11 Most slow-moving landslides in the Three Gorges Reservoir (TGR) region of China are characterized by pre-existing shear surfaces. The large deformation within the shear zones usually gives rise to clastic soil formation. The creep properties have large in uence on the kinematic feature of landslides. In this paper, we report an in-situ direct shear creep test carried out in the shear zone of a reactivated slow-moving landslide in the TGR region. Correspond- ingly, some laboratory ring shear creep tests are carried out to interpret the movement pattern of this landslide. The shear zone soil exhibits similar non- attenuating creep responses in both the in-situ direct shear and laboratory ring shear creep tests. At the same stress level, however, the in-situ direct shear creep test yields a larger rate of creep displacement due to shearing along the landslide direction. In the ring shear creep tests, at the prepeak stage, the critical creep stress that triggers creep failure is slightly lower than the peak shear strength but much larger than the residual strength; at the postfailure stage, the critical creep stress of the shear-zone soil is equal to the residual shear strength. The rate-dependent residual shear strength may account for the stepwise movement pattern of the landslide. Keywords: Huangtupo, in-situ creep test, Clastic soil, Slow-moving 12 landslide, Ring shear test 13 Corresponding author. E-mail: wangjinge@cug.edu.cn, wei.wu@boku.ac.at 1. Introduction 14 Deep-seated and slow-moving landslides often involve large-scale mass 15 movements with relatively small displacements (P anek et al., 2011, 2016). 16 They are, however, often known to be the cause of severe damage to struc- 17 tures and infrastructure (Cascini et al., 2009; Di Maio et al., 2013; Fern andez- 18 Merodo et al., 2014; Li et al., 2019, among others:). According to reported 19 statistics, there are some 1,726 landslides, with a total volume of 1.3339 20 1011m3in the TGR region in China (He et al., 2008), and most of them 21 can be classi ed as slow or very slow-moving landslides. For instance, Miao 22 et al. (2014) examined the movement rate of 21 landslides in the TGR area 23 and found out that 6 landslides move at rates ranging from 6 cm per year 24 to 1.5 m per year, while the other 15 display creeping displacement with an 25 average sliding rate of less than 6 cm per year. These characteristics usu- 26 ally make slow-moving landslides dicult to detect, and as a consequence, 27 many roads, buildings, and tourist resorts were constructed in areas prone 28 to slow-moving landslides in the TGR region. 29 China Three Gorges Reservior AreaChongqingYichangThree GorgesBeijing Huangtupo Landsldie N Yangtze River Figure 1: The TGR region and location of the Huangtupo landslide in China, base map from Wang et al. (2018a) A typical case is the Huangtupo landslide in Badong District, a small 30 county along the Yangtze River in China. This area is well known for its 31 landslide-prone geological conditions and large-scale landslides that have led 32 to the relocation of thousands of residents (see Figure 1). The Badong County 33 seat has experienced relocation twice in 3 decades. The old county seat was 34 2 moved to the Huangtupo area due to the rising water level following the 35 impoundments of the Gezhou Dam in 1982. After the relocation, however, 36 the Huangtupo area was identi ed as a large-scale landslide in 1992 (Deng 37 et al., 2000). The slow movement of this landslide has destroyed construc- 38 tion and roads on this slope. As a consequence, the town was moved to an 39 area approximately 10 km west of Huangtupo after 2008. Though the dis- 40 placement rate of the sliding mass is not large enough to cause a sudden slide 41 into the adjacent Yangtze river, the continuous creep displacement has posed 42 a great threat to the safety of surrounding residents and the Yangtze river 43 waterway. The large size of Huangtupo landslide makes it an ideal study 44 object to better understand the mechanisms of similar slides in this area. In 45 2012 the Badong eld test site was established by the Three Gorges Research 46 Center for Geohazards (TGRC). The test site includes a large-scale tunnel 47 with auxiliary adits and an extensive in-situ monitoring system. 