopes of the excavations may become unstable
and slide into the excavations before they can be backfilled. This can significantly increase
the expected excavation quantities;
Adjacent structures and buildings alongside the excavation may be adversely affected by the
removal of the side support if not adequately protected;
Suitable storage areas need to be identified locally for the disposal of the excavated peat;
The new embankment can act as a linear drain and affect the hydrology of the area
Diagram of a long section through an embankment being constructed by the progressive displacement
method.
Diagram of a long section through an embankment being constructed by the progressive displacement
method.
Diagram of a long section through an embankment being constructed by the progressive displacement
method.
In this method a standard embankment is constructed up to the edge of the peat deposit and then
driven across the peat by end tipping, normally helped by a surcharge at the point of the
advancing embankment to maximize the local displacement weight. The effect of the combined
weight of the embankment and the surcharge is to cause a shear failure in the peat/soft soil ahead
of the embankment, and a ‘displacement’ of the peat/soft soil to the side of the advancing
embankment.
Diagram of a typical cross section through an embankment formed by the progressive displacement
method.
Diagram of a typical cross section through an embankment formed by the progressive displacement
method.
Diagram of a typical cross section through an embankment formed by the progressive displacement
method.
A disadvantage of the progressive displacement method is that waves of displaced peat/soft soil
will be formed at the sides and front of the embankment as it is being placed, and these can act
passively to prevent the displacement continuing. Displaced peat/soft soil can also have an affect
on adjacent structures and buildings, even at some distance from the main axis of the
displacement. Adjacent structures, within 5 times the peat/soft soil depth, should be identified
and considered before the works commence.
Diagram showing effect of displaced peat and soft soil on adjacent buildings
Diagram showing effect of displaced peat and soft soil on adjacent buildings
Diagram showing effect of displaced peat and soft soil on adjacent buildings
Once a progressive displacement is started within a peat deposit it will usually be able to be
continued, provided that the embankment height above the surface of the peat is held constant by
adding further fill material. It may however be necessary in some marginal locations to remove
the displaced peat in front of the embankment, and deposit the material off the road line, to
ensure that the displacement continues. The recommended practice in Sweden is to excavate out
the top layer of the peat just to get the displacement going (as in Partial excavation in section
7.4.2). The displaced peat at the sides of the embankment can add to the overall stability of the
embankment by acting as pressure berms. [Link to section 7.5.2]
Once the displacement has been completed the surcharge is normally left in place for a sufficient
period (usually months) to further force the consolidation of any remaining trapped pockets of
peat/soft soil. This will ensure that the completed embankment is ‘bedded down’ before the final
road construction layers are placed. The amount of displacement achieved during an
embankment drive will be a result of a number of factors, all interdependent:
the weight of the new embankment versus the strength of the underlying peat/soft soil
the shape and volume of the embankment versus the depth and type of peat/soft soil to be
displaced
the topography of the hard layers below the road line
All of these elements need to be known and quantified before the quality of the displacement can
be assured.
As with the replacement method, care needs to be taken to avoid trapping pockets of peat/soft
soil below the embankment during the displacement drive. Progressive displacement is best used
when it is known that the topography of the underlying hard layer can permit the embankment to
move forward downhill without trapping pockets of peat/soft soil. If the direction of advance of
the embankment can be controlled ‘downhill’ it is possible to prevent situations that would cause
peat or other soft material to be trapped under the embankment on the ‘uphill’ side of the
direction of travel.
Diagram showing the direction of advance of a progressive displacemnet being controlled ‘downhill’ to
prevent peat/soft soil being trapped under the embankment on the ‘uphill’ side of the direction of travel.
Diagram showing the direction of advance of a progressive displacemnet being controlled ‘downhill’ to
prevent peat/soft soil being trapped under the embankment on the ‘uphill’ side of the direction of travel.
Diagram showing the direction of advance of a progressive displacemnet being controlled ‘downhill’ to
prevent peat/soft soil being trapped under the embankment on the ‘uphill’ side of the direction of travel.
It is normal practice to take proving cores through the completed embankment at the end of the
displacement to check if the displacement has been successful. Where peat/soft soil pockets are
detected it is usual to either allow time for the trapped peat/soft soil to consolidate under an
addtional surcharge, or to blast out the material from below the embankment by strategically
placed explosives.
The partial excavation method is particularly useful where the top layers of the peat deposit are
very fibrous or woody. Where these layers exist they can act as a surface reinforcement to the
peat and resist the displacing forces of the embankment. In these circumstances the fibrous layers
can be excavated out and the remaining peat displaced by the embankment assisted by a
surcharge. This method has been successfully used in Finland for replacement depths of 10-12
metres.
By water jetting
Water jetting involves pushing water jet lances into the base of the peat ahead of the
embankment front to locally increase the water content of the peat, and reduce its shear
resistance. The lances are then slowly withdrawn whilst the water is being pumped into the
ground. This maximises the volume of peat treated.
