Civil Engineering Reference
In-Depth Information
Chapter 12
Rate of Foundation Settlement
The settlement of a foundation in cohesionless soil and the elastic settlement of a foundation in clay can
be assumed to occur as soon as the load is applied. The consolidation settlement of a foundation on clay
will only take place as water seeps from the soil at a rate depending upon the permeability of the clay.
12.1 Analogy of consolidation settlement
The model shown in Fig.
12.1
helps to give an understanding of the consolidation process. When load is
applied to the piston it will be carried initially by the water pressure created, but due to the weep hole
there will be a slow bleeding of water from the cylinder accompanied by a progressive settlement of the
piston until the spring is compressed to its corresponding load. In the analogy, the spring represents the
compressible soil skeleton and the water represents the water in the voids of the soil; the size of the weep
hole is analogous to the permeability of the soil.
Consolidation attained at t
ime t
The degree of consolidation U
,
,
=
Total consolidation
12.2 Distribution of the initial excess pore pressure, ui
i
If we consider points below the centre of a foundation it is seen that there are three main forms of pos-
sible ui
i
distribution.
Uniform distribution can occur in thin layers (Fig.
12.2a
), so that for all practical purposes ui
i
is constant
and equals
Δ
σ
1
at the centre of the layer.
Triangular distribution is found in a deep layer under a foundation, where ui
i
varies from a maximum
value at the top to a negligible value (taken as zero) at some depth below the foundation (Fig.
12.2b
(i)).
The depth of this variation depends upon the dimensions of the footing. Figure
12.2b
(ii) shows how a
triangular distribution may vary from ui = 0
i
=
0 at the top of a layer to ui = 0
i
=
a maximum value at the bottom;
this condition can arise with a newly placed layer of soil, the applied pressure being the soil's weight.
Trapezoidal distribution results from the quite common situation of a clay layer located at some depth
below the foundation (Fig.
12.2c
(i)). In the case of a new embankment carrying a superimposed load, a
reversed form of trapezoidal distribution is possible (Fig.
12.2c
(ii)).
12.3 Terzaghi's theory of consolidation
Terzaghi's first presented this theory in 1925 and the most practical work on the prediction of settlement
rates is based upon the differential equation he evolved. The main assumptions in the theory are as follows.
