# Research on Bidirectional Reservation Method for Anti-Permeation Geomembrane Slack
## 3 Analysis and Comparison of Different Methods
### \"Z\" Cross-Type Two-Way Centralized Lay-Up Relaxation Amount Method Analysis  
The geomembrane is cross-folded in both directions, as shown in Figure 7. The long red tube under the membrane is a substitute for the neoprene rod, which is softer than the neoprene rod and makes it easier to realize the second Z fold. For the convenience of description, the two directions of folding are the X and Y directions, as shown in Figure 7a,b. The specific folding steps are as follows: (1) take the line where the Y-axis is located as the crease for the first zigzag fold and (2) take the line where the X-axis is located as the crease for the second zigzag fold. The final formation of the Z crossover geomembrane, as shown in Figure 7b, is a bidirectional reservation method.  
The key to the bidirectional Z-cross-shaped geomembrane reservation method is if and how easily the folded part can be separated when subjected to external stresses. To explore the effectiveness of the geomembrane bidirectional cross-Z-shaped reservation method, a bidirectional tensile test is conducted, as shown in Figure 7c,d.  
The test results show that the geomembrane can be separated well after cross-Z-shaped folding and that the whole geomembrane is relatively flat after being separated. To carry out the test effectively, the neoprene rubber gang that prevents the geomembrane from breaking is replaced with a PE hose to achieve a 180-bend effect when the geomembrane is folded for a second time. However, the neoprene rods used in practice do not achieve a  
Figure 6: Test results of Program II. (**a**) Preliminary determination of wiping method, (**b**) geomembrane-laying effect after preliminary wiping, (**c**) final cross-recess shape, (**d**) final geomembrane-laying effect, (**e**) slope foot curvature measurement, (**f**) diagonal two-arc slope connecting length measurement.  
180-degree bend. In addition, once the geomembrane slack is separated, the geomembrane cannot be restored to its original state once the force is removed. Moreover, the slackened part of the geomembrane may form folds under pressure, thus shortening the service life of the geomembrane.  
Analysis of \"Z\" and Groove Combination-Type Two-Way Pre-Set Concentration Laying Slack Amount Method  
Both the cross-Z-shaped and cross-groove bidirectional reservation methods have shortcomings, among which the cross-Z-shaped technique cannot achieve 180-degree bending when folding the second time. Once the slack in the geomembrane is separated when force is applied, the geomembrane cannot be restored to its original shape when the force is removed. The cross-groove method requires a large amount of excavation, increasing the civil construction cost to a certain extent. Due to the disadvantages of the cross-groove and bidirectional cross-Z-shaped reservation methods, the two methods were integrated, and a bidirectional and concentrated combined Z- and groove-shaped laying reservation method is proposed.  
To explore the effects of the bidirectional reservation of the combined Z- and groove-shaped concentrated laying reservation method, an outdoor laying test is conducted, as shown in Figure 8. The test is conducted by excavating a U groove that is 2 m in length, 0.15 m in width, and 0.15 m in depth and by laying a piece of HDPE geomembrane that is 2 \(\times\) 2 m in size and 0.8 mm in thickness in a unidirectional Z fold, as shown in Figure 8.  
Figure 7: Test results of “Z” cross-type two-way lay. (**a**) One-way fold, (**b**) two-way fold, (**c**) stretching process, (**d**) stretching completed figure.  
As shown in Figure 8, the bidirectional reservation method of combined Z- and groove-shaped geomembrane slack can avoid wrinkles in the cross-groove-shaped slack and neoprene rods that cannot complete 180-degree bending in the cross-Z-shaped slack. Moreover, since this bidirectional reservation method requires only one groove for excavation, the civil construction volume is reduced by half relative to the cross-groove shape.