A Study On Key Techniques Of Tunnelling In Karst Area With Gas

To control the density of gas within the safety criterion is the key to ensure the safety of tunneling through gas strata. When tunnneling in karst and gas strata, it is important to predict the gas density hebind the working face, to reduce the gas density by ventilation, especially to establish a monitoring and warning system of gas in the tunnel in order to know the gas density distribution in real time.This study investigated the key technologies of tunneling in karst and gas strata by using an integrated scheme of theoretical analysis, numerical modeling, field gas monitoring, and establishing the construction technial measures etc., based on the project features of Xiaofanping Tunnnel passing Karst and gas strata. The work done and major achievements are as follows.(1) By modeling the ventilation using FLUENT, it is found that at the low gas-section, a set of axial flow fan with 135kw in power using low grade air velocity can achieve a gas density of less than 0.5% at 1000s after the blasting, which can meet the requirment for tunneling. At the high gas section, a set of axial flow fan with 135kw in power using high grade air velocity can achieve a good air quality at 1800s after the blasting, and the construction workers can then work in the tunnel.(2) The ahead drilling is an important tool to detect difficult geologies such as cavity and gas. In Karst strata with gas, integrately utilising geology analysis, geophysical emthods and ahead drilling is effective to predict faults, cavity and gas ahead the working face, which would be extended in practical.(3) Based on the features of Xiaofanping Tunnel with Karst and gas, a gas monitoring system including gas detecting manually and automatical gas monitoring is established, the application of the system in tunneling, can effectively avoid the occurrence of gas accidents. This system can provide a good referrence for similar projects in the future.