This paper presents an experimental inv estigation of the loading-relaxation behaviors of reconstituted shear-zone soils 6 through drained triaxial tests. A multistage loading-relaxation approach is adopted to p erform this test. The test aims to study two 7 main issues: the in ﬂuence of relaxation on the mechanical response during reloading; and the in ﬂuence of strain rates, loading increments, 8 and the relaxation time on the relaxation characteristics. The test resu lts indicate that the stress-relaxatio n behavior of shear-zone soil is 9 dependent on the stress and strain levels. The loading patterns prior to the stress-relaxation process affect mainly the viscoelastic behavior 10 of the soil and subsequently in ﬂuence the initial relaxation behavior. Moreover, a reloading after a stress-relaxation process gives rise to a 11 higher deviatoric stress owing to the viscoplasti c hardening. It is further shown that the loadin g-relaxation behavior s can be interpreted by 12 the viscoplastic theory. DOI: 10.1061/(ASCE)GM.1943-5622.0001949 .© 2021 American Society of Civil Engineers . 13 Author keywords: Huangtupo; Stress relaxation; Shear-zone soil; Triaxial compression test; Strain rate; Viscous behavior. 14 Introduction 15 Most reactivated slow-moving landslides in the Three Gorges 16 Reservoir (TGR) area of China are characterized by preexisting 17 shear zones ( Miao et al. 2014 ;Wang et al. 2018b ). The large de- 18 formation within the shear zones usually contributes to the major- 19 ity of the whole landslide motion owing to the viscous behavior of 20 the shear-zone soils. In general, the concept of viscous behaviors 21 consists of three aspects, namely, rate dependence, creep, and 22 stress relaxation ( Feda 1992 ;Liingaard et al. 2004 ;Augustesen 23 et al. 2004 ). Rate dependence and creep deformation can be widely 24 observed in slow-moving landslides. It is reported that when af- 25 fected by seasonal rainfall and periodic changes in water level, 26 most slow-moving landslides exhibit a creep deformation charac- 27 terized by a stepwise trend ( Wang et al. 2018b ). The change in strain 28 rate in the shear zones could, in turn, affect the mobilized shear re- 29 sistance of the shear-zone material and consequently in ﬂuence the 30 creep deformation of the landslide ( Hu et al. 2018 ). Owing to the sig- 31 niﬁcance of this property, a series of works have been carried out to 32 investigate the rate dependence (e.g., Scaringi et al. 2017 ;Hu et al. 33 2018 ) and creep properties (e.g., Li et al. 2017 ;Tan et al. 2018 ; 34 Wang et al. 2018b ,2018c ,2020 ) of shear-zone soils taken from 35 slow-moving landslides in the TGR. Compared with the tests on 36 rate dependence and creep to date, there is insuf ﬁcient experimen- 37 tal data for quantifying the stress-relaxation behavior of shear-zone 38 materials. For the comprehensive investigation of the viscous39 behavior of the shear-zone soil, performing stress-relaxation 40 tests under various stress paths is indispensable. 41 Conventional relaxation tests we re mainly conducted in the un- 42 drained condition to obtain a pure relaxation process with constant 43 strain. This situation, however, is unlikely to occur in a slow- 44 moving reactivated landslide. Inst ead, while undergoing creep de- 45 formation, the shear-zone material in the landslide may experience 46 relaxation processes if the shear zone is subjected to massive ex- 47 posure, e.g., excavation near the shear zone ( Wang et al. 2018b )o r 48 toe cutting owing to construction on the landslide area ( Deng et al. 49 2016; Troncone et al. 2014 ). In these cases, a complex stress state 50 is imposed on the shear-zone material and loading-relaxation cou- 51 pling is likely to occur. There are several reports on the loading- 52 relaxation coupling effect for reconstituted clay in odometer tests 53 (Bagheri et al. 2019 ) and triaxial tests ( Wang et al. 2017 ). In these 54 tests, the loading-relaxation coupling effects signi ﬁcantly in ﬂu- 55 ence the viscous behavior of the tested materials. For example, 56 the rate and magnitude of the relaxed stresses increase with the in- 57 crease in the prerelaxation strain and stress levels, and strain rates 58 (B a g h e r ie ta l .2 0 1 9 ). Likewise, a stress-relaxation process is 59 bound to affect the subsequent reloading behavior in a multistage 60 loading-relaxation process. However, there is a lack of relevant 61 research on shear-zone soils. Therefore, how the stress relaxation 62 and loading in ﬂuence each other in the shear-zone soils remains 63 unknown. To this end, stress-relaxation tests combined with load- 64 ing are anticipated to be carried out to ﬁll the research gap of time- 65 dependent properties of shear-zone soils. 