technical note

A simple stability check for piling rigs

David Corke, director, DCProjectSolutions

Introduction
Although manufacturers have to
design piling rigs to comply with
Eurocode minimum stability
requirements, this cannot simulate
rig behaviour on all sites with a range
of working platform conditions
and all possible combinations of
operation and loading.
Cranes are generally equipped
with safe load indicators, but for
piling rigs, due to their variable
geometry and relatively complex
modes of operation, there has only
been a limited application of such
devices for specific types of use,
such as some driven piling.
Electronic systems for
monitoring and safely controlling
piling rig operations are becoming
more available, but may be an
optional item rather than being
fitted as standard. At present, there
is no available simple means of
checking rig stability for site specific
operations. This paper presents the
Rig Stability Factor as a proposed
solution.
Overturning of a piling rig may
be caused by inadequate bearing
capacity of the working platform
provided. However, a combination
of the rig configuration and Eurocodes BS EN 996:1995 that must be complied with by requirements for drilling and
excessive loading applied by +A3:2009, Piling Equipment. the rig manufacturers. These two foundation equipment.
winches or hydraulic rams may also Safety requirements; and BS EN Eurocodes are currently being The Eurocodes require that
result in a rig overturning, without 791:1995+A1:2009, Drill Rigs – redrafted into a single combined operational manuals provided
any bearing capacity failure of the Safety, set out requirements for document with the purpose with each rig include information
working platform. stability of piling and drilling rigs of specifying common safety on loading and stability, but

300 Maximum track load

Characteristic loading (kN/sq m)

Minimum track load

400
Track load (kN)

250

200 300 All load is on this track

when the rig body is at 90o
to the axis of the tracks
150
200
No load is on this track
100 when the rig body is at 90o
to the axis of the tracks
100 ie the track is about to lift
50 off the ground

0 0
0 15 30 45 60 75 90 0 15 30 45 60 75 90
Angle of rig body to axis of tracks (deg) Angle of body to axis of tracks (deg)

Figure 1: Characteristic track loading for penetrating. Figure 2: Track loads for penetrating.
BRE load case 2 BRE load case 2

ground engineering september 2011 45

cylindrical failure mode. The equation for the ultimate plate bearing pres-
2500

APPLIED TEST PRESSURE (kPa)

sure (pult) is as follows:
pult = (cu x Nc) + (2 x D2 x γ x Kp.Tanδ)/B
Where cu=sub-grade shear strength, Nc=bearing capacity factor (6.17),
2000
D=platform thickness, γ=platform material density, Kp.Tanδ=punching
shear coefficient from BR 470.
Use of this test to assess the angle of friction of the platform material will 1500
depend on the reliability of the sub-grade shear strength used in the design
or derived from subsequent testing.
However, as a plate of similar diameter to the track width has a bearing 1000
area smaller than the effective bearing area of the track loading, the response
of the plate test will not be directly representative of the response of the
platform to the rig loading applied to the full loaded track area. 500
Acceptability criteria for tests on a particular site should be considered
in advance of testing, taking into account the loading to be applied and the
0
sensitivity to the possible variability of the sub-grade and platform material 0 2 4 6 8 10 12
parameters..
PLATE SETTLEMENT (mm)
Case studies
Case Study 1: Plate testing on granular working platform material Figure 3: Working platform material plate loading test
(maximum particle size 75mm), with a 300mm diameter plate (see
figure 3). 400

STRESS UNDER PLATE (kPa)

