How to choose tunnel lining materials
Broadly speaking, the artificial support structure can be collectively referred to as lining, which can be divided into primary support and secondary support (secondary lining). Tunnel lining materials should first have sufficient strength, durability, impermeability, corrosion resistance and frost resistance. In addition, from an economic point of view, it should also meet the requirements of obtaining local materials, reducing the cost, convenient construction and easy mechanized construction.
Advantages: Good integrity, can be poured on site, can also be prefabricated in the processing field, and can be mechanized construction. Additives can be added to the cement to improve the performance of the concrete.
Disadvantages: Can not bear the load immediately after pouring, it needs to be kept in good health, and the mold can be demolished after reaching the predetermined strength, and it takes up more templates and arches. Ordinary concrete has poor corrosion resistance.
(1) For railway tunnels, in order to save cement, schist concrete is allowed to be used for the lining of side walls in good rock sections (the content of schist should not exceed 20% of the total volume). In addition, when there is over-excavation at a site other than 1m above the arch line, the over-excavated part can also be backfilled with rubble concrete.
(2) The selected stone should be hard, its strength grade should not be lower than MU40, and the stone with cracks and easy to weather should not be used to ensure the quality.
(3) The lining of highway tunnels is generally not used, and rubble concrete can be used for the inverted or over-excavated part. The dosage of rubble shall not exceed 30% of the total volume.
It is mainly used in open excavation sections such as tunnel doors and open tunnel linings, or when the tunnel passes through seismic areas, biased pressure, through fault fracture zones or silt, quicksand and other bad geological sections.
Precast stone and concrete blocks
Advantages: it can obtain materials on site, reduce the cost, ensure the thickness of the lining and bear the load earlier, save cement and formwork, and have better durability and corrosion resistance.
Disadvantages: poor integrity, many joints are prone to water leakage, poor waterproof performance, construction is mainly manual operation, difficult to mechanize construction, labor and time, slow construction progress, and high masonry technical requirements. Retaining walls of cave doors, retaining walls, and curbs of exposed lines can still be used. The strength grade of block stones should not be lower than MU60, and the strength grade of blocks should not be lower than MU20. Stones with cracks and easy weathering should not be used.
Shotcrete is made by spraying concrete dry mix, accelerator and water onto the clean rock surface at high speed with concrete sprayer.
It has high compactness and can quickly close the fissures of surrounding rock. It is closely attached to the rock surface and has high early strength. It can quickly seal the rock surface and support it. In addition, fiber materials can be added to the shotcrete to improve its performance. It is an ideal lining material.
In ordinary railway tunnels, shotcrete can be used as the middle inner lining, but its strength grade is not lower than C20, the cement grade used is not lower than 32.5, and ordinary Portland cement is preferred. The fine aggregate is made of hard and durable medium sand or coarse sand, the fineness modulus should be greater than 15, and the moisture content of the sand should be controlled within 5%-7%; the coarse aggregate should be made of hard and durable pebbles or gravel, and the particle size should not be larger than 15mm.
Anchor rod and steel frame
Anchor rod is a rod-shaped member inserted into the surrounding rock mass to reinforce the surrounding rock. It can be divided into mechanical anchors, bonded anchors and prestressed anchors. The diameter of the anchor rod should be 20～32mm, the material of the rod should be HRB335, HRB400 steel; the material of the backing plate should be HPB235 steel. The strength of various cement mortars used for bolts should not be lower than M20. The reinforcement mesh material can be HPB235 steel with a diameter of 6-12mm.
The steel frame is a section steel support or grid steel support placed in the primary support or secondary lining in order to strengthen the support rigidity.
The steel frame used for the initial support should be made of H-shaped, I-shaped, U-shaped steel, or steel pipe or rail.
In shield construction, fabricated materials are often used as lining materials, such as large prefabricated blocks of reinforced concrete and cast iron prefabricated blocks with reinforced ribs.
In the construction of shed-type open caves (referred to as shed-holes), prefabricated slabs or beams can be used to assemble plate-type sheds or beam-type sheds.
When using the new Austrian method of construction, in order to waterproof, anti-falling and beautiful requirements, you can also add an off-wall structure. Commonly used materials are corrugated steel arched large assembly preforms, and sometimes glass fiber reinforced plastic can be used instead of steel.
Various decoration materials can be pasted on the inner surface of the lining in order to improve the lighting, waterproof, ventilation, aesthetics, sight induction or reduce noise in the cave.