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607 Settlement of driven piles

Report ID: 607

Published: Newsletter No 46 - April 2017

Report Overview

A railway platform was constructed on piled foundations and it was noticed that one pair of piles and their associated cross head settled soon afterwards.

Report Content

A railway platform was constructed on piled foundations and it was noticed that one pair of piles and their associated cross head settled soon afterwards. The platform was not open to traffic at the time of the failure as the route was out of use. Subsequent load testing of other pile crossheads revealed further defective piles leading to further platform settlement. The platform in the area of the failure was dismantled and the piles re-driven. The subsequent investigation found that the installation of the piles was inadequately supervised to ensure that the intent of the pile design had been met. This was that the piles should be driven until they were founded on the underlying bedrock or a specific “Set” value had been achieved. This could have been because the supervisor was inadequately briefed or failed to undertake adequate checks. Dynamic testing of the piles failed to reveal that the piles did not comply with the design intent and did not provide sufficient support for the station superstructure. It is considered possible (although unproven) that this was caused by a “false set” phenomenon which can occur in some ground conditions. It was found during investigation that the use of images of signatures in electronic files used for quality assurance records provided no assurance that the specified checks were carried out by the person stated, on the date stated. If such records are not trustworthy, they have no value.

Comments

It appears that a combination of inadequate consideration of the design requirements and inadequate site supervision lead to the installation of piles which were too short and did not meet design criteria. For this to happen on a whole row of piles is unusual and may be the reason why a “false set” situation was considered as a possible reason. On the other hand, the load on the piles was presumably low so the ground in which they were installed must have been poor. It is essential that an adequate inspection and test plan is agreed with the Designer, with adequate supervision on site to ensure that design intent is assured. Dynamic pile tests may not provide a direct measure of pile settlement under loading, but the results may be better relied upon if a thorough site investigation report supports the design assumptions.  ‘False set’ readings may occur for piles driven into cohesive soils when insufficient time has been allowed between pile driving and pile testing for the dissipation of pore water.

 The issue of electronic copy signatures in the QA process is surely not what was intended and illustrates another problem with over-reliance on computer technology. A false sense of security can result from the assumption that those in a QA process are competent. In another case untrustworthy records were detected after a small area of a structure failed (despite perfect QA record). The records for the rest of the structure were deemed to have no veracity and the entire structure had to be strengthened as if all hidden detail, which could not be verified in the as built state, was defective. Had the original records been obviously wrong or absent the problem would have cost £x to remediate; apparently the final bill was £16x.


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