While a soil mass may be stable in the sense described above it may
nevertheless undergo deformationas a result of changes in loading or drainage
conditions. A limited amount of deformation occurs with no net volume
change, and is thus comparable with the elastoplastic behaviour of many non-
particulate materials. The most significant soil deformations, however, usually
involve volume changes arising from alterations in the geometric configuration
of the soil particle assemblage, e.g. a loosely packed arrangement of soil par-
ticles will on loading adopt a more compact and denser structure. Where the
soil particles are relatively coarse, as with sands, such a change occurs almost
immediately on load application. In saturated clayey soils, however, volume
changes and settlement due to external loading take place slowly through the
complex hydrodynamic process known as consolidation (Section 2.3.3).
The effective stress, , can be calculated from equation (2.1) if the
total stress, , and porewater pressure, uw, are known. While the total
stress at a point may be readily determined by statics, the local porewater
pressure is a more complex variable. In fine-grained clay-type soils the
value of uwfor applied increments of total stress will depend upon the
properties of the soil mineral skeleton and the pore fluid and will be
strongly time dependent. The immediate (t0) response of porewater
pressure in a particular soil to various combinations of applied total
stresses is described through the concept of pore pressure coefficients.
From consideration of volume changes in a soil element under
applied total stress (Fig. 2.4), the change in pore pressure ∆u 3 due to an
applied change in minor principal stress of ∆ 3 can be expressed as
∆u 3 B∆ 3 (2.2)
whereBis an empirical pore pressure coefficient.
If the major principal total stress, 1 , is also then changed, by an
increment∆ 1 , the corresponding change in pore pressure, ∆u 1 , is given by
∆u 1 AB(∆ 1 ∆ 3 ) (2.3)
whereAis a further empirical coefficient.
The overall change in pore pressure, ∆uw, due to changes in both 3
and 1 is then given by
∆uw∆u 3 ∆u 1 B[∆ 3 A(∆ 1 ∆ 3 )]. (2.4)
Pore pressure coefficients AandBpermit estimation of effective stresses
resulting from predicted or known changes in applied stress. In view of the
importance of effective stresses in controlling soil behaviour, the coeffi-
cients are essential predictive tools in the solution of many soil-
engineering problems. They are determined in special laboratory triaxial
shear strength tests (Section 2.3.2).
48 EMBANKMENT DAM ENGINEERING
