Page 216 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 211

11.2.5 Excess Pore Water Pressure and Shear Strength
Pushing soil grains together, as occurs under a foundation load, reduces the pore
space and causes a corresponding increase in the pore water pressure in a
saturated soil, until the water can escape and the system returns to equilibrium.
The temporarily increased pore pressure affects intergranular contact pressures
and friction the same as does static pore water pressure, and eq. (11.2) still applies.

The response of pore water pressure to external loading is a critical relationship in
geotechnical engineering. It explains why rapid loading of saturated clay soil, as
under the wheel of a car, can result in the car being stuck in the mud, whereas if
the same load is applied after the soil dries out there is no problem.

The rate of loading is particularly important for large structures such as earth
dams, so the pore water pressure is monitored using observation wells or piezo-
meters. If the pore water pressure is too high for the structure to be safe,
construction is stopped until there has been sufficient time for part of the excess
pore water pressure to drain out. The heavy stone castles of Europe survived
because they were built over time spans of hundreds of years instead of being
plunked down in a matter of months.

Settlement occurs as excess pore pressure declines and more load is transferred
from the water to the soil grains, causing the soil to compress. This time-related
compression is called consolidation. Consolidation of soil takes time, which
depends in part on the permeability of the soil. The time function was analyzed
by Karl Terzaghi and is presented in a later chapter.

Structures built on saturated sand are of less concern because sand has a
sufficiently high conductivity for water that it can drain out as a load is applied.

Excess pore water pressure also develops during formation of a sedimentary
deposit as more load is slowly applied at the surface. Very high pore water
pressures have been measured in deep oil wells, still left over from sedimentation
and enhanced by expansion of the pore water as a result of the increase in
temperature with depth, called a ‘‘geothermal gradient.’’

11.2.6 Is an Artesian Condition the Same as Excess Pore
Water Pressure?
This can be argued, because there is no question that an artesian condition
represents an excess of water pressure that can be relieved by drainage. However,
the cause is not compression of the soil voids, but water moving downward from
an elevated position through a confined porous soil layer such as a sand or gravel,
much as if the water were moving in a pipe. The source normally is a hill where the
groundwater table naturally is higher. An artesian condition therefore does
not qualify as excess pore pressure, because the pressure is in equilibrium with

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