1 Overview Urban rail transit project is a systematic project covering civil structure construction and system equipment manufacturing and installation. Among its many components, concrete construction is the last unit project of mass concrete construction in rail transit engineering. At the same time, the rail professional engineering also includes the installation of rails, fasteners, turnouts and ancillary facilities. Track professional engineering is in the center of the whole project construction, which plays an important role in connecting the preceding with the following (civil construction) and connecting (equipment installation). In the whole construction of rail transit project, rail transit is a very important milestone, which indicates that the whole project has entered the stage of equipment installation. The progress management of track engineering not only affects its own construction, but also promotes and optimizes civil construction (optimizing operation sequence) and equipment installation (providing convenient conditions), which plays a vital role in the progress management of the whole project. 2 Daxing Line Track Project Schedule Management 2- 1 Project Overview Daxing Line is a rapid rail transit line connecting Daxing New City and the central city, and together with Metro Line 4, it forms the north-south rail transit trunk line in Beijing. There are 1 1 stations and 1 depot on the whole line. The total length of the main line is 2 1-756km, of which the length of the underground line is 17-427km, accounting for 80-1%; The length of transition section is 0-703km, accounting for 3-2%, and the length of elevated section is 3-626km, accounting for16-7%; The length of access line is about 1-002km, in which the length of underground line is 0-565km, the length of transition section is 0- 186km, and the length of ground section is 0-250km. The minimum curve radius of the main line is 300m, which is located between Huangcun Railway Station and yihezhuang station. The maximum gradient of the main line is 26‰, which is located between Xihongmen Station and Gaomidian North Station. The maximum line slope of the access section line is 35‰. 60kg/mU7 1Mn rail is selected for the main line, 50kg/mU7 1Mn rail is selected for the station line, and 1/40 rail superelevation is adopted. The underground section of main line and auxiliary line adopts short sleeper monolithic ballast bed and DT ⅵ 2 fastener, the overhead section adopts short sleeper monolithic ballast bed and DT ⅶ 2 fastener, and the trapezoidal sleeper section adopts elastic strip II- 1 fastener; Gravel ballast bed and DT Ⅵ1fastener are used for the ground line and test run line of the depot, and DT Ⅵ1fastener is used for the line outside the depot. The main line and auxiliary line adopt No.9 single turnout, single crossover and crossover of 60kg/m rail; 50kg/m track No.7 single-track fork and crossing line are adopted for the station line. The whole track project is divided into two construction blocks, with the main line mileage K 14+500 as the demarcation point. The track-laying single-track kilometer of two sections * * * is 63km, of which the main track single-track kilometer is 43-4, and the depot track single-track kilometer 19-6. In response to the call of Beijing Municipal Party Committee and Municipal Government to speed up the construction of Beijing urban rail transit, five new lines, including Daxing Line, have accelerated the pace of construction. Different from other lines, the trial operation of Daxing Line is directly connected with Metro Line 4, which has no precedent in the history of urban rail construction in China and puts forward higher requirements and challenges for the construction of various disciplines including rail engineering. 2-2 Brief introduction of track construction process For the whole urban rail transit project, the track professional construction can be mainly divided into two sub-unit projects: the main line and the depot or parking lot. Generally speaking, because the parking lot and the depot are relatively independent, the track usually adopts gravel ballast bed, which can be carried out independently during construction, and the site conditions are good, which can increase the input of labor. Comparatively speaking, the main track is usually an integral ballast bed, and the influence of civil construction progress on construction must be considered during construction. At the same time, the completion of the main track can provide convenient conditions and construction benchmarks for equipment installation, which has a great impact on the overall project. Therefore, the author takes the overall ballast bed construction of the main line as an example. The construction process of track engineering is: construction preparation → integral ballast bed construction → rail welding and unit rail locking → contact rail and protective cover installation → auxiliary facilities installation and track fine adjustment → pre-acceptance and rectification → completion acceptance and delivery. In the whole track construction cycle, the integral ballast construction accounts for the largest proportion, reaching more than half of the whole construction process, and it is also the focus of track engineering construction progress management. In China, the track laying method is generally adopted in the construction of integral track bed of urban rail transit. The construction of monolithic ballast bed mainly includes seven small steps: foundation treatment, track laying by track laying machine, binding and welding of stray current-proof terminal steel bars, track transportation positioning, track state adjustment and control, formwork support adjustment, and concrete pouring and maintenance. Among them, before concrete pouring, it must be checked and approved by the supervision engineer before subsequent construction can be carried out. (1) Foundation treatment Before foundation treatment, take the rail surface elevation as the baseline, detect the height of the track structure, and confirm that the bottom surface of the whole ballast bed is not less than the design height. The basement of the track area shall be chiseled by pneumatic pick (the shield interval is not chiseled), and the sundries and garbage shall be cleaned immediately after chiseling. (2) Installation of Track Paver Track Paver is a machine for lifting track rows, steel bars, concrete and other materials. It is frequently used in the parts where the track car cannot travel (curing