What is Soil Subgrade Reaction in Raft?

Introduction #

Soil subgrade reaction, often denoted as the k-value, is a parameter used in structural engineering to characterize the stiffness of the soil beneath the raft foundation. 

The subgrade reaction is typically measured in force per unit length per unit settlement, expressed in unit kN/m³. 

Impact on structural behavior of raft foundation and the superstructure:

1. Soil types

• Different types of soils have different subgrade reactions.

2. Stiffness

• The higher the subgrade reaction, the stiffer the soil is, underneath the raft foundation. Stiffer soil can distribute loads more effectively and reduce deflection or settlement of the foundation.

How do engineers obtain the values of subgrade reaction? #

Subgrade reactions are determined from various methods; plate load tests, field observations or using empirical correlations based on soil properties. In Joseph E.Bowles book on the Foundation Analysis and Design, the subgrade coefficient values may be obtained using the approximations provided in Table 9.1. This table is used as a guidance in MiLA Structure to let the users define the stiffness of the soil beneath the raft foundation.

To get to this parameter, users may input a raft foundation, and in the properties, specify the subgrade coefficient according to the soil types underneath the foundation. To get to the table, users may point the cursor to the info button. 

Table 9.1 Shows the range of subgrade reaction for different soil types

The figure shows the effect of raft deflection with different values of subgrade coefficient. Left; 1000kN/m³, right; 5000kN/m³