Shear Strength of Soils - Elements of Soil Mechanics - page 123

ent upon the type of soil, its previous stress history, the drainage conditions, the form of construction and
the form of loading. Obviously the shear tests chosen to determine the soil strength parameters to be
used in a design should reflect the conditions that will actually prevail during and after the construction
period.
The variations of strength properties of different soils are described below.
Sand and gravels
These soils have high values of permeability, and any excess pore water pressures generated within them
are immediately dissipated. For all practical purposes these soils operate in the drained state. The appro-
priate strength parameter is therefore φ ′ , with c ′ = 0.
In granular soils the value of φ ′ is highly dependent upon the density of the soil and, as it is difficult to
obtain inexpensive undisturbed soil samples, its value is generally estimated from the results of the in situ
tests.
In the UK, the standard penetration test (see Chapter 6) is the one most used and a very approximate
relationship between the blow count N and the angle of internal friction φ ′ is shown in Fig. 4.34. It should
be noted that the corrected value for N, i.e. (N 1 ) 60 described in Chapter 6, can be used in conjunction
with Fig. 4.34, and that the value obtained approximates to φ t , the peak triaxial angle obtained from
drained tests.
Other factors, besides the value of N, such as the type of minerals, the effective size, the grading
and the shape of the particles are acknowledged to have an effect on the value of φ ′ , but in view of
the rough-and-ready method used to determine the value of N, any attempt at refinement seems
unrealistic.
Silts
These soils rarely occur in a pure form in the UK and are generally mixed with either sand or clay. It is
therefore usually possible to classify silty soils as being either granular or clayey. When there is a reason-
able amount of clay material within the soil there should be little difficulty in obtaining undisturbed samples
for strength evaluation. With sandy silts, estimated values for φ ′ can be obtained from the results of the
standard penetration test.
Fig. 4.34 Relationship between N and φ ′ .