# Field and Numerical Investigation of Taihu Resort Cut Slope Failure in Suzhou, China
## 3 Study Landslide Area  
In the previous section, the geotechnical profile of the Taihu slope of Section 6 (S6) before and after excavation was presented in Figure 3. Hence, after the excavation process, the base layer of the slope consists of strongly weathered quartz sandstone, while the over-top layer consists of soil interbedded with clayey silt layers. After a detailed investigation of the Taihu landslide and a comprehensive study of the monitoring data to understand the failure mechanism, the final damage occurred mainly in the silty clay layer of the slope, which was caused during the monsoon rainfall in 2013. Further, the failure of the slope is divided into two phases. The first collapse phase was caused by local slumping at the toe of Section 6 (S6) of the slope in November 2012, with cracks occurring along the slope body. The second overall failure phase of the slope was caused during the monsoon rainfall in June and July 2013, which led to the total failure, including the shotcrete cover on the slope at Section 6 (S6).  
Figure 4 illustrates the landslide in Taihu slope photos and a cross-section at the first collapse phase. According to the field investigation, two months after completing the excavations and treatment of the slope, the first collapse occurred over the toe on the middle portion of the slope (S6) after moderate rainfall (20 mm/day \(\leq\) rainfall \(\leq\) 70). It should be noted that heavy rainfall was recorded with a precipitation intercity of 130.9 mm in three days at the end of November 2012, and the groundwater table rose near the slope toe, which caused swelling in the slope body and cracks in the shotcrete (Figure 4a). The tension cracks occurred along the slope body, 7-15 m long, with openings of 10-15 mm at the middle portion of the slope surface, and continued to propagate. Accordingly, a collapse at the toe of the slope was triggered. As shown in a close-up view (Figure 4b,c), it can be seen that the first slip surface failure mainly occurred within the argillaceous siltstone layer, including a critical crack along the slope toe.  
Figure 4: _Cont._  
Before the slope failure occurred, there were many intermittent tension cracks in the slope surface. After the constant rain of 20 mm/day \(\leq\) rainfall \(\leq\) 180 mm/day during the monsoon rainfall from March 2013 to July 2013, with maximum daily precipitation occurring in May, the groundwater table level rose significantly. A seepage occurred along the whole slope surface, as shown in Figure 5a. Therefore, the tension cracks expanded in the following days significantly at Section 6 (S6), which led to the occurrence of the second failure phase of the slope (Figure 5a,b). Thus, a gully with a thickness of about 5-7 m was formed in the slope body. The scheme of the failure mechanism and the slip surface along Section 6 (S6) based on the measurement data and the final collapse at the end of June 2013 can be observed in Figure 5c. Accordingly, the geotechnical investigation showed that the total displacement of the sliding mass was about 11 m with an inclined angle of 70\({}^{\circ}\)-75\({}^{\circ}\). Hence, the damage to the slope body was significant.  
Figure 4: Taihu Resort landslide photos of (**a**) first phase of slope collapse in November 2013; (**b**) close view of the slope cracks and collapse characteristics; (**c**) cross-section of the Taihu Resort landslide at the first landslide.  
Figure 5: \(Cont.\)