Engineering Geology

Chapter 5
enough cement to develop cohesion but still break down into their component grains. At low
stress levels, locked sands undergo high rates of dilation. Dilatancy becomes suppressed as
the level of stress increases, since the asperities on the surfaces of individual grains are
sheared through rather than causing dilation. They have peak frictional strengths considerably
in excess of those of dense sand, with residual angles of friction varying between 30 and 35∞.
Liquefaction is a phenomenon wherein a mass of soil loses a large percentage of its shear
resistance and flows in a manner resembling a liquid until the shear stresses acting on the
mass are as low as the reduced shear resistance. The basic cause responsible for the liquefaction
of saturated sands is the build-up of excess pore water pressure due to either cyclic
or shock loading of the sand. As a result, the grains of sand are compacted, with a consequent
transfer of stress to the pore water and a reduction of stress on the sand grains.
If drainage cannot take place, then the decrease in volume of the grains causes an increase
in pore water pressure. If the pore water pressure builds up to the point where it is the same
as the overburden pressure, then the effective stress is reduced to zero and the sand loses
strength with a liquefied state developing. In loose sands the pore water pressure can
increase rapidly to the value of the overburden or confining pressure. If the sand undergoes
more or less unlimited deformation without mobilizing any notable resistance to deformation,
then it can be described as having liquefied. However, Norris et al. (1998) pointed out that
a loose sand does not lose all strength during liquefaction. Loose sands at low confining
pressure, and medium and dense sands undergo only limited deformation due to dilation
once initial liquefaction has occurred. Such response is referred to as ranging from
limited liquefaction (in the case of loose and medium dense sands at low confining pressure)
to dilative behaviour (in dense sands).
Silts and Loess
The grains in a deposit of silt often are rounded with smooth outlines. This influences their
degree of packing. The latter, however, is more dependent on the grain size distribution within
a silt deposit, uniformly sorted deposits not being able to achieve such close packing as those
in which there is a range of grain size. This, in turn, influences the porosity and void ratio
values, as well as bulk and dry densities (Table 5.8).
Dilatancy is characteristic of fine sands and silts. The environment is all important for the
development of dilatancy since conditions must be such that expansion can take place. What
is more, it has been suggested that the soil particles must be well wetted, and it appears that
certain electrolytes exercise a dispersing effect, thereby aiding dilatancy. The moisture content
at which a number of fine sands and silts from British formations become dilatant usually
varies between 16 and 35%.
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