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CROSS Safety Report

Sudden hole in piling mat

Report ID: 566 Published: 1 April 2016 Region: CROSS-UK

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Overview

A reporter describes how a 450mm diameter hole appeared in a piling mat whilst digging a trench that was 0.5m away.

Key Learning Outcomes

For construction professionals:

  • There may be circumstances when holes may suddenly appear on site without warning, and personnel should be aware and on the lookout for warning signs

  • Competent supervision during construction works can help to identify unexpected issues when they arise

Full Report

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This concerns a near miss on a site where a piling wall was being constructed. The reporter says that whilst digging a trench a 450mm diameter hole appeared in the pile mat about 0.5m from the guide wall (Figure 1). As the grab was being lowered into the 55m deep trench, bentonite from the wall panel gushed out from the hole. This was found to be 1.2m deep and went under the guide wall and into the panel trench.

Image
Figure 1: hole shown to the right of the guide wall

Steel plates were introduced into the front of the guide wall (piling platform side) and the opening filled with concrete. The reason for the hole appearing is not known although one possibility is that the bentonite wash from the grab being continually withdrawn created a vacuum. This pulled in loose granular material from beneath the guide wall thus causing a cavity which extended into the piling platform.

The integrity of the pile mat was then checked to ensure that work could safely continue. Had there been personnel in the vicinity of the hole when it appeared the consequences could have been serious.

Expert Panel Comments

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Excavations always carry some risk partially because of unknown or unidentified conditions below the surface. The lesson to be learned from this is that there may be circumstances when holes may suddenly appear, and personnel need to be aware and on the lookout for warning signs.

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