At the maximum applied test pressure of 2,250 kPa, the loading is not show- y = 98.975In(x) + 22.988
ing any signs of approaching failure. Even if the maximum applied test
pressure was taken as an ultimate value it would be indicating an angle of
300
frictionof 48°, i.e actual angle of friction greater than 50°. The working
platform design was based on an angle of friction of 40° for the platform
material and the resulting platform thickness required was 450mm. If the
platform design could have been based on an angle of friction of 50° the 200
platform thickness could have been reduced to 300mm.
PBT1
Case study 2: Plate testing on loose granular made ground sub-grade PBT2
100
with 450mm diameter plate PBT3
Load displacement curves from a series of plate bearing tests on a loose PBT4
granular made ground sub-grade were extrapolated to notional failure using
a log-linear curve fitting technique, (see figure 4). The ultimate bearing stress 0
was defined as that which induced 10% R plate settlement. Given the vari- 0 5 10 15 20 25 30 35
ability in the test response, a low bound ultimate bearing stress of 330kPa PLATE SETTLEMENT (mm)
was taken and this allowed an angle of internal friction of 40˚ to be dem-
onstrated. The piling platform was subsequently calculated to be the default Figure 4: Plate bearing tests on loose granular sub grade
minimum of 50% of the piling rig track width.
500
STRESS UNDER PLATE (kPa)

Case Study 3: A working platform for a tracked mobile crane was

tested using a 300mm diameter plate
400
The load displacement response, shown in Figure 5, was found to be less
stiff than that of the clay sub-grade beneath, which was separately tested,
and significant strain hardening observed in the test made interpretation 300
uncertain. The back calculated friction angle for the platform based on an
ultimate bearing stress of 150 kPa was found to be 38˚.
The designer’s assumed value was 45˚. On further examination, it became 200
apparent that the platform material had not been compacted other than
nominally beneath the tracks of the mobile crane. After re-compaction,
the platform was re-tested and the design value of angle of internal friction 100
shown to be satisfactory.
0
Conclusions and Recommendations 0 2 4 6 8 10
The working platform design process set out in BR 470 is sensitive to the
angle of friction of the granular platform material and the shear strength of PLATE SETTLEMENT (mm)
the sub-grade.
If conservative input design parameters are used then the platform thick- Figure 5: Plate bearing tests on uncompacted platform
nesses produced will also be conservative.
More detailed reporting of shallow depth soil strengths from site inves- References
tigations would improve the economics, reliability and safety of working
platforms. If appropriate for the site and project, preliminary testing of the 1. Working platforms for tracked plant. BR 470. Building Research Establish-
platform material will provide data from which the angle of friction of the ment. 2004.
platform can be derived, and then used for the platform design. 2. Ground Engineering as potential end uses for recycled and secondary aggregates.
This will not only ensure that the design is economical, but being based on 3. The Waste & Resources Action Programme. June 2004.
actual data it will be more reliable and safer than a design based on guessed Working Platform Design Sensitivity. Federation of Piling Specialists website.
and untested parameters, even if they appear to be conservative. www.fps.org.uk
Verification of design parameters improves the economics and safety 4. BS 1377 Methods of Test for Soils for Civil Engineering Purposes, Part 9, In-Situ
of working platform design, and plate loading tests are recommended as Tests. 1990
the preferred means of testing the sub-grade and the platform material for 5. Two and three-dimensional bearing capacity of footings in sand. Lyamin
design validation and construction quality control purposes. AV et al, Geotechnique 57, No.8, 647-662, 2007.

ground engineering february 2010 31

Proceedings of the Institution of
Civil Engineers
Geotechnical Engineering 161
December 2008 Issue GE6
Pages 279–281
doi: 10.1680/geng.2008.161.6.279

Paper 14201 J. A. Charles D. Corke H. D. Skinner

Keywords:
design methods & aids/piles &
piling/temporary works

Briefing: Working platforms for tracked plant

J. Andrew Charles, Building Research Establishment, UK, David Corke, DCProject Solutions, UK
(previously Bachy Soletanche) and Hilary D. Skinner, Ramboll Whitbybird, UK (previously BRE)

