Standard penetration test
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The standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. The test procedure is described in ISO 22476-3, ASTM D1586 and Australian Standards AS 12126.96.36.199.
The test uses a thick-walled sample tube, with an outside diameter of 50 mm and an inside diameter of 35 mm, and a length of around 650 mm. This is driven into the ground at the bottom of a borehole by blows from a slide hammer with a mass of 63.5 kg (140 lb) falling through a distance of 760 mm (30 in). The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm (6 in) up to a depth of 450 mm (18 in) is recorded. The sum of the number of blows required for the second and third 6 in. of penetration is termed the "standard penetration resistance" or the "N-value". In cases where 50 blows are insufficient to advance it through a 150 mm (6 in) interval the penetration after 50 blows is recorded. The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.
The main purpose of the test is to provide an indication of the relative density of granular deposits, such as sands and gravels from which it is virtually impossible to obtain undisturbed samples. The great merit of the test, and the main reason for its widespread use is that it is simple and inexpensive. The soil strength parameters which can be inferred are approximate, but may give a useful guide in ground conditions where it may not be possible to obtain borehole samples of adequate quality like gravels, sands, silts, clay containing sand or gravel and weak rock. In conditions where the quality of the undisturbed sample is suspect, e.g., very silty or very sandy clays, or hard clays, it is often advantageous to alternate the sampling with standard penetration tests to check the strength. If the samples are found to be unacceptably disturbed, it may be necessary to use a different method for measuring strength like the plate test. When the test is carried out in granular soils below groundwater level, the soil may become loosened. In certain circumstances, it can be useful to continue driving the sampler beyond the distance specified, adding further drilling rods as necessary. Although this is not a standard penetration test, and should not be regarded as such, it may at least give an indication as to whether the deposit is really as loose as the standard test may indicate.
The usefulness of SPT results depends on the soil type, with fine-grained sands giving the most useful results, with coarser sands and silty sands giving reasonably useful results, and clays and gravelly soils yielding results which may be very poorly representative of the true soil conditions. Soils in arid areas, such as the Western United States, may exhibit natural cementation. This condition will often increase the standard penetration value.
Correlation with soil mechanical properties
Despite its many flaws, it is usual practice to correlate SPT results with soil properties relevant for geotechnical engineering design. SPT results are in-situ field measurements, and not as subject to sample disturbance, and are often the only test results available, therefore the use of correlations has become common practice in many countries.
Overcoming SPT Problems
The NSWS was developed with the specific aim to encounter the recent weather abnormalities and natural hazard, saving human lives. It is compact, weighs only 70kg, and highly-mobile, suited to measure the ground in the crowded residential areas.  It costs about only half of what used to cost with the conventional SPT test and triaxial compression test. 
Features for detecting weak spots:
- it can measure very soft zones, converted N-value of zero in the ground that had been considered difficult.
- It has 1.08cm interval, far finer than SPT
- SPT conducts the test every 50cm, and 30cm interval out of 50cm is tested so the rest, 20cm, is not measured; that means 40% of an entire hole is unknown. NSWS does not suffer from such a limitation.
- NSWS can penetrate the ground diagonally.
Features for conducting in-situ shear test and sampling at a different hole:
- In-situ shear test capability, the result of the joint research with NARO and Okita-Ko Co.,Ltd
Features for conducting Stability Analysis:
- NSWS can prepare converted N-value, density, in-situ shear data for Stability Analysis.
- Since NSWS enables multi-point surveying due to its diagonal penetration capability and high-mobility. The multiple spots on the weak layers can be analyzed.
Problems with SPT
The following is the summary of the article. The article is based on the Japanese civil industry and they might not apply to other countries.
1. This test uses a boring machine to present soil strength as N-value by driving a sampler into the ground for 30 cm by blows from a slide hammer, and N-value resolution is per 1m so data only represents 30% of entire depth.
Comment: Someone pointed out that this is the case of Japan, and not the US.
2. Measured data are neither linear scaling nor normal distribution, but logarithmic normal distribution.
Comment: Soil strength is very wide ranged from the N-value zero to super hard rock, and if the researchers would like to be able to analyze the wide range of data, the exponential scale will be easier.
3. The method employees such a brute force method counting a number of blows of a sledge hammer, it cannot collect accurate data for weak soil layers.
Comment: The hammer uses the accelerated force, and strong force tends to drive away everything. One can tell that there is a great difference in how the force is applied to a matter using a sledgehammer pounding or using a drill and slowly squeezing out. The results of the boring machines are limited to whole numbers for a specific driving interval. This is very true, and the use of sledgehammer is the source of the problem.
Standard Penetration Test blow counts do not represent a simple physical property of the soil, and thus must be correlated to soil properties of interest, such as strength or density. There exist multiple correlations, none of which are of very high quality. Use of SPT data for direct prediction of liquefaction potential suffers from roughness of correlations and from the need to "normalize" SPT data to account for overburden pressure, sampling technique, and other factors. Additionally, the method cannot collect accurate data for weak soil layers for several reasons:
- The results are limited to whole numbers for a specific driving interval, but with very low blow counts, the granularity of the results, and the possibility of a zero result, makes handling the data cumbersome.
- In loose sands and very soft clays, the act of driving the sampler will significantly disturb the soil, including by soil liquefaction of loose sands, giving results based on the disturbed soil properties rather than the intact soil properties.
- Zatuwa, Monosagu (2005) Beware, soft ground and the standard penetration test (in Japanese) Public Works Research Institute
- University of Missouri - Rolla Class notes on the SPT.
- Inazumi, Shinya (2011). "In-Situ Ground Surv eying by the NSWS Testing Machine" (PDF). Int. J. of GEOMATE.
- Inazumi, Shinya. "A Presentation of Soil Investigation Example for Residentaial Embankment on Inclined Rock Mass and a Proposal of Reinforcement Material and Method".
- "Simplified strength analysis method for the slope of embankment such as a pond,in-situ rotation shear test(BST probe), Test manual(proposal) 1st edition" (PDF) (in Japanese). National Agriculture and Food Research Organization of Japan. 2013.
- "The National Agriculture and Food Research Organization of Japan".
- "NSWS(Nippon Screw Weight System)". Okita-Ko Co.,Ltd.
- "2) The Analysis Method for Ponds and Levees Preparing for Heavy Rain, and Earthquake test" (in Japanese). National Agriculture and Food Research Organization. 2012.
- "Public Works Research Institute of Japan". Public Works Research Institute of Japan.
- Be careful! Weak layers and Standard Penetration Test
- Kulhawy, F. H.; Mayne, P. W. (August 1990). Manual on Estimating Soil Properties for Foundation Design. Ithaca, New York: Electric Power Research Institute. pp. 2–17 to 2–26. EL-6800.
- Youd, T. L.; Member, Asce, I. M. Idriss, Chair; Fellow, Asce, Ronald D. Andrus, Co-Chair; Arango, Ignacio; Castro, Gonzalo; Christian, John T.; Dobry, Richardo; Finn, W. D. Liam et al. (2001). "Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER∕NSF Workshops on Evaluation of Liquefaction Resistance of Soils". Journal of Geotechnical and Geoenvironmental Engineering 127 (10): 297–313. doi:10.1061/(ASCE)1090-0241(2001)127:10(817).