https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html OHSA requirements for excavations OSHA technical manual https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html OSHA Trenching and Shoring OSHA Trenching & Shoring Guide https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html OSHA standards https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html Trench Safety OSHA standards OSHA technical manual OSHA manual OSHA standards OSHA technical manual