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I-4,2 B-829 DESCRIPTION I-4.2.1 General Description The B-$29 High Explosives Burn Pits was constructed in 1955 and was closed in accordance with the DTSC-approved closure plan in 1998. The operational facility consisted of three separate burn pits and an open-air burn unit known as the "Iron Horse Furnace." The burn area was constructed on a flat surface created by cutting and filling bedrock along the nose of a ridge formed on three sides by steep slopes of the natural drainage system. The burn pits were excavated 3 to 4 feet into the graded, level pad. Except for Pit 3, which had a sloping ramp approximately 35 feet long, each pit was approximately 20 feet wide at the bottom and 30 feet long with sloping sides. The walls of Burn Pits I and 2 were fill material. At least one wall of Burn Pit 3 was also fill material. The bottoms of Pits 1 and 2 were within 1 to 2 feet of bedrock, and the bottom of Pit 3 was bedrock. All three of the pits were unlined. The earth -filled portion of the burn pits was not compacted other than the compaction from operation of a bulldozer during their original construction and from settlement over time resulting from the weight of the fill material. The closed facility consists of an engineered cap that is designed to prevent the release of hazardous wastes left in place to the environment. The uppermost soil layer of the cap is vegetated with native grass in order to protect the cap from erosion. Storm water run-off is diverted off the cap into a drainage channel in order to protect the perimeter of the cap. I-4,2.2 Physical Description The size of the B-829 Facility is approximately one-half acre. From the bottom up, the components of the cover system consisted of the following: • General backfill within each of the three open burn pits. • 2 -ft -thick compacted general fill (90% ) of ASTM D-1557. • CETCO geosynthetic clay liner. • 60 -mil, textured high-density polyethylene (HDPE) geomembrane. • Double -sided geocomposite drainage layer. • 8-oz/sy geotextile filter in the perimeter subdrains. • Drainage rock and 4 -in. HDPE pipe within the perimeter subdrains. • Shotcrete-lined drainage ditches and associated culvert s. • Outfall drain pipe, T -pipe fitting, an d riprap. • 24 ft-thick vegetative cover soil hydroseeded with native grass species. LLNL Site 300 Contingency Plan 14 4 January 2019 Appendix 1-4, Building 829