If a piling rig overturns there can be a risk of casualties. ground-supported working platforms constructed of granular
Working platforms are critical for plant stability, and a material, so that an acceptable level of safety is achieved. A
good practice guide to the design, installation, secondary objective of the guide is that safety should be
maintenance and repair of ground-supported working achieved without unnecessary or excessive expenditure.
platforms for tracked plant has been prepared. A
simplified approach to the design calculations has been The guide is not intended to replace or reduce the designer’s
based on a punching shear failure mechanism. The input, but rather to promote the implementation of minimum
platform design is an integrated package, from track design, installation and maintenance standards. The guidance
loading through to geotechnical design. has sought to avoid being over-prescriptive, as this might limit
the scope for innovative solutions, and does not in any way
NOTATION limit the responsibilities of those parties involved in the
cu undrained shear strength of cohesive subgrade provision and operation of a working platform, who must
D depth of platform material exercise their own knowledge, experience and judgement.
Kp tan
punching shearing resistance coefficient
L effective track length of plant 2. GOOD PRACTICE GUIDANCE
Nc bearing capacity factor for cohesive subgrade Following an introductory chapter, the guide has four chapters
Nªp bearing capacity factor for granular platform that deal with, respectively, site conditions, design, installation
material and, finally, operation, maintenance and repair.
R bearing resistance
Rc bearing resistance of cohesive subgrade when no The strength of the near-surface ground is critical for the
platform is present design of the working platform, whereas for most construction
Rp bearing resistance of working platform when projects it is the strength at greater depths that is important.
platform is sufficiently deep that bearing The near-surface strength is unlikely to have been determined
resistance is unaffected by underlying cohesive in the site investigation for the construction project, and it may
subgrade be necessary to augment information obtained for the
sc , sp and sª shape factors; functions of W and L construction project with further investigation specifically for
W track width of the plant the working platform. The desk study and site inspection are
ªp bulk unit weight of platform material critically important, and should be carried out with the
9 angle of shearing resistance of granular material platform in mind; it is particularly important to locate any
weak and variable areas. Care must be taken in backfilling trial
1. INTRODUCTION pits to avoid creating additional hazards.
Working platforms are critical for plant stability, and while
most ground-supported working platforms perform well, The design of a working platform is a geotechnical design
overturning of rigs has occurred more frequently than it should. process, and should be carried out by a competent engineer
Although serious incidents involving working platforms are who has full access to the health and safety plan, including the
relatively rare, when they do occur there can be a risk of desk study and all other site investigation information. The
casualties. A good practice guide for ground-supported working quality of the granular platform material is crucial, and it
platforms for tracked plant has been prepared at the instigation should be such that all the performance requirements, in terms
of the Federation of Piling Specialists (FPS). A contract was let of compactibility, durability, trafficability and drainage, can be
by FPS to the Building Research Establishment Ltd (BRE) to met. Materials that will undergo significant crushing when
prepare the guide under the direction of a Steering Group trafficked should be avoided for works of any significant
appointed by FPS. The guide was published in June 2004. 1 duration, because drainage may be impaired. Sometimes it may
be economic to incorporate geosynthetics to strengthen the
The principal objective of the guide is to facilitate the design, working platform as an alternative to using a greater depth of
specification, installation, operation, maintenance and repair of granular material. A separating geofabric between a clay

