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

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The study investigated shear resistance at the soil–concrete interface of diaphragm walls and bored piles, focusing on filter cakes formed by bentonite or polymer support fluids. Existing design guidance assumes that filter cakes reduce interface friction, but earlier field and laboratory findings were inconsistent, particularly for polymer systems.<br /><br />Researchers combined field investigations at three diaphragm-wall sites with small-scale direct shear tests on artificial soil–bentonite mixtures and large-scale diaphragm-wall element tests. The work examined filter-cake composition and thickness, bentonite content, soil fines, penetration time, and support-fluid type.<br /><br />Field samples showed that bentonite filter cakes consisted mainly of particles from the surrounding soil, with thicknesses varying from roughly 0.3 to 6 cm. Bentonite made up about 10–20% of the cake in many cases. Laboratory tests found that shear resistance decreased as bentonite content increased; for cakes containing 10–20% bentonite, friction angles were approximately 20–28°. Results also depended on the grading of suspended fines and on adequate consolidation.<br /><br />In large-scale tests, bentonite filter cakes produced friction angles of about 25–30°, while thin cakes formed by ANH/CMC polymer solutions produced values around 35–40°. Polymer cakes showed no reduction in resistance with increased exposure time, and inner cakes could provide additional stability in fine- to medium-grained sand.<br /><br />Overall, the findings suggest that current recommendations are too conservative for the tested conditions. Interface resistance depends strongly on filter-cake composition and formation, so realistic representation of the cake is important in design.

Keywords

shear resistance

soil-concrete interface

diaphragm walls

bored piles

filter cakes

bentonite

polymer support fluids

direct shear tests

friction angle

interface friction