Further soil samples can be retrieved from greater depths by pushing the sampler back down the same hole. A schematic of a direct-push soil sampler is shown at the right. The sample tube is then pushed to obtain the soil sample and the complete tube and push-rod retracted to the ground surface to retrieve the sample. The sampler is pushed closed-ended to the desired sample depth, then the push-rod is retracted a short distance to expose the open-ended small diameter sample tube. After a standard CPT, the truck can be moved a short distance to one side (typically a few feet) to obtain selected samples based on the detailed CPT profile. These are often small diameter tube samples that resemble a standard 10 or 15 cm2 cone penetrometer but without the internal electronics. What many engineers and geologists often don’t know is that most CPT trucks are also equipped to push soil samplers. These are usually 20 to 25 ton push capacity vehicles designed to push cones in a highly efficient and effective manner, where 600-800 feet of CPT can be carried out in one day! It is common in North America that the CPT is performed using customized CPT trucks. Samples can then be collected in an intelligent selective manner based on the actual soil profile, as defined by the CPT. The preferred way to take soil samples is immediately following the CPT, when the soil stratigraphy is known in great detail. This often results in many samples being obtained in soil layers that do not represent the critical layers for the given project. A common problem with conventional drilling and sampling is that samples are usually taken at regular depth intervals, commonly every 5 feet.

However, it is possible to collect small diameter, disturbed soil samples with the same CPT pushing equipment immediately after the CPT. In the strictest sense this statement is true during a Cone Penetration Test (CPT) you do not collect soil samples. Engineers often state: “The biggest disadvantage of the CPT is that it does not collect soil samples”. This first column is about a pet peeve of mine. (Gregg) in their southern California location (Signal Hill) and take this opportunity to contribute to the Gregg GeoNews via this regular column. I recently joined Gregg Drilling & Testing, Inc. Hopefully, you may have also read the CPT book “CPT in Geotechnical Practice” which I co-authored with Tom Lunne and John Powell. Welcome to Robertson’s Remarks! My name is Peter Robertson and many of you may know me as an academic/researcher (c/o University of British Columbia then University of Alberta, Canada) via my publications about in-situ testing, especially the Cone Penetration Test (CPT).

Undisturbed soil sampler