# Study on Shielding Effect of the Pile Group in a Soft-Soil Foundation
## 2 Materials and Methodology  
### Samples  
The study is based on a high-speed rail project in Taicang, a city in eastern China on the alluvial plains of the lower Yangtze River, where the terrain is flat and open, and the foundation of the site is relatively homogeneous silty clay with a thickness of more than 30 m. To simulate the mechanical behavior of the soft soil in FLAC\({}^{\text{3D}}\), a modified Cam-Claymodel built into the software was employed in the finite element analysis, as this model has been validated to be suitable for describing the pile-soil interaction in soft-soil areas, and a limited number of input parameters are required [38,39]. Since the soil of the site is relatively homogeneous, the foundation is simplified into a homogeneous soft soil in the model in order to more intuitively analyze the disturbance mechanism of pile installation, and the parameters of the foundation soil are obtained through the following experiments.  
Natural density, natural moisture content, and specific gravity were measured using the ring knife method, drying method and specific gravity bottle method, and the initial porosity ratio was converted via the above three indexes.  
The critical failure ratio (\(M\)) is the slope of the critical state line, which is related to the effective stress friction angle of the Mohr-Coulomb yield function. To obtain the effective stress friction angle, a series of triaxial shear tests were performed using the TSY10-1.0 strain-controlled triaxial apparatus (Figure 1a), which was manufactured by Jiangsu Yongchang Science and Education Instrument Manufacturing Co., Ltd. in Yongchang, China. The testing system comprises a pressure chamber, axial compression equipment, a surrounding pressure system, back pressure system, pore water pressure force measurement system, as well as an axial deformation and volume change measurement system. During the experiment, four soil samples with dimensions of \(\Phi\) 391 \(\times\) 800 mm were saturated using the vacuum saturation method. Confining pressures of 100, 200, 300, and 400 kPa were, respectively, applied to the specimens, while the shear rate was set at 0.0064 mm/min. The confining pressure was modulated through a hydraulic servo controller, and axial deformations were automatically collected. The experiment was concluded upon reaching an axial strain of 20%.  
The compression index (\(C_{c}\)) and rebound index (\(C_{s}\)) of soil are the other two essential parameters that characterize soil deformation. They were successfully obtained from the K\({}_{0}\) consolidation compression-unloading test conducted on the soil sample. The test was performed on a sample with a diameter of 6.18 cm and a height of 4 cm, following the vacuum saturation method. The GJY-type K\({}_{0}\) consolidation apparatus, manufactured by Nanjing Ningxi Soil Instrument Co., Ltd. in Nanjing, China, was used for this experiment, as shown in Figure 1b. All air bubbles were eliminated from the sealed pressure chamber and pressure measurement system before the test was started. The water pressure inside the sealed pressure chamber was not allowed to exceed 3 kPa. During the test, lateral pressure was exerted through the water pressure in the enclosed compression chamber. Vertical loads of 12.5, 25, 50, 100, 200, 300, 400, 300, 200, 100, 50, 25, 12.5 and 0 kPa were sequentially applied to the top surface of the sample using calibrated weights, and the vertical displacements were measured using displacement sensors. The duration of each load level was governed by a vertical deformation rate of at least 0.01 mm per hour.  
Figure 1: (**a**) Triaxial test system; (**b**) K\({}_{0}\) consolidation meter.  
The reference porosity ratio (\(e_{1}\)) is the porosity ratio on the NCL line when the pressure applied is 1 kPa. The over-consolidation ratio (\(R\)) is set to 2 through the survey data of the site. Therefore, a summary of the soil model parameters is provided in Table 1.