The California Department of Transportation (Caltrans), using state and federal funding and local funding from the Trinity County Transportation Commission, plans to conduct a geotechnical investigation on State Route (SR) 299 in Trinity County between post miles (PM) 15.30 and 15.50 and conduct a geophysical investigation on SR 299 in Trinity County between PM 22.15 and 22.50. The purpose of the geotechnical investigation is to collect data on subsurface conditions at the location of a proposed retaining wall that would be constructed as part of the Del Loma Pavement/Down River Turnouts Project. The geotechnical investigation is needed because the data collected would be used to design the foundation of the retaining wall. The purpose of the geophysical investigation is to characterize subsurface conditions at a proposed turnout along the westbound lane at PM 22.2 and at a proposed borrow site along the westbound lane between PM 22.3 and 22.5.
Four borings would be drilled to a depth of approximately 100 feet within Caltrans’ right-of-way in the eastbound lane. The borings would be drilled with a hollow-stem auger if subsurface soil is dry (the diameter of hollow auger borings would be approximately 7 inches) or with a mud rotary if subsurface soil is saturated (the diameter of mud rotary borings would be approximately 4 inches). The work would take approximately 5 weeks to drill the borings and collect data (no instrumentation would be placed into the borings). Upon completion of work, all borings would be backfilled to the ground surface with neat cement in accordance with the Trinity County Department of Environmental Health and California Well Standards Bulletin 74-81 and 74-90. Closure of the eastbound lane of SR 299 would be required while drilling borings.
The GS field investigation work will consist of performing two non-invasive geophysical methods of imaging the near subsurface materials in a linear profile. The two methods are Seismic Refraction Tomography (SRT) and Electrical Resistivity Tomography (ERT). Data for SRT and ERT are acquired using staked geophones and electrode extensions. The geophones and electrodes are inserted into the ground at specific intervals and connected through a cable to a battery-powered data collector. For the SRT method, the geophones record very small vibrations generated by an energy source. Away from the immediate location of the SRT source, the vibrations are imperceptible and are only detectable by the data collecting seismograph. A more detailed description of the effects of the SRT method source Is provided below.
Location, Access, and Hours of Work
It is proposed to perform up to four SRT lines and two ERT lines. It is anticipated that access to proposed survey locations will be by foot from the roadway at the toe of the slope. Foot access utilizing safety harnesses and ropes may be required due to the steepness of the ground surface in areas. All work is typically conducted during daylight hours, Monday through Friday, and may be modified with proper notification to increase the efficiency of the studies (i.e. 1-day work week with less mobilization).
Footprint and Area of Related Ground Disturbance
24 or more geophones (small 2” square sensors mounted on a ½” x 3” spike) and electrode extensions are placed at the ground surface. The geophones and electrodes are removed after work is completed. The SRT geophone spikes and ERT electrode extensions maintain secure ground contact and leave no lasting impression in the ground surface. Ground and vegetation disturbance is isolated to foot-traffic areas and locations where the sources are employed. The extent of the SRT method source disturbance at the ground surface increases with source strength. An effort is made to conceal disturbed brush and tamp down disturbed soil to return it to its original condition. Additional discussion of SRT method source effects is provided below. Minor brush cutting is occasionally required for access and is minimized wherever possible. Effort is made to restore visual impacts at the end of the survey.
SRT Source Effects
Two types of SRT method sources are proposed on the project site and they are the
sledgehammer and striker plate, and the down-hole seisgun. The number of source
locations (called “shots”) required typical ranges from 5 to 20. For either source, away from the primary zone where sources are employed (greater than 10 feet away) vibratory ground strain is purely elastic and is on the order of one part in a million. The vibration is imperceptible beyond the immediate vicinity of the shot and lasts much less than one second. No discernable lasting effects on flora or fauna have ever been observed during Caltrans seismic operations. A more detailed description of each source is provided below.
Hammer and Striker Plate
The hammer and striker plate source consist of a 12- to 16-lb sledgehammer struck against a metal or HDPE plate placed on the ground. This creates the least ground disturbance (a dent or divot in ground in the shape of the plate), but the greatest noise of the two methods. The noise from the hammer striking the plate is about 107 dB within 10 feet of the source, so hearing protection is normally required by the operator. However, sound pressure fall-off with distance from the source is significant and drops below 85 dB (OSHA action level) within 60-70 feet of the source and reaches background within 120-140 feet.
The down-hole seisgun is a compact source that uses an industrial cartridge fired in a minimum 1.5-foot deep water-filled hole. The hole is created by manually driving a 2½-inch diameter gad bar into the ground, or by using a hand auger. Seisgun activity may leave an area of disturbed earth up to 2 feet in diameter. This area is tamped down to return it to its original condition. There are no appreciable effects outside that diameter. With well-prepared shot holes, the highest anticipated noise generated consists of a muffled “thump” of approximately 80 dB. Typically, the shots are barely audible.
Staging of construction vehicles and equipment for the geotechnical investigation would occur in two existing graveled turnouts along the eastbound lane of SR 299 between PM 15.30 and 15.50. Staging of construction vehicles and equipment for the geophysical investigation would occur in an existing graveled turnout along the westbound lane of SR 299 at PM 22.2.
The total area of topsoil disturbance associated with the geotechnical and geophysical investigation would be negligible. The geotechnical investigation does not require the use of a borrow site. 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 Translab facility in Sacramento. The material would then be disposed of at a landfill which has previously undergone environmental review and is approved for use. The geophysical investigation does not require the use of a borrow or disposal site.
The geotechnical investigation and geophysical investigation would be performed entirely within Caltrans’ right-of-way on federal land that is owned by the United States Forest Service and managed by the Shasta-Trinity National Forest. Work on federal land would require a Letter of Concurrence from the United States Forest Service.
No conflicts with utilities are anticipated.
No permits from regulatory agencies would be required.
Caltrans has performed a review of resource records and databases and consulted with applicable agencies and individuals. Additional coordination with the Forest Service may be required for work occurring on federal land. Environmental commitments to be implemented are described in the Environmental Commitments Record prepared for the project.