Movements Around Transit Tunnels in Mixed Ground

This paper describes the ground movements measured at a Test Section during construction of twin rapid transit tunnels in Cambridge, Massachusetts. The Test Section was located in an area of rock, soft ground and mixed face tunneling, with the alignment of the twin tunnels approximately 100 feet below ground surface. Overburden soils consist primarily of a very dense, saturated glacial till containing cobbles and boulders, with a weakly metamorphosed, fractured shale bedrock below. Instrumentation at the Test Section was installed in three cross-sections: one with the tunnel headings entirely in rock, a second with the tunnel headings in soft ground, and a third in a mixed face area. The field measurements are analyzed to show the effects of ground losses at the tunnel headings vs. distance away from headings, the effects of single vs. twin tunnel construction, and the effects of mixed face vs. rock and soft ground tunneling on ground movements

Submarine landslides along the mixed siliciclastic-carbonate margin of the Great Barrier Reef (offshore Australia)

Submarine landslides on modern mixed siliciclastic-carbonate margins are poorly understood compared to their counterparts in other settings. We present a synthesis of four representative submarine landslides types along the Great Barrier Reef margin, the largest extant mixed siliciclastic-carbonate province in the world. The investigated examples are 5–31 km in length, extend over 18–528 km2, and have remobilized an estimated 0.025–32 km3 of sediments. They display morphological features corresponding to debris avalanches and slides. The estimated timing of two dated landslides is coincident with deglaciations corresponding to the transitions MIS 12–11 and MIS 2–1. Large seismic events were the most likely triggering mechanism for the landslides, where high pore water pressure in examples close to paleo-deltaic systems could also have preconditioned the eventual failure. A potential preconditioning factor, yet to be confirmed, is the geologic control associated with alternating mixed siliciclastic and carbonate sediments in the failed lithologies. The Gloria Knolls Slide is large enough to have significant tsunamigenic potential. Tsunami simulations show that this landslide would produce a sizable tsunami under present-day sea level conditions, with coastal run-up heights of 0.5–2 m. We highlight a reef buffering effect due to broader-scale shelf bathymetry and the complex structure of coral reefs