concrete ballast bed, section where track rows are in place to be poured, steel bar binding section, etc.) and has a great effect. ). Therefore, it is required that the track laying and dismantling of the track laying machine is convenient and quick, convenient for manual handling in the hole, and stable and safe to walk. The track length of the track-laying machine should be adjusted according to the actual construction situation, and it is generally advisable to advance the steel mesh laying section by 50m. (3) Binding of Ballast Reinforcement and Welding of Stray Current-proof Terminal The integral ballast reinforcement fabric is cut and processed at the track-laying base, and then bound and welded on site. The longitudinal reinforcement shall be mixed according to the designed ballast block length, the reinforcement shall be bound in the track-laying base, and then hoisted to the flat car. When laying, the track-laying machine will hoist it from the flatbed car to the laying area. After the layout is dispersed, adjust the spacing of steel mesh and tie it, and weld it as required. The binding process of steel bars on the integral track bed of overhead lines is the same as this, and the spacing arrangement of steel bars and the welding of stray-proof terminals are carried out according to the design requirements. (4) Track row transportation and positioning [1] Track row assembly shall be carried out at the track laying base. Before assembling the track layout, calculate the track layout according to the design documents and specifications, and prepare the track layout table. The assembled rail rows are numbered according to the laying sequence, and loaded in the order that the first laid rail row is at the top and the second laid rail row is at the bottom. The rail car transports the rail row to the vicinity of the working face, and then it is hoisted in place by the rail row track laying machine. (5) Adjustment and control of track state After the track row is initially put in place by the track paving machine, the track state is preliminarily adjusted by the lead screw of the track support frame based on the construction benchmark. Then connect the front and rear rails with temporary joints, tighten the bolts, and keep the rail gap butt joint. After the coarse adjustment of the track row is completed, before pouring the ballast concrete, use the corrected track ruler with an allowable error of 0~0-5mm, and finely adjust the position, elevation and direction of the track according to the benchmark, so that the geometric dimensions of all tracks can meet the specified requirements. (6) Formwork supporting and adjusting the ballast concrete in the underground section is constructed in two parts, namely, the ballast concrete under the short sleeper and on both sides of the central ditch is constructed first, and then the concrete at the bottom of the ditch and the two outer sides are constructed. Therefore, before 1 pouring, it is necessary to support four formwork on both sides of the short sleeper. The pouring sequence of ballast concrete and the position of formwork are shown in figure 1. The focus is on the formwork on both sides of the central groove of the ballast bed, which is related to the appearance quality of the finished product. The positions of the outermost two templates can be adjusted according to the actual situation to facilitate the vibrator to extend in. There are two integral ballast beds with belt-shaped rail bearing platforms on the overhead line, and the steel bars and rail rows can be poured in place once they are adjusted. After the track row is in place, the template can be supported, and the template can be adjusted again after the track adjustment is completed. (7) Supervision Engineer Inspection The concealed works must be inspected by the supervision engineer, and the construction unit can enter the next working procedure-concrete pouring after it is confirmed to be qualified and signed. (8) The whole ballast concrete pouring and curing concrete pouring shall be carried out by track-laying reserved openings or other blanking openings, which shall be transported to the pouring position by a track transporter, lifted and poured by a track-laying machine, vibrated by an internal vibrator and plastered manually. After the concrete pouring reaches the strength for the first time 1 time, dismantle the formwork and rail row support frame. After concrete pouring, maintenance measures should be taken to prevent concrete cracking. Only when the concrete strength reaches more than 70% of the design strength can the rail car pass through. The rest of the concrete pouring can be arranged flexibly according to the progress of the project, but it should be completed before the installation of the contact rail. After the construction of all integral ballast beds on the working face is completed, the rail joints shall be welded. When 25m fixed-length rail is welded into 900~ 1500m long rail (unit rail), stress relief and locking are needed [3]. Welding, stress release and locking of rail joints are important links in jointless track construction, and are the key points of track construction quality control, which directly affect the safety and use function. In the construction schedule management, rail joint welding is the key line that affects the total construction period, and stress relief and rail locking can flexibly arrange the construction opportunity without affecting the total construction period. When the rail is welded into a long rail strip, the section where the rail welder passes is not affected, and the contact rail and its protective cover and ancillary facilities can be installed. Under normal circumstances, in order to achieve the node goal of "three-rail connection", the construction unit generally installs the contact rail and its protective cover first, and then installs the auxiliary facilities after the fine adjustment of the contact rail and the rail. After the construction of the whole project is completed, the supervision organization will conduct pre-acceptance after the construction unit has passed the self-inspection. The construction unit shall rectify the problems found in the pre-acceptance, provide the self-inspection report to the supervisor after the completion, and put forward suggestions for the completion acceptance and delivery to the operation unit to the construction unit after the re-inspection by the supervisor is qualified.
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