At Chartland, we are constantly looking at innovative building techniques that represent an improvement on traditional construction methods in terms of quality, speed and ease of build and performance. An example of this is in basement construction.
Whilst several local authorities now have policies in place to prevent the “iceberg” basements that have caused much controversy in the more exclusive parts of London, they are generally dismissed by the volume housebuilders for new build housing.
They do however afford the potential for better site utilisation and can provide a good way of making use of sloping sites, where the extra cost in the foundation work can be offset by creating extra living space.
There are a number of construction methods that one can use for basements in a new build situation. These include:
REINFORCED OR MASONRY WALLS
This tends to be the most economical option for small-scale basement construction. Both types rely on steel reinforcement – either reinforcing bar in the void of a hollow block or mesh reinforcement for cavity wall constructions that is tied to the steel fabric mesh cast within the raft. The void / cavity is then filled with concrete.
This type of construction requires a high standard of workmanship because each mortar bed in between each block, is a joint and when placing concrete into the cavity between the blocks, care needs to be taken to ensure the masonry shuttering does not blow as a result of the pressure that is exerted by the weight of the concrete, as it is poured. For this reason, the pour needs to be done in rotation around the building to allow a degree of cure before filling higher. This has the potential for joints to form between pours which can also affect the integrity of the structure.
In this system, concrete is poured into a metal formwork, which is then struck after the concrete has cured. This provides a robust form of construction that will usually demonstrate a high degree of natural water resistance.
Alternatively an insulated (ICF) formwork can be used. In this case the formwork is provided by two layers of polystyrene that are tied together with a plastic or metal ties, into which concrete is then poured.
There are many advocates of this system. However again, workmanship is important and care needs to be taken during the pour to avoid the risk of the walls distorting or even bursting open at pressure points. As with all basements it is important to get the waterproofing right as the ties joining the formwork can provide a pathway for water ingress.
TWIN WALL PRECAST PANELS
This construction method is not so common in the UK but is widely used in Europe.
It is in effect a hybrid solution that provides the speed and quality of precast concrete with the structural and waterproof reliability of a continuously poured in-situ concrete structure. It also provides a higher standard of internal finish compared to the other methods described above.
Each wall unit comprises of two leaves of precast concrete with a cavity between, linked by a lattice of steel reinforcement which act as permanent concrete formwork. Once the units and water bars are in place the cavity is filled with in-situ concrete to complete the structural wall.
Andrew Turvill has experience of basement construction having specified and project managed a development that included a twin wall precast basement.
The details of this scheme can be seen in the attached factsheet. In a future article we will look at the various methods and standards of waterproofing.
Author Andrew Turvill