Subgrade Modulus for Raft Analysis

The most important analysis parameter for raft on soil is the "Subgrade Modulus", the so-called K value.

In general, higher value of K represents soil with higher stiffness. Hence the settlement induced is smaller. Local concentration of bearing (high bearing pressure) under the loading point (columns/ walls) is expected.

On the other hand, lower value of K represents weak soil and larger settlement is expected. The bearing pressure would be smaller as the loading will be "absorbed" by the deformation of raft and soil.

The procedure of determine the value of K (in unit of kN/m³) is as following (assuming soil without consolidation):

(1) Obtain the Young Modulus of the Soil by Ground Investigation Works.

(2) Build a computer model (say, by SAFE) with a first trial of K value (say, ranged from 10,000 to 50,000 for typical soil).

(3) Input all the loading on top of the raft and have analysis of the raft.

(4) Obtain the bearing stress profile for the raft.

(5) Based on the computer output, estimate the settlement based on elastic theory.

(6) Obtain a new value of K by K= stress/ settlement.

(7) Have iteration of K value by repeating steps 2-6 until the results converged.