Working Hypothesis, Special Laboratory Tests, Working Tools, Analysis of the Monitoring of a Pilot Embankment Built on Soft Clay in Santos with Wick Drains and its Application to the Final Design

Abstract

Routine in-situ and laboratory investigations carried out for the basic design failed to provide reliable values for compressibility and consolidation parameters of soft clay layers at the site of a terminal to be built in Santos. A pilot embankment was designed and built divided in three equal areas, two with wick drains in a square mesh at spacings of 1.2 m and 2.4 m respectively and one with no drain. The basic working hypothesis adopted by the authors was that, in the field, primary and secondary consolidation occur simultaneously. High quality standard and special (long term and relaxation) oedometer tests provided reliable values of Cc , Cr and σ’p and the OCR value of 2.1 as equivalent to the end of secondary consolidation allowing to estimate the total primary and secondary compressions of one of the most compressible layers (layer 6). No of the shelf tool is available to backanalyze the measured compression of a soft clay layer based on the adopted hypothesis. The first tool tested, the ε vs. log(σ’v) Bjerrum type abacus with lines of equal values built from the oedometer tests results, proved to be non applicable. The second tool tested, the same type of abacus extrapolated from the first one for the field conditions through Taylor and Merchant’s theory also proved to be non applicable. Both the third tool tested, the fitting of a theoretical Taylor and Merchant type curve to the measured compression curve in the area with no drain and the fourth tool tested, the fitting of a theoretical curve obtained through a method tailored by the authors designated “Primary Barron + secondary pseudo Taylor and Merchant” to the measured compression curves in the areas with wickdrains proved to provide excellent conformity of the theoretical and measured curves. The cv and ch field values thus obtained are of the same order of magnitude as the laboratory values and show the same trend to decrease when the effective stress increases, contradicting the current creed based on backanalysis through Asaoka’s method, i.e. considering that secondary consolidation only starts after primary consolidations ends, that field cv and ch values are commonly 10 to 100 times higher than laboratory values. Based on their results the authors conclude that the excellent conformity of the theoretical and the measured curves obtained with the working hypothesis they adopted, the results of the laboratory tests they performed and the tools they used for the back-analysis, leads to the conclusion that the working hypothesis, the laboratory tests and the tools proved to be very efficient and trustworthy in leading to reliable compressibility and consolidation parameters of the soft clay, and will be equally efficient and trustworthy when used for the final design of the improvement of the foundation soft clay layer