Standard Test Method for Measuring Normalized Standard Penetration Test (SPT) Value, N60, of Sands

Importancia y uso:

5.1 This standard is used to obtain more reliable SPT N-values than those that can be obtained using Test Method D1586/D1586M, which allows greater variability in equipment, calibration requirements, and hammer operation. The method described herein requires automatic or trip hammers, energy measurements, and borehole quality checks, and it restricts drilling methods to assure more repeatable N-values. Rope and cathead hammer operation is not allowed. This test method should be specified when N Values are used for geotechnical design purposes.

5.2 SPT N-values are used to estimate engineering properties of strength and compressibility of soils. The test is more useful in sands where recovery of intact samples for laboratory testing is difficult. SPT can be used in clays but is unreliable in soft clays. Reliable design requires N-values adjusted to a 60 % drill rod ETR, N₆₀, due to the wide variability of hammer systems. An ETR of 60 % is representative of the average ETR used to develop many soil property correlations. Design for sand liquefaction evaluation requires further normalization of N-values to a reference stress level (see Appendix X1).

5.2.1 SPT is used in a wide variety of soils from sand to clays both above and below the water table (refer to 4.3 D1586/D1586M) but is generally not applicable to gravels and cemented or partially lithified soils that result in refusal. This test method requires use of fluids for drilling of loose sands below water table as they become unstable. This test can be applied to unsaturated soils with other drilling methods that have a stable borehole and fluid drilling requirements waived. When applying SPT to unsaturated soil the user must consider any changes to properties if the soil becomes saturated during drilling or after construction.

5.3 This test method requires measurement of the drill rod energy, EFV, and determination of the energy transfer ratio, ETR, using Test Method D4633. This measurement is performed by instrumenting drill rods just below the impact anvil at the surface. A hammer system may be adjusted to any desired ETR depending on project requirements by changing hammer drop height. For example, many automatic hammers run at fast rates of 50 BPM (blows per minute) and deliver high energy. The speed may be reduced to allow for detailed recording penetration rates or to meet other project requirements.

5.3.1 It may not be practical to measure EFV continuously, in which case hammer systems are measured periodically either onsite at critical projects or on a regular schedule (see 10.2). Once ETR is known for a specific hammer, it is possible to check hammer performance by monitoring blow count rates and hammer impact velocities to verify they are operating consistently.

5.4 Use of this practice provides a disturbed soil sample for identification and for laboratory testing such as water content (Test Method D2216) and Soil Classification (Test Methods D2487, D2488). The classification information is commonly used to develop site stratigraphy.

Note 1: The quality of the result produced by this standard is dependent on the competence of the personnel performing it and the suitability of the equipment and facility used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some, but not all, of those factors.

Practice D3740 was developed for agencies engaged in laboratory testing, inspection, or both, of soils and rock. As such, it is not totally applicable to agencies performing this field test. Users of this test method should recognize that the framework of Practice D3740 is appropriate for evaluating the quality of an agency performing this test method. Currently, there is no known qualifying national authority that inspects agencies that perform this test method.

Subcomité:

D18.02

Volúmen:

04.09

Palabras clave:

liquefaction; N60 value; N-value; SPT; Standard Penetration Test;