By blasting
Blast assistance to aid displacement can be carried out by a number of means:
“trench shooting”
Diagram of blast assistance by toe shooting from the Norwegian Road Research Laboratory 1990
Diagram of blast assistance by toe shooting from the Norwegian Road Research Laboratory 1990
Diagram of blast assistance by toe shooting from the Norwegian Road Research Laboratory 1990
“toe shooting”
Diagram of blast assistance after completion of filling from the Norwegian Road Research Laboratory
1990
Diagram of blast assistance after completion of filling from the Norwegian Road Research Laboratory
1990
Diagram of blast assistance after completion of filling from the Norwegian Road Research Laboratory
1990
and “underfill blasting”
Diagram of a sequence of underfill blasting from the Road Research Laboratory, Road Research
Technical Paper No 40
Diagram of a sequence of underfill blasting from the Road Research Laboratory, Road Research
Technical Paper No 40
Diagram of a sequence of underfill blasting from the Road Research Laboratory, Road Research
Technical Paper No 40
These methods are described in greater detail in the ROADEX II report “Dealing with Bearing
Capacity Problems on Low Volume Roads Constructed on Peat”.
This section will look at 6 groups of methods that utilise the underlying peat as a load bearing
layer. These are:
7.5.3 Reinforcement
7.5.5 Piling
7.5.6 Stabilisation
Fibrous peats are ideally suitable to stage loading as they have excellent initial properties of high
compressibility and permeability. Amorphous peats can also benefit from the technique but the
timescales for the stages will be longer. The rate of stage loading is normally determined by the
rate of dissipation of porewater from the peat matrix. This can be estimated from the basic peat
properties but is best done by monitoring the settlement with settlement plates, or by directly
reading of piezometers in the field.
Significant settlements can be incurred during the stage loading operations and these should be
known and their effects understood with reasonable accuracy at the design stage so that they do
not come as a shock to the engineering staff on site. Preloading is generally considered to be a
cost effective solution for peat depths of up to 4m. The method can of course be used for greater
depths than this but the surcharge required will be that much larger and take longer to achieve
the desired effect.
Preloading
Preloading is a method that improves the strength of peat by accelerating its consolidation so that
the peat can be capable of supporting the intended load earlier. Peat is well suited to preloading
as it has a very high permeability in its natural state and compresses in a relatively short time
when loaded. The principle is relatively simple. A load in excess of what is required is placed on
the peat and allowed to settle until it reaches the predicted settlement. Once this settlement has
been reached the excess load is removed and the service load left on a strengthened foundation.
Diagram showing the effect of preloading a peat mass. There may be a slight recovery of the peat after
the removal of the load.
Diagram showing the effect of preloading a peat mass. There may be a slight recovery of the peat after
the removal of the load.
Diagram showing the effect of preloading a peat mass. There may be a slight recovery of the peat after
the removal of the load.
Preloading with a surcharge is generally considered to be the most economical method of road
construction in the Northern Periphery, and results in what is commonly called a “floating” road.
The method is normally restricted to thin embankments close to the natural ground, usually
limited to an embankment height of 2-3m above the adjacent peat level. The surcharge load is
usually constructed using temporary stockpiles of construction materials, such as sub-base or
roadbase materials, planned for use elsewhere on the road. The cost of using these types of
surcharges is therefore cost neutral in the overall costing of the project.
The amount of surcharge needed to achieve the desired settlement will be a function of a number
of things such as the type and depth of peat, its water content, the ground water level, distribution
of load, etc. Each installation will be unique requiring a geotechnical assessment of stability,
settlement and increase in strength. Experience in Sweden suggests that the unloaded in-service
embankment weight should be 80% of the surcharged embankment after taking buoyancy effects
into consideration. This equates to a nominal 25% surcharge over the weight of the final
embankment ignoring the effects of buoyancy over time. A good preloading exercise will aim to
maximise the length of time the preload is in place and have a large unload, preferably more than
0.5m above the finished road level.
7.5.2. Load modification
The ‘load modification’ group covers those methods that alter the load distribution of a road
embankment to better suit the existing strength of the peat.
Profile Lowering
‘Profile lowering’ means that the route vertical alignment is lowered at the design stage to suit
the strength of the underlying peat (normally no more than 3m above the peat level).
Cross-section showing the effect of reducing the height of an embankment through a profiling lowering
exercise.
Cross-section showing the effect of reducing the height of an embankment through a profiling lowering
exercise.
Cross-section showing the effect of reducing the height of an embankment through a profiling lowering
exercise.
This method can be extremely cost effective both in time and materials used and is certainly
worth considering for schemes crossing areas of peat.
Stabilising Berms
Stabilising berms, also known as ‘counterweight berms’ or ‘pressure berms’, are used to widen
the base of an embankment to distribute the embankment load over a greater surface area and
increase the factor of safety of the embankment against slip failure. As with all structures over
peat a stabilizing berm must firstly satisfy its own stability requirements and be constructed, and
monitored, in stages to remain in a stable condition at all times.
Slope Reduction
‘Slope reduction’ is similar to the addition of pressure berms and is used to produce a wider
embankment, a greater distribution of load over the foundation area, and a longer more deep
seated potential failure slip circle in the underlying peat.