48 Since the completion of the Badong eld test site, research on the internal 49 structure and kinematic feature of the Huangtupo landslide has been intensi- 50 ed. Through U-Th dating, Tang et al. (2015a) reported that the Huangtupo 51 riverside sliding masses might undergo at least two periods of movement. 52 Subsequently, Wang et al. (2018a) proposed a double shear surface model for 53 the Huangtupo No. 1 Riverside sliding mass based on the geological logs ob- 54 tained from the excavation of the tunnel group and subsequent borehole logs. 55 On the other hand, Liu et al. (2013) and Tom as et al. (2014) analyzed the de- 56 formation of the Huangtupo landslide based on InSAR data. The upper part 57 of the landslide is a ected by seasonal displacements caused by rainfall, while 58 the lower part is a ected by periodic water level uctuation in the reservoir. 59 Furthermore, Wang et al. (2014) proposed a concept model for analyzing the 60 relation between the episodic landslide movement and the variation of the 61 hydrological conditions of the landslide. Tang et al. (2015b) investigated the 62 movement pattern of the Huangtupo landslide during groundwater variation 63 and seasonal rainfall. Wang et al. (2016) compared the super cial and deep 64 deformation of the Huangtupo landslide and identi ed that the water level 65 variation and seasonal rainfall have di erent e ects on the movement of the 66 Huangtupo landslide. The aforementioned studies give rise to an improved 67 understanding about the deformation mechanisms: the Huangtupo landslide 68 moves stepwise with episodic accelerating and decelerating phases. How- 69 ever, in uenced by hydrological boundary conditions, such as rainfall and 70 the groundwater table, the creep deformation is driven predominately by the 71 weight of the large sliding mass. 72 3 It is well known that the displacement rate of a landslide is the result 73 of a dynamic equilibrium between destabilizing and stabilizing forces, which 74 also depends on the viscous behavior of the involved soil (Van Asch, 1984; 75 Di Maio et al., 2013; Tan et al., 2018). Hence, in addition to the hydro- 76 logical conditions, it is believed that the creep property of the shear-zone 77 soil plays an important role in the interpretation of the mechanism behind 78 the movement pattern of the landslide. To date, there has been insucient 79 research on the creep properties of the shear-zone soil. Therefore, a series of 80 laboratory creep tests over various stress ranges have been performed by the 81 colleagues at TGRC. Li et al. (2017) reported the creep properties of intact 82 shear-zone soil in the triaxial stress state. Later, Wang et al. (2018b) investi- 83 gated the residual state creep properties of undisturbed shear-zone soil using 84 a ring shear apparatus. In most cases, however, there still remain diculties 85 in the laboratory tests for dealing with the coarse particles in the shear-zone 86 soil. In-situ tests carried out in the shear zone of giant landslides are rare be- 87 cause such tests are rather time-consuming and the access to the soil at large 88 depth is dicult. Fortunately, the excavation of the tunnels in the Badong 89 eld test site exposed several shear zones, which provide ideal conditions for 90 in-situ tests, e.g., the in-situ triaxial compression test(Tan et al., 2018). For 91 further investigation of the viscous behavior of the shear-zone soil subjected 92 to various stress paths, more creep tests both in the eld and laboratory are 93 required. 94 In this paper, we report an in-situ creep test performed at the Huangtupo 95 landslide to investigate the creep properties of shear-zone soil by means of 96 direct shear creep tests. Particularly, the direct shear creep test is performed 97 in the shear zone of this landslide with the shearing direction parallel to 98 the inclined shear surface. The viscous behavior of the shear-zone soil is 99 examined over various shear stress ranges. Additionally, two ring shear creep 100 tests, namely a prepeak ring shear creep test and a postfailure ring shear 101 creep test, are carried out in the laboratory with the same soil. The test 102 results are compared with those obtained by triaxial creep tests reported 103 in the literature. Based on the test results, the factors that in uence the 104 measurement of the critical creep stress and the relation between the critical 105 creep stress and landslide movement are discussed. 