Construction Plan for Tunnel Concrete

Construction Plan for Tunnel Concrete

1. Project Overview

This tunnel is a two-way four-lane highway tunnel with a total length of 1800m and a maximum depth of 260m. The tunnel lining adopts a composite structure of C35 shotcrete (initial support) and C40 cast-in-place concrete (secondary lining). The designed thickness of the secondary lining concrete is 50cm, with a single pouring length controlled at 12m and a single pouring volume of approximately 180m³. The working space inside the tunnel is limited, and there is a certain slope (maximum slope of 1.5%). It is necessary to focus on solving the problems of concrete transportation, placing, and density control during pouring, while meeting the safety protection and environmental protection requirements of tunnel construction.

2. Construction Preparation

2.1 Technical Preparation

（1）Organize technical personnel to carefully study the tunnel design drawings and geological survey reports, clarify the technical parameters such as concrete strength grade, impermeability grade (P8), initial setting time (≥12h), and slump (160-200mm for cast-in-place concrete, 100-130mm for shotcrete), and compile special pouring operation instructions.

（2）Conduct technical disclosure meetings for the difficulties in pouring such as dense reinforcement areas and vault parts of the tunnel lining, clarify the key requirements such as placing points, vibrating methods, and pouring sequence to the construction team, ensuring that personnel at all positions master the operating points.

（3）Communicate with the concrete mixing plant in advance to provide detailed requirements for concrete mix proportion design, including the selection and dosage of cement (P.O42.5R), aggregate (crushed stone with particle size of 5-25mm, medium sand with fineness modulus of 2.3-3.0), admixtures (retarding high-efficiency water reducer), and mineral admixtures (Class II fly ash), ensuring that the mixing plant conducts trial mixing according to the requirements and issues qualified mix proportion reports.

2.2 Material Preparation

（1）Concrete raw materials are uniformly purchased by the mixing plant. Before construction, it is necessary to inspect the quality certification documents of cement, aggregate, admixtures and other materials, conduct on-site testing of aggregate moisture content, and adjust the construction mix proportion in a timely manner; after the concrete is transported to the site, the slump of each truck is tested, and unqualified concrete is strictly prohibited.

（2）Prepare sufficient auxiliary materials for pouring, including Φ50/Φ30 inserted vibrators (no less than 6 units per working face, including 2 spares), concrete test molds (150mm × 150mm × 150mm, 3 pieces per group, 1 group of standard curing test blocks for every 100m³), geotextiles (for concrete curing), and rain-proof cloth (to cope with sudden rainfall).

2.3 Equipment Preparation

（1）Concrete mixing equipment: 2 sets of HZS120 concrete mixing plants (single plant production capacity 120m³/h) are selected, equipped with 2 sets of aggregate bins, 1 set of cement silo (50t capacity), 1 set of fly ash silo (30t capacity) and admixture storage tank, ensuring continuous supply of concrete for single pouring and avoiding construction joints due to material interruption.

（2）Concrete transportation equipment: According to the distance between the mixing plant and the tunnel portal (calculated as 5km, with a one-way transportation time of about 15min) and the pouring speed (15m³/h), 8 sets of 12m³ concrete mixing transport trucks are configured, each equipped with a GPS positioning system to monitor the transportation route and arrival time in real time, ensuring that the concrete arrival temperature (5-30℃) and initial setting time meet the requirements.

（3）Concrete trailer pump configuration (core equipment):

 Selection: The main pump in the tunnel is 1 set of ROOBON HBT80C concrete trailer pump (theoretical delivery capacity 80m³/h, maximum delivery pressure 18MPa), matched with Φ125mm high-pressure wear-resistant delivery pipe (wall thickness 5mm). This pump has the characteristics of large displacement and high pressure, which can meet the rapid delivery demand of 12m section lining concrete in the tunnel; 1 set of HBT60C trailer pump (theoretical delivery capacity 60m³/h, maximum delivery pressure 16MPa) is reserved at the tunnel portal. In case of main pump failure, it can be switched within 30min to avoid pouring interruption.

