## 3 Materials and Methods  
### Sample Preparation  
The granite residual soils tested in the present study were obtained from underground tunnel excavation interval areas in Xiamen city (latitude 24 deg26' N, longitude 118 deg04' E). In consideration of the disturbing characteristic of granite residual soils, undisturbed samples were used for testing. The basic parameters of residual soils were measured in laboratory and given in Table 1. The density, water content, liquid limit, plastic limit and plasticity index of the soil were measured to be 1.98 g/cm\({}^{3}\), 58.6%, 28.9% and 29.7%, respectively. According to XRD diffraction pattern measurements performed using an AXS D8-Focus X-Ray Diffraction instrument produced by Bruker Company from Germany, it is found that the soil is largely composed of clay minerals (e.g., kaolinite and illite) as well as non-clay minerals (e.g., quartz and a trace of hematite). Figure 1 shows the grain compositions of residual soils which were measured in the laboratory by referring to the standard ASTM-D422, 2007 [37]. As an intermediate soil between coarse-grained and fine-grained soils, the granite residual soil contains high proportions of clay and gravel particles in its particle size distribution. By conducting the permeability and porosity measurement using the KXD-II combined tester, the porosity and permeability of undisturbed residual soils were ascertained.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \begin{tabular}{c} **Density** \\ **(g/cm\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} **Moisture** \\ **Content** \(w\)** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Porosity** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Permeability** \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Afterberg Limits (\%)** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Mineral Composition (\%)** \\ **(\%)** \\ \end{tabular} \\ \hline \(1.98\) & 26.03 & 0.3301 & 0.942 & 58.6 & 28.9 & 29.7 & 53.6 & 5.2 & 38.5 & 2.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic parameters of the tested residual soils.  
During artificial freezing process, the cooling effect kept the soils around tunnel pipes at low-temperatures. The temperature dropped as the distance between pipes and soils decreased as shown in Figure 2. Generally, the freezing temperature at the surface tunnel pipe was controlled around \(-30\)\({}^{\circ}\)C. Namely, along the perpendicular direction to the tunnel axis, which is the heat conducting direction, the soil varying to its location encountered a freezing and thawing cycle under a certain low-temperature [38]. In order to simulate the freezing circumstance of the frozen-wall by laboratory experiments, it was simplified into four low-temperature groups of \(-5\)\({}^{\circ}\)C, \(-10\)\({}^{\circ}\)C, \(-20\)\({}^{\circ}\)C and \(-30\)\({}^{\circ}\)C, in consideration of the freezing effects on microstructures in residual soils in this experimental investigation. Before the experiments, four groups of cylinder samples from undisturbed residual soils (38 mm in diameter and 80 mm in height, respectively) were prepared. Then, the tested sample was frozen up to \(-5\)\({}^{\circ}\)C, \(-10\)\({}^{\circ}\)C, \(-20\)\({}^{\circ}\)C and \(-30\)\({}^{\circ}\)C respectively and kept frozen for 48 h in a programmable cryogenic freezer. Subsequently, the frozen sample with different temperature was defrosted at a room temperature of \(25\)\({}^{\circ}\)C for 24 h and fully saturated by using a vacuum saturation device for 72 h.