Using tunnel boring data to augment the geological model

A detailed borehole investigation has recently been undertaken for an extension to the Thames Water ring main in north London. Results of this were presented in a preconstruction geological model. This formed the basis for tunnel designers to assess ground conditions prior to construction and implement mitigation design and/or contingency plans in the event of adverse geological conditions being encountered. Previous sections of the ring main were constructed using open-faced tunnelling techniques. These allowed for inspection and continuous record of ground conditions at the tunnel face, enabling an identification of the geology and prediction of potentially adverse ground ahead. In facilitating such close geological supervision, tunnelling benefited from significant programme savings – in particular, a reduction of risk to the health and safety of underground personnel. In constructing the tunnel extension, a closed-face tunnel boring machine was opted for; this largely concealed the exposure of geology in situ during the excavation cycle. Apart from a limited number of tunnel face inspections, an indication of the geology was formed from observation and logging the spoil arising from the tunnelling machine, aided by an assessment of the performance of the machine itself in response to changes in geology over the alignment. These activities were necessary in validating the preconstruction geological model and in providing assurances to the on-site design and construction teams.