Evaluation of monitoring items for adverse ground conditions in subsea tunneling

Abstract

Three monitoring items, i.e. displacement vector orientation, preceding displacement, and water inflow, were compared to determine which one best provided supplementary and valuable information on the ground ahead of a subsea tunnel face when used with probe drilling. The geotechnical factors affecting tunnel stability were selected from the case studies on the construction of subsea tunnels, and six representative types of adverse ground condition were built with combinations of the factors. The capabilities of monitoring items were compared in depression and weakness zone types that were selected as major adverse ground conditions with three criteria, i.e. the capability of categorizing the type of adverse ground condition, the early-warning time, and the response capability. A three-dimensional finite element analysis program was used to simulate the process of subsea tunnel excavation, and the Analytic Hierarchy Process was used to select the optimum monitoring item. A comparison of the results showed that the vector orientation was the optimum item for categorizing ground type, and the preceding displacement and the water inflow possessed the best capability for early warning according to ground type. The response capability of water inflow was assessed as the best for three types of weakness zones, and that of vector orientation was best for depression type. In 13 cases where the priorities of comparison criteria were different, the vector orientation and the water inflow were respectively chosen in six cases, and there was little difference between the two items in the case where the criteria were equally important. While the application of one item alone may be vulnerable to a specific adverse ground condition, the monitoring capability could be overall improved by the adoption of both items.