# Application of a Geotextile in the Treatment of Post-Subsidence in Karst Areas
## 2 Research Methodology  
### Large-Scale Model Test  
To evaluate the influence of the anchorage length and buried depth of a given geotextile on the treatment of subgrade subsidence in karst areas, large-scale model tests have been conducted by [PERSON] et al. [31; 32]. The sinkhole prototype used in this work was that used for a project regarding the treatment of subgrade subsidence at Xihuan Road in Hechi City, Guangxi, China. The dimensions of the subsidence were 1.5 m in length, 1.2 m in width, and 4 m in depth. The site consisted of sandy soil. A large model test box with an effective volume of 1.5 m \(\times\) 0.6 m \(\times\) 1.5 m (length \(\times\) width \(\times\) depth) was used in the model tests. It consisted of a main frame made of channel steel and a front side made of tempered glass with a thickness of 25 mm to observe the soil deformation by using particle image velocimetry (PIV) equipment. The similarity ratio of the scale model was 1:5, corresponding to a scaling factor \(N\) of 5.0. The physical quantities of the model and prototype would satisfy Equation (1) based on the similarity analysis given by [PERSON] et al. [33]. \[I_{p}=NI_{m} \tag{1}\]  
where \(I_{p}\) denotes the physical quantities of the prototype, \(I_{m}\) denotes the physical quantities of the model, and \(N\) is the scaling factor. Table 1 lists the common scaling factors in the scale model test. The soil used in the model test was sand from Li River in Guilin and had a density of 1.68 g/cm\({}^{3}\), a moisture content of 1.03%, a cohesion value of 0.2 kPa, and an internal friction angle of 32.8\({}^{\circ}\). It was suggested by [PERSON] et al. [34] that gauze is a reasonable substitute for a geotextile prototype according to a similarity criterion. The medical gauze considered had a tensile strength of 3.22 kN/m and a tensile stiffness of 48 kN/m. The thickness of the medical gauze was 0.16 mm and the weight per unit area was 60 g/m\({}^{2}\). Due to the similar values of property parameters with a prototype geotextile according to the similarity factors (Table 1), the medical gauze was chosen to simulate a prototype geotextile in the large-scale model tests. Table 2 lists the large-scale model tests conducted by [PERSON] et al. [31; 32]. The subsidence width \(B\) was set as 300 mm for the model test according to the dimension of the prototype and the scaling factor \(N\) was equal to 5.0. Tests T1, T2, T3, and T4 represent tests with anchorage lengths for the geotextile (denoted as \(L\)) of 600 mm, 300 mm, 150 mm, and 75 mm, respectively, which were conducted to study the effect of the anchorage length of the geotextile on the treatment of subsidence [31]. The anchorage ratio \(L/B\) in Table 2 is the ratio of the anchorage length of geotextile \(L\) to the subsidence width \(B\). Test T5 was a parallel test conducted without a geotextile, and thus the anchorage ratio \(L/B\) equals 0. By comparing test T5 with T1 to T4, the effect of the use of a geotextile was evaluated. Comparing with test T1, test T6 was conducted with a buried depth of 500 mm to study the effect of the buried depth of the geotextile on the treatment of subsidence, as shown in Table 2 [32]. \begin{table}
\begin{tabular}{c c c} \hline
**Quantity** & **Unit** & **Scaling Factor (\(I_{p}\):\(I_{m}\))** \\ \hline Length & m & \(N\) \\ Force & N & \(N^{2}\) \\ Mass & kg & \(N^{3}\) \\ Density & kg/m\({}^{3}\) & 1 \\ Stress & Pa & 1 \\ Strain & \% & 1 \\ Tensile load & N/m & \(N\) \\ Secant stiffness & N/m & \(N\) \\ \hline \end{tabular}
\end{table} 
Table 1: Scaling factors in the scale model test. Figure 1 shows the model size (unit: mm) and layout of instruments including six earth pressure cells and six strain gauges in every model, and more details could be found in the work by [PERSON] et al. [31,32]. Soil pressure cells T1, T2, T3, T4, T5 and T6 were placed at the distance of 0 mm, 100 mm, 200 mm, 300 mm, 500 mm and 700 mm, respectively from the center of the subsidence area, which were used to measure the soil pressure beneath the geotextile.