Subsurface Settlement Research Articles

Subsurface settlement profiles above tunnels in clays

Subsurface settlement profiles above tunnels in clays

Subsurface settlement profiles above tunnels in clays. Technical note : mair, R J; Taylor, R N; Bracegirdle, A GeotechniqueV43, N2, June 1993, P315–320

Subsurface settlement profiles above tunnels in clays. Technical note : mair, R J; Taylor, R N; Bracegirdle, A GeotechniqueV43, N2, June 1993, P315–320

Subsurface settlement profiles above tunnels in clays

Designers assessing the possible effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop, and how these relate to surface settlement profiles. The effect on piled foundations or existing tunnels depends on the width of the subsurface settlement profile and on the magnitude of the settlement. This note uses available field measurements and centrifuge model test data to determine how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays. The results reveal a fairly consistent pattern of behaviour: the width of the subsurface settlement troughs at depth is significantly greater than would be predicted by assuming a constant trough width parameter K of 0.5. Smaller more realistic subsurface settlements and strains will be predicted if account is taken of the proportionally wider trough widths at depth.

Instrumentation for Test Fill in Hong Kong

An offshore test fill was constructed over very soft marine clay to help develop and prove construction techniques for a large proposed reclamation project. The fill and foundation soils were heavily instrumented to monitor their performance during and after construction. The instrumentation was used to measure the rate and amount of settlements, lateral movements, and excess pore pressures. It consisted of pneumatic and hydraulic piezometers, settlement plates and pipes, pneumatic settlement gauges, inclinometers, subsurface settlement anchors, and surface movement points. Two major constraints of the instrumentation program were the large vertical strains (approximately 40%) anticipated and the need for overwater/underwater installation of many of the instruments. Special installation and protection measures were developed to overcome these constraints. Performance aspects of the instruments are discussed and typical plots of the data are presented. Additional field testing was undertaken during the monitoring period to verify and supplement the data being obtained from the instruments. This testing is discussed briefly.

US tunnel rim nears full circle : Byrd, T New Civ Engr17 Jan 1985, P29–31

Mount Baker Ridge Tunnel is being built in Seattle, to an external diameter of 25.3 m from the perimeter inwards by the stacked drift methd. Small horshoe section drifts at 2.9 m centres are being driven around the bore's circumference then filled with concrete to form a permanent lining. The 400 m long tunnel is being bored to take two roadway levels of a new interstate highway through the ridge in the least destructive manner. Piled circular access pits were firstly built at both portals, and then broken through using a roadheader. Subsurface settlement, which was limited to 25 mm was found to be 75 mm on the second drift at a depth of 60 m. This level was repeated in the third drift without any explanation, but the specified level has not been exceeded since. On average, 15 days are taken to reach the far portil when the 55 ton backhoe shields are carried round by truck. Concrete is pumped along a slick line suspended from the crown and withdrawn as the drift fills. A design review board and a disputes review board have been set up to settle any unexpected issues on site. (TRRL)