Pipe jacking is an underground pipeline construction method developed after shield construction. It does not require excavation of the surface layer, and can cross highways, railways, rivers, ground buildings, underground structures, and various underground pipelines. Water supply and drainage, gas, electricity, communication and other pipelines are increasingly used in the construction of pipelines. However, the construction equipment, technology, and management level of the pipe jacking construction unit are uneven, and some problems often occur during construction, which affects the construction quality. This article takes the pipe-jacking construction with mud-water balance method as an example, analyzes the problems and reasons often encountered in pipe-jacking construction, and proposes some prevention measures. I. Characteristics and technology of pipe jacking construction
1. Construction characteristics of pipe jacking
(1) Road closure is not required for advanced pipe jacking. The selected working wells and receiving wells are located at idle places, and traffic on the road will continue as usual during jacking.
(2) No excavation, backfilling and pavement repair are needed, so compared with large excavation construction, the total project cost can be reduced.
(3) The mud-water circulation method construction avoids the adverse phenomena of mud-water overflow and muddy pile-up, and ensures that the natural environment is not polluted and can achieve civilized construction.
(4) The advanced pipe jacking method is fully automatic remote control, which does not require workers to work in the pipeline, which not only eliminates the heavy physical labor of the workers, but also provides sufficient guarantee for the safety of the workers.
(5) During the advancement of advanced pipe jacking, the excavator and the surrounding area of the pipeline are between active earth pressure and passive earth pressure, so there is very little interference with the original soil, so it will not cause the road surface to settle or crack, and avoid road surface repair.
2. Pipe jacking construction process
Pipe jacking technology is a kind of pipe laying technology. There is no need to dig the ground. A hydraulic jack with large thrust can be used behind the pipe jacking machine with remote control device, so that the tunneling machine and the pipeline that follows it can pass through the soil layer. To reach the pre-designed position, this is called the pipe jacking project. Excavation occurs in front of the pipe jacking machine, and the excavated material is discharged by the pump through the mud circulation system and reaches the surface. The basic principle of pipe jacking construction is: excavate two foundation pit wells from the ground, then place the pipe joints from the working well, and push the pipe joints from the work well through the jacking of the main jack or relay jacking machinery. Pass through the hole, pass through the soil layer to the reserved mouth of the receiving well, and then pass through the reserved mouth of the receiving well to form the construction of the pipeline.
At present, the construction methods commonly used in pipe jacking are divided into open manual hand-dug and sealed mechanical pipe jacking methods. Among them, the commonly used construction methods for mechanical pipe jacking are mud water balance and earth pressure balance. Pipes commonly used in pipe construction include concrete pipes, steel pipes, and glass-sand-jacketed steel pipes.
At present, the commonly used pipe jacking construction method is the mud-water balanced pipe jacking construction method. Its construction principle: the soil, sand, and stone are broken by the turning function of the steering head. Water is injected from one steel pipe, mixed into a slurry, and the slurry is sucked out by another steel pipe. , Put the slurry in a centrifuge to dewater and centrifuge, then unload the dry soil to the place where the dump truck transported the waste soil, and the separated water is returned to the water storage tank for repeated use. The main equipment used in the construction is information technology and fully automatic mud-water balance pipe jacking machine.
Common problems encountered in pipe jacking construction and prevention measures
1.The pipeline axis deviation is too large
The deviation between the pipe jacking axis and the design axis is too large, which will cause the pipe to bend, and even cause damage to the pipe joints and leakage of the joint.
(1) Cause analysis.
① The frontal resistance of the formation is not uniform, which causes the tool tube to be subjected to uneven force and forms a guide deviation.
② Displacement or unevenness of the back of the jacking pipe causes the jacking force line of force to shift.
③ The jacks are not synchronized, or the jacks have a large difference in jacking force, or the installation accuracy is not enough, resulting in deviation of the jacking force resultant line.
(2) Preventive measures.
① Before the construction of the pipe jacking, the geological situation of the pipeline passing zone shall be investigated carefully, and a force measuring device shall be set up to guide the correction. Correction should be carried out in accordance with the operation methods of regular measurement, regular correction and small amount of correction.
② Use the same type of hydraulic jack to make the jacking force, stroke and speed consistent, and keep the jacking force line and the center line of the pipeline coincide.
③ Strengthen the control of the construction quality of the back of the pipe jacking to ensure that the back does not shift, and the back should be flat to ensure the installation accuracy of the jacking equipment.
④ The jacking curve should be drawn at any time during the jacking process to guide the jacking correction.
2. Ground subsidence and uplift
During or after the construction of the pipe jacking, ground subsidence or uplift occurs within a certain range on both sides of the pipeline axis, which affects the buildings around the pipeline and public facilities such as road traffic and pipelines, and even endangers normal use and safety.
(1) Cause analysis.
① Settlement and uplift caused by unbalanced earth pressure on the front of the roadheader. From the principle of the earth pressure balanced roadheader, when the roadheader is fully cut and stirred, a soil body with greater plasticity and fluidity is formed in the earthen bunker. When the frontal earth pressure is controlled between passive earth pressure and active earth pressure, the ground will sink or swell. In fact, due to the large changes in soil quality, there are often large differences in the control based on theoretical calculations, which cannot be correctly grasped, resulting in unbalanced soil pressure and settlement. In addition, due to the inadequate adaptability of some earth pressure balancing roadheaders to the soil, for example, the cutting area of the cutter head is too small, the advance speed or the rotation speed of the screw conveyor cannot be adjusted, which makes the earth pressure control unfavorable or inconvenient, resulting in earth pressure instability and settlement With bulge.
② Settlement and uplift caused by the peripheral space of the pipeline. The peripheral gap of the pipeline is caused by the rectification or curve advancement of the roadheader, because the section area of the pipeline formed during the correction and curve advancement is larger than that of the pipeline, and the gap is filled by the surrounding soil to cause settlement. Currently, thixotropic mud friction reduction technology is commonly used in pipe jacking. The outer diameter of the roadheader is 2 to 3 mm larger than the outer diameter of the pipe in order to form a slurry jacket. If the grouting is not timely, it will cause settlement.
③ Settlement and uplift caused by friction between pipeline and surrounding soil. When the pipeline is advancing, there is friction with the surrounding soil. This friction often causes shear disturbances in the soil, causing the soil to move and causing ground subsidence. In the case where the shape of the pipe section is not uniform, the interface is uneven, or the pipeline is not straight, the shear disturbance will be aggravated, which will increase the ground subsidence and uplift.
④ Settlement and uplift caused by leakage at pipe joints. When the sealing ring of the pipe joint is improperly installed or damaged by uneven force at the pipe end, and the sealing of the pipe joint is excessively caused by poor sealing, the joint leakage is more likely to occur, and the soil and soil will be lost. This loss of soil layer will definitely cause the ground Settlement and uplift. In addition, the leakage of the pipe interface also causes the loss of thixotropic mud, which greatly reduces the effect of supporting the soil and reducing friction, and may also cause the settlement and uplift of the two reasons mentioned above.