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800 Retaining wall excavation collapse

Report ID: 800

Published: Newsletter 55 - July 2019

Report Overview

A 6m deep excavation formed for constructing a retaining wall collapsed due to insufficient propping.

Report Content

On a domestic project in a major UK city, a deep excavation collapsed due to insufficient propping, says a correspondent.

The existing property formed part of a 1980s terrace and was constructed on reinforced concrete ground beams and piles. The project was to build a residential basement under the existing property. The basement size was approximately 12m long by 6m wide by 6m deep and the soil investigation showed made ground with decent gravel at around 5m deep.

The design for the basement was a separate reinforced concrete box, isolated from the piled structure to prevent differential movement of the terrace. The basement walls were to be cast using the hit and miss underpinning methodology, with 1.2m wide pins as this fitted well with cutting steel mesh sheets in half. The pins were designed to be stable when propped at the base by the basement slab. In the temporary case, the design of the pins required cross propping.

The designer had suggested that the pins were to be cast in two drives. The first drive to a depth of 3m forming a box around the site, with the pins cross propped. The second drive would then be to the full depth, with the pins again cross propped and the basement slab then cast. The designer had provided a temporary works design for this construction methodology.

From the beginning of construction, the contractor increased the width of the pins from 1.2m to up to 3m, according to the reporter. The designer told the contractor not to do this. The client then altered the brief during construction and asked for the basement length to be extended to the edge of the site. For this extension, the contractor did not excavate in 1.2m wide pins, but instead excavated across the whole 6m width of the site. The designer had not provided a temporary works design for the 6m wide excavation. The contractor did provide some temporary works in the form of steel trench sheets with walling beams and braces back to the concrete underpins already cast.

The reporter goes on to say that the temporary works for the basement extension was an ad hoc on-site provision, which was not designed, drawn or checked, and that the structural elements used were undersized. The 152 UCs used for the temporary works buckled as the entire excavation collapsed. There was a manhole at the edge of the site where the basement was extended towards. The manhole had been leaking, so this might have contributed to the collapse, says the reporter.

The reporter believes that it was most fortunate that no site staff were killed.

Comments

CDM 2015 makes clear the importance of ensuring that construction work is properly planned, managed and monitored. This applies to both temporary works as well as permanent works. When changes occur, which often happens, co-operation and communication between the various parties is equally important.

BS 5975:2019 gives recommendations for temporary structures on building sites, with practical guidelines on design, specification, construction and the use and dismantling of falsework. Whilst compliance with BS5975:2019 is not a legal requirement, it does provide an authoritative industry guide to the management of temporary works.

The Temporary Works Forum (TWf) recently published Information Sheet 6: The safe management of temporary works: The basics for small and medium-sized enterprises (SMEs). This advice provides a summary of the key components of BS5975:2019 for those managing temporary works in SMEs.

The HSE’s Section Information Minutes (SIMs) on The management of temporary works in the construction industry suggests that for smaller contractors, the principles of BS5975 should be in place if not the formal and specific procedures, in particular:

  • Ensuring a suitably competent temporary works designer/adviser is in place to supply an engineered solution.
  • Adequate information flow.
  • Design checking to an appropriate level.
  • Suitable verification of correct erection of the temporary works and someone overseeing and co-ordinating the whole process.

 

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View other CROSS reports published in Newsletter 55

Images

Unpropped face which collapsed (with 2m long staff at the base)



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