湖北省恩施地区滑坡岩土体的工程地质特性研究
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摘要
恩施市位于湖北省西南部,毗邻清江河。恩施市的地理位置为北纬30°18′00″,东经109°28′60″,覆盖面积为3976 km~2。武汉至恩施的直线距离约为468 km。恩施及其周边的地形复杂,90%以上为多山地形;西北和东南部分海拔较高,东北和西南部分海拔较低。恩施的中心部位为江河流域。恩施位于云南—贵州高原东部延伸部位,该地区地形起伏,在云南—贵州高原地区滑坡地质灾害发生频繁,威胁到人民的生命财产安全,阻碍了当地经济建设的发展,而且对社会的发展产生了不良的影响。
恩施市属于中纬度的亚热带季风气候,受季节影响较大。天气温暖潮湿,并伴随着大量的降雨。
本文的主要研究重点是恩施市和宣恩县岩土体材料的工程地质特性。
研究区域正在被城市建设的发展所改造,同时不稳定的斜坡导致了分布较广的自然灾害。诱发保扎、杨家山和双龙洞地区滑坡发生破坏的原因和滑坡相关参数正在进行研究。
本研究的目的是:通过对以下内容进行研究,确定导致滑坡地质灾害的主要因素。主要的研究内容有:地质条件(岩性,层面及沉积结构)、构造地质、影响岩体和地形(通过分析坡角和海拔高度)的地质结构(断层、褶皱、结构面、边界和不连续面)、软岩和硬岩的关系、风化程度、岩石的矿物改造、降雨的影响、雨水向坡体内的渗透、粘土矿物聚集和主要粘土矿物的识别(通过X射线衍射分析XRD),最后确定滑坡体内岩土体的物理性质。
在调查期间,分别在室内和室外对地形图和地质图进行分析,对斜坡的坡角、岩性、岩体的走向和倾角以及结构面进行了测量;确定了风化程度和层面。在野外采集了研究区内的岩样,对岩样的纹理、颜色、矿物组成和胶结材料(砂岩)进行了分析。通过对典型岩样的薄片进行仔细研究,可以得出它们的物质组成以及风化程度。另外,测定了岩体的比重、体积密度、吸水率以及泊松比。
土样采自于滑坡沉积地域中以及泥岩地层中,进而用来进行XRD分析。确定了土样的液限塑限、粒径分配、天然含水量和比重。
研究区域及其周边区域主要由沉积岩和变质岩组成。该地区的特征是褶皱和断层,在褶皱和断层附近,海拔中等和海拔低的山同时存在,而且一部分陡的地壳是相对稳定的。一些滑坡位于活断层之间,例如杨家山滑坡位于恩施和建始活断层之间。
大部分的结构面可以用紧闭、部分张开或者有明显的空穴来描述。有时结构面是空的,被粘性土和(或者)岩石碎片填充。结构面的表面是波状的、平滑的、风化的或粗糟的,结构面的分布样式接近于构造面。对于紧密结构面而言,破裂用肉眼可以容易识别,但是结构面的侧壁结合的很紧密以至于不能看到沿着破裂的空洞。被方解石(石灰石)和泥质(砂岩和粉砂岩)填充的结构面比较常见。结构面走向的测量通过绘制玫瑰花图来反映,目的是确定结构面的主要倾向。玫瑰花图显示了结构面的主要倾向为NNE(保扎滑坡),NNW(屯保附近的清江河以及杨家山滑坡),而且研究区域内主要倾向为NNW。另外,结构面的来源属于构造结构面。
XRD分析结果表明:粘土矿物中伊利石最多,其次是高岭石、绿泥石和少量的叶蜡石,其中叶蜡石仅出现在保扎滑坡和杨家山滑坡。通过将XRD结果和塑性液限结果进行分析可以得到,低至中等塑性的无机粘性土表明了三个滑坡的土体中存在非贵粘性土。
研究区域内5种岩石类型的孔隙度值指明了:保扎和杨家山滑坡的泥岩的孔隙度最高,同时双龙洞岩样的孔隙度中等。高的孔隙度值是由岩石组成所决定的。页岩的孔隙度值中等,而且高于砂岩、灰岩和粉砂岩,其中粉砂岩的孔隙度最低。
保扎滑坡中砂岩和灰岩的低孔隙度是由于硅含量高造成的。保扎滑坡中高的硅含量是由于硅化过程造成的,同时该硅化过程是由于中泥盆纪断层影响了该区域。该现象也可以用来分析其它滑坡内断层附近的灰岩和粉砂岩。
泥岩具有最高的吸水率,平均值范围为(8.85-5.64%),其次为页岩(4.92-1.44%)、粉砂岩(2.94-0.89%)、砂岩(1.61-0.61%),最后为灰岩(0.45-0.38%)。