Layout: The trailer pump is set on a flat hardened ground 50m away from the pouring section in the tunnel (the ground is hardened with C20 concrete, 15cm thick, 15㎡ in area). 1.5m operating space is reserved on both sides of the pump body, and a protective shed is set up above (erected with Φ48mm steel pipes, 2.5m high, covered with rain-proof cloth) to prevent water dripping from the top from affecting the equipment operation; the feeding port of the trailer pump is aligned with the discharge port of the mixer truck, and a temporary receiving hopper (volume 1.5m³) is set to avoid concrete leakage.

Pipeline connection: The delivery pipe extends from the outlet of the trailer pump to the placing boom, arranged in a 'horizontal section + climbing section'. A fixed bracket (made of ∠50×5 angle steel) is set every 3m in the horizontal section, and an anti-skid bracket is set every 2m in the climbing section (tunnel slope 1.5%). The pipeline joints are fastened with double nuts and wrapped with sealing tape to prevent grout leakage; Φ100mm hose is used within 5m near the placing boom to facilitate adjusting the placing angle.

2.4  Concrete placing boom configuration (core equipment):

 Selection: 1 set of ROOBON R15 hydraulic placing boom (maximum placing radius 15m, rotation angle 360°, effective placing height 8m) is selected. This type of ROOBON placing boom can cover all areas of the vault, arch waist and side wall of the 12m pouring section without frequent movement, improving pouring efficiency. - Installation: The base of the placing boom is fixed on the special load-bearing beam on the top of the tunnel lining formwork trolley (the load-bearing beam is welded with 2 I40b I-steels and rigidly connected with the main truss of the trolley). 4 pieces of 10mm thick steel plates (size 200mm × 200mm) are set between the base and the load-bearing beam to disperse the pressure; after the placing boom is installed, the levelness (error ≤0.5°) should be adjusted, and the placing radius and rotation range should be calibrated with a total station to ensure no blind area.

 Commissioning: After installation, no-load commissioning is carried out to check the operation status of the slewing mechanism, luffing mechanism and lifting mechanism to ensure smooth movement without jamming; then full-load commissioning (delivering clean water + mortar) is carried out to check the pipeline tightness and placing accuracy of the placing boom, focusing on testing the feeding speed of the vault placing port (controlled within 0.5m³/min) to avoid concrete impacting the formwork. 5. Vibrating equipment: In addition to inserted vibrators, attached vibrators (ZW-10 type, power 1.5kW, 4 units arranged on each side) are pre-embedded in the side wall and arch waist of the formwork trolley, which are used in conjunction with inserted vibrating to solve the problem of inadequate vibrating in dense reinforcement areas; both vibrators and attached vibrators are equipped with spare motors to ensure quick replacement in case of failure.

2.5 On-site Preparation

（1）Cleaning inside the tunnel: Before pouring, clean up debris and accumulated water in the tunnel, chisel the base layer of the pouring section (the surface of the initial support shotcrete) (chisel depth 5mm, exposing fresh concrete surface), rinse it with high-pressure water gun, after draining the accumulated water, lay a 20mm thick cement mortar with the same mix proportion (without coarse aggregate) to prevent poor bonding between the bottom concrete and the base layer.

（2）Formwork inspection: Check the positioning accuracy of the lining formwork trolley (axis deviation ≤5mm, elevation deviation ≤3mm), tightness (5mm thick sponge strips are pasted at the formwork joints to prevent grout leakage) and support stability (4 radial supports are set on each side, and anti-floating anchor rods are set at the bottom). Use a total station to recheck the outline size of the formwork to ensure it is consistent with the designed lining thickness.

