Large soil fills can often become a controlling geotechnical design factor

Vol. 109 | February 08, 2017

In a world of big box structures, strip malls and value engineering it may be interesting to know that when more than 8 feet of soil fill is placed on a typical single-story strip mall project, the load applied by the fill often becomes the controlling geotechnical design factor.

All soils, whether they are natural or placed by construction, compress when they are loaded.  In fact, fill soils can compress significantly under its own weight, often 1-2 percent of their original thicknesses. So, an 8-foot high fill could compress close to 1-2 inches.

Two significant aspects to actually noticing this movement is the time that was taken to place the fill and the time between the placement of the fill and the construction of structures placed on the fill. Recently placed fills can have 5-15 percent air and 10-25 percent water (by volume) in the matrix of the soil.

The air in the soil allows the mixture of soil to compress more rapidly. When the water in the soil begins to take on pressure as a result of loads placed on the soil, the pressure can only dissipate at a rate proportional to the time it takes for water to flow through the soil.  In sands and gravel, that rate is very fast and is typically measured in hours. In clays, however, water may only flow at rate of 1 foot per year or slower.Therefore, it may take clay soils, weeks, months or even years to fully compress as a result of adding load to fill soil.

With these nuances in the construction of soil fills, it is important that the owner/designer/builder engage a geotechnical professional that can guide a project through the requirements of constructing a soil fill.

Kevin C. Miller, P.E.
Chief Engineer, Geotechnical Engineering Services

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Related topics: 2017 | Geotechnical