Raft Analysis Specification

When modelling a raft supported on soil some information is needed to clarify how the soil is considered.

Definition

Rigid Boundary Level

The soil layers are defined by a top soil level. This defines the level at which the ground can be considered rigid. Typically the rock level.

Maximum Allowable E Ratio

In a soil the E value typically varies with depth through a soil profile. The E ratio determines how many sub-layers are needed within a single soil layer in the profile. The larger the ratio the fewer the number of sub layers and the coarser the sol analysis.

Mindlin Method

The Mindlin analysis method opens up a number of options:

• Use weighted average – In this method, the average Young’s modulus and Poisson’s ratio are calculated between the centroid of each load and the displacement point at which the horizontal displacement is required.
• Use greatest stiffness – In this method, the maximum Young’s modulus that occurs between the centroid of each load and the displacement point is used for the calculations. The Poisson’s ratio is set to that of the stratum where the greatest E is found.
• Use stiffness at displacement point – In this method, the stiffness at the displacement point is used for calculating horizontal displacements. The soil properties between the load and the displacement points are disregarded. This method should be used with caution.

Boussinesq Method

A Boussinesq analysis requires that a global Poisson’s ratio for the soil.