1. 1 project introduction

Renhe Cableway Bridge is located in Maoba Town, Ziyang County, Ankang City, Shaanxi Province. A temporary bridge, with a total length of 220 meters and a span of 200 meters, was built for the construction of Maoba extra-long tunnel in MC2 contract section of expressway test section at the junction of Ankang and Sichuan and Shaanxi.

Maoba Tunnel is located on the right bank of Renhe Reservoir. The reservoir is like a natural moat, which blocks all the materials and equipment under construction from the provincial road S3 10 to the other side. There are mountains and hills on both sides of the strait. The cross section of the bridge body at the proposed bridge site is a U-shaped valley, and the natural slope of the mountains on both sides is 40~50 degrees, mostly rocks. The exposed strata in the bridge site area are thin to medium-thick microcrystalline limestone and carbonaceous argillaceous slate of the Cambrian Maobaguan Formation, and the rock mass is hard.

On the left bank of the temporary bridge, the end of the main cable is directly anchored to a certain depth in the rock, and on the right bank, the anchor cable is embedded in the bedrock. The anchor cable is made of the same material as the main cable. One end of the anchor cable is anchored to a certain depth in the rock after treatment, and the other end is connected with the main cable through a connector. The main cable consists of 46 roadway cables, sidewalk cables and stabilizing cables, including 28 roadway cables, 6 sidewalk cables and stabilizing cables 12. The main cable is φ 48-6 * 37+FC- 1770, and the whole bridge is equipped with two sets of wind cables * * φ 24-6 * 37+FC- 1770. A cross of 70mm, 24 * *, and transverse width 1770 shall be set every 8 meters in the whole bridge.

1.2 main technical standards

1.2. 1 Vehicle load: 50T single vehicle, limited to one vehicle;

1.2.2 crowd load: when vehicles pass, there are no more than 50 people on the bridge;

1.2.3 Bridge deck width: carriageway width is 4.5m, and sidewalks on both sides are1.1m respectively;

1.2.4 speed:10 km/h;

1.2.5 design service life: 3 years;

1.2.6 sag of main cable under heavy load: take span 1/37, that is, 200/37 = 5.405m 。

2 ropeway bridge construction characteristics

Cableway bridge is a semi-permanent bridge, which can be used as both an ordinary bridge and a construction bridge. It has the characteristics of low cost and fast construction. It is suitable to be erected on rivers where other types of bridges are difficult to erect, such as high mountains and steep slopes, acute water depth, many floating objects and large sediment erosion at the river bottom. Play its unique role, so it is more suitable for the temporary construction of civil transportation, hydropower system and engineering in southwest China.

The cableway bridge is divided into five parts according to the different functions of each part, namely, anchorage, abutment, connecting and adjusting components, load-bearing structure and deck system.

Anchorage is the most critical part of cableway bridge, bearing the weight of the whole bridge. Anchors can be divided into gravity anchors and ground anchors according to their mechanical characteristics. Gravity anchor anchors the steel cable into the mass reinforced concrete poured on both sides, mainly relying on the self-weight of concrete, and the amount of abutment masonry is large. Ground anchor is to anchor the steel cable into the mountains on both sides of the river, and rely on the anchoring force of bedrock to bear the tension of the steel cable, which requires good rock quality on both sides. The ground anchor method is adopted for the people and ropeway bridge because the bedrock on both sides of the mountain is good.

3 Key technologies of cableway bridge construction

The construction technology of cableway bridge is mainly based on its own characteristics of low cost, short construction period and convenient construction. According to the actual situation on site, organize and optimize the construction scheme in time, emphasize the scientificity and rationality of interspersed construction, and give full play to its own characteristics.

The main working procedures of cableway bridge include excavation of abutment and ground anchor pit, reinforced concrete construction of abutment, anchorage construction, main cable tensioning and deck installation. The main technologies of each process are described as follows.

3. 1 Main excavation techniques for abutment and ground anchor pit

Before abutment excavation, it is necessary to further accurately measure the axis of the bridge, the axis of abutment on both sides of the river and the excavation line of ground anchor, establish a survey control network, set up permanent leveling points, and carry out survey control at any time during excavation.

Large blasting is strictly prohibited in the excavation of abutment foundation and ground anchor pit, especially in ground anchor pit, presplitting blasting or smooth blasting should be adopted to ensure the bearing capacity of bedrock, and overbreak of ground anchor pit is strictly prohibited. The abutment foundation should be cleared of the covered weathered layer, so that the abutment body is placed 0.5~ 1m below the intact and stable rock stratum.

3.2 Key points of abutment reinforced concrete construction technology

The abutment of Renhe cableway bridge is made of C20 plain concrete, the cap is made of C25 reinforced concrete, and the anchor pad is made of C30 reinforced concrete.

