Shear strength is enhanced by increased suction, i.e. a lower degree
of saturation, on account of the stabilizing influence of the increased inter-
granular forces which are generated. Apparent cohesion is increased, but
there is often little change in angle of shearing resistance. In this lies the
explanation for the seemingly greater stability of steep slopes in unsatu-
rated soils. It will be evident that a subsequent increase in water content
towards saturation will alter the suction, and hence reduce stability of the
slope to the point where failure can occur. It may be noted that the effect-
ive stress relationship remains valid for partially saturated soils at high
values of Sr
Collapse compression, which occurs quickly when the water content is
increased, is a characteristic feature of unsaturated soils. The intergranular
forces produced by soil suction create a metastable structure with a high void
ratio; as saturation is approached the reduction in suction diminishes the
intergranular forces, causing compression of the soil. If saturation is rapid
there will be a correspondingly rapid reduction in soil volume, i.e. collapse
compression of the soil will occur, which can give rise to serious problems.
With certain clays, if saturation occurs at a reduced stress level undesirably
large expansive strains may be developed, with denser soils displaying
greater volume instability as swelling is highly dependent upon density.
The permeability of unsaturated fine-grained soils is very much
dependent upon suction level. Reduction in the degree of saturation,
particularly at values of Sr90%, can significantly influence the matric
suction and hence the intergranular contact forces.
The apparent permeability of the unsaturated soil may then change
by several orders of magnitude. The characteristic curve descriptive of the
suction. degree of saturation relationship for a soil is therefore basic to a
comprehensive description of the fluid flow processes within an unsatu-
rated soil. Characterisation of the permeability of the latter is correspond-
ingly of much greater complexity than for the soil in a fully saturated state.
Considered in context with the embankment dam, fine-grained cohesive
earthfills will invariably be in a partially saturated state at time of placing.
One illustrative instance of the significance of partial saturation
arises with respect to compaction of a rolled core, i.e. whether it is advan-
tageous to compact on the dry side of optimum (see Section 2.3.5).
Opinion on this remains divided, but the risk of so doing is that the com-
pacted soil will have a granular structure with larger pores and so be more
prone to subsequent collapse compression and settlement. Analytical
studies made for the Limonero dam (Spain) included comparison of the
predicted effects of compacting the core wet and dry of optimum and a
study of the influence of the soil microfabric. It was determined that the
wet core showed less collapse compression and more effective dissipation
of pore water pressure than the dry core, the latter showing a sharp
wetting front and increased collapse compression accompanied by little
dissipation of pore water pressures (Gens, 1998).
amelia
(Amelia)
#1