Back Analysis and Performance of a Cantilever Portion of Excavation in Mixed Soil Profile-Paper

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The paper examines the performance of a 6.5 m unsupported cantilever section of a 600 mm thick diaphragm wall constructed for a 12 m deep basement in Chennai. The mixed soil profile comprised clay layers over dense silty sand and weathered rock. Two levels of ground anchors were planned, with the first installed at 6 m below ground level. Because the wall was temporarily unsupported during excavation to the first anchor, its movement and groundwater conditions were closely monitored.<br /><br />Four inclinometers and four piezometers recorded wall deflections and groundwater levels. WALLAP analysis, initially based on a groundwater level 2 m below ground and average design soil parameters, predicted a maximum wall-top deflection of about 50 mm. Measured movements were considerably smaller, reaching approximately 10–20 mm, with a maximum of about 20 mm. During construction, groundwater was around 5 m below ground level, and the excavation period was under three months.<br /><br />Back analyses explored soil drainage behavior, groundwater level, soil strength and stiffness, and wall embedment. Drained analysis predicted greater deflection (70 mm) than undrained analysis, while lowering the groundwater level in drained conditions reduced the prediction to 57 mm. Increasing estimated soil strength and stiffness reduced predicted movement to about 35 mm. The study concludes that short-term undrained behavior was appropriate for the clayey soils and that groundwater lowering had little effect on movements under these conditions.<br /><br />The lower-than-predicted field deflections were attributed partly to the greater stiffness of the wall designed for the full-depth excavation and to its 700 mm socket into weathered rock, which helped provide fixity. The findings highlight the importance of considering soil stiffness, construction duration, wall penetration, and actual groundwater conditions when predicting retaining-wall movements.

Keywords

diaphragm wall

cantilever excavation

ground anchor

wall deflection

groundwater level

inclinometer

piezometer

undrained behavior

soil stiffness

WALLAP analysis