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Technical paper: Mohr Coulomb error correction

Andrew N Schofield, Cambridge University. This paper was first published in GE’s August 1998 edition.

Tenaghl and the Mohr Coulomb error

In the 1st ISSMFE Conference (1936) proceedings, Terzaghi writes that newly over-consolidated clay strength fits Mohr’s rupture hypothesis (he used the word hypothesis, not criterion). He quotes data for drained shear box tests which his research student Hvorslev fits to the Mohr Coulomb equation. They are wrong. They contradict Coulomb’s paper which, in three separate places, states that “I’adherence est nulle dans les terres nouvellement remuees” (newly worked soil has no cohesion), and they have no data for soil strength on the wet side of critical states (cs).There is no “true” cohesion on the dry side of the critical state. The peak strength of dense soil paste is due to interlocking and friction among the soil particles.

When soil flows, many soil particles change partners, and there is no time to bond particle to particle. It is only when soil is left to age and creep that bonds develop at particle contacts and turn it into soft rock. Renewed strains destroy this strength. Broken bonds do not resist ground failure mechanisms. Bonds are not remade until there has been time for ageing and creep, long after a failure event. Terzaghi’s Mohr Coulomb error is to suppose that peak strength seen in Hvorslev’s dense, newly remoulded shear test samples indicates strong cohesive bonds, when it really indicates dense packing of particles. Figure 1 shows how the peak strengths are caused by particle interlocking. Strength in Hvorslev’s tests depends on packing geometry, not on chemistry of bonds. 

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