Influence of shearing rate on the residual strength characteristic of 1 three landslides soils in loess area 2 3

19 In order to investigate the effect of the shearing rate on the residual shear strength of slip zone soils, a series ring shear tests 20 were carried out on slip zone soils from three landslides in loess area at the two shearing rates (0.1mm/min and 1 mm/min). 21 The slip zone soil specimens used in present study were from the northwest of China. Results indicated that the shear 22 displacement to achieve the residual stage for specimens with higher shearing rate is greater than that of the lower rate. 23 Relationship between the residual friction coefficients and normal stress shows that the residual friction coefficients for all 24 specimens under the lower normal stress were greater than that under the higher normal stress at two shearing rate. 25 Furthermore, the difference in the residual friction angle фr at the two shearing rates, фr (1)фr (0.1), under each normal 26 stress level were either positive or negative values, with the maximum absolute value of фr (1) фr (0.1) reach up to 2.218°. 27 However, the difference фr (1) фr (0.1) under all normal stresses was negative, which indicates that the residual shear 28 parameters reduced with the increasing of the shearing rate in loess area. 29 30 31 32


1． Introduction
Residual strength of soil is of great significance for evaluating the reactivating potential of the slope, in which consists of pre-existing sliding surface.Residual strength of a landslide soil is defined as the minimum constant value of strength along the slip plane, in which the soil particles are reoriented and subjected to sufficiently large displacements in relatively low shearing rate (Skempton, 1985) .
Numerical studies have been done to assess the residual strength through the laboratory tests using ring shear tests and reversal direct shear tests (Chen and Liu, 2013;Vithana et al., 2012).It is a generally accepted fact that the measurement of the residual strength is most preferred done with a ring shear test since it allows the soil specimen be sheared at unlimited displacement which can simulate the field conditions more accurately (Lupini et al., 1981;Tiwari and Marui, 2005;Bhat, 2013;Sassa et al., 2004).Until now, several relationships between the residual strength and soil index parameters have been reported in the literature with a wide range of soil by using various kinds of ring shear apparatus (Hoyos et al., 2014;Jiang et al., 2016;Kimura et al., 2015;Li et al., 2013;Skempton, 1964).Furthermore, many studies have shown that the shearing rate may or may not affect the minimum value of soil strength at residual states (Suzuki et al., 2007;Grelle and Guadagno, 2010;Gonghui et al., 2010;Bhat, 2013;Tika and Hutchinson, 1999;Lemos, 1985;Morgenstern and Hungr, 1984;Tika, 1999).
From the high shearing rate aspect in the geotechnical literatures, Morgenstern and Hungr (1984) carried out ring shear tests on two types of coarse sand in high velocity and found that the frictional behavior was not affected by either the velocity or the normal stress.However, there were many researchers asserted that the effect of shearing rate on the shear behavior of soil cannot be ignored.For example, Skempton (1985) , Tika and Hutchinson (1999) found that the faster shearing rate above 100 mm/min may bring about great qualitative changes in the residual behavior.Moreover, Tika et al. (1996) conducted fast ring shear tests on a wide range of natural soils and concluded that there are three types of rate effects on the residual strength, namely, a positive rate effect (the residual strength of soil at fast rate is higher than that of the slow rates), a neutral rate effect (the residual strength of soil is independent of the shearing rate) and a negative rate effect ( the residual strength of the soil at higher speed is lower than that of the lower speed).Recently, Gratchev Ivan and Sassa (2015) reported that the residual strength of the clay decrease with the shear rate increase from 0.2 to 5 mm/s.
On the other hand, in the slow shearing rate range, Skempton (1985) reported that variation in the value of the residual friction angle for shearing rates in a range of 0.05 to 0.35 mm/min was less than a 5% and concluded that the impact of shearing rate on the residual strength of clay is almost negligible within slow rate displacement.Similarly, Bhat (2013) concluded that there is hardly increase in residual strength of kaolin clay with the shearing rate ranging from 0.233 mm/min to 0.586 mm/min.Furthermore, Yokota et al. (1995) showed that residual strength is not affected by shearing rate lower than 1.01 mm/min in ring-shear tests.Except the above studies, other similar results were also found in clays that the residual strength is independent of the shearing rate (Chen and Liu, 2013;Tiwari and Marui, 2001).However, Suzuki et al. (2001) has reported the shearing rate ranging from 0.02 to 2.0 mm/ min significantly affected the residual strength of kaolin clay and mud stone.Moreover, Gonghui et al. (2010) also has reported that the residual shear strength of the weathered serpentinite is positively dependent on the shear rate in the slow rate.
On general, the effect of the shearing rate on the residual strength of the soil has not been sufficiently studied in high and slow shearing rate range.Furthermore, except for a few studies, researchers have not widely reported the impact of the shearing rate on the residual strength of loess soil in relatively lower shearing rate range from 0.1mm/min to 1 mm/min.However, it should be noted that the residual strength parameters obtained from using different shearing rate may be adopted to provide a guide for designing some precision engineering which require high accuracy of the design parameters, thus, the effect of the shearing rate on the residual strength of soils should be fully understood to determine the parameters with high reliability.In addition, residual strength of soil plays a key role in assessing the stability analysis and evaluating the reactivation potential of landslides which consists of pre-existing slip plane surface.Therefore, accurate determination of the residual strength parameters and their dependence on the shearing rate may affect the stability evaluation of landslides.Thus, it is necessary to study the residual strength variation of loess in rate of shearing in order to have a good understanding of the suitable approach for the residual strength measurement.
In this backdrop, the present study investigated the effect of the shearing rate on the residual strength of soil samples obtained from three landslides in loessic-developed areas at two different shearing rates (0.1mm/min and 1 mm/min) by using a ring shear apparatus.The main objective of this study was to examine the change in the residual strength parameters of loess at different shearing rates and their relationship with the normal stress in naturally drained ring shear tests.

