Lateral Earth Pressure - Elements of Soil Mechanics - page 217

the values of these strength parameters vary with both the soil type and the drainage conditions. For
earth pressure calculations, attention should be paid to the following.
Sands and gravels
For all stages of construction and for the period after construction the appropriate strength parameter is
φ ′ . It is appropriate to take c ′ as being equal to zero.
Clays
The manner in which a clay soil behaves during its transition from an undrained to a drained state depends
upon the previous stress history of the soil and has been described in Chapter 4 .
Soft or normally consolidated clay
During and immediately after construction of a wall supporting this type of soil the vertical effective stress
is small, the strength of the soil is at a minimum and the value of the active earth pressure exerted on to
the back of the wall is at a maximum. After construction and after sufficient time has elapsed, the soil will
achieve a drained condition. The effective vertical stress will then be equal to the total vertical stress and
the soil will have achieved its greatest strength. At this stage therefore the back of the wall will be sub-
jected to the smallest possible values of active earth pressure (if other factors do not alter).
Obviously it is possible to use effective stress analyses to estimate the value of pressure on the back of
the wall for any stage of the wall's life. A designer is interested mainly in the maximum pressure values,
which occur during and immediately after construction. As it is not easy to predict accurate values of pore
water pressures for this stage, an effective stress analysis can be difficult and it is simplest to use the
undrained strength parameters in any earth pressure calculations, i.e. assume that φ = 0° and that the
undrained strength of the clay is c u .
As mentioned in Chapter 4 , the sensitivity of a normally consolidated clay can vary from 5 to 10. If it is
considered that the soil will be severely disturbed during construction then the c u value used in the design
calculations should be the undrained strength of the clay remoulded to the same density and at the same
water content as the in situ values.
If required, the final pressure values on the back of the wall, which apply when the clay is fully drained,
can be evaluated in terms of effective stresses using the effective stress parameters φ ′ (c ′ = 0 for a nor-
mally consolidated clay). Soft clays usually have to be supported by an embedded wall (see Chapter 8)
and water pressures acting on the wall must be considered in the design.
Overconsolidated clay
In the undrained state negative pore water pressures are generated during shear. This simply means that
this type of clay is at its strongest and the pressure on the wall is at its minimum value during and imme-
diately after construction. The maximum value of active earth pressure will occur when the clay has reached
a fully drained condition and the retaining wall should be designed to withstand this value, obtained from
the effective stress parameters φ ′ and c ′ .
With an overconsolidated clay, c ′ has a finite value (Fig. 4.32) but, for retaining wall design, this value
cannot be regarded as dependable as it could well decrease. It is therefore safest to assume that c ′ = 0
and to work with φ ′ only in any earth pressure calculations involving overconsolidated clay. The assumption
also helps to allow for any possible increase in lateral pressure due to swelling in an expansive clay as its
pore water pressures change from negative (in the undrained state) to zero (when fully drained).
Silts
In many cases a silt can be assumed to be either purely granular, with the characteristics of a fine sand,
or purely cohesive, with the characteristics of a soft clay. When such a classification is not possible then