Systematic Laboratory and Field Study on the Shear Resistance of the Soil-Concrete Interface of Diaphragm Walls and Bored Piles-Paper

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This study investigates how bentonite suspensions and polymer solutions affect shear transfer at the concrete–soil interface of diaphragm walls and bored piles, focusing on non-cohesive soils.<br /><br />Researchers examined filter cakes sampled from three construction sites and conducted systematic laboratory tests. Small direct-shear tests assessed how bentonite content and soil grading influence friction. Larger tests simulated fluid penetration, filter-cake formation, tremie concreting and consolidation, then measured interface shear resistance using bentonite suspensions and two polymer types.<br /><br />Field samples generally contained only about 10–20% bentonite, with the remaining solids largely derived from soil fines. Small-scale tests showed that friction decreases as bentonite content increases. Large-scale tests found wall friction angles of approximately 25–30° for bentonite-supported interfaces; fine-grained soil layers may justify using the lower end of this range. Realistic filter cakes containing soil minerals provided substantially greater resistance than tests using pure bentonite. For the non-cohesive soils studied, results did not show a consistent decline in resistance with longer fluid exposure.<br /><br />Polymer-supported interfaces generally achieved higher friction angles, around 35–40°. Polymer tests produced thin filter-cake-like layers, and some polymer–additive combinations appeared to bond soil particles near the concrete surface. Superabsorbent additives were associated with somewhat lower resistance.<br /><br />The findings suggest that current design guidance may be conservative for concrete interfaces with non-cohesive soils. The authors recommend further field validation—particularly for finer-grained soils, longer exposure periods and polymer-supported construction—before revising international design or execution recommendations.

Keywords

bentonite suspension

polymer solution

shear transfer

concrete-soil interface

diaphragm walls

bored piles

filter cake

non-cohesive soils

interface friction angle

laboratory shear tests