The Impact of Long Opening Times on Shaft Friction in Cohesive Soil-PPT Presentation

PPT Presentation

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The presentation examines how long excavation opening times affect shaft friction in cohesive soils, focusing on London Clay and diaphragm wall panels. Earlier bored-pile and barrette load tests showed a good relationship between shaft friction and log time. Extrapolation to one hour suggested an adhesion factor of about 0.7, while the factor declined as time increased, potentially because of clay swelling and softening.<br /><br />Pocket penetrometer tests were conducted beside two diaphragm wall panels. At approximately 2.5 cm from the wall, normalized penetration resistance was 0.61 after an opening time of 62.5 hours and 0.34 after 157 hours, indicating greater softening with longer exposure. The tests also showed softening extending roughly 25 cm into the clay.<br /><br />A numerical model simulated panel excavation, slurry pressure, concrete placement, consolidation and a subsequent load test. Excavation reduced stresses and water pressures; bentonite fluid entering the clay and swelling at the exposed surface reduced undrained shear strength. The modeled strength profile, using a clay permeability of 10⁻¹¹ m/s, broadly matched the penetrometer results. The model further indicated that strength and adhesion factors continue to decrease over long opening times.<br /><br />The findings suggest that prolonged exposure can reduce diaphragm-wall shaft resistance. Pocket penetrometers provide a useful means of measuring near-wall softening, while numerical modeling can help assess its effects on strength and mobilized shaft friction.

Keywords

excavation opening time

shaft friction

cohesive soils

London Clay

diaphragm wall panels

adhesion factor

pocket penetrometer

clay softening

numerical model

bentonite slurry