Soil penetration testing (SPT) is important to do before a building foundation is about to be poured. Foundations built above soils that have poor soil density can encounter structure problems later, especially in the event of an earthquake, for example. The standard penetration test for soil density focuses on measuring the resistance of soil to penetration.
Core Sampling Penetration
The standard penetration test (SPT) consists of using a 140-pound hammer to drive a split spoon sampler into the soil's surface. Drop the hammer a total of 30 inches with each blow to the sampler. Repeatedly hammer the split spoon sampler--a tool with a 2-inch diameter tube attached to the bottom of an 18-inch length of pipe--6-inch depths at a time, until the total 18-inch depth is reached.
Count the total number of blows for each separate 6-inch distance covered. Discount the first 6-inch blow tally number (from 0 to 6 inches), but sum up the last two-blow tally numbers together (from 6 inches to 12 inches and 12 inches to 18 inches). This final tally of the total blows needed to propel the split spoon sampler from the soil's depth of 6 inches to that of the 18-inch depth represents the number of blows per foot--also known as the soil penetration number. Soil penetration numbers indicate a soil's density through resistance to penetration.
Although there is only one standard penetration test for soil density, there is another testing method used to calculate soil density: the Proctor Test. Once penetration of the soil has occurred, remove a soil sample. In the proctor test, you drop a weight on the removed soil several times. Weigh the soil material and then dry it in the special soil oven for 12 hours. Now you can determine water content of the soil. Since water makes up a portion of soil, measuring it reveals if too much water is present to sustain foundations without their becoming liquefied in the event of an earthquake.
The Proctor Test aids in avoiding other potential liquefaction problems. For example, liquefaction can occur when the soil isn't dense enough to withstand compaction efforts later, like during additional building efforts.
Once the standard penetration test for soil density has been done to ensure engineers can build in an area, they still need to keep checking the soil as the building process continues. Field tests can help with these types of soil density questions. Field test options include sand cones, balloon dens meter, Shelby tube and the nuclear gauge. Your use of these field testing methods ensures soil density continues to be enjoyed during the development taking place.
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