66 This paper presents the results of multistaged compression- 67 relaxation tests on a shear-zone soil taken from a reactivated slow- 68 moving landslide in the TGR area. The coupled effects of loading 69 patterns and relaxation behavior are investigated with two main is- 70 sues: (1) the inﬂ uence of relaxation on mechanical response of re- 71 loading; and (2) the dependence of relaxation properties on loading 72 patterns, including prerelaxation strain rates, strain levels, and load- 73 ing increments. The mechanism of the loading-relaxation coupling 74 effect is discussed with the help of viscoplastic theory. The test re- 75 sults provide insights and data for the development of constitutive 76 models for modeling the viscous behaviors of shear-zone materials 77 under complex loading conditions.1Associated Professor, School of Engineering, China Univ. of Geosci- ences, Wu Han, 430074, PR China. 2Postdoc Research Fellow, Institut für Geotechnik, Univ. für Bodenkul- tur Wien, Feistmantelstrasse 4, A-1180 Vienna, Austria (corresponding au- thor). Email: shun.wang@boku.ac.at 3Lecturer, School of Engineering, China Univ. of Geosciences, Wu Han 430074, PR China. 4Professor, Institut für Geotechnik, Univ. für Bodenkultur Wien, Feist- mantelstrasse 4, A-1180 Vienna, Austria. Note. This manuscript was submitted on July 15, 2020; approved on October 21, 2020 No Epub Date . Discussion period open until 0, 0; separate discussions must be submitted for individual papers. This tech-nical note is part of the International Journal of Geomechanics , © ASCE, ISSN 1532-3641. © ASCE 1 Int. J. Geomech.This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. This material may be found at https://doi.org/10.1061/(ASCE)GM.1943-5622.0001949 78 Background 79 The Huangtupo landslide, located in Badong County, Hubei Prov- 80 ince, China (Fig. 1), is one of the largest (69.34 × 106m3) and the 81 most complex landslides in the TGR ( Deng et al. 2000 ;Tang et al. 82 2019 ). Four different sliding masses, with separate shear zones, 83 were detected in the landslide ( Wang et al. 2014 ,2016 ,2018a ). 84 The large size and complex structure of Huangtupo landslide make 85 it an ideal study object to better understand the mechanisms of sim- 86 ilar slides in this area. In 2012, the Badong experimental station, in- 87 cluding an intensive monitoring network and an investigation tunnel 88 withﬁve adits, was established by the Three Gorges Research Centre 89 for Geohazards. The adits exposed several shear zones of this land- 90 slide for scienti ﬁc experiments, such as in situ triaxial creep and di- 91 rect shear tests ( Wang et al. 2020 ;Tan et al. 2018 ). 92 It is noteworthy that the shear zones in the investigation tunnel 93 will be exposed to the air for a very long time owing to the time- 94 consuming in situ mechanical tests. Without a lining support, stress 95 redistribution will take place in the soil, and the exposed shear zone 96 will move into the tunnel, leading to considerable stress relaxation 97 in the shear zone. Moreover, the stress-relaxation phenomenon was 98 also observed in in situ direct shear test. As shown in Fig. 1(d), for 99 example, a signi ﬁcant stress relaxation occurred at the end of the 100 in situ direct shear test when the shear displacement ceased. This101 large amount of stress relaxation was ascribed to the accumulation 102 of structural defeat over time in the soil under the sustained strain. 103 The stress was anticipated to be decreasing with time. However, 104 further relaxation was not measured in the in situ test. According 105 to the rheology theory of stress relaxation, the stress may decrease 106 continuously within a relatively long period. The stress state and 107 the observed stress relaxation in the shear zone bring about the ne- 108 cessity of performing triaxial relaxation tests to investigate the vis- 109 cous behavior of the shear-zone material. 110 Multistaged Compression-Relaxation Tests 111 Test Material and Specimen Preparation 112 Some intact samples were taken from the shear zone exposed in 113 the testing tunnel of the Badong ﬁeld test site. The image in 114 Fig.1(c)shows the site for the soil sampling. Theses intact samples 115 were used to obtain the basic physical characteristics of the shear 116 zone, such as the mean natural water content ( w=14.23%), the 117 mean bulk density (ρ =2.25 g/cm3), and the mean plasticity index 118 (Ip=18.76). According to the grain size analyses shown in 119 Fig. 2(a), the shear-zone soil typically comprises ﬁne-grained 120 soil (silt and clay) with a signi ﬁcant fraction of coarse-grained (a) (c) (d) (b) F1:1 Fig. 1. (a) Location; (b) plan view of the Huangtupo landslide with the investigation tunnel (base image from Google Earth 2020); (c) pro ﬁle section F1:2 A–A′of No. 1 Riverside sliding mass showing the sampling location; and (d) the investigation tunnel and observed stress relaxation during in situ F1:3 direct shear tests. [Adapted (a) and (c –d) from Wang et al. 2020 ; image (c) Google Earth Image © 2020 CNES/Airbus.] © ASCE 2 Int. J. Geomech. 121 particles, constituting approximately 20 –50 wt.% (wt. denotes 122 mass fraction) of the material. The argillaceous particle content 123 is 50 and 80 wt.