New Jersey has more than 450 roadway-adjacent rock slopes, all of which are inventoried and assessed using the Federal Highway Administration's (FHWA) Rockfall Hazard Rating System. The number one most hazardous rock site in the state is on I-80 westbound in the Delaware River Gap. The rock slopes range in height from 80 to 140 feet above the roadway, which is cut into the toe of Mt. Tammany. The rock slopes, adjacent Delaware River, and narrow highway configuration present a challenge to making the roadway safe from slope instability or rockfall, while maintaining the beauty of this Wild and Scenic River area.
To perform final geotechnical design for this site, our team coordinated a $2-million drilling and geophysical program to assess the soil and bedrock characteristics across the site. This information was used to inform the design of blasting, catchment ditches, retaining walls, and other geotechnical improvements.
I-80 westbound is a narrow highway configuration that is cut into the toe of Mt. Tammany and adjacent to the Delaware River.
New Ground for Horizontal/Vertical Inversion
We used two methods of surface geophysics which were cost efficient, and a quick and effective way of investigating large areas inaccessible to conventional drilling equipment. Where soil is present, seismic refraction was used to map and characterize the bedrock, however a large talus field-containing large boulders with little soil and a substantial amount of interstitial voids-covers the eastern portion and highest hazard section of the site. Due to the voids in the material, seismic refraction surveys can't be used in these type of deposits, so horizontal/vertical inversion was utilized as an alternate method in this area.
The test itself involves a surface unit-which is around the size of a tissue box-that is placed on the ground at specific grid points where you want to obtain point data. The unit uses ambient vibrations of the site-like passing trucks or pile driving in the area-to stitch together a shear wave velocity profile of the soil column. Once the information is gathered, dispersion curves are used to invert the horizontal and vertical shear wave velocities to assess the depth of hard, resonating layers-typically bedrock.
As with most geophysical methods, this technology shouldn't be the sole method of acquiring subsurface data, as certain conditions, such as shallow groundwater or dense layers overlying soft layers can skew the data. For this project, a series of control borings were drilled in accessible locations within the tested area to "ground-truth" the data.
Using surficial seismic methods for the I-80 westbound project is allowing us to tailor the way the project is designed and constructed to offer a solution that meets the needs of the various stakeholders.
The surface unit used in horizontal/vertical inversion.
The Future of Horizontal/Vertical Inversion
While originally developed for the mining, geologic, and geo-environmental industries, surficial geophysics are becoming more common in transportation due to their low cost and the type of information that they can provide. Horizontal/vertical inversion provides information regarding the shear wave velocity of the soil profile and depth to bedrock. However, it can't provide information regarding the quality of bedrock or detect the presence of faults, discontinuities, caverns, voids, or abandoned mines. As such, other methods, or conventionally drilling should be used in combination with this method if difficult conditions are present, or if additional information is desired.
The future of the horizontal/vertical inversion method and geophysics is bright as they're becoming commonly used tools in the geotechnical engineer's toolbox. We're currently proposing to use this method on other large-scale transportation projects, based on the successful use of this method on one of our more difficult projects.