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Standard Penetration Tests (SPT)

The standard penetration test (SPT) primarily allows to determine the strength and deformation parameters of soils. The standard penetration test is based on a sampling device being punched by a ram weight m [kg] that is being dropped down from a height h [m] on to an anvil or punching head. The number of blows N, needed to penetrate a sampling device by a co called interval of penetration depth, is called the penetration resistance.

The results of standard penetration test are presented as a number of blows N over a certain distance, to which the device (ram with a head or anvil) punched into the soil or rock, respectively. This distance is called the interval of penetration depth d [m]. The value of this parameter is commonly assumed being equal to 0.3 m. For some types of tasks this value can be changed.

Energetic ratio of testes device Er [%] represents the ratio of real energy Emeas and the calculated energy Etheor of the ram.

Other important parameters to evaluate standard penetration test are so called correlations or correction factors (e.g. loss of energy due to length of the system of rods, influence of overburden in sands, etc.). The current design methods based on the principle of SPT tests have an empirical character and therefore it is necessary to use the corresponding parameters correctly modified. The program assumes the following two ways of adopting the correction factors:

  • correlation CN for vertical stress σ'V - represents the influence of weight of overburden in sands. The values of the correction factor CN greater than 1.5 must not be used (according to EN ISO 22476-3 recommendations).
  • user’s correlation λ [-] - represents the loss of energy due to the length of the system of rods for sandy soils. This correction factor can be specified in the programs in the range of (0.5 - 1.0).

Table of built in types of correlations

Type

Type of consolidation

Relative compactness lp [%]

Correlation factor CN

Type 1 - EN ISO 22476-3 (Tab. A2)

Normally consolidated

40 - 60

Type 2 - EN ISO 22476-3 (Tab. A2)

60 - 80

Type 3 - EN ISO 22476-3 (Tab. A2)

Over-consolidated

-

Type 4 - EN ISO 22476-3

Normally consolidated sands

-

Type 5 - FHWA (1998), Peck (1974)

-

-

where:

σ'V

-

Effective vertical stress

The result of standard penetration test is its process plotted as a graph. The evaluation of standard penetration tests (SPT) are used as input parameters for the analyses in the "Micropile" program (Bustamante method).

Import of SPT

The results of standard penetration tests (SPT) can be imported into the program by inserting the file in different formats (eg. *.TXT, *.CSV, *.XLSX, *.ODS).

Literature:

EN ISO 22476-3: Geotechnical investigation and testing - Field testing. Part 3: Standard penetration test, 2005.