## 2 Project Background
### Geology and Soil Parameters  
The shield section of Line 7 primarily traverses soft soil layers, including powdered sand, silt with sandy silt, and sandy silt. The strata separating the new and existing tunnels consist of silt and sandy silt. The geotechnical investigation report for Hangzhou Metro Line 7 provided the fundamental and model parameters of soil mass, which were derived from a combination of indoor soil tests, in situ tests, and hydrogeological tests.  
The first layer was miscellaneous fill, which was loose and dry\(\sim\)slightly wet. This layer consisted primarily of clay and gravel and had a thickness of approximately 3 m. The second layer consisted of powdered sand that was very wet and slightly dense-medium-dense, with a small amount of thin layer of viscous soil with a thickness of 8.4 m. This was followed by silty sand with sandy silt, a saturated, medium-dense state that was locally slightly dense. This soil composition contained mica debris and, occasionally, clay clumps with a thickness of approximately 4.0 m. The fourth layer consisted of sandy silt, which was wet, slightly dense, and 3.4-m thick, with low dry strength and toughness, and contained mica debris. The fifth layer was a silty clay layer with high dry strength and toughness, localized inclusions of silty soil, and a thickness of approximately 4.3 m. The sixth layer was a clay layer, primarily made of soft plastic and local fluid plastic, with high dry strength and medium toughness, containing organic matter with a thickness of approximately 4.8 m. Layer 7, with a thickness of 16.5 m, was puddingstone in a saturated and dense state, containing a gravel content of roughly 60% to 70% with good roundness and a clay content generally less than 5%. Layer 8 consisted of 3.1 m of strongly weathered rock, featuring an argillaceous structure and a thickly layered structure. The integrity of most organizational structures was compromised, with an incomplete core, developed cracks, soft rock, and local inclusions of weathered rock blocks. Layer 9 was the final layer, which was a 12.5 m moderately weathered rock. It had an argillaceous structure with relatively developed cracks, primarily composed of quartz, feldspar, and rock debris. The cementing material was the parent clay of the water cloud with minor iron content, resulting in soft rock.  
The analysis of the natural soil weight revealed that the miscellaneous fill soil layer was 17.5 KN/m, while the soil weight of the other layers was greater than 19 KN/m.  
Figure 2: Planned tunnel view of the project.  
Notably, the strongly weathered rock layer and moderately weathered rock layer were particularly dense, each with a unit weight of 25 KN/m. Regarding soil cohesion, the upper four soil layers exhibited relatively low cohesion, all under 10 Kpa. In contrast, the layers below the transition from sandy soil resulted in a significant increase in cohesion, ranging from 25 to 28 Kpa. The boulder layer exhibited the lowest cohesion, while the weathered rock had the highest cohesion, with values ranging from 50 to 100 kPa. The inner friction angles of the second to fourth layers, as well as the seventh to ninth layers, were relatively high, ranging from 28\({}^{\circ}\) to 42\({}^{\circ}\). In contrast, the internal friction angles of the remaining layers did not exceed 20\({}^{\circ}\), a distinction primarily attributable to the differences between sandy soil and cohesive soil, as shown in Table 1.