Pipe-curtain method is a new type of underground excavation technology which uses pipe jacking machine to build large-section underground space. Its principle is that on the basis of single steel pipe jacking, the steel pipes are connected through the lock mouth, and water-stopping agent is injected at the lock mouth to form a sealed water-stopping curtain. When using the pipe curtain method, because the excavation of soil or box culvert is carried out under the protection of the pipe curtain, the ground settlement can be significantly reduced and the stability of the excavation surface can be increased during construction. At the same time, because the pipe curtain has the function of isolating groundwater, there is no need to reduce the groundwater level during construction. The jacking construction technology of box culvert by pipe curtain method and the box culvert propulsion construction technology in soft soil area are discussed in detail.

A, pipe curtain jacking construction technology

(A) technical measures for pipe jacking in and out of the hole

1, soil reinforcement

During construction in soft soil area, because the soil layer where the pipe curtain section is located is basically plastic soil with high water content and low strength, in order to ensure the stability and waterproof requirements of the hole when the pipe curtain steel pipe enters and exits the hole, to avoid the influx of soft soil and groundwater into the working well, resulting in surface subsidence, and to increase the stability of the soil behind the starting well, soil reinforcement measures are needed. The requirements for soil reinforcement are:

The unconfined compressive strength of (1) reinforced improved soil should be greater than 1.2MPa, but not too high, so as not to cause excessive jacking force and affect jacking accuracy.

(2) The reinforcement range shall be considered as 3m. In the process of reinforcement, the surface should be tracked and monitored, and the surface uplift should be controlled within 20 mm

2, pipe jacking hole-forming technical measures

(1) Tunnel construction

Before leaving the tunnel, observation holes must be opened to ensure that the water-stopping performance of the reinforced soil meets the design requirements, and to prevent mud from pouring into the tunnel due to poor reinforcement effect. If the soil reinforcement can't meet the design requirements, it needs to be reinforced twice, and the pipe jacking can only be carried out after the safety is confirmed.

In order to prevent the phenomenon of kowtowing when the roadheader goes out of the hole, the following measures can be taken: adjust the resultant force center of the main jack in the back seat, encrypt the deviation measurement when the roadheader goes out of the hole, and correct the deviation with the jack in the back seat once the tendency of kowtowing is found.

(2) the hole water stop device

In soft clay and muddy clay areas, groundwater is also the main factor affecting the construction of pipe curtain. In areas with high groundwater level, in order to avoid water gushing and sand gushing in the process of propulsion, impervious propulsion head should be selected as far as possible, or water-stop grouting should be carried out during propulsion. In order to effectively prevent groundwater and lubricating mud from flowing into the working well, it is necessary to set up an effective water stop structure at the entrance.

3, pipe jacking into the hole technical measures

When the nose is about to reach the receiving well, it is necessary to accurately measure the attitude position of the nose, and try to meet the requirement that the rubber flange is concentric with the nose. After the pipe jacking, connect the first pipe jacking with the tail and the steel ring embedded in the borehole wall and seal it.

(2) Prevention and control of pipe curtain leakage

Although polymer slurry is injected between the pipe curtain interfaces before excavation, the pipe curtain interfaces may still leak with excavation. At this point, according to the location of the leakage point, measures should be taken respectively:

(1) The leakage point is in the hollowed-out part: at this time, drill holes in the joints with an electric drill, and directly inject oil-soluble polyurethane under pressure. It can also be blocked with steel plates and then injected with polyurethane.

(2) The leakage point is in front of the excavation site: send someone to enter the steel pipe by car and inject polyurethane slurry at the predetermined position. If it happens on the side wall and the depth is large, grouting can be carried out on the ground vibrating tube as appropriate. It is worth pointing out that in order to protect the ground greening, it is not allowed to inject cement slurry into the soil above the pipe curtain.

(3) Drag reduction of thixotropic mud

In jacking construction, the application of thixotropic mud is an important measure to reduce jacking resistance. During jacking, a certain amount of drag-reducing mud is pressed into the outer wall of the pipe through the grouting holes reserved at the hinge and pipe joint of the pipe jacking machine, and a mud jacket is formed around the pipe to reduce the amount of drag-reducing mud pressed into the outer wall of the pipe joint, thus reducing the jacking force during jacking. The formation of mud jacket is directly related to the effect of drag reduction.

(4) Steel pipe welding

In order to improve welding quality and welding speed, automatic welding technology can be adopted, supplemented by manual welding, and automatic welding is the main method.

Due to the thick steel pipe of the pipe curtain, there is a single-sided groove at the head and no groove at the tail of the pipe joint, so that the stress area of the pipe joint will not be reduced when it is pushed in, and the groove is 45.

After the pipe joint is hoisted into the well, it should be spliced first. When splicing, weld several "legs" at the tail of the previous pipe joint. By driving the iron wedge into the leg, the head of the pipe joint of the latter section can be reshaped and connected with the tail of the previous section. When splicing, there should be a gap of 1 ~ 2 mm between the front and rear pipe sections, which is conducive to firm welding.

(5) Treatment measures after pipe curtain jacking

(1) Replace thixotropic mud.

When two adjacent steel pipes are jacked, the thixotropic mud is replaced. The replacement material is pure cement slurry to have sufficient connection strength. The replacement process is carried out in turn, and the thixotropic mud is completely discharged into the steel pipe through the reflux hole. During replacement, the grouting pressure and grouting quantity of cement slurry must be strictly controlled.

(2) the steel pipe lock mouth water stop processing

In order to prevent the box culvert from leaking slurry when jacking, after the thixotropic slurry replacement is completed, polyurethane slurry is injected into the lock mouth by using the reserved grouting hole. At the same time, enough tracking grouting holes should be reserved for secondary grouting of local leakage points when the box culvert is jacked in.

(3) Concrete filled steel tube

Before the box culvert is jacked, the steel pipe of the pipe curtain shall be filled with concrete without shrinkage and vibration, so as to increase the longitudinal stiffness of the pipe curtain and avoid local stress concentration and yield.

(4) Connection between steel pipe end and working well

After pouring concrete, the steel pipe mouth is welded and closed, and then the steel plate and the reserved steel plate of the continuous wall are welded into a whole.

Second, the box culvert propulsion construction technology

The jacking construction of large section box culvert in soft clay is difficult, and the pipe curtain jacking construction scheme can be adopted according to the specific engineering conditions.

At present, the jacking construction methods of large-section box culvert widely used abroad mainly include FJ method and ESA method. FJ construction method connects the box culverts on both sides through steel strands, and alternately pulls the box culverts on both sides through rear series core oil cylinders or relay jacks, or sets reaction wall to install steel strands, and one side of the box culverts is pushed and pulled; ESA construction method depends entirely on the reaction force of box culvert itself.

The main characteristics of the above two construction methods are digging first and then advancing, so the soil in front must be reinforced to maintain the stability of the working face. At the same time, when the box culvert is advanced, it is necessary to excavate the pilot tunnel and lay the track. It also takes a lot of space to make box culvert; But there is no need for a reaction back and a reaction frame; The box culvert is pushed in sections, so the total propulsion force is small, the number of jacks and steel strands is reduced, and the reliability is high.

Box culvert jacking method is widely used in China, with simple technology and relatively mature technology, but the abutment soil needs to be strengthened and the abutment structure is prone to instability. When a large prefabricated site cannot be provided due to road restrictions; At the same time, due to the project cost, the soil in the pipe curtain is not reinforced and the pilot tunnel cannot be constructed. At this time, there are four schemes for box culvert: propulsion method, traction method, push-pull method and propulsion method with relay room.

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