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Design and performance of two piled-raft foundations.

Thorne, Anthony Martin. (1991) Design and performance of two piled-raft foundations. Doctoral thesis, University of Surrey (United Kingdom)..

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Abstract

The thesis examines two projects, a grain silo at Corby, Northamptonshire, and a store with a basement at Basildon, Essex. The two sites are examined from their conception, through the site investigation, design, construction and finally reports on the completed structures. In the case of the grain silo at Corby the contract was termed "fast track" which meant that the designers and contractors were on a very tight schedule to complete the structure. The thesis discusses the implications that this had on the decisions reached regarding the foundation and pile design as the site investigation specification had not included obtaining parameters for the founding limestone stratum and the underlying Lias Clay. The first pile test at the Corby site resulted in a failure and it showed that heave was a problem for the closely spaced piles. The piling contractor stated that pre-boring to any depth was not necessary. The results from a re-tapping exercise show that pile heaves were the norm and that even partial pre-boring made little difference to the magnitude of the heave. The complex was monitored by precise levelling. The indications from settlement v log(time) plots are that only about 40% consolidation had occurred at the last date of measurement. Even so the indications are that the design method overestimated the magnitude of the initial settlement by at least a factor of 4. A simple back analysis would suggest that this is primarily a result of a low value of Young's Modulus calculated from the laboratory work. It is also suggested that a better understanding of the stiffness of the overall structure would lead to a better prediction of across slab differential settlement. The Basildon case record looks at the design principles behind allowing substantial load sharing between the piles and the raft. The thesis highlights the fact that there was not the correct guidance for the site investigation to produce those parameters necessary for a particular design. A stiffness value for the London Clay was not determined and the ground water conditions in the area of a deep basement were not monitored prior to construction. A long term design condition of maximum water pressure was present during construction when there were minimal dead loads to resist the upward force. It is felt that the construction method of taking all the dead load through the falsework scaffold system helped to resist the slab doming the water pressure induced. As a result the piles were taken into tension at a stage in the construction process when they were designed to be in compression. The thesis proposes that the water pressure present has domed the slab such that the raft cells have broken contact with the London Clay and as a result are now weighing themselves in the form of "negative" effective stress readings. The method of design has predicted safe loads in the piles with the exception of the corner piles. The design load for the corner piles appear to have been exceeded. The compressive steel in the pile was able to resist a tensile load of this magnitude and no damage resulted.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors :
NameEmailORCID
Thorne, Anthony Martin.UNSPECIFIEDUNSPECIFIED
Date : 1991
Contributors :
ContributionNameEmailORCID
http://www.loc.gov/loc.terms/relators/THSUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Depositing User : EPrints Services
Date Deposited : 09 Nov 2017 12:16
Last Modified : 09 Nov 2017 14:44
URI: http://epubs.surrey.ac.uk/id/eprint/843889

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