Lightweight Fill
Lightweight fill is normally used to reduce the overall weight of an embankment and thereby
reduce the permanent stresses on the foundation. Embankments constructed with a lightweight
fill core are usually installed in conjunction with a temporary surcharge load to accelerate
consolidation and settlement.
Lightweight fills are normally only used as part replacements of embankments due to their high
cost and are generally restricted to those sections that cannot be economically constructed by
other means. A good lightweight fill material, in addition to being light, should also be durable,
resistant to decay, easy to place and compact, have a good compressive strength with low
compressibility, and be environmentally friendly. Some of the most popular lightweight
materials being used in the Northern Periphery are:
Dry Bulk
Density Density
Material kg/m³ kg/m³ Comments
LECA
Manufactured product. Lightweight aggregate produced by heat expansion of c
Lightweight Expanded Range of densities due to water absorption. Normally requires 0.6m of road co
Clay Aggregate 300-900 650-1200 May be difficult to compact if unconfined.
PFA
700- 1300- By-product of coal fired power stations, Naturally cementitious, especially use
Pulverised fuel ash 1400 1700 bridge abutments.
1000- 1400- By-products of heavy industry, steel furnaces, etc. Generally at the ‘heavy’ end
Slag 1400 1800 materials. Leachates can pose environmental problems.
500- 1100-
Aerated slag 1000 1700 Foamed by-product formed by quickly quenching molten slag in water.
650-
1000
1400-
Volcanic ash 1700 Natural material (particularly useful in Iceland).
100-300
Fresh wood is not recommended as it is difficult to compact. Aged bark can ha
Bark/woodchip 800-1000 properties and be
Manufactured product. Extremely light, generally produced in blocks, relativel
EPS Expanded 100 for 100kPa minimum compressive strength. Installations are usually capped with a
polystyrene 20 design Requires protection from petrol, fire and UV light.
Waste concrete products from precast concrete production, e.g. broken blocks,
Concrete waste 500-600 750-100 concrete, etc.
600- 1000- Manufactured product. Pre-foam added on site to ready-mixed mortar, 4MPa m
Foamed concrete 1800 1800 compressive strength
Past installations still exhibiting 20% buoyancy after 10 years submergence, no
Compressed peat bales 200 600-800 available.
Horticultural peat 200 500-800 “Garden peat bags”, laid flat as bulk fill, assume 800kg/m3 for long in situ den
Foamed glass Recycled glass product manufactured from waste cathode ray tubes, stable, ine
“Hasopor” 100-500 100-500 compressive strength 6-12 MPa.
Waste tyres bales 500-650 500-650 Waste tyres compressed into bales and bound with galvanized wires.
The low densities of some lightweight products are not however always a benefit in
embankments over peat as their light weight can pose buoyancy problems particularly in location
with high water tables.
The most popular lightweight materials today are LECA and EPS. The major advantage of EPS
is its low density of 20 kg/m³ although it is generally given a higher design value of 100 kg/m³
for stability and settlement calculations to allow for some water absorption over time. EPS
blocks are easy to transport and handle (up to 100m³ can be transported on a single vehicle) and
their only disadvantage, other than their production costs, appears to be that they can be
susceptible to petrol and chemical attack. This is usually catered for in careful detailed design.
EPS for roadworks is usually specified at a compressive strength of 100 kPa to limit local
pavement deflections under wheels. The completed installation is normally capped with a 100-
150mm reinforced concrete slab topped by a 500mm gravel road base to try to tie the
construction together and provide a heat storage mass to counter any variations in icing
conditions along the finished carriageway between lengths of normal construction and EPS
blocks.
EPS 2 Photograph of a concrete capping slab being poured on top of a completed EPS installation
EPS 1 Photograph of EPS blocks being placed in a lightweight embankment on a prepared base (NRRL)
EPS 2 Photograph of a concrete capping slab being poured on top of a completed EPS installation
EPS 1 Photograph of EPS blocks being placed in a lightweight embankment on a prepared base (NRRL)
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Lightweight forestry by-products such as bark, woodchip and sawdust wastes from the timber
industry have regularly been used a lightweight fills in the ROADEX Partner areas across the
Northern Periphery. These materials are normally installed with a covering layer of a low
permeability material, such as clay or topsoil, to keep them moist and isolate them from the
effects of the atmosphere. When forest by-products are left exposed to air they can decay and be
prone to spontaneous combustion if incorrectly handled.
Photograph of woodchip being used as a lightweight fill on a forest road in Scotland (Forestry
Commission)
Photograph of woodchip being used as a lightweight fill on a forest road in Scotland (Forestry
Commission)
Photograph of woodchip being used as a lightweight fill on a forest road in Scotland (Forestry
Commission)
Offloading
‘Offloading’ basically involves the removal of heavyweight material from an existing road
construction and its replacement with something lighter.
Illustration showing the ‘Offloading’ principle of removing the weight of the existing emabankment and
replacing it with something lighter
Illustration showing the ‘Offloading’ principle of removing the weight of the existing emabankment and
replacing it with something lighter
Illustration showing the ‘Offloading’ principle of removing the weight of the existing emabankment and
replacing it with something lighter
The aim of offloading is to produce a reduction in load on the underlying peat down to less than
its existing bearing capacity. Normally designers aim to achieve a reduction of load of 1/2 to 1/3
of the original embankment. If this can be achieved the new carriageway can be expected to be
relatively settlement free for the rest of its service life.
