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Karst cave causes geological disasters in tunneling engineering, which brings great risks to subway construction. The microtremor survey has become one of the main methods for karst cave detection in urban areas due to its convenience and great detection depth. However, the microtremor survey lacks high-frequency signals, resulting in poor detection accuracy on the shallow layer. An advanced method called mixed-source surface wave (MSW) method is proposed by imposing active sources during the continuous passive surface wave survey. MSW is applied to the detection of beaded karst caves (i.e. two or three karst caves at different depths in the same location) in Ji'nan, China. The influences of ambient noise, array type, and other factors on the detection accuracy were studied. Experimental results show that the detection depth and accuracy of MSW are much better than those of passive surface wave method. The inversion accuracy of linear array perpendicular to road is better than the triangle array and L-shaped array. The accuracy of the linear array along the road is worst. The detection accuracy of MSW is similar to that of combined active–passive surface wave methods. However, the basic principle of MSW is easier to understand, and its operation is more convenient. The two-dimensional measurement results show that the MSW method can well reflect the size and location of the beaded karst cave and provide important references for the practical engineering.

Various diseases and failures inevitably appear on expressway roadways in water-rich strata under the long-term erosion of water. It is very difficult to support the surrounding rock of a water-rich roadway because water will corrode the anchorage bond and weaken the surrounding rock mass. In the process of supporting, damage and fracture of anchor bolt often appear in water-rich roadway. In order to study the stability analysis of a support anchor bolt in the process of surrounding rock fracture evolution and the relationship between the prestressed value and the length of the anchor bolt, this paper studied the fracture evolution law of surrounding rock and the progressive debonding law of the bolt are studied by RFPA3D numerical simulation and used MATLAB software to calculate and draw several graphs to reveal the mechanism by analytical method. The following main conclusions were drawn: (1) the change and attenuation of the surrounding rock stress have a certain influence on the stability of the supporting bolt. The existence of confining pressure (horizontal stress) has a significant impact on the ultimate pullout force of anchor bolts. (2) With the gradual destruction of the surrounding rock, the shear stress, horizontal stress, and vertical stress in the surrounding rock are gradually reduced to zero, and the change speed of the surrounding rock is fast at the shallow surface and slow at the deep. (3) The interface shear stress tends to a low stable value after debonding, which means the value of friction resistance is relatively stable in different positions. (4) The frictional resistance after interface debonding is an important condition to maintain the balance of higher anchorage force. If there is no friction resistance, when the axial force of the anchor bolt reaches the initial critical value, the interface debunking process will develop catastrophically and cannot be stabilized until complete failure, even if the axial force no longer increases.