TM 5-852-4/AFM 88-19, Chap. 4
Figure 4-68. Conditions for bearing capacity analysis of square raft (by CRREL). Desired/actor of safety = 2.0. A uniform
load can be assumed for closely spaced individual footings or piers, or the area beneath piling.
qu = 1.3 c Nc + y D Nq + 0.4 yB N7
qu = 1.3 c(5.7) + 115(7)(1.0)
The value of "c" varies at different depths with the soil
qu = 1.3 c (5.7) + 2000
temperature and the soil type. A failure surface can be
qu = 7.41 c + 0.40
assumed as shown in figure 4-68 and a "weighted" value
Using cohesive stress of 2.0 T/fr' for silt at 30F (from
for "c" used, e.g.
table 4-4 in paragraph 4-4, neglecting safety factor):
qu = 7.41 (2.0) + 0.40 = 14.82 + 0.40 =
FS = 15.22(30x30) 7.61 (>2)
This is very safe, but creep settlement may govern.
Making creep settlement estimate. Using charts
from TM 5-818-1 for uniform load (based on Boussinesq
equations) the stress distribution beneath the centerline
of the footing (δz) is tabulated in table 4-9 and shown in
cn = cohesion for zone n
figure 4-69 (δz = increase in the stress in the soil
ARCn = arc length contained in zone n
neglect weight of soil).
A conservative procedure is to use the smallest cohesion
of the soil zones involved. Neglecting internal friction (Nλ
Nc = 5.7
Nq = 1.0
D = 7