Abstract:

Mangbang basin is a SN directed fault depression basin which was shaped when Gaoligong Mountains intensely upheaved at the end of Himalayan movement in Pliocene period, where carbonaceous siltstone of Mangbang group, a kind of soft half-diagenesis rock, is well distributed. The Baoshan-Tengchong high way section of Burma Road will pass through this basin. Field geology survey indicates that this kind of carbonaceous siltstone is characterized by weak diagenesis, easy disintegration and strong structural strength, which can cause the engineering geological problems of slope failure and roof fall of surrounding rock in tunnel. Laboratory test results show a high natural water content, high porosity, low expansibility and high organic content of it. Due to organic cementation, striking difference can be obtained in the engineering properties of natural rock and dry rock after immersion in water, which indicates the enhancement in water stability and reduction in swelling potential of dried rock. Based on typical project example, in-situ shearing test was conducted on the carbonaceous siltstone, and the results indicate that it has the characteristics of high shear strength and little difference between peak and residual strength, which is one of the main factors for the development of large unstable slope body and creeping landslide in this kind of rock stratum.

Key words: Mangbang group, carbonaceous siltstone, half-diagenesis rock, engineering geology