Unbraced Vertical Cut in Clay

The following problem for verification is the slope failure described by Tschebotarioff (1973), and reviewed in Duncan, J.M., Wright, S.G., and Brandon, T.L., 2014. Soil Strength and Slope Stability, Second Edition. It is an unbraced 31.5 foot high vertical excavation in a varved clay, which has been assigned a unit weight of 120 pcf, and an undrained shear strength of 1050 psf.

The geometry and material parameters are shown below.



The review of this unbraced excavation included:
  • a comparison with the closed-form solution for a plane slip surface through a vertical slope in cohesive soil, which gave:
  • a comparison with Janbu's charts (Φ = 0) solution, which gave:
  • a comparison with several computer programs using the Bishop's Simplified method, which gave a factor of safety of 1.06.


The computer programs indicated tensile forces (ie. a negative base normal force) at the upper part of the slip suface. Analyses were completed with the inclusion of a tension crack at 1 ft intervals. The published results and Mobislope results for the varying tension crack depths are shown below.

Published Results

MobiSlope Results

Crack Depth (ft)
FOS
0
1.06
1
1.03 - 1.04
2
1.01 - 1.02
3
0.98 - 0.99
4
0.96
Crack Depth (ft)
FOS
0
1.066
1
1.038
2
1.012
3
0.986
4
0.962

MobiSlope analyses were conducted using Bishop's Simplified method and using the Auto Search with an auto resolution tolerance of 2. The analysis results summary and heat safety map for the analyzed slip surfaces of the 4 ft deep tension crack case are shown below.



Download the MobiSlope file with the analysis results for this verification example here.