Page 215 - Geotechnical Engineering Soil and Foundation Principles and Practice

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

2
2
Answer: Total stress is ¼ 3.05 125 ¼ 381 lb/ft (1 19.6 ¼ 19.6 kN/m or kPa). The pore
2
water pressure is 3.05 62.4 ¼ 190 lb/ft (1 9.81 kPa). Then
2
0
¼ 381 190 ¼ 191 lb=ft 19:6 9:81 ¼ 9:79 kPað Þ
2
2
0
In the cgs system ¼ 1 2.0 ¼ 2.0 Mg/m , u ¼ 1 1.0, and ¼ 2 1.0 ¼ 1.0 Mg/m .
It will be noted that the effective stress for a submerged soil having this unit weight is
almost exactly one-half of the total stress, which is a useful approximation.
Example 11.2
Repeat the above calculation for effective stress using the submerged unit weight of
the soil.

Answer: As discussed in the preceding chapter, submerged unit weight at saturation
equals total unit weight minus the unit weight of water:
sub ¼ 125 62.4 ¼ 62.6 lb/ft 3
(19.6 9.81 ¼ 9.79 kPa). Then
2
0
¼ 3:05 62:6 ¼ 191 lb=ft 1 9:79 ¼ 9:79 kPað Þ
The two answers are the same, and spreadsheet calculations sometimes are simplified by
using submerged unit weight for the part of the soil that is below a groundwater table to
calculate effective stress.

11.2.3 Does Pore Water Pressure Affect Shearing Stress?

Pore pressure acts normal or perpendicular to grain surfaces and therefore
subtracts from compressive stress. How does pore water pressure influence
shearing stress? Under static conditions water has no shearing resistance and is
not influenced by pressure within the water. It is only by exerting pressure, which
tends to separate the soil grains, that pore water pressure affects the shearing
strength of soil.

11.2.4 Effective Stress at the Bottom of a Lake or Ocean

Let us assume that soil is submerged under 1000 m of water; does eq. (11.2)
still hold true? It might intuitively be assumed that the weight of the water would
push the soil grains together. On the other hand it can be argued that the
high water pressure will push them apart. Actually, both occur and balance
out. That is, pressure from the water column adds both to the total stress, , and
0
adds equally to the pore water pressure, u, in eq. (11.2), so effective stress, ,
remains the same. This explanation for low densities of sea bottom soils was
first recognized by a geologist, Lyell, and later quantified by Terzaghi in the
effective stress equation.

The same observation can be made by applying Archimedes’ principle: as a
buoyant force equals the weight of water displaced, it is not affected by the weight
of water underneath or on top of a submerged object.

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