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Chapter 3 Urban Tunnel Project (4. Shallow Burial and Underground Excavation Construction 5. Safety and Quality Control of Urban Tunnel Projects)

3.4 shallow burial and hidden excavation construction

3.4.1 Construction method of shallow burial and hidden excavation

Full-section method, step method, single-sided wall guide pit method, double-sided wall guide pit method, middle partition method, cross middle partition method, middle hole method, side hole method, column hole method

1. Full-section excavation method
  • Stable soil quality and small section
  • Top-to-next molding
  • The number of disturbances to the surrounding rock is reduced, the process is simple, the geological conditions are strict, and the surrounding rock must have sufficient self-stabilization ability.
2. Step excavation method
  • Tunnel with better soil quality
  • Excavation in step by step on the upper and lower working faces
  • Enough working space and fast construction speed, flexible and changeable, and highly applicable

3.4.2 Shallow buried and concealed excavation construction technology

1. Work well construction technology

(1) Construction preparation

  • Survey of underground pipelines and structures, determine protection plans, reinforcement monitoring
  • Manual excavation of cross trenches to determine that no pipeline is being excavated
  • The vertical shaft is equipped with a rainproof canopy, waterproof wall, safety guardrail, and railings of no less than 1.2m. Warning signs are required, and non-staff are prohibited from entering.

(2) Lock ring beam

  • The upper part should be equipped with retaining walls, earth nail walls or grille steel frames + sprayed concrete temporary enclosure structures.
  • Do not over-diggle, and slope support should be done
  • The concrete strength of the ring beam reaches more than 70% of the designed strength, so that the vertical shaft can be excavated downward
  • The lock ring beam and the grille should be connected according to the design requirements, and the well wall shall not fall off.

(3) Improve the system
(4) Excavation and support of the shaft

  • Take groundwater control and strata pre-reinforcement measures
  • Waterproof walls are installed at the wellhead, the surrounding ground should be hardened, and drainage measures should be taken
  • Symmetric, layered, and chunk excavation.Support as you dig，Dig the surrounding area first, then dig the middleOrder of
  • The initial support should be closed into a ring as soon as possible.
  • Requirements for review of strength and thickness of jet concrete
  • Install temporary support in time
  • NoNot digging. Timely bottom cover
  • Strengthen observation and monitoring
2. Horsehead Door Construction Technology
  • Buried concealed beams and concealed piles, drilled into advance small conduits, grouting and reinforcement
  • Before breaking, do a good job of converting the support system of the Matou Gate shaft and tunnel
  • Before excavation construction, take strengthening measures
  • Break the vertical shaft wall in sections, break the first arch, side wall, and last bottom plate in sequence
  • The tunnel at the Matou Gate should be closely arranged with three-grid steel frames and door ring beams to be installed outside the hole door.
  • The same vertical shaft of the Matou door shall not be constructed at the same time. After the tunnel on one side is dug 15m, it shall be lowered first and then high.
  • Strictly implement it. Pipe ahead, strict grouting, short excavation, frequent measurement, strong support, early closure
  • Strengthen monitoring.
  • Stop excavation and spray concrete in time to seal the palm surface.
3. Advance pre-support and pre-reinforcement construction technology

(1) Grouting and reinforcing of the small catheter ahead

  • Set along the outline of the tunnel arch, single-layer and double-layer advance small conduits can be used according to the formation conditions.
  • The length and diameter should meet the design requirements, generally 3-5m
  • Pass through the abdomen of the steel grille, the rear end is supported on the erected steel grille and welded firmly. Horizontal support lap length shall not exceed 1m
  • The disturbance to the formation should be minimized as much as possible
  • The end is cone-shaped, and the tail end is equipped with steel bar reinforcement hoops, and the plum blossom shape of the pipe body does not have 6-8 overflow holes.
  • The grouting materials can be ordinary cement single liquid slurry, modified water glass slurry, cement-water glass double liquid, slurry and other grouting materials.

(2) Deep hole grouting reinforcement technology

  • Groundwater conditions, geological conditions, slurry type, determine the grouting range
  • The deviations in the hole position, angle and depth of the grouting port should comply with the specifications
  • The length combination of grouting sections should be 10-15m considering the formation conditions and groundwater conditions. And reserve the thickness of the slurry wall
  • Grouting materials and types should be considered comprehensively and determined by experiment
  • Arrange in full sections, semi-sections and other ways
  • Segmented grouting, reversing segmented grouting
  • Check and confirm the grouting effect
4. Initial support construction

(1) Grill processing and installation

  • Made according to the drawings, welded
  • The first grille arch frame should be tested assembled.Construction unit, supervision unit, design unitAfter passing the joint acceptance, batch processing can be carried out
  • Grille arch frames and steel mesh are classified and stored, marked, and anti-rust measures are taken.
  • The arch foot shall not be installed on empty soil. A pad should be added under the connecting plate to reduce the sinking of the arch frame.
  • For soil layers with poor self-stabilization ability, locking anchor pipes should be made at the arch foot, and the locking anchor pipes should be welded firmly with the grille arch frame. Grout in time after injecting.

