Consolidation Settlements and Pore Pressure Dissipation

by Demetrious C. Koutsoftas, (M.ASCE), Dames & Moore, San Francisco, United States,
Raymond K. H. Cheung, Dames & Moore, San Francisco, United States,



Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments

Abstract: A test fill was constructed off the coast of one of the outer islands in Hong Kong to evaluate the consolidation characteristics of a soft plastic clay deposit known as Hong Kong mud. Extensive instrumentation was installed to monitor surface and subsurface settlements of the soft mud, as well as pore pressures at various depths in the mud. The results of the measurements were analyzed, correlating the observed pore pressures with corresponding settlements and their distribution with depth. The test fill involved a main test area with four quadrants, each 50 m×50 m in plan. One of the quadrants served as control to evaluate the consolidation of the mud without drains. Plastic band chains were used in two of the other quadrants and large diameter (500 mm) sand drains were used in the fourth quadrant, to accelerate the consolidation of the mud. At various times after filling, piezoprofile tests were carried out to supplement the measurements made with permanent piezometers, to better define the distribution of pore pressures with depth. From the pore pressure profiles, settlements were calculated using 1-dimensional compression theory. The distribution of calculated settlements with depth was compared with measured settlements. There was good agreement between the measurements and the calculated settlements. A value of compression ratio, CR, of 0.40 was backcalculated from these analyses. Settlements and pore pressures were also measured at four areas outside the main test area. They included an untreated area (where no drains were installed) and three areas where band drains had been installed at 1 m, 2 m and 3 m triangular spacings, respectively. Measured excess pore pressures and consolidation settlements are discussed. In three areas of the test fill where the primary consolidation settlements had been completed, the measurements showed substantial residual excess pore pressures. The causes for and significance of the residual pore pressures are discussed.

Subject Headings: Pore pressure | Soil settlement | Consolidated soils | Pressure measurement | Soil pressure | Mud | Drainage | Fills | China | Asia | Hong Kong

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