Settlement of sewer pipes in soft clay installed in a trench

Excavation for installation of sewer pipes in soft clay often require the use of retaining walls and in Sweden the most commonly used method is sheet piling. A few weeks after installation, the sewer pipe often show considerable settlement and the effect is more pronounced for trenches supported by sheet piles compared to shallow open excavations. The settlement sometimes results in reinstallation of the pipes causing extensive costs for both the contractor and the owner. This paper presents results from a full-scale test in soft clay, where 30-meters of sewer pipes were installed in a 3 m deep trench supported by sheet piles. The instrumentation included inclinometers, bellow-hose-settlement-gauges, and piezometers. Measurements were made through the different stages of installation up to 19 weeks after completion. The purposes of the measurements were mainly to study the following elements; elastic heave, deformations of the retaining wall, settlements and time effects.