%. The ﬁne-grained portion can be classi ﬁed as 124 silty clay with low to medium plasticity and silt-clay mixtures 125 of low plasticity [see Fig. 2(b)]. 126 The material in the shear zone is characterized as gravelly soil 127 with inhomogeneously embedded coarse particles. It is reported 128 that the inhomogeneity of the shear-zone soils has a signi ﬁcant in- 129 ﬂuence on their viscous behaviors ( Wen and Jiang 2017 ). More- 130 over, previous experiments with intact shear-zone soil of this131 landslide usually give rise to very scattered results (Wang et al. 132 2018b ;Li et al. 2017 ;Tan et al. 2018 ;Wang et al. 2020 ). To high- 133 light the loading-relaxation coupling effects, therefore, remoulded 134 soil with a particle size less than 5 mm was used in this study. Cylin- 135 drical specimens of 50 mm diameter were prepared with equal 136 heights of 100 mm for the tests. The ratio of the specimen diameter 137 to the maximum particle size is 10, which satis ﬁes the requirement 138 for minimizing the boundary effect. The remoulded soil was mixed 139 with water to reach a natural water content and, then, the material 140 was stored in a covered container for 2 days. To achieve better homo- 141 geneity, the specimens were compacted into six layers using a split 142 mould with a circular cross-section to achieve the desired bulk den- 143 sity of 2.25 g/cm3and an initial void ratio of ei=0.49. During speci- 144 men preparation, the specimens were vacuumed for 12 h before being 145 saturated with deaired water. B-value checks were performed before 146 the triaxial tests, and values of 0.95 or greater were obtained, indicat- 147 ing high degrees of saturation of the specimens. 148 Test Plan 149 Two types of tests were performed: conventional consolidated 150 drained triaxial (CD test, including an unloading-reloading test, 151 i.e., UR test), and combined loading-relaxation tests (LR tests). 152 The program for all involved tests is given in Table 1. The exper- 153 imental work was performed on a triaxial testing system from 154 GDS Instruments. Prior to the LR tests, the CD and UR tests were 155 carried out to determine the stress-strain relationships of shear zone 156 soil under the drained triaxial compression condition. The CD 157 and UR tests were strain rate controlled with a constant strain 158 rate of ˙εa=0.0002/min during loading. The con ﬁning pressure 159 of the UR and CD2 tests was set to be 1,000 kPa with a constant 160 pore water pressure of 300 kPa, thereby providing an effective conﬁ n- 161 ing pressure of 700 kPa. This con ﬁning pressure and pore pressure 162 were calculated according to the burial depth of the shear-zone soil 163 and underground water level in the sliding masses. An additional 164 three CD tests (CD1, CD3, and CD4) with effective con ﬁning pres- 165 sures of 400, 1,000, and 1,300 kPa were performed for comparison. 166 The LR tests were carried to investigate the loading-relaxation ef- 167 fects that occurred in the shear zone. The soil in the shear zone may 168 experience a stress relaxation after loading with different rates of dis- 169 placement, loading time, and loading interval. The effects of these 170 factors on the loading-relaxation behavior of the shear zone soil 171 can be evaluated using a single specimen in the LR test ( Sheahan 172 et al. 1994 ). An axial strain-controlled approach was employed dur- 173 ing the relaxation tests, in which the axial strain was kept constant 174 when the desired strain was reached, but the radial strains were 175 free to evolve during the testing process. This boundary condition 176 is consistent with that of the soil in the shear zone. 177 Four LR tests, namely, LR1, LR2, LR3, and LR4, were 178 performed under the same con ﬁning pressure as the CD2 test.(a) (b) F2:1 Fig. 2. (a) Grain size distribution of intact and remoulded soils; and F2:2 (b) plasticity of remoulded soil without coarse particles. (Data from F2:3 Wang et al. 2020 .) Table 1. Test program of all tests on sliding zone soil T1:1 σ3 u σ′3 Relaxation strain Strain rates Relaxation time, t T1:2 Test name (kPa) (kPa) (kPa) ɛa(%) ˙εa(/min × 10−2) (h) T1:3 CD1 700 300 400 — 0.02 — T1:4 CD2 1,000 300 700 — 0.02 — T1:5 CD3 1,300 300 1,000 — 0.02 — T1:6 CD4 1,600 300 1,300 — 0.02 — T1:7 UR 1,000 300 700 — 0.02 — T1:8 LR1 1,000 300 700 3, 6, 9, 12, 15, 18, 21 0.02 24 T1:9 LR2 1,000 300 700 3, 6, 9, 12, 15, 18 0.02 –1.0 24 T1:10 LR3 1,000 300 700 1, 3, 6, 12, 15, 17, 18 0.02 24 T1:11 LR4 1,000 300 700 3, 6, 9, 12 0.02 72 © ASCE 3 Int. J. Geomech. 