When VTrans (Vermont Agency of Transportation) personnel undertook the bridge replacement project at the interchange of I-89, US 2 and US 7 in Colchester, they knew they would need to bring their A-GaME. The agency had used Cone Penetration Testing (CPT) in the past to supplement traditional boring methods, but based on its understanding of the regional geology, site history and perceived likelihood of encountering soft clay deposits, they decided to incorporate CPT as a key component of the subsurface investigation for the project.
According to Stephen Madden, geotechnical engineer at VTrans, the use of CPT to provide high-quality and near continuous data had the following benefits:
- Aided in seismic site classification
- Improved estimation of undrained shear strength
- Allowed for direct design of driven pile foundations
- Allowed for a higher resistance factor to be selected for stability analyses, resulting in a more efficient foundation design
The geotechnical investigation program included CPTu, Seismic CPT (SCPTu) and traditional borings. (The “u” indicates that pore pressure measurements were taken). The boring/CPTu location plan was created such that CPTu locations were adjacent to borings at the planned bridge abutment locations and near the I-89 northbound US 2 interchange. CPTu soundings replaced borings along the I-89 southbound off ramp to US 2. The CPTu locations were strategically selected for areas where deep glaciolacustrine clay was anticipated to potentially influence the seismic design of the bridge at infrastructure components, and where the presence of tall embankment fills could lead to settlement and stability issues.
“The data obtained from the CPTu and SCPTu were used to justify selecting a higher resistance factor for stability analyses of the MSE walls at the proposed abutment locations due to the well-defined subsurface stratigraphy and geotechnical parameters,” explains Madden. “Additionally, we were able to use the measurements from the SCPTu soundings to designate a Seismic Site Class of D and Seismic Performance Zone 1, both of which resulted in time and cost savings in the design of the replacement structure over traditional borings alone.”
While the use of CPT offered several benefits to the site investigation and subsequent analysis and design for the project, there were challenges including the presence of obstructions, identification of the refusal layer, and execution of dissipation testing in low-permeability clay. Lessons learned from these challenges will be helpful for future projects in similar soils.
In the end, CPT provided value above and beyond that of SPT borings due to the continuous profile and the improved reliability that the CPT data provided. What’s more, the CPT program was implemented over a single day and resulted in direct costs savings of $6,025.
So, what’s the key takeaway according to VTrans?
When developing a geotechnical exploration program, it is critical to understand the capabilities and limitations of the exploration technology and the specific characteristics of the project site. One size does not fit all! The reward for this effort on the I-89 interchange project was the time and cost savings realized for VTrans that resulted from a successful and efficient exploration program.
Information for this article was provided by Stephen Madden of VTrans, and Melissa E. Landon, Ph.D., P.E., and Chris Benda, P.E., of Golder Associates USA Inc., Member of WSP.
Original source: U.S. DOT Federal Highway Administration. June 14, 2022 newsletter