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CROSS Safety Report

Embankment stability concerns after heavy rainfall

Report ID: 3 Published: 1 November 2005 Region: CROSS-UK

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Overview

During the construction of a building the top edge of the steepest section of an embankment moved outwards and downwards after a period of heavy rainfall.

Key Learning Outcomes

For construction professionals and designers:

  • Careful consideration needs to be given to the stability of slopes and embankments at all stages of construction

  • During construction operations, be aware of the drainage scheme and maintain necessary drainage at all times

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A development required the creation of a flat surface on a sloping site. This was achieved by excavating chalk from a nearby hill and placing and compacting it in layers on the slope. The intended angle at the edge of the embankment was about 70o but when detailed plans were drawn up it became necessary to increase the size of the site so the slope at one end was nearly vertical.

To give stability the slope was reinforced with a proprietary material between the layers of fill.

A building was erected on the site with two faces a few metres from the edges of the embankment. Foundations were piled to bedrock below the fill.

At a stage of construction when the building was complete, but not all of the drainage was installed, there was a prolonged period of heavy rain fall. Water falling on the roof of the building was discharging through a drainpipe directly onto an area near the edge of the embankment.

The top edge of part of the steepest section of the embankment moved outwards and downwards and this was attributed to the effects of water on the chalk fill. The downpipe was then connected to the drainage system and the top of the embankment was sealed against further water ingress. 

There were concerns that the embankment could have been weakened to the extent that factors of safety against failure could have been compromised. As there was pedestrian access to the foot of the embankment this could have been serious.

There were concerns that the embankment could have been weakened to the extent that factors of safety against failure could have been compromised

Accordingly, the embankment was monitored for movement until investigations and repairs could be made. These included a new detail for the area between the edges of the building and the top of the embankment to prevent abnormal volumes of water penetrating the fill.

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