Pioneering Small Conduit

Advanced small conduit is a construction technique used during tunneling operations, primarily for pre-supporting soft and broken zones, shallow burial sections, tunnel mouth bias pressure sections, sand layers, sand gravel sections, and fault broken zones.

Supporting Principle

1 The advanced small diameter pipe is a highly effective auxiliary construction method for stabilizing the excavation face. In the construction of soft and fractured rock formations, the advanced small diameter pipe plays a reinforcing role in loose rock strata, enhancing the stability of loose and soft surrounding rocks after grouting. This is beneficial for maintaining the stability of the surrounding rocks between excavation completion and the completion of initial support, preventing instability, damage, and eventual collapse of the surrounding rocks.

2 Advanced small conduit grouting is suitable for tunnel arch soft surrounding rocks, loose and unbound soil layers, sand layers with poor self-stabilization ability, as well as crushed rock layers composed of sand and gravel (cobble) stones.

3 Advancing small conduit grouting can alter the condition and stability of the surrounding rock. After the grout is injected into soft, loose strata or fractured rock containing water, it can come into close contact and solidify. The grout, by filling and fracturing, displaces water and air between soil particles and within rock fissures, taking their place. After a certain period of time, it solidifies, bonding the original loose soil particles or fissures into a single mass, forming a new structure with high strength and excellent waterproofing properties, significantly improving the loose and fractured condition of the surrounding rock.

Parameters

The determination of various parameters for the construction of advanced small conduits should be based on the geological conditions of the surrounding rock boundary, the condition of the surrounding rock, the form of support structure, and the cross-sectional dimensions of the tunnel. Generally, the construction of advanced small conduits is set along a 120-degree range of the excavation contour line. In most cases: the length of the small conduit L = height of the upper step + 2m. The diameter of the small conduit: 38-50mm. The front section of the small conduit is made into a cone shape about 10cm long, and a steel reinforcing hoop with a diameter of 6-8mm is welded at the end. The external insertion angle is generally controlled between 10 and 15 degrees. Grouting pressure is controlled at about 2MP. The radius of the slurry diffusion is generally 0.5m. The grouting speed is controlled at 50-100L/MIN. The overlapping length of each small conduit in each cycle is controlled within 1m.

Manufacture

Generally, seamless steel tubes with diameters ranging from 38 to 50mm are used for production.

2. The front end of the small conduit is shaped into a cone about 10cm long, and a steel hoop with a diameter of 6~8mm is welded at the tail end. No holes are drilled within 100cm from the rear end, and the remaining part is arranged with 6mm diameter overflow holes in a plum blossom pattern every 20~30cm.

Installation

可采用 piercing or direct insertion methods.

Drill holes using a drill, with a hole diameter 10-20mm larger than the diameter of the small conduit, and the depth of the hole is determined by the length of the conduit.

If insertion of the导管 is difficult, a hammer drill with a thrusting function can be used to push the casing in.

Blow out debris from the hole using a blow tube.

4 Small conduit ends are wrapped with cotton yarn to ensure a tight fit between the small conduit and the drilled hole, and the open end is sealed with cotton yarn.

After the 5 small conduits are installed, they must be enclosed with shotcrete within a certain range surrounding them. The thickness of the shotcrete should be controlled between 5-8 cm.