# Research on Bidirectional Reservation Method for Anti-Permeation Geomembrane Slack
###### Abstract  
Geomembrane relaxation can effectively prevent the geomembrane from being damaged by tensile stresses caused by temperature changes and uneven settlement of the foundation. Existing and commonly used reservation methods, such as wave-shaped, groove-shaped, and Z-shaped techniques, are unidirectional and cannot resist multidirectional tensile stresses in geomembranes. Therefore, we propose three methods for reserving bidirectional geomembranes, including the cross-groove-shaped, cross-Z-shaped, and combined Z- and groove-shaped techniques. Additionally, we investigate the key issues of these methods and their practicality through field tests and theoretical analyses. The results of the study show that the cross-groove-shaped technique is prone to geomembrane folding at the corners of the cross-intersection, and it is necessary to set a wiping angle of approximately 35\({}^{\circ}\) to solve the problem. The cross-Z-shaped technique does not require grooving and reduces the amount of civil construction performed. However, the neoprene rods cannot be folded 180\({}^{\circ}\) or restored after being separated once. The combined Z- and groove-shaped technique has no folding phenomenon and does not require 180\({}^{\circ}\) of folding. With a lack of folding, including 180\({}^{\circ}\) folding, the number of grooves can be reduced by half, but the cross-Z-shaped specimen cannot be restored after separation. In practical applications, it is recommended to use the cross-groove-shaped method or to choose a suitable bidirectional reservation method according to the actual requirements. geomembrane; slackness; bidirectional reservation; groove-shaped; Z-shaped +
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of the foundation under the membrane causes tensile deformation of the geomembrane. When the uneven settlement of the foundation is severe, the geomembrane faces the risk of stretching and fracture. For example, without setting the amount of relaxation, the geomembrane face barrier bedding layer of a high rockfill dam on a deep cover is susceptible to cracking and damage due to excessive bending deformation [13]. The temperature stresses in HDPE geomembranes vary with the ambient temperature and, when the ambient temperature decreases, the modulus of elasticity and shrinkage values of HDPE geomembranes sharply increase, resulting in temperature stress [14; 15]. The settlement of the soil under the geomembrane causes large geomembrane folds, and the maximum strain of the geomembrane increases as the settlement of the soil under the geomembrane increases [16]. When the temperature stress and tensile stress are large due to uneven settlement of the foundation, tearing damage of the geomembrane and leakage of the reservoir water may occur, thus affecting the overall safety of the reservoir. To avoid this phenomenon, the relevant specifications stipulate that a proper amount of slack should be left when laying the geomembrane [17]. Based on actual engineering experience, the slack allowance should be controlled between 1% and 1.5% [18; 19], and the construction details of the project stipulate that the slack expansion joints reserved for geomembranes during laying should be set every 100 m [20; 21]. The existing geomembrane slack-laying methods mainly include wave-shaped [22], Z-shaped [19], and groove-shaped techniques, these slack reservation methods are described in Section 2.1 below. These existing geomembrane slack reservation methods can only address the tensile stress from a fixed direction when the geomembrane is subjected to a fixed direction, while the direction of tensile stress due to environmental factors is random. Thus, the existing geomembrane slack reservation methods cannot resolve tensile stresses originating from different directions.