Chapter 56 Rafts and piled rafts

Rafts can provide a very cost-effective foundation solution. Their design requires a careful assessment of the soil–structure interaction. Many factors, including soil and raft stiffness, superstructure and sub-structure stiffness, local yield and soil nonlinearity and time-dependent effects, can influence differential settlement and structural forces in the raft. Errors associated with assuming that a raft sits on a bed of uniform soil ‘springs’ are discussed, and a practical alternative is outlined.

Piled rafts, which are a hybrid foundation comprising a raft and piles, can offer many benefits. Currently there is little guidance in the codes and there is confusion about appropriate design methods. Piled-raft behaviour is potentially very complex. Significant simplifications are possible, which facilitate practical design and greater confidence in analysis output. A key aspect is a clear definition of two different types of piled raft: the raft-enhanced pile group and the pile-enhanced raft (previously known as a raft with settlement-reducing piles). The design concepts for each type of piled raft are very different. The intermediate zone between these two types should be avoided.