Stability of Slopes - Elements of Soil Mechanics - page 408

13.3 Slope stability design charts
Design charts offer a means of rapidly determining an approximate factor of safety of a homogeneous,
regular slope. These slopes rarely exist in reality but nonetheless many embankments and cuttings can
be considered as being of this form, if only to offer a simple and rapid, yet very approximate solution to
assessing their stability. Once an approximate factor of safety is found from a design chart approach, it
would be normal practice for a more rigorous analysis to then be performed to establish a more accurate
and reliable measure of the safety of the slope.
13.3.1 Taylor's charts: rapid determination of F for a homogeneous, regular slope
It can be shown that for two similar slopes made from two different soils the ratio c m / γ H is the same for
each slope provided that the two soils have the same angle of shearing resistance. The ratio c m / γ H is
known as the stability number and is given the symbol N, where c m = mobilised cohesion, γ = unit weight
of soil, and H = vertical height of slope.
For any type of soil, the critical circle always passes through the toe when β > 53°. In theory, when
φ = 0° (in practice when φ < 3°) and β > 53°, the critical slip circle can extend to a considerable depth
(Fig. 13.7c ).
Taylor ( 1948 ) prepared two set of curves that relate the stability number to the angle of the slope: the
first (Fig. 13.24) is for the general case of a c ′ - φ ′ soil whilst the second (Fig. 13.25) is for a soil with φ = 0°,
a slope angle of less than 53° and with a layer of stiff material or rock at a depth DH below the top of
the embankment. D is known as the depth factor and, depending upon its value, the slip circle will either
emerge at a distance nH in front of the toe or pass through the toe (using the dashed lines the value of
n can be obtained from the curves).
Fig. 13.24 Taylor's curves for rapid assessment of F (for φ = 0° and β < 53°, use Fig.13.25).