Systematic laboratory and field investigations on the soil-concrete interface of diaphragm walls and bored piles with focus on the influence of wall roughness-Paper

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The study investigates how wall roughness, support-fluid exposure, and fluid–soil interactions affect shear resistance at diaphragm-wall and bored-pile interfaces.<br /><br />Field observations and laboratory direct-shear tests show that greater concrete-wall roughness increases interface shear strength, even when a filter cake separates the soil from the concrete. In fine-grained soils, filter cakes were generally absent or thin, allowing wall roughness to dominate. In silty and sandy soils, cakes often exceeded the roughness depth; in gravel, discontinuous cakes and coarse grains could permit direct soil–concrete contact.<br /><br />Longer fluid exposure (roughly 24–30 versus 72–78 hours) did not consistently change filter-cake thickness or bentonite content. Fluid regeneration and slurry composition nevertheless mattered: effective regeneration could produce cakes with higher bentonite content, while differences in particle loading influenced cake formation and properties.<br /><br />Bentonite suspensions and polymer solutions interact differently with soil. In tests on sand, polymer solutions formed more cohesive, stable contact zones and could stabilize penetrated soil. Tests on clay indicated that polymer solution affected soil uptake, stiffness, consistency, and shear strength less than bentonite suspension, while both performed better than water. These results are preliminary and may not represent all field conditions.<br /><br />Overall, interface performance depends on the combined effects of wall roughness, cake thickness and composition, soil type, fluid properties, and construction practices. The authors call for further research, particularly on cohesive soils, to clarify these interactions and support improved design guidance.

Keywords

diaphragm-wall interface

bored-pile interface

wall roughness

shear resistance

filter cake

bentonite suspension

polymer solution

soil-fluid interaction

direct-shear tests

fluid exposure time