An offloading exercise on a low volume road over peat in the Scottish Highlands using bales of waste
tyres as a lightweight fill replacement
An offloading exercise on a low volume road over peat in the Scottish Highlands using bales of waste
tyres as a lightweight fill replacement
An offloading exercise on a low volume road over peat in the Scottish Highlands using bales of waste
tyres as a lightweight fill replacement
7.5.3 Reinforcement
Embankments can be reinforced, or “stabilised”, by a number of materials each governed by
their own particular technologies. The area of embankment reinforcement is probably one of the
more dynamic areas in road construction at the present time, and new manufacturers and new
materials continue to appear regularly in the technical press. Six groups of reinforcement will be
considered in this section
1. Geotextiles
2. Geogrids
3. Timber rafts
4. Concrete rafts
5. Galvanised steel sheeting
6. Steel mesh reinforcement of pavement layers
1. Geotextiles
A great deal of discussion has centred round geotextiles and their application to the two types of
road construction over soft ground, i.e. the ‘thin’ construction of roads and pavements and the
‘thicker’ construction of embankments. What is generally accepted is that for thin fills the
geotextile will act as a separator and filter, and the particular material should be chosen with
these properties in mind.
Diagram of a geotextiler under a thin embankment acting a separator between the peat and fill material
Diagram of a geotextiler under a thin embankment acting a separator between the peat and fill material
Diagram of a geotextiler under a thin embankment acting a separator between the peat and fill material
In the case of thicker fills the geotextile or geogrid will perform more of its true reinforcement
role and a suitable grade of reinforcement material should be selected. Here it will be necessary
for the designer to establish that there will be sufficient friction generated between the geotextile
and fill and underlying soil to resist the forces created.
Diagram of a geotextile below a thick embankment performing a reinforcement role.
Diagrams of typical geogrids and interlock. In a normal road construction roadstone aggregates are very
effective in compression but not good at resisting tensile forces. A geogrid on the other hand is excellent
at resisting tensile forces and dealing with tensile effects.
Diagrams of typical geogrids and interlock. In a normal road construction roadstone aggregates are very
effective in compression but not good at resisting tensile forces. A geogrid on the other hand is excellent
at resisting tensile forces and dealing with tensile effects.
Diagrams of typical geogrids and interlock. In a normal road construction roadstone aggregates are very
effective in compression but not good at resisting tensile forces. A geogrid on the other hand is excellent
at resisting tensile forces and dealing with tensile effects.
Interlocking forms a composite stabilised layer between the geogrids and aggregate, creating an
increased stiffness in the geogrid that helps to distribute the loads over a wider area than that of a
road without geogrids. A geogrid stabilised floating road does not eliminate settlement but the
geogrid makes it better able to distribute the loadings across the width of the geogrid and as such
helps to reduce differential settlement over weaker areas.
The key to achieving effective interlock with a geogrid is having the correct size and shape of
aggregate relative to the geogrid being used. This will depend, among other factors, on the
relative geometry of the geogrid and the aggregate. Ideally, there should be such an intimate
match between the geogrid and the aggregate that the interlocked “composite layer” is created.
Round gravels, moraines and large stones are not therefore generally suitable for use in the
interlock layer. “As dug” material may be suitable for use in the interlock area provided that it is
sufficiently well graded and angular to produce interlock.
Photograph of double layer geogrid installation being laid, 450mm apart, using crushed rock aggregate to
creat the interlock.
Photograph of double layer geogrid installation being laid, 450mm apart, using crushed rock aggregate to
creat the interlock.
Photograph of double layer geogrid installation being laid, 450mm apart, using crushed rock aggregate to
creat the interlock.
A system of two geogrids usually produces a stiffer road structure than that of a single geogrid
and this can help to minimise differential settlement across the peat. A separator grade geotextile
should be used below the base geogrid where there is the possibility of fine materials migrating
into the aggregate layer. Fines from the subgrade soil can reduce the effectiveness of the
geogrid/aggregate interlock and consequently affect the performance of the finished road.
Subgrade soils with a fines content >15% should be considered as a fine material and treated
with a separator geotextile.
3. Timber rafts
Timber raft construction is the oldest method of strengthening embankments over peat. The
technology has been around for many years and involves laying an interlocking platform of local
forest materials on the peat surface to support and distribute the loads of the new embankment
until such time as the underlying peat can gain sufficient strength to support the embankment on
its own.
Cross-section showing a timber mattress/grillage.
The simplest method uses bundles of locally available woody material (fascines), as the
structural elements. These are laid alongside each other on the peat surface before being overlaid
with a suitable filling material.
Bundles of fascines stacked at the side of a floating gravel road on peat
Bundles of fascines stacked at the side of a floating gravel road on peat
Bundles of fascines stacked at the side of a floating gravel road on peat
Timber grillages are the heavy equivalent of fascines and are designed to provide a resistance to
bending in the base of the embankment. In their simplest form they can comprise a single
platform of logs (corduroy) laid side by side at right angles to the road line.
