At the Arcadia flood risk management project, in Arcadia, Wisconsin, geotechnical staff are gathering data using a unique method of subsurface exploration. The Cone Penetrometer Test, or CPT, is one method used to identify and characterize soils. The CPTs were conducted with assistance from the Savannah District geotechnical and geology branch. “We benefited from their expertise and cooperation,” said Greg Wachman, senior geotechnical engineer.
In CPTs, a device with a conical tip and metal sleeve measure penetration resistance as it’s pushed into the ground. Those measurements are used to characterize the soils’ engineering properties. For example, the forces on the device as it’s pushed through a soft clay are very different from those as it’s pushed through a dense sand, Wachman said. The device also records pore water pressure, which aids in understanding soil permeability and groundwater characteristics.
CPTs vs. soil borings
A CPT is most useful when used together with standard soil borings, Wachman explained. A soil boring drills into the ground to retrieve physical samples. In contrast, with a CPT, the soil is never seen. CPTs are significantly faster than standard borings and provide continuous test data with depth. With a soil boring, samples are collected about every 5 feet, or change in material, so it’s possible to miss important information. One limitation of the CPT, due to excessive friction, is that it may not be extended to the same depth as a soil boring. The CPTs at Arcadia are being pushed to approximately 60-70 feet, whereas a soil boring can be performed in excess of 100 feet.
“By doing some CPTs next to soil borings – where we know what the soils are – we can increase the likelihood that we are correctly interpreting the CPT data at locations where we don’t have any borings,” Wachman said. “Ultimately, by combining the test data from soil borings and CPTs, we assign engineering properties to the soils and analyze the foundations for things like seepage, stability and settlement.”
Seepage, stability and settlement
Wachman’s job is to ensure that the levees and floodwalls for the Arcadia project have a stable foundation that can withstand the forces produced by a flood that reaches the top of protection. Soil can behave in unexpected ways when it is subject to water forces, he explained.
Three soil mechanisms that the geotechnical engineers are concerned with are seepage, global stability and settlement. With seepage, the concern is that the forces produced by water flowing through soil beneath a levee or floodwall could start to erode the soil and undermine the foundation, leading to failure.
When global stability is analyzed, they are interested in ensuring that stresses produced by construction of the levee or floodwall or by flood loading do not exceed the foundation soil strength. If the soil strength is exceeded, the structure could experience a collapse, Wachman said.
Settlement of the foundation, which can happen over a period of years, needs to be considered to ensure the structure is tall enough to provide the designed level of protection, he added.
“We’re getting a lot of useful data out here, and we have a lot of really good tools to help us construct something that will help the city of Arcadia,” Wachman said.
The city of Arcadia has a population of about 3,000 people and is home to Ashley Furniture. The city is in the driftless area in Wisconsin – one of the few regions in the upper Midwest that was not covered in ice during the last ice age – which is reflected in the topography of the region. Part of the city is located in the flood plain of the Trempealeau River and is vulnerable to flooding.
Under the Continuing Authorities Program, the Corps is working with the city of Arcadia to design and construct a system of levees and floodwalls that will provide flood protection to the city. The project is estimated at approximately $37 million and is currently in the design phase.