## 5 Strength Deterioration Mechanism Analysis and Discussion  
Rockfill material is a dispersed system composed of soil particles with different particle sizes and shapes. Under the action of compaction, the coarse particles contact each other to form the soil skeleton, and the fine particles fill the pores of the coarse particles with cementation. The close contact structure of the coarse and fine particles gives the soil the ability to resist shear deformation. The shear strength of the rockfill material is composed of the apparent cohesion and friction strength of the soil [34]. The apparent cohesion reflects the adhesion degree of fine particles in the soil and the interlocking strength of coarse particles. The friction strength is a macroscopic representation of the frictional interaction of soil particles on the shear surface and the degree of directional particle arrangement. Figure 15 shows the shear surface characteristics of a soil sample with 61.34% coarse grain content after the reciprocating shear under the normal stress of 500 kPa. It can be seen that under the reciprocating shear, the block particles distributed on the shear surface of the soil crush, the effect of the squeezing occlusion between the coarse particles weakens, and the directionally arranged fine particles on the shear surface significantly increases, resulting in the continuous deterioration of the soil shear strength. The meso-mechanism of the shear strength deterioration of rockfill materials under reciprocating shear is shown in Figure 16.  
Figure 16: Schematic diagram of deterioration mechanism of shear strength of rockfill materials under reciprocating shear. (**a**) Deterioration of apparent cohesion. (**b**) Deterioration of friction strength. Figure 15: Shear surface characteristics of soil sample after reciprocating shear. Figure 16a illustrates the deterioration of the apparent cohesion of the rockfill material under reciprocating shear. A change in the original spatial distribution and contact mode of the soil particles when the shear stress is generated in the soil can be seen. In order to obtain a larger shear space, the soil particles will squeeze, roll, and rotate to change the position, thus forming a shear surface in the soil. However, with the continuous increase of shear stress in the soil, the particle occlusion is closer, the stress concentration is more obvious, and the number of cracks inside the block stone increases. Furthermore, the reciprocating shear accelerates the particle breakage, with an increasing number of internal fractures. In the alternating changes of push and pull environments, these fractures gradually open, widen, deepen, and penetrate. As a result, the disintegration of the block particles continuously occurs, and the particle edges are gradually smoothed, with increased roundness. The interlocking effect between the particles continuously weakens, and the apparent cohesion of the soil body further deteriorates. In addition, the fine particles in the rockfill material have good water-holding properties. The fine particles are dissociated and negatively charged by water. After forming an electric field, the cations in the aqueous solution are tightly adsorbed on the surface of the fine particles through electric field attraction to form a bound water membrane, exerting a certain bonding effect between the fine particles. With the increase of the number of reciprocating shears, the compactness of the shear surface of the soil decreases, the pores between the soil particles increase and connect with each other, and the electric field attraction is insufficient to overcome the gravitational water flow, resulting in the decrease of the binding strength between the fine particles and the decrease of the apparent cohesion of the soil. Figure 16b illustrates the particle distribution characteristics on the shear surface of the rockfill material under reciprocating shear.