Mariposa Grove Geotech Testing

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This project would investigate percolation rates and feasibility of septic leach field locations proposed in the Restoration of the Mariposa Grove of Giant Sequoias DEIS. The project would also test the seismic and soil bearing capacity for structural components of the plan. Geotechnical work would consist of exploratory borings, installing piezometers, and digging test pits. Work would take place at the South Entrance, the Lower Mariposa Grove, and the Upper Mariposa Grove.

There would be an estimated 11 borings and 24 test pits, in total. The borings would be 8 inches in diameter, and up to 25 feet deep. The test pit excavations would be approximately 2 feet wide and up to 9 feet deep. Phase I testing (May and June 2013) includes 4 borings with piezometers and 12 test pits for soil evaluation and percolation tests. Phase II (fall 2013 or spring/summer 2014) includes an estimated 7 additional borings and up to 12 additional test pits in the Grove. Investigations in Phase II would be based on Phase I findings.

An all-terrain track-mounted drill rig and rubber tire backhoe would drill the borings and excavate the test pits. This rig would travel about 1,000 feet off the paved road on the abandoned Washburn Wagon Road to access the sites at the South Entrance. The Upper Grove sites would be accessed off a trail. A water truck would likely be used to provide water for percolation tests. Equipment would be staged at the South Entrance parking area.

Contracted work would be as follows:
• Obtain clearance with Underground Service Alert
• Prepare a Site-Specific Health and Safety Plan
• Excavate and sample 12 test pits in May of 2013 to a depth of about 9 to 10 feet by using rubber-tired backhoe. In fall 2013 or spring/summer 2014, conduct up to 12 additional test pits and 7 borings.
• Drill and sample 4 test borings to a maximum depth of 25 feet below existing grades or to practical refusal. Drill the borings with an all-terrain track-mounted drill rig equipped with 8-inch-diameter, hollow-stem augers.
• Construct one 2-inch-diameter piezometer (aka monitoring well) in each borehole that encounters water. The 25-foot-deep, piezometer would be constructed with 15 feet of non-perforated, polyvinyl chloride (PVC) piping and 10 feet of 0.01-inch, machined slotted, PVC for the screen interval. The filter pack around the screened interval would consist of clean sand. A sanitary seal consisting of a one to two foot plug of bentonite chips would be placed above the sand filter pack. The remainder of the borehole would be grouted with a slurry of neat-cement, bentonite, and water in accordance with Mariposa County Environmental Health Department (MCEHD) requirements.
• Perform two percolation tests at each of the four potential disposal areas, in the test pits. Percolation tests also may be required in shallow borings, per MCEHD requirements.
• Protect pits that need to be open overnight with fencing or covers so that animals will not fall into them.
• Backfill and tamp test pits with excavation soils and restore to grade after testing. Spills or hydraulic leaks are not anticipated; however, if observed, these affected soils along with soil cuttings from the drilling activities would be placed in metal drums for disposal.
• Use on-site pine needles and surface organics to recover test pit locations and areas disturbed by the equipment.
• Phase II explorations would be based on the results of Phase I explorations. It is estimated that Phase II may include up to 6 test pits and 5 borings at the South Entrance area, three test pits and two borings at the Lower Grove area and 3 test pits in the Upper Grove.