SDSU proposes to undertake a limited geotechnical investigation within the San Diego River in conjunction with the proposed Fenton Parkway bridge. The geotechnical investigation is intended to benefit the City of San Diego, as its purpose is to inform engineering and design specifications required for the proposed bridge. The proposed bridge would be constructed on real property owned by the City of San Diego, and upon the completion of construction, would be owned, operated, and maintained by the City of San Diego.
The geotechnical investigation would involve assessment of subsurface soils at two boring locations (see Figure 1). The boring locations were selected to be near the anticipated proposed bridge pier columns. The borings would be approximately 8 inches in diameter and approximately 150 feet in depth and would not result in removal of existing vegetation. An area of approximately 20 feet by 40 feet around each boring location would be needed to maneuver the equipment associated with conducting the borings (see Figure 1). Additionally, to further avoid permanent disturbances to vegetation, plywood, steel plates, and/or rubber mats would be laid down in these areas and along proposed paths of travel to each area in order to avoid the need for vegetation removal. All spoils and drilling mud would be sealed in drums or stored in a roll-off bin and disposed of off-site. The drums would be temporarily stored in the river channel during the field work and hauled out at the same time the equipment is demobilized. Consistent with County of San Diego Department of Environmental Health guidelines, backfill for the boreholes would include cement-bentonite grout. Up to 5 feet of native soil will be placed at the top of each borehole to resemble the existing conditions prior to drilling.
Equipment that is anticipated for the work in the river channel includes the following: track- or rubber-tired truck-mounted drill rig and associated drill tooling (e.g., augers, drill rods, drill bits, mud pan); four-wheel drive telehandler; tracked skid steer loaders to move supplies and equipment in the river channel; plywood, steel plates, cribbing, prefabricated timber walkways, rubber mats, and/or steel plates to provide access to the drilling locations and support the drilling rig outriggers; 55-gallon drums and/or roll-off bins for storing boring cuttings and drilling mud; large water totes and associated water and grout piping/hoses; and hand tools and equipment for the collection and storage of soil samples. At the end of each workday, all equipment and tools would be turned off and secured within the river channel. Drip pans would be placed under all equipment when not in use or during refueling. All equipment will be demobilized and removed prior to the commencement of rain events significant enough to forecast that the river within or immediately abutting the boring work area will rise above the low flow channel into the area of work. Any equipment stored within the river would be affixed with drip pans to avoid potential fuel leakage exposure to the river environment.
Consistent with the City’s Water Pollution Control Plan requirements (City of San Diego 2023), temporary best management practices (BMPs) would include the use of visqueen and fiber rolls around the work areas, as needed. Implementation of these and other appropriate BMPs would ensure no runoff or siltation resulting from activities would occur in the river channel. All equipment would be stored within the river channel for the duration of the drilling. Any equipment stored within the river would be affixed with drip pans to avoid potential fuel leakage and exposure of the river environment.
Pursuant to the Biology Guidelines of the San Diego Municipal Code’s Land Development Code (2018), a Qualified Biologist would be retained to monitor all geotechnical boring activity to ensure no impacts to biologically sensitive areas occur. Biological monitors will direct work to be performed in locations that will not disturb sensitive environmental resources, as illustrated on Figure 1. The qualified biologist will accompany the drilling team to confirm all staked locations or slightly modify them based on changed conditions to locate worksites consistent with the methodology described above (maximize bare ground, avoid trees and need to remove or trim vegetation). In addition, the work would be heavily monitored by cultural resources (also experienced in paleontological resource monitoring), and hydrology/stormwater resource experts to ensure that sensitive environmental resources are not impacted during the approximately 2- to 3-week investigation period. There is no reason to believe the proposed work could disturb cultural or paleontological resources because there would be no grading, and ground-disturbing work is limited to drilling two 8-inch boreholes on bare land.
A crane will be used to lower the equipment and supplies for the work that cannot be carried by hand into the river channel. Due to the location of the active San Diego River channel, a crane setup is required. It will be located outside of the river channel in an existing construction staging area within the southwest portion of Mission Valley River Park. The crane setup location is devoid of sensitive habitat, as shown in Figure 1, and can swing above the river channel to deliver equipment and supplies into the work area from the air, avoiding the need to traverse sensitive habitat in and around the river channel. A water truck will be staged near the crane location to supply water to the drilling rig with hoses as needed.
The geotechnical boring activities are anticipated to occur over a period of approximately 2 to 3 weeks. Once geotechnical investigations are complete, all equipment would be removed. The drilling start date will depend on the status of environmental, right of access, Department of Environmental Health permits, subcontractor availability, weather, and conditions in the river channel. The drilling would occur between September 16 and January 14 to avoid any potential impacts to nesting and/or special-status bird species. Further, as described above, equipment would be dropped into the work area via crane, which will ensure that existing habitat in and around the river channel would not be affected through the use and maneuvering of equipment (see Figure 1). The aforementioned plywood, steel plates, and/or rubber mats would be used to move equipment from the crane drop location to drilling work areas. Use of these materials would ensure that existing vegetation would not be removed or otherwise destroyed through equipment movement along the temporary access paths in the river channel.