179 The LR1 test can be regarded as a reference test, which is hereafter 180 referred to as the regular relaxation test . The strain rates of the LR1 181 tests were ˙εa=0.0002/min, and relaxation duration was 1 day for 182 each relaxation process. The axial strain level increments between 183 two adjacent relaxation tests were 3%. On the other hand, the effects 184 of loading rate, relaxation time, and prestrain interval on the reload- 185 ing characteristics of shear zone soil were explored through LR2, 186 LR3, and LR4 tests, respectively. Compared with the LR1 test, the 187 conditions for the stress relaxation are variable in the LR2, LR3, 188 and LR4 tests. Therefore, the term irregular relaxation test is used 189 to refer to these tests. Speci ﬁcally, the LR2 test was designed for in- 190 vestigating the effects of strain rate at the loading process on the re- 191 loading response of shear zone soil. In the LR3 test, multistage 192 stress-relaxation tests were carried out with different strain incre- 193 ments. The LR4 test was performed to study the relaxation time 194 on the mechanical response of shear zone soil. To this end, the relax- 195 ation duration was extended from 1 to 3 days. 196 Experimental Results 197 Conventional and Unloading-Reloading Tests 198 The stress-strain results of the CD and UR tests are shown in Figs. 3 199 and4, respectively. In general, all specimens exhibit strain harden- 200 ing behavior during shearing in the CD tests. The volumetric strain 201 increases with the increase in the conﬁ ning pressures. The critical 202 state is not achieved in all CD tests. This may be attributed to the 203 situation of the remoulded shear-zone soil being normally consol- 204 idated. In the initial state, the soil in the shear zone of the Huang- 205 tupo landslide is heavily consolidated; thus, the critical state can be 206 easily achieved in the intact samples under the triaxial compression 207 loading condition (Li et al. 2017 ). In the UR test, both the unload- 208 ing and reloading curves exhibit obvious nonlinearity. With an in- 209 crease in the axial strain, the hysteresis loops caused by the 210 unloading-reloading cycles change only in size whereas the shapes211 remain unchanged. Compared with the CD2 test, a noteworthy en- 212 hancement in the strength is observed after each unloading- 213 reloading cycle. This strengthening behavior is most likely due to 214 the drainage process that occurred during the unloading-reloading 215 cycles. As shown in Fig. 4(b), contractive volume changes are ob- 216 served during the unloading-reloading cycle, leading to a larger 217 volumetric strain in the UR specimen. This observation indicates 218 that more pore water was drained from the UR specimen. 219 Regular Relaxation Test 220 Fig.5(a)shows the results of the LR1 test compared with the CD2 221 test. The deviatoric stress shows a monotonic increase to the ultimate 222 state plateau. There were seven stress-relaxation and reloading steps 223 involved in the LR1 test. It is worth noting that a switch of the stress 224 evolution from strain hardening to strain softening at a certain strain 225 level was observed in the LR1 test. At each stress-relaxation stage, 226 the deviatoric stress experienced a signi ﬁcant decrease to a residual 227 value, which is smaller than that measured in the CD2 test. On the 228 other hand, each reloading gives rise to a peak deviatoric stress 229 that is signi ﬁcantly greater than that obtained from the CD2 test at 230 the same strain level. Obviously, a hardening effect occurred during 231 the previous relaxation process. 232 Fig.5(b)presents the total volume change of the CD2 test and 233 the LR1 test. The former shows a linear trend with axial strain, 234 whereas the latter exhibits a multistep curve. Compared with 235 the volume variation of the CD2 test, the LR1 test gave rise to 236 a remarkable contraction during the relaxation steps rather than 237 the loading phases. In addition, the volume change caused by 238 loading can be extracted from the total volume strain, which is 239 shown by the dashed line. In contrast to the volume change of 240 the CD2 test, the presumed volume change by loading in the 241 LR1 test shows ﬁrst contraction and then dilatancy at εa=9%, 242 which shows an excellent agreement with the transition of stress 243 from strain hardening to strain softening.(a) (b) F4:1 Fig. 4. Deviatoric stress ( q) and volumetric strain ( ɛv) against axial F4:2 strain (ɛ a) for the UR test.(a) (b) F3:1 Fig. 3. Deviatoric stress ( q) and volumetric strain ( ɛv) 3against axial F3:2 strain ( ɛa) for CD tests. © ASCE 4 Int. J. Geomech. 244 Fig. 6shows the stress-relaxation curves in terms of devia- 245 toric stress versus relaxation time. The ﬁrst stress-relaxation 246 stage occurred at a strain level of 3% under an effective mean 247 stress p′=777.6 kPa. Six subsequent relaxation tests were coupled 248 with reloading. The results reveal that the stress rates decrease 249 with time leading to a stabilized stress state dependent on the ap- 250 plied strain amplitude. Each stress-relaxation curve shows similar 251 time-dependent characteristics, and the relative amplitude of the 252 deviatoric stress decrease Δq/qappear to be independent of the 253 applied strain. This result con ﬁrms that the viscosity in ﬂuence is 254 independent of the stress and strain. Similar result was reported 255 in the stress relaxation of saturated clay under undrained triaxial 256 condition ( Hicher 2016 ).257 Irregular Relaxation Test 258 In the following, we