Stress Distribution in Khon Kaen Loess under Spread Footing

 

Pairoj Yodsa-nga*

Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen, Thailand Tel. +66-43-202846 Fax. +66-43-202847
*Corresponding Author e-mail: paiyod2513@gmail.com

Watcharin Gasaluck

Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen, Thailand Tel. +66-43-202846 Fax. +66-43-202847
e-mail: watgas@kku.ac.th

Pongsakorn Punrattanasin

Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen, Thailand Tel. +66-43-202846 Fax. +66-43-202847.
e-mail: ppunrattanasin@gmail.com

 

 ABSTRACT

Loess is a non-plastic fine grained soil which occurs extensively in the Northeast Thailand. The present research, which involved field testing and numerical modeling, was developed in order to study the pattern of stress distribution in loess due to surcharge loading. In the field, plate bearing tests were carried out at a 3-m depth in a natural deposit of loess instrumented with five small pressure cells to measure the vertical stresses caused by plate loading. Parallel to the field tests, at specific locations in the loess, the stresses due to given surface loads were predicted using three well-known numerical methods (Boussinesq’s, Walter’s and a linear finite element model). The above numerical methods have an implied assumption that the contact pressure beneath the loading plate is uniform, although this may not be realistic. The vertical stress predictions from Boussinesq and finite element methods were found to be inconsistent with the measured values probably due to unrealistic assumptions of linear stress-strain models. However, the finite element method is powerful enough to give reasonable predictions if an appropriate constitutive model is used. Walter’s method, where a non-linear stress strain approach was used, yielded the most accurate predictions. Both the measured vertical stresses and the predictions from Walter’s method were interpreted to represent the pattern of variation of the influence factor with location in the loess as well as intensity of the surface loading. In order to improve the accuracy of predictions, it is suggested that the numerical methods should include foundation-structure interaction effects, which would allow realistic estimation of the contact pressure pattern.

Keywords: Loess, Stress Distribution, Boussinesq’s method, Walter’s method, finite element method.

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