HDPE geomembrane, a widely used seepage control material in geotechnical engineering, boasts excellent chemical stability, low permeability, and good mechanical properties. However, during long-term service, the stress relaxation characteristics in the soil and rock media significantly affect the durability of its seepage control effect. Stress relaxation refers to the phenomenon where the stress in a material gradually decreases over time under constant strain. This process is closely related to the molecular structure of high-density polyethylene, environmental conditions, and the interaction with the soil and rock media.
The molecular structure of HDPE geomembrane is characterized by the coexistence of crystalline and amorphous regions. The crystalline regions provide strength and rigidity, while the amorphous regions impart a certain degree of flexibility. In soil and rock media, the geomembrane must withstand complex stresses such as overburden loads, uneven foundation settlement, and temperature changes. Initially, the stress is mainly borne by the crystalline regions, but over time, the molecular chains in the amorphous regions gradually slip and rearrange, causing stress to shift to the amorphous regions. This process manifests as macroscopic stress relaxation, resulting in a gradual decrease in the stress level of the geomembrane. If stress relaxation is too rapid or excessive, it may lead to insufficient contact pressure between the geomembrane and the soil/rock medium, resulting in interfacial debonding or localized deformation, creating a potential breeding ground for seepage channels.
The properties of the soil/rock medium significantly influence the stress relaxation behavior of HDPE geomembrane. For example, the creep characteristics of soft soil foundations exacerbate stress relaxation of the geomembrane because the slow deformation of the soil continuously alters the stress state of the geomembrane. Conversely, a firm rock foundation provides stronger constraint on the geomembrane, slowing down the stress relaxation rate. Furthermore, changes in pore water pressure within the soil/rock medium also affect the stress distribution of the geomembrane through infiltration. Under prolonged high water head, the geomembrane may undergo creep due to the continuous action of water pressure, further accelerating the stress relaxation process.
Temperature fluctuations are another key factor affecting the stress relaxation of HDPE geomembrane. As a polymer material, high-density polyethylene is temperature-sensitive; the mobility of its molecular chains increases with increasing temperature. In high-temperature environments, the molecular chains in amorphous regions are more prone to slippage, leading to a faster stress relaxation rate. Under low-temperature conditions, the movement of molecular chains is hindered, slowing down the stress relaxation process. However, extreme low temperatures can lead to material embrittlement, reducing its puncture and tear resistance, thus indirectly affecting its seepage prevention effect. Therefore, in cold regions or seasonally frozen soil areas, special attention needs to be paid to the low-temperature performance of geomembranes and its coupling effect with stress relaxation.
The impact of stress relaxation on the long-term seepage prevention effect of HDPE geomembrane is mainly reflected in two aspects. First, stress relaxation may lead to increased tensile deformation of the geomembrane, especially in areas of localized stress concentration. Excessive deformation may exceed the elastic limit of the material, causing permanent plastic deformation, or even leading to membrane rupture. Second, stress relaxation weakens the friction between the geomembrane and the soil medium, reducing interface stability. In scenarios such as slopes or landfill slopes, interface debonding may cause geomembrane sliding or misalignment, forming seepage loopholes.
To mitigate stress relaxation of HDPE geomembrane and ensure long-term seepage prevention, a comprehensive approach is needed, encompassing material selection, design optimization, and construction control. At the material level, creep resistance can be improved by adjusting molecular weight distribution or adding nanofillers. At the design level, the thickness and width of the geomembrane should be rationally determined to avoid localized stress concentration. At the construction level, the flatness of the base layer must be strictly controlled to reduce damage to the membrane from sharp objects, and appropriate protective layers should be used to distribute external loads.
The stress relaxation characteristics of HDPE geomembrane in soil and rock media are a crucial factor influencing its long-term seepage prevention performance. A deeper understanding of the molecular mechanisms, environmental dependence, and engineering effects of stress relaxation can provide more scientific guidance for the design, construction, and maintenance of geomembranes, thereby providing reliable and durable seepage barriers for geotechnical engineering.
