Pore pressure, effective stress and drainage - The Mechanics of Soils and Foundations - page 77

The processes of undrained loading followed by consolidation can be represented
by an experiment with a packet of crisps. Put the packet on the table and put a mass of
a few kilos on it. The packet will inflate as the pressure in it increases. This represents
undrained loading. Puncture the packet with a pin. The air will escape and as the
pressure in the packet reduces the mass settles and you can hear crisps breaking as
load is transferred from the air pressure to the crisps. This represents consolidation.
The analogy is not exact but the experiment nicely illustrates the processes. If the crisp
packet is punctured before the mass is applied the air escapes immediately and the load
is taken by the crisps. This represents drained loading.
In the simple examples of drained and undrained loading illustrated in Figs. 6.10
and 6.11, the increment of loading was positive so that the soil compressed as water
was squeezed out. Exactly the same principles apply to unloading where the increment
is negative and the soil swells as water is sucked in by the negative excess pore pressure.
You should sketch diagrams like Figs. 6.10 and 6.11 for an increment of unloading.
Remember the final steady state pore pressure at the end of consolidation u ∞ need
not be same as the initial steady state pore pressure u 0 before the undrained loading.
The excess pore pressure which causes consolidation is the difference between th e
current pore pressure and the final steady state pore pressure so after a long time u ∞
is always zero. Sometimes only the external water levels are changed as, for example,
when a dam is filled or emptied. In this case there will be consolidation in the soil as
the pore pressures adjust to the new external water levels.
Consolidation is any process in which effective stresses change as excess pore pres-
sures dissipate towards their long term steady state values. If excess pore pressures
are positive, effective stresses increase with consolidation and the soil compresses. On
the other hand, if excess pore pressures are negative, effective stresses decrease with
consolidation and the soil swells.
6.10 Rates of loading and drainage
When distinguishing between drained and undrained loading it is relative rates of
loading and seepage that are important, not the absolute rate of loading. Seepage of
water through soil, which will be covered in more detail in Chapter 14, is governed
by the coefficient of permeability k . Figure 6.12 illustrates seepage with velocity V
Figure 6.12 Seepage of water through soil.