Design - Buoyancy Raft.

Introduction
The buoyancy raft works on a similar principle to that of a floating structure where the support for the raft is mainly obtained by displacing the weight of earth or overburden by the volume of a large voided foundation.

The raft  is described previously and is often economically achieved by making use of the voids as a basement structure (see Fig. 13.42).

Fig. 13.42 Buoyancy foundation/basement.

It is designed so that sufficient overburden is removed to allow the superstructure load to be applied to the ground with little or no increase in the original stress which existed on the sub-strata prior to excavation and construction. Thus the structure floats like a ship – which displaces water equal to its own weight.

The bottom slab can form the basement of the proposed building, and be combined with the ground slab and retaining walls to act as the raft. It can also be of cellular form (see Fig. 13.43).

Fig. 13.43 Cellular buoyancy foundation.

The raft design takes into account any eccentricity of load and aims to keep differential settlements and tilting within acceptable limits (see Fig. 13.44), which shows how eccentric resultant loads can be caused by the basement projections, producing a uniform bearing pressure.

Fig. 13.44 Eccentrically loaded buoyancy raft.

Since buoyancy foundations are expensive compared to more traditional forms they tend only to be used where suitable bearing strata is at too great a depth for other more traditional alternatives. For this reason the foundation tends to be restricted to sites on very deep alluvial deposits such as soft sands and silts and where loads on the foundations can be kept concentric. Examples of such building types would be schemes where deep basements can be economically incorporated into the design or where underground tanks are required.
The cases therefore where the engineer would adopt such solutions tend to be limited.

2 Sizing the design
The overall sizing of the design would generally involve:

(1) Calculating the depth plan size and centre of gravity required for the overburden removal to suit structural buoyancy.
(2) Comparing the results of (1) with the requirements for tanks or basements to suit client’s needs.
(3) Calculating the water pressure for (1) to check for  flotation.
(4) Combining the requirements of (1), (2) and (3) into a mutually suitable voided foundation.
(5) Designing the external walls, floors, roof and separating wall elements for the pressures, bending moments and shear forces including any projections to prevent flotation.