Before concrete pouring, check whether the position, size and elevation of the basement plane conform to the design drawings; Determine whether the foundation soil can ensure the stability of abutment; The basement is clean without water and sundries; Check whether the embedded parts are installed firmly and accurately.

Concrete is mixed in the mixing station and poured continuously in layers. The pouring thickness of each layer is controlled at about 30cm, and it is vibrated with built-in vibrator. Vibrating rods are inserted into the lower concrete by 5cm, and the insertion spacing is less than the action radius of vibrating rods. Vibration leakage and heavy tamping are not allowed. When vibrating, observe that the concrete no longer sinks and the surface is flooded. When the level is shiny, you can slowly pull out the vibrator. Pulling out too fast will lead to voids in concrete. After pouring is completed, cover it in time and water it for curing.

3.3 Key points of anchorage construction technology

The ground anchor method is adopted for the anchorage of this project, and its main working procedures include drilling, anchor cable anchorage section treatment and anchor cable grouting.

3.3. 1 drilling precautions

When DTH percussion drill is used to drill holes, the hole position, hole diameter, hole depth and angle direction must be constructed in strict accordance with the design. Scaffolding shall be erected for the drilling rig to walk and place according to the site topography, and the drilling depth shall exceed the designed length of anchor cable by 0.5m, so as to prevent insufficient hole depth caused by debris settlement at the bottom of the hole. According to the drilling situation and the blown rock powder, make detailed construction records, including lithology, rock mass integrity, groundwater, cracks, rock powder color, particle size, quantity, hole depth, accidents, etc. When drilling, high-pressure wind should be used to blow the hole frequently, and the accumulated layer (hole depth less than 6 meters) can be protected by filling cohesive soil, and the hole can be swept up and down to prevent the hole from falling and blocking.

After drilling, pull out the drill bit, check the hole depth with polyethylene pipe, clean the hole with high-pressure air, then pull out the polyethylene pipe and block the orifice with cement paper.

Handling of special circumstances:

Water seepage: when blowing holes, all large particles are blown out without dust, indicating that there is water seepage in the holes and rock powder has adhered to the hole wall. At this time, if the hole is deep enough, inject clean water and wash the hole wall with high-pressure air to make the hole wall clean. If the hole depth is not enough, in order to prevent rock powder from blocking the drill pipe, stop drilling immediately, pull out the drill pipe, and then drill after washing the hole. Sometimes there is a lot of water seepage in the cave, and mud and gravel are sprayed out. As long as the impactor works, drilling can continue. If the water inlet of the impactor doesn't work, pressure grouting should be taken to block the water before drilling.

Hole collapse: When drilling meets strongly weathered rock stratum or rock mass fracture zone, hole collapse often occurs. The sign of hole collapse is that rock powder is blown out of the hole, undisturbed, not crushed by drill bit, stale and dull. At this point, stop drilling immediately, pull out the drilling tool and grout the borehole wall. Before grouting, clean water and high-pressure wind are used to wash the mud in the hole, and the grouting pressure of cement-glass dual-liquid grouting is 0.4MPa, and then drilling is carried out again after 24 hours.

3.3.2 Matters needing attention in handling anchorage section of anchor cable

The anchorage section of steel cable can be made on site or transported after prefabrication in other places. Firstly, according to the specific depth of the anchor hole and the length of the anchor section, the steel wire rope in the anchor section is divided into strands, and the oil-soaked hemp core in the steel wire rope is removed. After splitting, the oil stain on the surface of steel strand shall be scrubbed clean with fine sand, and wiped clean with homespun before paying off the cable. Pay attention to the oil pollution and be sure to clean it.

Within the scope of the anchorage section, a centering isolation bracket is worn every 0.72m to form an anchorage bundle, and a certain gap is left between the steel hinge lines to ensure the grouting can be compacted. A fastening ring iron wire is tied every 0.24m to form a corrugated anchorage section. Finally, add a guide cap on the end cover of the anchor cable, and hang a label according to the hole number for storage after the production is completed.

3.3.3 Key points of down-cable grouting technology

Before lowering the anchor cable, clean the hole again with high-pressure air in the hole, and then put the anchor cable in. The locating piece on the anchor cable sheath centers the anchor cable to prevent the anchor cable from twisting and bending. Grouting pipe should enter the tunnel with the anchor cable. When the grouting pipe head is 5 ~ 10 cm away from the bottom of the hole, stop advancing when the anchor positioning retaining ring reaches the orifice, and check whether the exhaust pipe is unblocked again.

The cement slurry is pure cement slurry, the cement strength is not less than 42.5Mpa, and the water-cement ratio is determined by experiments. Exhaust grouting is used for grouting, slurry is injected from the bottom of the hole, and air is discharged from the anchor hole. The slurry mixer should stir evenly, the slurry should be used up before initial setting, and no sundries should be mixed. Grouting should stop after the anchorage section is filled. Only after the cement slurry meets the design requirements can the steel wire rope be tensioned.

