Residual Strength: Does BS 5930 Help or Hinder?

Abstract

BS 5930 offers little assistance to engineers wishing to use residual strength parameters in slope stability analysis. It wrongly suggests the ring shear gives lower parameters than the shear box.BS 5930 does not mention the fact that the residual strength is stress dependent, hence the failure envelope is curved and the parameters must be assessed using an appropriate effective normal stress. For this reason the correlation charts relating ϕ′R to plasticity index or clay content need replacing with a series of charts in which these properties are plotted against ϕ′R values obtained at a number of effective normal stress loadings. Even then such correlations should be treated with caution.

Correlation of large displacement drained shear strength of landslide soils measured by direct shear and ring shear devices

“…Townsend and Gilbert (1973) concluded that the residual shear strength values measured in the DSD and the torsional RSD for remoulded specimens were not significantly different. The overestimation of the residual shear strength parameters in the direct shear test has also been documented by other researchers (Stark and Vettel 1992;Skempton 1985;Hawkins and Privett 1986;Stark and Eid 1994;Watry and Lade 2000). In these reports, it has been agreed that in the DSD, proper particle reorientation cannot occur without physical disturbances to the shear surface, as opposed to the case of the RSD.…”

“…However, except for a few studies, researchers have not widely addressed the use of artificially polished specimens in direct shear tests to measure residual shear strength. Related works or references to such procedures are documented in a few previous studies that dealt with the use of the direct shear and triaxial tests to measure residual shear strength (Skempton 1964(Skempton , 1985Chandler 1966;Kenny 1967;Durham 1976;Townsend and Gilbert 1976;Bromhead and Curtis 1983;Anayi et al 1988;Hawkins and Privett 1986;Kalteziotis 1993;Meehan et al 2010). Skempton (1985) postulated that although cut plane specimens would enable the measurement of residual shear strength for intact specimens, the reversal motion of the direct shear apparatus hinders the complete particle reorientation along the shear plane.…”

“…Anayi et al (1988) reported that the modified Bromhead ring shear tests produced lower values of residual shear strength than the reversal shear box tests. Hawkins and Privett (1986) reported that even with the pre-cut and polished shear plane, maximum ten reversals were found to be frequently necessary to establish the lowest residual conditions. Also, according to them, it is logical that even in that case, the changes generated in the internal soil structure would inhibit the development of a true residual state.…”

“…However, due to cost limitations of the RSD, the use of the DSD may be more appealing than the RSD to many practitioners worldwide (Stark and Eid 1994;Stark and Contresars 1996;Wan and Kwong 2002;Duncan and Wright 2005;Meehan et al 2010). It has been found that by preparing pre-cut specimens (Skempton 1964;Chandler 1966;Kenny 1967;Bromhead and Curtis 1983) and polishing the cut surface of the two halves prior to actual shearing (Chandler 1966;Hawkins and Privett 1986;Meehan et al 2010), it may be possible to achieve the residual shear surface conditions that are somewhat similar to the shear surface conditions produced by the RSD. Hawkins and Privett (1986) speculated the inability to achieve the true residual conditions even after many reversals in the DSD by using pre-cut and polished specimens.…”

“…It has been found that by preparing pre-cut specimens (Skempton 1964;Chandler 1966;Kenny 1967;Bromhead and Curtis 1983) and polishing the cut surface of the two halves prior to actual shearing (Chandler 1966;Hawkins and Privett 1986;Meehan et al 2010), it may be possible to achieve the residual shear surface conditions that are somewhat similar to the shear surface conditions produced by the RSD. Hawkins and Privett (1986) speculated the inability to achieve the true residual conditions even after many reversals in the DSD by using pre-cut and polished specimens. They emphasized that the data from the two devices should ideally be compared at the normal stress level to avoid the problem posed by the curvature of the stress envelope at lower normal stresses.…”