A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering in Geotechnical and Geoenvironmental Engineering, School of Engineering and Technology

Thesis (Ph. D.) - Asian Institute of Technology, 2011

This study focused on investigation on the current practice of foundation design forhigh-rise buildings in Bangkok and exploring possibility to improve by adopting the piledraft foundation concept. An interview survey with design firms revealed that the currentdesign practice was categorized into two groups. In the first group, the design was madewith collaboration between structural engineers and geotechnical specialists of which thework may not involve in-depth geotechnical analysis. In the second group, the design wassolely in the hand of structural engineers. The current design practice followedthestructural design approach which considered only checking limiting stress capacity offoundation without in-depth settlement analysis and consideration of soil-structureinteraction. Piles were designed to carry the whole building load although in many casesrafts were built at the level of stiff to hard clay layers. The conventional method ofcombined stress analysis and plate-on-spring method were popularly used to analyze pileloads and raft stresses. The results did not yield a cost-effective design outcome. A reviewon soil parameters in assessment of pile capacity and stiffness of piles was also made in thestudy. Over the year, the acceptable pile capacity increased with the maturing in bored pileconstruction technique and new development in piling technology.To explore the benefit in adopting the piled raft foundation design concept to high-rise building design in Bangkok, a comparative study on the results given by differentmethods of analyses were made namely the conventional combined stress method, piledfoundation by the plate-on-pile spring method, and piled raft foundation by the plate onspring method and by the 3D continuum FEM method viaPLAXIS 3D Foundation. The 3DFEM analysis gave more realistic result in load shared by piles, settlement, and raftbending moment. The load shared by piles was reduced to about 70-80% when the piledraft design concept and analysis was considered when the raft was placed in the stiff claylayer for buildings with 2-3 basement levels. The number of piles could be significantlyreduced particularly if the reducedsafetyfactor of piles per the true piled raft foundationconcept was adopted. Besides conducting a parametric study on influencing factors onpiled raft performance in Bangkok, a comparative analysis was made on a selected case ofhigh-rise building foundation design that utilizes a raft over piles. The actual design wasdone by the conventional method and the plate on pile springs method. Analysis along theline of piled raft foundation concept via the 3D FEM analysis showed that the load sharedby piles was only 84% if the raft base level was deepened. Number of piles could besignificantly reduced. The load distribution among piles in the raft was drastically variable.Apparently, the actual design following the current design practice in Bangkok yielded ahighly conservative outcome. Potential improvements for general pile foundation practicewere outlined.