滇西芒棒盆地芒棒组碳质粉砂岩的工程地质特性

滇西芒棒盆地是在上新世喜山运动后期高黎贡山强烈隆升过程中形成的近SN向的断陷盆地,在盆地内发育芒棒组碳质粉砂岩等半成岩,规划建设中的中缅公路保山-腾冲高速公路段从该套地层中通过。野外调查发现:该套碳质粉砂岩具有成岩差、易崩解、结构性强等特性,容易引发边坡失稳、隧洞围岩冒顶等工程地质问题。室内试验测试表明:该类岩石具有天然含水量高、孔隙度大、低膨胀性和有机质含量高等特点。由于有机质的胶结作用,天然岩体和干燥岩块浸水后的性状显著不同,岩体干燥后具有水稳性增强和膨胀势降低的特性。结合典型工程实例,开展了原位直接剪切试验。试验结果表明:碳质粉砂岩具有抗剪强度较高、峰残降差小等特点,是该类地层中大型不稳定斜坡体和蠕滑型滑坡发育的主要因素之一。

Engineering Geology Characteristic of the Carbonaceous Siltstone in Mangbang Basin,West of Yunnan Province

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.