present the results of the irregular relaxation 259 tests, i.e., the LR2, LR3, and LR4 tests. The effects of loading 260 rate, loading increments, and relaxation time on the mechanical re- 261 sponse of the combined LR tests are investigated. The results of the 262 irregular relaxation tests are compared with the LR1 test. 263 Dependence on Loading Rate 264 Fig.7shows the deviatoric stress and volumetric change obtained 265 from the LR2 test. The comparison between the LR1 and LR2 266 tests reveals that the deviatoric stress decreases with increasing strain 267 rate during the reloading steps. These negative rate effects are most 268 likely owing to the generation of pore water pressure in the specimen 269 at high strain rates. This situation leads to a decrease in the deviatoric 270 stress. In addition, the volumetric strain evolution of the specimen is 271 the result of the combined effects of a change in the effective mean 272 pressure during the relaxation process and the viscous properties of 273 the material. A similar switch of the strain-hardening to stain- 274 softening characteristic is observed in the LR2 test. It is also found 275 that the volumetric strain of each step varies with rate of strain. 276 The volumetric strain of the LR1 test is greater than that of the 277 LR2 test with a variable rate of strain, as shown in Fig. 7(b). 278 Fig.8shows the comparison of stress-relaxation responses ob- 279 tained from the LR1 and LR2 tests. Distinct relaxation behaviors 280 are observed in these two tests. For the LR1 test, with the same load- 281 ing rate at each reloading stage, the relaxation test starting at a higher 282 strain level will give rise to a relatively smaller stress-relaxation ratio. 283 Compared with the LR1 test, the LR2 test obtains much smaller 284 stress-relaxation ratios for all relaxation tests. In contrast to the 285 LR1 test, in the LR2 test, the relaxation starting at a smaller strain 286 level obtains a larger stress-relaxation ratio at the beginning of stress 287 relaxation. After 5 h, all relaxation curves intersect at a transition 288 point. Subsequently, the LR2 test exhibits the same relaxation trends 289 as that observed in the LR1 test. This experimental observation (a) (b) F5:1 Fig. 5. Comparison between the CD2 test and the LR1 test: (a) devia- F5:2 toric stress versus axial strain; and (b) volumetric strain versus axial F5:3 strain. F6:1 Fig. 6. Relaxation of deviatoric stress in the LR1 test at different strain F6:2 levels.(a) (b) F7:1 Fig. 7. Comparison between results of LR1 and LR2 tests: (a) devia- F7:2 toric stress versus axial strain; and (b) volumetric strain versus axial F7:3 strain. © ASCE 5 Int. J. Geomech. 290 indicates that the strain rate at the previous loading stage has a re- 291 markable in ﬂuence on the relaxation response. This effect, however, 292 gradually disappears after the transition point. 293 Effect of Loading Increment 294 Fig.9shows the comparison of the LR1 and LR3 tests. The differ- 295 ence in loading increments gives rise to different mechanical re- 296 sponses. For example, the LR3 test yields a larger deviatoric stress 297 at the axial strain ranging from εa=2−9%. The difference in devia- 298 toric stress, however, is compensated at εa=9% after the LR1 test 299 experiences the same number of relaxation processes as that in the 300 LR3 test. Thereafter, the LR1 and LR3 tests gain almost the same de- 301 viatoric stress, until the LR3 test undergoes one more stress relaxa- 302 tion at εa=17%. Although the LR3 test experienced a larger 303 volume change during the entire loading-relaxation process, as 304 shown in Fig. 9(b), it is believed that the stress curve of the LR3 305 test will coincide with that of the LR1 test after εa=18%. In addi- 306 tion, the LR3 test also shows a switch of strain hardening to strain 307 softening in the stress-strain curve, whereas this transition point ap- 308 pears at εa=12% rather than εa=9% in the LR1 test. 309 The comparison of stress relaxation responses from the LR1 and 310 LR2 tests is presented in Fig. 10. A similar stress-relaxation behavior 311 is observed in these two tests. Likewise, the transition point appears at 312 approximately 3 min, earlier than that observed in the LR1 test, after 313 the relaxation test starts. Compared with the LR1 test, the LR3 test 314 experienced one more relaxation process at the earlier stage of axial 315 strain. This additional process gives rise to a strengthening effect to 316 the soil. As a result, at the same strain level, the stress-relaxation ratios 317 in the LR3 test are larger than those measured in the LR1 test. 318 Effect of Relaxation Time 319 Previous investigation has shown that neither test LR2 nor test 320 LR3 exhibited an obvious third stage of stress relaxation. This 321 is because the stress was relaxed for only 1 day in both tests. 