Study On Destruction Characteristics And Investigation Techniques Of Expansive Soil Cutting Slope In Ningming Highway

A great number of studies showed that failure of cutting slope is the major problem during the highway construction in the expansive soil area in China at present. The behavior of the residual expansive soil found in Ningming basin has been studied and the deformation characteristics of swell-shrink zone and failure behavior of landslide in this region were investigated by the author.Firstly, in slope surface drought-wet cycle affect region the soil water content is often less than the plastic limit, montmorillonite in lattice swell-shrink primarily, causing its serious swell-shrink deformation. Some characteristics of the residual type expansive soil in drought-wet cycle affect region such as microstructure, montmorillonite mineral content, specific surface area size, ESP values, etc had been found will be overall increase decay varied with the weathering depth, make its high moisture content, porosity ratio, low dry density characteristics, coupled with directional fabric in macro and microscopic characteristics, slope deformation and destruction of the zone finally are controlled by such geologic structure as shallow weathered zone interface, bedding and so on.Secondly, Through statistical measuring analysis of the bedding-like and joint-like structural surface morphology elements in the cutting slope, and comparative analysis the Stereographic projection statistical results about the edge slope angle between landslide slope and natural slope, the double activity model of residual type of expansive soil is established based on structure surface control theory. This model break through the traditional uniform weathered zone theory, propose a new theory that is this type of slope have two activities layer, the upper soil layer and the underneath rock layer, and the upper layer controlled by the interface between A soil layer and B soil layer, and the underneath layer controlled by the interface between soil interface and (strong and weak) weathered zone interface, the lower limit controlled by the bottom interface of the perched water.Thirdly, The step landslide failure mode is established through two surveys, considering the slope deformation observational data, temperature data, moisture data and the form factors of landslide and expansion and contraction structural surface, the double sliding failure model of residual type of expansive soil is established. This model proposed and verified a new view that the landslide of residual type of expansive soil controlled by the double expansion and contraction activities, and has a development regular of shallow, gradual, double slide and successive evolution. This new view provides essential geological theory to solve the long-standing problem of residual type of expansive soil cut slope exist in Nanning, Baise, Ningming, Shangsi ect. in Guangxi province always landslide, and this new view also provides essential geologic theory to develop the new method of flexible support structure.Fourthly, To accurately monitor the relative expansion and contraction deformation of the mudstone residual type of expansive soil in different depths within 1.0m from the excavation slope table by embedded displacement sensor in the shallow well, and to get two different depths of new expansion and contraction active zone when excavation ( that is the depth of upper soil layer is 2.34m, and the depth of lower Weathered rock is 1.21m) by using the time effect statistical analysis method proposed by this paper and established the aging model of double active zone. This model and the double active model which get from the Structure surface statistics confirmed each other, directly reveal the reasons for traditional slope protection structures which the effective depth less than 1m failure protect in the expansive soil slope, and give valuable and reliable parameters to similar expansive soil slope protection design.Fifths, The depth of soil and rock activities zone(2.12m,1.21m)calculated by suction method correspondence with the depth measured by shallow well displacement meter respectively, this conclusion confirmed the double active zone time effective model, and prove the depth obtained by suction method is exactly the depth of the wet and dry cycles significantly affected zone, also is the surface soil of slope responding to the dry-wet cycles on short-term climate. Shallow well displacement method provides a new practical method to observe the depth of the active zone.Sixths, This paper proposed a new method that can measure the depth of wet and dry cycles significantly affected zone fast, the test results is repeatability and close to the result which is got by the humidity coefficient method and expansion and contraction structural surface geological notation method, and verified by the tracking observations at the same time, so, it is a scientific method. Because it is more rapid, reliable, economical, convenient, etc. than existing methods, it has great application prospect. Using the test results of this method to design the slope reinforcement will be more economic.