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Skempton Method (CPT)

This solution for cohesive soils use a Skempton theory, where the bearing capacity of foundation soil is given by a formula:

It is recommended to use a safety factor FS = 3 when calculating the bearing capacity using this method.

Where:

Rd

-

bearing capacity of foundation soil

Kc

-

load inclination factor

Nc

-

Skempton bearing capacity factor

Su

-

undrained shear strength

γ

-

unit weight of the soil in the footing bottom

d

-

depth of footing bottom

The formula is derived for imperial units [tsf, tcf, ft] - the program calculates automatically in the units used in the program.

Where:

H

-

horizontal component of resultant load

V

-

vertical component of resultant load

This method should not be used for ratio H/V > 0,4.

Skempton bearing capacity factor Nc depends on the ratio of effective width bef and effective length lef and on the ratio of footing bottom depth d and effective width bef.

Determination of Nc factor (From Peck, Hanson a Thornburn, 1974)

Undrained shear strength Su is determined as follows:

Where:

qc

-

average value of cone penetration resistance

σv0

-

average value of vertical stress measured at depths from footing base to 1,5.bef bellow the footing base

Nk

-

Cone factor (range <10;20>)

Where:

qc1

-

average value of cone penetration resistance measured at depths from footing base to 0,5.bef bellow the footing base

qc2

-

average value of cone penetration resistance measured from the depth 0,5.bef bellow the footing base to 1,5.bef bellow the footing base

The influence of inclined terrain and inclined footing bottom is considered same way as in Schmertmann method.

Literature:

FHWA-SA-91-043: THE CONE PENETROMETER TEST

Bridge Engineering Handbook (Wai-Fah Chen, Lian Duan, 1999)