CROSS Safety Report
Sudden hole in piling mat - update
This report is over 2 years old
This is further information about report 566 which highlighted how a sudden hole appeared in a piling mat.
The report at that stage was not conclusive about the cause of the hole and did not reflect the conclusions of later investigation. The report has therefore been updated.
Key Learning Outcomes
For construction professionals:
If there are discrepancies or a lack of clarity on the design intent seek clarification from the designer prior to commencing works
Regular monitoring of works as scheduled is critical to ensure they are in accordance with the design intent as demonstrated in the report
For temporary works designers:
Consider how the design intent can be effectively communicated to contractors on site, particularly on high risk or complex projects
On projects where there is a high level of uncertainty over the design approach, consider attending site to oversee the works
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Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-UK page.
This is further information about report 566 which highlighted how a sudden hole appeared in a piling matt. The report at that stage was not conclusive about the cause of the hole and does not reflect the conclusions of later investigation. The report has therefore been updated as follows.
Diaphragm walling, with a toe level 60m below ground level was being constructed within a dewatered cut-off box of sheet piled and slurry walls. The cut-off box was toed into the underlying clay and was 120m by 40m on plan. Trench stability calculations had shown that the groundwater level should be a minimum of 2m below the level of the bentonite support fluid and dewatering of the granular soils above the clay was initially carried out from a number of dewatering points.
During the excavation of two panels, in incidents about one week apart, holes hydraulically linked to the panels opened up in the adjacent piling mat. The panels were backfilled with granular material, so as to make safe and allow for further investigation. Initially the possibility of bentonite wash causing a vacuum was suspected.
Several factors identified
It is now considered that the hole was formed by a failure to maintain the required head difference between bentonite level and groundwater level. This was likely caused by some combination of the following factors:
The bentonite level not being maintained during the excavation process
De-watering and groundwater level monitoring arrangements implemented not being sufficient to draw down levels throughout the cut-off box
Lack of clarity with regards to the targeted design dewatered groundwater level in a number of temporary works design documents
Another possible contributing factor was the panel being left open over a weekend in a period of very wet weather
The importance of regular monitoring
Additional deeper dewatering points were installed following an investigation and the two panels were subsequently re-dug, as were the remainder of the diaphragm wall panels, without any further holes have occurring in the piling mat.
A regular monitoring regime was implemented for checking the bentonite level and groundwater level and function of the pumps throughout this period. In conclusion the reporter's firm believes that temporary works schemes reliant upon dewatering need their design intent clearly communicated and a robust approach adopted to implementation and monitoring on site.
the reporter's firm believes that temporary works schemes reliant upon dewatering need their design intent clearly communicated and a robust approach adopted to implementation and monitoring on site
Expert Panel Comments
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Lack of communication of the design intent through adequate drawings and other design documentation is a recurring theme. The CDM Regulations 2015 should strengthen this area with the introduction of the role of principal designer, whose obligations include the following:
Work with any other designers on the project to eliminate foreseeable health and safety risks to anyone affected by the work and, where that is not possible, take steps to reduce or control those risks
Ensure that everyone involved in the pre-construction phase communicates and cooperates, coordinating their work wherever required
Liaise with the principal contractor, keeping them informed of any risks that need to be controlled during the construction phase
In this case coordination of designs, clear communication of dewatering requirements, and communication of monitoring requirements should have provided the robust approach needed.
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