Page 247 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
242 Geotechnical Engineering

is to place the structure on a raised gravel pad that has a relatively high thermal
conductivity so that it cools from the outside. This and other arrangements are
discussed by Phukan (1991).

11.11 SUMMARY

Positive pore water pressure weakens soil by reducing friction between soil grains.
Pore pressure therefore is subtracted from total stress to obtain effective stress.

Capillary action draws water up into a fine-grained soil, creating a capillary
fringe. Soil in the capillary fringe may be saturated, so saturation is not a criterion
for locating a groundwater table. A groundwater table is measured in an open
boring where there is no capillary effect. Pore water pressures in a capillary fringe
are negative relative to atmospheric pressure.

Under static conditions, pore water pressure increases linearly with depth below a
groundwater table, whereas soil weight-induced stresses increase with depth below
the ground surface.

Compressing saturated soil increases its pore water pressure. The increase over
static groundwater conditions is excess pore water pressure, which like static pore
water pressure acts to decrease effective stress and friction.

Capillary action is a lead actor in frost heave.

Permafrost conditions require special considerations and will become more
prominent as mineral resources are developed in Arctic areas.

Problems

11.1. Why is the term ‘‘matric potential’’ now preferred over ‘‘capillary
potential’’?
3
11.2. A soil unit weight above the groundwater table averages 115 lb/ft and
3
below the groundwater table averages 112 lb/ft . The water table is 6 ft deep
below the ground surface. Prepare a plot of total and effective stresses to
a depth of 20 ft.
11.3. Distinguish between artesian water pressure and excess pore water
pressure.
11.4. (a) Calculate the height of rise of pure water in a clean glass tube whose
inside diameter is 0.15 mm, using T ¼ 7.75 mN/m. (b) What is the height of
rise if the tube is sufficiently dirty to produce a wetting angle of 158?
11.5. If the outside diameter of the tube in Problem 11.4 is 0.35 mm, what is the
compressive stress in the walls of the tube in part (a) of that problem?

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