Geological setting of landslide sites
Soil samples from three reactivated landslides in the northwest of China were selected for this study.Soil samples used for the ring shear tests and index measuring tests predominantly consist of loess deposits and were collected in a disturbed condition.For convenience, the names of landslide sites were abbreviated into Djg, Ydg, and Dbz. Figure 1 shows the study sites and some views of the landslides.

Dingjiagou landslide
The Djg landslide, located at the mouth of Dingjia Gully in Yan'an of China, is geologically composed of upper loess and lower sand shale in the Yan-chang formation.The dustpan-shaped landslide is inclined to the east, with its inclination 75.85 • .The landslide is 350 m in width, 180 m in length, 70 m in elevation.The average thickness of slip mass is around 20 m, and the volume of landslide totaled approximately 105 x 10 4 m 3 .The slip mass is mainly constituted by loess, whereas the sliding bed consists of sand shale in Yan-chang formation.The thickness of the sliding zone varied from 30 to 50 cm.The front lateral region of the main slide section of the Djg landslide, where the sampling was performed, was found to be silty clay.

Yandonggou landslide
The Ydg landslide is located in the Qiaogou town of Yan'an in Shaan xi province of China.The top and the toe altitude of the landslide are about 1165 m and 1110 m above the sea level, with the height difference between the toe and the top of landslide about 55 m.The slides have well-developed boundaries with the main sliding direction of 240 。 and slope angle of 30 。 .From the landslides profile, the sliding masses from top to bottom were classified by Q3 loess, Q2 loess and clay soil, respectively.Multiple landslide activities had occurred in this site, and the soil samples used in this study were collected from Q2 loess stratum within the slide ranged from 4.5 to 18 m in height.

