## 4 Results and Discussion
### Deformation and Cracking of the Protective Layer  
Figure 13 show the pasting position and number of strain gauges on the surface of the protective layer of concrete. No data were collected at the No. 6 measurement point because of a strain gauge failure during the process of model making. Figure 14 show the curve of the measured strain at each measuring point on the surface of the protective layer of concrete with the uniform load at the top of the model. During the initial stage, the No. 1, No. 2, and No. 3 measuring points on the upper surface of the protective layer were under a compressive strain state, which increased with increasing load. The No. 4 and No. 5 measuring points on the bottom side were under a tensile strain state. When the top load reached 0.5-0.6 MPa, the strain values at all five measuring points abruptly changed.  
By analyzing the boundary conditions along with the mechanical and deformation characteristics of the protective layer, during the initial loading stage under uniform pressure, the protective layer was under a tension state at the bottom and a compression state at the top. Thus, the compressive strain was measured at the No. 1, No. 2, and No. 3 measuring points. The tensile strain was measured at the No. 4 and No. 5 measuring points. When the load reached 0.5-0.6 MPa, the measured value rapidly decreased to 0 because of cracking at the middle part of the bottom of the protective layer and the detachment failure of the strain gauge at the No. 1 measuring point. With the extension of the crack, the whole stiffness of the protective layer decreased, and the bending momentum decreased. Thus, with further increase in load, the deformation no longer increased, and the strain values at the No. 2, No. 3, No. 4, and No. 5 measuring points stabilized.  
Under the top flexible pressure, the flexural deformation at the bottom of the model was continuously transmitted to the top. This caused the tensile stress at the middle  
Figure 14: Strain-load curve of each measuring point on the surface of the protective layer.  
Figure 13: Position of strain gauges on the surface of the protective layer. (1)—position of measuring point No. 1; 2—position of measuring point No. 2; 3—position of measuring point No. 3; 4—position of measuring point No. 4; 5—position of measuring point No. 5; 6—position of measuring point No. 6).  
part of the bottom of the protective layer to reach the tensile strength, which resulted in cracking and failure. With a further increase in load, the crack extended to the top, and the width of the crack increased. However, when the composite geomembrane was subjected to tensile load, the crack extension became restricted despite the increase in load. Figure 15 show the final cracks in the protective layer. The maximum width of the final cracks was 4.0 mm. The crack extended close to the top edge but did not fully penetrate. After cracking, the protection layer did not have dislocation deformation because of the limitation of the composite geomembrane and the suitable rigidity of the polyurethane bonded aggregate cushion layer at the bottom. Due to the limitation of the test conditions, the real water seepage pressure was not simulated. In a real situation, seepage pressure and the phreatic surface will develop in the granular transition layer and dam body if the protective layer cracks [35]. That may cause the larger flexural deformation of the cushion and lead to further tension failure of the geomembrane. Therefore, besides paying attention to the strength and deformation of the cushion and transition layer, the gradation of granular materials may be designed according to the requirements of seepage stability and filtration criteria.