In recent years, with the continuous development of science and technology, the level of building construction has been continuously improved and the construction technology has been constantly updated. Arch cover construction is a new construction technology, as the name implies, it is the combination of arch cover. In view of the combination of cover excavation and arch construction in subway stations, it is mostly used in subway stations constructed by underground excavation and drilling and blasting. The following briefly discusses the application and technology of arch cover construction in subway station construction.
A, the main building arch cover method construction technology
The construction steps are as follows:
Step 1: Main body side pilot tunnel and crown beam construction.
Excavation of main tunnel 1 and main tunnel 2 for initial support, the two pilot tunnels are staggered not less than 5m. Mortar anchor and lock-foot anchor are set outside the main tunnel to reinforce the surrounding rock of the big arch foot. When the two guide holes are connected, they will be used as crown beams.
Step 2: Arch construction of primary support of main body.
The excavation of main tunnels 3 and 4 will serve as the first supporting arch of the station vault, and the excavation arrangement of main tunnels 3 and 4 should be kept at about 30m. Plain concrete is used to backfill the spare parts of main tunnel 1 and main tunnel 2. Then, the second layer of initial support is applied to the arch. (The second layer of primary support is added as temporary secondary lining to increase the overall strength of the arch.)
Step 3: Earthwork excavation of the station and prestressed anchor cable construction.
Demolition of temporary support, layered excavation, excavation and construction of prestressed anchor cable on the side wall of the station, in order to strengthen the stability of the rock under the crown beam. At the same time, we should pay attention to the loose blasting of the side wall to ensure the integrity of the rock mass under the crown beam.
Step 4: Construction of the second lining of the station main body
The ground of the station is lined with concrete and part of the side wall concrete, then the main side wall is lined with concrete, and finally the middle plate and column.
Step 5: Construction of the second lining arch of the main body.
The concrete structure with the second lining of the vault is constructed in sections to complete the main construction of the station. When making the second lining concrete of vault, the bracket of middle plate should be reserved to strengthen the bearing capacity of middle plate.
Second, the arch cover construction technology control points
1, blasting control of urban underground excavation drilling and blasting construction. Follow the principle of "advance prediction (advance piping, strict grouting), short footage, quick closure, strong support, weak blasting, diligent measurement and quick feedback". Strictly adopt microseismic controlled blasting, use low detonation velocity explosives and millisecond detonators to jump, and reduce the vibration intensity and influence on the surrounding of blasting construction section by controlling the maximum amount of primary explosive in a single section. The core cutting adopts the comprehensive control blasting technology of throwing blasting to minimize the disturbance to the surrounding rock, make full use of the strength of the surrounding rock to maintain the stability of the rock mass, effectively control the surface subsidence, control the over-excavation and under-excavation of the surrounding rock of the station, and achieve a good contour formation.
2. Strengthen ground monitoring measurement and observation of vault settlement and side wall convergence in the cave. Its purpose and significance lies in comparing the monitoring data with the predicted value, judging whether the previous construction technology and parameters meet or reach the expected requirements, and at the same time realizing the control of the next construction technology and construction progress, so as to effectively realize information construction; At the same time, monitoring can ensure the normal operation of surrounding buildings, roads and pipelines during excavation.
3. When the main earthwork of the station is excavated in layers, attention should be paid to the stability control of the rock stratum under the crown beam. In order to ensure the stability of the rock mass of the lower side wall of the crown beam when the earthwork of the station is excavated in layers, loose blasting should be adopted for the side wall to ensure the integrity of the rock mass of the lower side wall of the crown beam. At the same time, in order to prevent shale from sliding or cracking due to unreasonable bedding structure angle, anchor or anchor cable construction should be carried out for the side wall while the earthwork in the station is excavated in layers to increase the stability of the side wall rock mass.
Third, the application scope of arch cover method and its comparison with existing mature construction methods
1, scope of application. The arch cover method is generally suitable for urban main roads with surrounding rock grade above grade 4 and good geological conditions, and it is not allowed to adopt the cover excavation method or the open excavation construction of subway stations. The commonly used "middle hole method", "side hole method" and "PBA method" are mostly suitable for soft rock and soil layers.
2. Advantages and disadvantages of construction methods. The common construction methods of underground excavation station construction are "middle hole method", "side hole method" and "PBA method", which have been successfully implemented, but there are also some defects and deficiencies. The "middle hole method" is the current middle hole method. The beam-column support system is established first, and then the side hole is constructed. "Side hole method" means that there are two side holes first, and then the middle hole is constructed. "PBA method" takes the small pilot tunnel as the pile beam to form the main force transmission structure, and excavates the inner pit under the hidden arch cover.
The disadvantages of "side hole method", "middle hole method" and "PBA method" are as follows: ① there are many pilot holes, many procedures, many blasting times and many times of disturbing strata; (2) The support is complicated, the initial support is dismantled, and the amount of abandoned works is large; ③ There are many connection points between pilot tunnels, and the supporting system is weak; (4) Slow progress, high cost and much waste; ⑤ "PBA method" is difficult to ensure the verticality of retaining piles. The advantage is that the tunneling process is relatively safe.
The advantages of "arch cover method" are as follows: ① fewer pilot holes, fewer procedures, fewer blasting times and fewer disturbance times; (2) The support is simple, the initial support is removed less, and the engineering quantity is wasted less; ③ There are few connection points between pilot tunnels, and the supporting system is guaranteed; (4) After the arch cover is formed, it can work in a large area, with high efficiency and shortened construction period; ⑤ For moderately weathered slate surrounding rock, piling is not required; ⑥ Pre-stressed anchor cable is used in the construction of station side wall, and there is no temporary support to ensure the construction safety. The disadvantage is that the strength of surrounding rock is high and the weak blasting of the lower side wall of crown beam is difficult to control.
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