CPT investigations for roadway applications
Investigators at work
"There are a lot of new tools for us to use as engineers," said Glenn Engstrom of Mn/DOT's Office of Materials, "and we need to figure out how to use them and fit them in our applications. That is what cone penetrometer testing (CPT) is about."
In a little "sales pitch" for geotechnical investigations, Engstrom said half of all claims are related to geotechnical issues. Fixes are expensive, and the cost of overdesign is also a problem. "It gets expensive when you don't have the right information," he said.
While the cone is not new—in fact, it was used in 1930 or so in Europe—it is becoming more useful now with computer technology to record the data. CPT offers many advantages: there's no lab work and it gives immediate results; it is 5 to 10 times faster than drilling; and it gives a continuous soil profile and pore water pressure measurements. "It's a great tool for preliminary investigations," he said. "It's definitely the way to go."
The system works by pushing a stainless steel rod into the ground at 1 inch per second. It gives instantaneous measurements of tip stress, sleeve friction, pore pressure, push rate, and push force. An inclinometer indicates if the rod is straight or crooked. "Sometimes we get too much information—we need ways to analyze and store it all," Engstrom said.
To meet this data management need, Mn/DOT has developed a map-based database using ArcIMS to store all this information and make it widely available. The Web-based archive includes data from prior subsurface investigations, and background maps can be changed to USGS Quads or aerials photos. "Our goal is to allow anyone to use the database to look across the state and find the boring you want," he said. Look for it this fall from the Geotechnical Engineering Section.