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org/ at University of Colorado Boulder on December 7, 2016

Large diameter plate tests on weathered in-situ Chalk

M.C. Matthews1 & C.R.I. Clayton2
1
University of Surrey, UK (e-mail: m.matthews@surrey.ac.uk)
2
University of Southampton, UK

Abstract
he prediction of the settlements of spread

T foundations on in-situ chalk is best carried out

using empirical methods, based on the simple
behavioural model developed by Burland & Lord
(1970) using results from plate tests. However, the data
available to support this approach remain somewhat
limited, stratigraphically, geographically and in terms of
size and level of loading. This paper reports the results of
nine large (1.8 m) diameter plate tests carried out on
three weathered chalks with widely different intact den-
sities and strengths. The results are interpreted in terms
of the Burland and Lord model, and despite the differ-
ences in the chalks at these sites, are found to be broadly
in agreement with the parameters deduced on the basis
of the pioneering work carried out at Mundford by Ward
et al. (1968). In addition the paper provides some
additional data on creep settlements, confirming that the
long-term settlements on chalk can be considerably
larger than those predicted on the basis of short-term
loading tests.

Keywords: Chalk, in-situ tests, plate-bearing tests, stiffness, Fig. 1. The Chalk outcrop in England, showing the geographi-
weak rocks cal locations of the plate tests.

The settlements of foundations on chalk are best pre- presented in Lord et al. 2002) to develop a new method
of settlement prediction. The purpose of this short paper
dicted using empirical methods (Kee 1974; Lord 1990;
is to provide detail and background to these tests and
Lord et al. 1994), based on the model of behaviour
draw together the results with other long-term testing
developed by Burland & Lord (1970) using data from
(Matthews 1993).
plate tests carried out on the chalk at Mundford (Ward
The objective of the plate-testing programme de-
et al. 1968). Because of the fractured nature of the chalk
scribed in this paper was to obtain realistic estimates of
(Lake & Simons 1975), and difficulties in extrapolating the behaviour of foundations placed on near-surface,
the results of small-diameter plates to large diameter highly weathered but structured chalks of differing
foundations (Lake & Simons 1970), it has long been intact stiffness. Tests were therefore carried out at
recognized that to provide representative parameters, three selected sites in England, near North Ormsby,
loading tests must be sufficiently large. Because of Lincolnshire, Leatherhead, Surrey, and Needham
its non-linear load-settlement behaviour (Ward et al. Market, Suffolk (see Fig. 1).
1968) loading should be controlled, and carried out to
sufficiently high levels to identify yield.
Despite the above, to the authors’ knowledge the Load-settlement behaviour of
detailed results of only four controlled large-diameter the chalk
(defined here, arbitrarily, as greater than 1 m diameter)
loading tests on chalk (the Mundford tank (Ward et al. On the basis of 865 mm plate tests carried out at
1968), two slab tests at Luton (Powell et al. 1990), and a Mundford, Burland & Lord (1970) found that the
test on structureless chalk at Salisbury (Burland et al. average bearing pressure-settlement curves for the more
1983) have previously been reported in the literature. fractured chalks were markedly non-linear but could
The data from an additional nine 1.8 m diameter tests be idealized by the simple bi-linear model shown in
carried out by the authors therefore represent a major Figure 2. This idealization permits the stress-settlement
part of the database used by Lord et al. (1994) (and behaviour to be described using four parameters:
Quarterly Journal of Engineering Geology and Hydrogeology, 37, 61–72 1470-9236/04 $15.00  2004 Geological Society of London
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62 MATTHEWS & CLAYTON

