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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

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