Significance and Use
4.1 This test is the most frequently used subsurface exploration drilling test performed worldwide. Numerous international and national standards are available for the SPT which are in general conformance with this standard. The test provides samples for identification purposes and provides a measure of penetration resistance which can be used for geotechnical design purposes. Many local and widely published international correlations which relate blow count, or N-value, to the engineering properties of soils are available for geotechnical engineering purposes.
4.1.1 Incremental SPT sampling is not a preferred method of soil sampling for environmental or geohydrological exploration unless the SPT N-value is needed for design purposes. Continuous sampling methods such as Direct Push Soil Sampling (Guide ), or continuous coring using Hollow-Stem Augers (Practice ) or Sonic Drills (Practice ) provide the best continuous record of lithology. Continuous sampling can be performed with SPT samplers, but it is slow compared to other methods, and N values may unreliable (see ). Sampling for detailed lithology can be reduced by using screening tests such as geophysics and Direct Push profiling tests such as Cone Penetrometers (Test Method ), Dynamic Cone Penetrometer, or electrical resistivity probe.
4.2 SPT N values are affected by many variables allowed in the design and execution of the test (see ). Investigations of energy transmission in SPT testing began in the 1970’s and showed that differing drop hammer systems provide different energies to the sampler at depth. There are so many different hammer designs that it is important to obtain the energy transfer ratio (ETR) for the hammer system being used according to Test Method . ETR of various hammer systems has shown to vary between 45 to 95 % of maximum Potential Energy (PE). Since the N-value is inversely proportional to the energy delivered, resulting N values from different systems are far from standard. It is now common practice to correct N values to an energy level of 60 % of total (PE), or N60 values as presented here and in Practice . In this standard it is not required to report ETR or N60 but strongly advised to be noted and reported if available. If ETR of the hammer/anvil/rod system is known, the hammer PE can still vary after calibration, thus it is essential that hammer drop heights/rates be monitored to confirm consistent performance. Report any occurrence of hammer drop heights that do not meet the required value of 30 in. [750 mm] during testing. Using previous ETR data for a hammer system does not assure that it will perform the same on the current project. If onsite ETR is not obtained, be sure to check hammer drop height/rates to assure the hammer is operating the same as when previously checked.
4.2.1 Other mechanical variables and drilling errors can also adversely affect the N value as discussed in . Drilling methods can have a major effect on testing (see ). While the SPT hammer system is standardized knowing ETR, drilling methods are not, and a variety of drilling methods can be used.
4.3 SPT is applicable to a wide range of soils. For nomenclature on soil in terms of N-value refer to for consistency of clays (cohesive soils) and relative density of sands (cohesionless soils) as proposed by Terzaghi and Peck and used commonly in geotechnical practice. SPT drilling can be performed easily using a variety of drilling methods in denser soils but has some difficulty in softer and looser soils. This test method is limited to non-lithified or un-cemented soils and soils whose maximum particle size is approximately one-half of the sampler diameter or smaller. Large particles result in higher blow counts and may make the data unsuitable for empirical correlations with finer soils. For example, chamber tests on clean sands have shown coarse sands have higher blow counts than medium fine sands (see ). In gravelly soils, with less than 20 % gravel, liquefaction investigations may require recording of penetration per blow in an attempt to extrapolate the results to sand blow counts (see ). Soil deposits containing gravels, cobbles, or boulders typically result in penetration refusal, damage to the equipment, and unreliable N values if gravel plugs the sampler.
4.3.1 Sands—SPT is widely used to determine the engineering properties of drained clean sands during penetration. Obtaining “intact” soil samples of clean sands for laboratory testing is difficult and expensive (see thin walled tube, Practice ), so engineers use penetration results in sands for predicting engineering properties ( ). and provides some estimated properties of sands. There are problems with SPT in loose sands below the water table since they are unstable during drilling. Practice provides restricted drilling methods for SPT in loose sands for evaluating earthquake liquefaction potential. Practice method relies on mud rotary drilling, casing advancers, and fluid filled hollow-stem augers.
