TM 5-818-1 / AFM 88-3, Chap. 7
m. The degree of plastic yielding beneath an excavation
is governed by the stability number N expressed as
a. Empirical design earth pressure diagrams
developed from observations are shown in figure 14-11.
N = γH/su
In soft to medium clays, a value of m = 1.0 should be
applied if a stiff stratum is present at or near the base of
where γ, H, and su, are defined in figure 14-11.
the excavation. If the soft material extends to a sufficient
depth below the bottom of the excavation and significant
exceeds about 4, m < 1.0.
plastic yielding occurs, a value of 0.4 should be used for
U. S. Army Corps of Engineers
Figure 14-10. Types of support systems for excavation.