3.7 Derived values of geotechnical parameters
3.7.1 Derived values for calculations of bearing capacity and settlement of spread foundations
(1)P When the bearing capacity or the settlement of a spread foundation is evaluated from CPT results, either a semi-empirical method or an analytical method shall be used.
(2)P When a semi-empirical method is used, all the features of the method shall be taken into account.
(3) When the sample analytical method for bearing capacity of annex B in ENV 1997-1 is ued, the sundrained shear strength of cohesive soils cu, may be determined from the following relation:
or, in the case of CPTU, preferably by:
with Nk or Nkt estimated from local experience.
(4) When the sample analytical method for bearing capacity of annex B of ENV 1997-1 is used, the angle of shearing resistance φ' may be determined from the cone resistance qc, on the basis of local experience, taking into account depth effects, when relevant.
An example of sample values for quartz and feldspar sands is given in annex B.1 to estimate a value of φ' from qc, for the bearing capacity of spread foundations, when depth effects do not need to be taken into account.
(5) More elaborated methods may also be used for determining φ' from qc, taking into account the effective vertical stress, the compressibility, and the overconsolidation ratio.
(6) When the adjusted elasticity method of annex D of ENV 1997-1 is used for calculating settlements of spread foundations from CPT results, the correlation between cone resistance qc and the drained (long term) Young's modulus Em depends on the nature of the method; the semi-empirical elasticity method, or the theoretical elastic method.
(7) An example of a semi-empirical method for calculating settlements in cohesionless soils is given in annex B.2.
(8) When a theoretical elastic method is used, the drained (long term) Young's modulus Em may be determined from cone resistance qc, on the basis of local experience. An example of sample values for quartz and feldspar sands is given in annex B.1 to estimate a value of Em from qc.
(9) Correlations between the oedometer modulus Eoed and the cone resistance qc may also be used when calculating settlements of spread foundations.
The following relation between the oedometer modulus Eoed and qc is then often adopted:
Eoed = α × qc
a is a correlationfactor estimated on the basis of local experience
An example of a correlation is given in annex B.3.
3.7.2 Pile foundations
(1)P When the ultimate bearing resistance of piles is evaluated from CPT results according to 18.104.22.168 (4)P of ENV 1997-1, calculation rules based on established correlations between the results of static load tests and CPT results shall be used.
(2) In annex B.4 an example is given for the assessment of the bearing resistance of a single pile on the basis of qc-values from a CPT.