Photograph of a wind farm road on a timber grillage
Timber grillages are currently not so popular as geotextiles or geogrids in the Northern Periphery
due to their high labour input and the cost of timber, but they can be competitive if suitable
timber is available locally. They should not be forgotten however as many roads in the Northern
Periphery still rest on grillages and these will require maintenance or widening in the future.
4. Concrete rafts
Reinforced concrete rafts or slabs were used very successfully in Scotland and Ireland from the
1920’s through to the 1950’s.
Photograph of the construction of a concrete floating road over Killimister Moss, Caithness, Scotland in
the 1950’s
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They were generally built in a series of slabs 200mm thick, doubly reinforced with edge
strengthening and were either constructed directly on to the peat surface or on top of a regulating
layer of sub-base material. They were stiff structures, much more so than other strengthening
systems such as mattresses, geosynthetics or grillages and needed minimum road construction
layers to distribute the traffic loads. Many of these concrete rafts still remain in service over deep
blanket bogs deposits in northern Scotland providing a stable load bearing platform for modern
traffic.
GPR long section of a 200mm thick concrete raft, constructed 1960. The concrete slab is shown in blue.
(The Highland Council)
GPR long section of a 200mm thick concrete raft, constructed 1960. The concrete slab is shown in blue.
(The Highland Council)
GPR long section of a 200mm thick concrete raft, constructed 1960. The concrete slab is shown in blue.
(The Highland Council)
A modern derivative of the reinforced concrete raft is the lightweight “foamed concrete” raft.
“Foamed concrete” is mixed on site by adding foam to a previously mixed concrete mortar. This
is done by mixing the selected foaming agent with water in a foam generator on site to produce a
“pre-foam”. The pre-foam is then folded into the prepared cement mortar in a concrete mixer to
produce the foamed mortar ready for placing. This is normally done by pumping direct to the
point of use. Compressive strengths of foamed mortars can range from 900 to 1500 kg/m² and
are dependant on the amount of pre-foam added and the cement content of the final material.
Photograph of lightweight foamed concrete raft used as a floating housing road in the Netherlands.
Photograph of lightweight foamed concrete raft used as a floating housing road in the Netherlands.
Photograph of lightweight foamed concrete raft used as a floating housing road in the Netherlands.
Normally 7mm corrugated steel plate is used with a zinc coating for corrosion protection. Sheets
can be installed transverse or parallel to the road line. It is considered that sheets installed
crosswise give a better bearing capacity and rutting resistance whereas sheets installed along the
roadline appear to be better at dealing with longitudinal depressions and frost heave.
Photograph of a road construction over peat using box profile galvanised sheeting from Temmes, Finland
(Finnra)
Photograph of a road construction over peat using box profile galvanised sheeting from Temmes, Finland
(Finnra)
Photograph of a road construction over peat using box profile galvanised sheeting from Temmes, Finland
(Finnra)
The EU REFLEX Project showed how steel meshes could prolong the service life of a road and reduce
maintenance costs by reducing the frequency and cost of individual rehabilitation measures across the
life of the pavement.
The EU REFLEX Project showed how steel meshes could prolong the service life of a road and reduce
maintenance costs by reducing the frequency and cost of individual rehabilitation measures across the
life of the pavement.
ROADEX recommends that a steel grid should always be installed in the lower layer of a road
structure over peat. This helps stiffen the structure and improves its load distributing properties
by tying everything together. The steel grid should extend over the full transverse width of the
road without joints (i.e no joints parallel to the line of the road). Experience has shown that
cracks will develop at such longitudinal joints or at the ends of the grids. Designers should
therefore ensure that the grid extends the full width of the road. Transverse joints between
adjacent grids along the road may be lapped or unlapped. Finland does not lap these joints,
Scotland laps them sometimes. Steel grids over culverts, pipes or cables may cause problems for
future maintenance operations and should be omitted at these locations or carefully designed to
prevent future problems.
In a normal embankment, without vertical drainage, the excess pore water pressures have to
migrate substantial distances before they can dissipate. With a vertically drained soil the
maximum distance to a drainage path is half the horizontal distance between the drains (normally
1.0 - 1.5m). This short drainage distance between the bands means that any excess pore water
pressures can be released more rapidly from the peat thereby quickening the transfer of the
embankment load to the soil skeleton.
The installation process normally consists of a grid of drainage elements (usually geotextile
bands) being driven vertically into the soil by a mandrel which is then retracted leaving the drain
in place.
Diagram showing the drainage paths with and without vertical drainage.
Diagram showing the drainage paths with and without vertical drainage.
Diagram showing the drainage paths with and without vertical drainage.
Vertical drainage in peat is generally only necessary for the more amorphous types of peat and
particularly when these are underlain by thick clay layers. Fibrous peats can usually be expected
to dissipate any excess porewater pressures quickly enough without having the need to resort to
additional vertical drainage acceleration measures.
