The bearing capacity of a soil, often termed its stability, is the ability of a soil to carry a load without failure. The load carrying capacity of sand varies not only with it's strength but also with the magnitude and distribution of the load.
The variables that govern the final bearing capacity of a soil (sand) are: particle angularity; relative density; porosity; particle-size distribution; and water content (matric potential).
Holubec and D' Appolonia (1973) found the increase of angularity of sand particles had a positive impact on the strength of the soil. Angularity is a measure of the curvature of the comers to the average curvature of the particles. Comparing sands with increasing angularity, they found a positive correlation between angularity and the resultant friction angle. Sands with greater angularity have a greater friction angle and are more stable.
The relative density of a cohesion less soil is defined by its degree of compaction. In other words, relative density describes how close to maximum compaction a particular sand might be. Many have shown a direct correlation between relative density and friction angle. As relative density increases so too does friction angle and stability increases.
Relative density and porosity are inversely related in as far as bulk density and porosity are inversely related. As density increases, porosity decreases, and bearing capacity (stability) increases. Of course, maintaining porosity (and particularly macroporosity) for rapid drainage and maintaining oxygen content in the soil is extremely important for turfgrass growth and vigor. Therefore, a balance between density, porosity, and bearing capacity needs to be achieved.
The importance of particle-size distribution has already been discussed. The last variable deemed to be important is the water content of the soil (sand). Dry sands and saturated sands have no pore water tension. But as sand dries from saturation, tension develops which causes the soil to behave as if it possesses cohesion. As turfgrass managers very seldom do we allow soil water content to vary from field capacity and luckily enough that is close to the point were maximum strength from pore water tension is found. Therefore, water content is a variable we manage for turf grass growth and not for maximum soil stability.
Utilizing the ultimate bearing capacity equation the theoretical supporting capacity of the selected sands were determined as a function of the angle of internal friction and the water tension. Figure 1 shows how many pounds distributed over 3" by 12" (about shoe size) surface area could carry. We found curvilinear relationships between friction angle and bearing capacity. As friction angle increased so too did the bearing capacity. The actual number of pounds of bearing capacity are not as important (these are modeled numbers) as the relative amount from one sand to another. Given TDS 2150 has a friction angle of about 28 degrees and 2NS sand has a friction angle of approximately 33 degrees, 2NS will support about 1.9 times more weight than TDS 2150 under the same environmental conditions.
Figure 1. Modeled bearing capacity at the water content of field capacity.
Bearing Capacity at Field Capacity
900 800 -
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Angle of Internal Friction (deg)