106 4 2. Description of the landslide and in-situ creep test site 107 This section is concerned with describing the position and creep move- 108 ment of shear zone where the in-situ creep test was carried out. In addition, 109 the environment of the in-situ test site is introduced. 110 2.1. Shear surfaces distribution 111 The Huangtupo landslide developed at the south bank of the valley of 112 the Yangtze River with the bedrock dipping towards the valley. The crown 113 elevation of the landslide is approximately 600 m.a.s.l., while its toe varies 114 from 50 to 90 m.a.s.l. The toe is submerged in the Yangtze River, with 115 water levels varying from 145 to 175 m, as regulated by the Three Gorges 116 Dam. The composite landslide covers a total area of 1.35106m2, with a 117 mobilized volume of nearly 6.93107m3, according to the investigation in 118 2001; therefore, it is one of the largest reservoir landslides in the TGR region. 119 More details concerning the geological structure, lithology, and hydrological 120 conditions of this landslide can be found in the literature (Deng et al., 2000; 121 Wang et al., 2018b, 2016; Tang et al., 2015a,b, 2019). 122 Yangtze RiverHZK5Erdaogou JC1 JC2 JC3JC5 JC6 JC7JC8 JC9 JC10Sidaogou Rivines Investigation tunnel Section line Investigation borehole Inclinometer borehole Visible boundary of slding mass Buried boundary of slding massInvestigation tunnelShear surfaces of Huangtupo landsldie NSandaogou No.2 Riverside Sliding mass No.1 Riverside Sliding massGarden Spot landslide Substation landslide 175 m water level Figure 2: The plan view of the Huangtupo landslide with the 3D model of the shear surfaces based on the data from the 2001 investigation 5 According to the previous investigation in 2001, the Huangtupo landslide 123 is a complex mass formed by multiple sliding masses that occurred over a 124 period of at least 100,000 years (Tang et al. ,2015a). The plan view of the 125 Huangtupo landslide is shown in Figure 2. On the surface, the landslide is 126 divided by the Sandaogou Valley into two groups of sliding masses. Each 127 group is composed of two sliding masses, one on top of the other, with addi- 128 tional recent sliding masses developing adjacent to the edges of the landslide. 129 The foreside sliding masses, the No. 1 and No. 2 Riverside sliding masses, 130 are adjacent to the Yangtze River with their toes being submerged in the 131 river. 132 Inclinometer profile of HZK5 in 2008 HZK5 BT3 Distance(m)Elevation(m) A A’0 10 20 30 40 500 20 40 60 8011. 2007 12. 2007 01. 2008 02. 2008 03. 2008Displacement (mm)Depth (m) 145175 Slickenside in the shear zone 0 100 200 300 400 500 600 700 800 900 400 300 200 100 0 400 300 200 100 0 Figure 3: Section A-A' in No. 1 Riverside sliding mass: ¬limestone; ­pelitic limestone; ®dense soil and rock debris; ¯loose soil and rock debris; °sliding zones with possible slip surfaces; ±Mudstone fraction zone, and the inclinometer pro le of borehole HZK5, inclinometer data from Jian and Yang (2013). The spatial distribution among the shear surfaces shown in Figure 2in- 133 dicates that the four sliding masses are sliding along separate shear surfaces. 134 The geological pro le of the No. 1 Riverside sliding mass shown in Figure 3 135 indicates that it is a chaotic mass consisting of either loose or dense soil and 136 rock debris (Wang et al., 2014). The inclinometer pro le of borehole HZK5 137 shows that signi cant horizontal displacement has occurred at depth of 54 138 m and 76 m. This implies a double shear-surface structure of this landslide. 