（3）Safety protection: Set up an access control system at the tunnel portal to strictly prohibit non-construction personnel from entering; set up emergency lighting (36V safety voltage) and fire-fighting equipment (2 pieces of 4kg dry powder fire extinguishers + 1 fire bucket) every 50m in the tunnel; set up safety warning signs above the pouring working face, and operators wear safety helmets, non-slip shoes and reflective vests, and fasten safety belts when working at heights (such as placing boom commissioning).

3. Concrete Pouring Construction Process

3.1 Concrete Transportation and Acceptance

（1） The mixing plant mixes concrete according to the construction mix proportion, with each batch mixing time ≥90s. After mixing, it is transported to the tunnel portal by the mixing transport truck. During transportation, the mixing drum keeps rotating at a slow speed of 3-5r/min to prevent concrete segregation.

（2） After the concrete arrives at the site, the test personnel test the slump (once per truck). If the slump loss is too large (less than 160mm), an appropriate amount of admixture with the original mix proportion can be added to the mixer truck (dosage ≤10% of the original dosage), and the slump is re-tested after rapid mixing for 2min, and unloading is allowed only if it is qualified; if the slump exceeds the standard or the concrete has initially set, unloading is strictly prohibited and it is treated as waste.

3.2 Start-up and Commissioning of Trailer Pump and Placing Boom

1. Before starting ROOBON trailer pump, check the oil level in the oil tank, the pressure of the hydraulic system, and the insulation of the motor. After confirming that it is normal, pump clean water (about 10m³) into the delivery pipe to flush the inner wall of the pipeline; then pump 1:2 cement mortar (the dosage is 1.2 times the volume of the pipeline, about 0.8m³) to lubricate the pipeline and prevent concrete from bonding with the pipe wall.

2. The operator of the placing boom and the operator of the trailer pump establish intercom communication (using explosion-proof walkie-talkies to ensure clear signals in the tunnel). ROOBON placing boom is adjusted to the initial placing position (bottom of the side wall), and the trailer pump starts to ROOBON pump concrete at a slow speed (initial displacement controlled at 20m³/h). After the concrete flows out smoothly from the outlet of the placing boom, the displacement is gradually increased to 50-60m³/h (to avoid pipeline blockage caused by excessive displacement).

3.3 Layered Pouring and Vibrating

（1）Pouring sequence: Adopt the principle of 'from bottom to top, left-right symmetry, layered pouring'. First pour the side wall (from the bottom to the arch waist, height 3m), then pour the vault (from the arch waist to the vault, height 2m). The thickness of each layer is controlled at 30-50cm, and the interval time between adjacent layers is ≤2h (to ensure that the lower layer of concrete has not initially set), avoiding cold joints.

（2） Placing control: When pouring the side wall, the discharge port of the placing boom is ≥30cm away from the inner wall of the formwork to avoid concrete directly impacting the formwork; when pouring the vault, adopt the method of 'multi-point placing, slow advancement', set a placing point every 3m, and control the feeding speed at 0.3-0.5m³/min to prevent voids in the vault.

（3） Vibrating operation: - Inserted vibrator: When vibrating, follow the principle of 'quick insertion and slow extraction'. The insertion depth is 5-10cm deeper than the lower layer of concrete, the vibrating spacing is ≤30cm (not exceeding 1.5 times the radius of action of the vibrator), and the vibrating time at each point is 20-30s until no bubbles overflow from the concrete surface and cement slurry emerges. Leakage and over-vibration are strictly prohibited. - Attached vibrator: When the side wall is poured to the position of each vibrator, turn on the attached vibrator, each vibrating time is 15-20s, and work alternately with the inserted vibrator, focusing on vibrating the dense reinforcement areas and formwork corners to ensure concrete density.

3.4 Special Control for Vault Pouring

（1）The vault is a weak part of pouring, and the 'in-mold grouting' process is used to assist in densification: A grouting hole (Φ50mm) is reserved every 3m at the vault of the formwork trolley. After pouring to the vault, continue to pump concrete until the grouting hole overflows with concrete, then close the grouting hole; before the initial setting of the concrete (4-6h after pouring is completed), press and inject cement slurry (water-cement ratio 0.5, pressure 0.3-0.5MPa) through the grouting hole to fill the possible gaps in the vault.

