Slab On Ground Analysis
Posted Oct. 28, 2005

SoilVision Systems Ltd. has recently undertaken a significant research study into the analysis of slab-on-ground structures. The research project resulted in additions to the SVFlux and SVSolid software packages to allow the analysis of concrete slabs placed on clays with the potential for shrinkage and heave due to climatic influences.

The changes to the SVFlux (seepage) and SVSolid (stress/deformation) software packages allow analysis of the potential stress changes under a concrete slab as a result of extended evaporation events or precipitation events. The finite element packages allow a comprehensive analysis of the problem using conventional theories of seepage flow (Richard’s equation) and stress/analysis.
The input of soil constitutive parameters allows the calculation of volume change resulting from changes in suction or net normal stress.



The unsaturated constitutive model is implemented in both the 2D and 3D formulations of SVFlux and SVSolid and therefore allows analysis of concrete slabs in 2D or full 3D spatial coordinates. The implementation of the Modified Penman equation in SVFlux allows for calculation of actual evaporation and the analysis of the impact of climatic events on the stress changes beneath a concrete slab. Both the seepage portion of the problem and the stress-deformation portion of the “slab-on-ground” problem can be viewed as independent “boundary value” problems. As such, it is necessary to provide realistic boundary conditions with respect to the seepage analysis and also with respect to the stress-deformation analysis. The approach is theoretical but is consistent with the most commonly advocated engineering approaches to geotechnical problems.



History

Lightly loaded structures constructed on expansive soils are often subjected to severe distress subsequent to construction, as a result of changes in the (negative) pore-water pressures in the soil (i.e. matric suctions). Structures most commonly damaged are roadways, airport runways, small buildings, irrigation canals, spillway structures, and all near ground surface structures associated with infrastructure development. Slabs-on-ground form a classic example of a light structure that suffers distress due to changes in matric suction. Changes in the (negative) pore-water pressure can occur as a result of variations in climate, change in the depth to water table, water uptake by vegetation, removal of vegetation or the excessive watering of a lawn.



The problems associated with expansive soils have been addressed in many international and regional conferences. There were three Symposiums on Expansive Clays (from 1957 to 1960), seven International Conferences on Expansive Soils (from 1965 to 1992), three International Conferences on Unsaturated Soils (from 1995 to 2002) and several other regional conferences. The research literature shows that the prediction of heave associated with the wetting of an expansive soil has received more attention than any other problem involving unsaturated soils (Fredlund, 2000).

The worldwide interest in research on expansive soils in the last four decades has resulted in numerous methods being proposed for the prediction of heave. The heave prediction methods are based on either one-dimensional oedometer test results or on direct matric suction measurements (Fredlund and Rahardjo, 1993). Although an analytical tool for the prediction of heave is extremely important, there has been a slow advancement in the development of such a tool for solving practical engineering problems. Historically, there does not appear to be a computer program that has been written and widely accepted for the prediction of heave in expansive soils.

The SVFlux and SVSolid finite element computer programs represent the first comprehensive implementation of stress/deformation constitutive models with unsaturated soils volume change theory which allow solution to the slab-on-ground problem. If you are interested in receiving more information on this subject, please contact us here.