The net resistance of the soil is deduced from qnet the stress associated with the net resistance of the ground under a foundation superficial. To calculate it, several methods are possible:
• methods based on in situ tests:
- the pressiometric method based on the pressiometric test (paragraph 2.1.1);
- the penetrometric method based on the static cone penetrometer (paragraph 2.1.2);
- other methods based on in situ test results under certain conditions (paragraph 2.1.3);
• methods based on soil shear properties (paragraph 2.1.3).
The methods presented here are fully applicable only to soles with a horizontal base. Otherwise, the reader is invited to consult articles D.2.1 (2) and E.2.1 (2) as well as Annex F of standard NF P94-261 subject to this guide.
Summary Tables and Flowcharts 1, 3, 4 and 5 in Annex A summarize the determination respectively parameters related to the sole, the procedures for checking the lift, determining qnet and coefficients reduction of lift related to the inclination of the load and the presence of an embankment.
2.1.1 - Pressiometric method
The pressiometric method uses the Ménard pressiometric limit pressure. In this case, the constraint qnet speaks thus:
[Formula D.2 .1]
With:
kp: pressiometric lift factor the calculation method of which is given in paragraph 2.2.4. of this chapter;
ple*: equivalent net limit pressure for which the calculation method is given in paragraph 2.2.2. of this chapter;
iδ: lift reduction coefficient related to the inclination of the load for which the calculation method is given in paragraph 2.5.2 of this chapter;
iβ: lift reduction coefficient related to the proximity of an embankment whose calculation method is given in paragraph 2.5.3. of this chapter.
2.1.2 - Penetrometric method
The penetrometric method uses peak resistance values qc derived from static cone penetration tests. In this case, the qnet constraint is expressed as follows:
[Formula E.2 .1]
With
kc: penetrometer lift factor the calculation method of which is given in paragraph 2.3.4. of this chapter;
qce: equivalent peak strength for which the calculation method is given in paragraph 2.3.2. of this chapter;
iδ: lift reduction coefficient related to the inclination of the load for which the calculation method is given in paragraph 2.5.2 of this chapter;
iβ: lift reduction coefficient related to the proximity of an embankment whose calculation method is given in paragraph 2.5.3. of this chapter.
2.1.3 - Other methods
Mention may be made of the method for the analytical calculation of qnet from the soil shear strength properties (c' and φ' or cu ) measured in the laboratory on unengineered field samples. For this type of calculation, it is necessary to refer to Annex F of standard NF P94-261. This empirical method is not presented here since it is little used in France.
It is possible to use numerical models based on soil shear strength properties. For this, the reader is invited to consult article 9.3.2 of standard NF P94-261.
Other methods of determining qnet from in situ tests other than the pressiometer or static penetrometer are possible, but this requires determining a model coefficient γR;d validated by a set of static loading of surface foundation. The criteria to be met are described in clause 9.2 (4) of the standard NF P94-261.