322 Thereafter, in the LR4 test, the duration of the stress-relaxation 323 stage was extended to 3 days, whereas the other conditions 324 were kept the same as in the LR1 test. 325 The coupling effects of relaxation time on the mechanical behav- 326 ior of shear-zone soil during reloading are shown in Fig. 11. In com- 327 parison with the LR1 test with 1-day relaxation duration, the time 328 effect is more pronounced in the LR4 test with 3-day relaxation. 329 A ss h o w ni nF i g . 11(a) , the reloading gains much higher deviatoric330 stress under the strain level of 6%. For the subsequent reloading at 331 higher strain levels, the difference due to the longer previous relaxa- 332 tion duration becomes more visible. It is interesting to note that the 333 deviatoric stress q1is almost equal to q2, with q1andq2being the 334 deviatoric stress at the beginning of the third reloading state in 335 the LR1 test, and at the end of the ﬁrst reloading state in the LR4 336 test, respectively. Fig. 11(b) presents the volume change of the LR1 337 and LR2 tests. Clearly, the LR4 test gained much greater volume con- 338 traction than that in the LR1 test during the entire relaxation duration. 339 Nevertheless, for the loading process, the LR1 test obtained a slightly 340 larger volume contraction, as shown by the dashed line in Fig. 11(b) . 341 Fig. 12shows the comparison of stress-relaxation responses 342 from the LR1 and LR4 tests. A similar trend of stress relaxation 343 is observed in the LR1 and the LR4 tests, with the former giving F10:1 Fig. 10. Comparison of stress-relaxation results in the LR1 and LR3 F10:2 tests (TP: transition point). F8:1 Fig. 8. Comparison of stress-relaxation results in the LR1 and LR2 F8:2 tests (TP: transition point).(a) (b) F9:1 Fig. 9. Comparison between results of LR1 and LR3 tests: (a) deviatoric F9:2 stress versus axial strain; and (b) volumetric strain versus axial strain. © ASCE 6 Int. J. Geomech. 344 rise to larger stress-relaxation ratios. This is attributed to the 345 strengthening effect caused by the relaxation process. In contrast 346 to the LR2 and LR3 tests, the transition point in the LR4 tests co- 347 incides with that observed in the LR1 test. Although a longer relax- 348 ation duration gives rise to a large initial stress for the next stage of 349 stress relaxation, the previous relaxation time may not signi ﬁcantly 350 inﬂuence the initial response of the subsequent stress relaxation. 351 Mechanical Theory 352 From the test results presented in the previous section, it is found 353 that (i) the mechanical behaviors under reloading are in ﬂuenced354 by previous relaxation processes, and (ii) stress-relaxation behavior 355 is independent of the stress and strain levels, but can be in ﬂuenced 356 by preceding loading patterns and relaxation history. In the follow- 357 ing, the loading-relaxation coupling effects are discussed through 358 two aspects: the viscoplastic hardening effect and the effect of 359 the preceding loading pattern. 360 Viscoplastic Hardening Effect 361 The whole process of a loading-relaxation test is illustrated in 362 Fig.13, in which a loading-relaxation curve can be divided into 363 three stages, namely, a preliminary stage (line OA), a relaxation 364 stage (line AB), and a reloading stage (line BCD ). The stress-strain 365 curve of the preliminary stage maintains the same behavior as that 366 in a conventional triaxial compression test, as shown by the curve 367 OAE. In the relaxation stage, the deviatoric stress relaxes with time. 368 Although the axial strain remains unchanged, the volumetric strain 369 still increases owing to the radial strain. Therefore, the radial strain 370 presents the whole volume change, and it is obviously larger than 371 that caused in the preliminary stage. 372 The viscoplastic hardening can be clearly observed in the re- 373 loading stage (line BCD ), which leads to a signi ﬁcant strengthen- 374 ing in the deviatoric stress in comparison with the loading of in the 375 conventional triaxial test (line OE). The stress-strain response in 376 this stage can be further divided into two parts: the viscoelastic 377 loading (line BC) and the viscoplastic loading (line CD), which 378 give rise to deviatoric stress increments ΔqrevandΔqvp, respec- 379 tively. In the viscoelastic loading part, the tangent stiffness 380 (slope of line BC) is signi ﬁcantly greater than that in the conven- 381 tional triaxial tests. Beyond point C,t h ei n ﬂuence of relaxation on 382 the stress-strain response begins to weaken. The trend of the stress- 383 strain curve (line CD) becomes identical to that in the conventional 384 triaxial compression test (line AE). 385 The reason for the aforementioned viscoplastic hardening might 386 be attributed to structure adjustment, particle rearrangement, and 387 drainage under con ﬁning pressure during the stress-relaxation 388 stage. The viscoplastic hardening can be explained on the basis 389 of the elastoviscoplastic theory. It is assumed that the strain incre- 390 ment tensor can be decomposed into an elastic and a viscoplastic 391 components: dε=dεrev+dεvp(1) (a) (b) F11:1 Fig. 11. Comparison between the results of LR1 and LR4 tests with 1- F11:2 and 3-day relaxation duration, respectively: (a) deviatoric stress versus F11:3 axial strain; and (b) volumetric strain versus axial strain. F12:1 Fig. 12. Comparison of stress-relaxation results in the LR1 and LR4 F12:2 tests (TP: transition point). F13:1 Fig. 13. Typical stages of the combined loading-relaxation test (data F13:2 from CD2 and LR1 tests). © ASCE 7 Int. J. Geomech. 