Borehole drilling

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The Bohrlochrammsondierung (BDP, Borehole Dynamic Probing) is a Sondierverfahren the subsoil in the geotechnical . In the case of a borehole, a conical probe tip is driven into the bottom of the borehole using a method standardized in DIN 4094-2, and the number of blows is measured.

The procedure is based on the US standard penetration test , but no soil samples are taken. The process competes with pile driving and pressure sounding .

The probe has an outside diameter of 50.5 ± 0.5 mm and thus a cross-sectional area of ​​20 cm². The inside diameter is 35 mm. A drop weight of 63.5 ± 0.5 kg is used for ramming, which is dropped from a height of 76 ± 1 cm (≈ 30 inches).

First, half a foot (around 15 cm) is rammed in to penetrate the layer that is believed to have been disturbed by the drilling. Then another foot (around 30 cm) is rammed deep and the punch numbers are recorded. Only the number of strokes for the lower 30 cm is of interest for the evaluation. The number of blows is referred to as the N-value and is indicated with the depth of the pile-driving (here 30 cm): The total penetration depth of the probe is thus 1½ feet, i.e. approx. 45 cm. From this it becomes clear that the method can only be used meaningfully in one borehole.

If the ground is so soft that a penetration depth of more than 15 cm is achieved with one blow, the penetration depth is measured after this blow.
In hard soils, DIN 4094-2 specifies an upper limit of 150 impacts for borehole drilling, after which the test is terminated. Often, however, when 50 strokes are reached within a 15 cm section, the procedure is terminated and the depth of penetration noted in order not to overload the device and to limit the effort.

In DIN 4094-2, diagrams are also given that correlate the number of impacts of the BDP with those of the pile-driving soundings and the peak pressure of the pressure sounding, and diagrams that connect the results of the SPT with the stiffness modulus and with the density of the soil.

literature

  • DIN 4094-2: Field investigations, part 2: Drill hole drilling, May 2003
  • Klaus-Jürgen Melzer, Ulf Bergdahl: Building ground investigation in the field . In: Grundbau-Taschenbuch 2001, Ernst and Son, Volume 1

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