Design, Modelling and Construction of a 12-Sided Diaphragm Walled Shaft-Paper

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The project involved constructing a sewage pump station and detention tank in Officer South, Victoria. The original design used reinforced-concrete walls within sheet-piled cofferdams, but diaphragm walling was selected as an alternative.<br /><br />Because diaphragm wall panels must meet in straight, aligned joints, the pump-station shaft could not be built as a true circle. Instead, it was formed as a twelve-sided polygon. Unlike a circular shaft, which largely resists uniform external pressure through hoop compression, the polygon could experience significant in-plane bending, especially where soil pressures differed around the shaft.<br /><br />The design team used a two-stage finite-element approach because suitable 3D analysis was unavailable. First, an axisymmetric model estimated horizontal effective earth pressures and vertical bending moments with depth. Next, horizontal slices of the shaft were modelled in plane strain, with gravity disabled and the calculated pressures applied as boundary loads. Analyses at several depths considered variations in soil stiffness and pressure. The resulting bending moments and shear forces informed the reinforcement design.<br /><br />The diaphragm-walled shaft and detention tank were successfully constructed. Inclinometers monitored wall movements during excavation. Measured movements followed a broadly similar profile to the predictions, but were generally smaller, indicating that the ground was stiffer than assumed. Maximum measured movement was under 4 mm. The final predicted and measured profiles agreed particularly well through much of the wall depth.<br /><br />The case study demonstrates how a practical 2D modelling method can be used to design reinforcement for an unusual diaphragm-walled shaft, and how instrumentation can validate the analysis and reveal conservative assumptions.

Keywords

sewage pump station

detention tank

diaphragm wall

finite element analysis

polygonal shaft

sheet-piled cofferdam

reinforced concrete

earth pressure

wall movement monitoring

inclinometer measurements