392 where dɛrevanddɛvpdenote the viscoelastic and the viscoplastic 393 strain increment tensors, respectively. To interpret the viscoplastic 394 hardening effect, we assume a time-dependent current yield surface 395 fand a reference surface f′that are mobilized during the loading 396 phase, such as proposed by Yao et al. ( 2015 ). When the stress 397 goes beyond the current yield surface f, the viscoplastic strain is cre- 398 ated. Meanwhile, hardening will develop and the current yield sur- 399 face will evolve according to hardening function. The viscoplastic 400 increment tensor is determined by dεvp=Φ∂g ∂σ(2) 401 where Φis the viscoplastic scalar multiplier, which is dependent 402 on the stress increment, the distance between the current stress 403 point, and the reference stress point and real-time increment 404 (Yao et al. 2015). The direction of the viscoplastic strain is dic- 405 tated by the potential g. The interpretation of the viscoplastic 406 hardening effect is shown in Fig. 14. The stress states are obtained 407 from the LR1 test with εa=3%−6%. The viscoplastic hardening 408 effect is explained as follows: 409 (a) In the preliminary stage, the material undergoes viscoplastic 410 strain if the stress state goes beyond the current yield surface 411 f; thus, both the reference surface and the current yield sur- 412 face expand accordingly because of the accumulation of vis- 413 coplastic strain. 414 (b) During relaxation, the stress state is located outside the refer- 415 ence yield surface f′. Owing to the distance between the current 416 and the reference surfaces, the stress relaxes until the stress 417 state moves back to the reference yield surface (A′→A), but 418 the current yield surface fis further pushed and expanded 419 owing to the viscoplastic strain increment Δϵvp. 420 (c) In the reloading stage, the load is applied following the same 421 stress path as that in the preliminary stage. During the initial422 viscoelastic loading ( A→A′), the stress state moves from Ato 423 A′. Because AA′is inside the current yield surface f, the load- 424 ing strain Δϵrevis reversible. 425 (d) With further loading, the stress state moves back the limit 426 yield surface f. Viscoplastic strain increment Δϵvpwill be pro- 427 duced again. Consequently, both the reference surface f′and 428 the current yield surface fexpand forward. The current stress 429 state is located outside the reference surface, and stress relax- 430 ation is bound to occur owing to the over-stress. 431 According to the above discussion, it is evident that the relax- 432 ation stage indeed has a signi ﬁcant effect on the mechanical re- 433 sponse in the reloading. In addition, the viscoplastic hardening is 434 time dependent, that is, a longer relaxation stage would give rise 435 to a larger increase in the deviatoric stress in the subsequent re- 436 loading stage (see test LR3). Moreover, the viscoplastic hardening 437 is independent of the strain level (see test LR4). This result is also 438 conﬁrmed by Hicher ( 2016 ) in stress-relaxation test under undrained 439 condition. Apparently, the viscoplastic hardening is a result of the in- 440 crease in the radial strain during a relaxation stage. As the specimen 441 had not reached a vanishing value in radial strain, the radial strain 442 may transform into another time-dependent process, i.e., creep, 443 owing to the con ﬁning pressure. In fact, the mechanisms that take 444 place during stress relaxation that lead to displacements and strains 445 are fairly complicated. 446 Effect of Preceding Loading Pattern 447 Two indexes, the stress-relaxation ratio Rand the initial relaxation 448 velocity vri, are adopted to evaluate the effects of preceding loading 449 patterns on the relaxation behavior. The stress-relaxation ratio Ris 450 deﬁned as the ratio of the deviatoric stress at any time, qr(t), to 451 the stress at the beginning of the stress relaxation, qi(e.g., Yin 452 and Graham 1989 ;Yin et al. 2014 ;Hicher 2016 ):(a) (b) (c) (d) F14:1 Fig. 14. Illustration of viscoplastic hardening (units: kPa): (a) preliminary loading; (b) stress relaxation; (c) initial reloading; and (d) subsequent F14:2 loading. The stress states are obtained from the LR1 test with εa=3%−6%. © ASCE 8 Int. J. Geomech. R(t)=qr(t) qi(3) 453 The initial relaxation velocity vri,r eﬂecting the activity of inter- 454 nal stress reduction and transition (Wang et al. 2017 ), is de ﬁned as vri=Δqr tr(4) 455 where Δqris the change in the deviatoric stress before the transition 456 time tr. For the sake of simplicity, the transition time tr=60 s is 457 