The influence of suction changes on the stiffness of railway formation

Abstract

Despite compacted soils being unsaturated, geotechnical design is predominantly based on saturated soils,
despite it being known that differences exist between the behaviour of saturated and unsaturated soils.
Suction in soils has been shown to increase its strength and stiffness with the increase dependent on the
degree of suction, soil type and particle arrangement. Future climate change predictions suggest an
increase in average global temperatures leading to longer drier summers and wetter winters resulting in
seasonal fluctuations in suction, such that unsaturated soil behaviour will become more important.

Transport infrastructure such as railway tracks, founded at shallow depths, are likely to be unsaturated
and influenced by variations in suction. A key parameter in assessing the performance of soils is stiffness;
in particular shear modulus, however, although suction affects shear modulus, limited research has been
conducted into its influence. This research, therefore, considers how variations in suction influence the
small strain shear modulus of railway formation material.

Soil water characteristic curves were determined, using the pressure plate apparatus and filter paper
technique, for four materials covering the spectrum of railway formation materials encountered on the
COALlink line in South Africa. Shear modulus measurements of specimens prepared from two of these
materials were determined using a Stokoe resonant column apparatus. Suction was controlled by
preparing specimens at different water contents, with the suction independently measured using the filter
paper technique.

Results showed that shear modulus was significantly influenced by suction and exhibited a complex
behaviour. For specimens tested at their preparation water content shear modulus increased with
increasing suction up to an optimum value, and then reduced as suctions exceeded this optimum value
forming a bell-shaped curve. The influence of suction was seen to be greater at lower net normal stresses.
Although this behavioural pattern was similar for both materials tested in the resonant column apparatus,
the peak value of shear modulus increased with clay content. Importantly, the marked changes in shear
modulus due to suction coincided with the suction range predicted in-situ.

Tests were also conducted on specimens prepared at different water contents but the same density, airdried
to very low water contents. The shear moduli of these air dried specimens were markedly higher
than those of the dry and unsaturated specimens tested at their preparation water contents. Variation in
measured shear modulus between specimens led to the conclusion that different particle arrangements
between the specimens may have also contributed to the changes in shear modulus and were dependent
on the preparation water content. At high water contents the clay particles became evenly distributed
around the sand particles when the specimen was prepared, whilst, at low water contents the clay formed
lumps during preparation of the material mix and became embedded with the sand particles during
formation. CT scanning undertaken to look at the particle arrangement highlighted regions throughout the
specimen where sand particles were embedded in lumps of clay and silt particles.

These results are the first to demonstrate the importance of suction and its variation on the small strain
shear modulus behaviour of railway formation material.

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Identifiers

ORCID for C.R.I. Clayton: orcid.org/0000-0003-0071-8437

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