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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

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