Type of Testing
Sinkhole testing should involve two phases:
The first phase involves a detailed inspection of your home. The testing company employee will visit your home for a few hours. He or she will walk through the home and look at damages, make a drawing of the layout of your home depicting the locations of the damage. He or she may note any indentations, depressions or holes in the yard, too.
The tester will also likely perform some type of geophysical tests, too. A geophysical test uses advanced technology to allow scientists to form a picture of ground conditions below the surface. A very loose comparison is that geophysical testing is like taking an x-ray of the ground - just as an x-ray can see through your skin and show bone structure, geophysical testing seeks to see below ground to show soil structures.
GPR, stands for Ground Penetrating Radar. GPR shoots radar signals down through the ground to take a picture of the below ground surface. Different types of soils may show different GPR results, so it may allow scientist to detect subsurface anomalies - that is, conditions below the ground that don’t match other areas below the ground. A sinkhole, for instance, is likely to show up as anomalies on GPR.
ER stands for Electrical Resistivity testing. In ER, a string of probes are inserted into the ground in a straight line, and each probe measure the time it takes for an arc of electrical current to travel from one probe to another. The length of time can tell scientists about subsurface conditions, and can possibly indicate anomalies or strange subsurface conditions.
An anomaly below ground is exactly what the name implies - something abnormal. Generally speaking you would expect the subsurface conditions to not vary much over the course of your yard. If they do, it can be an indicator of something abnormal occurring below the ground. Although not the decisive factor in determining sinkhole activity, a GPR or ER anomaly can indicate areas for further or additional investigations. For instance, a geologist may want to drill a deep SPT boring in the location of a GPR or ER anomaly.
SPT stands for Standard Penetration Test. An SPT is a deeply drilled bore hole that can go down hundreds of feet into the ground. SPTs drill for approximately three and one half feet, and then stop drilling. The drill is pulled out of the ground and a hollow metal tube, called a "split spoon sampler," is inserted on a rod into the bottom of the drill hole. The drillers hit the rod attached to the hollow metal tube the a one hundred forty pound hammer to try to advance, or hammer, the tube into the ground. They drillers count the number of times they hit the tube with the hammer to cause the tube to sink six inches into the ground. The number of times the tube is hit with the hammer is referred to as the "blow count." The drillers will try to advance the tube six inches three times in a row, counting the blow count for each six inch increment separately. After the tube has been sunk the full six inches three times (or eighteen inches in total), the rod and tube are removed, and the drill is reinserted into the tube. The hollow tube is then opened up, and samples of the soils are taken and placed into a sample jar for later labrotory analysis. Drilling is conducted for another three and one half feet, and the process repeats itself.
Blow counts measure the density of the soils below ground at different depths. Typically three separate blow counts are recorded for the first interval of six inches, the second interval of six inches, and then finally the third interval of six inches. The results of the first six inch interval is discarded (this is called the seeding blow), and the number of blows of the second and third interval are added together to create an "N-Value."
The N-Value, determined during SPT testing, give a numerical indication of the density of the soils at different depths. N-Values range from a high of 50 (which is very dense, solid soil), to a low of 0, known as a WH or WR (standing for Weight of Hammer or Weight of Rod). WH or WR indicate either a void, or soil so loose that it cannot support hardly any weight at all. WH, WR and very low N-Values are often present in sinkhole conditions.