Schmertmann Method (CPT)
This solution for cohesionless soils use a Schmertmann 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 |
Kq, Kγ | - | reduction factors | |
γ | - | unit weight of the soil in the footing bottom | |
bef | - | effective footing width | |
Nq, Nγ | - | bearing capacity factors | |
d | - | depth of the footing bottom |
The formula is derived for imperial units [tsf, tcf, ft] - the program calculates automatically in the units used in the program.
where: | qc1 | - | average value of cone penetration resistance measured at depths from the footing base to 0.5*bef below the footing base |
qc2 | - | average value of cone penetration resistance measured from the depth 0.5*bef below the footing base to 1.5*bef below the footing base |
where: | iq, iγ | - | load inclination factors |
sq, sγ | - | footing shape factors | |
dq | - | footing depth factor | |
bq, bγ | - | footing bottom inclination factor | |
gq, gγ | - | ground inclination factor |
Inclination factors iq and iγ are determined as follows:
where: | H | - | horizontal component of resultant load |
V | - | vertical component of resultant load | |
lef | - | effective footing length | |
θ | - | angle between horizontal component of the resultant load H and the longer axis of footing (in the direction lef) |
determination of angle θ
This method should not be used for ratio H/V > 0.4.
Footing shape factors sq and sγ are determined as follows:
where: | φ | - | angle of internal friction in the footing bottom |
Footing depth factor dq is determined as follows:
Footing bottom inclination factor bq and bγ are determined as follows:
where: | η | - | angle of the footing bottom inclination |
Ground inclination factor gq and gγ are determined as follows:
where: | β | - | angle of ground inclination |
Literature:
FHWA-SA-91-043: THE CONE PENETROMETER TEST
Bridge Engineering Handbook (Wai-Fah Chen, Lian Duan, 1999)