5.6 Reporting of results
(1)P In addition to the requirements given in 2.6 the report shall include the following information:
- penetration of the sampler under static weight, if significant;
- number of blows required for each 150 mm increment of penetration (including the seating drive);
- if the drive is terminated before the full penetration in any of the 150 mm interval: record of the depth of penetration for the corresponding 50 blows;
- the original N-value and the corresponding depth interval and soil description;
- the corrections applied, if any, and the corrected N-value;
- boring method and diameter;
- location of the water table, if known, and level of the water or drilling fluid during the preparation and execution of each test;
- dimensions and weight of drive rods used for the penetration test;
- type of hammer and release mechanism and weight of the drive head;
- the energy ratio ERr;
- the N-values should be reported in the boring log at each corresponding elevation, with a graphical representation.
5.7 Derived values of geotechnical parameters
5.7.1 General criteria
(1) When dealing with cohesionless 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 capacity 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 in clayey soils. 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. Nevertheless it may sometimes be used in a quantitative way, under well known local conditions, when directly correlated to other appropriate tests.
5.7.2 Bearing capacity in sands
(1) Bearing capacity problems may be solved through the determination of the density index, and the effective angle of shearing resistance, N'.
5.7.2.1 Derived values for the density index
(1) In annex D.2 an example is given of the relationship between N60 and (N1)60 versus the density index ID.
(2) The resistance of sand to deformation is greater the longer the period of consolidation. This "ageing" effect is reflected in higher blow counts.
(3) Overconsolidation increases the blow counts, for the same values of ID and F'v.
(4) In annex D.2 the effect of both ageing and overconsolidation is illustrated through some sample correlations.
5.7.2.2 Derived values for the angle of shearing resistance N'
(1) The angle of shearing resistance may be derived directly from the value of N through empirical correlations.
(2) It can also be estimated using the density index as an intermediate parameter. In annex D.3 a sample correlation of this type is shown.
5.7.3 Settlement of spread foundations in sand
(1) When a purely elastic method is used, the drained Young's modulus Em may be derived from the N-values through empirical correlations.
(2) Alternatively, the density index may be derived based on the N60-value, and then use an appropriate correlation to obtain Em, through the density index.
(3) The direct methods are based on comparisons of the N-values and results of plate loading test or records of measured settlements of foundations. Allowable bearing pressures 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.
(4) A sample method for the calculation of the settlements originated by spread foundations in sand is given in annex D.4.
5.7.4 Pile foundations in sand
(1)P When the ultimate bearing resistance of piles is evaluated from SPT results according to 7.6.3.3 (4) of ENV 1997-1, calculation rules based on established correlations between the results of static load test and SPT results shall be used.
