Recently, in order to effectively reduce the occurrence of risk accidents, lower economic losses, casualties and environmental impact of projects, and ensure the safety of engineering construction and urban operation, the Ministry of Housing and Urban-Rural Development (MOHURD) compiled and issued the Key Points for Technical Risk Control of Large-scale Engineering. In the face of technical risks, what preparations should construction units make?
　　Summary of Risk Categories
　　1. Foundation
　　Risk of pile foundation fracture, risk of high fill soil foundation sliding, risk of high cut slope instability, risk of deep foundation pit slope collapse, risk of pit bottom sudden surge, risk of underground structure floating.
　　2. Large-span structures
　　Risk of overall structural overturning, risk of cracks in super-long and super-large cross-section concrete structures, risk of fracture in super-long prestressed tensioning, risk of collapse of large-span steel structure roofs, risk of damage to large-span steel structure roof panels by strong winds, risk of collapse of steel structure support frames, risk of collapse during sliding (jacking) installation of large-span steel structures.
　　3. Super high-rise structures
　　Risk of collapse and falling of core tube formwork system, risk of instability, overturning and falling of external climbing tower crane on core tube, risk of collapse and falling of steel structure trusses in super high-rise buildings, fire risk during construction.
　　4. Shield tunnel
　　Risk of shield launching/arrival, risk of shield machine cutter head failure, risk of shield chamber opening, risk of shield machine lifting, risk of shield empty pushing, risk of crossing risky geology or complex environment during shield construction, risk of mud-water discharge system failure, risk of soil loss during excavation in upper-soft lower-hard strata, risk of misalignment, water and sand inrush during tail grouting, risk of segment installation mechanism failure, risk of rockburst in open shield excavation in hard rock.
　　5. Mined tunnel
　　Risk of portal excavation, risk of multi-drift construction arching excavation, risk of temporary support removal in large cross-section, risk of enlarged section construction, risk of upward excavation, risk of drill and blast excavation, risk of crossing risky geology or complex environment, risk of collapse accident, risk of water and sand inrush accident, risk of underground pipeline damage accident.
　　Responsibilities of Construction Units
　　(1) Implement the main responsibility.
　　1) Strictly implement the main responsibilities of all parties involved in engineering construction. Further improve the quality and safety management system and responsibility system, fully implement the quality and safety responsibilities of all parties, especially strengthen the primary responsibility of the construction unit and the main responsibility of the survey, design, and construction units.
　　2) Strictly implement the responsibility of project leaders. Strictly execute the quality and safety responsibility regulations for project leaders of the five parties: construction, survey, design, construction, and supervision, and strengthen the quality and safety responsibilities of project leaders.
　　3) Strictly implement the responsibility of practitioners. Strengthen personal practice management, implement the quality and safety responsibilities of registered practitioners, standardize practice behavior, promote the establishment of personal practice insurance system, and increase accountability for practice.
　　4) Strictly implement the lifelong responsibility for project quality. Further improve the lifelong quality responsibility system, strictly implement the written commitment to lifelong quality responsibility, permanent signage, quality information archives, and other systems, and increase accountability for quality.
　　(2) The construction unit shall formulate a targeted special construction organization design (including risk pre-control measures and emergency plans) before commencement, and implement and track the quality and safety risks throughout the construction process according to the pre-control measures and emergency plans, while recording and archiving relevant materials.
　　(3) Select regular raw material suppliers; strengthen quality inspection of raw materials, and conduct sampling tests when necessary. Continuously improve and enhance survey methods and means, improve the quality of survey results, and improve design quality; timely feedback information on abnormalities found during construction or discrepancies with survey and design. Promote new construction technologies. Accelerate the promotion and application of advanced construction equipment and intelligent equipment. Strengthen the construction of supervision teams, ensure the personnel and funding of supervision agencies. Conduct inspections on the staffing and funding guarantees of supervision agencies.
