4.5 Flexible dilatometer test (FDT)

4.5.1 Objectives

(1) The objective of the flexible dilatometer test is to measure in-situ the deformability of rock (rock dilatometer test, RDT), and soil (soil dilatometer test, SDT) from measurements of the radial expansion of a borehole section under a known uniform radial pressure applied by means of a cylindrical dilatometer probe.

(2)P The test consists of inserting a cylindrical probe, having an outer expandable flexible membrane, into a borehole, and measuring, at selected time intervals or in a semi-continuous manner, the radial displacement of the borehole while inflating the probe under known radial pressure.

(3) The RDT should be used mainly in soft and hard rock formations while the SDT should be used primarily in soft to stiff soil to obtain profiles of deformability variations with depth.

(4) The results of cylindrical dilatometer tests may be used to determine the deformation and creep properties in-situ when testing intact rock.

(5) In fragile or clayey rock, and in fractured or closely jointed formations, where core recovery is poor or inadequate for the purpose of obtaining representative samples for laboratory testing, the cylindrical dilatometer test may be used for rapid index logging of boreholes and for comparisons of relative deformability of different rock strata.

4.5.2 Specific requirements

(1)P When planning a test programme for a project, the specific requirements of the device to be used shall be specified.

(2)P The tests shall be carried out and reported in accordance with a test method that conforms to EN ISO 22476-5.

(3)P Any deviations from the requirements given in EN ISO 22476-5 shall be justified and in particular their influence on the results shall be commented upon.

4.5.3 Evaluation of test results

(1)P In addition to the requirements given in 4.2, the field and test reports according to EN ISO 22476-5 for the specific test type shall be used for evaluation purposes.

(2) The interpretation of flexible dilatometer tests requires that the Poisson's ratio of the soil or rock should either be known or assumed.

4.5.4 Use of lest results and derived values

(1) The results of dilatometer tests may be used to check the serviceability limit state of spread foundations on soil or rock through a deformation analysis.

(2) When performing a deformation analysis, the Young's modulus of elasticity (E) may be taken equal to the dilatometer modulus (EFDT) on the assumption that the soil or rock is linearly elastic and isotropic.

(3)P When an indirect or analytical design method is used, the geotechnical parameters of shear modulus shall be derived from the dilatometer curve using methods relevant for that particular lest type.

4.6 Standard penetration test (SPT)

4.6.1 Objectives

(1) The objectives of the standard penetration test are the determination of the resistance of soil at the base of a borehole to the dynamic penetration of a split barrel sampler (or solid cone) and the obtaining of disturbed samples for identification purposes.

(2)P The sampler shall be driven into the soil by dropping a hammer of 63,5 kg mass onto an anvil or drive head from a height of 760 mm. The number of blows (N) necessary to achieve a penetration of the sampler of 300 mm (after its penetration under gravity and below a sealing drive) is the penetration resistance.

(3) The test should be used mainly for the determination of the strength and deformation properties of coarse soil.

(4) Valuable additional data may also be obtained in other types of soil.

4.6.2 Specific requirements

(1)P The tests shall be carried out and reported in accordance with EN ISO 22476-3.

(2)P Any deviation from the requirements given in EN ISO 22476-3 shall be justified and in particular its influence on the results of the test shall be commented upon.

4.6.3 Evaluation of test results

(1)P In addition to the requirements given in 4.2, the field and test reports according to EN ISO 22476-3 shall be used for evaluation purposes.

(2)P Existing design methods of foundations based on the SPT are of empirical nature. Equipment-related operating methods have been adapted to obtain more reliable results. Therefore, the application of appropriate correction factors for interpreting the results shall be considered (see EN ISO 22476-3).

(3)P The energy ratio (Er) has to be known for the equipment if the results are to be used for the quantitative evaluation of foundations or for the comparison of the results. Er is defined as the ratio of the actual energy Emeas (measured energy during calibration) delivered by the drive-weight assembly into the drive rod below the anvil, to the theoretical energy (Etheor) as calculated for the drive-weight assembly. The measured number of blows (N) shall be corrected accordingly (see EN ISO 22476-3).

(4) In sands, the energy losses due to rod length and the effect of effective overburden pressure should be taken into account accordingly (see EN ISO 22476-3:2005, A.2 and A.4).

(5) Other corrections should be considered, such as taking into account the use of liners (see EN ISO 22476-3:2005, A.3) or the use of a solid cone.

4.6.4 Use of test results and derived values General criteria

(1) When dealing with sands, a wide empirical experience in the use of this test is available, such as for the quantitative evaluation of the density index, the bearing resistance and the settlement of foundations, even though the results should be considered as only a rough approximation. Most of the existing methods are still based on uncorrected or partly corrected values.

(2) There is no general agreement on the use of the SPT results in clayey soil. In principle, it should be restricted to a qualitative evaluation of the soil profile or to a qualitative estimate of the strength properties of the soil.

(3) The SPT results may sometimes be used in a quantitative way in clayey soil under well-known local conditions, when directly correlated to other appropriate tests. Bearing resistance of spread foundations in sands

(1) If an analytical method for the calculation of bearing resistance is used, the effective angle of shearing resistance (φ') may be derived from SPT results.

NOTE For examples of analytical methods for the calculation of bearing resistance, see EN 1997-1:2004, Annex D.

(2) The value of φ' may be derived empirically from:

  • direct correlations with SPT results;
  • correlations with density index, where the density index is derived from SPT results.

NOTE 1 See for example F.1 and F.2.

NOTE 2 The relationships given in F.1 can be used to determine the density index I0 from either N60 or (N1)60, (see EN ISO 22476-3).

(3) The resistance of sand to deformation is often increased the longer the geological period of consolidation. This "ageing" effect is reflected in higher blow counts and should be taken into account.

(4) Over-consolidation should be taken into account because it increases the blow counts, for the same values of ID and σ'v0.

NOTE 1 In F.1, some sample correlations arc shown by the means of which the effect of both ageing and over-consolidation can be taken into account.

NOTE 2 When correcting for over-consolidation and ageing effects, the resultant derived φ' values, using the density index, from the correlations in F.2 can be conservative. Settlement of spread foundations in sand

(1) If a purely elastic design method is used, the drained Young's modulus of elasticity (E') may be derived from the N-values through empirical correlations.

(2) Alternatively, the density index may be derived based on the N60-value. Then an appropriate correlation may be used to obtain E' through the density index.

(3) The direct design methods arebased on comparisons of the N-values and results of plate loading tests or records of measured settlements of foundations. Allowable bearing resistance for a maximum settlement of 25 mm or the settlement corresponding to a given applied pressure can be obtained through the corresponding procedures with reference to the width of the footing, its embedment in the ground and groundwater table position.

NOTE The simple method for the calculation of the settlements caused by spread foundations in sand, as given in F.3, can be used. Pile bearing resistance in sand

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

Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing