Rate and OC clays is opposite. Additionally, the

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Last updated: March 12, 2019

Rate effect on the peak andresidual shear strength of normally consolidated (NC) soils has been investigatedcomprehensively. Differently from NC soils, the shear surface structure of over-consolidated(OC) soils can be affected by the dilation process when shearing. Thisinfluence will exhibit differently at different shear displacement rates and mayaffect the shear strength. Therefore, the rate dependency of both peak and residualstrength of OC soils may be different from that of NC soils.

In this study, a number of ring shear tests wereconducted on both NC and OC kaolin clays under different shear displacementrates to investigate the rate dependency of shear strength. The artificial overconsolidationcondition of clay samples was created in the ring shear box. The variation ofcohesion and friction angle under different shear displacement rates was alsoclarified. In addition, the multi-stage procedure decreasing the normal stresswas applied to determine the residual strength of OC clays.

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The test resultsindicated that similarly to NC clays, the positive rate effect on residualstrength also exhibits in OC clays, but with the less magnitude. The rate effecton the peak strength of NC and OC clays is opposite. Additionally, thevariation in cohesion and friction angle at the peak and residual state of OC claysunder different shear rates is different. The multi-stage procedure reducing thenormal stress can be used to determine the residual strength of OC clays atshear displacement rates equal to or less than 0.

5 mm/min. Keywords:rate effect; over-consolidated clays; peak strength; residual strength;single-and multi-stage procedure.               1. Introduction        The existence of over-consolidated (OC)soils is popular in nature and large-scale landslides. It can be formed by the stressrelease, the change of groundwater level, secondary consolidation (aging), chemicalbonding and desiccation stress (Hanzawa and Adachi,1983).

Many of OC clays are fissured, jointed, and contain slickensidesas well (Skempton, 1964). Thus, the shearstrength of OC clays has received much attention for evaluating and predictingthe slope stability (Skempton, 1964, 1970, 1985; Stark,1995; Stark and Eid, 1997). The shear behaviour of OC and normallyconsolidated (NC) clays are different. At peak state, the shear strength of OCclays is often higher than that of NC clays, and the shear behaviour is morebrittle. At post-peak state, for NC clays, the strength loss is due to theorientation of clay particles in a direction parallel to the shear surface. ForOC clays, the post-peak strength is decreased by an increase in water content dueto dilation and by the orientation of clay particles parallel to the shear surface(Skempton, 1970). The constant minimum shearstrength in the post-peak state after a large shear displacement is defined asthe residual strength (Skemptom, 1964). In thedetermination of residual strength by laboratory testing, the use of OC samplesis also suggested to reduce the vertical settlement and the soil leakage (Stark and Eid, 1993; Stark, 1995).

        In the geotechnicalliterature, it has been noted that the residual shear strength is a fundamentalcharacteristic of soil. It is mainly affected by the clay mineral, the plasticitycharacteristics, the size and shape of particle, the pore water chemistry, the effectivenormal stress and the shear displacement rate (rate effect). It is almostindependent of stress history or overconsolidation ratio (OCR) (e.

g., Skempton, 1964; Lupini et al., 1981; Vithana etal., 2012; Li et al., 2017; Xu et al., 2018). The rate effect on the residualshear strength of soils has been thoroughly investigated for many years. It hasbeen observed that the shear strength at the residual state may or may notdepend on the shearing speeds.

In particular, Tikaet al. (1996) have indicated three types of rate effect on the residualstrength of soils: neutral, negative, andpositive. Previous investigationsindicated that the magnitude of the rate effect on residual strength may dependon the effective normal stress (e.g., Carrubba andColonna, 2006; Kimura et al., 2013; Gratchev and Sassa, 2015); the clayfraction, the plasticity index (Suzuki et al.,2001, 2009); and the soil density (Li andAylin, 2013).

However, most of the researches on the rate dependency ofresidual strength base on the NC soils. Hong et al.(2011) investigated the effect of cyclic loading on the residualstrength of over-consolidated silty clay.

The test results presented that thechange of residual strength with shear displacement rates after a cyclicloading at low OCRs was higher than that at a higher OCR. This indicates thatthe shear zone structure under different OCRs affects the rate effect onresidual strength of soils. Vithana et al.

(2012)revealed that the shear surface structure of OC soils may be disturbed by thedilation. Thus, although the OCR seems to have no effect on residual strength,it may affect the magnitude of the rate dependency of residual strength becausethe shear zone structure at different shear rates will influence the residualstrength.        Besides the ratedependency of residual strength, the rate effect on the peak strength of bothNC and OC clays was also examined. The rate dependency of the peak strength ofNC soils has been comprehensively investigated using triaxial test, directshear box test, and ring shear test. However, most of the researches on therate dependency of the peak strength of OC soils use the triaxial test (e.

g., Lefebvre and Lebouef, 1987; Sheahan et al., 1996;Mun et al., 2016).

Therefore, the rate effect on the residual and peakstrength of OC soils in ring shearing needs to be investigated. In addition,the relationship between OCRs and the rate dependency of shear strength needsbe clarified.        In this study, aconventional ring shear apparatus (Bishop-type) (Bishopet al., 1971) was employed. A series of ring shear tests were performedon reconstituted kaolin clay with different overconsolidation ratios (OCRs)under different shear displacement rates.

The main objectives of this study areto examine the rate dependency of both peak and residual strength of OC clays andtheir relationship with OCRs in ring shearing. The variation of cohesion andfriction angle at both peak and residual state of OC clays at different shearrates was also clarified. In addition, the multi-stage procedure decreasing thenormal stress was also applied to determine the residual cohesion and frictionangle of OC clays. The residual strength from this procedure was compared withthe test result of single-stage procedure which was conducted on individual OCsamples.

 

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