（2）Spray concrete

  • Continuous and uniform
  • Spray from bottom to top, and then slice in segments and layers, and after the last final set
  • Control the water-cement ratio to avoid flowing and falling. Rebound material is strictly prohibited
  • The thickness of the spray concrete protective layer of the grating arch frame and steel mesh should meet the design requirements

(3) Grouting behind initial support

  • After the initial support is closed, backfilling and grouting is promptly carried out. The grouting operation point and the excavation working surface should be kept at a distance of 5-10m.
  • Backfill grouting was performed multiple times. Radar monitoring means to detect backfill effects.
5. Waterproof layer construction
  • A plastic waterproof plate and buffer layer are composed of. Layout after initial support is stable
  • First lay the buffer layer, non-woven fabric or polyethylene foam. Fixed with a concealed nail ring, with the overlap width not less than 50mm. When laying waterproof boards, use a press welding machine to slurry plastic waterproof board to weld the concealed nail ring.
  • Fixed point. spacing. The arch should be 0.5-0.8m and the side wall should be 1-1.5m. The bottom should be 1.5-2m, with large concave and convexity, and encrypted fixing points
  • The width of the waterproof plate should not be less than 100mm. The lower plastic waterproofing plate presses against the upper plastic waterproofing plate. The number of overlapping layers of waterproof board shall not exceed 3 layers
  • The plastic waterproof plate adopts double welds, and the effective width should not be less than 10mm.
  • Zoned grouting system.
  • When installing a plastic waterproofing plate waterproofing layer in sections, sealing measures are taken at both ends.
6. Secondary lining construction
  • Combined steel formwork or formwork trolley formwork system, and prepare special plans.
  • The pumping mold building is built, the side walls on both sides are vibrating with insertion vibrators, and the bottom is vibrating with adhesion vibrators.
  • Symmetric on both sides, pouring horizontally. No horizontal or inclined joints shall be present
  • After completion, perform backfill grouting behind the secondary lining. Radar detection performs cavity monitoring.

3.5 Safety and quality control of urban tunnel engineering

3.5.1 Key points of safety technical points of urban tunnel engineering

1. Underground pipeline protection

(1) Survey on engineering geological conditions and current conditions pipelines
(2) PreparationUnderground pipeline protection plan
(3) Pipeline modification and protection measures

2. Key points for safety technical control of open foundation pits
  • When the enclosure structure uses underwater pouring concrete, the strength should be increased by one level, and the elevation of the design documents at a height of 300-500mm
  • The earth excavation should be coordinated with the internal support, and the excavation should be 0.5m below the position specified in the design document, and the internal support should be constructed.
3. Key points for safety technical control of tunnel construction in shallow buried and hidden excavation method
  • Precipitation or water stop measures meet the conditions of waterless operation
  • Grouting construction shall not pollute groundwater
  • The soil reinforcement of the Matoumen arch should be followed in time with the excavation of the working well.
  • In the step construction process, the steps should be excavated first and then the steps should be lowered. The lower steps should be excavated only after the deformation of the support structure in the early stage of the arch and the sprayed concrete reaches 70% of the designed strength.

3.5.2 Key points of quality control of urban tunnel engineering

1. Open excavation construction quality control

(1) Key points for quality control of foundation pit enclosure structure construction
（2）Foundation pit excavationKey points of construction quality control
(3) Key points of structural construction quality control
(4) Foundation pit backfilling
(5) Main structure waterproof construction

2. Key points of construction quality control of shallow burial and hidden excavation method

(1) Construction quality control indicators of shallow buried and hidden excavation method
(2) Quality control of earth excavation and initial support construction
(3) Quality control of waterproofing and secondary lining construction

3.5.3 Seasonal construction measures for urban tunnel engineering

1. Key points of winter construction control

(1) Waterproofing project

  • The waterproof concrete must not be less than P8, and the freezing resistance level must not be less than F300.
  • The mold entry temperature shall not be less than 5 degrees. Ingredients with antifreeze, take thermal insulation and moisturizing care
  • The coating waterproof layer shall not be constructed at a construction temperature below 5 degrees.
  • The ambient temperature of the coil cold pasting method and self-adhesive method should not be lower than 5 degrees. Hot melt welding should not be less than 10 degrees.

(2) Steel support engineering

  • Anti-slip spacer pads should be added when the steel cable tied and lifted in direct contact with the component.
  • The columns, main beams, and large supporting components should be installed at negative temperatures immediately and permanently fixed immediately after the position is correct.

(3) Spray concrete
Antifreeze-type ready-mixed spray concrete dry material

2. Construction during rainy period
  • The coating and coil waterproofing layer is strictly prohibited from rainy days, foggy days, or above level five strong winds.
  • If there is a possibility of rainfall, the coating has been done and protection work is done in a timely manner
3. High temperature construction
  • The coating waterproof layer shall not be constructed when the construction ambient temperature is higher than 35 degrees or when the sun is exposed to the sun.
  • Pouring is completed, moisturizing and moisturizing are carried out in time.