Geotechnical Engineering 161 Issue GE6 Briefing Charles et al. 279

 subgrade and the platform may improve long-term the tracks should be calculated in accordance with EN
performance by preventing the granular platform material from 791:1996 Drill rigs—safety and EN 996:1996 Piling equipment—
being punched into the clay. safety requirements. Using the weights of the various
components of the rig or crane, the overturning moments can
The platform should be installed to an appropriate specification be calculated for the range of operations that will be carried
that ensures that the design strength of the platform material is out, including standing, travelling, handling, penetrating and
achieved, and there should be adequate supervision by extracting, for different jib and mast orientations. The bearing
competent staff during platform installation. Careful pressure distributions, which can be calculated from the
observation during placement of the working platform material overturning moments, may be triangular or trapezoidal, but
may reveal weak areas of the subgrade that require design radically simplified stress distributions are recommended for
modifications. Where required, quality control tests should be use in the design calculations. Non-uniform loading
carried out. distributions can be transformed into equivalent uniform loads
over a reduced areal extent using the method described by
A formulation of good practice is of value only where it is Meyerhof. 2 For a more recent review of the problem, see
applied with careful supervision, control and monitoring of the Foundoukos and Jardine;3 further information on loading can
platform under appropriate contractual arrangements. also be found on the FPS website, www.fps.org.uk.
Throughout its working life, the platform should be under the
day-to-day control of appropriately experienced site staff. Two loading situations are considered.
Contractual arrangements should ensure that the platform is
adequately inspected, controlled and maintained. Site control (a) Case 1 loading applies to the situation when the rig or
should be adequate to ensure that it is not used by plant for crane operator is unlikely to be able to aid recovery from
which it has not been designed, and that working areas are an imminent platform failure.
clearly marked. Problems may arise where working platforms (b) Case 2 loading applies to the situation when the rig or
are also used as haul roads, because there is likely to be an crane operator can control the load safely, for example by
incompatibility between the two uses. releasing the line load, or by reducing power, to aid
recovery from an imminent platform failure.
The integrity of the working platform should be preserved at
the original designed standard throughout its working life. The guide gives recommended partial factors to be applied to
Inadequate performance of working platforms may be due to the loads derived according to EN 996:1996, which are
poor maintenance, and many problems experienced with rig converted to an equivalent uniform loading.
instability can be associated with failure to adequately
maintain and repair a working platform rather than with The design calculations indicate the depth of working platform
inadequacies in specification or installation. Figure 1 illustrates that is required for the given soil and loading conditions.
the hazard presented by a poorly backfilled excavation. Where a working platform of relatively shallow depth is placed
on a weak subgrade, a simple approach to the design
calculations can be based on the analysis of punching failure,
3. ROUTINE DESIGN CALCULATIONS as illustrated in Figure 2. The bearing resistance R is considered
The guide has two appendices: the first provides an approach to be the sum of the shear required to punch through a vertical
to routine design calculations, with some worked examples, plane in the granular platform material and the bearing
and the second gives guidance on the determination of capacity of the subgrade. It is considered that this mechanism
characteristic values of soil parameters. can be applied to the design of granular platforms overlying
subgrades of cu ¼ 20–80 kPa.
Working platforms are subject to complex loading conditions,
and it is not easy to reflect in the design calculations the Using the simplified analysis for a footing punching through a
loading that will actually be experienced. The pressure under dense granular layer overlying soft clay, the following

Loaded area

W L

Platform Punch D

Subgrade

Poorly backf illed excavations

Bearing capacity failure

Fig. 1. Hazard posed by poorly backfilled excavations Fig. 2. Punching failure mechanism

280 Geotechnical Engineering 161 Issue GE6 Briefing Charles et al.

expression is obtained for the bearing resistance of a platform
on a cohesive subgrade with a low water table. Rp
Granular material alone
cuNcsc
5
ªp D2
1 R ¼ cu Nc sc þ K p tan
sp
W

4
There are two limiting conditions.

(a) When D ¼ 0, the bearing resistance is simply that of the

R
cohesive subgrade 3
cuNcsc

2 Rc ¼ cu Nc sc
2

(b) When D is large, the bearing resistance is simply that of

the platform material when unaffected by the underlying
Rc
clay layer 1 Subgrade alone
cuNcsc

