1 Objectives

The main purpose of constructing basements are :

(a) to provided additional space,

(b) as a form of buoyancy raft,

(c) in some cases, basements may be required to reduce net bearing pressure by the removal of the soil.

2 Buoyancy Foundation

To give an example of the effect, excavation to a depth of 4.5m for the basement relieves the soil at foundation level of a pressure of about 80KN/m2. The substructure itself weighs about 25KN/m2; thus a loading of 50KN/m2 can be placed on the basement before any additional loading causing settlement comes onto the soil at foundation level. A bearing pressure of 50KN/m2 is roughly equivalent to the overall loading of a 4 storey block of flats or offices.

The term basement and buoyancy raft need explanation. Basement if in effect a form of buoyancy raft, however, it is not necessarily designed for the purpose.

The main function of a basement in the building is to provide additional space in the building for the owner, and the fact that it reduces the net bearing pressure by the weight of the displaced soil may be quite incidental. In some cases, basements may be required for their function in reducing net bearing pressure and advantage is taken of this to provide additional floor space in the sub-structure.

The true buoyancy raft, however, is a foundation which is designed solely for the purpose of providing support to the structure by the buoyancy given by the displaced soil without regard to utilise the space for any other purpose.

3 Construction Methods

Three different constructional methods are discussed for the construction of deep basements.

(a) in excavations supported by timbering or sheet piling.

(b) in excavation supported by reinforced concrete diaphragm wall constructed in advance of the main excavation.

(c) in excavations supported by a contiguous bored pile wall constructed in advanced of the main excavation.

Construction in excavations supported by timbering or sheet piling

This is a suitable method of construction for sites where insufficient space is available around the excavation to slope back the sides.

If the soil conditions permit withdrawal of sheet piling for re-use elsewhere this method of ground support is very economical compared with the alternative of a diaphragm wall.

Sheet piling comprises a row of piles interlock with one another to form a continuous wall which may be temporary or permanent.

It is manufactured by different firms and in general consisting of rolled steel sections with interlocking edge joints.

Common materials are timber, reinforced concrete and steel.

Disadvantages :

(a) noise (b) direct effect of the driving on the subsoil immediately surrounding the site.

Construction in excavations supported by reinforced concrete diaphragm wall

A diaphragm wall is constructed by excavation in a trench which is temporarily supported by a bentonite slurry. On reaching founding level steel reinforcement is lowered into the trench, followed by placing concrete to displace the bentonite.

Suitable for sites where obstructions in the ground prevent sheet piles from being driven and where the occurrence of ground water is unfavourable for other methods of support.

The method is also suitable for sites where considerations of noise and vibration preclude driving sheet piles and where ground heave and disturbance of the soil beneath existing foundations close to the margins of the excavation are to be avoided.

The bentonite slurry has the following properties:

(a) supports the excavation by exerting hydrostatic pressure on the wall

(b) has the ability to form almost instantaneously a low permeability membrane

(c) suspend sludgy layers building up at the excavated base

(d) allows clean displacement by concrete, with no subsequent interference with the bond between reinforcement and set concrete.

Construction in excavations supported by a contiguous bored pile wall

This is a line of bored piles installed close together or touching. Smaller diameter piles called micro-piles are installed to close the gaps between the main piles and the gaps between the micro-piles and main piles are grouted.

- useful in built-up areas where noise and vibration should be limited.

- useful in industrial complexes where access, headroom or a restriction on vibration may make other methods such as steel sheet piling or diaphragm walling less suitable.