Effect of Prior Cyclic Loading on Triaxial Compression Strength of Sliding Zone Soil of the Huangtupo Landslide

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Introduction
e Hubei Yangtze River Economic Belt, located in the center of the Yangtze River basin, is the demonstration zone for the emergence of ecological civilization in central China.
erefore, it is essential to study the catastrophic mechanism and prevention technology of landslide disasters in the ree Gorges Reservoir Area (TGRA) of the Hubei Yangtze River Economic Zone. e cyclic stress amplitudes and loading sequences on soil did affect the characterization of soil [1][2][3][4]. In particular, there is an urgency to investigate the catastrophic mechanism of landslide under cyclic loadings, such as atmospheric rainfall, the rise or fall of reservoir water levels, and especially, traffic loadings and earthquakes. e influence of periodic rainfall infiltration on sliding mass is mainly reflected in the dynamic process of increasing bulk density and attenuated matric suction, both of which lead to a lowered landslide stability safety factor [5]. e rise or fall of reservoir levels results in periodic changes in the phreatic line and affects the seepage force of groundwater [6]. e dynamic loading caused by expressways and railways leads to the accumulated deformation of landslide mass [7]. During an earthquake, the slip mass produces horizontal and vertical inertial forces under the action of earthquake acceleration, which eventually results in a landslide [8,9]. In addition, a higher magnitude earthquake may yield landslide in the stage of cumulative failure deformation under microseisms, and it also has the tendency to cause integral sliding deformation [10].
As the properties of sliding zone soil control the stability of landslide, a lot of in situ and indoor static strength tests on sliding zone soil have been carried out by scholars, including a direct shear test [11], triaxial compression test [12], ringshear test [13], rheological test [14], and dynamic strength test, such as hollow cylinder test and dynamic triaxial test [15]. Monitoring data [16] shows that the Huangtupo landslide is in creep state, and the sliding zone soil is in residual strength state. For a slowly sliding landslide, the sliding zone soil is in the state of residual strength, which is generally 70% of the shear strength [17,18]. e residual strength is further reduced under the action of earthquake [19]. ereby it is urgent to study the dynamic response of the sliding zone soil in this residual strength state and analyze the catastrophic mechanism of the landslide under cyclic loading.
Currently, there are many ongoing studies on the formation mechanism of landslide in the ree Gorges Reservoir Area. Studies of the influence of cyclic loading with long period, small amplitude, and multifrequency on sliding zone soil with residual strength state, however, are scarce. e cyclic loading is likely to trigger more landslides. erefore, on the basis of previous research results, this paper exhibits the experimental research carried out on the strength and deformation characteristics of sliding zone soil under different confining pressures, different cycle periods, and different number of cycles, in order to better explain the deformation mechanism of the Huangtupo landslide.

Landslide Investigation
e Huangtupo landslide is located in Xinling Town, Badong County, Hubei Province ( Figure 1). According to a preliminary investigation, detailed investigation, and expert argumentation by the Changjiang Water Resources Committee in 1992, the Huangtupo landslide was confirmed to be an ancient large-scale landslide accumulation with complex history, which seriously threatened the safety of lives and property of the nearby people [20]. Consequently, the Huangtupo landslide relocation project was officially launched by Badong County in February 2009. is was completed by the end of May 2017; a total of 15713 people from 4475 households relocated from the landslide mass. Houses, shopping malls, workshops, and other buildings were demolished on the landslide, leaving only the highway connected with the G348 National Highway. ere are still buses and trucks passing through the road all year around, which produces continuous cyclic loading on the Huangtupo landslide.
e Huangtupo landslide, formed from the mudstone, argillaceous siltstone, and limestone strata of the Triassic Badong Formation, is a multistage geological hazard mass composed of paleo-landslide mass and landslide accumulation mass. e Huangtupo landslide is composed of No. 1 and No. 2 riverside sliding masses distributed along the river. is super large and super deep paleo-landslide deposit has an area of 135.8 × 10 4 m 2 and a volume of 6934 × 10 4 m 3 . e No. 1 riverside sliding mass takes up an area of 32.5 × 10 4 m 2 and has a volume of 2255.5 × 10 4 m 3 , an average thickness of 69.4 m, and a maximum thickness of 95.27 m. At the end of 2012, the China University of Geosciences (Wuhan) built a large field comprehensive test site in Badong County on the No. 1 riverside sliding mass to study the evolution mechanism of landslides, which includes a 908 m long main tunnel and 5 test branches of different lengths [16]. e tunnel had a horseshoe shaped section of 5 m × 3.5 m. Monitoring data on surface deformation, test tunnel deformation, and deep deformation within the Huangtupo landslide revealed that the sliding zone soil is subject to low frequency and long period cyclic loading under the actions of reservoir level rise or fall and cyclic loading by traffic. erefore, to be able to produce a stability analysis and deformation evolution of the Huangtupo landside, it is of great significance to study the stress-strain relationship of sliding zone soils under different confining pressure, cycle periods, and number of cycles.