Plate test equipment

At the three sites the maximum discontinuity spacing
likely to be encountered in the stressed zone beneath the
large-diameter plate tests on the weathered chalk was
estimated as between 60 mm and 200 mm. Lake &
Simons (1975) have suggested that the plate diameter
should be six times the maximum fracture spacing in
order to test a volume of rock containing representative
discontinuities. This suggests a minimum plate diameter
of between 360 mm and 1200 mm. However, because of
the uncertainties involved in extrapolating from smaller
to larger loading areas, and in order to reflect the
behaviour of full-scale foundations, it was considered
necessary to use the largest plate available, which was
1800 mm diameter. Three tests were carried out at each
site, to allow some judgement of the variability of the
compressibility within each type of chalk (Matthews
Fig. 2. Idealized pressure-settlement curve for a plate or 1993).
foundation on Chalk (after Burland & Lord 1970 and Lord The maximum stress applied to the plate tests at
et al. 1994).
Mundford was 1600 kN/m2 (Burland & Lord 1970).
This was high enough to define the post yield stiffness in
Ei initial modulus their chalk, which had a similar fracture state to that at
Ey post-yield modulus
the authors’ test sites. But since it was thought that the
qe average applied stress at onset of yield
high-density chalk might yield at a higher stress than its
qy ‘yield’ stress (based on the establishment of general
Mundford equivalent, a loading rig which could provide
yield Ey)
a higher stress was considered necessary. The 5000 kN
The initial load-settlement behaviour is approximately loading frame (Fig. 3) designed and constructed by the
linear with settlements more or less fully recovered on Building Research Establishment, which could apply a
unloading. This is characterized by an initial tangent stress of about 1800 kN/m2 to the 1800 mm diameter
modulus Ei. At an applied stress typically between 200 plate, was used at all the test sites.
and 400 kN/m2 settlements begin to increase more rap- A row of nine 5m x 200 mm dia. anchors were used to
idly and are not fully recovered on unloading. This onset hold down each spreader beam of the loading frame at
of yielding, thought to result from localized crushing of each of the nine test locations. A manually operated
asperities (Matthews 1993; Lord et al. 1994) is denoted hydraulic jack was used to apply the load to the plate. A
by the stress qe. Further loading results in more curva- locking ring type jack was selected since this prevented
ture of the stress-settlement curve until a linear portion jack retraction should hydraulic oil leakage occur when
is once again entered, characterized by a tangent modu- the equipment was unattended, overnight. A disc spring
lus Ey. At this stage the material is assumed to be unit was incorporated into the loading column to pro-
undergoing general yield due to slip and more wide- vide sufficient compliance so that plate settlement would
spread crushing of asperities. If the average applied lead to minimal loss of load.
stress-settlement curve for this linear stage is projected Each plate-loading test was expected to run over a
back it will intersect the applied stress axis at a point period of up to two weeks. The sites were generally
denoted by qy which is termed the ‘yield’ stress. remote and were not secure, with no access to electrical
In practise the accurate measurement of tangent power apart from that which might be provided by
modulus Ei is often difficult as settlements are generally a portable generator. The instrumentation used to
small at applied stresses less than 200 kN/m2. Insufficient measure the applied load and plate settlement therefore
care in bedding the plate on the chalk as well as thermal needed to be reliable, robust and require minimal or no
movements can lead to significant errors in settlement electrical power. The instruments also needed to be
measurements at such low applied stresses. In general it reasonably temperature stable since temperature
is considered that Ei reflects the initial behaviour at changes of between 0 and 15oC were to be expected
stresses less than 100 kN/m2 and hence its use in predict- during a test.
ing settlements at stresses greater than this could lead to A mechanical cantilever beam type load cell was
under-prediction. As a result Lord et al. (1994) suggests selected for measuring the applied load at the top of the
the use of a secant modulus Es at an applied stress of loading column (Fig. 3). The load was registered on a
200 kN/m2 be used instead of Ei. dial gauge that was mounted in the load cell itself, and
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PLATE TESTS ON WEATHERED CHALK 63

Fig. 3. Plan and elevation of BRE. 5000 kN plate load testing equipment.

