Evaluating Ko in Champlain clays with hydraulic fracture tests

Hydraulic fracture tests using piezometer tips 10–500 mm long were performed in Champlain clays at five different sites with overconsolidation ratios (OCR) varying from 1.6 to 4.8. Before the test, the piezometer fluid was replaced by a methylene blue solution (10 g/L) to dye the fissures resulting from the test. After the hydraulic fracture, the soil around the piezometer tips was recovered by means of a large-diameter sampler. For 300 and 500 mm long piezometers, the fissure pattern was radial and vertical, even in deposits with an OCR above unity, confirming that hydraulic fracturing is related to the total stress acting on the cavity walls. The results for the five sites define for the Champlain clay deposits a Ko–OCR relationship that gives higher Ko values than generally proposed in the literature. Key words: hydraulic fracture, piezometer tip, sensitive clay, fissure, stress ratio Ko, overconsolidation.