139 6 To investigate the shear surface distribution in this landslide, a investi- 140 gation tunnel was constructed crossing the No. 1 Riverside sliding mass. As 141 shown in Figure 4(a), the investigation tunnel in the Badong eld test site 142 includes a main tunnel and ve branch tunnels. The arc-shaped main tunnel 143 is 908 m long. Five branch tunnels named BT1{BT5 are connected to the 144 main tunnel. The excavation of the tunnel group exposed weak interlayers 145 and shear zones located at both the main tunnel and the branch tunnels BT3 146 and BT5. More details can be found in the literature (Wang et al., 2018a; 147 Tang et al., 2015a) 148 JC8 JC1 JC7JC5 JC6 JC10JC9 JC3JC2HZK5C303C501C006C010 C012 C302C3010 50 150 300 m (b) (c)(a) Main tunnelBT3, creep test site BT5 No.1- 2 sliding mass No.1- 1 sliding mass N NMain tunnelN 0200 100 100200300400500200 300 400 500 600 700300 800 900600 No.1-1 sliding mass No.1-2 sliding mass Yangtze River 200 200 150 150250 250300350 300 JC5 JC6JC8 JC9 JC10 JC7JC3JC2JC1 Figure 4: The Huangtupo No. 1 Riverside sliding mass (a) plan view (b) 3D topographic model. Modi ed after Tang et al. (2015a) and (c) 3D view of the double shear-surface model, after Wang et al. (2018a) According to the U-th dating results by Tang et al. (2015a), the ages 149 of the shear-zone soil in BT3 and BT5 are 40 ka and 100 ka, respectively. 150 Thus, the shear zones in BT3 and BT5 are not the same layer; therefore, 151 the Huangtupo No. 1 Riverside sliding mass can be further divided into 152 secondary sliding masses (No. 1-1 sliding mass and No. 1-2 sliding mass). 153 Based on this knowledge, Wang et al. (2018a) proposed a double shear surface 154 model of the Huangtupo No. 1 Riverside sliding mass using the geological 155 data obtained from the excavation of the tunnels and the recent investigation 156 boreholes. The 3D topographic model, and 3D view of this double shear 157 surface model are shown in Figure 4(b,c). From this model, it is easy to 158 7 show the location where the in-situ creep test was carried out. 159 2.2. Movement of the shear zone 160 In-situ GPS monitoring indicates that the No. 1 Riverside sliding mass 161 exhibits steady creeping, with a movement rate range from very slow (17.2 162 mm/year) to extremely slow (12.8 mm/year) in the direction of 20northeast 163 (Tom as et al., 2014; Tang et al., 2015a). Since the completion of the tunnel 164 group in December 2012, this movement has resulted in many cracks in the 165 lining and on the pavement of the tunnel. Several extensometers have been, 166 therefore, installed in the tunnel to measure the propagation of these cracks 167 since 2013. The distribution of the extensometers is shown in Figures 6. The 168 extensometers were installed in the tunnel in locations where the shear zones 169 were exposed and vertical to the cracks to obtain the width variation. Hence, 170 the development of cracks provides direct insight into the movement pattern 171 of the Huangtupo landslide. 172 28/07/2013 28/03/2014 28/11/2014 28/07/2015 28/03/2016 28/11/20160246810121416 Rainfall and water levelCumulative displacement (mm)C301 120 m C302 125 m C303 130 m C306 138 m C006 715 m C010 745 m C012 760 m C501 5 m 020406080100120140160180 water level (m) rainfall (mm) Figure 5: Crack development in the tunnel group together with variation in TGR water level and rainfall Figure 5 shows the development of partial cracks in the tunnel group from 173 August 2013 to December 2016, together with variation in the groundwater 174 table and seasonal rainfall. The monitoring results of eight cracks, including 175 3 cracks in the main tunnel (C006, C010, and C012), 4 cracks in BT3 (C301, 176 C302, C303, and C306), and 1 crack in BT5 (C501), are presented. During 177 the monitoring period, the crack C501 obtained a deformation of 15 mm 178 in the last four years, yielding the largest rate of displacement among these 179 cracks. The other cracks attained relatively smaller displacement. The cracks 180 C301, C302, C303 and C306 grew 1.65, 3.16, 5.84 and 3.60 mm, respectively, 181 8 while C006, C010, and C012 grew 4.37, 2.47, and 5.80 mm, respectively. The 182 development of the cracks indicates that the movement rate of the shear zone 183 in the No. 1-2 sliding mass is larger than that in the No. 1-1 sliding mass. 184 On the other hand, the groundwater table and seasonal rainfall can in u- 185 ence the development of the cracks. The propagation of the cracks coincides 186 with the periods of intensive rainfall and signi cant declines in the TGR 187 water level. The development of the cracks shows intermittent phases of 188 acceleration and deceleration in the apparent dip of the shear surface and 189 exhibits a stepwise creep tendency (Wang et al., 2018b). This result is in ex- 190 cellent agreement with the super cial monitoring data measured from GPS 191 and InSAR (Liu et al., 2013; Tom as et al., 2014; Tang et al., 2015b; Wang 192 et al., 2016), that is, the landslide has experienced alternating accelerat- 193 ing and decelerating movements (Wang et al., 2018b). The main conclusion 194 drawn from this analysis is that the creeping movement of the Huangtupo 195 landslide is predominantly governed by its self-weight. A ected by some ex- 196 ternal factors, the soil within the shear zone is undergoing a stepwise creep 197 displacement. This movement pattern leads to the necessity of performing 198 in-situ creep tests to investigate the viscous behavior of the shear zone ma- 199 terial. 200 2.3. In-situ creep test site 201 After the construction of the investigation tunnel, two testing tunnels 202 were excavated and advanced in the BT3 and BT5 at the 138-m and 20.1-m 203 sections, respectively, for further in-situ tests and intact soil sampling. The 204 creep tests were supposed to be carried out in the testing tunnel either in 205 BT3 or BT5; see Figure 6(a). 206 The shear zone in BT3 is usually 0.5- to 1.2-m thick, which is much 207 thicker than that in BT5. Moreover, the hydrological condition in BT3 is 208 more suitable for the in-situ test, while groundwater has passed through the 209 shear zone in BT5, and water bursting has caused severe collapse on the 210 tunnel face. Therefore, the testing tunnel in BT3 was selected as the in-situ 211 creep test site, as shown in Figure 6(b). Several samples were prepared in the 212 east direction of the testing tunnel in BT3. To this end, the left side of the 213 testing tunnel was excavated, leaving a platform with shear-zone soil above 214 the shear surface. The shear-zone soil was then prepared to block samples 215 for the in-situ creep tests. To mimic the landslide movement as realistically 216 as possible, the in-situ creep shearing was supposed to be carried out parallel 217 to the shear surface, as shown in Figure 6(c). 218 9 (a) (b) (c)BT3testing tunneltesing area tesing tunnel in BT3 tesing tunnel in BT5BT5 Main tunnel k0+000 +100 +200 +300BT1 BT2 BT4 BT5BT3 +400 +500 +600 +700 +800 k0+908.0Slide mass Bed rock In situ creep test 300 cm300 cmR = 150 cmBT3 C306 C303C302 C3301 C501C010C012 C006Extensometer exposed shear surfaceFigure 6: (a) The tunnels in the Badong eld test site, (b) creep test area in the testing tunnel in BT3, and (c) the direct shear device xed on the shear surface τσn (a)σ3 σ1σ3(d) σ3 σ3σ1 (b)(a) block sample for the in situ direct shear creep test(b) block sample for the in situ triaxial compression creep test(c) ring-shaped sample for the ring-shear creep test(d) cylinder sample for the triaxial compression creep test Note: no intention to show the exact dimension Shear surface bedrock soil matrix gravelσnτ(c) Gravel Soil Matrix Crack Shearing direction Figure 7: The in-situ test site in the testing tunnel: the relative location of (a) in-situ direct shear creep test, and (b) in-situ triaxial creep test; the sampling position of the (c) ring shear sample, and (d) laboratory triaxial creep sample It is worth noting that, based on this testing platform, the TGRC has 219 proposed a comprehensive test plan including both in-situ and laboratory 220 creep tests using various testing approaches, e.g., in-situ and laboratory tri- 221 axial compression creep tests and in-situ and laboratory shear (direct shear 222 and ring shear) creep tests. The in-situ test site for sampling and in-situ tests 223 are shown in Figure 7. The laboratory and in-situ triaxial compression creep 224 tests were reported by Li et al. (2017) and Tan et al. (2018), respectively. In 225 10 the following, therefore, we report the experimental works of the in-situ and 226 laboratory shear creep tests. 227 3. Experimental works 228 3.1. Test material 229 Some intact samples were taken from the shear zone exposed in the testing 230 tunnel. Laboratory tests were then carried out to determine their basic 231 physical properties. As presented in Figure 8(a), the shear-zone soil typically 232 comprises ne-grained soil (silt + clay) with a signi cant fraction of coarse- 233 grained particles, constituting approximately 20 to 50 wt.