Engineers Australia

Designing stiffened raft footing on non-engineered fill

M. Schult, A. Haque and P. G. Ranjith


This paper presents the results of numerical analyses of a stiffened raft footing on non-engineered fill (NEF) located in the Melbourne suburb of Chelsea. Analyses have been undertaken using a simplified approach with linear elastic parameters in a commercial Finite Element (FE) program (STRAND6) followed by a more comprehensive analysis using a non linear constitutive model in a commercial Finite Difference (FD) program (FLAC3D). The results of the numerical analyses and findings of a simplified empirical design approach known as the “Soft Spot” method are compared with the field settlement behaviour. The results indicate that the “Soft Spot” method is unable to predict the field behaviour of the raft, in particular the behaviour of the raft slab panels, while the FLAC3D program predicts the raft behaviour more closely. Furthermore, the efficiencies of using additional concrete (such as the haunching, increasing slab thickness) are evaluated and it is found that an increase in slab panel thickness is a more efficient alteration which reduces differential settlement of the raft footing. The use of haunches is observed to reduce the shear stresses compared to no haunches; however the developed shear stresses in both the cases are well within the requirements of AS 2870 (1996). An aspect ratio (length/width) of unity for the raft footing is also found to be more effective in reducing the surface deflection and total settlement of raft over NEF.