Many construction units blindly thicken double-wall casing resulting in unnecessary cost. Double-wall casing adopts an outer wall + inner wall composite structure with clear functional division, which can be scientifically matched according to pile depth, geology and construction technology to adapt to various full-rotary follow-up working conditions.
The outer wall of double-wall casing is the core load-bearing and wear-resistant structure, bearing rotary friction and lateral soil pressure. Standard configuration of 12+8mm and 16+8mm is applicable for conventional soil layers and piles within 40m. For complex conditions such as karst caves, thick pebbles, ultra-deep piles and coastal high water pressure, the outer wall is thickened to 16–20mm to enhance deformation and wear resistance.
The inner wall ensures smooth pipe interior, reduces drilling tool friction and guarantees smooth lowering of reinforcement cages and pouring pipes to avoid jamming. Conventional 8mm inner wall adapts to more than 90% of pile working conditions. 10–12mm inner wall is only adopted for long-term hard rock drilling and high-frequency turnover projects.
Reasonable matching of inner and outer wall thickness of double-wall casing ensures construction safety and controls engineering material cost without excessive thickening.