An experimental study of hydraulic fracture and erosion
This thesis concerns an experimental investigation of hydraulic fracturing when the water pressure is increased rapidly in a borehole and development of a possible simple method for identifying erodible clayey soils. Case histories of hydraulic fracturing in embankment dams and boreholes are reviewed. It is found that hydraulic fracturing in dams is often associated with rapid reservoir filling and zones of low stresses. Previously proposed criteria for hydraulic fracturing are outlined. It is found that no existing theory adequately explains fracturing observed in laboratory tests. Characteristics of dispersive clays and the methods of identifying them are reviewed. It is found that no single test provides results that have a satisfactory level of reliability. Hydraulic fracture was examined in the laboratory on samples of puddle clay from the core of Cwmernderi Dam in modified Bishop and Wesley triaxial cells. Various sets of tests examined the effects of sample geometry, loading rate and overconsolidation on the initial fracturing pressure and examined the effects of subsequent reconsolidation time on refracturing pressures. For the analysis of these tests results, simple criteria for hydraulic fracturing are proposed. The test results suggest that hydraulic fracturing for undrained conditions occurs when the drained strength of the soil element at the edge of a borehole is reached. For fully drained conditions, fracturing occurs when the soil element at the edge of the borehole reaches the no-tension cut-off. A new test, the cylinder dispersion test, was developed and a series of tests were carried on samples of puddle clay from several British dams and on various other soils. The results of these tests demonstrate the major influence of pore water chemistry on the true cohesion and dispersion properties of the soils.