26The failure envelopes sketched in Figure Q2.7c show that •φ'critis constant (= 32.5°, closer to φ'peak–ψmax= 32°than the estimate of 35°based on φ'peak– 0.8 ×ψmax) because there is no dilation at the critical state •φ'peakreduces as the normal effective stress σ' increases, because the amount of dilation needed to reach the appropriate (critical) specific volume is reduced. In design, it may be safer to use the critical state strength φ'critthan the peak strength φ'peak, because •the peak strength depends on the extent to which the soil is dense in relation to the critical state under the effective stress conditions at failure. It is not a soil constant, and is unlikely to be the same throughout the mass of soil involved in a potential failure mechanism •it is unlikely that the peak strength will be mobilised simultaneously throughout the soil mass; instead, progressive failure at an average strength rather lower than the peak may occur.
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