Preliminary Geotechnical Engineering Report - Tamsin Ridge
Recommendations
Mine area. Either (a) exclude arrays and equipment from the undermined area (Quote starts. about 165 acres of the 240
fenced acres in the East Block, or roughly 20 to 25 MW of the Project's capacity Quote ends.), or (b) carry out a mine
subsidence investigation before final design: 12 or more additional borings to the coal, downhole camera logging
and a geophysical survey, followed by a grouting program under any area kept in the layout. Grouting of shallow
workings typically costs several million dollars for an area of this size and takes 4 to 8 months. Option (b) still
leaves residual risk.
Foundations. Design driven W6x9 or W6x12 piles for 8 to 10 feet of embedment, with pre-drilling and grouted
piles where refusal is met. Add a corrosion allowance or galvanizing for a 35-year life. Substation and BESS
foundations may bear on stiff clay or rock at 3 to 4 feet below grade, outside the mine area and after the Phase II
environmental sampling at the former tank battery.
Access roads and grading. Use 12 inches of crushed aggregate over geotextile on the silty clay. Avoid cuts
deeper than 3 feet on the Sallow Creek valley walls. Strip and stockpile topsoil separately; the soil survey maps
most of the West Block as prime farmland.
Further work. Final design needs borings at the substation, BESS yard and switching station, thermal resistivity
tests along the collection routes, and the mine investigation above.
Tobias Wren, P.E., Principal Geotechnical Engineer, Graywacke Geotechnical, LLC.
Graywacke Geotechnical, LLC - Project G23-0219 Page 3 of 3
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