Local jurisdictions require that safety factor calculations for static slope stability and infinite slope stability be based on residual, drained shear strength parameters obtained using a direct shear machine. I am consistently getting values for cohesion around 100 to 200 psf, and values for phi around 22 to 26 degrees, for well compacted fill materials (92% modified proctor) with fines contents ranging from 20 to 45 percent. Test samples are both in-situ (obtained from a hand-driven california drive), and remolded (to avoid potential disturbance). Use of these values will result in unsatisfactory safety factors for infinite stability and static stability of high 2:1 (h:v) slopes. Competitors seem to be reporting values up to 50% greater. Any suggestions on how to obtain more suitable (and economical) shear strength values (i.e. shear rate, normal load, sampling method, etc)?
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