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CROSS Safety Report

Unsafe excavation practices

Report ID: 1026 Published: 30 July 2021 Region: CROSS-UK


Overview

A builder undermined the existing footing of a masonry wall without being aware of the risks.

Key Learning Outcomes

For contractors and builders:

For civil and structural design engineers:

  • Ensure that the requirements of CDM Regulations 2015 for site specific hazards (such as the risk of undermining existing structures) have been clearly communicated to the principal contractor during the project planning stage
  • Consider how the design risks, such as the risk of undermining existing structures, can be effectively communicated to contractors on site

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A reporter was called to inspect a foundation excavation and found the situation as shown in Figure 1. The builder said ‘I haven’t undermined the foundation at all.’  The outer face of the excavation was neatly in line with the outer face of the 225mm wall to the neighbour’s house; whose footing can be seen to the right. 

When told that this was highly dangerous, their response was ‘I’ve been doing it like this all my life’.  The reporter told them that if he continued in this manner then it would probably bring his life to an early conclusion.

A masonry wall adjacent to an excavated trench has been undermined
Figure 1: Unsafe excavation undermines masonry wall

The builder did not understand that:

  • the load was now concentrated on a footing that was only two thirds of its original width

  • the bearing capacity of the outer edge was almost zero, due to the lack of lateral resistance

  • there is a natural tendency for any trench to fail under the weight of the soil alone never mind with a foundation load applied to its edge

There was no appreciation of the risk to their own life never mind the risk to the householders’ lives.

Emergency propping was immediately installed, and the new foundation constructed a day later, and nothing untoward occurred on this occasion. The reporter can recall many cases where the outcome wasn’t as happy; the number of lives lost or forever changed by such practice would be a sobering statistic worth knowing.

Emergency propping was immediately installed, and the new foundation constructed a day later, and nothing untoward occurred on this occasion

The problem was ignorance. Not of soil mechanics as much as a lack of awareness of the limitations of knowledge. This is why the licencing of builders is important; not to set a bar which prevents conscientious people entering the business, but to give something which can be taken away for failure to train, supervise, and educate themselves and their employees.

Expert Panel Comments

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Undermining any foundation to the degree shown is an absolute failure without the sanction of an engineer with a method statement and risk assessment. Normally a method statement would give an indication of a type of hit and miss sequence with a limit on width exposed of between 1 and 1.2m depending upon the wall construction and ground condition.

Awareness is key to improving safety

Training for excavations gives different risk profiles for depths of excavation and therefore solutions for propping, though the circumstances in which the work is being undertaken does need to be taken into account for the risk assessment and mitigation measures. As the report says awareness is key and despite having done it this way all of their career is no guarantee that today will not be a bad day for the builder.

As a generalised statement, the key issue with construction – anything from work in the ground to erecting a steel frame, is to appreciate that at any intermediate stage prior to completion, possibilities of instability exist and possibilities of design conditions (for strength) are likely to be more adverse than in the final assembled state. Thus design as a whole must consider how a project will be built so that the issues of strength and stability at all intermediate stages can be verified. Frequently this will require dialogue and consultation between those charged with designing and those charged with executing. 

Thus design as a whole must consider how a project will be built so that the issues of strength and stability at all intermediate stages can be verified

Training for small building firms

As is the case with so many reports of this type there will probably not have been any engagement with an engineer, so it is essential to provide more training for small builders, modelled on what is carried out by larger companies. Licensing, which is often talked about, may be associated with training but would require extensive consultation and legislation. Of more immediate use would be publicity about risk at the level where it would be seen by small builders. For example, notices and leaflets in builders’ merchants.

HSE offers extensive advice on excavations, see: Structural stability during excavations, which begins with the statement:

‘The law says you must prevent danger to workers in or near excavations. To maintain the required precautions, a competent person must inspect excavation supports or battering at the start of the working shift and at other specified times. No work should take place until the excavation is safe.’

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