Large-diameter tunnelling collapses: recovery design and execution

Abstract

A number of large-diameter tunnels with completed excavation areas up to approximately 150 m2 were mined in weak sedimentary rock and soils using sequential excavation and support (SES) methods. The ground is weak to extremely weak. Tertiary sedimentary rock and soil, poorly cemented, locally folded and faulted, and has moderate to high groundwater tables. Excavation was carried out using hydraulic excavators and breakers. Owing to the low strength of the ground mass and the large excavation cross-section, the section was mined in three stages - a top heading with a height of about 5-7 m, followed by a bench with a height of about 3-4 m and an invert with a height of about 2 m. The total excavation height and span reaches maximums of approximately 12-13 and 14-15 m respectively. During or immediately after each excavation stage, initial support, termed the ''outer lining'', is installed to support and stabilize the excavated cross-section prior to installation of a cast concrete inner lining. The outer lining consists of steel ribs (lattice girders or H-profiles), shotcrete with welded wire fabric (WWF), rock dowels. Depending on the ground quality pre-support of the upper part of the tunnel (crown zone) ahead of the excavation face is required. This comprises forepoling with various combinations of steel bars, pipes, sheets and grouting. Six collapses of part or most of the tunnel crown occurred during the tunnel drives. Four occurred during the top heading stage and two during bench or invert excavation and support installation. The length of the collapsed sections varied from 20 to 277 m. Two of the collapses can be referred to as ''small'', one medium and three ''large''. Remedial measures were designed to allow re-mining through the collapsed lengths using the same excavation method (SES) and, thus, essentially the same tunnelling equipment and personnel. The re-design processes comprised post-collapse/schedule reviews and evaluations, site investigations, ground treatment and re-design of the outer lining. The processes required considerable time and careful management to avoid and mitigate potential delays to tunnel completion. The process of design management, design and execution of the recoveries is the subject of this paper. A tunnel cross-section with typical pre-support and outer lining required for the re-mining is shown in the underlying figure. At the time of preparing the paper the re-mining of the collapses were complete or well underway. Placement of the inner lining had been completed in two of the tunnels and final design of the inner linings in the other tunnels was underway. This process requires the evaluation of the performance of all the geological, geotechnical and monitoring data measured during the excavation of the top headings. The performance to date shows that SES soft ground tunnelling methods can be applied to the re-excavation of large span excavations through collapsed ground provided an extensive program of excavation pre-support is undertaken followed by careful excavation and tunnel monitoring. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.