# Prediction of Ground Surface Settlements Induced by EPB Shield Tunneling in Water-Rich Soft Strata
## 2 Project Overview
### Geological and Geotechnical Conditions  
The water-rich soft deposits of Changzhou were formed during the Pleistocene and Holocene epochs under the scouring of the Yangtze River and Taihu Lake. Figure 2 depicts the geological formation, soil types, and hydrogeology of the top 80.5 m deposit, which is the highest depth reached by human building operations. The deposit is a multiaquifera-aquitard system, as shown in Figure 2, with silty soil, clayey soil, and sandy soil overlapping. The deposit up to a depth of 80.5 m contains 10 geotechnical soil layers according to the soil properties and grain size distribution (labeled in circled numbers from 1 to 1 in Figure 2). Each layer is further subdivided into sublayers on the basis of the color of the soil, for a total of 31 sublayers. The buried depth of each layer varies depending on its location.  
The geological and hydrogeological formation of each layer within the surveyed depth is described in the following. The top deposit is a top crust layer composed of silty clay, which is a phreatic aquifer with a groundwater level of 0.5-2.8 m below the surface. Following is a soft clay layer (the first soft clay layer). The top crust layer and the first soft clay layer were formed in the Holocene period. Next is a stiff clay layer (the first stiff clay layer). Underlying the first stiff clay layer is the first sand layer, which is mainly composed of silt and silty sand. Most normal metro tunnels are built in the first sand layer, and it is a low-pressure artesian aquifer (I1) with a groundwater level of 3.15 to 5.45 m. The following layer is the second soft clay layer, which is a saturated and plastic layer with a high clay concentration. The second soft clay layer is the softest layer in Changzhou, where many below river crossing tunnels are built and are very sensitive to disturbance. The next layer is the second stiff clay layer, in which some below the river crossing tunnels are also built. The following are the third soft clay layer and the second sand layer, which are the deepest excavation positions for the EPB shield machine. The second sand layer contains a silt sublayer (8)\({}_{1}\)) and a silty sand sublayer (8)\({}_{2}\)). They are the second low-pressure artesian  
Figure 2: Geology and hydrogeology in Changzhou.  
aquire (I2), with a groundwater level from 4.23 to 8.13 m. Top to bottom, layers from the first stiff clay to the third soft clay layer were formed in the upper Pleistocene period, and they are the densely distributed areas of the metro lines. Deeper areas include several similar layers, such as soft clay layers, sand layers, and stiff clay layers, which were formed in the middle Pleistocene period, and they are usually located deeper than metro tunnels.  
Figure 3 shows the geotechnical parameters of the water-rich soft strata in Changzhou. The physical parameters, including water content, unit weight, void ratio, plasticity index, and liquidity index, were determined using laboratory tests conducted in accordance with the Chinese standard [15], which is identical to the British Standard [16]. The compression index was determined using a normal oedometer test. Layers 5 and 8 are classed as sandy layers by the Unified Soil Classification because of their high sand content. The other layers with a significant silt concentration are classed as the clay layer. The permeability of clayey soils was determined using oedometer testing, and the permeability of the sandy soil was determined using pumping testing.