So, what is the difference between SPT and CPT? At a glance, the answer is quite simple. One starts with an S, and the other starts with a C! While technically correct, both are used in soil test borings, but each method yields different results.

Howell utilizes a Geoprobe 7822DT drill rig, which initially came with Standard Penetration Testing (SPT) capabilities. As you may have read in the “Data 2.0 and Other Drilling Updates” newsletter, Howell recently installed Cone Penetration Testing (CPT) tooling on the same Geoprobe drill rig, so now we have the capabilities and experience to do both types.

SPT testing has been conducted since the early 1900’s and utilizes hollow-stem augers to drill through soil layers, and split-spoon sampling to retrieve soil samples. Both allow us to perform lab testing to classify soil types. SPT gives blow counts every 6 inches to determine how the density and consistency of the material change with depth, so we can locate any soft or very hard areas that may impact construction and costs on a project. SPT is optimized for sandy soils.

CPT utilizes direct push with a specialized cone that can measure inch by inch through a soil profile to obtain more data points and provide a more accurate profile. The data collected consists of tip pressure, sleeve pressure and pore water pressure, which are used to classify the soil without taking any samples. You may be thinking, “More data points are better, so why not just use CPT everywhere?”  The answer is site specific. CPT does not collect soil samples, which are needed in some circumstances and the cone can be very sensitive to obstructions such as fill, gravel, cobbles or boulders, which could lead to a potential break.

While SPT can be used on most sites with density over 6 inches, augers drill through cobbles and variable fill layers to bedrock and potentially beyond with rock coring. CPT testing is most useful in fine-grained soils such as clays, silts and some sands, providing more detailed, inch-by-inch results.

So, which one should be used on your site? Well, you can leave that to Howell Environmental. We conduct preliminary investigations prior to arriving on-site, and with our experience in your area, we can determine what makes the most sense for your project.