3 Rp ¼ 0:5ªp WNªp sª
0
0 1 2 3
Equation 1 used in conjunction with the limiting values given D
in Equations 2 and 3 indicates an increase in bearing resistance W
(R) with increasing depth of the platform (D), as illustrated in
Figure 3 for some typical conditions. The graph Fig. 3. Typical increase in bearing resistance (R) with
is in the form of a dimensionless plot of bearing resistance, increasing depth of platform (D) for well compacted granular
R/(cu Nc sc ), against platform depth and loaded width (D/W). platform material on soft clay subgrade
When the platform depth D ¼ 0, the bearing resistance is a
minimum, corresponding to the subgrade alone, Rc /(cu Nc sc ) (see
Equation 2); when D is large (corresponding to a very thick appraised by an appropriately experienced engineer, and in
platform) the maximum bearing resistance is that provided by situations where the routine design calculations are not
an infinite depth of platform material, Rp /(cu Nc sc ) (see appropriate it may be necessary to carry out more sophisticated
Equation 3). For the conditions that apply in Figure 3, the forms of analysis.
latter state is reached when D/W ¼ 2.5 and Rp /(cu Nc sc ) ¼ 5.6.
Between these two limits the bearing resistance of the
combined system can be determined using Equation 1. REFERENCES
1. BUILDING RESEARCH ESTABLISHMENT. Working Platforms for
The guide recommends values for the punching shear Tracked Plant: Good Practice Guide to the Design,
coefficient, Kp tan
, for a range of 9 as well as shape factors Installation, Maintenance and Repair of Ground-Supported
for this coefficient and for the subgrade bearing resistance. Working Platforms. Building Research Establishment,
Watford, 2004, BR470.
A simple addition to the capacity of the platform is suggested 2. MEYERHOF G. G. The bearing capacity of foundations under
to take account of the effect of geosynthetic reinforcement. eccentric and inclined loads. Proceedings of 3rd
International Conference on Soil Mechanics and Foundation
This approach to the calculation of the required depth of the Engineering, Zurich, 1953, 1, 440–445.
working platform contains many simplifying assumptions, 3. FOUNDOUKOS M. and JARDINE R. J. The effect of eccentric
which cannot fully represent actual soil behaviour. The method loading on the bearing capacity of shallow foundations.
described in the guide has been calibrated against other design Proceedings of BGA International Conference on
methods for strip loadings and the experience of FPS members. Foundations: Innovations, Observations, Design and
However, the results of calculations should be critically Practice, Dundee, 2003, pp. 297–305.

What do you think?

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Geotechnical Engineering 161 Issue GE6 Briefing Charles et al. 281

 Dalwadi and Dixon

4 CASE STUDIES onal geogrids was proposed and constructed using

the available recycled granular fill (6F5 Capping). In
4.1 Crossrail C310 North Woolwich, London this instance, site conditions dictated that the MSL
layer incorporated a non-woven geotextile separator
Crossrail is Europe’s largest construction project
placed directly below the bottom layer of geogrid on
with £19 billion invested to help improve the capi-
the formation to provide the additional function of
tal’s rail infrastructure. Based on the provided infor-
separation, to control any upward migration of fines.
mation by Hochtief Murphy JV, The North Wool-
The upper 300 mm thick granular material was re-
wich Portal constructed as part of Contract C310 in
placed with rolled concrete in accordance with the
East London by Hochtief Murphy JV provides a
client’s requirement. It was acknowledged by the
structure for the reception of the Tunnel Boring Ma-
construction team that despite the presence of very
chines (TBM) for the Thames tunnel driving from
low strength soils and high rig loadings, the working
Plumstead and a transition from the over ground to
platform designed with load transfer approach and
the underground sections of Crossrail. A working
stabilised with hexagonal geogrid was extremely sta-
platform was required to safely support various Pil-
ble and performed as required.
ing Rigs and associated plant within the North
Woolwich Portal over a length of 200 m for the con-
struction of the Diaphragm Walls, Secant CFA Piling
Walls and Rotary Bored piles
The ground profile varies across the area of work-
ing platforms. The upper crust comprises Made
Ground mainly granular material, underlain by allu-
vial clay and peat of variable thicknesses. Due to the
site constraints the existing Made Ground layer was
required to be excavated to achieve the necessary Bottom layer of hexagonal stabilizing geogrid placed on
top of geotextile separator
head room for the piling operation and other site ac-
tivities. So the piling platform was required to be
constructed on the alluvium clay foundation which
4.2 Stoke Quay, Ipswich
can be described as a very soft to firm dark grey
slightly sandy clay. Based on the soil test results, an An apartment block and town house development
undrained shear strength of 15 kN/m2 for alluvial was planned at Stoke Quay, Ipswich, UK. The
clay and peat material was adopted for the working apartment required an 18,000 sq.m. piled foundation
platform design. The design approach given in as the ground conditions were very poor. The cleared
BR470 is not applicable where the undrained shear site which has had several former uses was covered
strength is less than 20 kN/m2. Thus an alternative with a layer of made ground underlain by sand and
approach was required to be adopted to provide sta- gravel glacial deposits on geological strata. The made
ble, safe and economical working platform. In 2012 ground could be regarded as cohesive having un-
the JV commissioned Tensar International to carry drained shear strength of 30 kPa and granular made
out the stabilised working platform design. Extensive ground with angle of internal friction of 28-30 de-
research background for the stabilised granular mate- gree. The ground water regime was locally tidal and
rial with hexagonal geogrids and years of experience considered at 1 m depth for design.
in similar conditions meant that an alternative to the An initial design assessment was carried out ac-
BR470 analysis method could be adopted. Load cording to BR 470 guidelines with and without struc-
spread approach through the Mechanically Stabilised tural geosynthetics. The proposed thickness for non-
Layer (MSL) was adopted to assess the thickness of stabilised platform and with geosynthetic reinforce-
working platform over a challenging ground and ment (Refer- figure 6) having short term tensile
loading conditions. These rigs induced high track strength (T ult) of 30 kN/m was 1135 mm and 900
loads including a Load Case 2 equivalent UDL pres- mm respectively. A working platform thickness in
sure of 355kPa. The MSL with three layers of hexag- range of 1 m was considered uneconomical and time

