Pioneering Small Conduit

Advanced small diameter pipe jacking is a construction technique used during tunneling projects, primarily for pre-supporting in soft and fractured zones, shallow buried sections, portal bias sections, sand layers, sand gravel sections, and fault fractured zones.

Supporting Principle

The超前小导管 is an extremely effective auxiliary construction method for stabilizing the excavation face. In the construction of soft and fractured rock formations, the超前小导管 serves to reinforce loose rock layers, enhancing the stability of loose and soft surrounding rocks after grouting. This is beneficial for maintaining the stability of the surrounding rocks between the completion of excavation and the completion of initial support, preventing instability, damage, and eventual collapse of the surrounding rocks.

2 Advanced small diameter conduit grouting is suitable for tunnel arch soft surrounding rocks, loose, unbound soil layers, poorly self-stabilizing sand layers, and broken rock layers of sand and gravel (cobble) stones.

3 Advanced small conduit grouting can alter the condition and stability of the surrounding rock. After the grout is injected into the soft, loose strata or fractured rock with water content, it can come into close contact and solidify. The grout, through methods such as filling and splitting, replaces the water and air between soil particles and rock fissures, occupying their positions. After a certain period of time, it solidifies, bonding the original loose soil particles or fissures into a single mass, forming a solid with a new structure, high strength, and good waterproofing properties, significantly improving the loose and fractured condition of the surrounding rock.

Parameters

The parameters for the construction of advanced small conduits should be determined 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 the excavation contour line within a 120-degree range. Typically: the length of the small conduit L = height of the upper step + 2m. The diameter of the small conduit: 38-50mm. The front end of the small conduit is made into a cone shape about 10cm long, with a steel hoop welded at the end with a diameter of 6-8mm. The external insertion angle is generally controlled between 10 and 15 degrees. Grouting pressure is controlled around 2MP. The radius of the slurry spread is generally 0.5m. The grouting speed is controlled at 50-100L/MIN. The overlap length of each small conduit cycle is controlled within 1m.

Manufacturing

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 with a length of approximately 10 cm, and a steel hoop with a diameter of 6~8 mm is welded at the tail end. No holes are drilled within 100 cm from the rear end, with the remaining portion having diameter 6 mm overflow holes arranged in a plum blossom pattern every 20~30 cm.

Installation

May be installed by piercing or direct insertion.

Drill holes using a drill, with a diameter larger than the small conduit by 10-20mm, and the depth depending on the conduit's length.

If insertion of the导管 is difficult, you can use a hammer drill with a jacking function to force the casing into place.

Blow out debris from the hole using a blow tube.

4 Small conduit is wrapped with cotton yarn at the end to ensure a tight fit with the drilled hole, and the opening is sealed tightly with cotton yarn.

After installation of the 5 small conduits, ensure that the surrounding area within a certain range is sealed with shotcrete. The thickness of the shotcrete should be controlled between 5-8 cm.