Specimen Preparation.
e sample used in this test is sliding zone soil from the 3 # test branch of No. 1 riverside sliding mass of the Huangtupo landslide, and its basic physical and mechanical property indexes are shown in Table 1. Results of XRD analysis show that the main mineral components of the sliding zone soil are Calcite (30%), Quartz (20%), Illite (29%), Montmorillonite (11%), Chlorite (5%), and Feldspar (5%). e grain size distribution curve of the sliding zone soil is demonstrated in Figure 2, showing a large number of coarse-grained groups in the sliding zone soil. Coarse-grained groups play an important role in mechanical behaviour [21][22][23]. In order to retain the coarsegrained components as much as possible and truly reflect the composition of sliding zone soil, a sample with 61.8 mm diameter and 125 mm height was chosen. e preparation and saturation of samples were performed in the laboratory as shown in Figure 3. A mass of 10 kg sliding zone soil was weighed, air-dried, crushed, and sieved to reach a maximum allowable particle size of 6.18 mm. After measuring the moisture content removed by air drying, the required water volume was calculated, and purified water was evenly sprayed on the dried sample. e soil samples were put into a plastic bucket and sealed for 24 h. Soil samples were weighed and compacted in five layers according to the dry density of natural samples. After each layer was compacted to the required height, the surface was roughened, and the second layer of soil sample was added until the last layer was compacted.
e saturator with a sample was placed in a suction cylinder for air extraction. When the vacuum pressure was close to -101.3 kPa, the vacuum-control state was continued for 10 h, and then the purified water was slowly injected. When the saturator was completely submerged by water, vacuum-control device was stopped, the vacuum of the extraction cylinder was released, and the sample was kept saturating for 24 h.

