Civil Engineering Reference
In-Depth Information
10
9
) in the durations, or rates, of loading or unloading in
large range (more than
×
these examples.
In any geotechnical calculation or analysis it is absolutely essential to state whether
the calculation is for drained or undrained loading, and we will discover that different
analyses are required for each in later chapters. What is important is the relative rates
of loading and drainage - is there enough time during the loading to allow drainage
to occur or is the loading so fast that there will be no drainage? Of course, in reality,
neither condition will be satisfied absolutely and decisions must be made as to whether
the construction is more nearly drained or undrained.
Many engineers will assume that loading or unloading of a coarse-grained soil will be
drained and of a fine-grained soil will be undrained. These assumptions are adequate
for loading rates which are not at the extremes of those in Table 6.3. Very rapid
loading of coarse-grained soil is likely to be undrained. Earthquakes, pile driving and
ocean waves may generate excess pore pressures in sands which can cause liquefaction
failures and which explain the change of pile capacity after a delay in driving. Very
slow loading of clay slopes due to natural erosion is likely to be drained and pore
pressures and slope angles of many natural clay slopes correspond closely to the fully
drained, steady state conditions.
6.11 Summary
1. In soils total stresses arise from the weight of the soil (including the soil grains
and the pore water) and any other external loads from foundations, walls and free
water. There are also pore pressures in the water in the voids.
2. The stresses that govern soil behaviour are effective stresses given by
τ
=
τ
and
σ
=
σ
−
u
. As a result soils are affected equally by changes in total stress and pore
pressure.
3. Pore pressures may be either positive below the water table or negative (suction)
above it. Where pore pressures are negative effective stresses are larger than total
stresses. The maximum suctions which can occur in saturated soil depend on the
grain size of the soil.
4. Volume changes in saturated soil can only occur as water seeps through the
pores and the rate of seepage is governed by the coefficient of permeability
k
.
If soil is loaded slowly, compared with the rate of drainage, the pore pressures
remain constant and volume changes occur during the loading, which is called
drained.
5.
If soil is loaded quickly, compared with the rate of drainage, the volume remains
constant, excess pore pressures arise and the loading is called undrained. Sub-
sequently, consolidation occurs as the excess pore pressures dissipate and water
seeps from the soil, causing volume changes.
Worked examples
Example 6.1: Calculation of vertical stress
For the measuring cylinder of sand des-
cribed in Example 5.4 (see Fig. 5.8), the total vertical stress, the pore pressure and the
