Geotextiles and Geomembranes 22 (2004) 415–437 Factors affecting field instrumentation assessment of marine clay treated with prefabricated vertical drains A. Arulrajah a, *, H. Nikraz a , M.W. Bo b a Department of Civil Engineering, Curtin University of Technology, Bentley Campus, Perth, Australia b Bullen Consultants Ltd., UK Received 3 July 2003; received in revised form 9 September 2003; accepted 21 September 2003 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 settlementofthereclaimedlandisplaceduntiltherequireddegreeofconsolidationofthesoft 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 ofassessmentisusedintheprediction.Itisapparentthatasthetimeintervalincreases,acut- 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 ARTICLE IN PRESS *Corresponding author. 11, Jalan 36/70A, Desa Sri Hartamas, Kuala Lumpur 50480, Malaysia. Tel.: +60-12-2069437; fax: +60-3-23000745. E-mail address: aarulrajah@maxis.net.my (A. Arulrajah). 0266-1144/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2003.09.001