Civil Engineering Reference
In-Depth Information
Figure 26.6
Strength of unsaturated soils.
marked loading shows what happens as the net stress is increased at a given water
content. The gas compresses and the degree of saturation increases: the strength
may increase or decrease depending on the relative contributions of
c
and
φ
in Eq. 26.13. At very large total stress the initially unsaturated soil may become sat-
urated, or nearly so, in which case the strength will converge on the envelope for
saturated soil.
You can determine the strength surface shown in Fig. 26.6 from a set of triaxial
or shear tests and you can use these to do total stress analyses for stability of slopes
and retaining walls. But if the water content rises due to rainfall or changes in the
groundwater the strength will reduce. The worst that can happen is the soil becomes
saturated and then you can use the analyses given in Chapter 21.
σ
tan
26.9 Settlement of foundations on unsaturated soil
In Sec. 22.11 I discussed settlement and heave of shallow foundations on saturated
soil due to changes of groundwater resulting from such things as dewatering, changes
of vegetation and leaking drains. These apply also to settlement and heave of foun-
dations on unsaturated soil but in this case there is another important mechanism for
settlement.
In soil with a low degree of saturation the water forms matrix water bridges, which
act like glue bonding the grains together. As a result the soil can be very loose but
still relatively stiff and strong so shallow foundations can have large allowable bearing
pressure and small settlements. If the soil becomes wetter the matrix water bridges are
lost as the water changes from that shown in Fig. 26.3(a) to that shown in Fig. 26.3(b).
The initially loose bonded soil now becomes unbonded and its strength and stiffness
decrease. Its load factor and allowable bearing pressures decrease and there will be
settlements which are known as wetting collapse settlements.