The normal process involves laying a surface layer of free draining material to acts both as a
working platform and horizontal drain. The vertical drains are installed through this layer in a
triangular or square pattern, of which the square grid is generally the easiest to control but has
the largest drainage path for equal centres.
CFA (continuous flight auger) piles are increasing in popularity in the Northern Periphery and
can be very competitive with good production rates. The piles are formed by boring a
‘continuous flight auger’ into the ground that supports the sides of the hole with the soil within
the auger. When the auger reaches the required depth a sand-cement grout or concrete is pumped
down through the hollow stem of the auger as it is withdrawn up the shaft. Reinforcement is
placed immediately the auger has been withdrawn from the hole. CFA piles are available from
300mm diameter to 900mm diameter and can be driven to 30m deep.
Irrespective of the pile type chosen pile groups through peat are usually topped with 1 of 3 types
of cap: either a continuous concrete slab or individual concrete pile caps or a geotextile/concrete
cap combination.
Diagrams showing cross-sections of piling arrangements with continuous and individual piling caps, with
vertical or inclined piles.
Diagrams showing cross-sections of piling arrangements with continuous and individual piling caps, with
vertical or inclined piles.
Diagrams showing cross-sections of piling arrangements with continuous and individual piling caps, with
vertical or inclined piles.
Photograph of precast concrete pile caps placed on top of prepared concrete piles.
Photograph of an arrangement of cast-in-place pile caps on concrete piles.
Photograph of precast concrete pile caps placed on top of prepared concrete piles.
Photograph of an arrangement of cast-in-place pile caps on concrete piles.
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Good practice normally requires a pile group to be self supporting, i.e. as if the peat was not
there at all and ignoring any side resistance which may come from the peat. Inclined piles,
sometimes called "raking piles", are used to give added horizontal resistance where the
completed pile installation is expected to be affected by horizontal forces. Finland uses 2 or 3
rows of raking piles in all piled embankment installations as it is considered that future loadings
on the adjacent peat could result in horizontal forces on the piles.
Geosynthetics can also be used as pile caps and design philosophies are now available which
matches size and centres of caps to suitable strength geosynthetics fabrics to produce a ‘load
transfer platform’ rather than a rigid concrete slab.
Diagrams of the use of geosynthetics in pile caps and load transfer platforms.
Diagrams of the use of geosynthetics in pile caps and load transfer platforms.
Diagrams of the use of geosynthetics in pile caps and load transfer platforms.
In this process the ‘load transfer platform’ usually comprises one or more layers of geosynthetic
reinforcement and aggregate laid across the tops of the pile caps under the base of the proposed
embankment. As the embankment is constructed layer by layer on this 'platform' soil arching
occurs across the pile caps that transmits the embankment load into the piles and down to the
firm layer.
Photograph of an exploratory excavation through an existing load transfer platform on concrete piles
through peat. This clearly shows the underlying circular pile caps, the geosynthetic envelope enclosing a
crushed rock aggregate filling, topped by the main embankment fill.
Photograph of an exploratory excavation through an existing load transfer platform on concrete piles
through peat. This clearly shows the underlying circular pile caps, the geosynthetic envelope enclosing a
crushed rock aggregate filling, topped by the main embankment fill.
Photograph of an exploratory excavation through an existing load transfer platform on concrete piles
through peat. This clearly shows the underlying circular pile caps, the geosynthetic envelope enclosing a
crushed rock aggregate filling, topped by the main embankment fill.
Timber piles have been used for low volume roads over peat in the past and recently they have
been gaining in popularity in Sweden as a method of settlement reduction in clay and silt. They
could therefore play a similar role in locations where peat overlies clay or clayey silt.
Photograph of wooden piles being used to reduce settlement in a road widening over clay in Sweden
Photograph of wooden piles being used to reduce settlement in a road widening over clay in Sweden
Photograph of wooden piles being used to reduce settlement in a road widening over clay in Sweden
Timber piles have also been used on forest roads where the availablity of cheap timber on site
makes the method extremely attractive and cost effective.
Photograph of a wooden pile installation capped with a steel grid on a forest road in Ireland. (Coillte
Teoranta)
Photograph of a wooden pile installation capped with a steel grid on a forest road in Ireland. (Coillte
Teoranta)
Photograph of a wooden pile installation capped with a steel grid on a forest road in Ireland. (Coillte
Teoranta)
So far mass stabilization projects in the Northern Periphery have been carried out using a mixing
tool mounted on an excavator boom. A typical stabilized ‘block’ in road improvements normally
comprises 8 to 10 square metres in plan and 3 to 5 metres in depth and is usually surcharged with
0.5m to 1m of fill material immediately after the completion of mixing to compress the stabilised
material and increase its strength. This surcharged area in turn acts as the working platform for
the machine for the next section.
Cross section through stabilised peat, Source: N Jelisic, Mass stabilization of peat in road and railway
structures.
Cross section through stabilised peat, Source: N Jelisic, Mass stabilization of peat in road and railway
structures.
Cross section through stabilised peat, Source: N Jelisic, Mass stabilization of peat in road and railway
structures.