4.3.2 Clays—SPT is easy to perform in clays of medium to stiff consistency and higher using a variety of drilling methods. SPT is unreliable in soft to very soft clays because the clay, yields or “fails” under the static weight of the rods alone, or weight of rods and hammer before the test is started. This problem is accentuated by the heavier weights of automatic hammer assemblies (see ) but can be alleviated with automatic hammers which are designed to float over the anvil (see ). There is such a large variation in possible N values in soft clays it is well accepted that SPT is a poor predictor of the undrained shear strength of clay. It is recommended to evaluate soft clays with more appropriate methods such as CPT (Test Method ), vane shear (Test Method ), and/or Thin-Wall Tube sampling (Practice ) and laboratory testing.
4.4 Hammer Drop System—SPT can be performed with a wide variety of hammer drop systems. Typical hammer systems are listed below in order of preference of use:
(1) Hydraulic automatic chain cam/mechanical grip-release hammers
(2) Mechanical trip donut hammers
(3) Rope and cathead operated safety hammers
(4) Rope and cathead operated donut hammers
4.4.1 Automatic and trip hammers are preferred for consistent energy during the test. Automatic chain cam hammers are also the safest because the hammer is enclosed, and the operators can stand away from the equipment. If the rope and cathead method is used, the enclosed safety hammer is safer than donut hammer because the impact anvil is enclosed. For more information on hammer systems, consult .
4.5 Drilling Methods—The predominant drilling methods used for SPT are open hole fluid rotary drilling (Guide ) and hollow-stem auger drilling (Practice ). Limited research has been done comparing these methods and their effects on SPT N values (see ).
4.5.1 Research shows that open hole bentonite fluid rotary drilling is the most reliable method for most soils below the water table. Hollow-stem augers had problems with saturated loose sands since they must be kept full of fluid. The research also showed that driven casing using water as the drilling fluid, can adversely influence the SPT if the casing is driven close to the test depth interval. Use of casing combined with allowing a fluid imbalance also causes disturbances in sands below the water table. Fluid filled rotary casing advancers (Guide ) are included as an allowable drilling method for loose sands in Practice .
4.5.2 SPT is used with other drilling methods including reverse circulation, sonic drilling, and direct push methods practices. There are concerns, undocumented by research, with direct push (Guide ), sonic drilling (Practice ), and reverse circulation methods using heavy casing drive hammers (Guide ), that the extreme dynamic loading and vibrations could disturb some soils such as sands and soft clays past the seating interval. The professional responsible for the investigation should evaluate SPT under these conditions and if drilling disturbance is suspected, then N values can be checked against other drilling methods in section or deploy the alternate drilling method through and ahead of the casings.
4.5.3 SPT is also performed at shallow depths above the groundwater table using solid stem flight augers (Practice ), but below the water table borings may be subject to caving sands. Solid stem borings have been drilled to depths of 100 ft or more in stable material.
4.5.4 SPT is rarely performed in cable tool or air rotary drilling.
4.6 Planning, Execution, and Layout—When SPT borings are used, often there are requirements for other companion borings or test holes to be located near or around the SPT boring. In general, borings should be no closer than 10 ft [3 m] at the surface for depths of up to 100 ft [30 m]. A minimum would be as close as 5 ft [2 m], but at this spacing, boreholes may meet if there is significant vertical deviation.
4.6.1 Test Depth Increments—Test intervals and locations are normally stipulated by the project engineer or geologist. Typical practice is to test at 5 ft [1.5 m] intervals or less in homogeneous strata. If a different soil type in the substratum is encountered, then a test is conducted as soon as the change is noted. It is recommended to clean out the borehole a minimum cleanout interval of at least 1 ft [0.25 m] past the termination point of the previous test depth between tests to assure test isolation and to check drill hole condition for the next test. Therefore, the closest spacing for typical practice of SPT is 2.5 ft [0.75 m]. The cleanout between test intervals can be adjusted by the user depending on borehole conditions and design data needs such as hard soils or thin strata. The practice of performing continuous SPT for N-value determination is not recommended but can be done with careful cleanout before testing. The borehole must be cleaned out between tests (see ). At continuous spacing, with no additional cleanout depth, N values may be adversely affected by disturbance of previous sample driving especially in softer soils but the effect his not known. Some practitioners like to overdrive the sampler an additional 0.5 ft [0.15 m] to gain additional soil sample for a total drive interval of 2.0 [ 0.6 m]. This is acceptable if the N-value remains the sum of the 0.5 to 1.0 ft [0.15 to 0.3 m] intervals of the drive interval and reasonable cleanout is performed between tests.
