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https://hdl.handle.net/2142/19016
Description
Title
Soil improvement by vertical drains
Author(s)
Lo, Dominic O. Kwan
Issue Date
1991
Doctoral Committee Chair(s)
Mesri, Gholamreza
Department of Study
Civil and Environmental Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Civil Engineering
Language
eng
Abstract
Vertical drains are used in soft clays and silts to accelerate primary consolidation. In this study a practical method for predicting the magnitude and rate of settlement and pore water pressure dissipation of soft clays under embankment type of loading is described. A computer procedure, ILLICON, was developed within the framework of the traditional concepts of soil mechanics for consolidation analysis of soft clays with or without vertical drains. ILLICON allows multi-layer analysis with each individual layer having its own compressibility and hydraulic properties which may vary during consolidation. The vertical drain may have well resistance and the effect of drain installation is taken into account by assuming a smear zone at the drain periphery.
ILLICON was applied to analyze field settlement and pore water pressure observations at Ska-Edeby test field in Sweden, Changi airport in Singapore and at the proposed replacement airport at Chek Lap Kok in Hong Kong. Detailed interpretation and evaluation of the consolidation properties of soft clays, including the compressibility parameters, preconsolidation pressure and coefficient of permeability is presented. These properties can be determined from the standard oedometer test with direct permeability measurements.
Vertical drain performance, which is expected to depend on the quality of drain and the installation procedure, has been evaluated in terms of well resistance and smear effects. In this investigation the discharge capacity of vertical drains is analyzed in terms of a well resistance factor which is a function of the discharge capacity of drains as well as the horizontal permeability of the soil adjacent to the drain and the maximum drainage length of the drain. It is shown that for typical soil conditions and loads, a threshold value of well resistance factor defines a drain with no well resistance. The magnitude of minimum discharge capacity required for no well resistance, computed from the threshold value of the well resistance factor, are compared with the mobilized discharge capacity back-calculated for four major construction projects that employed prefabricated drains. The results of analyses indicate that at three sites the mobilized discharge capacity was generally at least as large as the minimum discharge capacity required for no well resistance. At the fourth site the mobilized discharge capacity was less than that required for no well resistance. However, the minimum discharge capacity required for no well resistance decreased during consolidation and the wick drains performed satisfactorily.
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