sand-prevention pillar防砂煤柱
sand-proof pillar防砂煤柱
pillar against sand防砂煤柱
safety pillar against sand防砂煤柱
set safety pillar prevent sand防砂煤柱留设
coal pillar for sand proof防砂煤柱
anti-sand & anti-collapse coal pillar防砂防塌煤柱