The strength of the stabilised soil depends on the type and quantity of binder as well as the
properties of the natural soil. A typical undrained shear strength for stabilised peat normally lies
within the range of 50 – 150 kPa. The Figure below shows some typical peat constituent
components and volumes in the various stages of a stabilisation project. The figures shown were
measured in Swedish case history Sw7, Road No 44 between Uddevalla and Trollhättan.
Diagram showing some typical peat constituent components and volumes in the various stages of a
stabilisation project. From the left, the 4 columns are: Column 1: The natural unloaded peat, water
content 2000 %, void ratio of 26. Column 2: The stabilised peat immediately after stabilising with
200kg/m³ of cement. The volume increased by approx 20%. Void ratio = 6.8. Column 3: The stabilised
constituents after 6 months curing and consolidation with a surcharge of 3m. Void ratio = 5.4. Column 4:
Comparable preloading operation, without stabilisation. Need a longer time for preloading with larger
settlements. Void ratio after consolidation approx 10.9.
Diagram showing some typical peat constituent components and volumes in the various stages of a
stabilisation project. From the left, the 4 columns are: Column 1: The natural unloaded peat, water
content 2000 %, void ratio of 26. Column 2: The stabilised peat immediately after stabilising with
200kg/m³ of cement. The volume increased by approx 20%. Void ratio = 6.8. Column 3: The stabilised
constituents after 6 months curing and consolidation with a surcharge of 3m. Void ratio = 5.4. Column 4:
Comparable preloading operation, without stabilisation. Need a longer time for preloading with larger
settlements. Void ratio after consolidation approx 10.9.
Diagram showing some typical peat constituent components and volumes in the various stages of a
stabilisation project. From the left, the 4 columns are: Column 1: The natural unloaded peat, water
content 2000 %, void ratio of 26. Column 2: The stabilised peat immediately after stabilising with
200kg/m³ of cement. The volume increased by approx 20%. Void ratio = 6.8. Column 3: The stabilised
constituents after 6 months curing and consolidation with a surcharge of 3m. Void ratio = 5.4. Column 4:
Comparable preloading operation, without stabilisation. Need a longer time for preloading with larger
settlements. Void ratio after consolidation approx 10.9.
Table of costs for the rehabiltation options shown above. The costs given are in Swedish crowns for the
finished road structure.
Table of costs for the rehabiltation options shown above. The costs given are in Swedish crowns for the
finished road structure.
These different methods will however give quite different final road products and the costs for
maintenance of the completed roads will vary depending on the form of road achieved. The
relative maintenace needs of the range of types of roads on peat is beyond the scope of this
lesson and all that can really be said is that the likely future maintenance needs should be one of
the factors that should be taken into account when deciding on a project involving peat.
Stages 1-3 can cost around 2-4% of the overall cost of a project, depending on the size of the
works, but is always money well spent in producing the best long term solution. Sufficient
resources should always be allocated to these stages to ensure that all problems underlying the
damaged road sections are correctly diagnosed, and the most appropriate solution chosen.
Importantly, any data gathered is never just a “single-use” investment. Data, once collected, is
capable of being utilized in many ways for many years not only in the design and rehabilitation
of roads but also for the performance management of the completed road structure over its
lifetime. With good monitoring and record keeping the data can be kept on file for future works
to be used as a reference source, increasing the collected experience, and identifying any trends
in the growth of defects. It is recommended that all of the information collected should be saved
with its linkages, GPS or similar, so that it can be re-accessed and assessed jointly again in the
future.
With the collected survey data in place the next phase, an “integrated analysis”, can be
commenced to understand the underlying problems.
The practicalities of site management will of course have a significant affect on what
rehabiliation options will be possible:
Traffic management – can the road be closed, is it wide enough for single lane traffic, is a
temporary bypass road possible alongside the existing road
Execution – how will the works be tackled. A rehabilitation using steel grids will need to
have the grid installed across the full width of road, approx 30cm deep, without a longitudinal
joint. This may not be possible with live traffic flows and it may be necessary to build on top
of the existing road and accept that settlement that will happen.
Construction vehicles – how will the heavy construction traffic affect the road? Work cycles
will have to be planned in detail to ensure that sufficient strength is avaialble within the road
to support the planned construction activities.
The final decision on the choice of rehabilitaion option will be a balance of the available
engineering solutions and their effects.
In this, lowering the groundwater table, such as with a new roadside drain, can have the same
effect as adding weight to the road. When the groundwater table is lowered below a road on peat
the hydrostatic uplift on the road, its “buoyancy”, is reduced and the drained road thereby
becomes effectively heavier. This results in a heavier load being placed on the underlying peat
and consolidation and settlement being triggered.
In the example above, lowering the water table by 1.0m will cause an increase in load of 8kN/m²
on the peat. Lowering the water table by a more modest 0.5m will cause an increase in load of
4kN/m². These changes in water level will take time to happen, and the embankment will
respond incrementally as the water table is lowered, but the effect in the long term will be the
same. The load on the underlying peat will be increased and a settlement will occur.