Dabuzi landslide
The Dbz landslide is located in the middle part of Shaanxi province (about 108 • 51'36'' east longitude and 34 • 28'48'' north latitude), China, which is a semi-arid zone dominated by loessic geology.In this region, the investigated site is classified as a typical loess tableland with quaternary stratum.The sedimentary losses in this area are grey yellow, and the exposure stratum  The fact that the residual shear strength is independent of the stress history was reported by many researchers (Bishop et al., 1971;Vithana et al., 2012;Stark Timothy et al., 2005).Thus, disturbed loess samples from each of the three landslides weighing about 25 kg were collected from the slip surface soil of each slide and used to determine residual shear strength.
The soil samples were air-dried and then crushed with a mortar and pestle, and subsequently processing it through 0.5 mm sieve.Distilled water was added to the soil samples until desired density and water content were obtained.The physical parameters such as natural moisture content, specific gravity, bulk density, plastic limit, and liquid limit were determined in accordance with the Chinese National Standards (CNS) GB/T50123-1999 (standards for soil test methods) (SAC, 1999), but clay size was defined to be less than 2um followed ASTM, D422 (ASTM, 2007).Each soil sample was separated into clay (sub 0.002 mm), silt (0.002-0.075 mm), and sand (0.075-0.5 mm) fractions.The physical indexes of the soil are listed in Table 1.
The grain size distribution of soil was measured using a laser particle size analyzer Bettersize 2000 (Dandong Bettersize Instruments Corporation, Dandong, China).The sieved soil samples were used to determine particle size distribution.In this study, soil samples were treated with sodium hexaphosphate, serving as a dispersant, to disaggregate the bond between the particles.The results show that the clay fraction in Djg landslide soil (24%) is more than two times than that from Ydg (9%) and Dbz (9.1%).Furthermore, the particle size analyses illustrates that the percentage of silt-sized soil in three landslides ranged from 75.66% to 87.4%.In addition, Ydg landslide soil consists of the greatest percentage of the sand fraction which reaches up to 10.55%.
In present study, a total of twenty four specimens were tested at two shearing rate (0.1mm/min and 1 mm/min) and under normal stresses ranged from 100kN/m 2 to 400kN/m 2 in a ring shear apparatus.Notes: ρd= dry density; w=moisture water content; ρ= bulk density; GS = specific gravity; WL=liquid limit; Wp= plastic limit

Testing apparatus
The advantage of a ring shear test apparatus to measure residual shear strength including its ability to allow unidirectional shearing of a soil specimen (Bishop et al., 1971;Tika, 1999;Suzuki et al., 2007;Bromhead, 1979).Thus, a ring shear apparatus was used in this study.
An advanced ring shearing apparatus (SRS-150) manufactured by GCTS (Arizona, USA) was adopted in ring shear tests and the photos of apparatus were shown in Fig. 2, which consists mainly of a shear box with an outer diameter of 150 mm, an inter diameter of 100 mm and the maximal sample height of 250 mm.The shearing box consists of the upper shearing box and the lower shearing box.In the shearing process, the upper shearing box keeps still while the lower one rotates.The apparatus, which provides effective specimen area of 98 cm 2 , is capable of shearing the specimen for large displacement in single direction.The annular specimen is confined by inside and outside metal rings.Moreover, the specimen is confined by bottom annular porous plates and top annular porous plates in which have sharp-edged radial metal fins which protrude vertically into the top and bottom of the specimen at the shearing process.The normal stress, shearing strength and shearing displacement can be monitored while shearing by computer.The measurement features of the ring shear apparatus employed in this study are described as follows: shearing rate range from 0.001 to 360 degrees per minute, 10 kN axial load capacity, 300 N.m continuous torque capacity, maximum normal stress of 1000 kN/m 2 .

Testing procedure
In present study, reconstituted samples of the sub 0.5 mm soil fraction were used in the testing as it was reported that the residual strength of the soil was unaffected by its initial structure (Bishop et al., 1971;Vithana et al., 2012).Specimens were first prepared by adding distilled water to the air-dried soil until the saturated moisture contents of the three landslide soils were obtained.Then, specimens were kept in a sealed container for at least one week to fully hydrate.Specimens are then reconstituted in the ring-shaped chamber of the apparatus by compaction.In order to make the sample uniform while packing, the sample was placed in three layers, and each layer was tamped under a vertical stress which is lower than the given normal stress to achieve the design height.The final height of the specimen in the ring shear apparatus after tamp varied but was typically about 20 mm (to achieve a specific bulk density).The specimen was then consolidated under a specific effective normal stress in a range of 100kN/m 2 to 400kN/m 2 until required consolidation was achieved.Then, the consolidated specimen is subjected to shearing under constant normal stress by rotating the lower half of the shear box attached to a gear, while the upper half remains still.In ring shear tests, the normal stress at the shearing was the same as at consolidation stage.
In this study, ring shear tests were performed in a single stage under drained condition and the samples were subjected to shear until the residual state was achieved.Drain condition of the shearing process is provided by two porous stones attached on the top and the bottom platen of the specimen container.As for soil specimens with low permeability, the rate of excess pore pressure generation in the shear box may exceeded that of pore-pressure dissipation, this type of condition is identified as naturally drained condition in previous studies (Okada et al., 2004).Furthermore, Tiwari (2000) asserted that it was acceptable to use a shearing rate below 1.1 mm/min to simulate the field naturally drained condition.Thus, shearing rates of 0.1 mm/min and 1 mm/min were used in this study to simulate the naturally drained condition of the slip zone soils.

