Abstract：Longtan tunnel is located on the section from Baishiping Town to Enshi City，Hubei Province，of Hurong(Shanghai—Chengdu) highway. It is about 8.6 kilometers long，the second longest highway tunnel in China，and its depth is about 530 meters. It is designed to penetrate the strata consisting of developed karst caverns，faults，and fractures caused by tectogenesis. So the initial stress in the surrounding rock is high；and the geological conditions along the tunnel are very complicated. It is a very typical deep and extra long tunnel under complicated geological conditions. To study the stability of this tunnel，many in-situ measurements on the surrounding rock have been taken. Through the in-situ measurement，the distribution characteristics of the internal displacements，the bolt axial force and the contact pressure between lining and surrounding rock are studied. And the influencing factors are analyzed in detail. Based on the analyses of the measurement，some useful conclusions are drawn to the stability of the tunnel；and some suggestions are made to modify and optimize design and construction scheme. This study will offer some useful references for construction of tunnels in the similar geological conditions.