adopted in this work. 458 Fig.15(a) presents the stress-relaxation ratio Rafter 24 h of stress 459 relaxation. In general, the stress relaxation ratio Rincreases with 460 axial strain. For example, for the LR1 test, the ratio Rincreases 461 from 0.5 to 0.7 as the axial strain increases from 3% to 15%, respec- 462 tively, and then remains unchanged despite further increase in the 463 axial strain. The in ﬂuence of the loading rate on the relaxation behav- 464 ior becomes larger with increasing axial strain. For instance, with an 465 increase in the preceding loading rate from 0.0005/min at εa=9% 466 to 0.005/min at εa=15%, the difference in Rbetween the LR1 467 and LR2 tests enlarged from 0.03 to 0.11, respectively. The stress- 468 relaxation ratio R, however, suggests that an increase in relaxation 469 time results in a lower relative stress reduction during the relaxation470 stages. The results suggest that stress relaxation may signi ﬁcantly in- 471 ﬂuence the reloading behavior through relaxation time. 472 As shown in Fig. 15(b) , the initial relaxation velocity de ﬁned in 473 Eq. (4) increases with increasing axial strain in all tests, with the LR4 474 and LR2 tests giving rise to the highest and the lowest values, respec- 475 tively. The LR4 specimen experiences much longer relaxation dura- 476 tion with an accumulation of viscoplastic hardening in the soil. The 477 specimen, therefore, exhibits more elastic behavior among the four 478 specimens. In contrast, the LR2 specimen is sheared with an increas- 479 ing strain rate. The shearing with a high strain rate to some extent 480 spoils the microstructure of the specimen and, thus, leads to a weak- 481 ened effect on the viscoelastic response of the soil. 482 Conclusions 483 In this paper, we report on the loading-relaxation coupling effect of 484 reconstituted shear-zone soil of a slow-moving landslide by perform- 485 ing a series of drained triaxial tests. A strong loading-relaxation cou- 486 pling effect is observed in the experiments, and the following 487 conclusions can be drawn: 488 (1) A reloading after a stress-relaxation process gives rise to a 489 higher deviatoric stress owing to the viscoplastic hardening 490 effect. The viscoplastic hardening effect is time dependent, 491 that is, a long-term relaxation process achieves the similar 492 hardening effect as several cumulative short-term relaxation 493 processes. In addition, the behavior upon reloading after re- 494 laxation is initially in an viscoelastic regime, which can be 495 enhanced by increasing the relaxation time. 496 (2) The preceding loading patterns affect mainly the viscoelastic 497 behavior of the soil, and subsequently in ﬂuence the initial re- 498 laxation behavior. The loading increments and relaxation du- 499 ration inﬂ uence the subsequent relaxation behavior through 500 the viscoplastic hardening effects. In terms of the relaxation 501 duration, we found that longer previous relaxation leads to 502 observable differences in the initial drop of the stress as 503 well as the relative stress reduction. 504 (3) The loading-relaxation coupling can be interpreted with elas- 505 toviscoplastic theory, provided the viscoplastic strain con- 506 tribute to the expansion of the current yield surface. This 507 relaxation-induced strain is largely dependent on the radial 508 strain during the relaxation process. 509 It should be noting that different landslide processes may lead to 510 various shear zones and the soil within the shear zones are usually 511 inherently inhomogeneous in coarse particles and macrostructures. 512 These features may in ﬂuence the viscous behaviors of the shear- 513 zone soils. Moreover, the soils may experience more complex stress 514 and strain paths, e.g., creep along with stress relaxation, under the 515 in situ conditions. Therefore, our conclusions are limited to the re- 516 constituted shear-zone soil of the Huangtupo landslide. To de- 517 velop a general constitutive model for such soils, further 518 experimental investigations on the viscous behavior of intact 519 shear-zone soils under complex loading conditions are required. 520 Data Availability Statement 521 The data for the triaxial compression behavior of the soil is available 522 inﬁgshare with the identi ﬁerhttps://doi.org/10.6084/m9. ﬁgshare 523 .13077632 . 524 Acknowledgments 525 Open access funding provided by the University of Natural 526 Resources and Life Sciences Vienna (BOKU). This work was(a) (b) F15:1 Fig. 15. (a) Relaxation ratio Rafter 24 h stress relaxation; and (b) ini- F15:2 tial relaxation rate vriafter 60 s stress relaxation. © ASCE 9 Int. J. Geomech. 527 funded by the National Natural Science Foundation of China 528 (Grant Nos. 41772304 and 41602313); the H2020 Marie 529 Sk łodowska-Curie Actions RISE 2017 HERCULES (778360) 530 and FRAMED (734485), and the Erasmus +KA2 project Re-built 531 (2018-1-RO01-KA203-049214). The corresponding author wishes 532 to thank the Otto Pregl Foundation for ﬁnancial support in Austria. 533