3.4 Main Cable Tension Technical Points

The main cable is the load-bearing component of the whole bridge. Before the main cable is tensioned, the cable should be tested, and the main cable can be connected only after it is qualified. Tension-type connection regulator is welded by combined steel and processed in the factory. The two ends of the regulator are connected with the main cable through pulleys, and the middle is connected with two long screws. Tension jack can be used to adjust the tightness of main cable.

Each main cable is towed from the left bank abutment to the right bank abutment by a simple cable crane and catwalk. First, connect the main cable with the anchor cable on the right bank with the connection regulator, and pretension the main cable with the winch system and pulley block on the right bank to make the empty vector of the main cable slightly smaller than the design vector, and clamp the cable clamp.

Empty cable vector is one of the key control parameters of ropeway bridge, which is obtained by calculating the minimum temperature design load. The sag of the main cable is completed by using the connection regulator located on the right bank. When the main cable is laid halfway, the sag adjustment of the main cable can be started. After adjustment, the following requirements must be met: first, the main cable vector is equal to the calculated value; Second, the elevation deviation of the same section of the main cable is not greater than15 mm; ; Third, the main cable vector should be in a relatively stable state after adjustment. The above general requirements can only be met after adjustment for more than five times.

3.5 Key points of bridge deck installation technology

3.5. 1 Key points of steel beam installation technology

Steel beams play a balance and stability role in cableway bridges. At the same time, the lane cable, the sidewalk cable and the stabilizing cable are connected together, so that the main cable is uniformly stressed. The steel beam structure of Renhe cableway bridge is mainly welded by 16Mn steel plate into I-shaped section, the section size is 500mm×200 mm, the length of steel beam is 13 m, and one steel beam is set every 8m on 200m span.

The technical difficulties in the installation and construction of steel beams lie in the hoisting and distribution of steel beams and the connection between steel beams and main cables. The construction method we adopt is to hang the steel beam on the stable steel cable with simple support and hoist respectively, locate it with φ 18.5 mm steel cable according to the designed spacing, and then pull the beam from the abutment to the corresponding position in turn. Fine-tune the bridge deck after installation, and tighten the connecting bolts after the installation adjustment is qualified, so that the steel beam and the main cable become a whole.

3.5.2 Key points of bridge deck installation technology

The deck board of Renhe cableway bridge is made of imported deciduous pine, which is processed on site and used after anti-corrosion treatment. It is strictly forbidden to have decayed, oversized bug eyes, edge skins, etc. The bridge deck installation sequence is from the right bank abutment to the left bank abutment according to the actual situation; The bolt holes on the deck slab should be the same as the bolt diameter or according to the design requirements; The bridge deck on the rigid beam should choose better wood; After the bridge deck is installed, the transverse direction of the bridge deck should be kept horizontal, and its transverse slope should not be less than 2 degrees, otherwise the length of the main cable should be readjusted.

4 some experience

Renhe Cableway Bridge is a temporary bridge for the construction of Maoba extra-long tunnel in MC2 contract section of expressway experimental section at the junction of Ankang and Sichuan and Shaanxi. It is the land transportation channel for Maoba tunnel construction. Construction started on April 2007 10 and was completed on May 2007 16. It took only 37 days from the excavation of abutment and ground anchor pit to the construction and opening to traffic. On May 9, 2007, the design, supervision and construction units jointly carried out a continuous load test on the temporary bridge. Under the design load of 50 tons, the sag of the main cable is 5.340 meters, which is less than the allowable value of 5.405 meters. Other indicators have also met the design requirements and passed the acceptance test successfully. The monitoring and measurement results show that the system runs well and plays a key role in the material transportation of Maoba tunnel construction. In the process of carrying out the construction task of cableway bridge, I have the following experiences:

4. 1 cableway bridge is characterized by simple structure, short construction period and high technology content. When constructing in mountainous areas, steep cliffs, deep valleys and rapids are often encountered. Therefore, when designing the construction technical scheme, we should fully consider the influence of geological conditions on the construction, especially on the excavation quality of abutment and ground anchor foundation pit, and take corresponding technical preventive measures.

4.2 The key technology of cableway bridge construction lies in the installation method and technical measures of main cables. The operation parameters of cableway bridge are mainly influenced by the installation parameters of main cables. When installing the main cable, it is necessary to track and measure the change of empty cable vector and empty cable vector. Especially the vector value of empty cable directly affects the sag parameters of bridge. Set up tracking measurement near the abutment, use level and total station to measure and analyze the results.

4.3 The installation speed of main cables and steel beams depends on scientific installation technology. During the construction of this project, the installation of the whole superstructure was completed in only 18 days.

4.4 In today's engineering construction management, the owner, design, supervision and construction unit are responsible for each other and cooperate with each other. According to the actual situation on site, multi-party consultation and scientific decision-making are the keys to fast and good engineering construction.