值得注意的是,当把饱和的保扎滑坡和杨家山滑坡内的页岩放置于水中,不到半天的时间它将分解成小的部分。分解开始发生于软弱部位,例如层面,易裂结构以及微观结构面。对于风化的岩石和新鲜的岩石该现象都会发生。另外,一些泥岩岩样是从水平层面开始破裂的,并没有分解为更小的颗粒。试验表明:两种岩石的工程性质主要受水的控制,在降雨期间它们将发生破坏而且导致滑坡。
降雨的强度和持续时间是决定滑坡发生位置的重要因素。另外,其他因素例如地形、土的厚度、滑坡外貌以及滑坡防治措施也是很重要的。在暴雨情况下,将会形成具有坚固表面的冲积物。冲积物集中于泻洪渠道,并取决于不同下坡路径的表面特征。当洪水发生时,水将以最快的速度渗透洪水区域,直至洪水消退。滑坡对降雨的水力学响应涉及到复杂的、瞬间饱和—非饱和的相互作用,该作用常常导致水位的升高。地下水流动的增加可以作为滑坡破坏的诱发机制。
岩性(岩石类型)是控制地貌的基本因素。从某种意义上说,地貌的类型和形成速率取决于岩性和地下岩土体的风化特征。因此,滑坡的形成和岩性有着直接的关系。值得注意的是,地质结构决定类型、分布、地下水的状态和流动规则,另外地下水的流动将影响或者诱发滑坡。
在研究区内各种各样的风化现象比较活跃。之所以活跃是应为水的存在,包括降雨,泉水,死水或者小溪。小溪的存在使得岩土体内富存水。因此,它也是岩体含水量增加的主要原因。大部分的岩体为微风化—中风化,在一些位置岩体是强风化的,例如页岩。
滑坡内的风化现象主要是由胶结的破坏、砂岩和粉砂岩支撑的移动以及灰岩的分解造成的。胶结材料被移动或改变。泥岩基质和砂岩的风化是由于粘性土的分解和淋溶作用形成的。氧化铁胶结物易于和水结合形成氢氧化物,同时存在砂岩内铁质的流失,从而形成凝固物或者其他的聚集物。岩相学指出了斜长岩和K—长石中存在矿物迁移的现象,形成了一些粘性土。矿物迁移是由于水渗入裂隙,层面或者岩石中的空隙造成的。风化作用的影响随着深度递减,而且该影响受大气情况和地下水的影响。岩体中的化学改造受水的作用而增强。
综上所述,本文的结论如下:在研究区域内影响滑坡最重要的因素是强烈的降雨,延长的潮湿天气的持续时间,岩性以及岩石的风化程度。如果没有防护措施,滑坡发生的频率和滑坡的尺度将增加,因为老滑坡的复活将诱发新的滑坡。
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.
恩施市属于中纬度的亚热带季风气候,受季节影响较大。天气温暖潮湿,并伴随着大量的降雨。
本文的主要研究重点是恩施市和宣恩县岩土体材料的工程地质特性。
研究区域正在被城市建设的发展所改造,同时不稳定的斜坡导致了分布较广的自然灾害。诱发保扎、杨家山和双龙洞地区滑坡发生破坏的原因和滑坡相关参数正在进行研究。
本研究的目的是:通过对以下内容进行研究,确定导致滑坡地质灾害的主要因素。主要的研究内容有:地质条件(岩性,层面及沉积结构)、构造地质、影响岩体和地形(通过分析坡角和海拔高度)的地质结构(断层、褶皱、结构面、边界和不连续面)、软岩和硬岩的关系、风化程度、岩石的矿物改造、降雨的影响、雨水向坡体内的渗透、粘土矿物聚集和主要粘土矿物的识别(通过X射线衍射分析XRD),最后确定滑坡体内岩土体的物理性质。
在调查期间,分别在室内和室外对地形图和地质图进行分析,对斜坡的坡角、岩性、岩体的走向和倾角以及结构面进行了测量;确定了风化程度和层面。在野外采集了研究区内的岩样,对岩样的纹理、颜色、矿物组成和胶结材料(砂岩)进行了分析。通过对典型岩样的薄片进行仔细研究,可以得出它们的物质组成以及风化程度。另外,测定了岩体的比重、体积密度、吸水率以及泊松比。
土样采自于滑坡沉积地域中以及泥岩地层中,进而用来进行XRD分析。确定了土样的液限塑限、粒径分配、天然含水量和比重。
研究区域及其周边区域主要由沉积岩和变质岩组成。该地区的特征是褶皱和断层,在褶皱和断层附近,海拔中等和海拔低的山同时存在,而且一部分陡的地壳是相对稳定的。一些滑坡位于活断层之间,例如杨家山滑坡位于恩施和建始活断层之间。
