Abstract
Three Iowa landslides analyzed on the basis of Borehole Shear Test (BST) data are reported. The soils included CL and ML loess and glacial till, and CH clay shale. Backcalculations from BST friction angles measured in the shear zones consistently indicated that cohesion after sliding is near zero, suggesting that after-slip equilibrium must derive mainly from internal friction in the shear zone. A unique advantage of the BST is that it can measure Ф that is in effect at the time of sliding. Stage BST's were performed on a 28-ft high lagoon embankment of compacted CH soil over random fill, to determine if the lagoon could be safely put into service. 14 BST shear envelopes were obtained in 2 days and gave sufficient data for probabilistic determinations of the failure risk based both on the range in individual strengths and the range in predicted means. The stage BST separately measures c and Ф means and variabilities, not possible when tests are performed on individual samples that may or may not share a common shear envelope.
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