was read visually. Recorded values were corrected to Optical precise levels using a parallel plate micrometer
allow for the self-weight of the loading column and plate and an invar staff are known (Cooper 1971; Deumlich
components. 1982) to provide a resolution and accuracy better than
In order to assess the magnitude of differential settle- 0.1 mm and have the advantage of good temperature
ment a total of 10 settlement measurements were made stability, and hence this was chosen as the main
using two independent measuring systems (see Fig. 4). measurement system. A Zeiss Jenar Ni 007 level
Four measurements were made using precise levelling of met these criteria. This instrument had a resolution
dome headed nails grouted into the plate to measure of 0.005 mm and a claimed instrument accuracy of
plate movement relative to a set of at least two tempor- +/
0.05 mm.
ary benchmarks. Six dial gauges were used to measure Dial gauges with a stroke of 25 mm and a resolution
movement relative to a datum bar (see Fig. 4). The use of 0.03 mm were chosen for the secondary settlement
of a sub-plate assembly (Marsland & Eason 1973) was
measurement system. A rectangular scaffold frame fixed
considered, but was rejected because of:
to the ground at four points located approximately 1 m
(a) the increased complexity during installation and the away from the edge of the plate was constructed under
need to provide power on site; the plate loading rig, to act as a reference from which to
(b) doubts over the ability of the system to anchor itself mount the dial gauges. The size of this reference frame
effectively; was kept as small as possible and was located entirely
(c) an inability to determine the stress changes required between the main beams of the loading frame in order
for the interpretation of the results. that temperature and wind effects could be minimized. It
Based on a review of published data the maximum was appreciated that the four datum posts would be
expected value of initial modulus, Ei was about subject to ground movements associated with the plate
1000 MN/m2. Assuming the plate to be rigid a plate settlement. However, an assumption of isotropic elas-
settlement of about 0.1 mm was calculated under an ticity suggested that movements of the reference system
applied pressure of 100 kN/m2. It was therefore necess- would be small relative to the average plate settlement if
ary for the instrumentation systems to have an accuracy the datum posts were kept 2 m away from the centre of
and resolution somewhat better than this. the plate. It was expected that the increase in stiffness of
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64 MATTHEWS & CLAYTON

Fig. 4. Layout Plan of plate test showing precise levelling points for plate settlement measurement and layout of dial gauges.

the ground with depth would further restrict the lateral period of at least 5 days. The plate was then lowered
extent of the settlement trough (Gibson 1967). The space onto a liquid plaster base spread onto the set concrete to
available over the plate allowed only six dial gauges to bed the plate well in (Fig. 3). (During dismantling of the
be used, which were positioned on the metal radial plate it was found that a layer of plaster less than 15 mm
stiffeners of the plate. thick formed between the plate and the concrete sur-
faces). A significant gap (generally 100 to 200 mm) was
left between the edge of the plate and the pits, which
Site preparation were generally square or rectangular.

At all the test sites it was necessary to remove either

topsoil or surface debris, before the 1800 mm diameter Test procedure
plate could be placed. At North Ormsby and Needham
Market the pits were between 100 mm and 250 mm Three 1800 mm diameter plate tests were carried out at
deep. At Leatherhead the pits were taken down to the each site. Each plate test was loaded in 100 kN/m2
lower part of the structureless chalk, until clay was increments, with each load being held for a period of
absent in the matrix. This required the pits to be between 24 hours. Plate settlement measurements were taken
500 mm and 750 mm deep. regularly during this period.
All the test pits were hand dug to minimize distur- An examination of published plate loading test data
bance. Loose fragments and remoulded chalk were on chalk at Mundford (Ward et al. 1968; Burland &
removed using a combination of compressed air and a Lord 1970) had suggested that a stress increment of
brush. Once cleaning was complete, up to 100 mm of about 100 kN/m2 would be sufficiently small to define
concrete blinding was placed on the chalk surface. The the yield point and provide enough data to determine
surface was prepared by using a steel float to produce the initial modulus, Ei. A maximum average plate stress
a smooth finish, and left under cover to harden for a of approximately 1800 kN/m2 could be applied to the
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PLATE TESTS ON WEATHERED CHALK 65

