Question

1. A 20-ft-deep cut for a basement excavation is specified to be sloped at 1:2 (one horizontal to two vertical). The upper 40 ft of soil at the site consists of a homogeneous clay with a density of 90 pcf and an unconfined compressive strength averaging 2400 psf. The water table is at great depth. Can the cut be made? If so, what is the factor of safety for the slope? Is the slope safe enough for normal work to progress in the excavation without danger?

Answer 1

We have

H = 20 ft

Slope angle = tan-1(2/1) = 63 degrees

Cohesion = c = unconfined strength/2 = 2400/2 = 1200 psf

(clay)

From stability chart

For

Stability factor = N = 5.1

Also Stability factor is given by

Factor of safety for slope is

Since Fs > 1

Slope is safe enough for normal work to progress in the excavation without danger