68th Canadian Geotechnical Conference and 7th Canadian Permafrost Conference

ESTIMATION OF THE SMALL STRAIN STIFFNESS OF GLACIAL TILL USING GEOPHYSICAL METHODS AND BAROMETRIC LOADING RESPONSE

Session: Insitu Testing and Instrumentation II / Essais in situ et instrumentation II

Ruth Harley, Queen's University Belfast (United Kingdom)
Paolo Bergamo, Queen's University Belfast (United Kingdom)
David Hughes, Queen's University Belfast (United Kingdom)
Shane Donohue, Queen's University Belfast (United Kingdom)
Laura Carse, Queen's University Belfast (United Kingdom)
Lee Barbour, University of Saskatchewan (Canada)

Stiffness values in geotechnical structures can range over many orders of magnitude for relatively small operational strains. The typical strain levels where soil stiffness changes most dramatically is in the range 0.01-0.1%, however soils do not exhibit linear stress-strain behaviour at small strains. Knowledge of the in situ stiffness at small strain is important in geotechnical numerical modelling and design. The stress-strain regime of cut slopes is complex, as we have different principle stress directions at different positions along the potential failure plane. For example, loading may be primarily in extension near the toe of the slope, while compressive loading is predominant at the crest of a slope. Cuttings in heavily overconsolidated clays are known to be susceptible to progressive failure and subsequent strain softening, in which progressive yielding propagates from the toe towards the crest of the slope over time. In order to gain a better understanding of the rate of softening it would be advantageous to measure changes in small strain stiffness in the field. Seismic geophysical surveys result in very small strains and have been shown to provide estimates of small strain elastic moduli. Furthermore, loading efficiency theory uses the pore pressure response of grouted piezometers to barometric pressure fluctuations to estimate the compressibility of the formation. Barometric loading efficiency gives a direct measure of stiffness from continuous monitoring of pore pressures, whilst seismic surveys provides a means of mapping stiffness values both spatially, and temporally. This paper outlines the techniques used to investigate small strain stiffness in a glacial till cutting in Northern Ireland, using seismic surveys and barometric loading efficiency theory and presents some preliminary findings.