Rate and OC clays is opposite. Additionally, the

Rate effect on the peak and
residual shear strength of normally consolidated (NC) soils has been investigated
comprehensively. Differently from NC soils, the shear surface structure of over-consolidated
(OC) soils can be affected by the dilation process when shearing. This
influence will exhibit differently at different shear displacement rates and may
affect the shear strength. Therefore, the rate dependency of both peak and residual
strength of OC soils may be different from that of NC soils. In this study, a number of ring shear tests were
conducted on both NC and OC kaolin clays under different shear displacement
rates to investigate the rate dependency of shear strength. The artificial overconsolidation
condition of clay samples was created in the ring shear box. The variation of
cohesion and friction angle under different shear displacement rates was also
clarified. In addition, the multi-stage procedure decreasing the normal stress
was applied to determine the residual strength of OC clays. The test results
indicated that similarly to NC clays, the positive rate effect on residual
strength also exhibits in OC clays, but with the less magnitude. The rate effect
on the peak strength of NC and OC clays is opposite. Additionally, the
variation in cohesion and friction angle at the peak and residual state of OC clays
under different shear rates is different. The multi-stage procedure reducing the
normal stress can be used to determine the residual strength of OC clays at
shear 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.

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1. Introduction

        The existence of over-consolidated (OC)
soils is popular in nature and large-scale landslides. It can be formed by the stress
release, the change of groundwater level, secondary consolidation (aging), chemical
bonding and desiccation stress (Hanzawa and Adachi,
1983). Many of OC clays are fissured, jointed, and contain slickensides
as well (Skempton, 1964). Thus, the shear
strength of OC clays has received much attention for evaluating and predicting
the slope stability (Skempton, 1964, 1970, 1985; Stark,
1995; Stark and Eid, 1997). The shear behaviour of OC and normally
consolidated (NC) clays are different. At peak state, the shear strength of OC
clays is often higher than that of NC clays, and the shear behaviour is more
brittle. At post-peak state, for NC clays, the strength loss is due to the
orientation of clay particles in a direction parallel to the shear surface. For
OC clays, the post-peak strength is decreased by an increase in water content due
to dilation and by the orientation of clay particles parallel to the shear surface
(Skempton, 1970). The constant minimum shear
strength in the post-peak state after a large shear displacement is defined as
the residual strength (Skemptom, 1964). In the
determination of residual strength by laboratory testing, the use of OC samples
is also suggested to reduce the vertical settlement and the soil leakage (Stark and Eid, 1993; Stark, 1995).

        In the geotechnical
literature, it has been noted that the residual shear strength is a fundamental
characteristic of soil. It is mainly affected by the clay mineral, the plasticity
characteristics, the size and shape of particle, the pore water chemistry, the effective
normal stress and the shear displacement rate (rate effect). It is almost
independent of stress history or overconsolidation ratio (OCR) (e.g., Skempton, 1964; Lupini et al., 1981; Vithana et
al., 2012; Li et al., 2017; Xu et al., 2018). The rate effect on the residual
shear strength of soils has been thoroughly investigated for many years. It has
been observed that the shear strength at the residual state may or may not
depend on the shearing speeds. In particular, Tika
et al. (1996) have indicated three types of rate effect on the residual
strength of soils: neutral, negative, and
positive. Previous investigations
indicated that the magnitude of the rate effect on residual strength may depend
on the effective normal stress (e.g., Carrubba and
Colonna, 2006; Kimura et al., 2013; Gratchev and Sassa, 2015); the clay
fraction, the plasticity index (Suzuki et al.,
2001, 2009); and the soil density (Li and
Aylin, 2013). However, most of the researches on the rate dependency of
residual strength base on the NC soils. Hong et al.
(2011) investigated the effect of cyclic loading on the residual
strength of over-consolidated silty clay. The test results presented that the
change of residual strength with shear displacement rates after a cyclic
loading at low OCRs was higher than that at a higher OCR. This indicates that
the shear zone structure under different OCRs affects the rate effect on
residual strength of soils. Vithana et al. (2012)
revealed that the shear surface structure of OC soils may be disturbed by the
dilation. Thus, although the OCR seems to have no effect on residual strength,
it may affect the magnitude of the rate dependency of residual strength because
the shear zone structure at different shear rates will influence the residual
strength.

        Besides the rate
dependency of residual strength, the rate effect on the peak strength of both
NC and OC clays was also examined. The rate dependency of the peak strength of
NC soils has been comprehensively investigated using triaxial test, direct
shear box test, and ring shear test. However, most of the researches on the
rate 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 peak
strength of OC soils in ring shearing needs to be investigated. In addition,
the relationship between OCRs and the rate dependency of shear strength needs
be clarified.

        In this study, a
conventional ring shear apparatus (Bishop-type) (Bishop
et al., 1971) was employed. A series of ring shear tests were performed
on reconstituted kaolin clay with different overconsolidation ratios (OCRs)
under different shear displacement rates. The main objectives of this study are
to examine the rate dependency of both peak and residual strength of OC clays and
their relationship with OCRs in ring shearing. The variation of cohesion and
friction angle at both peak and residual state of OC clays at different shear
rates was also clarified. In addition, the multi-stage procedure decreasing the
normal stress was also applied to determine the residual cohesion and friction
angle of OC clays. The residual strength from this procedure was compared with
the test result of single-stage procedure which was conducted on individual OC
samples.

 

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