RESPONSE OF A PIPELINE TO NEARBY DEEP TRENCHING IN BOULDER CLAY

Renewal, replacement, or new construction of sewers that involves deep trenching may lead to differential ground movement with an associated risk of damaging nearby shallow buried service pipelines. A series of trenching trials are being carried out in a range of ground conditions to evaluate this risk and in this particular study the response of a pipeline to adjacent deep trenching in boulder clay is investigated. An instrumented 100mm diameter cast-iron pipeline was installed at a cover depth of 0.75M some 2 months prior to deep trench excavation. It was located parallel to the projected trench and 2.25M from one face. Bending strains up to a maximum of 130 x 10 to the minus six were measured in the pipeline during its installation. During the excavation of the adjacent deep trench, peak bending strains of 70 x 10 to the minus six and pipe settlements and lateral movements, each of about 10mm, were recorded. No significant changes occurred during reinstatement of the deep trench. Measured movements of the pipeline were correlated with those evaluated from strain data using a semi-graphical integration technique. Using this technique, an assessment of the effect of nearby trenching on the likely performance of nearby service pipelines in boulder clay, compressible alluvium, and London clay has been produced. (Author/TRRL)