By Kristen Skopeck, Albuquerque District Public AffairsDecember 18, 2012
The U.S. Army Corps of Engineers is partnering with Eastern Sandoval County Arroyo Flood Control Authority (ESCAFCA) to examine the existing conditions of the spoil bank levees within the Town of Bernalillo, N.M.
The agencies are working under the Corps' Planning Assistance to States authority, which requires a 50 percent cost share, and will result in a final geotechnical report. Ultimately, the results of the study will be incorporated into a flood damage reduction study the Corps and ESCAFCA will start in 2013 to address the existing Rio Grande flood plain in Bernalillo.
A team of Corps specialists from Albuquerque and Kansas City districts began test drilling on Bernalillo's spoil bank levee Nov. 28, using a cone penetrometer test rig (CPT) to study the existing conditions of the levee soils and the foundation soils.
"When we design for a new levee we have to drill on the levee centerline, the landside toe, and the riverside toe at a minimum of every 1,000 feet," said Albuquerque Project Geologist Jennifer Denzer. "Traditionally, our district always defaults to auger drilling with standard penetration tests (SPT)."
With SPT, a 1.5 foot sampler is driven into the ground every 2.5 feet and collects a sample. That soil sample is taken to a lab for classification. That method is generally the fastest and cheapest of the traditional drilling methods but does not always work well in loose wet sands, which is what the Corps expected to find when drilling the levees near the Rio Grande. Therefore, Albuquerque Project Manager Michael Martinez coordinated with Kansas City District, in Kansas City, Mo., who has in-house drilling crews and a CPT rig.
"The CPT rig is not a traditional drilling rig," Denzer said. "It's a probe that is hydraulically pushed down the hole with up to 40 tons of force from the truck. The CPT has a tip with a cone and a sleeve that measures resistance from the soils to give a soil classification without taking samples."
Denzer said the benefits of using a CPT rig is that it gives a continuous log of every hole, which auger holes do not, is much faster, works very well in loose wet sands, there are no samples that need testing, there are no cuttings brought up that need disposal, and there is much less disturbance to the current levee and no risk of damaging, the levee, which can happen with other drilling methods.
The team will wrap up work with the CPT rig Dec. 11; they will begin traditional augur drilling the next day to supplement data collected. The soil study work will be finished by Dec. 20.
"It is great that a drilling crew is here from the Corps' Kansas City District and is using a more efficient method (the CPT rig) to measure resistance from the soils and to give a soil classification without taking test samples," Martinez said. "There are no CPT rigs in the state of New Mexico, and we heard Kansas City had successfully done CPT work on their levees and ones in Sacramento, so this project will help us learn the ins and outs of the process here."
In all, the Corps is drilling 18 CPT holes and approximately 10 auger holes at Bernalillo's spoil bank levee, but the results are expected to be representative of the spoil bank levee in the area north and south of the Town. Martinez said that additional drilling will likely be required on the spoil bank levee north and south of the Town during the flood damage reduction project.
"Since a big portion of Kansas City's cost, or for using any CPT rig, would be the mobilization here, being able to use this on a very large job with tens of miles of levee would be very beneficial," Denzer said. "And, with using an in-house, Corps' crew, the quality of the work is often better, and there are no short cuts taken to increase profits."