Foundation

The CPT Method of Determining Ultimate Bearing Capacity

Cohesionless Soils
Relationship Between qc, Dr and 0
Relationships between the static cone penetration resistance qc and 0 have been developed by Robertson and Campanella (1983b), Fig. 9.15. The value of $ can therefore be determined with the known value of q . With the known value of 0, bearing capacity factors can be determined and  hence the ultimate bearing capacity. Experience indicates that the use of qc for obtaining 0 is more reliable than the use of N.
Bearing Capacity of Soil
As per Schmertmann (1978), the bearing capacity factors N and N for use in the Terzaghi bearing capacity equation can be determined by the use of the equation
eq 12.41
 
where qc = cone point resistance in kg/cm2 (or tsf) averaged over a depth equal to the width below the foundation.
Undrained Shear Strength
The undrained shear strength cu under 0 = 0 condition may be related to the static cone point resistance qc as [Eq. (9.16)]
eq 12.42
 
When once cu is known, the values of qm and qna can be evaluated as per the methods explained in earlier sections.

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