Bearing Capacity of Soil

To answer your questions in the order they were posed:

1. The CBR is a timed loading test of granular soils for pavement design. It only tests the upper layer, the few inches below the loading piston. It is not used for foundation design.

2. The density of soil is related to its strength and somewhat related to the bearing capacity.

The most common way of determining the bearing capacity of the soil is by means of the Standard Penetration Test (SPT)-ASTM-D1586, driving a 2" OD x 2 foot long standard split spoon sampler into the soil with an energy equivalent of a 140 lb hammer falling 30 inches, and recording the blows to advance the sampler. Usually it is recorded over the entire 2 foot run and the middle 12 inches recorded as the "N" number. A soil sample for laboratory or field classification is obtained in the sampler at the same time it is advanced. The sampler is emptied after each run of the test, the samples placed into moisture-proof containers, then the sampler is re-assembled, tripped back down the hole and advanced the next 2 foot run. The SPT is performed using a soil boring rig while at the same time advancing a test boring which keeps the hole open to the sampling depths and provides additional information about the subsurface condiitons. The total depth of the sampling depends on the proposed foundation loading, type of structure and site geology. Of particular concern in northern climates are the conditions at 4 to 6 feet below the surface as these depths correspond with the frost depths and typically shallow foundations are constructed at these depths, provided a decent bearing capacity is achieved there. One can go into various bearing capacity charts with the N number and obtain a preliminary value for an allowable bearing capacity. Of special concern is the presence of any weak layers and/or the water table below the planned depth of the foundation, as these will cause a reduction in the allowable bearing capacity. The actual design bearing capacity is dependent on not only the "N" value but the presence of a water table, the soil type, the site geology, the presence of weaker layers, the proposed foundation loadings and the settlement tolerance of the structure. It may be, that considering the type of structure and soil conditions, a shallow foundation type is not ther best choice, then the designer will have to consider an alternative "deep" foundation such as piles or caissons, and these require an entirely different set of subsurface considerations. A considerable amount of engineering judgement goes into the whole bearing capacity process from the depths and types of test borings to the actual determination of the design bearing capacity and ultimate foundation type. Thus, for a structure of any degree of critical importance, it is best to rely on a geotechnical engineer to make these determinations.

Dave Sanford, PE

Geotechnical Engineer.