Test Procedure.
e instrument used in the test was the KTL triaxial automated system developed by Xi'an KTL Instruments Company Limited. e instrument consists of a motor-controlled loading frame, a triaxial pressure chamber, load and pore pressure sensors, an eight-channel data acquisition instrument, a pressure volume controller, and other components, as shown in Figure 4. e single loading test is consolidated undrained triaxial compression test; the test procedures are as follows: (1) Calculate the B-value until it is equal to or greater than 0.95. (2) Consolidation. Considering the buried depth of sliding zone soil, the confining pressures are designed to be 100 kPa, 200 kPa, and 400 kPa, respectively. e loading rate is 10 kPa/min. Once the confining pressure is stabilized, the drainage rate of sample is monitored. When the drainage rate is less than 0.01 mm 3 /min, the main consolidation stage is completed. (3) Close the drain valve and start the motor. When the deviatoric pressure equals 3 kPa, it means that the load sensor contacts with the sample top. Record the axial displacement and calculate the height of the sample after consolidation. (4) Load and set the axial strain rate to 0.06%/min until the axial strain reaches 20%.
As for cyclic loading, the test procedures are as follows: (1) Firstly, determine the shear strength.
(2) When the deviatoric stress reaches 70% of the shear strength, cyclic loading is applied. e amplitude of cyclic loading is 5% of the shear strength.
(3) Periods of 10 s, 60 s, and 90 s are selected as cycle period for different cyclic loading tests. e numbers of cycles are 10, 100, and 1000, respectively. (4) If the axial strain of the specimen is less than 20% after cyclic loading, continue to load the specimen at the rate of 0.06%/min until the axial strain reaches 20%. (5) When shear is completed, reduce the confining pressure and pore pressure to zero, turn off the motor, remove the pressure chamber cover, and describe the shape of the specimen after failure. e specific test scheme is shown in Table 2. According to creep test results, the residual strength (q r ) of the sliding zone soil was determined as about 70% of the shear strength [13]. When the deviatoric stress reached 70%    Advances in Civil Engineering of the shear strength, cyclic stress was applied to the specimen, and the stress amplitude (q amp ) was 5% of the shear strength. Subsequently, the influence of confining pressure (σ 3 ), cycle period (T), and number of cycles (N) on the strength characteristics of sliding zone soil was studied.
Under the consolidated undrained condition, cyclic loading can change the pore water pressure and the internal structure of sliding zone soil, resulting in a reduction of shear strength. To compare the stress difference of monotonic loading and multistage loading, the failure stress ratio     Advances in Civil Engineering R f is defined as shown in the following equation and is used to study the influence of prior stress on sliding zone soil strength characteristics.
where q f indicates consolidated undrained compressive strength under monotonic loading and q cf denotes consolidated undrained compressive strength under cyclic loading. e cyclic failure stress ratio R f can reflect the influence of cyclic loading on the consolidated undrained strength of sliding zone soil. e typical triaxial test scheme is shown in Figure 5. e test process can be divided into three stages: (I) static loading stage with strain-controlled mode; (II) cyclic loading stage with stress-controlled mode; and (III) static loading stage with strain-controlled mode.

Monotonic Loading Test.
Firstly, a strain-controlled consolidated undrained triaxial compression test was carried out to obtain the stress-strain curve and shear strength of sliding zone soil. For confining pressures of 100 kPa, 200 kPa, and 400 kPa, the stress-strain curves are shown in Figure 6. e stress-strain curves clearly show strainhardening characteristics, and there is no obvious peak strength in the curves. Failure stress (q f ) is often taken to correspond to the deviatoric stress at 15% axial strain. e shear strength of the saturated sliding zone soil under three different confining pressure is 219.70 kPa, 229.89 kPa, and 252.19 kPa, respectively. When the confining pressure is 100 kPa and 200 kPa, respectively, the pore water pressure increases during axial loading process, which indicates that the sliding zone soil is in the state of contractancy, and the effective shear strength is 219.36 kPa and 228.80 kPa, respectively. However, in the confining pressure of 400 kPa, the pore water pressure decreases to a negative value, indicating that the sliding zone soil is in the state of dilatancy, and the effective shear strength becomes 253.88 kPa. As the change of pore water pressure is not obvious during the process of axial loading, we aim to investigate the influence of different cycle periods and number of cycles on the shear strength from the perspective of total stress.

