# Instability Risk Assessment for Deep Excavation of Soil-Rock Combinations Containing Groundwater
## 3 Case Study  
### The Project  
The excavation of Qingdao Metro Anshan Road Station is located at the northwest corner of the intersection of Anshan Road and Shandong Road, and the form is in the shape of an \"L\", as shown in Figure 2, and is the deepest station in Qingdao, with a maximum excavation depth of about 42 m. Figure 3 shows the geologic profile of YDK7 + 495.533 - YDK7 + 670.333 of the Anshan Road station excavation. The topography of the area is relatively flat, and the geology is a typical composite stratum of Qingdao area with upper soft and lower hard strata. While the upper soil layer mainly consists of fill, powdery clay, and coarse sand, the lower bedrock is dominated by granite with different degrees of weathering, partially intruded by lamprophyre, granite, and cataclasite. Under the influence of the Guanshan Fracture and Qingdao Mountain Fracture, a number of tectonic fracture zones have been formed at the site, as shown in Figure 4, which are characterized by the development of rock joints and fissures, rock fragmentation, and instability. The depth of the groundwater is about 2.2-4.8 m, which is mainly stored in the fill soil layer and coarse sand layer with strong permeability, recharged through atmospheric precipitation and groundwater seepage. The station was excavated using top-down construction, and the support system is a composite support form, consisting of steel pipe piles, cables, and rock bolts.  
During excavation, the existence of fractured rock and joints in the tectonic fracture zone is prone to destabilizing the slope of the excavation. Meanwhile, the surrounding environment of the excavation is complex, being adjacent to schools, residential areas, and viaducts, as shown in Figure 2. The north and northwest sides of the station are adjacent to the school's depot and a residential building, respectively, with a space of 12.5 m and 24 m. And on the west of the excavation are Anshan 4 th Road 2A and Qingdao 16 th Middle School, with the shortest distances of 20 m and 36.2 m to the excavation. Shandong Road Viaduct and Hang-An Expressway Viaduct are located to the east and south of the excavation, and the distance between the excavation and them is 24.9 m and 22.2 m,respectively. Additionally, there are 17 underground pipelines within the construction impact area. In particular, as an interchange station, Anshan Road Station has a complex structure with 21 supporting units containing a large number of corners, as shown in Figure 5, which leads to collision of anchors and difficulties in construction control. For the above reasons, during the construction, while ensuring the safety of the excavation itself, it is also necessary to ensure the safety of the surrounding buildings, which greatly increases the difficulty of the project. In fact, the excavation has resulted in cracks in the surrounding ground and buildings, as shown in Figure 6. Therefore, an effective stability assessment is urgently needed to ensure the safety of the excavation.  
Figure 2: Plan of Anshan Road station.  
Figure 4: Crushed area.  
Figure 3: Geological profile.