The preferred option with roads on peat is therefore not to affect the established hydrology of the
area.
a) The standard rehabilitation structure. Where the problem is limited to the construction layers
(minor settlements, cracking, alligator cracking) and the rehabilitation can be accommodated
within the road construction layers without adding additional load;
b) The lightweight structure. Where the problem is limited to the construction layers (moderate
settlements, cracking, deformations) and the rehabilitation can be accommodated within the road
construction layers with the use of lightweight material;
When using steel grid reinforcement in the rehabilitation structure (and this is recommended in
all road rehabilitation on peat), the weight of the steel grids must also be included in the weight
of the rehabilitation layers. Steel grid reinforcement over culverts, pipes or cables may cause
problems for future maintenance operations and should be omitted at these locations or carefully
designed to prevent future problems.
When used appropriately, a lightweight structure can restore the road profile to its previous level
without adding weight to the road and, where circumstances permit, even allow the grade line to
be raised. The lightweight material in the new structure should be enclosed by a suitable
geotextile separator selected to suit the lightweight fill and strong enough to resist punching by
the base course material aggregate.
It is recommended that a minimum of 400mm of road construction material is placed on top of
the lightweight material to act as a structural layer. This depth will also provides a heat storage
mass to counter any variation in icing conditions along the finished carriageway between
sections of normal road construction and lightweight fill. This is a major consideration in areas
with long cold winters such as in the Northern Periphery.
For this to happen a ‘transition wedge’ must be constructed in the sound material before crossing
on to the peat. A typical transition wedge is shown below:
8.4.3. Embankment settlement
This section deals with the typical road rehabilitation problem of a floating road settling into peat
using the general principle of ‘causing no further harm’. In this case the problem lies deeper
than the construction layers and the rehabilitation has to include the replacement of some, or all,
of the road embankment with lightweight material.
The Lightweight Embankment Replacement Structure
The lightweight embankment replacement structure generally follows the same sequence as that
of the lightweight structure, but with greater awareness of the need to protect the existing water
table and hydrostatic uplift effects on the embankment. These forces are essential to preserve the
established equilibrium in the peat and every effort should be made to understand the
implications of the changes being proposed and their effects on the permanent works before
commencing work.
The construction sequence for the lightweight embankment replacement structure is, as before:
A common method to widen a road on peat is to excavate out the adjacent peat for the widening
and build the new width of road on the exposed firm layer in the recommended fashion, the so-
called “legs” solution.
This can however be an expensive practice for roads on peat, particularly for ‘floating roads’
over deep peat, and can also pose real problems for the road if the new widened area is allowed
to act as a linear drain through the peat. The new fill material can dewater the peat below the
existing road and cause unnecessary settlement, consolidation and deformation to the road.
A cheaper, and less harmful, solution for widening ‘floating’ low volume roads over peat is by
preloading.
1. dig new intercepting ditch 10m off the old road and use the excavated peat to refill the
existing roadside ditch
2. remove any fine materials from the road shoulders, approx 200mm deep
3. lay a separator grade geotextile on prepared shoulder and reform the cross-section with good
material
4. lay 5m wide reinforcement grade geotextile below the area to be preloaded
5. commence preloading of the road widening in 1m stages until the designed preloading height
is reached
6. leave preload in place for 90 days and monitor performance by means of settlement plates
7. remove the excess preload material after the designed settlement has been achieved
8. construct the widened road layers
Widening using preloading (Swedish Transport Administration) The red line indicates the new
road position. This has been drawn above the existing road for ease of reference.
This method can be a cost effective solution where the existing road construction has become
stable enough over its lifetime to permit its retention in the new works. A geotechnical input will
be needed for this type of widening to estimate the height and duration of the preloading required
together with the likely predicted settlement.
Ideally the new road structural layers should be constructed within the depth of the existing road,
as in the “Standard Rehabilitation Structure” in 8.4.2 (a), to avoid adding new load to the peat. If
however a higher, or heavier, road is to be built on top of the widened embankment the effects
of the increased loadings have to be understood.
An additional benefit of this form of widening is that it can usually be carried out on the existing
road without affecting traffic flows. A typical road widening project using the preloading
principle, as carried out by The Swedish Road Administration and the Icelandic Road
Administration, is shown below:
8.4.5. Reinforced overlays of paved roads
Reinforced overlays can be useful rehabilitation solutions for deformed and cracked paved roads
where the settlement of the carriageway is not a major consideration. They have been used with
good results on rural low volume roads in the Scottish Highlands and in Ireland both with
polyester geogrids and twisted steel mesh. The basic structure for these installations is shown
below:
The screen below shows the results of a steel reinforced overlay on a single track paved road in
northern Scotland. The steel reinforced section extends from 20540 to 20640 and can be seen in
the GPR plot as a blurry area indicating noise feedback from the grid. The improvement in
pavement performance can be seen in the plots of the FWD taken before the project (black lines)
and with the steel grid (red lines). The steel grid used was a Maccaferri twisted steel
“Roadmesh”.
Reinforced replacement structure for paved roads
If existing bituminous layers are thick enough to allow them to be milled, the exercise can also
be carried out within the weight of the existing road. In this circumstance the process is as
follows: