The effects and progressive failure in clay slopes
The stability of over consolidated fissured clays and clay
shales is a central problem in Soil Mechanics. In this thesis
the problem has been approached using field evidence; the actual
failure surfaces of slips being analysed by the generalised
stability method rather than the most critical circle or wedge for
a failed slope being determined.
development has resulted.
A different emphasis on slip
Case records of over 50 slips have been analysed in a spectrum
of argillaceous sediments ranging from the Oligocene to the Carboniferous.
The London Clay, Oxford Clay and Lias Clays provided the
bulk of the data, with the support of several Cretaceous Clays,
and mention is made of the Coal Measure shales, bentonitic shales
and, shales weakened by flexural slip. Laboratory tests were undertaken.
These included classification tests on nearly all slips,
except those in London Clay, and several sets of direct shear tests
on Oxford Clay, two of which illustrated the importance of directional
(sedimentary) properties on the path to the residual strength.
From the analysis the time effects on the stability of soils
were obtained. Decay in strength with time was found to be
predictable in the London Clay and, to a lesser extent, the Oxford
Clay, although the Lias Clays were more erratic. In general, it
was found that the initial failure of a slip was explainable with
the average angle of friction equal to, or greater, than the laboratory
peak value, and that the decay in strength - at least over
periods of the order of 100 years - was a drop in the effective
cohesion term only, ie. a softening process. This general trend
may not apply where any form of progressive failure occurs. The
modes of progressive failure do, however, appear to be very limited
and deformation, due either to lateral expansion towards a cutting
or simple swelling of susceptible layers, needs to be severely
constrained in order to develop strains of sufficient magnitude to
produce residual conditions. Some information on the average drop
in strength due to movement of slips is given, and it has been found
that, a slip will generally need to be reactivated once or twice to
give average conditions near to the residual. An attempt has also
been made to show the time development of progressive failure mechanisms
in some London Clay slides.
Generally good agreements were obtained between the residual
angle of friction of reactivated slides and that found by conventional
laboratory tests, e.g. direct shear or tests on cut planes,
However, most slides take place at very much smaller values of the
average effective normal stress than are used in laboratory tests and
the implications of these two facts, combined with the possibility of
curvature of the failure envelope, are mentioned. Occasional wide
discrepancies between the field and laboratory values of the residual
angle of friction were also encountered; these perhaps being due to
bands of different clay minerals in the slope or excessive movements.
In Section I of the thesis the generalised stability analysis
is described and the effect on the results of several variables and
assumptions is investigated. Comparisons are also made between
this method and the other conventional (hand) methods of stability