Evaluation of ground loosening behavior and mechanical properties of loosened sand associated with underground cavities

Subsidences in ground surface are caused by collapses of underground cavities by internal erosion with ground water percolation around and through the cavity. During the last few decades, there is an increased record of road cave-in accidents. Specially, many road cave-ins were caused by underground cavities developed through defects in buried sewer and drain pipes. Researchers have found that, there is a tendency of forming cave-ins when the age of buried pipe exceeds 25 years. Hence, vulnerability of road cave-in accidents is increasing all over the world, with most of the buried sewer and drain lines reaching their lifespan. Although modern techniques like Ground Penetrating Radar (GPR) and Geoelectric Tomography are available to detect the subsurface cavities, those technologies can only identify air filled voids separately from soil and having many other limitations. Maximum depth which can accurately detect is limited to few meters; in clayey and high moist soils this will be further limited to 1-1.5m maximum. Therefore, it is very difficult to detect cavities in deeper ground and it can ultimately cause ground subsidence by expansion towards the surface. Cavities are generally associated with process of loosening around it. Therefore, it will be better in detecting the loosened soil conjunction with air-filled cavity, rather than targeting air-filled cavity only. Thereby, even cavities in deeper ground can be detected by signs of capturing the loosened region in the subsurface. This will lead to improve the accuracy and efficiency of current methods in inexpensive manner. At present, cavity formation, extents of loosening and mechanical properties of loosened soil around cavity are not well addressed. There are several objectives of this study. First is to understand the extents of loosening with respect to the potential cavity size and to identify the parameters which affect the scale and rate of loosening. An attempt was made to create loosened soil in laboratory by dissolving water soluble material of pre-selected size and shape in Toyoura sand. The above mentioned objectives were achieved by conducting series of two-dimensional model tests by introducing a potential cavity and forming loosening by expanding the cavity by water infiltration and drainage. Effect of cavity size, density, ground compaction and rate of drainage on loosening and cavity expansion was studied through the two dimensional tests. As the second part of the study, quantitative study on mechanical properties of loosened soil was evaluated by triaxial tests after forming ground loosening inside the specimen. Simultaneously, influence of density, cavity size and location on scale of loosening was studied by changing conditions of the experiment. In each test, variation of axial strain, radial strain and volumetric strain during process of loosening was obtained and Poisson's ratio and shear modulus of loosened sand was evaluated. Furthermore, extent of loosening in triaxial specimen was studied by X-ray CT scan, in order to compare the different behaviour in two dimensional and three dimensional responses. Results obtained for Toyoura sand from two dimensional model tests show that, height of the loosened region is extending up to five times of cavity height and radial expansion is limited compared to vertical expansion. Furthermore, It was found that ground collapse around the potential cavity was higher when the cavity was located above the ground water table rather than beneath the water table. Cavity formed in compacted or dense soil was more stable compared to loose soil and extents of loosening were proportional to cavity size. Further observed that, suction supports the stability of cavity and abrupt failure of cavity ceiling was caused by rapid drainage. Extent of loosening was nearly limited to two times of the initial cavity height in triaxial tests. Density has affected the axial and radial deformations caused by loosening and lower density (Dr=37%) samples were observed with four times larger strains than medium dense samples (Dr=63%). Similarly, lower confining pressure (50kPa) shows nearly three times larger deformations than high confining pressure (100kPa). Loose sand and coarse sand (Silica) was subjected to further propagation of loosening under repetitive infiltration on already loosened soil. Evaluated Poisson's ratio of loosened sand shows complex variation throughout the entire test and difficult to understand the relationship with scale of loosening. This might be due to non-uniform distribution of soil and voids. Parametric study showed that Young's modulus of loosened region was 50-60% of the non-loosened region. Overall stiffness of ground with loosening was 70-85% of stiffness of controlled specimen. As a conclusion the cavities in subsurface are more vulnerable for cave-in and loosening expands at least twice of the cavity height. Overall stiffness of the ground is decreasing significantly even by having a smaller void.

報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 修士 ; 学位の種類: 修士（工学） ; 学位記番号: ; 研究科・専攻: 工学系研究科社会基盤学専攻