4.7 This test method provides a Class A and B soil samples according to Practice which is suitable for soil identification and classification (Practices and ), water content (Test Methods ), and specific gravity tests (Test Methods ). The soil can be reconstituted for some advanced laboratory tests. The small-diameter, thick wall, drive sampler will not obtain a sample suitable for advanced laboratory tests such as those used for strength or compressibility from the core. Consult Guide for samplers that provide laboratory grade intact samples.
Note 1: The reliability of data and interpretations generated by this practice is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice generally are considered capable of competent testing. Users of this practice are cautioned that compliance with Practice does not assure reliable testing. Reliable testing depends on several factors and Practice provides a means of evaluating some of these factors.
Practice was developed for agencies engaged in the 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 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.
1.1 This test method describes the procedure, generally known as the Standard Penetration Test (SPT), for driving a split-barrel sampler with a 140 lb [63.5 kg] hammer dropped 30 in. [750 mm] to obtain a soil sample for identification purposes, and measure the resistance of the soil to penetration of the standard 2 in. [50 mm] diameter sampler. The SPT “N” value is the number of hammer blows required to drive the sampler over the depth interval of 0.5 to 1.5 ft [0.15 to 0.45 m] of a 1.5 ft [0.45 m] drive interval.
1.2 Test Method is generally necessary to measure the drill rod energy of a given drop hammer system and using the measured drill rod energy, N values can be corrected to a standard energy level. Practice uses Test Methods and and has additional requirements for hammers, hammer energy, and drilling methods to determine energy corrected penetration resistance of loose sands for liquefaction evaluation.
1.3 Practice is a similar procedure using a larger diameter split barrel sampler driven with a hammer system that may allow for a different hammer mass. The penetration resistance values from Practice do not comply with this standard.
1.4 Test results and identification information are used in subsurface exploration for a wide range of applications such as geotechnical, geologic, geoenvironmental, or geohydrological explorations. When detailed lithology is required for geohydrological investigations, use of continuous sampling methods ( , , ) are recommended when the incremental SPT N value is not needed for design purposes (see ).
1.5 Penetration resistance testing is typically performed at 5 ft [1.5 m] depth intervals or when a significant change of materials is observed during drilling, unless otherwise specified.
1.6 This test method is limited to use in nonlithified soils and soils whose maximum particle size is approximately less than one-half of the sampler diameter.
1.7 This test method involves use of rotary drilling equipment (Guide , Practice ). Other drilling and sampling procedures (Guides and ) are available and may be more appropriate. Considerations for hand driving or shallow sampling without boreholes are not addressed. Subsurface investigations should be recorded in accordance with Practice . Samples should be preserved and transported in accordance with Practice using Group B. Soil samples should be identified by group name and symbol in accordance with Practice .
1.8 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice , unless superseded by this test method.
1.8.1 The procedures used to specify how data are collected/recorded and calculated in the standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of these test methods to consider significant digits used in analysis methods for engineering data.
1.9 Units—The values stated in either inch-pound or SI units [presented in brackets] are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard. Reporting of test results in units other than inch-pound shall not be regarded as nonconformance with this practice. SI equivalent units shown herein are in general conformance with existing international standards.
1.10 Penetration resistance measurements often will involve safety planning, administration, and documentation. This test method does not purport to address all aspects of exploration and site safety.
1.11 Performance of the test usually involves use of a drill rig; therefore, safety requirements as outlined in applicable safety standards (for example, OSHA regulations, NDA Drilling Safety Guide, drilling safety manuals, and other applicable local agency regulations) must be observed.
1.12 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.13 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.