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Construction Process Control Of Trenchless Pipe Jacking Mud-water Balance Method

July 27, 2022

With the acceleration of the urban process in my country, the construction scale of underground pipelines is getting larger and larger, and the awareness of environmental protection of the whole people has been enhanced. The pipe jacking technology plays an increasingly important role in the construction of underground pipelines. It can pass through urban roads, ground buildings, underground structures and various underground pipelines without excavating a large area of ​​the ground. It has the characteristics of small size and minimizes the disturbance to residents and traffic disturbance, which is very suitable for the complex hydrogeological conditions in urban construction and the construction under the condition that the excavation is restricted by the external environment.

1. Engineering overview and hydrogeological conditions

Binhe Road Sewage Pipe Jacking Project is located on Binhe Road on the east side of Liuyang River, Furong District, Changsha City. There are bridge pier pile foundations and numerous underground pipelines around the location where the sewage pipeline passes through. There is a 10,000-volt high-voltage pole at the meter. The construction site is only 40m away from the Liuyang River. The geological survey report clearly states that the river water has a certain hydraulic connection with the groundwater in the site, showing a mutual replenishment relationship, with a slight pressure bearing, mainly occurring in the round gravel, and the water volume is relatively abundant. Due to the influence of climate change and atmospheric precipitation, the water level is stable between 3 and 7.5m below the ground, while the sewage pipeline is located at a position of 5 to 6m above the ground, which is in the groundwater affected area. .

2. Project Implementation

2.1 Track Control

(1) Initial jacking
The pipe jacking machine is the first to enter the soil, and the special deviation correction section is spliced ​​at the tail after the entry. During the initial jacking, the uniformity of the excavation should be ensured, and the initial deviation should be strictly controlled. At the same time, the initial deviation should be effectively controlled by adjusting the center of the resultant force of the rear seat jack, so that the initial state of the machine head remains stable and the axis remains straight.
(2) Anti-twist during jacking
On the other end where the equipment and the pipe are installed, a counterweight with the same weight should be configured to ensure that the weight on the left and right sides of the pipe can be balanced during the jacking process of the pipe. Also, avoid twisting due to human factors as much as possible.

2.2 Underground control measures

(1) Set up an exhaust pipe in the pipe, and transport several axial fans to the wellhead by means of belt relay. If the alarm device shows a display, it means that there are toxic and harmful gases in the pipe, and the exhaust system should be opened immediately, and the pipe Toxic and harmful gases in the purifier are discharged, and at the same time, the program-controlled telephone is used to contact the gas supply room to increase the actual supply of purified gas.
(2) Build a perfect hot-fire system. On-site operators are not allowed to use open flames at will, and a certificate should be issued before using them. At the same time, there should be full-time safety officers on site to supervise and manage, and there should be no smoking.
(3) If the excavated soil contains biogas, and the area is large and the concentration is high, the construction should be suspended immediately, and a detailed and reasonable and feasible plan should be formulated in consultation with the technical personnel. implement.

2.3 Pipe jacking receiving well into the hole

When the roadheader and the opening of the receiving well are close to each other, the elevation and axis should be reviewed. After a complete review and confirmation, all relevant preparatory work should be done, such as setting up the receiving platform. After the tool head enters the well wall, close the grouting valve, remove the sealing door on the receiving well, and pass through the well wall when the pipe jacking is carried out. After it is completely on the shelf, make a permanent water stop.

