Experimental Study On The Subsurface Erosion Of Loess In The Zi Chang Area Of Shan Xi Province

The subsurface erosion of loess is a critical problem that causes the loss of water and soil in loess area. Meanwhile, the loess cave caused by the subsurface erosion is also one of the main problems of geological engineering. With the promotion day by day of construction of the West Region, its hazard to the engineering construction becomes more and more remarkable. It is imperative to clarify its action mechanism and quantificational evaluation in order to make prevention and cure in accordance with own characteristic. Combining the program of " The Influence of the Subsurface Erosion of Loess on the Pipeline Engineering for Transmitting Natural Gas from the West to East ", this paper does the research on the action mechanism of the subsurface erosion of Loess, and makes a pilot study on the quantification of it.Since subsurface erosion of loess is represented in the form of loess cave, basing on the forefathers' study on the formation mechanism of loess cave, and with the combining of their results on the characteristics and structural features of loess, this paper analyses its action mechanism and each influencing factor . Finally, with the research and analysis above, this paper brings forward two guidelines on the study of the quantification of the subsurface erosion of loess: modulus and ratio of subsurface erosion.Basing on the analysis of the action mechanism of the subsurface erosion of loess, with the field survey in Zi Chang area—part of the pipeline engineering for transmitting natural gas from the west to east—and the combining of exploration and reconnaissance of slopes near Xin Zaihe Village in Shan Xi Province, this paper figures out the two ways in which the subsurface erosion of loess may affect the pipeline engineering for transmitting natural gas from the west tothe east——exposure and no-supporting of pipeline. And the no-supporting of pipeline can bedivided into three different types: the ones caused by underlying carves, vertical carves and flow soil.Basing on the guide line of quantificational evaluation of the subsurface erosion of loess and with the combining of the ways in which influence the pipeline engineering for transmitting natural gas from the west to east, author brings forward the idea of experimental study on the subsurface erosion of loess. According to the requirement, author designs and makes the experimental equipment for subsurface erosion and assists the making of modal equipment.After getting the basic guide lines of physical mechanics of soil samples, author uses the tester refitted from the permeater No.Nan-55 to make subsurface erosion experiment on the loess taken from the Xin Zaihe Village where the loss of water and soil is extremely great. Through the experiment, author gets several conclusions as follows:1) The changing process of permeability coefficient of undisturbed loess with closed crack and remolded loess with different moisture content before the running-through of subsurface erosion is obtained. Permeability coefficient of the undisturbed loess has the tendency of rising as the increase of pressure. However, the permeability coefficient of horizontal undisturbed loess falls after reaching its peak, quite different from that of the vertical one. The permeability coefficient of remolded loess before the running-through of subsurface erosion changes with moisture content in three different ways: rise with of pressure, change little with pressure, and reduce with pressure.2) Meanwhile, get running-through critical permeative pressure caused by subsurface erosion of different soil samples and remolded loess with different moisture content. And find out that running-through critical permeative pressure is mainly controlled by cracks of soil samples. And it is also controlled by their density and structure.3) The characteristics of subsurface erosion of loess are different in different periods of the process. During a period of time when water enters into carves, the quantity of subsurface erosion reaches its peak, later reduces as time goes by, and stays in a fixed number at last4) Get the subsurface erosion speed of different soil samples with different entering water discharge, and establish the formula about the relation between subsurface erosion ratio and entering water discharge of undisturbed loess and remolded loess. Through the experiment, author figures out that the formula obtained indoors is similar to the one got in field test, both of them are in exponential relation. Subsurface erosion ratio of remolded loess and entering water discharge has polynomial relation.As one of the ways, which may influence the pipeline engineering for transmitting natural gas from the west to east, author makes simulative test on the unsupported pipeline with caves nearby which stand vertically by using the self-made modal tester. And at last, author gets the process that subsurface erosion influences pipelines under this condition, and also the developmental tendency of the length of pipeline, which is unsupported, with the variation of time and discharge of entering water.With the analysis and conclusion of related data and the combining of the field test and indoor experiments, this paper for the first time makes research on the quantification of the erosion. It will impel the study of subsurface erosion of loess from qualitative level to quantificational one in some ways.