1800 mm dia. plate by the 5000 kN loading frame. It was vertical sets, giving rise to trapezoidal-shaped blocks. All
decided to hold each 100 kN/m2 increment of loading the discontinuities had rough walls and apertures gener-
constant for a period of about 24 hours, rather than ally less than 3 mm. The spacing of the sub-horizontal
applying each load until movements reduced to less than discontinuities increased with depth to a much greater
a pre-determined rate of creep. extent than the sub-vertical discontinuities. Within 1m
Plate settlements were determined immediately before of the ground surface the spacing of the sub-horizontal
the application of each loading increment and again discontinuities was generally less than 50 mm, giving rise
shortly afterwards, once the rate of settlement had to tabular blocks of flaggy chalk (see Fig. 5a). The
reduced sufficiently. Subsequent readings were taken at fracture-block system was found to be loose within the
irregular intervals several times each day. The dial top 2 m, such that blocks of chalk could easily be
gauges were read at regular intervals during each stage removed from excavations without breakage. Below 1 m
of every test, to provide time-settlement data. The last depth the blocks became more equi-dimensional and the
settlement measured before changing the load was used flaggy chalk graded into blocky chalk. In general the
to construct average plate settlement versus applied chalk below the area used for the plate loading tests was
pressure graphs of the type shown in Figure 2. Average found, on the basis of fracture spacing observed on
applied pressure was calculated by dividing the load on adjacent quarry faces and drill-hole logs, to be equiva-
the plate by the plate area (based on its diameter of lent to Mundford (Ward et al. 1968) Grade IV at the
1800 mm). ground surface and Grade III below 2 m. According to
the recent CIRIA classification (Lord et al. 2002) it
would be graded as High density Grade C4/5 near the
The Chalk surface, becoming High density Grade B3 with depth.

Sketches of the topmost 4 m of the chalk at the three test

sites, as typically observed in trial pits and adjacent Leatherhead (intermediate density chalk)
quarry faces, are shown in Figure 5. These were logged
(Fig. 5b)
by qualified engineering geologists who, in addition to
recording geometry, made records of fundamental char- This test site was located in the grounds of Esso’s UK
acteristics such as fracture spacing, aperture, infill and headquaters at Ashtead near Leatherhead, Surrey. It
hardness. These records were initially interpreted in was situated on a spur between two of the numerous dry
accordance with the classification of Ward et al. (1968), valleys that cut the northern slopes of the valley occu-
and subsequently interpreted in terms of that of Lord pied by the River Rye, a tributary of the River Mole.
et al. (2002). The outcrop at this site is the Seaford Chalk Formation,
part of the Upper Chalk and is characterized by numer-
North Ormsby (high density chalk) ous nodular flint bands. Tests carried out on samples
(Fig. 5a) from the site gave an average dry density of 1.54 Mg/m3,
and an unconfined compressive strength of 3.1 MN/m2.
The test site was situated on the topmost level of a Its intact stiffness, derived from unconfined triaxial tests
disused chalk pit located on the escarpment of the with local strain measurement, was found to be
Lincolnshire Wolds about near Louth. The tests were 9.7 GN/m2 at an axial strain of 0.001% and 7.4 GN/m2
carried out in the lowest part of the Burnham Chalk at an axial strain of 0.01%.
formation, which is equivalent in age to the upper part The rock mass at this site was observed in three deep
of the Middle Chalk of Southern England (Kent & trial pits dug below each plate test location. The ground
Gaunt 1980). The Burnham Chalk is characterized by comprised up to 0.5 m of top soil, over 0.5 to 1.0 m of
numerous continuous bands of tabular flint, which reach structureless (Mundford Grade V, CIRIA Grade Dc)
a maximum frequency towards the base of the forma- chalk. This overlays about 1.0 m of structured flaggy
tion. At the test site the flint bands were generally about chalk (Mundford Grade IV on the basis of fracture
100 mm thick with a spacing of 1 to 2 m. Tests carried spacing), which graded downwards into a more blocky
out on samples from the site gave a uniformly high dry chalk in which the discontinuity spacing increases with
density of 1.84 Mg/m3, and an average unconfined com- depth. The structureless chalk was characterised by
pressive strength of 12 MN/m2. Its intact stiffness, coarse gravel to cobble size fragments of chalk in a
derived from unconfined triaxial tests with local strain matrix composed of sand and fine gravel size chalk
measurement, was of the order of 17 GN/m2 at an axial fragments mixed with top soil. The topsoil was absent
strain of 0.001%, and 13 GN/m2 at an axial strain of in the lower part of the structureless chalk and the
0.01%. rock mass displayed some structure as it graded into
The rock mass was observed in the quarry face the underlying flaggy chalk. The plate was located in
adjacent to the test area. It comprised four sets of this lower zone of structureless chalk, between 400
discontinuities; one sub-horizontal set and three sub- and 450 mm above the structured chalk below. The
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66 MATTHEWS & CLAYTON

