A widely used soil testing procedure is the Standard Penetration Test (SPT).
This test is still used because of it’s simplicity and low cost. It can provide useful information in very specific types of soil conditions, but is not as accurate as a Cone Penetration Test. Here’s more information about this basic soil testing procedure.
For this test, a sample tube, which is thick walled to endure the test environment is placed at the bottom of a borehole. A heavy slide hammer (140 lbs) is dropped repeatedly 30 inches onto the top of the sample tube, driving it into the soil being tested. The operation entails the operator counting the number of hammer strikes it takes to drive the sample tube 6 inches at a time. Each test drives the sample tube up to 18 inches deep. It is then extracted and if desired a sample of the soil is pulled from the tube. The borehole is drilled deeper and the test is repeated. Often soil recovery is poor and counting errors per interval may occur.
The number of hammer strikes it takes for the tube to penetrate the second and third 6 inch depth is called the ‘standard penetration resistance’, or otherwise called the ‘N-value’. The standard penetration resistance offers a gauge of the soil density of soils which are hard to pull up with simply a borehole sampling approach. You can imagine pushing a sample tube into gravel, sand or silt and struggling to recover samples that are useful for analysis. Coupling the standard penetration test with borehole drilling and sampling can be an improvement for understanding certain soil types underground.
This basic soil testing procedure gives reasonably consistent results in fine-grained sands and is not as consistent in coarse sands or clays. It can be useful in areas that are difficult to access with any vehicles for very preliminary testing of near surface conditions. For more accurate data and resulting analysis of soil conditions that will be more useful for engineering purposes, a cone penetration test should be specified with minimal SPT field verification.
To review the ASTM (formerly known as the American Society of Testing & Materials and now known as ASTM International) standard for Standard Penetration Testing, ASTM D5877-11 click here.
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