Oh the horror! An old salt like me had to go camping during our summer holiday. Our daughter had her birthday during our road trip in the USA and she wanted to celebrate it by camping in the woods. Complying to her wishes we pitched a tent and roasted marshmallows. Meanwhile my mind was frantically searching for familiar clues to connect to my maritime heritage. Hammering down the tent pegs, it dawned to me: putting up a tent is basically a simple Standard Penetration Test.
Standard Penetration Test is one of the easiest soil investigations you could do1. All you need is a pipe and a hammer. You count the number of blows to hammer the pipe down and you have an indication of the effort it takes to cut the soil. This method completely ignores sophisticated parameters as e.g. undrained shear strength, porosity or internal friction angle. It is very crude in its results. On the other hand, the basic principle of driving the pipe into the ground is very similar to the cutting action of the pick points on a cutter. As such, it is a very good indicator for the performance of a cutter head. This is also the reason, why for initial discussions about the performance of a CSD, the SPT is a good starting point to ask the client. He might have a report like this already available, or he can easily perform the tests. Also ‘Sandy’ accepts SPT values for an indication of the soil quality2.
Be aware, that SPT’s are often not very deep. Of course, a full soil investigation report with a Cone Penetration Test is much more valuable. We can always translate the results from a CPT report into a SPT value. But the SPT information is not covering all the parameters to translate this to a CPT. Sometimes even an SPT report fails. And then it might be useful to discuss with the client on a qualitative level about the soil condition. Usually people have actually touched the soil, or at least can paint a mental picture of the soil conditions and these criteria might help to use the same descriptive language.
Hammering a rod into the soil is a relative cheap and quick method to collect the soil consistency. It can be performed everywhere, anytime, under most conditions. That is why it was also selected for the soil investigation on one of the most remote locations imaginable. Although it is still on my wish list of dream destinations, the prohibitive price tag of the ticket will prevent it for me to pitch up my tent over there. I just have to revel in the camping adventures of Neil and Buzz.
- ASTM: Standard Test Method for Standard Penetration Test
- Sandy, Dredge Finder
- Astronaut Edwin Aldrin takes a core-tube sample