Abstract. Construction of the 276 and 312 ha reclamation on the Holocene marine clay deposit in Jakarta Bay is on-going. Settlement has been measured during the construction in order to satisfy the requirement of residual settlement. This paper briefly discusses limitation of accuracy of settlement prediction based on purely parameters derived from laboratory and in situ testing, in particular the Holocene marine clay. A reliable 'observational' method which makes full use of the monitoring data is described and estimates reliable residual settlement. More accurate settlement method is then also briefly described taken into account complex loading history. Finally, the factors, which are source of inaccurate settlement predictions and its implications, are included in the discussion.

The northern Semarang area is prone to a land subsidence problem as the underlying soil consists of a thick layer of normally consolidated clay which also exacerbated by groundwater extraction to meet water demand. The downdrag impact on a pile structure due to the water extraction from two major aquifers that have been used for domestic consumption was investigated. A 3D-finite element analysis was performed to a raft foundation with steel pipe piles. The study used Banger polder data and soil characteristics based on previous investigations. The evaluation was done by comparing the results of negative skin friction (NSF) and total settlement within 50 years period under continuous extraction and one without. From the result, the corner pile located farthest from the pump mobilized NSF of 64% of the total negative skin friction which was highest than middle and interior-corner piles of 28% and 20%, respectively. The groundwater extraction was also found to contribute to almost double the settlement in 50 years in the non-piled area, from 0.24 m without extraction to 0.42 m accounted for. Also, the empirical 0.75Lpile for the maximum neutral point of the pile is in agreement in the result of this study.

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