## 5 Conclusions  
This study proposed a geomembrane surface barrier for a high rockfill dam on deep overburden. The proposed geomembrane surface barrier uses polyurethane bonded aggregates as the cushion material. The mechanical and deformation property of the barrier under uniform water pressure was investigated using a self-developed structure model test device. The mechanical and deformation performances of each layer of the structure and the interaction mode between adjacent layers were obtained by external videos and internal sensor monitoring. The following conclusions were drawn:  
1. Under the maximum uniform water pressure of 0.708 MPa, the cushion layer of polyurethane bonded aggregates showed good flexibility during the entire process of structural deformation. The cushion layer maintained good contact and coordinate deformation with the upper protective layer and the lower transition layer without discontinuous deformation (e.g., cracking, detachment, and dislocation). This confirmed that the polyurethane bonded aggregates are suitable as the cushion material for a geomembrane surface barrier for a high rockfill dam on deep overburden.
2. The protective layer cracked when the relative deflection at the bottom of the model reached 1/70. However, because the composite geomembrane at the bottom of the protective layer was subject to tensile load, the crack stopped expanding with the increasing load, and no through crack was formed. The protective layer of concrete was cast in situ on the composite geomembrane. The cement slurry completely infiltrated into the geotextile on the surface of the composite geomembrane during the setting process. This formed a good bonding interface between the protective layer and the composite geomembrane. There was no obvious dislocation or sheer  
Figure 15: Final cracks of the protective layer of concrete.  
displacement between the protective layer and the composite geomembrane during the process of flexural deformation of the entire structure.
3. There was sheer displacement between the composite geomembrane and the cushion layer of polyurethane bonded aggregates during the flexural deformation of the whole model. The sheer force between the protective layer and the cushion layer was transferred to the composite geomembrane, which made the composite geomembrane bear tensile stress and produced tensile strain. However, during the loading process of the model, the strain of the composite geomembrane was always very small. When the protective layer of the concrete cracked, the maximum tensile strain of the composite geomembrane was only 295 \(\mu\varepsilon\). This low maximum tensile strain indicated that the surface barrier on the dam surface could ensure the safety of the composite geomembrane in the event of flexural deformation of the dam.  
Writing--original draft preparation, H.W.; writing--review and editing, L.F., Z.T. and Y.S.; All authors have read and agreed to the published version of the manuscript.  
The research was supported by the National Natural Science Foundation of China (No. 51409083 and 51379069).  
Not applicable.  
The authors would like to thank the anonymous referees whose comments helped us improve the presentation of this paper.  
The authors declare no conflict of interest.