During a basement construction there are 2 types of underpinning design to be considered:
1. Traditional mass concrete underpinning
Mass concrete underpins can be designed to form the walls of basement structures although this type of underpinnining will not have sufficient strength to retain the lateral load of the soil. This particular type of underpin is in most cases used to transfer vetical loads only. In this case the mass concrete underpins will not be sufficiently strong to act as a retaining wall hence permanenet horizontal propping is usually needed to provide support.
Mass concrete underpinning is not usually employed during a basement construction, however upon the party wall process this option can overcome various disputes during the Party Wall process.
1. Reinforced concrete underpinning
A reinforced concrete underpin will most likely be designed by the structural engineer as this will achieve both functions of dealing with vertical as well as lateral loads without the need for any permanent lateral propping. Although slightly more expensive to install, the use of reinforced concrete underpinning will significatnly simoplify the basement construction resulting in an overall saving and due to its design this increases subsequent space available for development below ground. Under the Party Wall etc Act 1996, a building owner requires the written consent of an adjoining owner in order to use special foundations on their basement construction project.
The underpinning method used is constructed in sections each no wider than 1000mm in a hit and miss procedure following the sequence of the structural enfineer. This method of construction reduces the amount of potential ground movement and so minimises the effects of settlement of the adjacent structures.
The design and construction methodology, as described above, deals with the potential risks and ensures the excavation and construction of the proposed basement will not affect the structural integrity of your property or any of the adjoining properties.