　　Key Points of Risk Control
　　Foundation
　　During excavation, mechanical collision and brutal pile cutting are strictly prohibited; compare and select schemes such as anti-slide piles with counter-pressure berms outside the toe, slowing down slope rate, adding retaining walls and reinforced soil treatment, and select the best or combined design scheme; monitor the high fill filling process and determine appropriate construction control parameters.
　　When conditions permit, cut off the hydraulic connection between the confined water layers inside and outside the pit as much as possible, and isolate the confined aquifer; local deep pits within the foundation pit should be reinforced with cement-soil mixing piles or jet grouting piles, and their construction quality should be ensured; determine the number of减压 dewatering wells and the depth of filter heads through calculation, and verify through pumping tests; the confined water level observation wells inside the pit should be set up separately and continuously observe and record the water head elevation; during excavation, protective measures should be taken to ensure the integrity of the dewatering wells; open the dewatering wells in time according to the predetermined excavation depth, and ensure the effectiveness of the dual power supply system.
　　Communicate with the design to determine the number and construction method of drainage holes, and seal the wells according to the specified time; the covering soil should be compacted in layers, and the soil compactness should meet the design requirements.
　　Large-span structures
　　Strengthen settlement observation during construction, control uneven settlement of the foundation; pay attention to the coordination of adjacent foundation pit excavation to avoid the impact of excavation unloading on the completed foundation structure.
　　During steel structure assembly, measures should be taken to eliminate welding stress and control welding deformation; do a good job in anti-corrosion and anti-rust treatment of steel structures; consider the impact of temperature changes on the steel structure roof.
　　Wind pressure resistance tests should be conducted on metal roofs, considering many influencing factors; ensure good construction quality of the interlocking parts, significantly improve the ultimate bearing capacity, and use high-quality mechanical locking for metal roofs; for special parts, metal roof panels with wind resistance reinforcement clips can be used to improve wind resistance.
　　The sliding support should be designed and calculated before determining the erection scheme. In load design, the impact of sliding traction should be considered, and sliding tests can be conducted if necessary; the support should be erected by professional scaffolders and can only be put into use after quality and safety inspection and acceptance; during sliding, the internal force and deformation of the support should be monitored to ensure they do not exceed the specified limits; the stiffness and overall stability of the steel structure under sliding (jacking) construction conditions should be checked, and if insufficient, contact the designer to appropriately increase the cross-section of structural members or take other measures to strengthen stiffness; increase the construction camber value during steel structure assembly.
　　Super high-rise structures
　　Before installation and removal of the overall steel platform equipment, the sequence and method of installation and removal should be formulated according to the force characteristics of the system components and the position of blocks or segments, and temporary supports should be set up according to force requirements, and the stability of the installation and removal process of block and segment components should be ensured.
　　When the vertical support limit devices of the筒架 support system and steel beam climbing system of the overall steel platform equipment are placed on concrete support brackets or steel structure support brackets, the actual compressive strength of the concrete structure at the support location should meet the design requirements and should not be less than 20MPa; when the steel column climbing system of the overall steel platform equipment is supported on the concrete structure, the actual compressive strength of the concrete structure should meet the design requirements and should not be less than 15MPa; during the climbing operation of the overall steel platform, the bottom isolation gate should be 50mm away from the wall, the steel platform system, suspended scaffold system, and formwork system should be free of foreign objects, and the chain of the formwork chain hoist should be free of hooks; the overall steel platform equipment should be equipped with displacement sensing systems and gravity sensing systems. At least 2 automatic wind speed recorders should be installed for construction operations, and the construction process should be controlled according to the wind speed monitoring data against the design requirements.
