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

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The paper examines how extended excavation times soften cohesive soil beside piles and diaphragm-wall panels, reducing shaft friction. This issue is increasingly important as deep foundations become larger and excavations can remain open for several days.<br /><br />The authors combine reported pile and barrette load tests, penetrometer measurements beside London Clay diaphragm-wall panels, and numerical modelling of panel construction. At the Victoria Road Crossover Box, panels open for 62.5 and 157 hours had normalised penetration resistances of 0.61 and 0.34, respectively. Applying a base adhesion factor of 0.7 gives estimated adhesion factors of 0.43 and 0.24. Softening was strongest close to the panel and extended about 25 cm into the soil.<br /><br />A coupled-consolidation model using the BRICK soil model reproduced the observed softening when a very low permeability of 10⁻¹¹ m/s was assumed. The analysis indicates that excavation causes swelling and reduced effective stress; after concreting, pore-pressure dissipation gradually restores strength. Modelling a primary panel affected by excavation of a neighbouring panel produced a numerical load-test adhesion factor of 0.51, while the average mobilised adhesion factor along the panel was about 0.47. A standalone barrette experienced less softening than a panel influenced by a successive panel.<br /><br />The results support a reduction in shaft adhesion with increasing construction time, but penetrometer readings at one depth cannot reliably predict a wall’s full-depth performance. The authors recommend further load tests and modelling, including shorter opening times, mechanical disturbance during excavation, and measurements of concrete pressure and setting time.

Keywords

excavation time

cohesive soil softening

shaft friction

diaphragm wall panels

pile load tests

London Clay

penetrometer resistance

coupled consolidation

adhesion factor

deep foundations