At the end of August 2020, our contractors at WSP performed cone penetration tests (CPTs) at the West Biddle Dam site, and collected soil samples. The test is called a cone penetration test because it consists of a cone attached to a rod that is pushed into the soil at a steady rate with the use of a CPT machine. The purpose of this test is to measure the resistance of the soil to penetration. In other words, how dense the ground is and what kinds of soils are there. The test can help us understand how the ground will respond to different levels of seismic events. CPTs can also help locate bedrock and the water table. Nine different tests were performed. This means we will know the exact measured soil conditions for those nine locations. Using the information from these nine CPTs, we can estimate the soil conditions around the rest of the dam and spillway below.
Many people were involved to perform this test. Three contractors operated the machine and collected the data, two engineers made sure that the information required for design was collected, and one archeologist checked any overturned soil to ensure the proper procedures would take place in the event something historic was uncovered.
Below are some images that describe the process.
Unloading the truck that will be performing the cone penetration tests (CPTs).
The CPT machine is set up for use. It is important that the machine is level. Anchors, like those pictured here, are drilled into the ground and help support the machine.
The front end of the CPT machine. The cone is threaded into the front part of the machine and slowly lowered into the ground.
The rod can be extended by adding more pieces, depending on how deep into the soil the CPT requires. The test is finished when rock bed is reached. We know when the rock bed is reached because there is so much resistance from the rock pushing back onto the machine that the machine will be lifted from the ground. Once the test is finished, the rod, cone, and anchors are removed from the ground and the machine can be moved to the next location.
After the CPTs are finished, soil samples can be taken using the same machine. A plastic Shelby tube is inserted into a hollow metal rod. The CPT machine hammers the metal rod into the Earth and the Shelby tube is filled with soil. Tubes can be filled until the rock layer is reached. The tube is taken from the metal rod and the soil can then be inspected.
Here, the bottom tube contains soil from the top of the earth (right side) to 5 feet below the surface (left side). The top tube contains soil from 5 feet to 9 feet below the surface. You can see how many different layers there are in just one soil sample and how the visual soil properties change quickly.
Here is another example of a soil sample starting at the surface going down 5 feet. On the right, you can see the “dirt” that we commonly think of as the ground below our feet. However, as you look to the left, you can see how the soil changes as we go deeper into the Earth.
Although we can make some observations about the soil and its properties with the naked eye, more information can be learned by sending the soil to a lab. After the collection of the soil, it was bagged and delivered to a lab to perform a variety of tests allowing us to further understand the properties and behaviors of the soils.