NF P94-261 (PMT)
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 |
p*le | - | equivalent net limit pressure | |
kp | - | bearing capacity factor | |
iδ | - | load inclination factor | |
ig | - | ground inclination factor |
Equivalent net limit pressure ple* is a geometric average of values of pressiometric limit pressure from the footing bottom to the depth of hr.
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:
or:
Values pLM,i for determining equivalent embedding depth De are measured between terrain and foundation bottom.
If De/B < 2 :
Calculation kp for strip, circle or square foundation:
Calculation kp for spread footing (ratio L/B)
Coefficients a, b, c, kp0 and kpmax depend on the type of soil according to the table:
Soil category | Variation curve of the bearing factor | Expression of kp | |||||
a | b | c | kp0 (De/B=0) | kpmax | |||
Clays & silts | Strip footing (B/L = 0) | Q1 | 0.20 | 0.02 | 1.30 | 0.80 | 1.022 |
Square footing (B/L = 1) | Q2 | 0.30 | 0.02 | 1.50 | 0.80 | 1.123 | |
Sand & gravels | Strip footing (B/L = 0) | Q3 | 0.30 | 0.05 | 2.00 | 1.00 | 1.393 |
Square footing (B/L = 1) | Q4 | 0.22 | 0.18 | 5.00 | 1.00 | 1.580 | |
Chalk | Strip footing (B/L = 0) | Q5 | 0.28 | 0.22 | 2.80 | 0.80 | 1.517 |
Square footing (B/L = 1) | Q6 | 0.35 | 0.31 | 3.00 | 0.80 | 1.768 | |
Marls Calcareous... | Strip footing (B/L = 0) | Q7 | 0.20 | 0.20 | 3.00 | 0.80 | 1.399 |
Square footing (B/L = 1) | Q8 | 0.20 | 0.30 | 3.00 | 0.80 | 1.598 |
Load inclination factor iδ is calculated as follows:
If δd < π/4:
If δd ≥ π/4:
Literature:
NF P94-261