Relationship Between Ncor and qu (Unconfined Compressive Strength)

Relationships have been developed between Ncor and qu (the undrained compressive strength) for calyey soil. This relationship gives the value of cu for any known value of Ncor. The relationship may be expressed as

qu = 2 Cu K Ncor (KPa)

where the value of the coefficient K may vary from a minimum of 12 to a maximum of 25.

Once qu is determined, the net ultimate bearing capacity and the net allowable bearing pressure can be found following Skempton's approach.

Strangely, you can use the Ultimate Bearing Capacity of soil at Failure, used by sauropod dinosaurs to determine the surface area of the bottom of their feet.

At soil failure, the Factor of safety is about 1.0 at about 5,000 pounds per square foot of horizontal surface area downward pressure. Sauropod foot prints thus determine the weight of the Sauropods in a reduced gravity environment of the geologic Past, which could be useful for construction projects on the Moon, or on Mars. On Earth, we tend to want a factor of safety of around 2.0 or higher, which reduces the surface pressure under foundations to 2500 PSF, or less.

The enemy of soils is water, and vibrations, and too high vertical, or horizontal loads. If you vibrate soils, they can flow down hill, or flow to reach a lower slope.

If you supersaturate soils, they can also flow downhill. The key is, design for the worst conditions expected. A wider foundation is generally safer than a narrow foundation, and the added cost is vastly less than the replacement cost of a failed structure.

This does not answer your question, but hopefully, it will get you to think about the importance of soil bearing capacity.