Effect of Confining Pressure on Stress-Strain Curves.
Test results of the monotonic loading and cyclic loading (100 cycle times) of sliding zone soil under different confining pressures are shown in Figure 7. For cyclic loading, it apparently does not change the stress-strain curves type of sliding zone soil, which shows a strain-hardening type. Nevertheless, because of the influence of cyclic loading, the dynamic stress level is lower than static stress level. Before the cyclic loading is carried out, the static loading is performed to reach 70% of shear strength. Figure 7 shows that the axial strain does not exceed 3% under various confining pressures when the deviatoric stress reaches 70% of shear strength.
In the stage I of loading under the same confining pressure, the static loading curve is the same level. In the stage II of loading, the stress-strain curve depends on the period of cyclic loading. When the period is 10 s, the stressstrain curve remains almost sinusoidal. With a period of 60 s and 90 s, the horizontal stress-strain curve changes from sparse to dense, indicating that plastic strain in the early phase of cyclic loading is larger, and in the later phase elastic strain is obvious. In the stage III of loading, the strain-strain curve of cyclic loading is lower than that in the monotonic loading. Obviously, the prior cyclic loading reduces the ability of sliding zone soil to bear loading. e failure stress ratio decreases with an increase in confining pressure (Figure 8). Shear strength and strength index c and φ of the sliding zone soil after 100 cycles are shown in Table 3    Advances in Civil Engineering 5 number of cycles. It can be observed that the cohesion of sliding zone soil under cyclic loading decreases by 13.94%∼ 21.81%, while internal friction angle increases by 1.69%∼ 35.81%. Hence, cyclic loading mainly affects the cohesive strength of sliding zone soil, which makes the clay particles slide and rearrange, which decreases the bond of clay particles and increases the friction forces between particles. e relationship between axial strain and the number of cycles under different confining pressures is shown in Figure 9. When the cycle period is T �10 s, total time T t � 10 × 100 � 1000 s with loop 100 times, the strain versus cycle number curve shows linear growth under different confining pressures. When the cycle time is T � 60 s, total time T t � 60 × 100 � 6000 s with loop 100 times, the strain versus cycle curve under confining pressure of 100 kPa has an attenuation stage and a stable stage, whereas the strain versus cycle curve under confining pressure of 400 kPa shows linear increase. When the cycle time is T � 90 s, total time T t � 90 × 100 � 9000 s with loop 100 times, the strain versus cycle curve can be divided into two stages: an attenuation stage and a stable stage. Taking the confining pressures of 100 kPa and 400 kPa as examples, the curve exhibits a stable stage after 53 and 38 cyclic loadings, respectively. Except for the influence of cyclic period, the total time of cyclic loading has a significant impact on the strain characteristics of samples. Axial strain (%) σ 3 =400 kPa, multi-stage loading σ 3 =400 kPa, monotonic loading σ 3 =200 kPa, multi-stage loading σ 3 =200 kPa, monotonic loading σ 3 =100 kPa, multi-stage loading σ 3 =100 kPa, monotonic loading Axial strain (%) σ 3 =400 kPa, multi-stage loading σ 3 =400 kPa, monotonic loading σ 3 =200 kPa, multi-stage loading σ 3 =200 kPa, monotonic loading σ 3 =100 kPa, multi-stage loading σ 3 =100 kPa, monotonic loading Deviatoric stress (kPa) σ 3 =400 kPa, multi-stage loading σ 3 =400 kPa, monotonic loading σ 3 =200 kPa, multi-stage loading σ 3 =200 kPa, monotonic loading σ 3 =100 kPa, multi-stage loading σ 3 =100 kPa, monotonic loading   Advances in Civil Engineering

Effect of Cycle Period on Stress-Strain Curves.
To further study the stress-strain characteristics of sliding zone soil under different cyclic loading and constant confining pressure of σ 3 � 100 kPa, consolidated undrained triaxial compression tests with different cycle periods (10 s, 60 s, and 90 s) and 100 cycles were carried out, as shown in Figure 10. It is observed that, under the same cycle number (N � 100), a short cycle period results in a decrease in shear strength, showing that the short cycle period decelerates the dissipation of excess pore water pressure and longer cycle period accelerates the dissipation of excess pore water pressure.
e cycle period has a significant impact on the failure stress ratio of samples. e increase of cycle period under the same number of cycles is followed by a reduction of failure stress ratio of samples with respect to prior cyclic loading, as indicated in Figure 11.
Taking 1000 cycles as an example, the axial cumulative strain curves of sliding zone soil under different cycle period are shown in Figure 12(a). When the specimen is subjected to a loading period of 10 s, the axial cumulative strain increases almost linearly. A loading period of 60 s causes a rapid rise in the axial strain within 100 cycles, followed by a steady reduction and tending to a stable level. When the specimen is loaded under the cycle period of 90 s, the axial strain increases rapidly within the first 60 cycles and then decreases slowly to a stable level. Figure 12(b) shows a typical dynamic stress-strain curve under N � 100 and T � 90 s. e axial strain of the sample is composed of elastic strain and plastic strain. For longer cycle period, during the early stage of loading, the plastic strain decreases significantly, followed by elastic strain. But for shorter cycle period, during the whole loading stage, the plastic strain remained at the same level because of the unfavorable dissipation of excess pore water pressure.