1399
Geotechnical Engineering for Infrastructure and Development

consuming by main contractor ISG plc. who sought

an alternative design using a stabilised platform ap-
proach. The design was carried out in accordance
with the BR470 guideline, where alternative design
methodologies can be used for the incorporation of a
‘structural geosynthetic ’. The variety of alternative
approaches that can be used include load spread, load
factor and bearing capacity improvement. To use an
alternative method, the requirements are:
1. The objective of safety is preserved Alternative
Working platform with hexagonal geogrid at Stoke
approaches are based on credible and representa- Quay. On-site crushed material was used extensively
tive research work
2. A person competent in both geotechnical engi-
neering and geosynthetics is made responsible
5 CONCLUSION
for the design
An alternative design approach- Load Spread The safety of working platforms for tracked plant is
method using a mechanically stabilised layer with crucial and BR 470 Guideline has been effective in
hexagonal structure geogrid was adopted to demon- raising industry awareness of this within the UK.
strate safe and economical design. Based on the in- However it is widely recognized that the analytical
formation, the working platform thickness was as- design methods used there can produce excessively
sessed for cohesive and granular ground conditions thick platforms, requiring large quantities of granular
and the following sections were produced for the crit- fill material with associated economic and environ-
ical rig track pressure of 270 kPa. mental costs as well as slower installation which can
impact on the overall site program.
BR 470 does recognise that alternative empirically
derived design methods may be acceptable. These
empirical methods however need to be backed up
with credible experience, research testing and moni-
tored performance. A load spread method of platform
design based on the stabilisation of granular fill using
hexagonal structure geogrids has been proven to pro-
duce safe working platform designs which require
relatively thin platforms. Furthermore this method
can be applied to platforms over very weak for-
mations with strengths below the range permissible
Figure 6. Working platform sections with BR470 and load spread method within BR 470

The platform was constructed in 2012 and a series

of plate load tests were carried out on the top of the ACKNOWLEDGEMENT
platform prior to piling operations to ensure that the
applied design pressures could be safely supported. The authors thank Hochtief Murphy JV, Crossrail
All deformations were found to be within acceptable and ISG Jackson for providing project information.
serviceable levels.
The alternative design approach utilising the stabi-
lisation function of hexagonal structure geogrids re-
duced the overall platform thickness by 65% com-
pared to the original design using BR470 method.

1400
 Dalwadi and Dixon

REFERENCES Watts K. & Jenner C.G. (2008) Large scale Laboratory Assess-
ment of Geogrids to reinforced Granular Working platforms, proc
4th European Geosysnthetics Conf. Edinburgh 2008.
European Technical Approval ETA12/0530, Report number KI-
BR470- Working platforms for tracked plant, 2004 WA K76041 issued by Kiwa Nederland B.V.
Use of ‘structural geosynthetic reinforcement’– A BRE review Tensar Case Study reference-Ref 319 for Stoke
seven years on, BRE2011 Quay_Ipswich_Piling_Platform
Jenner C.G.,Watts, G.R.A, Blackman and The performance of re- Tensar Case Study reference Ref 344 for Crossrail Working Plat-
inforced unpaved sub-bases subjected to trafficking form.

1401