Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 233

For site characterization, maximum use of past local experience is impor-
tant, particularly in the selection of an appropriate in-situ program. In new,
unexplored territories where the presence of carbonate soils is suspected, selec-
tion of an in-situ test program should draw upon any experience with carbonate
soils where geographical and environmental conditions are similar.

4.5.1 Strength
Soils usually cannot transfer stresses beyond a certain limit. This is called
the strength of the soil. The shear strength of soils is usually expressed by
Coulomb

' s relation between the maximum shear stress and the effective normal

stress:

τ max = c + σ′ tan ϕ (4.3)

where c is the cohesion and ϕ is the friction angle. For sands, c is usually neg-

ligible, so that is the only strength parameter.

For clays it is often most relevant to consider the strength in undrained con-
ditions, during which the effective stress remains constant. The undrained shear
strength is usually denoted by s u , and it is often considered irrelevant to what
degree this is to be attributed to cohesion or friction. The shear strength
parameters can be determined in the laboratory, for instance by tri-axial testing.
A simple and useful in-situ test is the cone penetration test (CPT), in which a
cone is pushed into the ground using hydraulic pressure equipment, while
recording the stress at the tip of the cone, and often also the friction along
the lower part of the shaft. The test is used in The Netherlands as a model
test for a pile foundation, and the results are used directly to determine the bear-
ing capacity of end-bearing piles, using simple scale rules.
The CPT can also be used to estimate the strength of a soil, however, by
using certain correlations. For a penetration test in sand, for instance, the bear-
ing capacity of the cone, q c , is, according to Brinch Hansen

ϕ

' s formula,

q c = s q N q σ′ = s q N q γ′ z (4.4)

where s q is a shape factor to express effective weight of the overburden, for
which one may use the formula:

s q = 1 + sin ϕ (4.5)

z is the depth, and N q is a dimensionless constant for which theoretical analysis
has given the value as in the following equation:

1 + sin ϕ

N q = exp ðπ tan ϕÞ (4.6)

1 − sin ϕ

The predicted cone resistances (q c ) for various types of sand at a depth of
10 m and 20 m are shown in Table 4.3 , assuming that = 10 kN/m 3 . The values γ