The California Department of Transportation (Caltrans), using state and federal funding, plans to conduct a geotechnical investigation on State Route (SR) 96 in Siskiyou County at post mile 71.23 (adjacent to the existing Scott River Bridge). The purpose of the geotechnical investigation is to collect data on subsurface conditions at this location. The geotechnical investigation is needed because the data collected would be used to evaluate the subsurface conditions to provide foundation recommendations for the future widening of the deck of the Scott River Bridge.
Drilling Operations and Proposed Boring Locations
The drilling operations would be performed by a Caltrans drill crew or a contracted drilling crew. The proposed subsurface investigation would consist of drilling three borings. Borings #1 and #3 would be drilled outside the ordinary high-water mark of the Scott River in uplands at or near highway grade. Boring #1 would be drilled east of the bridge and north of the roadway. Boring #3 would be drilled west of the bridge and south of the roadway. Boring #2 would be drilled within the ordinary high-water mark but outside the wet channel of the Scott River. The borings would not extend deeper than 130 feet below the ground surface.
Access to boring #1 and boring #3 would be from the existing roadway. Access to boring #2 in the riverbed would utilize an existing gravel road west of the bridge and north of the roadway.
The total area of topsoil disturbance associated with the geotechnical investigation would be negligible. The access route to boring #2 may require the removal of several small trees within riparian habitat along the Scott River.
Permits will be obtained from the County prior to drilling. Permits will also be needed from regulatory agencies (e.g., U.S. Army Corps of Engineers, North Coast Regional Water Quality Control Board, and California Department of Fish and Wildlife) prior to performing work associated with boring #2.
Drilling equipment at the site may consist of a truck-mounted, trailer-mounted, or all-terrain drill rig to complete the borings. The site set-up usually includes a water tender/truck, and a crew cab with small trailer. Also, one or two vehicles used by a geoprofessional will be on site to support the drilling operation.
Caltrans is proposing to initiate the drilling of the boring utilizing an 8.5-inch diameter hollow-stem auger until the proposed completion depth, or groundwater is encountered, whichever comes first. Groundwater is expected to be at ground surface for borings within the creek bed and within 25 from the surface for those boring located a road level. When groundwater is encountered, 5.25-inch diameter casing will be installed, and the drilling method will be changed to 4.5-inch diameter mud rotary with a self-casing wireline drilling method to a proposed depth not to exceed 130 feet.
With the advancement of the boring, Standard Penetration Test (SPT) samples will be collected at approximately 5-foot intervals. When cohesive soils are encountered, brass tubes will be used to collect samples from the California Modified Sampler. Within the mud rotary drilled methods, continuous sampling recovery of the soil materials may be achieved by utilizing “punch-core” barrel sampling with a finger-bit in addition to the Standard Penetration Test. All soil samples drilled will be collected for field identification and geotechnical laboratory testing. Soil samples not selected for testing and drilling cuttings will be containerized in mud drums for disposal.
The SPT is an in-situ dynamic penetration test used to assess the geotechnical engineering properties of the soil. The test uses a thick-walled sample tube, with an outside diameter of two inches and an inside diameter of 1.4 inches, and a length of approximately 25.6 inches (including the shoe). This tube is driven into the ground (at a desired sampling interval) within the borehole by blows from a hammer with a weight of 140 lbs. free falling 30 inches. The tube is driven 18 inches into the ground or until refusal is achieved.
All borings will be backfilled with grout immediately after drilling according to the Siskiyou County Department of Environmental Health. Caltrans will contract with a licensed C-57 driller to oversee and approve the destruction of the borings upon completion of work. In addition, a Certified Engineering Geologist or Professional Geologist will ensure proper destruction of the borings according to State law.
Drilling Fluid and Containment
To obtain quality soil/rock samples at the depths needed, advancement of the test boring may require the use of the mud rotary self-casing drilling system. This drilling system requires the use of drilling fluid to maintain the stability of the hole, bring drill cuttings to the surface, as well as lubricate and cool the drill bit. The drilling fluid typically consists of water, water mixed with bentonite, a clay-based powder. The drilling fluid consists of a natural sodium bentonite clay with quartz.
The drilling fluid is contained and recirculated through a closed system utilizing a drill rod and a mud tank. The mud tank will be positioned on the surface of the ground and serves as a settlement tank for the soil cuttings, which are periodically removed and placed in 55-gallon steel drums. Once the desired boring depth has been reached, the hole will be flushed with clear water to displace the drilling fluid back into the mud tank. The drilling fluid and water will then be pumped from the mud tank into 55-gallon steel drums for disposal. The 55-gallon steel drums will be removed from the job site and transferred to an appropriate staging area, usually a nearby Caltrans Maintenance yard for eventual disposal.
After drilling has been completed, select samples will be submitted to Caltrans’ Translab for geotechnical testing.
The drilling operation may take four to ten days to complete. The anticipated work schedule includes 10-hour days between Monday and Thursday. Work would be completed during daylight hours.
Traffic control consisting of a single-lane closure is anticipated to be required for the drilling of borings #1 and #3. No traffic control would be needed for the drilling of boring #2.
Equipment storage for the drilling operation is typically located at the local Caltrans maintenance yard. Typically, the drill rig, water tender, and trailer are stored at the nearest Caltrans yard at the end of each workday. The support truck and geoprofessional’s vehicles will be transported off-site at the end of each workday.
Soil (“cuttings”) removed from the boring holes would be placed into 55-gallon drums and temporarily stored at a nearby Caltrans maintenance yard or at the Caltrans Trans-Lab facility in Sacramento. The material would then be disposed of at a landfill that has previously undergone environmental review and is approved for use. The geotechnical investigation does not require the use of a borrow site.
The geotechnical investigation would be performed entirely within Caltrans’ right-of-way. No federal land is present within the project limits.
No new utilities would be installed and no conflicts with existing utilities are anticipated.