Abstract:
‘Modulus of subgrade reaction’ is the ratio between the pressure applied on the soil and the
corresponding settlement. There is no theoretical relationship found to obtain equivalent subgrade
modulus of layered soil. Top dense sand layer thickness, bottom loose sand layer thickness, strip
footing width and thickness are changed and equivalent modulus of subgrade reactions are obtained
by equivalent spring theory and weighted average method. These, equivalent subgrade modulus are
separately applied in Heteryni method equations in order find vertical settlement, bending moment
and shearing force along the medium length footings. PLAXIS 3D numerical models are developed
for same footing parameters and soil properties to compare the Heteryni method outputs.
Equivalent subgrade modulus using equivalent spring method is constant with top soil layer thickness
for a given footing width and footing depth. Weighted average method equivalent subgrade module
is non linearly increasing with top dense sand layer thickness for a given footing and bottom loose
sand layer thickness. Equivalent subgrade module for thinner footing depth is always greater than the
thicker footing for a given footing width and soil profile in both spring theory and weighted average
method. Settlement along footing obtained by equivalent spring method equivalent subgrade modulus
applied in Heteryni method equation is highly varying from weighted average method equivalent
subgrade module applied in Heteryni method equation and PLAXIS 3D model settlement output.
Equivalent spring method is considered as unsuitable to calculate the equivalent modulus of subgrade
reaction for layered soil stratum. Settlement difference between PLAXIS 3D method and weighted
average method equivalent subgrade module applied in Heteryni method equation shows up to 45
percentages and this difference cannot be negligible.
This study will shed a light in the theoretical relationship of equivalent subgrade module research
field as this would be the first attempt to check the behavior and suitability of equivalent subgrade
modulus of layered soil stratum.