大部分的结构面可以用紧闭、部分张开或者有明显的空穴来描述。有时结构面是空的,被粘性土和(或者)岩石碎片填充。结构面的表面是波状的、平滑的、风化的或粗糟的,结构面的分布样式接近于构造面。对于紧密结构面而言,破裂用肉眼可以容易识别,但是结构面的侧壁结合的很紧密以至于不能看到沿着破裂的空洞。被方解石(石灰石)和泥质(砂岩和粉砂岩)填充的结构面比较常见。结构面走向的测量通过绘制玫瑰花图来反映,目的是确定结构面的主要倾向。玫瑰花图显示了结构面的主要倾向为NNE(保扎滑坡),NNW(屯保附近的清江河以及杨家山滑坡),而且研究区域内主要倾向为NNW。另外,结构面的来源属于构造结构面。
XRD分析结果表明:粘土矿物中伊利石最多,其次是高岭石、绿泥石和少量的叶蜡石,其中叶蜡石仅出现在保扎滑坡和杨家山滑坡。通过将XRD结果和塑性液限结果进行分析可以得到,低至中等塑性的无机粘性土表明了三个滑坡的土体中存在非贵粘性土。
研究区域内5种岩石类型的孔隙度值指明了:保扎和杨家山滑坡的泥岩的孔隙度最高,同时双龙洞岩样的孔隙度中等。高的孔隙度值是由岩石组成所决定的。页岩的孔隙度值中等,而且高于砂岩、灰岩和粉砂岩,其中粉砂岩的孔隙度最低。
保扎滑坡中砂岩和灰岩的低孔隙度是由于硅含量高造成的。保扎滑坡中高的硅含量是由于硅化过程造成的,同时该硅化过程是由于中泥盆纪断层影响了该区域。该现象也可以用来分析其它滑坡内断层附近的灰岩和粉砂岩。
泥岩具有最高的吸水率,平均值范围为(8.85-5.64%),其次为页岩(4.92-1.44%)、粉砂岩(2.94-0.89%)、砂岩(1.61-0.61%),最后为灰岩(0.45-0.38%)。
值得注意的是,当把饱和的保扎滑坡和杨家山滑坡内的页岩放置于水中,不到半天的时间它将分解成小的部分。分解开始发生于软弱部位,例如层面,易裂结构以及微观结构面。对于风化的岩石和新鲜的岩石该现象都会发生。另外,一些泥岩岩样是从水平层面开始破裂的,并没有分解为更小的颗粒。试验表明:两种岩石的工程性质主要受水的控制,在降雨期间它们将发生破坏而且导致滑坡。
降雨的强度和持续时间是决定滑坡发生位置的重要因素。另外,其他因素例如地形、土的厚度、滑坡外貌以及滑坡防治措施也是很重要的。在暴雨情况下,将会形成具有坚固表面的冲积物。冲积物集中于泻洪渠道,并取决于不同下坡路径的表面特征。当洪水发生时,水将以最快的速度渗透洪水区域,直至洪水消退。滑坡对降雨的水力学响应涉及到复杂的、瞬间饱和—非饱和的相互作用,该作用常常导致水位的升高。地下水流动的增加可以作为滑坡破坏的诱发机制。
岩性(岩石类型)是控制地貌的基本因素。从某种意义上说,地貌的类型和形成速率取决于岩性和地下岩土体的风化特征。因此,滑坡的形成和岩性有着直接的关系。值得注意的是,地质结构决定类型、分布、地下水的状态和流动规则,另外地下水的流动将影响或者诱发滑坡。
在研究区内各种各样的风化现象比较活跃。之所以活跃是应为水的存在,包括降雨,泉水,死水或者小溪。小溪的存在使得岩土体内富存水。因此,它也是岩体含水量增加的主要原因。大部分的岩体为微风化—中风化,在一些位置岩体是强风化的,例如页岩。
滑坡内的风化现象主要是由胶结的破坏、砂岩和粉砂岩支撑的移动以及灰岩的分解造成的。胶结材料被移动或改变。泥岩基质和砂岩的风化是由于粘性土的分解和淋溶作用形成的。氧化铁胶结物易于和水结合形成氢氧化物,同时存在砂岩内铁质的流失,从而形成凝固物或者其他的聚集物。岩相学指出了斜长岩和K—长石中存在矿物迁移的现象,形成了一些粘性土。矿物迁移是由于水渗入裂隙,层面或者岩石中的空隙造成的。风化作用的影响随着深度递减,而且该影响受大气情况和地下水的影响。岩体中的化学改造受水的作用而增强。
综上所述,本文的结论如下:在研究区域内影响滑坡最重要的因素是强烈的降雨,延长的潮湿天气的持续时间,岩性以及岩石的风化程度。如果没有防护措施,滑坡发生的频率和滑坡的尺度将增加,因为老滑坡的复活将诱发新的滑坡。
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.
引文
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