Combined with engineering example, the design and construction method of double-layer air duct are introduced, the load-structure model is established, the plane calculation is carried out by using SAP84 program software instead of frame, the points that should be paid attention to in design are put forward, the information obtained from monitoring and measurement is used to guide the construction, and corresponding measures are taken in time to ensure the construction safety.
1 project overview
Southeast Nanfeng Road of Xinjiekou Station of Beijing Metro Line 4 is located at the intersection of Deng Yu Road and Xizhimennei Street, with heavy ground traffic. There are many underground municipal pipelines, such as sewage, water supply and municipal pipelines. Therefore, the air duct is constructed by mining method and designed as a double vault straight wall structure. The maximum buried depth of air duct is about 6m. The bottom plate of the air duct structure passes through 6-2 layers of silt, which belongs to grade VI surrounding rock; The rock and soil layers through which the roof of the structure passes are silty sand ③-3, silty clay ④ and silty soil ③, which belong to Grade V surrounding rock, easy to collapse, poor in stability and unable to form a natural stress arch. The rock and soil layers that the side walls of the air duct structure pass through are silty clay ⑥, pebbles ⑥, medium coarse sand ⑥-4, silty fine sand ⑥-3 and silty silt ⑥-2. The lower part of the side wall is mainly composed of sand and gravel soil, and the surrounding rock and soil have poor self-stability and are easy to collapse.
2 Construction method
The air duct is constructed by CRD method, which is divided into four layers and eight small pilot tunnels. The basic principles of "pipe feeding, strict grouting, short excavation, strong support, quick closure and diligent measurement" are strictly followed in the construction. The construction sequence of pilot tunnel is as shown in figure 1. In order to reduce the influence of large-scale excavation of the arch on the stratum, the construction adopts the method of step-by-step excavation, from large to small, temporary inverted arch and timely closure, as follows:
1) as the advanced support of arch, a row of large pipe sheds are used to protect the roof at the air inlet of shaft and station, and a small pipe is used as the advanced support, and the stratum is reinforced by grouting. 2) Distribute the excavated pilot tunnel soil, seal the initial support of the pilot tunnel, with a step length of 5m up and down, and seal the upper pilot tunnel face in time. 3) Remove the vertical temporary support at the bottom (the length of one removal is less than 6m) as the waterproof layer and part of the bottom plate. 4) Gradually dismantle some temporary inverted arches and temporary supports (the length of one demolition is less than 6m), and make waterproof layer and secondary lining in time. 5) Finally, the temporary inverted arch and vertical support are removed, and the waterproof layer and secondary lining are closed in time.
3 initial support parameters
C20 early strength shotcrete and small conduit are used for initial support. 42(3 pieces /m)L=3.0m, setting angle 10 ~ 15, longitudinal step 1.0m, and steel grating consisting of 4 pieces? Weld 25 steel bars with a grid spacing of 500mm. What is steel mesh? 6 @150mm×
150mm, double-layer layout, grid layout? 22 longitudinal Rachel tendons, with a circumferential spacing of 0.7m, are staggered inside and outside the main tendons.
4 structural calculation
4. 1 load combination
1) Strength calculation of basic composite members;
2) Crack resistance calculation of short-term effect composite members; 3) the strength of the accidental combination component; 4) Strength checking calculation of accidental composite components of civil air defense.
After trial calculation, the internal force of the structure under the action of earthquake load and civil air defense load has no control effect on the reinforcement of the structural section. In this way, the focus of seismic and civil air defense design is to take some structural measures to meet the requirements of the code.
4.2 Main load value
1) structural self-weight, reinforced concrete gravity γ=25kN/m3.
2) Vertical pressure of stratum: The overburden thickness of this tunnel is 6m, which is a shallow tunnel, and the weight of all soil columns above the calculated section is considered; Horizontal pressure of formation: Rankine earth pressure theory is adopted, active earth pressure is taken at the outer side of the construction stage, and static earth pressure is taken at the use stage.
3) Live load: the ground is overloaded by 20kPa.
4) Accidental load: 8-degree earthquake load and 5-level civil air defense load.
