Predicting Tunnel Squeezing Using the SVM-BP Combination Model

Rock squeezing has a large influence on tunnel construction safety; thus, when designing and constructing tunnels it is highly important to use a reliable method for predicting tunnel squeezing from incomplete data. In this study, a combination SVM-BP (support vector machine-back-propagation) model is proposed to classify the deformation caused by surrounding rock squeezing. We designed different characteristic parameters and three types of classifiers (an SVM model, a BP model, and the proposed SVM-BP model) for the tunnel-squeezing prediction experiments and analysed the accuracy of predictions by different models and the influences of characteristic parameters on the prediction results. In contrast to other prediction methods, the proposed SVM-BP model is verified to be reliable. The results show that four characteristics: tunnel diameter (D), tunnel buried depth (H), rock quality index (Q) and support stiffness (K) reflect the effect of rock squeezing sufficiently for classification. The SVM-BP model combines the advantages of both an SVM and a BP neural network. It possesses flexible nonlinear modelling ability and the ability to perform parallel processing of large-scale information. Therefore, the SVM-BP model achieves better classification performance than do the SVM or BP models separately. Moreover, coupling D, H, and K has a significant impact on the predicted results of tunnel squeezing.

Wedge stability analysis and rock squeezing prediction of headrace tunnel, Lower Balephi Hydroelectric Project, Sindhupalchok District, central Nepal

The wedge stability and stress analyses are important in tunnel stability assessment. The identification of the wedge stability and stress condition for the headrace tunnel suggests the required tunnel support in Lower Balephi Hydroelectric Project in Sindhupalchock District, central Nepal. The planned tunnel of the project is 4.5 min diameter and 4.2 km in length. The main litholog ies of the area along the tunnel axis are phyllite and phyllitic quartzite of the Kunchha Formation, Nawakot Complex. Wedge stability analysis in the headrace tunnel showed that the structural wedge would form due to excavation and can be stabilized with the help of rock bolting and shotcreting. Rock squeezing is predicted to occur in high tunnel depth in phyllite and it may be stabilized with the installation of roc k support consisting steel rib.