5.1 General

(1)P This method covers the determination of the resistance of soils at the base of a borehole to the dynamic penetration of a split barrel sampler and the obtaining of disturbed samples for identification purposes.

(2)P The basis of the test consists in driving a sampler by dropping a hammer of 63,5 kg mass on to 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 seating drive) is the penetration resistance.

(3) The test is used mainly for the determination of the strength and deformation properties of cohesionless soils, but some valuable data may also be obtained in other types of soils.

(4)P The tests shall be carried out in accordance with a method that complies with the requirements given in this section.

(5)P The test method used shall be reported in detail with the test results.

(6) The test method may be reported by reference to a standard.

(7)P Any deviation from the requirements given below shall be justified and in particular its influence on the results of the test shall be commented upon.

(8) Experience of deviations exists with respect to the following points:

  • use of a solid 60° cone instead of the standard shoe when dealing with gravelly materials;
  • use of a barrel with an internal diameter larger than that of the shoe, in order to place an inside liner flush with the shoe where the sample is recovered;
  • an encased hammer assembly operating directly on top of the sampler in the bottom of the borehole.

5.2 Definitions

(1)P drive-weight assembly: a device consisting of the hammer, hammer fall guide, the anvil, and the hammer drop system.

(2)P anvil or drive head: that portion of the drive-weight assembly which the hammer strikes and through which the hammer energy passes into the drive rods.

(3)P drive rods: rods that connect the drive-weight assembly to the sampler.

(4)P Energy ratio-ERr: ratio between the actual energy delivered into the drive rod, immediately below the anvil, and the theoretical free fall energy of the hammer, expressed in percentage.

(5)P hammer: that portion of the drive-weight assembly consisting of the 63,5 kg impact weight which is successively lifted and dropped to provide the energy that accomplishes the sampling and penetration.

(6)P N-value: the number of blows required to drive the sampler. The N-value, reported in blows per 300 mm, equals the sum of the number of blows required to drive the sampler over the depth interval of 150 mm to 450 mm from the base of the borehole (and after its eventual penetration under gravity).

(7)P N60-value: N-value corrected to a reference energy ERr of 60 %.

(8)P (N)60-value: N-value corrected to a reference energy ERr of 60 % and an effective vertical stress σ = 100 kPa.

5.3 Equipment

5.3.1 Boring equipment

(1)P The boring equipment shall be capable of providing a clean hole to ensure that the penetration test is performed on essentially undisturbed soil. The correction for the diameter must be considered.

(2) The diameter of the borehole should not be larger than 150 mm.

5.3.2 Sampler

(1)P The steel sampler shall have the dimensions indicated in figure 5.1 and shall be provided with a non-return valve with sufficient clearance to permit the free flow of water or mud during driving.

Longitudinal cross section of an SPT sampler
Figure 5.1: Longitudinal cross section of an SPT sampler (dimensions in mm)

5.3.3 Drive rods

(1)P The drive rods shall have a stiffness that prevents buckling during driving. Rods with a mass of more than 10,0 kg/m mass shall not be used.

5.3.4 Drive weight assembly

(1)P The drive weight assembly, of an overall mass not exceeding 115 kg, shall comprise:

  • a steel hammer of 63,5 kg ± 0,5 kg conveniently guided to ensure minimal resistance during the drop;
  • an automatic release mechanism which will ensure a constant free fall of (760 ±10) mm, a negligible speed of the hammer when released, and no induced parasitic movements in the drive rods;
  • a steel drive head or anvil rigidly connected to the top of the drive rods. It may be an internal part of the assembly, as with safety hammers.

5.4 Test Procedure

5.4.1 Preparation of the borehole

(1)P The borehole shall be clean and essentially undisturbed at the test elevation and without an upward water pressure gradient.

(2)P When boring bits are used, they shall be provided with side discharge and not with bottom discharge, from a safe distance of the test elevation.

(3)P When testing below the groundwater table, particular care shall be taken to avoid any entry of water through the bottom of the borehole, as this will tend to loosen the soil or even lead to piping. For this purpose, the level of the water or drilling fluid in the borehole shall be maintained at a sufficient distance above the groundwater level at all times, even during withdrawal of the boring tools. Withdrawal shall be performed slowly and with drilling tools providing enough clearance to prevent suction effects at the bottom.

(4)P When a casing is used, it shall not be driven below the level at which the test will start.

5.4.2 Penetration test

(1)P Lower the sampler and the drive rods (with the hammer assembly on top) to the specified bottom of the borehole. The sampler shall be penetrated over an initial or seating drive of 150 mm applying the 63,5 kg hammer falling 760 mm and the number of blows shall be recorded. Then the sampler in the same manner shall be driven over 2 increments of 150 mm. The number of blows needed, or 50 blows, which ever occurs first, shall be recorded during each of these increments. The total number of blows required for the 300 mm penetration after the seating drive is termed the penetration resistance (N).

(2)P If the sampler advances below the bottom of the boring under the static weight of the drive rods and hammer assembly on top, the corresponding penetration shall not be included as seating drive and this information should be reported. In no case shall any material reach the level of the non return valve.

(3)P Representative samples or samples recovered shall be placed in air-tight containers, clearly labelled with all pertinent data.

Eurocode 7 Geotechnical design — Part 3: Design assisted by fieldtesting