Strength deterioration of karst fillings under dry–wet cycles: Testing and modeling study

The strength of karst fillings is vital for the safety of tunnels in large-filled karst areas. Karst fillings in areas with seasonal rainfall often experience dry–wet cycle environments, which have a deteriorating effect on their strength and threaten the safety of underground projects such as tunnels. In this study, direct shear tests of dry samples (samples with the moisture content of \(w_{1}\)) and wet samples (samples with saturated moisture content) at different dry densities (\(\rho_{d}\)), the number of dry–wet cycles (\(n\)), and the lower-bound moisture content of dry–wet cycles (\(w_{1}\)) were performed to analyze the deterioration rule of \(c\) and \(\varphi\) of karst fillings. The causes of the strength deterioration of karst fillings were explained based on scanning electron microscopy tests. The deterioration rate was fitted using an exponential function, and a karst filling strength deterioration model (KFDM) was developed that integrated three influencing factors. The KFDM was used through FLAC3D to predict the deformation and safety coefficient of tunnels under dry–wet cycles. The following results were obtained: (1) dry–wet cycles had a significant degrading effect on \(c\) and \(\varphi\), and \(\rho_{d}\) and \(w_{1}\) had an important influence on the degradation; (2) the KFDM accurately predicted the \(c\) and \(\varphi\) values of karst fillings after dry–wet cycles; and (3) the tunnel deformation and safety coefficient exhibited cyclic changes during dry–wet cycles. The deformation increased gradually at increasing \(n\), and the safety factor increased in the opposite direction. These results provide insights into the prediction of the deformation and safety coefficient of tunnels in large-filled karst areas and can aid in developing long-term maintenance strategies to ensure safety.