## 2 A Geomembrane Surface Barrier for a High Rockfill Dam  
This study proposes a new type of geomembrane surface barrier that employs polyurethane bonded aggregates as the cushion material for high rockfill dams on deep overburden. Figure 1 show the specific structure composition and materials.  
As shown in Figure 1, the geomembrane surface barrier for a high rockfill dam is primarily composed of four layers: transition layer, cushion layer, composite geomembrane, and protective layer.  
1. Transition Layer: The transition layer is composed of coarse aggregates. The coarse aggregates of this new structure are filled and rolled to a thickness of 2-3 m, which is similar to the transition layer of a concrete faced rockfill dam. The particle size should match the lower rockfill and the upper cushion to meet the requirements of permeability and filtration. During construction, the transition layer and the dam rockfill are simultaneously filled and rolled into layers.
2. Cushion layer: The cushion layer is composed of situ-cast polyurethane bonded aggregates with a thickness of 6-10 cm and an aggregate particle size of 5-20 mm. After the rockfill and transition layer are filled, the polyurethane bonded aggregates are directly poured onto the surface of the transition layer. Then, it is formed after rolling and curing for two days. As the cushion adopts the cast-in-place construction method, it can be bonded with the crushed stone on the surface of the transition layer before the final setting. This method serves to maintain the stability of the cushion.
3. Composite geomembrane: The composite geomembrane is a sandwich structure comprising a geomembrane (with a thickness of 0.5-2.5 mm) layer laminated to one layer of needle-punched filament geotextile (with Mass per unit area of 250-500 g/m\({}^{2}\)) on both sides. After two days of curing of cushion layer, the composite geomembrane is directly laid onto the surface of the polyurethane bonded aggregate cushion. The sheer strength of the interface between the filament geotextile on the surface of the composite geomembrane and polyurethane bonded aggregate cushion maintains its stability [33].
4. Protective layer: The protective layer is composed of situ-cast plain concrete and has a thickness of 20-30 cm. The protective layer uses joint or fiber reinforcement to prevent cracking with the deformation of the dam body. Before the final setting of the situ-cast concrete, the cement slurry penetrates the fluffy filament geotextile on the surface of the composite membrane. This forms good adhesion with the composite membrane after the final setting.  
Figure 1: The proposed geomembrane surface barrier for a high rockfill dam ((**a**) protective layer; (**b**) composite geomembrane; (**c**) cushion layer; (**d**) transition layer; and (**e**) rockfill).  
In summary, the proposed geomembrane surface barrier for a high rockfill dam has the following advantages: (1) the cushion layer will not have large cracks and dislocations under large deformations and will not damage the geomembrane; (2) it can adapt to the deformation of the rockfill dam body on a deep overburden; (3) it can be applied to a rockfill dam with a steep slope; and (4) the structure is simple and easy to construct. Based on the above advantages, the geomembrane surface barrier may be successfully applied to a high rockfill dam on a deep overburden.