Civil Engineering Reference
In-Depth Information
The element contracts both horizontally and vertically under the actions of
∆
′
σ
3
and
Δ
σ
1
, which gradually
increase in magnitude as the excess pore water pressure,
Δ
u, decreases. Eventually, when
Δ
u
=
0, then
∆
′
=
′
=
σ
∆
σ
3
and
∆
σ
∆
σ
1
, and at this stage consolidation ceases, although secondary consolidation may
1
3
still be apparent.
If it can be arranged for the lateral expansion due to the change in shape to equal the lateral compres-
sion consequent upon the change in volume, and for these changes to occur together, then there will be
no immediate settlement and the resulting compression will be one-dimensional with all the strain occur-
ring in the vertical direction. Settlement by one-dimensional strain is by no means uncommon in practice,
and most natural soil deposits have experienced one-dimensional settlement during the process of deposi-
tion and consolidation.
The consolidation of a clay layer supporting a foundation whose dimensions are much greater than the
layer's thickness is essentially one-dimensional as lateral strain effects are negligible save at the edges.
11.3.2 The consolidation test
The apparatus generally used in the laboratory to determine the primary compression characteristics of
a soil is known as the consolidation test apparatus (or oedometer) and is illustrated in Fig.
11.6a
.
The soil sample (generally 75 mm diameter and 20 mm thick) is encased in a steel cutting ring. Porous
discs, saturated with air-free water, are placed on top of and below the sample, which is then inserted in
the oedometer.
A vertical load is then applied and the resulting compression measured by means of a transducer at
intervals of time, readings being logged until the sample has achieved full consolidation (usually for a
period of 24 hours). Further load increments are then applied and the procedure repeated, until the full
stress range expected
in situ
has been covered by the test (Fig.
11.6b
).
The test sample is generally flooded with water soon after the application of the first load increment
in order to prevent pore suction.
After the sample has consolidated under its final load increment the pressure is released in stages at
24 hour intervals and the sample allowed to expand. In this way an expansion to time curve can also be
obtained.
After the loading has been completely removed the final thickness of the sample can be obtained, from
which it is possible to calculate the void ratio of the soil for each stage of consolidation under the load
increments. The graph of void ratio to consolidation pressure can then be drawn, such a curve generally
being referred to as an e-p curve (Fig.
11.7a
).
It should be noted that the values of p refer to effective stress, for after consolidation the excess pore
pressures become zero and the applied stress increment is equal to the effective stress increment.
Transducer
Water
Porous
discs
Soil
Load
Load
(a) Consolidation apparatus
Fig. 11.6
The consolidation test.
