# Research on Bidirectional Reservation Method for Anti-Permeation Geomembrane Slack
## 2 Geomembrane Slack Methods  
### Introduction of Existing Methods  
To date, there is relatively little information on geomembrane slack reservation methods. In this paper, three existing geomembrane slack reservation methods are compiled based on existing data and combined with actual engineering experience. These techniques are wave-shaped [22], groove-shaped, and Z-shaped [19] slack reservation methods. As shown in Figure 1, schematic diagrams of the reservoir profile under the three geomembrane slack allowance methods are shown.  
1. Wave-shaped slack-laying method. When laying geomembranes at construction sites, geomembranes are designed to undulate, as shown in Figure 1a, similar to waves that form during water surface fluctuation. In this process, the amount of raised geomembrane is equivalent to the amount of slack reserved. Although the wave-shaped slack-laying method shown in Figure 1a requires no special structures, relatively little work, and easy operation strategies, the natural slack unfolded during laying without tensioning, which forms a natural wave, produces folds when subjected to overlying water load and backfill pressure. These folds reduce the service life of the geomembrane [23; 24; 25; 26].
2. Concentrated groove-shaped slack-laying method. Figure 1b shows that the concentrated groove-shaped method involves digging grooves at a certain distance before geomembrane laying. When laying the geomembrane, the reserved part is laid in the grooves dug beforehand. However, when the geomembrane is strained, the groove-shaped centralized laid geomembrane unfolds, making the lower part of the geomembrane in the grooves partially hollow and vulnerable to damage or destruction by water load pressure. Additionally, groove excavation increases the amount of earthwork and makes the gullies in the reservoir area not conducive to geomembrane laying or engineering construction.
3. Concentrated Z-shaped slack-laying method. Figure 1c shows the concentrated Z-shaped slack-laying method for reservation, in which the geomembrane is folded and laid at intervals, and the overall appearance is Z-shaped. After the reservoir is utilized, when the geomembrane is deformed due to certain factors, such as uneven foundation settlement and temperature changes, the Z-shaped geomembrane is separated. To prevent the geomembrane from folding, neoprene rods are added to the geomembrane bend. Since the reservation is Z-shaped, when the geomembrane deformation is pulled, one layer unfolds, meeting the requirements for reserving a geomembrane.  
The abovementioned methods of laying geomembrane slack are unidirectional allowances, which serve as a reserve against tensile stresses in the direction of the allowance. However, when the geomembrane is subjected to tensile stresses from directions apart from the reserved direction, the reserved slack cannot protect the geomembrane, and the geomembrane may have safety problems caused by tensile stresses. Theoretically, the amount of geomembrane slack laid in plain reservoirs should be bidirectional or even multidirectional to solve the geomembrane stresses caused by stretching in all directions. However, as shown above, the existing reservation methods are unidirectional, and no bidi  
Figure 1: Diagram of existing slack allowance methods. (**a**) Wavy slack laying, (**b**) groove slack laying, (**c**) Z slack laying.  
rectional (multidirectional) reservation methods have been found in existing research and practical applications, which greatly affects the safety of using geomembranes. Therefore, there is a need to improve the existing geomembrane slack reservation methods and to propose a new bidirectional reservation method to make the reserved geomembrane slack available in different directions for practical applications.