% of the material. 234 Figure 8(b) shows an X-ray CT (computed tomography) section of an intact 235 sample, where the outline of the gravel clasts are clearly distinguished from 236 the ne grained portion. Visual observations in the testing tunnel (see Figure 237 7) and from the CT image reveal that the gravel is typically subrounded to 238 angular and irregularly shape, with diameters varying from 0.5 to 2.5 cm. 239 Previous experimental studies evidenced that the interaction of particles in 240 coarse-graded soil, with 25 and 50 wt.% coarse particles, remains complex 241 (Jiang et al., 2016; Xu et al., 2007,2008), and thus, the viscous behavior 242 can be signi cantly in uenced. For example, the creep threshold stress and 243 the creep displacement leading to accelerated failure of shear-zone soil in the 244 residual state increases with coarse particle content (Wen and Jiang, 2017). 245 CT Image (cm)(b) 0.001 0.01 0.1 1 10 100020406080100 d=0.005 mmd=0.075 mmCumulative percentage (%) Grain size (mm)d=2 mm(a) 0 50 100 150 200020406080100120 MLPlasticity index: Ip=WL-WP Liquid limit: WL(%)A line: Ip=0.73(WL-20) B line: WL=50 MLOCL CLO CH CHO MHO MH(c) 0 10 30 20 40 Figure 8: Indices of physical properties of soil samples from Huangtupo landslide: (a) grain size distribution; (b) CT image of the shear-zone soil, and (c) plasticity of the ner soil On the other hand, the shear-zone soil contains between 50 and 80 wt.% of 246 ne material. It can be classi ed as silt-clay with low to medium plasticity 247 11 and silt-clay mixtures of low plasticity according to the Casagrande chart 248 shown in Figure 8(c). It has been reported that there exists a relationship 249 between the potential of creep in clays and the plasticity index; the coecient 250 of secondary compression C can be described by I pusing a linear function 251 (Mesri et al., 1973; Nakase et al., 1989; Jin et al., 2019). This implies that 252 the shear-zone soil may exhibit high creep potential under certain loading 253 conditions. 254 3.2. in-situ creep apparatus and instrumentation 255 A reliable shearing device is required for the long-term in-situ creep test 256 in the eld site. For this purpose, a direct shear apparatus was designed by 257 the TGRC. It was made of steel for reliability and comprises a reaction frame, 258 two servo-controlled, oil-actuated hydraulic jacks, a circulation cooling device 259 and the shear box. The shear box consists of two parts, i.e., an upper and a 260 lower box. The upper box has the dimension 0 :50:50:25 m, while the 261 low box was 0 :50:50:10 m. The hydraulic jacks can produce a maximal 262 shear force of 300 kN and a normal force of 500 kN. Both hydraulic jacks 263 have a 100-mm stroke. This implies that the maximal shear and normal 264 displacement is 100 mm. The physical dimensions and the arrangement of 265 the shear box, transducer valves, reaction frame and the loading cylinders 266 are shown in Figure 9. 267 Vertical force Horizontal force Oil-actuated 17.5 cm bore × 15 cm stroke loading cylinder I Ball bearing plate Shear plane Vertical displacement transducer Horizontal displacement transducerReaction support Reaction wall Loading cylinder IIHandle BedframeFhFv Intact sample (upper part) Intact sample (lower part) Figure 9: Con guration of large direct shear creep box 12 The in-situ creep test involved the assembly of the shear box on the shear 268 surface and shearing at controlled forces. To mimic the shear movement of 269 the landslide as realistically as possible, a block of sample, with near-vertical 270 sides to the shear surface, was prepared according to dimensions of 0.5 m  271 0.5 m0.35 m on the inclined shear surface. Then, the exposed interface was 272 atly grouted before the bedframe was xed. The bedframe had a height of 273 0.10 m and was used as the lower shear box, which accommodated the lower 274 part of the soil sample. The upper shear box was centered over the upper part 275 of the block sample and stabilized by two adjustable support legs connected 276 to the bedframe. The gap between the box and the block was lled with 277 wet coarse sand. Afterward, the loading cylinders, transducers, and data 278 acquisition system were instrumented. 