Fig. 5. Chalk profiles observed at the three test sites (a) North
Ormsby, (b) Leatherhead & (c) Needham Market.
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PLATE TESTS ON WEATHERED CHALK 67

structureless chalk on which the plate was founded was graded as Low density Grade B4/5 near the surface,
dense and clast dominant. becoming Low density Grade B3 with depth.
Two sub-vertical sets of discontinuities, trending NW
and NE, and one sub-horizontal set were observed in the
trial pits. The sub-horizontal discontinuities comprised Results
bedding planes and stress relief fractures. Evidence of
dissolution was observed, particularly along bedding The measurement and interpretation of deformations
planes. In general the bedding planes exhibited wavy caused by plate loading on a weak fractured rock such
walls, with apertures ranging from 0 to 8 mm. In the as chalk is not straightforward.
open sections the lower walls were often found lined (a) Disturbance during excavation (for example using
with putty chalk and in some cases rounded fine gravel hydraulic excavators or piling plant) may loosen the
size chalk fragments were seen. chalk, giving low measurements of stiffness. In this
The upper part of the flaggy chalk generally exhibited study great care was taken to minimize disturbance
a loose fracture block system but not to the same degree through hand excavation down to the test level.
as that observed at North Ormsby. In the lower part (b) Since the material under test is relatively stiff,
of the flaggy chalk and in the underlying blocky any bedding between the plate and the chalk can
chalk, blocks could not be removed without breakage, introduce significant errors. The test procedure
indicating a tight fracture block system. adopted avoided bedding through the use of
On the basis of structure and fracture spacing, the concrete blinding and a very thin plaster filling.
chalk at the Leatherhead site was considered equivalent (c) If the plate is placed in a pit or bore, a depth
to Mundford Grade V (0.5 m thick), overlying Grade IV correction factor is required, and for deeper tests the
(1.0 m thick), and Grade III–II material. According to effect of the gap or of bonding between the side of
the CIRIA classification it would be graded as Grade Dc the plate and the rock may be significant and
near the surface, becoming Medium density Grade B3/4 uncertain, as noted by Lord et al. (2002). The results
and then B3/2 with depth. reported here do not require depth correction since
the tests were performed in shallow pits, and a
significant gap was left between the outer edge of the
Needham Market (low density chalk) plate and the side of the pit.
(Fig. 5c) (d) The plate is to some extent flexible, and because the
ground is variable, differential settlement will occur.
This test site was located in a working chalk quarry on In this test 10 measurements of plate settlement
the southern side of the Gipping valley near the town of (four precise levels and six dial gauge readings) were
Needham Market, Suffolk. The outcrop here is the made to minimize this effect.
Culver Chalk Formation, part of the Upper Chalk and (e) The discontinuous nature of the chalk, and uncer-
is much younger than that found at the North Ormsby tainty with regard to the relative rigidity of the
site. It was characterized by a low flint content. Tests plate, means that interpretation on the basis of
carried out on samples from the site gave a very low simple stress distributions, for example assuming the
density (dry density=1.34 Mg/m3) and an average uni- chalk to be an elastic continuum and the plate to be
axial compressive strength of only 0.9 MN/m2. The rigid, cannot be relied upon. For this reason the
intact rock was easily crushed to a putty between the term ‘average applied plate pressure’ is used to
finger and thumb. Its intact stiffness, derived from describe the degree of loading on the chalk.
unconfined triaxial tests with local strain measurement, (f) The stress paths and strain levels imposed upon
was found to be 7.6 GN/m2 at an axial strain of 0.001% the chalk vary with depth, and distance from the
and 3.3 GN/m2 at an axial strain of 0.01%. centreline. Sophisticated interpretation of the stress-
The rock mass at this site was observed in three deep strain behaviour of the chalk is impossible. Even the
trial pits which were dug beneath the plate loading test use of a sub-plate assembly would not overcome the
locations. It was characterized by vertical and sub- difficulties, because the stress changes at measure-
vertical rough and often open discontinuities, together ment positions cannot be predicted with any confi-
with wavy sub-horizontal discontinuities that showed dence owing to the fractured, discontinuous nature
evidence of dissolution. There were also numerous of the rock.
minor horizontal and vertical impersistent, rough and (g) Settlement of the ground around the plate can affect
relatively tight fractures. The fracture block system at measurement datums, if these are placed too close.
this site was sufficiently tight that no blocks could be Where they are placed at a distance, and even where
removed from the face of the trial pits without breakage. the reference beams are constructed of Invar, tem-
On the basis of fracture spacing the chalk was Mundford perature effects lead to measurement instability. To
Grade IV at the ground surface, and Grade III below avoid this, the data reported in this paper were
1 m. According to the CIRIA classification it would be acquired using two independent measuring systems.
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68 MATTHEWS & CLAYTON