Load-structure model is adopted for structural calculation, and SAP84 program software is used for plane calculation instead of frame. The internal force of the secondary lining is calculated according to that the primary support bears the same load as the secondary lining. Because waterproof is not considered in the primary support, the secondary lining bears all the water pressure, and the earth pressure acts on the primary support. Considering that the performance of the primary supporting materials will gradually deteriorate and the stiffness will decrease under the long-term action, the secondary lining will bear 70% of the load. After calculation, it is found that the outside of the foot, the inside of the bottom span, the outside of the arch back and the inside of the vault are the most important.
5 Main technical measures
The construction method is only determined according to the key factors such as section form, surrounding rock grade and environmental conditions. For the existing structural span, the choice of foundation construction method is relatively mature. However, the main factors that determine the success of the project and the quality of construction technology are the adoption of auxiliary construction methods and the implementation opportunity of various measures to control ground displacement. The design of this project mainly considers the following points:
1) small duct grouting. Generally, the small conduit adopts F42 steel flower tube, with a length of 3m, and its grouting plus solid diameter is about 500mm. Pre-reinforcement grouting is one of the effective methods to ensure the self-stability of stratum. According to the formation properties, different muds are used. For the structure with the excavation span of small pilot tunnel less than 6m, the radius of reinforcement ring of 0.5m can ensure the self-stability of surrounding rock before the initial support completely reaches the bearing capacity. Controlling grouting pressure or grouting quantity is the key of grouting technology.
2) Lock the foot anchor. According to the size of the excavation span, setting a lock-foot anchor with a length of about 3m and grouting can effectively ensure that the supporting structure will not cause excessive overall settlement, especially for the structure composed of upper and lower pilot tunnels. When the upper pilot tunnel is constructed, the lower pilot tunnel will not be empty.
3) Backfilling and back grouting. Considering the gap after the initial branch in the design is not conducive to controlling the stratum displacement. Therefore, timely and effective backfilling is needed. Generally, when the distance from the working face is 3m, backfill grouting must be carried out behind the initial support, and the slurry is cement mortar.
4) Step size. The excavation length of each cycle is 0.5m, which is suitable for steel grating spacing, and the core soil is left, and the length of steps is not suitable, which may cause excessive stratum displacement and even stratum instability. In the design, the requirement of 5m distance between the excavation face of the upper and lower pilot tunnels is put forward.
5) One-time demolition length. For structures constructed by CRD method, the length of one demolition is also a key technical index. In order to avoid stratum displacement caused by structural stress state change, it is proved by theory and practice that the length of a demolition section must be less than 1D(D is the tunnel diameter), and a relatively safe value is adopted in this design, with each demolition of 6m.
6 monitoring measurement
Using monitoring and measuring information to guide construction is an important part of shallow excavation method. Through monitoring, the stability of stratum and the reliability of supporting system are judged, and corresponding measures are taken in time to ensure construction safety. In this project, the measurement items such as surface displacement, vault subsidence and convergence around the tunnel are set as mandatory items, and the earth pressure, soil displacement and supporting stress are selected as measurement items. When the change rate of horizontal convergence is less than 0. 12mm/d, the change rate of vault subsidence is less than 0. 1mm/d, and the measured displacement value is more than 80% of the estimated total displacement value, the tunnel can be considered as basically stable.
7 conclusion
Through the design of the southeast wind tunnel at Xinjiekou Station of Beijing Metro, we mainly get the following experiences and understandings:
1) Because the construction of air duct structure has caused many disturbances to surrounding rocks, especially the plastic zone with complicated stress exists at the top of the partition wall, special attention should be paid to the stress balance and stability of the partition wall in design and construction.
2) When CRD method is used for underground excavation of shallow-buried long-span structures, the size of the top block will ultimately affect the ground settlement. Practice has proved that the ground settlement caused by top construction accounts for 60%~70% of the final total ground settlement. Therefore, when designing the construction scheme, the principle of quick excavation and quick closure should be adopted.
3) During the secondary lining construction, the temporary supports shall be removed in sections, and each removal shall not exceed 6m. Excessive demolition will cause great settlement and deformation.
4) In general, the length of each grid steel frame should be controlled at 2m~4m. Too long and too heavy transportation will cause installation difficulties, too short will lead to many steel joints and great waste.
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