# Effect of Local Cyclic Loading on Direct Shear Strength Characteristics of Shear-Zone Soil
## 3 Materials and Methods
### Testing Methodology  
The cyclic direct shear tests were performed using an automated direct shear apparatus (Figure 4) from GDS Instruments (Hook, UK), which could perform standard/cyclic direct shear testing with full automation (ASTM D-3080). In cyclic direct shear tests, horizontal shearing could be applied at an (i) assigned rate of horizontal deformation, (ii) assigned rate of horizontal force change, or (iii) assigned cyclic loading. The system had a shear actuator, internal load cell, and data acquisition box. The system also had horizontal travel of \(\pm\)25 mm and displacement accuracy of 0.1% FSO.  
The specimen assessed with the cyclic direct shear test apparatus was approximately 75 mm \(\times\) 75 mm \(\times\) 30 mm. The remolded specimens were prepared to match the dry density and water content of the undisturbed specimens. To prevent material loss at the clearance between the two halves of the direct shear box, the clearance size was fixed to 0.2 mm using the gap screws.  
The samples in the cyclic direct shear test were loaded to 100 kPa, 200 kPa, and 400 kPa vertical stresses according to the buried depth and elevation of the shear-zone soil. For monotonic loading tests, the specimens were sheared at a steady rate of 0.02 mm/min throughout the test. The Huangtupo landslide is a revived landslide after the impoundment of the TGR, and the shear strength of the shear-zone soil falls to the residual value, about 60-70% of the maximum shear stress (\(\tau_{\mathrm{f}}\)) [28]. Thus, for the cyclic loading test when the shear stress increased to 65% of the maximum shear stress or specified shear strength under monotonic loading (stage I), cyclic loading (stage II) was applied. According to the water level fluctuation data [20], the stress amplitude of the cyclic loading was 5% of the shear strength, the cyclic frequencies were 0.011 Hz (90 s), 0.017 Hz (60 s), and 0.100 Hz (10 s), and the number of cycles were 10, 20, 100, and 1000. The dynamic stress ratios were 0.577, 0.540, and 0.368 for normal consolidation stress of 100 kPa, 200 kPa, and 400 kPa, respectively. After cyclic loading, the specimen was sheared monotonically (stage III) until the specific termination to reveal the effect of local cyclic loading on the shear stress of the shear-zone soil. A schematic diagram of monotonic loading (stage I), cyclic loading (stage II), and monotonic loading (stage III) is shown in Figure 5. In the absence of a peak strength, the shear stress at 10 percent relative lateral displacement is taken as a failure (ASTM D-3080). Table 2 shows the detailed cyclic direct shear test conditions for the shear-zone soil of the Huangtupo landslide.  
The shear strength ratio \(f_{\mathrm{cm}}\), which is expressed as the ratio of the shear strength \(\tau_{\mathrm{f}\mathrm{c}}\) in a cyclic direct shear test to the peak strength \(\tau_{\mathrm{f}\mathrm{m}}\) in a monotonic direct shear test on shear-zone soil under the same vertical stress is proposed as follows:  
\[f_{\mathrm{cm}}=\tau_{\mathrm{f}\mathrm{c}}/\tau_{\mathrm{f}\mathrm{m}} \tag{1}\]  
Figure 4: The GDS direct shear test equipment. (**a**) Components of shear test system; (**b**) mounting the specimen.