4.3 Cone penetration and piezocone penetration tests (CPT, CPTU, CPTM)

4.3.1 Objectives

(1) The objective of the cone penetration test (CPT) is to determine the resistance of soil and soft rock to the penetration of a cone and the local friction on a sleeve.

(2)P The CPT consists of pushing a cone penetrometer vertically into the soil using a series of push rods. The cone penetrometer shall be pushed into the soil at a constant rate of penetration. The cone penetrometer comprises the cone and if appropriate a cylindrical shaft or friction sleeve. The penetration resistance of the cone (q_c) as well as, if appropriate, the local friction on the friction sleeve shall be measured.

(3)P For electrical CPTs, all measurements shall be made by sensors contained in the cone penetrometer.

(4) For mechanical CPTs, the measurements are generally made remotely.

(5) The piezocone penetration test, CPTU, is an electrical CPT, which includes additional instrumentation to measure the pore water pressure during penetration at the level of the base of the cone.

(6) The CPTU results should be used mainly for the determination of a soil profile together with results from sampling by drilling and excavations according to Section 3 or in comparison with other field tests.

(7) The results may also be used for the determination of geotechnical parameters such as the strength and deformation properties of soil and soft rock provided penetration is possible, and for direct input to design methods, generally in coarse and fine soil but also possibly in other deposits.

(8) The results may also be used to determine the length of piles and their compressive or tensile resistance or the dimensions of shallow foundations.

4.3.2 Specific requirements

(1)P The tests shall be carried out and reported in accordance with a method that conforms to the requirements given in EN ISO 22476-1 for the electrical CPT and CPTU, or EN ISO 22476-12 for the CPTM.

(2)P When planning the test programme for a project, the following items shall be decided in addition to the requirements given in 4.2.1:

type of required cone penetration test according to EN ISO 22476-1 or EN ISO 22476-12;
depth and duration of pore pressure dissipation tests, if applicable.

(3)P Any deviation from the requirements given in EN ISO 22476-1 or EN ISO 22476-12 shall be justified and reported. In particular, any influence on the results shall be commented upon.

4.3.3 Evaluation of test results

(1)P In addition to the requirements given in 4.2, the field and test reports according to EN ISO 22476-1 or EN ISO 22476-12 shall be used for evaluation purposes.

(2) F Possible geotechnical influences on the penetration resistance shall be considered in evaluating the test results, e.g. in clays, the cone penetration resistance corrected for pore water pressure effects, (q_t), should be used in evaluation.

4.3.4 Use of test results and derived values

4.3.4.1 Bearing resistance and settlement of spread foundations

(1 )P If the bearing resistance or the settlement of a spread foundation is derived from CPT results, either a semi-empirical or an analytical design method shall be used.

NOTE See, for example, EN 1997-1:2004, Annex D or F.

(2)P If a semi-empirical method is used, all the features of the method shall be taken into account.

NOTE If, for instance, the semi-empirical method to determine the settlement of spread foundations From CPT results is used (see D.3), only Young's modulus of elasticity derived from q_c is, applied in this particular method as shown in the example.

(3) If the sample analytical method for bearing resistance of Annex D in EN 1997-1:2004 is used, the undrained shear strength of fine soil, (c_u) may be determined for a CPT from:

(4.1)

Or, in the case of a CPTU, from:

(4.2)

where

q_c is the cone penetration resistance

q_t is the cone penetration resistance corrected for pore water pressure effects;

N_k and N_kt are coefficients estimated from local experience or reliable correlations

σ_v0 is the initial total vertical overburden stress at the depth under consideration;

(4) If the sample analytical method for bearing resistance calculation of Annex D of EN 1997-1:2004 is used, the angle of shearing resistance (φ') may be determined from the cone resistance (q_c), on the basis of local experience, taking into account depth effects, when relevant.

NOTE 1 An example of ranges of values to estimate φ' from q_cfor quartz and feldspar sands is given in D.1, for estimating the bearing resistance of spread foundations when depth effects do nut need to be taken into account.

NOTE 2 In addition, an example for a correlation between φ' and q_c for poorly-graded sands is given in D.2. The correlation given in D.2 should be considered as giving a conservative estimate.

(5) More elaborate methods may also be used for determining φ' from q_c, taking into account the effective vertical stress, the compressibility, and the over-consolidation ratio.

(6) If an adjusted elasticity method is used for calculating settlements of spread foundations from CPT results, the correlation between cone resistance (q_c) and the drained (long term) Young's modulus of elasticity (E') depends on the nature of the method: the semi-empirical elasticity method, or the theoretical elastic method.

NOTE An adjusted elasticity method is given in EN 1997-1:2004, Annex F.

(7) Semi-empirical methods may be used for calculating settlements in coarse soil.

NOTE An example is given in D.3.

(8) When a theoretical elastic method is used, the drained (long term) Young's modulus of elasticity (E') may be determined from cone resistance (q_c), on the basis of local experience.

NOTE An example of sample values for quartz and feldspar sands is given in D.1 to estimate a value of E' from q_c.

(9) Correlations between the oedometer modulus (E_oed) and the cone resistance (q_c) may also be used when calculating settlements of spread foundations. The following relationship between E_oed and q_c is often adopted:

E_oed = α × q_c

(4.3)

where

α is a correlation factor depending on local experience.

NOTE An example of a correlation is given in D.4.

(10) When a theoretical elastic method is used to calculate the settlements of spread foundations, a stress dependant oedometer modulus (E_oed), based on q_c, may be used.

NOTE 1 For examples of theoretical elastic methods, see EN 1997-1:2004, Annex F.

NOTE 2 Examples of correlations between q_c and E_oed are given in D.5. The correlations given in D.5 should be considered as conservative estimates.

4.3.4.2 Pilebearing resistance

(1)P If the ultimate compressive or tensile resistance of piles according to EN 1997-1:2004, 7.6.2.3 or 7.6.3.3 is derived from CPT results, calculation rules based on locally established correlations between the results of static load tests and CPT results shall be used.

NOTE 1 An example for such correlations for coarse soil is shown in D.6.

NOTE 2 An example is given for the assessment of the compressive resistance of a single pile on the basis of q_c-values from a CPT in D.7.