Effect of Number of Cycles on Stress-Strain Curves.
Triaxial consolidation undrained tests with a confining pressure of 100 kPa and cycles of 0, 10, 100, and 1000 times were carried out with the aim of studying the influence of different cyclic times on the strength characteristics of sliding zone soil. e acquired stress-strain relationship is shown in Figure 13. When the cycle period and stress amplitude are fixed, after being subjected to different cycle times, the stress-strain curve of sliding zone soil still shows the strain-hardening type. e shear strength shows a decline when the axial strain reaches 15% under larger cycle times. Cumulative plastic strain increases with an increase in cycle number, but with a gradual reduction in its growth rate. Figure 14 shows that the failure stress ratio grows as the number of cycles increases. Taking T � 90 s as an example, the failure stress ratio is observed to increase by 5.93%, 10.54%, and 12.13% after 10, 100, and 1000 loading cycles, respectively. It can be seen that the accumulated plastic strain in prior cyclic loading increases the failure stress ratio.   Advances in Civil Engineering Furthermore, the dissipation of excess pore water pressure increased the effective stress and reduced the loss of shear strength.
Curves of the failure stress ratio (R f ) of sliding zone soil as a function of number of cycles (N) show that the relationship between R f and N approximately satisfies the following formula: where a, b, and c denote the fitting parameters, respectively. e logarithmic relation correlates with the failure stress ratio and the number of cycles in cyclic loading tests, which preliminary validates the applicability of logarithmic relation for sliding zone soil characteristics influenced by cyclic   10 Advances in Civil Engineering loading, providing a theoretical basis and guidance for the further understanding of strength characteristics of sliding zone soil.

Conclusions
In an effort to investigate the influence of prior cyclic loading on sliding zone soil stress-strain behaviours under a consolidated undrained triaxial compression test, a series of monotonic loading and cyclic loading tests were designed and carried out on the basis of the KTL Triaxial Automated System. e strength and deformation characteristics of soil specimens subjected to different confining pressure, cycle periods, and number of cycles were analyzed in depth. e conclusions are as follows: (1) For the sliding zone soil involved in the consolidated undrained triaxial compression test with different prior cycle periods and number of cycles, the stressstrain curves of monotonic loading and cyclic loading both exhibit strain-hardening characteristics.
(2) e deviatoric stress of cyclic loading is smaller than that of monotonic loading under different confining pressures because of the prior cyclic stress. As the confining pressure increases, the failure stress ratio decreases, indicating the effect of prior cyclic loading on shearing strength weakens under higher stress state. (3) For the curves of axial strain versus number of cycles, the axial strain first exhibits a precipitous rise and then remains stable under the cycle period of 90 s, total time T t � 90 × 100 � 9000 s with loop 100 times. But the axial strain shows a linear increase with the increase of number of cycles under the period of 10 s, total time T t � 10 × 100 � 1000 s with loop 100 times. e axial strain accumulation was influenced by not only the cycle period but also the total cyclic loading time.
(4) e logarithmic relation correlates with the failure stress ratio and the number of cycles in cyclic loading tests, which preliminary validates the applicability of logarithmic relation for sliding zone soil characteristics influenced by prior cyclic loading.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interests.