Project: Doing CPT Dissipation Tests and Sampling Simultaneously?

Legion Drilling was recently engaged to complete a multi-week site investigation involving geotechnical and environmental works with various rigs. In particular, our client made full use of a Geoprobe 77/78 rig, using it for either environmental push tubing, geotechnical boreholes with SPT and Shelby tubes, or Cone Penetration Testing with pore pressure measurements (CPTu).

Dissipation testing was conducted to investigate the hydraulic conductivity of clayey layers as well as their coefficient of consolidation. Dissipation tests basically consist of stopping the CPTu probe (or HPT or DMT) advancement at any target depth and monitor how pore pressure dissipates over time, which typically takes seconds to few minutes in coarse-grained materials and hours to days in fine-grained materials. In this project, the tests were taking several hours and most CPTu rigs would have had to stay in 'standby' with operators patiently waiting for the test to finish.

This is not the case with Geoprobe 77/78 rigs, where CPTu systems can be de-attached easily. At this site, an external battery was employed to power the CPTu probe and convert it into an 'standalone' system when the dissipation test was running. Then, one operator tracked the rig to other locations, completing additional boreholes and samples as the pore pressure kept decreasing at the CPTu location. After a few hours of good progress, the rig went back to finish the log and retrieve the rods. Everything with only one rig.

Running overnight dissipation tests (when the site is safe to leave the rig and equipment unattended) or using CPTu probes with pore pressure filters in different positions (i.e., 'u1') are also common solutions that our crews can implement to maximise productivity and quality, getting all the data you need.