Results and discussions
Twenty four specimens were tested to investigate the shear characteristics of the slip-zone soils in the ring shear apparatus.Tests results are shown in this section.

Shear behavior
Figures 3a, 4a and 5a show the typical shear characteristics of the slip-zone soils (shearing rate 0.1 mm/min and 1 mm/min) obtained from three different locations, where, the shear stress is plotted against the shear displacement at the normal stress ranged from 100kN/m 2 to 400kN/m 2 .It is a widely accepted fact that normal stress has effect on the shear behavior of the soil (Kimura et al., 2015;Stark Timothy et al., 2005;Stark et al., 2005;Eid, 2014), thus, the shear behavior of samples at the peak and residual stages, where, the determined peak friction coefficient as well as residual friction coefficient are plotted in Figure 3b, 4b, and 5b against the corresponding effective normal stresses.The friction coefficient is defined as the shear stress divided by the effective normal stress.
Figures 3a, 4a and 5a demonstrate that shear stress increases dramatically within small shear displacement and then reduces with shearing displacement, until residual conditions were achieved at large displacements.Furthermore, it is clear that peak strength as well as residual strength of the samples with high shearing rate is almost smaller than that of the samples with low rate.In Figures 3a, 4a and 5a, a clear drop can be seen, at any normal stress, for specimens obtained from all sites.It is obvious that Djg specimens showed greater peak-post drop than that of Ydg and Dbz specimens.According to the conclusion that the residual stage is attained if a constant shear stress is measured for more than half an hour (Bromhead, 1992), it can be seen that the shear displacement to achieve the residual stage for specimens with higher shearing rate is greater than that of the lower rate.For example, the minimum shear displacements for attaining residual condition for Djg specimens with low and high shearing rate were about 360mm and 650mm, respectively.Under the shearing rate of However, Dbz specimens require about 40mm and 60mm displacement to reach residual condition for low and high shearing rate, respectively.

Effect of normal stress on the friction coefficients
It can be seen from the Figures 3b, 4b and 5b that the friction coefficients (peak and residual) are higher at lower effective normal stress levels.For example, the peak and residual friction coefficient of Djg landslide soils at the shearing rate of 0.1mm/min reduced from 0.569 to 0.32 and from 0.3 to 0.262, respectively.Similarly, results obtained from other two landslides soils also showed that the friction coefficients decrease nonlinearly with the normal stresses.Furthermore, specimens with shearing rate of 0.1mm/min attained greater friction coefficients than that with shearing rate of 1mm/min.
In order to get an insight into the effects of the normal stress on the slip zone shear strength, the shear behavior of the soil sheared at the normal stress of 100kN/m 2 and 400kN/m 2 were selected for analysis.At the normal stress of 100kN/m 2 , Djg samples showed about 47.3% and 36.8%decrease in the friction coefficient from the peak friction coefficient at the shearing rate of 0.1 and 1 mm/min, respectively, which is greater than in the Ydg (about 9.8% and 10.3%) and Dbz (about 2.4% and 3.2%) samples.In Figures 3b, 4b and 5b, on average, it is obvious that the decrease of the friction coefficient from the peak strength to the residual strength in the Djg sample was almost 18.1% and 21.3% for the sample consolidated at normal stress of 400kN/m 2 under the shearing rate of 0.1mm/min and 1mm/min (Figure 3b), While the friction coefficient reduction in Ydg sample with low and high shearing rate were only about 4.1% and 4.8% (Figure 4b).And the friction coefficient reduction in Dbz samples with low and high rate were only approximately 5.6% and 6.0% (Figure 5b) from the peak strength, respectively.Based on the conclusion that the post-peak drop in strength of soil is only due to particle reorientation after the peak strength (Mesri and Shahien, 2003;Skepmton, 1964), the results in this study demonstrated that the Djg landslide soil existed the greater particle reorientation compared with that of other two landslide soils.

