Settlements Around Tunnels in Soil: Three Case Histories: Fid-Bau Portal

Settlements Around Tunnels in Soil: Three Case Histories

H. H. MacPherson

This report presents the results of three case histories of field observations of settlements around tunnels in soil. Two of the cases are twin, 20-ft (6-m) diameter, single-track tunnels for the Washington, DC. Metro System: (1) Section F2a, F Route, is a steel segment lined tunnel in interbedded sands and gravels and clays, typical of downtown Washington; (2) Section G1, with an expanded rib and lagging lining, is in transition from these deposits to a hard, fissured clay. The third case is a 9-ft (3-m) diameter sewer tunnel with an expanded rib and lagging lining driven in dewatered, dense sands at Rockford, Illinois. Detailed measurements of subsurface settlements at points 3 to 6 ft (1 to 2m) above the tunnel crowns are used to determine sources and magnitudes of lost ground. Where the tunnel face was controlled to prevent large losses, ground losses due to overcutting and plowing of the shield were about one-half of the total estimated ground loss; incomplete filling of the tail void was the next biggest source of loss. Ground surface settlement data, including widths, slopes, and volumes of the surface settlement troughs are reported for several cross-sections on each tunnel and for points along the tunnel centerlines. The relationship between ground loss and surface settlement volume, as shown by sand bin model test data is also reported. A procedure for estimating ground loss and surface settlement in advance of tunneling is suggested.