NF P94-261 (CPT)
This solution for cohesive and cohesionless soils use theory from the French standard NF P94-261, 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 |
qce | - | equivalent cone resistance | |
kp | - | bearing capacity factor | |
iδ | - | load inclination factor | |
ig | - | ground inclination factor |
Equivalent cone resistance qce is considered to a depth hr below the footing bottom and is calculated as follows:
- average cone resistance qcm is calculated
- the original cone resistance is limited to a maximum value of 1,3qcm
- the average value of the corrected cone resistance is determined
Depth hr depends on the type of load (design/service), the geometry of foundation and eccentricity of the load:
design load | service load | ||
Strip footing | |||
Circle footing | |||
Spread footing | |||
Bearing capacity factor kp depends on the type of soil, geometry of the foundation (ratio L/B) and equivalent embedding depth De.
Equivalent embedding depth De is calculated as follows:
Values qcc and qce for determining equivalent embedding depth De are measured between terrain and foundation bottom.
If De/B < 2 :
Calculation kc for strip, circle or square foundation:
Calculation kc for spread footing (ratio L/B)
Coefficients a, b, c, kc0, and kcmax depend on the type of soil according to the table:
Soil category | kc | ||||||
a | b | c | kc0 (De/B=0) | kc,max | |||
Clays & silts | Strip footing (B/L = 0) | Q1 | 0,07 | 0,007 | 1,30 | 0,27 | 0,348 |
Square footing (B/L = 1) | Q2 | 0,10 | 0,007 | 1,50 | 0,27 | 0,378 | |
Sand & gravels | Strip footing (B/L = 0) | Q3 | 0,04 | 0,006 | 2,00 | 0,09 | 0,141 |
Square footing (B/L = 1) | Q4 | 0,03 | 0,020 | 5,00 | 0,09 | 0,160 | |
Chalk | Strip footing (B/L = 0) | Q5 | 0,04 | 0,030 | 3,00 | 0,11 | 0,210 |
Square footing (B/L = 1) | Q6 | 0,05 | 0,040 | 3,00 | 0,11 | 0,240 | |
Marls Calcareous... | Strip footing (B/L = 0) | Q5 | 0,04 | 0,030 | 3,00 | 0,11 | 0,210 |
Square footing (B/L = 1) | Q6 | 0,05 | 0,040 | 3,00 | 0,11 | 0,240 |
Load inclination factor iδ is calculated as follows:
If δd < π/4:
If δd ≥ π/4:
Literature:
NF P94-261