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  • Clio Domenech

Sequence of Works Excavation Method for Safer Extensions

Today, we'll talk about a specific excavation method known as the "Sequence of Works," sometimes referred to as the "hit and miss" excavation. We will go over when a Sequence of Works method is required and what to do when a Party Wall Surveyor requests such design.

If you're planning a rear or front extension, there's a crucial step to take before diving into the work. Check whether your neighbour's property already has an extension, which is particularly relevant for terraced and semi-detached houses. This precaution is vital when existing buildings are close to the proposed excavation. In these house types, extensions often reach property boundaries, requiring careful consideration during nearby excavation work.

Excavation can alter the ground surface, creating slopes, mounds, or depressions. The extent of these changes depends on the excavation method and project scale. Once soil is displaced, the exposed area becomes vulnerable to erosion. To maintain the structural integrity of the excavation, proper shoring or supporting measures are essential.

In cases of poor soil quality, such as sandy gravel types, due to the lack of cohesion between the large particles, the risk of easy erosion increases. Additionally, poor soil quality indicates a low bearing capacity, making it unable to adequately support newly built structures. In such instances, building control may require deeper foundations to reach more stable soil layers capable of bearing the load.

To prevent these challenges, we employ the Sequence of Works method during excavation. This involves excavating a typical 1m x 1m area and immediately pouring concrete. This segmented approach reduces stress on trench walls compared to excavating the entire trench at once with heavy machinery, minimising the risk of a collapse.

A line drawing of numbered sections of excavation bays

Numbers indicate sequence of casting segments. All bays of the same number may be excavated and concreted at one time.

It's crucial to include details of this excavation method on the floor plan drawing and provide it to the contractor. In the resolution of a party wall dispute, a Party Wall Surveyor may request this drawing, especially if the Adjoining Owner has an existing extension built along or in proximity to the boundary line. This proactive method helps ensure a smoother construction process and mitigates potential conflicts with neighbours.

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