Different from open‑air construction sites, tunnel closed‑space environment brings multiple coupled risks for scaffolding systems. First, continuous mechanical‑equipment vibration and blasting shock load: in tunnel excavation progress, rock‑blasting and rock‑drilling machinery generate frequent impact vibration. Even small‑amplitude vibration accumulates for a long‑time, which may cause gradual slippage of wedge‑pin locking joints. Second, high‑humidity corrosive atmosphere: underground water seepage makes tunnel inner‑space maintain high relative‑humidity all year‑round, mixed with alkaline rock dust. The corrosive medium continuously attacks weld seams, cutting edges and tiny surface scratches of steel components. Third, space‑constrained layout restriction: tunnel cross‑section height and width are fixed, so scaffold layout cannot expand outward like open‑air building projects. Partial zone must adopt compact narrow‑bay frames, which raises higher requirements for component modular combinability. Fourth, bad on‑site storage condition: spare components are often stacked temporarily inside tunnel cavities, lacking ventilation condition, accelerating metal corrosion speed.
Material‑selection logic for tunnel scaffolding needs to adjust according to above‑mentioned coupled risks. For raw‑material steel tube, Q355B low‑alloy‑steel standards are recommended for heavy‑duty lining‑pouring support; Q235 carbon‑steel tubes can be applied for local operation platform with relatively light load. Surface anti‑corrosion treatment cannot only pay attention to tube outer surface. Weld seams, cutting openings, rosette root positions are corrosion‑vulnerable zones. Factory‑integral hot‑dip‑galvanizing (zinc‑coating ≥70 μm) is compulsory for tunnel‑purpose components. Partial repair painting after field cutting cannot achieve equivalent anti‑corrosion effect, so minimize field‑cutting behaviour as much as possible. In terms of accessories, wedge pins shall satisfy anti‑pull‑force ≥3 kN under vibration condition. The sleeve reserved inspection‑hole is a non‑ignorable structural detail, which helps constructors confirm full insertion of upper‑section standards inside tunnel dim lighting environment.
Frame‑erection and layout rules under tunnel working‑conditions have many differences compared with conventional projects. First, diagonal‑brace configuration principle: in tunnel inner frames, diagonal braces shall be arranged in full height and full‑bay. It is not allowed to reduce diagonal‑brace quantity only for saving steel consumption. Blasting vibration will amplify the defect of insufficient lateral rigidity, further triggering frame offset risk. Second, foundation treatment: tunnel bottom surface is not always flat hard ground. Local loose sediment, mud and weathered rock need cleaning, tamping or paving backing plates. Adjustable base jack cannot be directly placed on soft silt layer. Third, limitation of component combination: when building compact narrow‑bay frames in limited‑clearance zones, do not pursue too small bay‑size blindly. Keep reasonable spacing to satisfy worker passing and tool‑operation space. Fourth, distinction of different construction‑stage schemes: excavation‑stage tunnel‑face working‑platform pursues flexible disassembly, and cuplock scaffolding fits such demand; lining‑pouring high‑formwork emphasizes high bearing‑capacity and anti‑vibration, so ringlock scaffolding shall be taken as main system; tunnel‑renovation maintenance project often needs mixed collocation of two systems.
Acceptance and daily inspection points inside tunnel deserve special attention. Owing to poor illumination inside tunnels, visual omission happens easily. Acceptance shall focus on four major parts. ① Joint‑locking status: check whether wedge pins are hammered to self‑locking state, prevent partial pins in shadow zone not being hammered in‑place. ② Splicing condition of vertical standards: utilize sleeve inspection holes one‑by‑one to verify inserting depth of upper‑pole male‑end. ③ Foundation and jack status: check whether base jacks are completely bearing force, no hollow suspension phenomenon. ④ Corrosion spot‑check: focus on weld‑seam and cut‑edge positions, early detect local zinc‑layer damage and initial‑stage rusting. During construction cycle, periodic re‑inspection must be arranged after blasting operation, for vibration may cause subtle joint slippage invisible to naked eyes.
There are also several common mis‑operations worthy of warning for tunnel‑site management personnel. Do not reuse components with cut‑edges processed on‑site without integral galvanizing; do not arbitrarily cancel partial diagonal braces for speeding‑up erection; do not stack large quantities of spare scaffolding parts for long‑time inside damp tunnel cavity; do not ignore re‑inspection work after each blasting cycle.
Many overseas tunnel‑project accident investigation reports indicate that most scaffolding‑related hidden‑troubles do not root in raw‑material quality defects, but come from ignoring closed‑space special environmental factors. Procurement, acceptance and daily inspection shall fully consider tunnel‑specific vibration, dampness and space constraints. Only matching material selection, layout scheme and inspection mechanism with actual tunnel working‑conditions can lower scaffolding‑associated safety risks effectively.