Standard penetration test

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The Standard Penetration Test (SPT) is a probing method for ground investigation in geotechnical engineering . It is mainly used in the USA and is very widespread there, as the process is relatively cheap and can be combined with soil sampling. In Germany, however, ramming and pressure sounding are more common.

A similar method, which instead of taking samples only measures the number of blows when a conical tip penetrates the borehole, is the borehole- ramming (BDP) method regulated in DIN 4094-2 in Germany .

standardization

The method is standardized by the US ASTM (American Society for Testing Materials) (ASTM D 1586). There are also corresponding standards in other countries, such as the British standard BS 1377 or DIN EN ISO 22476-3.

Structure of the probe

The probe has an outside diameter of 2 inches (exactly 50.5 mm plus or minus 0.5 mm). The force or energy with which the probe is pressed in corresponds to a 63.5 kg hammer with a fall distance of 76 cm.

The drop weight can be arranged on the surface or in the borehole. In the case of the second method, the entire arrangement can be shielded so that undisturbed operation in the groundwater is possible.

execution

Protocol of the results of a standard penetration test in Florida , the N values ​​are plotted for two drill holes (dark and light solid lines)

In the case of SPT, a hollow probe tip is usually rammed into the ground in the borehole and the number of blows is measured for a specified penetration depth. 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.

Since the probe is hollow, a soil sample can also be taken during the probe . However, this procedure is not standardized in Germany.

evaluation

The impact numbers provide information on the density of the soil, but many other geotechnical soil parameters have also been correlated with the results of the SPT over time, for example stiffness modules . However, the samples were disturbed by the ramming.

application

The SPT is mainly used in countries where samples are mainly obtained using less expensive directional drilling methods, such as B. in the USA. In Germany, where other drilling methods are mostly used for sampling, however, according to DIN 4094-2, almost only a modified method is used in which the nozzle for soil samples is replaced by a cone point (which is the SPT (C) in DIN EN ISO 22476-3).

The following table provides guide values ​​for comparisons with other soil exploration methods for sandy soil:

Guide values ​​sand
Storage density very cool relaxed medium density tight very close
Related storage density (%) <15 15 to 35 35 to 65 65 to 86 85 to 100
SPT (N / 30 cm) <4 4 to 10 10 to 30 30 to 50 > 50
Pressure probe (MPa) <5 5 to 10 10 to 15 15 to 20 > 20
Lightweight ramming probe LRS 5 (N / 10 cm) <10 10 to 20 20 to 30 30 to 40 > 40
Heavy ram probe SRS 15 (N / 10 cm) <5 5 to 10 10 to 15 15 to 20 > 20

literature

  • Naval Facilities Engineering Command: Soil Mechanics Design Manual 7-01, Alexandria, Virginia, 1986, pdf (page 7-1-85)
  • JA McGregor, J. Michael Duncan Performance and Use of the Standard Penetration Test in Geotechnical Engineering Practice , Center for Geotechnical Practice and Research, Virginia Tech 1988
  • Chris Clayton : The standard penetration test (SPT): methods and use , Construction Industry Research and Information Association (CIRIA), 1995

Web links

Individual evidence

  1. ↑ DIN 4094-2: Field investigations, part 2: Borehole ramming , May 2003
  2. ↑ Standard Penetration Test, course at the University of Rhode Island, pdf
  3. ↑ Winfried Entenmann: Ground explorations , expert Verlag 2008, p. 194
  4. ↑ Wolfgang Sondermann, Klaus Kirsch, building ground improvement, in: Karl Josef Witt, Grundbau-Taschenbuch, 7th edition, Ernst and Son 2009, p. 120