Factors affecting field instrumentation assessment of marine clay treated with prefabricated vertical drains

Abstract

The use of prefabricated vertical drains with preloading option is the most widely-used ground improvement method for the improvement of soft clays in land reclamation projects. Surcharge of equivalent working load after taking into account submergence effect and settlement of the reclaimed land is placed until the required degree of consolidation of the soft clay is obtained. The assessment of the degree of consolidation of the marine clay is of paramount importance prior to the removal of preload. This analysis can be carried out by means of observational methods with the use of field settlement plates and piezometer monitoring. Field settlement monitoring data can be used to ascertain the settlement of the reclaimed fill from the time of initial installation. The field settlement data can be analysed by the Asaoka and Hyperbolic methods to predict the ultimate settlement of the reclaimed land under the surcharge fill. Back-analysis of the field settlement data by the Asaoka method will enable the coefficient of consolidation due to horizontal flow to be closely estimated. Piezometer monitoring data can be analysed to obtain the degree of consolidation of the improved marine clay. Back-analysis of the piezometer data will also enable the coefficient of consolidation due to horizontal flow to be estimated. The authors’ findings of the Asaoka method reveal that the magnitude of ultimate settlement decreases and the degree of consolidation subsequently increases as a longer period of assessment is used in the prediction. It is apparent that as the time interval increases, a cut-off time interval is obtained after which increasing time intervals would converge to the same magnitude of ultimate settlement. The authors’ findings of the Hyperbolic method reveal that the magnitude of ultimate settlement increases and subsequently the degree of consolidation decreases as a longer period of assessment is used in the prediction. The degree of consolidation predicted by the piezometers is found to be in good agreement with the Asaoka and Hyperbolic methods for the early period of assessment. However as the assessment period increases, the piezometer indicates lower degree of consolidation as compared to field settlement predictions. The aim of this paper is to highlight the significance and influence of various factors that affect predictions by the Asaoka, Hyperbolic and piezometer assessment methods.