　　Design points for temporary support (bracing) structures: 1) When the structural strength or stability reaches the limit and may cause overall damage to the main structure, reliable load-bearing supports or other safety measures should be set up; 2) Temporary support structures and measures during the construction stage should be checked for strength, stability, and stiffness according to the load effects of the construction condition, and the connection nodes should be checked for strength and stability; 3) If the temporary support structure is used as equipment load-bearing structure, such as sliding tracks, lifting brackets, etc., special design should be carried out; when the temporary support structure or measures have a significant impact on the structure, it should be submitted to the original design unit for confirmation.
　　The construction water supply riser can also serve as a fire standpipe, with a diameter not less than 100mm; a certain number of outdoor temporary fire hydrants should be set around the building, and indoor temporary fire hydrants, hoses, and nozzles should be set on each floor; evacuation channel facilities should be built in appropriate locations. While the inner and outer frames are constructed alternately, construction channels connecting each other can be set up outside the outer frame elevators, serving as work climbing facilities in normal times and as emergency evacuation channels for personnel in special circumstances; wood storage yards should be stacked in groups and piles, with fire access between groups; smoking and open flames are strictly prohibited in wood processing sites, and flammable materials such as shavings and sawdust should be cleaned up in time and dumped at designated safe locations.
　　Shield tunnel
　　The H-shaped steel in the SMW method pile of the enclosure structure should not be pulled out too early; before launching, check the installation of the portal sealing system; when designing the shield machine, sufficient foam inlets should be set up, and foam pipes should be cleared in time when blocked; before entering the chamber under pressure, grouting reinforcement must be carried out around the front body and cutter head of the shield machine to maintain the air pressure of the chamber working environment; before entering the chamber, conduct a pressure test first, the pressure level is determined by the soil depth at the working location, and the chamber can only be entered when the pressure is stable; the pressure chamber operation should not be too long, close the soil chamber door first, then reduce the pressure, and the pressure reduction must be slow and steady.
　　During the shield machine advancement, an appropriate amount of pea gravel needs to be accumulated in front of the cutter head and continuously replenished to provide sufficient reaction force for the shield empty pushing.
　　Grout in time to avoid insufficient grouting; on the other hand, avoid excessive grouting that splits the soil and causes surface grout leakage; control the shield attitude to reduce stratum disturbance; establish joint analysis of tunnel monitoring measurement and advanced geological prediction; set emergency remedial measures for adverse consequences.
　　Set up a sedimentation tank or sedimentation box in front of the slurry discharge pipe, and install vertical grilles; take measures to prevent mud cake; inject sufficient tail grease in time; set appropriate grouting pressure.
　　Mined tunnel
　　After excavating the portal, immediately construct the portal support grid frame, and if necessary, pre-reinforce the soil within the portal height range to improve the arch effect; strengthen the shaft wall before excavation, and densely arrange steel frames above and below the portal excavation and at the temporary invert positions; when excavating the cross passage, the densely arranged steel frames and shotcrete at the temporary invert position should not be removed, and the shaft should be sealed at the bottom before excavation; if necessary, grouting for backfill to prevent soil instability after removing the shaft wall, advanced pipe roof or single-layer or double-layer small conduit grouting parallel to the passage can be used according to the situation, and if necessary, circumferential grouting or full-face grouting of the upper bench can be carried out to withstand the lateral pressure of the shaft on the portal after excavation, a reinforcement ring should be constructed around the initial support of the cross passage before excavation; the lower shaft wall can only be removed for the next excavation after the cross passage has been excavated for a certain distance (generally greater than 10m); the temporary support can only be removed after the initial support of the cross passage has been fully ringed for a certain length; stress and strain observation should be carried out on the shaft wall in the portal excavation section.
　　Accurately measure the boundary position between the small cross-section and the large cross-section, use the step-by-step expansion method to expand the cross-section to the next cross-section, and then reverse excavate the transition section to the required height and width. During reverse excavation, do advanced support. During upward excavation, due to the steep slope, steps should be added to the completed section to facilitate personnel access, and small movable platforms can be used near the working face for operators, and ventilation management should be strengthened, ventilation volume increased, and fresh air supply ensured at the working face position.