# Application of a Geotextile in the Treatment of Post-Subsidence in Karst Areas
## 4 Results and Discussion
### Effects of Buried Depth of Geotextile
#### 4.2.3 Soil Displacement  
Figure 16 presents vertical displacement contours of the soil for different buried depths of the geotextile when the displacement of the subsided soil beneath the geotextile was 100 mm. The displacement legend is shown at the right side of each displacement contour. Darker colors can be seen for the semi-elliptical displacement contours when shallow buried depth values for the geotextile (e.g., \(H\) = 2 m) (Figure 16a) are used, thus representing great soil displacement and great settlement on surface. With the increase of the buried depth of the geotextile (e.g., \(H\) = 3 m, 4 m, 5 m, and 6 m) (Figure 16b-e), greater displacement develops towards the stable area and smaller surface displacement occurs. For example, the displacement developing towards the stable area under the buried depth of geotextile of \(H\) = 6 m was greater than that of \(H\) = 4 m. The surface displacement under \(H\) = 6 m was smaller than that of \(H\) = 4 m. A deeper buried depth of the geotextile has a greater effect on the treatment of subsidence. Figure 17 shows the surface settlement for different buried depths of geotextile \(H\) at the final state of subsidence. It decreases with the increase of buried depth of geotextile. In the subsidence area, the surface settlement was greater for buried depths of 2 m and 3 m as compared to deeper buried depths \(H\) (e.g., \(H\) = 4 m, 5 m, and 6 m). The area of deformation at the surface became larger, indicating that the surface deformation tends to be uniform with the increase of buried depth of the geotextile. To quantify the decrement on surface settlement with the increase of buried depth of geotextile, decrement ratio \(\beta\) was proposed as Equation (4), which represents the decrement of surface settlement under different buried depth \(H\) as compared to \(H\) = 2 m.  
\[\beta\ =\ \frac{D-d}{d}\times 100\%, \tag{4}\]  
where \(\beta\) is the decrement of surface settlement under buried depth \(H\) as compared to \(H\) = 2 m as a percentage, \(D\) is the surface settlement under buried depth \(H\) in mm, and \(d\) is the surface settlement under buried depth \(H\) = 2 m in mm. Figure 18 shows the decrement ratio of surface settlement under buried depth \(H\) as calculated by Equation (4). It shows that the decrement ratio of surface settlement is greater with the improvement of the buried depth of the geotextile, indicating that a greater buried depth contributes to reducing surface settlement. For example, the decrement ratios \(\beta\) are 29.14%, 48.19%, 57.98%, and 65.91% under buried depth values of 3 m, 4 m, 5 m and 6 m, respectively, meaning that the surface settlements under the buried depth \(H\) of 3 m, 4 m, 5 m and 6 m are reduced by 29.14%, 48.19%, 57.98% and 65.91%, respectively, when compared to \(H\) = 2 m.  
Figure 15: Vertical deflection of the geotextile in the subsidence area for different buried depths at the final state of subsidence. The differential surface deformations \(d_{s}/D_{s}\) are 0.72%, 0.41%, 0.19%, 0.15% and 0.13% for the conditions of \(H=2\) m, 3 m, 4 m, 5 m, and 6 m, which are smaller than the specified value of \(d_{s}/D_{s}\) of 1% according to BS 8006 [35]. Consequently, the application of a geotextile has a great effect on the treatment of subsidence and differential surface deformation is mitigated with the use of a deeper buried depth for the geotextile. Figure 16: Vertical displacement contours of the soil for different buried depths of the geotextile (\(H\)): (**a**) \(H=2\) m; (**b**) \(H=3\) m; (**c**) \(H=4\) m; (**d**) \(H=5\) m; (**e**) \(H=6\) m.  
To determine an appropriate depth for the geotextile, the minimum anchorage length of the geotextile, ensuring the separation of the geotextile from the subsided soil beneath the geotextile, was introduced.