6.4 Test procedure
(1)P The drive rods and the cone shall be driven vertically and without undue bending of the protruding part of the extension rods above the ground.
(2)P The penetrometer shall be continuously driven into the ground. The driving rate should be kept between 15 and 30 blows per minute, except when penetrating sand and gravel, where the driving rate may be increased up to 60 blows per minute. All interruptions longer than 5 minutes shall be recorded.
(3)P The rods shall be rotated 1 1/2 turns every 1,0 m.
(4) For rotating the rods, a torque measuring wrench (capacity > 200 Nm, graduations < 5 Nm) should be used.
(5) To decrease skin friction, drilling mud or water may be injected through horizontal or upwards holes in the hollow rods near the cone. A casing is sometimes used with the same purpose.
(6)P The number of blows shall be recorded every 100 mm for the DPL, DPM and DPH (N10) and every 200 mm for the DPSH (N20).
(7) The normal range of blows, especially in view of any quantitative interpretation of the test results, is between N10 = 3 and 50 for DPL, DPM and DPH and between N20 = 5 and 100 for DPSH. The rebound per blow should be less than 50 % of the penetration per blow. In cases beyond these ranges, when the penetration resistance is low, e.g. in soft clays, the penetration depth per blow may be recorded. In hard soils or soft rocks, where the penetration resistance is very high, the penetration for a certain number of blows may be recorded.
(8) For obtaining reliable results, the maximum investigation depths recommended are: 8 m for DPL, 20 to 25 m for DPM and 25 m for DPH.
6.5 Interpretation of test results
(1)P The test results shall be interpreted in one of the following ways:
- in terms of N10 for DPL, DPM, and DPH or N20 for DPSH;
- by determining the unit point resistance (rd) or the dynamic point resistance (qd) using the following formulae:
rd and qd axe resistance values in Pa;
m is the mass of the hammer in kg;
g is the acceleration due to gravity in N/kg;
h is the height of fall of the hammer in m;
A is the area at the base of the cone in m2;
e is the average penetration, imm per blow (0,1/N10 from DPLm DPM, and DPH, and 0,2/N20 from DPSH);
m' is the total mass of the extension rods, the anvil and the guiding rods, in kg.
(2) The value of rd is an assessment of the driving work done in penetrating the ground. To calculate qd-values, the rd-values are modified to take into account the inertia of the driving rods and the hammer after impact on the anvil. qd should thus allow comparison of different equipment configurations.
(3) Appropriate correction factors may be applied to take into account the friction on the rods.
(4) Energy losses are originated during driving due to the same factors described for the SPT test (5.5.2). Therefore, when this test is used for quantitative evaluation purposes, it is recommended to know by calibration the actual energy ratio ERr transmitted to the drive rods.
6.6 Reporting of results
(1)P In addition to the requirements given in 2.6 the test report shall include the following information:
- the type of dynamic probing: DPL, DPM, DPH or DPSH; all divergences from the normal procedures of applied these tests shall be described in detail;
- a graphic representation with respect to depth of the following data:
- number of standard blows to drive the cone 100 mm for the DPL, DPM and DPH or 200 mm for the DPSH, and/or the values of rd or qd;
- the maximum torque required to rotate the penetrometer at each test level (in Nm), if measured;
- all interruptions during the work, longer than 5 minutes;
- the use of any separate precaution against friction such as casing, drilling mud or water;
- the use of corrections to take account of friction along the rods, if any;
- details of any unusual event during driving, e.g. penetration without blows, temporary obstructions, artesian conditions.
6.7 Derived values of geotechnical parameters
(1) If dealing with cohesionless soils, it is possible to obtain correlations with some geotechnical parameters and in situ tests and use the results in a quantitative evaluation for foundations design, provided the friction along the rods is negligible, or duly corrected.
(2) If dealing with cohesive soils, the quantitative use of the results is more controversial and should be employed only under well known local conditions and supported by specific correlations. The skin friction during the test is a factor of special concern with this type of soils and should be duly taken into account.
(3) Several correlations have been established among the different dynamic probing tests and between them and other tests or geotechnical parameters. In some cases the friction along the rods has been eliminated or corrected, but the actual energy transmitted to the probe has not been measured. Therefore they cannot be considered valid in general. Nevertheless, some of them are included in annex E.