| Enshi City, is situated in southwest of Hubei Province, beside the Qingjiang River. It bounded by latitude 30°18' 00" N and longitude 109°28' 60" E. It is covered by total area of about 3976 km~2. The straight distance between Wuhan and Enshi is about 468 km. The land of Enshi District is complicated landform; more than 90% are mountainous areas; northwest and southeast are prefecture higher in elevation and the northeast and southwest are lower in elevation. The central area is a river basin in mountains. It is located in wrinkled terrain in eastern extended part of Yunnan-Guizhou plateau, where landslide happen frequently and causes threats to people's life and property and hauled the development of the local economic construction, also brings bad effect on the social development.It is situated in subtropical monsoon climate of middle latitude, easily affected by the season changing. It is warm and wet with plentiful rainfalls.The main focus of this research is the study of the geological and geoengineering properties of rock and soil materials from the landslides in Enshi District. The area under study is undergoing urban constructional development, with slope instability causing a widespread natural hazard. Landslide related-parameters and reasons which lead to the triggering of the landslides at the locations of Baozhe, Yangjia mountain and Shuanglongdong landslides are studied.The study has been conducted so as: to determine the dominant factor/factors which may lead to landslides, through the study of geology (lithology, bedding planes, and sedimentary structures), tectonic and geological structure (faults, folds, fractures, boundaries and discontinuities) which affects the rocks and geomorphology (through slope angle, aspect and elevation), the relation between the soft rocks and the hard one, the degree of weathering and mineral alteration of rocks, the effect of rainfalls and infiltrated rain water through rocks and soil of the slopes, identification of the clay mineral assemblage and the dominant clay mineral/minerals by using X-Ray Diffraction (XRD) analysis and determine the physical properties of soil and rocks at the location of the landslides. During the investigation: physiographic, topographic contour and geological maps are studied in the office and used during the fieldwork. Slope angle in relation to lithology, dip and strike of rocks and fractures were measured, degree of weathering and bedding planes are determined. In the fieldwork, rock samples are collected from the study area. Their texture, colours, mineral composition and cementing materials (sandstone) has been studied. Thin sections are made from representative rock samples and have been studied petrographically in order to know their mineral composition and degree of weathering. Specific gravity, bulk density, water absorption capacity and porosity has been determined.Samples of soils were taken from the depositional environment of the landslides and from the strata of the mudstones in order to conduct the XRD analysis. Atterberg Limits, grain-size distribution, natural moisture content and specific gravity are determined.The study area is composed of dominant sedimentary and a few metamorphic rocks. It characterized by folded and faulted belt, where both medium and low mountains exist and part of the hilly crust is relatively stable. Some of the slope sites are located between active faults such as Yangjia mountain landslide which is situated between Enshi and Jianshi active faults.Most of the studied fractures are described as tight, partly open or has clear visible cavities. Sometimes they are empty, filled by clays and/or rock fragments. Their surfaces are wavy, smooth, weathered or rough. Their set frequency increases with proximity to the tectonic plane. In case of tight fractures, the rupture is quite apparent with the naked eye, though the walls of the fractures are brought together to such an extent that it becomes impossible to see the cavity along the rupture. Blind and refilled fractures by calcite (limestone), mud (sandstone and siltstone) are not uncommon. Many measurements of fractures strikes are plotted using rose diagram, with the aim of establishing predominant orientation of the fractures. The rose diagram showed that, the dominant orientation is NNE (Baozhe landslide), NNW (Qingjiang River near Tunbao and Yangjia mountain landslide) and the general trend is NNW. The genesis of these fractures may be tectonic in origin.The results of the XRD analysis showed dominant Illite clay mineral through the study area, followed by kaolinite, chlorite and lesser pyrophyllite which is confined to Baozhe and Yangjia mountain landslides only. The results of the XRD were compared with the results obtained from the Atterberg limits. The low to medium plastic inorganic clay indicate the existing of inexpensive clays in the soil of the three landslides.The porosity values of the five rock types collected from the study area showed that, the sand mudstone collected from Baozhe and Yangjia mountain landsides