Fast shearing of pre-existing shear zones in soil

Abstract

The paper presents results from a laboratory investigation into the in¯uence of fast rates of displacement on the residual strength of soil. This in¯uence was studied in the ring shear apparatus. Shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement. Results from a wide range of natural soils are presented. All soils show initially an increase in strength when resheared at fast rates of displacement. The strength then tends to decrease with fast displacement to a minimum valueÐthe fast residual strength. Three types of rate effects on the residual strength are identi®ed: a positive rate effect in soils showing a fast residual strength higher than the slow residual; a neutral rate effect in soils showing a constant fast residual strength, equal to the slow residual, irrespective of rate of displacement; and a negative rate effect in soils showing a signi®cant drop in fast residual strength below the slow residual when sheared at rates higher than a critical value. The relation between the types of displacement rate effect and the basic shearing modes of slow residual strength (turbulent, transitional and sliding) is considered and the causes of the negative rate effect and the in¯uence of the apparatus on it have been investigated. The practical implications of the results in the context of slope stability and displacement piles are also discussed.

Rate-weakening friction characterizes both slow sliding and catastrophic failure of landslides

“…However, there are no published rate and state friction parameters from laboratory experiments on landslide material at relevant in situ stress conditions. Because the slip surfaces of landslides and faults are both governed by frictional sliding (14), we select nominal rate and state parameter values from the fault and soil mechanics literature (Table S1) (18,(44)(45)(46)(47)(48)(49). Because our focus here is on understanding the overall patterns of landslide motion, we consider small errors introduced by uncertainty in parameter values to be acceptable.…”

“…For the slow-moving landslide model runs, we use a larger d c that is consistent with fieldbased estimates of friction parameters on faults (35). We select μ 0 that characterizes weak, clay-rich materials (45,46,48,49) that are common to slow-moving landslides with well-developed slip surfaces (2). The shear modulus and shear wave speed are based on values measured for the Jurassic and Cretaceous Franciscan Complex, Northern California (50), a region well known for slow-moving landslide activity (5), and are notably lower than values reported for deep faults.…”

“…4 and Table S1). We use a μ 0 that characterizes rock and soil materials (18,44,45,48,49). The shear modulus and shear wave speed are based on values measured in glacial tills that are prone to catastrophic landslides (51) [e.g., the March 2014 Oso landslide in Washington, which killed 43 people (52)].…”

Dynamic thermo-poro-mechanical analysis of catastrophic landslides

“…First, he is addressing the issue of an initial increase in shear strength: that is, from slow residual (static) to the threshold and then to the fast peak value (Tika et al, 1996). As he states, this initial increase is due to 'viscous and suppressed-dilatancy effects'.…”

Residual Strength Characteristics of Naturally and Artificially Cemented Clays in Reversal Direct Box Shear Test

“…The relationship between the residual strength and the shear displacement rate for various soils has been examined using RDBST apparatus (Kenney, 1967;Ramiah et al, 1970;Skempton, 1985;Mitachi et al, 2003) and RST apparatus (Lemos et al, 1985: Skempton, 1985Yatabe et al, 1991;Tika et al, 1996;Suzuki et al, 2000). Skempton (1985) concluded that the variation in strength within a usual range of slow laboratory test (0.002-0.01 mm/min) was negligible.…”