In Figure 6 average settlements are given separately pressure (900 kN/m2) for some 40 days. The results, in
for the two independent sets of measurements, made terms of average plate settlement versus the logarithm of
with dial gauges and using precise levelling. It can be time, are shown in Figure 8.
seen that until yield there is good agreement be-
tween the two measurements. The greatest diver- Discussion
gence occurs during unloading – elsewhere, as
expected given the position of the dial gauge datum The general behaviour of large plates and instrumented
posts (Fig. 4), the dial gauge readings give average foundations on chalk, previously observed by Ward
settlements less than those calculated from levelling, et al. (1968), Burland & Lord (1970), Kee (1974),
but the difference is small. Burland & Davidson (1976), and Burland & Bayliss
(1990) for the more weathered near-surface chalk is also
seen in these tests. Initially the chalk behaves in a stiff
and more-or-less linear and elastic manner. Stiffening
as a result of bedding fracture closure (Barton 1986)
was not seen. After yielding, settlements increased
dramatically, and were largely irrecoverable.
Despite founding these large plates at the highest
possible level, the weathered chalk at all three sites
displayed very high stiffness before yield. The average
initial Young’s modulus for the nine tests was 593 MN/
m2, with a range from 328–1381 MN/m2. The linear
range extended to an average plate pressure of between
200 and 400 kN/m2, and settlements of the 1.8 m diam-
eter plate under an average applied stress of 300 kN/m2
in general did not exceed 2.3 mm. These figures suggest
that it should only be necessary to pile exceptionally
heavily-loaded foundations, such as for silos, when
building on structured chalk.
The large-diameter plate tests reported in this paper
were deliberately carried out on chalks with the widest
possible range of intact density, but with a similar high
degree of weathering, in an attempt to allow an assess-
ment of the influence of density variations on the mass
Fig. 6. Plate settlement as a function of applied pressure, compressibility of more-weathered chalks. Table 1 sug-
comparing results from dial gauges and precise levelling. gests that density has relatively little effect on either the
initial modulus (Ei) or the stress at the onset of yield (qe).
Given that the stress levels and strain paths in the As has already been seen (Fig. 7 and Table 1), the
discontinuous chalk mass must remain unknown, a Needham Market chalk was initially very stiff, despite its
simple interpretation of plate test data is necessary. The high porosity. It had the highest average Es value, and
plate-loading test should be regarded as a model footing also the highest average yield stresses (both qe and qy).
test, and average settlement plotted against average The average initial modulus of the hard North Ormsby
applied plate pressure. Stiffness values derived from such chalk was the lowest, because of its loose state. Post
curves are not true moduli but can be used to estimate yield the Needham Market chalk was extremely com-
foundation settlements, and as suggested by Ward et al. pressible, presumably as a result of its low intact yield
(1968) can be regarded as ‘figures of merit’. stress (Clayton et al. 2002), but the North Ormsby chalk
The results of the 9 plate tests are plotted in Figure 7, had a similar modulus to the Leatherhead chalk, despite
using data from the precise levelling. All tests were taken their different densities and intact stiffness.
significantly past yield, and at each site at least one plate The data in Table 1 suggest that, if anything, increas-
location was loaded to an average plate pressure greater ing intact strength and stiffness may be associated with
than 1 MN/m2. From these data values of Ei, Ey, qe and decreasing yield stress (qy). However, intact strength
qy (Burland & Lord 1970) were calculated, and are may be having an important effect on stiffness reduction
shown in Table 1. post yield, since this varies significantly between the
As with previous investigations, and because of time different types of chalk. The ratio Ei / Ey is on average
constraints, there was little opportunity to investigate 6.3, 9.2 and 44.3 for the North Ormsby, Leatherhead
the creep characteristics of the chalk. Most loads were and Needham Market plate tests respectively.
held for only about 24 hours. One plate test, on the Creep is obviously an additional factor that must be
North Ormsby chalk, was however left at its highest taken into account. On the basis of both the plate and
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PLATE TESTS ON WEATHERED CHALK 69

