normally consolidated (NC). If, on the other hand, previous effective stress
levels have been relieved, e.g. as a result of glaciation, the clay is described
asoverconsolidated(OC). The ratio of previous maximum effective stress
to present in situeffective stress is the overconsolidation ratio(OCR). NC
clays are relatively soft and compressible. Their undrained shear strength,
cu, developed where there is no relief of porewater pressure by drainage, is
proportional to the pressure under which they have consolidated, and
therefore increases with depth. OC clays, such as the glacial tills
(OCR1–3) frequently used in UK embankment earthfills, are relatively
stiff.
If the structure of certain cohesive soils is disturbed or remoulded, as
in the process of compaction of earthfill in an embankment, a significant
loss of shear strength may result. The ratio of undisturbed to remoulded
undrained strength at the same moisture content is defined as the sensitiv-
ity,S. The sensitivity of most UK clays and tills lies between 1 and 3. Clays
with values above 4 are referred to as sensitive.
Clay consistency may be loosely classified on the basis of undrained
cohesive shear strength, cu, as in Table 2.2.
2.3.3 Compressibility and consolidation
When load is applied to a soil mass compression and settlement may occur
in consequence of one or more of three mechanisms:
- elastic deformation of the soil particles;
- compression of the pore fluid;
- expulsion of the pore fluid from the stressed zone, with rearrange-
ment of the soil particles.
Soil particles and water are sensibly incompressible, and compression
or volume decrease in a saturated fine-grained soil due to applied stress or
52 EMBANKMENT DAM ENGINEERING
Table 2.2 Descriptive consistency of clay soils (BSI, 1999)
Consistency Undrained strength,
cu(kN m^2 )
Very stiff or hard 150
Stiff 100–150
Firm to stiff 075–100
Firm 050–75
Soft to firm 040–50
Soft 020–40
Very soft 0 20