recorded the highest values while the same samples from Shuanglongdong produced relatively medium values. The high values are attributed to the composition of the rock. Shale produced middle values and higher than the sandstone, lime stone and slitstone. The later gave the lowest values.The low porosity of the sandstone and the limestone from Baozhe landslide is due to the presence of the high silica content. The effect of the high silica leads to from the silicified sandstone of Baozhe landslide. The high silica content of Baozhe resulted from the process of the silicification due to the effect of the post Devonian fault which has affected the area. The same phenomona can be applied to some limestones and siltstones situated near to the faulted zones at the other landslide locations. The porosity of these rocks is similar to the porosity of the plutonic igneous rocks.The sand mudstone showed highest ability to absorb water, the recorded mean value ranges between (8.85-5.64%), followed by shale (4.92-1.44%), siltstone (2.94-0.89%), sandstone (1.61-0.61%) and finally limestone (0.45-0.38%).It is noticed that, the shale from Baozhe and Yangjia mountain when placed in water under the saturation condition, it disintegrates into small parts in period not less than half of a day. The disintegration start from the points of weakness such as the bedding planes, fissile structure and the microfractures. These phenomena occur to the two degree of weathering either weathered or the fresh rocks. Also, some samples of mud sandstone started to separate from the line of the clear horizontal bedding without disintegrating to smaller particles. This test indicates that, the geoengineering behaviour of these two rocks are highly controlled by water. The two rocks have the ability to collapse during heavy rainfall and may cause landslides.Rainfall intensity and duration are important in determining where landslide would occur. In addition, other factors such as geomorphology, soil thickness, slope aspect, and slope protection are also important. Substantial surface runoff will be generated during severe rainstorms. The runoff will be concentrated in the form of flooding, channeling of flow or combination, due to various ground features on its route downhill. Where flooding occurs, water in the flood pool allows infiltration at maximum rate to proceed over the flood area and to continue until the flood pool dissipates. The hydrologic response of a hillslope to rainfall involves a complex, transient saturated-unsaturated interaction that usually leads to a water table rise. An increase of groundwater flow can act as the triggering mechanism for slope failure.Lithology (rock type) exerts a fundamental control on geomorphological characteristics of the study area. The nature and rate of geomorphological process depend, to some extent, on the lithology and weathering characteristics of underlying materials. The landslide process, therefore, has direct correlation to lithology. It is also noted that geological structure decides the type, distribution, state and movement rule of groundwater, which affects or triggers a landslide.All types of weathering are very active in the study area. The main cause of the activation is the presences of water either in form of rainfall, springs, stagnant water or small streams. The presence of these streams in the area causes the enrichment of the soil and rocks with water. Therefore, it is the main responsible of increasing of the moisture content in the rocks. Most of the rocks range in their degree of weathering from slightly to medium weathering, in some localities rocks like shale are highly weathered. Weathering at the landslide sites consists largely of attack on the cement and removal of support of the sandstone and siltstone and decompose of the limestone, mudstone and shale. The cementing material are removed or altered. The clay matrix and sandstones weather by breakdown and eluviations of clay. Iron oxides cements tend to hydrate to hydroxides, and there is often migration of iron within the sandstones to form concretions or other accumulation is also observed. The petrographic study indicated that there is mineral alteration occurred to the plagioclase and K-feldspar, in addition to the formation of some clays. The alterations are attributed to the action of the penetrating water through fractures, bedding planes and porosity of the rock. The effect of weathering generally decreases with depth and is partly controlled by atmospheric conditions and ground water. Chemical alteration to the rocks is enhanced by the action water.The study concluded that, the most common triggering factors for the landslides in the study area is the intense rainfall, prolong periods of wet weather, lithology and the degree of weathering of rocks. The frequency and magnitude of landslide are expected to increase through the activation of old landslides and triggering of new ones if no remedial actions has taken to solve the problem.
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