Effects of shearing rate on residual strength parameter
For the representative samples described above, Figures 6, 7 and 8 show the relationships between the residual friction coefficient and the normal stress, and the residual strength parameters.The residual friction coefficient is plotted against the normal stress.The residual friction coefficient is defined as the residual shear strength divided by normal stress.It is widely recognized that the shear strength parameters including cohesion and friction angle (Terzaghi, 1951;Stark et al., 2005).
However, according to the previous studies, the residual angle of soils varies depended on the soil properties as well as the magnitude of normal stress provided the residual cohesion of soil is zero (Skepmton, 1964;Bishop, 1971;Kimura et al., 2014).
Thus, in this study, the residual frictions are calculated by Coulomb's law assumed the residual cohesion is zero.The residual strength parameters were defined as фr (0.1) and фr (1) at the low shearing rate and high shearing rate, respectively.
In case of Ydg soil sample, there was insignificant reduction in residual friction coefficients with the increasing of shearing rate for all normal stresses.found to be 15.003 °and 14.09 °, respectively.However, the residual friction angles фr (0.

Influence of the shearing rate on the residual friction angles according to soil properties
Figure 9 depicts the relationships between residual friction angles as well as the difference in the residual friction angles and soil properties including liquid limit (LL), plasticity index (Ip) and clay fraction (CF) at two shearing rates.
The residual friction angles at two shearing rates decreased nonlinearly with the increasing of the LL.As for the relationship between the фr and Ip, the фr under the low and high shearing rates decreases from about 32 °to 15 °with increasing the plasticity index from 11 to 16.With increasing of CF from 9% to 24%, the residual friction angles under low and high shearing rates were found to decrease.Interestingly, for Dbz and Ydg soils of which have similar percentage of clay fraction, the residual friction angles at both shearing rates varied.However, in the relationships between the difference in the residual friction angles and the soil properties, no clear correlations were found.
Nat. Hazards Earth Syst.Sci.Discuss., https://doi.org/10.5194/nhess-2018-270Manuscript under review for journal Nat.Hazards Earth Syst.Sci. Discussion started: 25 September 2018 c Author(s) 2018.CC BY 4.0 License.in this area has been divided into two stratigraphic units, namely, the upper late Pleistocene (Q3) loess and the lower mid-Pleistocene (Q2) loess, of which the Q3 loess is younger.The Q3 loess is closest to the surface and is up to approximately 12 m thick, while the thickness of Q2 loess may reach an upper limit of about 50 m(Leng et al., 2018).The loess in this area have well-developed vertical joints(Sun et al., 2009).The travel distance and the maximum width of the slip mass are roughly estimated to be 121.55m and 133.46 m, respectively.The armchair-shaped landslide shows an apparent sliding plane, with an area of approximately 15,660 m 2 and about 66.25 m maximum difference in elevation.The main direction of this landslide is approximately 355 • .The exposed slip zone in the side scarp of the landslide, where the sampling was done, was found to be entirely in the Q2 loess stratum of the Dbz landslide site.The thickness of narrow-band slip zone loess is less than 1.0 cm, inclined at around 65 • to the horizontal direction.Since the band of slip zone is thin, mix soils which consist of the slip zone soils, the very thin upper and lower parts of the loess are mingled together and served as the representative samples of the slip zone loess in this site.

Figure 6 .
Figure 6.Relationships between residual shear stress and normal stress, and residual strength parameter for Djg soil sample

Figure 8 .
Figure 8. Relationships between residual shear stress and normal stress, and residual strength parameter for Dbz soil sample

Figure 9 .
Figure 9. Relationships between residual shear parameter, the difference in residual shear parameter and the soil properties at two shearing rates
summarizes residual strength parameters including фr (0.1) and фr (1) of all specimens obtained from the ring shear tests in this study.As for the Djg samples, the residual strength parameter фr(0.1) and фr(1) for all normal stress were