Liu Changhong

 

 

Abstract: Under the premise of ensuring the traffic on the existing railway line, deep foundation pit construction brings many difficulties to the existing line traffic and deep foundation pit operation. This technology adopts double-row piles with crown beams at the top and uses reinforced tie rods to protect the existing subgrade.

 

Keywords: crown beam   tie rod   row pile   protection

 

1. Introduction

 

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

 

2. Characteristics of Double-Row Tie Rod Protection Piles

 

Row pile protection operations are carried out outside the clearance limit on both sides of the existing line, reducing interference with traffic. The tie rods pass through the top of the existing subgrade, reducing disturbance to the existing subgrade, ensuring the traffic safety of the existing railway and the smooth progress of deep foundation pit excavation.

 

3. Construction Plan

 

3.1 Overall Plan

 

The section of Harbin Hashuang South Road Reconstruction Project crossing 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 102.75m. In order to maintain normal operation of Wangsun Up and Down Lines, the Wangsun Up Line is moved to run parallel with the Wangsun Down Line at a line spacing of 5m, located above underpass section C. On both sides of underpass section C, parallel and perpendicular line protection piles (C30 reinforced concrete) are used to protect the railway subgrade, and then underpass sections A, B and D, E are excavated.

 

3.2 Layout of Railway Subgrade Protection Piles

 

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

 

 

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 mechanical equipment, materials, components, etc.

 

3.3.4 Labor Preparation

 

Formulate and determine labor demand plan, conduct three-level safety education for incoming personnel, special types of work must hold certificates, and introduce project conditions and construction technical disclosure to construction personnel. According to project characteristics, season, and surrounding environment, anticipate possible problems during foundation pit excavation and make emergency preparations. Based on construction experience, prepare for easily occurring unforeseen situations.

 

3.4 Construction Method

 

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 vertical, move the two legs of the Luoyang shovel to make it vertical, and the leg angle meets requirements.

② Hole forming with Luoyang shovel: The opening of the Luoyang shovel is centered on the wooden pile at the pile position, and timely correction is made if there is deviation. The pile diameter formed by the Luoyang shovel is 1.2m, with deviation not greater than ±20mm. During hole forming, check verticality every 5m and correct deviations promptly.

③ Hole inspection: Make a lifting ring with a diameter of 1180mm using steel bars. Lower it into the pile hole with a rope; if the lifting ring smoothly reaches the bottom of the pile, 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, gradually reverse points and lower, and do not deform the reinforcement cage. During lowering of the reinforcement cage, gradually adjust its direction.

(3) Concrete pouring construction

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

 

2. Crown Beam Construction

 

(1) Formwork

The crown beam adopts steel formwork. The formwork surface is required to be flat, dimensional deviations meet requirements, with sufficient stiffness, strength, stability, and convenient assembly and disassembly with tight joints and no grout leakage.

After applying release agent, installing and reinforcing the formwork, check that the axis and elevation meet requirements to ensure the formwork does not deform or displace under force during concrete pouring. The inside of the formwork is clean without debris, and the assembly is flat and tight.

(2) Reinforcement fabrication and installation

During installation, connect and embed 4 Φ32 round steel bars at each tie rod location with the main reinforcement of the crown beam and the main reinforcement of the protection piles. The embedment length is 1m to facilitate later welding of tie rods between protection piles on both sides. The accurate position of the reinforcement must be ensured during embedding.

(3) Concrete pouring

C30 concrete is poured in layers with a thickness not exceeding 30cm, compacted with an insertion vibrator, and immediately wrapped with plastic film and regularly watered for curing after completion.

 

3. Tie Rod Connection

 

The crown beams on both sides of the line are tied every 1.5m using 4 Φ32 round steel bars. 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 crown beams on both sides.

 

4. Foundation Pit Excavation

 

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

 

5. Line Maintenance

 

After foundation pit excavation, assign dedicated personnel to monitor line conditions, equip with line maintenance equipment, and monitoring personnel work 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 entire foundation pit construction process until backfilling is completed.

 

1. Layout of monitoring points and control points

(1) For foundation pit wall displacement, observation points, control points, and reference points are set according to safety level II.

(2) For railway line horizontal displacement monitoring points, 4 points are set on both side shoulders (2 points on each side) and one point every 19m on the crown beam to monitor the stability of the railway subgrade.

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

 

2. Monitoring methods

Select according to the actual site conditions. The site is a trapezoidal and parallelogram-shaped foundation pit, and the measurement methods are:

(1) Settlement observation

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

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

(2) Protection pile inclination observation

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

Instrument: plumb bob supplemented with 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, displacement and settlement comparison tables.

 

3. Observation frequency

Initial value observation is carried out before foundation pit support construction. During construction and excavation, observe once a day. When sudden settlement changes and external environment 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.