Environmental Engineering Reference
In-Depth Information
6.2
COMPRESSIBILITY OF SOILS AND EMBANKMENT MATERIALS
6.2.1
General principles
6.2.1.1
Within the foundation
The compression of soils in the foundation of a dam, as load from the dam is applied dur-
ing construction, has three components:
-
Initial “elastic” compression;
-
Primary consolidation, as the pore pressures induced by the weight of the dam are
dissipated;
-
Secondary consolidation, or creep, which takes place with no further change in pore
pressures.
The theory and principles involved are covered in soil mechanics text books and will
not be discussed here. There are some points to keep in mind:
(a) The in-situ vertical and horizontal stresses,
vo
and
HO
are affected by overconsoli-
preconsolidation pressure) of the founda-
tion soils. These are influenced by periods of erosion of the site, lower groundwater
levels than at present, desiccation from drying of soils exposed to the sun (or by freez-
ing) and, in some areas, glaciation.
(b) The coefficient of consolidation, along with the drainage path distance determines the
time for primary consolidation:
dation ratio (OCR
vp
/
vo
, where
vp
TH
C
2
t
(6.41)
v
where t
time for primary consolidation (years); T
time factor; H
drainage path
coefficient of consolidation (m
2
/year).
The coefficient of consolidation is dependent on the OCR (it is larger in the over-
consolidated range of stresses, than in the normally consolidated range) and by fabric
in the soil (e.g. fissures, fine layers of silt or sand interbedded with clay). Care must be
taken to test sufficiently large samples to include the fabric e.g. by using triaxial con-
solidation. However, in these tests, the side drains may become a control on the rate
of flow of pore water from the soil.
The drainage path H, is dependent on the thickness of the clay layer, and the pres-
ence of more permeable sandy, or silty layers within the clay. These can often best be
located by cone penetration (CPT) or piezocone (CPTU) testing.
(c) Consolidation within the over-consolidated stress range (O-A in
Figure 6.38
) results
in relatively small changes in void ratio (and hence consolidation) as the soil follows
the unload-reload index C
r
. Consolidation within the primary or normally consoli-
dated range (A-B in Figure 6.38) gives larger changes in void ratio as the soil follows
the compression index C
c
. Hence, to estimate settlements, it is important to clearly
define the pre-consolidation pressure
distance (metres) and C
v
vp
in oedometer testing.
It is recommended that the Schmertmann (1955) correction to C
c
be applied to allow for
sample disturbance. This involves selecting point 0 from the existing effective over burden
