Liu Changhong

 

 

    Abstract: Under the premise of ensuring the operation of existing railway lines, deep foundation pit construction brings many difficulties to both train operation and deep pit work. This technology uses double-row piles with a capping beam and tensioned steel bars to protect the existing subgrade.

 

Keywords: capping beam   tensioning   row piles   protection

 

1. Introduction

 

    The technology of using row piles to protect subgrade is relatively mature, but the double-sided tensioned protection form has not yet formed a complete technology, especially given the many safety restrictions for existing railways. This technology completes deep foundation pit excavation on both sides of the line without affecting railway operation.

 

2. Features of Double-Row Tensioned Protection Piles

 

    Row pile protection operations are carried out outside the clearance limits on both sides of the existing line, reducing interference with train operation. The tensioned steel bars pass through the top of the existing subgrade, reducing disturbance to the existing subgrade, ensuring the safety of existing railway operation and the smooth progress of deep foundation pit excavation.

 

3. Construction Plan

 

3.1 Overall Plan

 

    The section of the Harbin Hashuang South Road Reconstruction Project crossing the Wangsun Up and Down Lines at K1+549.1 adopts an underpass bridge construction. The underpass bridge is divided into five sections A, B, C, D, and E, with a total length of 102.75 m. To maintain normal operation of the Wangsun Up and Down Lines, the Wangsun Up Line is moved to run parallel with the Wangsun Down Line at a spacing of 5 m, located above Section C of the underpass. On both sides of Section C, parallel and perpendicular protection piles (C30 reinforced concrete) are used to protect the railway subgrade, and then Sections A, B and D, E of the underpass are excavated.

 

3.2 Layout of Railway Subgrade Protection Piles

 

    To ensure normal operation of the Wangsun Up and Down Lines and timely completion of the underpass bridge, two rows of 37 protection piles each (74 in total) are arranged parallel to the line on both sides of Section C, with a spacing of 1.5 m, diameter of 1.2 m, and length of 20 m. A capping beam of length 57 m × height 1.5 m × width 1.2 m is cast on top of the piles. At the pile tops within the capping beam, 4 Φ32 round steel bars are embedded every 1.5 m, welded to the main reinforcement of the capping beam and the main reinforcement of the protection piles, and passed through the ballast of the Wangsun Up and Down Lines to be tensioned with the protection piles on the other side. At both ends of these two rows of protection piles, perpendicular to the line, protection piles of diameter 800 mm and length 20 m are arranged. Near the line, within the range of the subgrade slope of 1:1.5, double-row piles are densely arranged. The longitudinal center spacing of the densified piles is 1 m, while other parts use protection piles with longitudinal center spacing of 1.6 m and transverse center spacing of 2.5 m evenly arranged. The tops of the double-row piles are connected by a cap beam of width 3.7 m and height 0.8 m.

 

3.3 Preparatory Work

 

3.3.1 Technical Preparation

 

    1. Familiarize and review drawings and relevant design documents.

    2. Understand terrain, geology, hydrology, and other conditions.

 

3.3.2 Construction Site Preparation

 

    1. Removal of obstacles

    2. Construction survey and setting out

    3. Ensure “three connections and one leveling”

 

3.3.3 Preparation of Construction Materials

 

    Before excavation, make a usage plan for required construction materials, complete ordering procedures, material transportation and storage, so as to timely guarantee required machinery, equipment, materials, components, etc.

 

3.3.4 Labor Preparation

 

    Formulate and confirm labor demand plans, conduct three-level safety education for personnel entering the site, require special operation personnel to work with certificates, and provide project introduction and construction technical disclosure to construction personnel. Based on project characteristics, season, and surrounding environment, anticipate possible problems during foundation pit excavation and make emergency preparations. Based on construction experience, prepare for likely unforeseen situations.

 

    3.4 Construction Methods

 

    1. Protection Pile Construction

 

    (1) Luoyang shovel hole-forming process

    ① Positioning of Luoyang shovel: Check whether the central axis of the Luoyang shovel is vertical to the center of the pile hole. If not, move the two legs of the Luoyang shovel to make it vertical, and ensure the leg angle meets requirements.

    ② Hole forming with Luoyang shovel: The opening of the Luoyang shovel is centered on the wooden stake at the pile position, and any deviation is corrected promptly. The pile diameter formed by the Luoyang shovel is 1.2 m, with a deviation not exceeding ±20 mm. During hole forming, check verticality every 5 m and correct any deviation promptly.

