Imaging geotechnical property changes during failure development of tropical residual soil slope

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Sadiq, Atiku Abubakar (2019). Imaging geotechnical property changes during failure development of tropical residual soil slope. University of Birmingham. Ph.D.

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Abstract

Devastating cases of slope failure have been reported in many parts of the world; many of which are in tropical countries where the natural and man-made slopes are mostly made up of red tropical residual soils. Most of the failures occurred following heavy rainfall or during flooding. For this reason, the abnormal change of the soil water content is presumed to be responsible for these failures. It becomes imperative, therefore, to understand the soil-water interaction and eventual failure process in tropical residual soil, if this problem is to be effectively tackled. To achieve this aim, a research was conducted on a systematically designed and constructed slope model of a simulated tropical residual soil slope.
Preliminary studies were conducted to assess the suitability of the selected material, methodology and instrumentation to be used in the experiment. This was followed by a trial experiment on a small-sized model with a surface area of 200 mm x 370 mm and a maximum (crest) slope height of 220 mm. This small size allowed easy handling and repeatability but could not accommodate enough instrumentation, due to the limited size. To remedy this deficiency, another experimental trial was conducted on a larger-sized model constructed in a square fibreglass box with surface area of 1315 mm x 1315 mm and the maximum height of this slope was 650 mm. Before conducting the final laboratory experiment, the model was analyzed and designed with the aid of modelling software (i.e. SLOPE/W, SEEP/W and SIGMA/W). The geotechnical tests and other preliminary laboratory studies conducted at the beginning of the study provided necessary inputs during numerical modelling. This numerical modelling produced a final workable model and provided an idea about the failure mechanism of the designed model. Finally, the main experiment was conducted on the designed model which was constructed in a large acrylic glass. The surface dimension of this model was 1000 mm x 1700 mm and the maximum slope height was 750 mm.
In all the trials, a slope failure was caused by supplying water through a supply chamber, provided by the side of the crest, and allowing it to move freely to the toe. The geophysical changes and physical deformation during failure were observed using an integrated system of electromagnetic sensors (5TE and MPS6), electrical resistivity, and cameras. From the results presented and discussed, it was understood that the gradual movement of water through the slope caused a gradual reduction in matric suction, with the consequent reduction of shear strength and eventual slope movement. The physical deformation began with an initial settlement and minor surface cracks; which continually progressed to excessive settlement and larger cracks, before the final forward movement. The excessive settlement prior to the final movement appears to be associated with a soil structure collapse induced by wetting; while the forward sliding is caused by the movement of water, which tends to pull the slope downwards. The experiment provided an improved knowledge of the slope failure mechanism in tropical residual soils and has established the suitability of geophysical methods for monitoring the stability of the slopes and embankments of the tropics.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):