279 The application of normal and shear forces were of crucial importance 280 for the creep test. To apply the normal and shear forces, two reaction walls 281 in the shear direction and the normal direction were built on the top of the 282 inclined shear surface and the left side of the tunnel lining, respectively (see 283 Figure 6). The shear force was applied directly to the upper box, at an 284 o set to the plane of shear, to facilitate the alignment and operation of the 285 device at the test site. The normal load was applied through a rigid top 286 plate, supported by the tunnel lining, to impose a normal stress on the block 287 sample. The plate is independent of the upper box and, therefore, free to 288 move in the shearing direction. During testing, the shear and normal forces 289 were maintained through feedback in the servo-control system; the shear 290 displacement of the upper shear box and vertical displacement of the loading 291 plate were monitored by high-precision transducers. The test was terminated 292 at a horizontal displacement of approximately 100 mm or when the failure 293 occurred in the block sample. The eld set up of the shear box on the shear 294 surface is shown in Figure 10. 295 3.3. Test program and procedure 296 The in-situ direct shear creep test is the so-called multistage creep test 297 that is commonly used in creep tests (Ladanyi and Johnston, 1973). In the 298 test process, the sample was consolidated until the rate of normal displace- 299 ment approaches the null under constant e ective normal stress of 0 n= 255 300 kPa, which is one-third of the normal component of the overburden stress on 301 the shear plane. The shear force was then brought rapidly up to an initial 302 creep level. It was then increased to the limit in several equal stress incre- 303 ments under constant e ective normal stress, each kept constant for several 304 13 (a) Data acquisition systemCooling system Shear device Loading system shear boxLoading cylinderFigure 10: (a) Field set up of the test and (b) the inclined xed shear box days. Five stages of shear stress were performed with di erent durations. 305 Speci cally, the tests of the rst two states were maintained for approxi- 306 mately 34 days, which was referred to as the long-term creep test. The 307 remainder of the creep tests took place for only two days for each stage, and 308 they were referred to as the short-term creep tests. 309 Table 1: The program of all creep tests Creep test name, 0 n= 255 kPa Shear stress (kPa) Time (hour) in-situ direct shear creep test (long term) 96.5, 101.2 760 in-situ direct shear creep test (short term) 109.2, 112.8, 116.4, 120.5 210 prepeak ring shear creep test (RS2) 155.4, 158.2, 161.5, 164.1 450 postfailure ring shear creep test (RS1) 164.1, 146.3 65 A pair of ring shear creep tests, namely, a prepeak ring shear creep test 310 and a postfailure ring shear creep test, were conducted in the laboratory. 311 These tests were designed to demonstrate the creep properties of shear-zone 312 soil under both prepeak and postfailure conditions (refer to Wang et al. 313 (2018b)). The results can be used as a supplement to the in-situ direct creep 314 shear test to interpret the movement pattern of the Huangtupo landslide. 315 The prepeak ring shear creep test used an intact sample (sample RS2), while 316 the postfailure ring shear creep involved a sample presheared to the residual 317 state after a large displacement of shear (sample RS1). To this end, two intact 318 ring-shaped samples, with a diameter of 200 mm and height of 50 mm, were 319 obtained from the in-situ creep test site; see the sampling position in Figure 320 7. By using a ring-shaped mold, the samples were made to a size of 100 321 mm inner diameter, 150 mm outer diameter, and 30 mm height. Compared 322 14 with the height of the sample, some gravel in the soil is too large, so we used 323 a steel needle to dig some large gravel out, leaving relatively ned-grained 324 material in the \intact" samples. During the test, the sample was sheared in 325 a container with wet cotton covered on the top of the container to mimic the 326 high relative humidity environment in the testing tunnel. The con guration 327 of the used ring shear apparatus is detailed in the