Fig. 7. Applied pressure v. observed settlement for the nine 1800 mm diameter plate tests, based upon precise levelling results
(a) North Ormsby results, (b) Leatherhead results & (c) Needham Market results.
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70 MATTHEWS & CLAYTON

Table 1. Stiffness parameters (Ei, Ey, qe and qy) derived from the 9 plate tests

Mass compressibility parameters Settlements

Initial modulus Onset of yield Yield stress Post-yield modulus at 300kN/m2 at 900kN/m2
Site and test No. Ei1 (MN/m2) Es1 (MN/m2) qe (kN/m2) qy (kN/m2) Ey (MN/m2)
300 (mm)
900 (mm)

North Ormsby
PNO1 328 351 200 299 58 1.3 13.9
PNO2 314 315 250 324 48 1.4 16.2
PNO3 454 409 200 319 68 1.2 11.5
average 365 358 217 314 58 1.3 13.9

Leatherhead
PLE1 463 463 200 363 57 1.3 12.6
PLE2 620 648 200 440 59 0.6 10.7
PLE3 635 635 200 364 69 0.8 10.3
average 573 582 200 389 62 0.9 11.3

Needham Market
PNE1 1381 724 400 576 30 0.6 14.0
PNE2 808 663 300 590 14 0.7 29.1
PNE3 338 371 200 399 13 2.3 41.2
average. 842 586 300 522 19 2.3 28.1

overall average 593 509 239 408 46 1.1 18.0

The equivalent Young’s modulus is calculated from the equation for a flexible circular loaded area on an isotropic homogeneous elastic half space:
E=/4. qD/?
. (1
2).I
Where q=applied stress, D=Plate diameter,
=Average settlement, =Poisson’s ratio, I=Influence factor (this is a controlled by flexibility of plate,
depth of embedment, partial loading of test pit) (see Lord et al. 1994)

the tank loading tests at Mundford it was concluded that which occurs at more than 2900 kN/m2. The Mundford
no measurable creep occurs in hard, high-density chalks, data suggested that the little creep undergone by the
even under loads as high as 1600 kN/m2 (Burland & Grade III chalk soon ceased, but that the creep de-
Lord 1970). Recent laboratory data using very-small flexions of grade IV and V chalk might ‘in the long term
local-strain measurement (Heymann 1998) similarly be considerably larger than the immediate deflexions’.
show that even for the high porosity Needham Market Burland (1975) reported that the ratio of the long-term
chalk there is insignificant creep below the yield stress, to short-term moduli for these materials was as low as
0.2, emphasising the importance of estimating creep
settlements when designing foundations on weathered
chalks.
Although the primary objective of this work was to
assess the influence of intact stiffness on immediate
settlement parameters, an opportunity did arise to make
some longer-term measurements. Whilst the third site
was being prepared for testing, the loading rig was left in
place at North Ormsby, on location PL3, with a load of
900 kN/m2 applied. The 24 hour settlement under
900 kN/m2 (loaded in 100 kN/m2 increments over a 9
day period) had been 10.3 mm, and this increased, as
shown in Figure 8, to almost 14 mm over the next 39
days. As noted by previous authors (Burland 1975;
Powell 1990), the settlement-time relationship is almost
linear when plotted on a logarithmic basis, suggesting
that settlement of the plate might approximately double
over a 30-year period. These data confirm that creep
must be taken into account when predicting the long
Fig. 8. Results of the long-term loading test at North Ormsby term settlements of foundations on weathered chalks
(average applied plate pressure=900 kN/m2). when under high applied stress levels (> qe).
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PLATE TESTS ON WEATHERED CHALK 71

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D, F. 1982. Surveying Instruments. Walter de Gruyter,
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Received 18 September 2003; accepted 17 February 2004.