Actes du colloque - Volume 2 - page 281

1152
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 3. Initial lime consumption (ILC) test.
Commercially available hydrated lime was used after its
suitability for soil stabilisation has been established. Chemical
analysis on the lime sample carried out in duplicate showed that
the relative proportion of calcium hydroxide to calcium oxide
was 4.88:1.00. The lime was mixed with clay in dry condition.
Plasticity tests were performed on London Clay mixed with
lime at percentages of lime of 0%-8% by dry unit of soil
respectively, for mellowing periods of 1 and 24 hours
respectively. These showed no change in the plasticity
characteristics of the lime-treated soil beyond 4% of lime
addition (see Fig. 2). Hence this was considered to be the
minimum necessary lime percentage for treating this clay. The
percentage was confirmed by initial consumption of lime test
results (see Fig. 3).
2.2 Specimen preparation
For the preparation of untreated London Clay samples the clay
powder was thoroughly mixed with water to achieve a water
content of 25.5% (the Proctor optimum) and left to hydrate in
sealed bags for 72 h. For the preparation of lime treated
samples, dry London Clay and hydrated lime powders were
thoroughly mixed and then the required amount of water was
added (27% and 32% i.e. dry and wet of Proctor optimum for
the lime treated soil). Static compaction was selected as the
best way of exerting sufficient control over the compaction
process of a clayey soil. In this experimental investigation both
types of specimen were compacted at the same target dry
density of 1.43 g/cm
3
, corresponding to the maximum standard
Proctor dry density of the London Clay soil. The compaction of
the triaxial specimens (76 mm height and 38 mm diameter) was
conducted in split-moulds of the appropriate dimensions. The
soil was placed in the mould in six equal layers and compressed
at a monotonic displacement rate of 1mm/min until the required
height was reached. The loading ram was then held in contact
with the soil for another 5 minutes to reduce the rebound upon
unloading. A similar method was adopted for the compaction of
the specimens for the SWRC tests. These were compacted in
standard oedometer cutting rings of 75 mm diameter and 20 mm
height used as compaction moulds. After compaction two
different methods of curing for the lime-treated specimens were
used, namely water curing and air curing. In the first method the
specimen was left in the mould to cure in contact with water for
the whole curing period. In the air curing method the specimen
was wrapped in several layers of cling film and stored in
controlled environmental conditions for the specified curing
period. To complete saturation after curing, back-pressure
saturation was applied regardless of the curing technique.
2.3 Triaxial testing
To assess the effect of cementation, indicative sets of different
triaxial testing results will be shown. These were performed on
specimens of London Clay and the corresponding lime-treated
London Clay specimens prepared and tested at a variety of
different conditions. All saturated specimens were sheared
drained after isotropic consolidation, following a
q
/
p’
=3 path.
For the saturated lime-treated specimens results based on two
different curing methods (air or water curing), and two different
percentages of lime will be shown. For partially saturated
specimens, results from four tests will be shown: a) two
compacted specimens of a treated (4% lime, air cured) and an
untreated soil respectively sheared as compacted (UU test); and
(b) two compacted specimens of a treated (4% lime, air cured)
and an untreated clay respectively, that were brought to a 300
kPa suction equilisation before testing and subsequently
isotropically consolidated under a net stress of 200 kPa and
sheared drained following a
q
/(
p-u
a
)=3 path, maintaining a
constant cell pressure and a constant suction of 300 kPa. Axis
translation was used to control the suction during testing. The
reason for showing results from two different test types was to
demonstrate that the effects of cementation were similar,
irrespective of the testing conditions.
2.4 Filter paper testing
The filter paper used in the present research to measure matric
suction (“contact” filter paper technique) was Whatman No.42
filter paper with a calibration formula according to Chandler
and Gutierrez (1986). The soil specimen was placed between
two Perspex disks. Filter papers were placed on each side of the
specimen, between the soil and Perspex disk interfaces. The soil
specimens were then tightly wrapped in multiple layers of cling
film and sealed bags and left in an insulated environment for
one week at a time. After this period the filter papers were
carefully removed and their water content was determined.
Subsequently, the soil specimens were left for air-drying until
the new target water content was achieved. They were then
wrapped again for the new moisture content measurement to be
performed one week later.
3 RESULTS AND DISCUSSION
3.1 Triaxial testing results
Figures 4 and 5 show indicative comparative stress-strain
relationships and volumetric strains of London Clay and lime-
treated London Clay specimens (saturated and partially
saturated specimens respectively) prepared and tested under
various conditions. It can be seen that for all types of tests the
London Clay specimens show a strain hardening behaviour for
both saturated and partially saturated samples irrespective of the
suction level, although there is an increase of strength with
increasing suction as expected. Conversely the lime-treated soil
shows a strain softening behaviour irrespective of the mode of
curing and testing, and this is consistent with the breakage of
the cementation bonds. The lime-treated soil became
increasingly stiffer, stronger but also more brittle with the
increase in lime percentage and also when it was air cured as
opposed to water cured. Although the three sets of tests in
Figure 5 are not directly comparable due to the different
preparation and/or testing procedures adopted, it can still be
seen that for the same net stress of 200 kPa, the strength of the
lime-treated specimens also increased with suction. In the
partially saturated state the brittle behaviour and strain softening
of the lime treated specimens was particularly pronounced.
Whereas this is the expected behaviour of a partially soil due to
the effect of suction in this instance the behaviour is presumably
due to the combined effect of suction and the breakage of
cementation bonds. Note that for all lime-treated samples
(saturated or partially saturated) dilation is clearly observed
after, rather than before the peak stress (especially in the 6% air
cured samples), i.e. the extra component of strength is not due
to the dilatancy as it would be in the case of a particulate
material; instead peak strength is mobilised well before the
maximum rate of dilation. As dilation only happens upon
softening, this could be related to the breakage of cementation
bonds and is consistent with the typical behaviour of soft rocks
(Vaughan 1993).
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