Civil Engineering Reference
In-Depth Information
Figure 26.9
Compaction curve.
26.12 Summary
1. An unsaturated soil contains soil grains, water and a gas, usually air, and it is
characterized by the degree of saturation
S
r
. The air may occur in discrete bubbles
or it may be continuous, in which case the water collects at points the contact
between the grains. For
S
r
in the range 0.25 to 0.85 both the water and the air are
continuous in three dimensions.
2. The simple effective stress described in Chapter 6 does not work for unsaturated
soils whose behaviour depends on the net stress (
σ
−
u
a
) and the matrix suction
u
w
).
3. Matrix suctions are generated by surface tension. They are related to the grain size
and for fine grained soils they can be very large. Suctions also vary with degree of
saturation but the relationship is different when the soil is drying from when it is
wetting.
4. The strength of unsaturated soil is related to the net stress and the matrix suction.
5. Foundations on unsaturated soils may settle or heave depending on whether the
bearing pressure is greater or less than a critical bearing pressure.
6. Compacted soils are unsaturated and the degree of saturation decreases as the
water content decreases. They should be compacted at a water content close to
the optimum when the dry density will be maximum.
(
u
a
−
Further reading
Fredlund, D. G. andH. Rahardjo (1993) SoilMechanics for Unsaturated Soils, Wiley, NewYork.
Mitchell, J. K. and K. Soga (2005)
Fundamentals of soil behaviour, 3rd edition
, Wiley,
New York.
