Professional Contributions

“The Life of a Traveling Geologist” CPT Pushing in Pure Michigan

Luke Ducey, WSP USA

t feels like yesterday that I got a phone call from the head of Geotechnology’s Geotechnical Division to ask what I had going on in November and December and if I might be available to do some Cone Penetration Testing (CPT) in Michigan for “a while.” At Geotechnology, only two people were trained to operate the massive, 40,000-lb CPT rig, and I was one of them. After being reassured I would be home for the holidays and not miss a thing, I said, “I’m in.”

A CPT rig is a direct push rig that pushes a cone tip vertically into the ground to collect load cell data—think of pushing a piece of spaghetti into a cake. Before starting a test location, it is important to have the utilities cleared, the cone hooked up, the rig leveled to ensure a stable push platform, computer software operational with the proper job number/hole number, and the cone cleaned and assembled.

As the ~$10,000 cone is pushed into the ground, it collects tip, sleeve, pore pressure, and inclination using load cells within the cone. Additionally, temperature and shear wave data can be collected. These data are processed and displayed on a

lscreen in real-time so that we can observe and assess our depth, soils strength, where groundwater is, cone inclination, and cone or sleeve refusal. This requires quick thinking and can be stressful because the cone is advanced at approximately 2 cm per second, which seems slow, but is fast enough that during difficult pushing, you have to pay close attention and be ready to stop the cone prior to damaging it. CPT operation is a two-person job because as the operator is monitoring the push and controller pad, the helper is attaching each additional rod as the push goes deeper and deeper. The helper makes sure that the cord, running through all the rods, does not get pinched or damaged, and attaches each rod (1 meter or 3.3 feet) one at a time until the target depth is reached or rig/cone refusal occurs. Once at the final depth, the operator switches gears, and the cone is then hydraulically pulled back up, cleaned, and placed on the rack until you start on the next hole. This particular CPT job was different from any other I had done. Starting with the location—neither of us had been up to Michigan for a job before, let alone somewhere so far north in the winter. Usually, when we are doing CPT for a new building, we will just do a few locations across the building footprint, but for this project they weren’t putting in buildings, they were building wind turbines—seventy-five of them. These wind turbines weren’t all just a couple of hundred feet apart, they were miles and miles apart so the project covered a large area and required tracking the CPT rig in and out of many cornfields. Mind you, the CPT rig has a maximum speed of 2.5 mph, so getting from one site to the next was…slow. To save time, instead of driving the rig (slowly) between each site, we decided to load and unload it onto a drop-deck trailer and transport the rig from site to site. However, each load and unload cycle still took an hour, resulting in a significant time sink. The proposed wind tur-

Figure 1: Geotechnology CPT rig tracking to a new location

Figure 2: View from inside of the CPT rig of the head press, rods going down to depth, and the computer/controller set up

Figure 3: The author taking a break to make a “pure” snow angel

bine sites were spread over an area of roughly 17 by 10 miles and usually located in the center of fields away from trees, sheds, and homes.

One unexpected roadblock we encountered on this project was getting the supplies we needed to continue operating. For each hole we pushed, we then set a temporary well in each location to check groundwater levels. But, to do so, we had to purchase hundreds of PVC pipes to assemble and install in each hole. We went to multiple stores looking for enough PVC pipe for the project, but many were closed for the holidays or because of snowy weather. We were a bit confused as to why the stores were closed for the “holiday” when it was still early November. We didn’t realize it was actually the opening day of deer season until a local told us. Everyone in central Michigan considered these days a holiday that businesses, schools, and supply stores shut down for! We made sure to keep our flashers on when driving down the road, safety vests permanently on, and the CPT rig lights on at all times after learning that. Luckily, we found a mom-and-pop shop that was open one of the days so we bought everything they had!

The other challenge of this project was the most obvious— the “pure” cold weather. Michigan winters chill to the bone no matter how many layers you are wearing. Each morning saw the same routine of putting on two pairs of socks, two pairs of pants/long underwear, three long-sleeve shirts, ear muffs, a knit cap, and then finally my coveralls and jacket. Then, it would begin to snow, sleet, and the winds would pick up, making it even more brutal on your body. Fortunately, the cab of the CPT rig does have a heater, which was usually kept at around 65 degrees inside, so coming out of freezing temperatures and high winds felt like stepping into a sauna. The whole trip consisted of putting clothes on and taking them off about ten times a day. The snow and ice made driving a Peterbilt semi-trailer truck and F-150 on the country roads a challenge at times. When the snow finally melted and the temperatures warmed up to the 40s, the fields turned into deep, wet mud, which led to us getting the support truck stuck MANY times. But with 40k pounds on steel tracks, you have something to pull yourself out.

At the end of the project, we were able to complete all the locations the client had requested. And in fact, we even went back again at a later time to do some more testing. Pushing in the stiff glacial till of central Michigan was tough. But with a lot of experience behind the rig, my partner and I overcame the stiff till, the cold weather, random holidays, ten-hour days/nights, and long drives between St. Louis, Missouri, and St. Louis, Michigan.

I hope this information and story were easy to understand and provide an awareness of all the activities that go into pushing with a CPT rig. If you have other questions, feel free to contact me through AEG or Luke.Ducey@wsp.com.