Standard Penetration Test (SPT)
Designation: ASTM D 1586
History and Development:
The history of SPT goes back to early 1900s. In 1902 Charles R. Gow who was owner of Gow Construction Company in Boston started making borings of 1 inch through a sampler to explore the soil properties. After that time engineers started using wash borings with cuttings to get information about soil properties. In early 1930s this method was standardized by Harry Mohr who was an engineer at Gow Constructions. In his experiment Mohr used a larger sampler and counted the number of blows per foot of penetration on an 18 inches deep sample. He used different diameters of the sample ranging from 1/8 to 3/8 inches. He used 140 pounds hammer and made it to fall freely from a height of 30 inches. The value of blows for the last 12 inches penetration was counted and was called as SPT blow count value. At the same time other firms also worked on the test. They used a variety of split-spoon and push tube samplers with outer diameters varying from 2 to 4.5 inches and hammer weight was 100 and 350 pounds. This modified split spoon sampler recovered 1 3/8 inches diameter samples. These driven samples were able to get important details of soil that could not be obtained using wash borings. In 1945, the sampler barrel was replaced by the introduction of size A hollow drill rods which were equipped with Jackbit threads. These rods replaced the old 1 inch extra pipe that was being used until that time. In early 1940s, Raymond Concrete Piles Co. used 22 inches long sampler and this apparatus became known as Raymond Sampler.
Further research on the method was carried out by Terzaghi and Cassagrande. They used split spoon for their experiment with the support of ASCE’...
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...de into the large voids and low penetration resistance is observed.
2- Excessive large resistance may be expected when the spoon is blocked by a large piece of gravel or when the piece of gravel sedge into the spoon.
Factors Affecting Results of SPT:
1- Effect of overburden pressure. Soils of same density will give smaller count near ground surface.
2- Use of a drive shoe that is badly damaged or worn from too many driving to refusal.
3- Failure to properly seat the sampler on undisturbed material in the bottom of the boring.
4- Inadequate cleaning of loosened material from the bottom of the boring.
5- Driving a stone ahead of the sampler.
6- Variations in the 30 inch drop height of the drive weight, since this is often done by eye.
7- Interference with the free fall of the drive weight by guides and/or the rope used to hoist the drive weight for successive blows.
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