2.4 Reduced formation movement

(1) Ensure the stability of the excavation surface
During the excavation and jacking process, effective anti-slump and anti-water gushing measures should be taken to keep the frontal soil stable, and at the same time, its original stress state should not be changed as much as possible.
The construction method used is a mud-water balance type. In order to balance the frontal soil, based on the properties of the soil, a retaining wall mud with satisfactory performance is applied in the sealed cabin. In the sealed cabin, the mud pressure should be controlled within the range of 1.0 to 1.1 times the static earth pressure, and at the same time, it should be adjusted in time according to the monitoring results of formation movement. Check and block all possible paths for slurry leakage. Focus on checking whether the tail of the tool tube is tight, so as to avoid the impact of slurry leakage on the pressure of the front slurry.
(2) Grouting in the gap around the jacking pipe
Due to the difference in the diameter of the tool tube and the pipeline, and when the tool tube is rectified, there will be gaps on the periphery of the pipeline, and grouting must be carried out during construction. In the process of grouting, the accuracy and adaptability of the point and pressure should be ensured, and correct and feasible methods should be adopted to ensure that all the gaps around the pipeline can be filled with the slurry. The grouting adopts thixotropic mud.
(3) Strictly control the eccentricity of the jacking force
Monitor at any time whether the compression of the joint joint during the advancing process is average, and calculate the stress distributed on the joint end and the eccentricity corresponding to the resultant jacking force according to the monitoring results. Based on this, the amplitude is corrected. Avoid pressure loss of pipe joints due to large eccentricity, or circumferential cracking in the middle of pipe joints, which makes it difficult to perform due support and lubrication, resulting in impossibility of propulsion and uneven settlement of the surface.
(4) Keep the back structure and soil of the pipe jacking pressure wall always in a stable state
On the basis of effectively ensuring the structural quality of the caisson, after the caisson is sunk to the designated position and the construction of the roof and floor is completed, the gap between the ground body and the side of the caisson should be filled, and the soil body behind the caisson should be filled. Check whether it is stable or not, and determine the ultimate jacking force in the jacking process accordingly, and strictly control the spacing between relays.
(5) Take effective sealing measures to prevent muddy water from entering the pipeline
Before starting the pipe jacking construction, the lap joint between the tool pipe and the pipe shall be checked to determine the annular rubber sealing device in the joint of each pipe joint and the joint between the relay and the joint end, the pipe joint itself and the pipe joint. Waterproofing of pre-embedded grouting holes. For reinforced concrete pipe sections, there should be a warranty. It is necessary to strictly prevent the collision of pipe joints during handling, including seam opening and pressure loss cracking that may occur during the deviation correction process.

2.5 Jacking Control Measurement

(1) Plane control
In order to ensure the continuity of the pipe jacking, the horizontal and vertical errors should not exceed 100mm, and ground wire points should be set near the wells at both ends. , and construct the corresponding plane control network. For the transmission of coordinate points from the uphole to the downhole, the connection triangle method is mainly used. The reference point used to control the jacking direction in the well can be embedded with a steel frame to form a fixed point, and the theodolite can be used to track and observe the plane deviation of the nose.
(2) Elevation control
According to the existing leveling points around the operation area, the second-level leveling line is laid, and the elevation pilot survey is carried out near the working well, and the elevation control point is set at the same time. Use a level and a leveling rod to carry out leveling measurements, and make round-trip observations. After the ground elevation is transmitted to the underground, hang it vertically with a steel ruler, tie a line hammer, and then observe the two levels on the ground and underground at the same time.
(3) Land subsidence observation
Use steel pipe to arrange the observation point at a distance of 10m from the Liuyang River embankment; the observation should be carried out every day during construction, and the settlement should not exceed 20mm.
(4) Pipe jacking attitude measurement
The push of the pipe jacking machine must be carried out according to the axis determined by the design, and the dynamic data of the pipe jacking should be observed in time, and the relevant parameters should be adjusted according to the observation results, so as to provide correct guidance for the pipe jacking push. A measurement target is set at the nose of the machine, and the 2” laser theodolite and DS3 level are located in the foundation pit to observe the measurement target, compare the observation results, and determine the correction value. The on-site operator corrects the deviation according to the tilt angle of the nose.

3. Conclusion

The restoration construction adopts the form of non-excavation, which has no impact on road traffic, and effectively prevents the impact of large-scale excavation on the environment. After the project is completed, the pipeline runs normally, and the upper roads, ground buildings and underground structures have no settlement or deformation, indicating that the construction has achieved the expected results.

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