    ③ Hole inspection: Make a hanging ring of diameter 1180 mm with steel bars. Lower it into the pile hole with a rope; if the ring reaches the bottom smoothly, the pile diameter meets requirements. Before concrete pouring, protect the pile hole against rain and cover it promptly to prevent falling accidents.

    (2) Quality control of reinforcement cage processing and installation

    During fabrication and installation, strictly follow specifications. During lifting of the reinforcement cage, use four-point double-hook slow lifting and gradually transfer points for lowering to prevent deformation of the cage. During lowering, gradually adjust the orientation of the reinforcement cage.

    (3) Concrete pouring construction

    Commercial concrete is transported to the construction site and poured using a tremie to ensure pile quality, and the pile top elevation is controlled to meet design requirements.

 

    2. Capping Beam Construction

 

    (1) Formwork

    Steel formwork is used for the capping beam. The formwork surface must be flat, dimensional deviations must meet requirements, and it must have sufficient rigidity, strength, and stability, with convenient assembly and disassembly and tight joints without grout leakage.

    After applying release agent and installing and reinforcing the formwork, check that the axis and elevation meet requirements, ensuring the formwork does not deform or displace under stress during concrete pouring. The inside of the formwork must be clean without debris, and the joints must be flat and tight.

    (2) Reinforcement fabrication and installation

    During installation, at each location, 4 Φ32 round steel bars for tensioning are embedded and connected to the main reinforcement of the capping beam and the main reinforcement of the protection piles, with an embedment length of 1 m, facilitating subsequent tensioning and welding of protection piles on both sides. The accurate position of the reinforcement must be ensured during embedment.

    (3) Concrete pouring

    C30 concrete is poured in layers with a thickness not exceeding 30 cm, compacted with an insertion vibrator, and promptly wrapped with plastic film and watered for curing at regular intervals.

 

    3. Connection of Tensioned Steel Bars

 

    Every 1.5 m along the capping beams on both sides of the line, 4 Φ32 round steel bars are used for tensioning. The Φ32 round steel bars pass through the ballast below the sleepers and above the subgrade, and the portion passing through the ballast is wrapped with plastic pipes, then welded to the reserved Φ32 steel bars in the capping beams on both sides.

 

    4. Foundation Pit Excavation

 

    Sections A, B and D, E of the underpass are two foundation pit work faces. Each pit uses 2 excavators and is excavated in three layers. Within 20 cm-30 cm from the protection piles, manual cleaning is used to prevent mechanical damage to the piles. After the concrete strength of Sections A and B reaches 100%, the second line transfer is carried out, and then Section C of the underpass is constructed.

 

    5. Line Maintenance

 

    After foundation pit excavation, assign dedicated personnel to monitor line conditions, equipped with line maintenance equipment, with monitoring personnel working in 24-hour shifts.

 

3.5 Foundation Pit Displacement Monitoring

 

    To ensure the safety of adjacent buildings during foundation pit construction, displacement monitoring of the foundation pit and adjacent buildings should be carried out throughout the construction process until backfilling is completed.

 

    1. Layout of monitoring points and control points

    (1) Displacement of the foundation pit wall: observation points, control points, and reference points are set according to safety level II.

    (2) Railway line horizontal displacement monitoring points: 4 points are set on the shoulders on both sides (2 per side), and one point every 19 m on the capping beam to monitor the stability of the railway subgrade.

    (3) At the top and bottom of the protection piles on both sides of the pit, a group of measurement points is arranged correspondingly (4 points per section) for horizontal and vertical monitoring of the pit and inclination monitoring of the protection piles.

 

    2. Monitoring methods

    Selected according to actual site conditions. The site has trapezoidal and parallelogram-shaped foundation pits, and the measurement methods are:

    (1) Settlement observation

    Use elevation observation to measure the settlement of the protection pile tops and capping beams, and whether uneven settlement is caused after foundation pit excavation.

    Instrument: DS3 level with specified accuracy, ring closure error calculation.

    (2) Inclination observation of protection piles

    Use vertical observation to measure the verticality of the protection piles.

    Instrument: plumb bob supplemented by steel ruler.

    (3) Horizontal displacement monitoring

    After measuring the coordinates of two monitoring points with a total station, perform coordinate inverse calculation to obtain the distance between the two points.

    (4) After each observation, submit observation data tables and displacement/settlement comparison tables.

 

    3. Observation frequency

    Before foundation pit support construction, initial value observation is carried out. During construction excavation, observe once a day. When sudden settlement or external environmental changes such as rainfall occur, increase observation frequency. After foundation pit excavation is completed, the number of observations can be adjusted according to actual conditions until backfilling is completed.