（2）During pouring, assign special personnel to observe the deformation of the vault formwork. If template displacement or bulging is found, immediately stop pouring, adjust the support, and then continue the operation; at the same time, use an ultrasonic detector to conduct real-time monitoring of the concrete density of the vault, and promptly handle any voids found.

 3.5 Construction Joint Treatment

If it is necessary to set construction joints due to special circumstances (such as equipment failure, interruption of concrete supply), the following requirements shall be met before the initial setting of the concrete:

（1）A 300mm wide and 5mm thick water-stop steel plate (material Q235, 100mm bent at both ends) is set at the construction joint position perpendicular to the lining axis. The center of the water-stop steel plate is aligned with the construction joint and fixed on the steel bar to ensure that the verticality deviation is ≤1°.

（2）After the concrete has finally set, chisel the surface of the construction joint, remove the laitance and loose aggregate, expose the fresh concrete surface. Before the next pouring, lay a 20mm thick cement mortar with the same mix proportion, and then continue pouring.

4. Concrete Curing and Formwork Removal

 4.1 Curing

（1）After the concrete pouring is completed, immediately cover it with geotextile after the surface is trowelled (about 4-6h), and use a sprayer to sprinkle water for curing, keeping the geotextile moist. The curing time is not less than 14d (adjusted according to the type of cement and ambient temperature, and the frequency of sprinkling water is increased in high-temperature weather).

（2）Keep the tunnel ventilated (turn on the axial flow fan, wind speed ≥0.5m/s) to avoid cracks caused by sudden temperature drop on the concrete surface; if the ambient temperature is lower than 5℃, use electric blankets + geotextile for covering and heat preservation, and prohibit sprinkling water for curing to prevent concrete from freezing.

4.2 Formwork Removal

（1）The formwork removal time is determined according to the concrete strength: The side wall formwork can be removed after the concrete strength reaches 50% of the designed strength (about 7d, determined by the compressive test of the same-condition test block); the vault formwork can be removed after the concrete strength reaches 100% of the designed strength (about 28d). It is strictly prohibited to remove the formwork in advance to avoid cracking of the lining.

（2）When removing the formwork, use manual cooperation with a crane to remove it slowly to avoid the formwork colliding with the surface of the lining concrete; after formwork removal, check the quality of the lining surface. If there are defects such as honeycombs and pockmarks, repair them with 1:2 cement mortar (before repair, chisel and rinse the defective parts), and re-cure for 3d after repair.  

5. Quality Control Points

（1）Raw material control: Sample and inspect the cement, aggregate, and admixtures of the mixing plant once a month to ensure that the material quality meets the specification requirements; the concrete slump is tested for each truck, and unqualified concrete is strictly prohibited.

（2）Pouring process control: The thickness of layered pouring is controlled by a marking ruler (one every 30cm), and the vibrating personnel wear timers to ensure that the vibrating time meets the requirements; assign special personnel to record the pouring time, volume, slump, and vibrating conditions to form a pouring ledger.

（3）Strength control: Standard curing test blocks and same-condition test blocks are placed according to the specifications. The standard test blocks are cured in a curing room with 20±2℃ and relative humidity ≥95% for 28d. The same-condition test blocks are cured in the same environment as the lining concrete to determine the formwork removal time and entity strength.

（4）Appearance and density control: After formwork removal, check the lining surface flatness (deviation ≤5mm/m) and verticality (deviation ≤3mm/m). Use an ultrasonic detector to conduct 100% density detection of the lining. If defects such as voids and interlayers are found, a special plan must be formulated for repair.

6. Safety and Environmental Protection Measures

 6.1 Safety Measures

（1）Equipment safety: Operators of trailer pumps and placing booms must hold certificates and receive safety training before taking up their posts; it is strictly prohibited to touch rotating parts during equipment operation. Regularly check the firmness of delivery pipe joints and support brackets, and immediately stop the machine for treatment if looseness is found.

（2）Electrical safety: The construction electricity in the tunnel adopts 'three-level power distribution and two-level protection'. The distribution boxes of trailer pumps, placing booms and other equipment are equipped with leakage protectors (leakage action current ≤30mA, action time ≤0.1s); the cables are laid overhead or buried (burial depth ≥0.7m) to avoid rolling damage.

（3） Emergency measures: Formulate emergency plans for equipment failures, concrete pouring interruptions, and fires in the tunnel, and equip with emergency materials (such as spare vibrators, flashlights, first-aid kits); organize emergency drills once a month to ensure that construction personnel master the emergency disposal procedures.

6.2 Environmental Protection Measures

（1）Noise control: Select low-noise models for trailer pumps, mixer trucks and other equipment. Close the portal access control during operation in the tunnel to reduce the impact of noise on the outside; construction is prohibited at night (22:00-6:00). If construction is necessary, a night construction permit must be obtained and surrounding residents must be informed.

（2）Wastewater treatment: The flushing wastewater from concrete transport vehicles is treated by a sedimentation tank (three-level sedimentation, total volume 50m³) and then used for site sprinkling and dust suppression, and is strictly prohibited from being directly discharged; the accumulated water in the tunnel is pumped to the external sedimentation tank by a water pump and discharged after reaching the standard.

（3）Solid waste treatment: Concrete waste residues, waste formwork and other solid wastes generated during construction are collected centrally and transported to the designated construction waste disposal site, and are strictly prohibited from being discarded at will; classified garbage bins are set up for domestic waste, which are regularly cleared and transported by the environmental sanitation department.

7. Emergency Plans

7.1 Emergency Disposal for Trailer Pump Failures

（1） If the trailer pump has hydraulic system failure or motor failure, immediately start the standby trailer pump, and at the same time organize personnel to remove the connection between the main pump and the pipeline, and complete the docking between the standby pump and the pipeline within 30min. Before docking, it is necessary to pump clean water to flush the residual concrete in the pipeline to avoid pipeline blockage.

（2） If the fault cannot be eliminated in a short time and the concrete has initially set, it is necessary to handle it according to the requirements of construction joints. After the fault is eliminated, re-pour, and at the same time chisel the poured concrete.

7.2 Emergency Disposal for Concrete Segregation

（1）If concrete segregation (aggregate sinking, mortar floating) is found after the mixer transport truck arrives at the site, unloading is strictly prohibited. An appropriate amount of admixture with the original mix proportion can be added to the mixer truck (dosage ≤10% of the original dosage), and the slump is re-tested after rapid mixing for 3min, and it can be used only if it is qualified; if the segregation is serious, it is treated as waste.

（2）If concrete segregation is found during pouring, immediately stop pumping, remove the hose at the outlet of the placing boom, clean the segregated concrete in the pipe, and continue pouring after replacing with new concrete.

7.3 Emergency Disposal for Fire in the Tunnel

（1）If a fire occurs in the tunnel (such as cable fire, protective shed fire), immediately stop the operation, organize personnel to evacuate to the safe area outside the tunnel, and at the same time turn on the smoke exhaust fan in the tunnel, close the portal access control to prevent the fire from spreading.

（2）Firefighters wear gas masks and use dry powder fire extinguishers or fire buckets to put out the fire. If the fire is large, call 119 immediately and start the tunnel fire protection system (such as fire hydrants, water curtain systems). This plan covers the core links of tunnel concrete pouring, especially the configuration details of trailer pumps and placing booms. If your project has special tunnel geological conditions (such as water-rich, high ground temperature) or other special requirements (such as the use of self-compacting concrete), please feel free to inform me, and I will further optimize the plan content.