Brief introduction: Box culvert refers to the culvert with reinforced concrete box joints in the tunnel. Box culvert is composed of one or more square or rectangular sections, and generally adopts reinforced concrete or masonry structure, but reinforced concrete is widely used. When the span is less than 4m, box culvert is adopted. For pipe culvert, reinforced concrete box culvert is a cheap substitute, and the pier and upper and lower plates are uniformly poured.
1 box culvert jacking construction technology
The machines and tools in box culvert jacking construction include: hydraulic system consisting of power instruments, operating instruments, executing instruments and auxiliary mechanisms; Force transmission system consisting of top iron, top column and distribution beam. The force transmission equipment in box culvert jacking construction is different because of the jacking mode and the number of hole spans. After the box back is completed, jack-up equipment should be installed and tested. Before jacking construction, relevant technicians should first check whether the preparatory work before jacking is completed. For example, whether the box meets the design strength, whether the line is reinforced, etc. After inspection and connection, the box and bottom plate should be separated. After all inspection work is qualified, formal jacking construction will be carried out. Start the high-pressure oil pump to make the jack generate jacking force due to hydraulic pressure and push the box forward. After the box body pushes a pick, the piston of the jack should return in time to ensure that the pick is opened next time. This cycle is carried out alternately until the box reaches the required position.
In order to ensure the jacking quality, it is necessary to control the two-hole box bridge on the same plane during the jacking process. After the box reaches the required position, the triangular area of the line needs to be backfilled immediately.
The following problems should be paid attention to during construction:
(1) When the box is jacked, it is necessary to constantly cope with the changes of observation points. If there is any problem, stop immediately and start construction after solving it. (2) In the process of jacking, the excavated working face shall be buried in time and shall not be exposed for a long time. (3) Monitoring subgrade during jacking. (4) Monitor the earth-moving machinery, and avoid direct collision when working offline. ⑤ During jacking construction, when the oil pressure of the oil pump rises by 5~ 10Mpa, the pump must be stopped for observation. Problems should be dealt with in time. 6. Before jacking construction, the oil output of each jacking oil pump must be debugged to ensure that each jacking oil pump can work normally at the same time. All landowners jacking construction, use theodolite and level tracking measurement. Once jacking, measure once, measure the direction at any time, and adjust the jacking force of the jacking pick according to the direction. The adjustment method is to adjust the jacking force on both sides. End up jacking construction, ejector pin and ejector pin should be on the same axis, so as to avoid instability caused by too long ejector pin.
2. Strengthen soft foundation treatment
Soft soil is characterized by high water content, large pores and low strength. It is precisely because of the above characteristics that the bearing capacity and stability of soft foundation are relatively poor. Therefore, the soft foundation must be treated to prevent the direct construction on the soft foundation from endangering the safety of buildings.
Soft foundation treatment generally adopts the following methods:
① By squeezing or vibrating, the void ratio of soft foundation can be reduced, so as to improve the foundation strength. In general, for soft foundations such as loose sand, when in shallow layers with optimal water content, manual or mechanical tamping and mechanical vibration rolling are adopted; Suitable for cohesive soil, gravel, miscellaneous fill, etc. Through strong external tamping force, the soft foundation is deeply consolidated, thus compacting the soil and improving the foundation strength; The shallow surface of soft foundation is compacted by the impact force generated by the falling of heavy hammer, thus forming a uniform and tough shell. This method is more suitable for unsaturated cohesive soil. (2) Take certain measures to reduce the pore water and void ratio of soft foundation, so that the pore water pressure of soil is also reduced, which leads to the consolidation deformation of soil, thus improving the settlement speed, the shear strength of foundation and the bearing capacity of foundation. Heap preloading method, vacuum preloading method and electroosmosis drainage method are all common drainage consolidation methods. ③ Replace soft soil with lime, gravel, sand and other materials, and form a foundation with surrounding soil at the same time, so as to reduce foundation settlement and improve foundation bearing capacity. Dynamic compaction replacement method, lime pile method and gravel pile method are all common replacement methods. (4) Using external force, cement, lime and other chemical materials are poured into soft soil foundation, which is formed after soil and materials are consolidated. Foundation can greatly improve the strength of foundation. This method is called cementation. Both high-pressure jet grouting method and grouting method belong to the category of cementing method.
3. Jacking construction of mass box culvert on soft foundation
3. 1 Jacking construction method of large volume box culvert on soft soil foundation According to the geological characteristics of the construction site, the subgrade within the influence range of jacking should be reinforced and grouted to prevent landslides including side and front during jacking. Grouting above the bottom plate should be interspersed with reinforcement to improve the bearing capacity of the foundation, and the reinforcement range should reach 2 meters above the basement. During grouting reinforcement, the formation pressure is 0.3~0.5mpa, and the working pit is supported by cast-in-place pile. The diameter of bored piles is 100cm, and the spacing between adjacent piles is about 150cm. The skateboard is built by manual excavation, and the surface of the skateboard is required to be flat and smooth, and the elevation error is less than 3mm.
In order to ensure that the back soil resistance can meet the jacking force, bored piles are used for the back piles to increase the compactness of the back soil, and the back river skateboard is integrated to prevent the skateboard from breaking during jacking construction. Reinforcement measures need to be taken in the process of jacking.
3.2 Jacking of large volume box culvert in soft soil foundation The following principles should be mastered: ① Jacking principle of box culvert. After the box is preformed, the jacking construction can only be carried out after the strength of the main body and protective layer of the frame reaches 100% of the design strength and the line is reinforced. ② Install steel blade feet. Steel blade angles are made of 20 mm steel plates. All cutting edges are connected by welding, and the height of welding seam is required to be greater than or equal to 8mm, and warping should be prevented during welding. When installing the bottom corner, the bottom surface forms an elevation angle with the surface of the bridge and culvert, which can facilitate soil cutting and prevent the bridge and culvert from being punctured. The angle of side edge should be larger than the end face of bridge and culvert to reduce jacking resistance. (3) The jacking force of the top picks is generally calculated as 60% of the rated jacking force of the top picks, and the top picks are usually distributed in pairs. ④ During jacking construction, starting the high-pressure oil pump is the jacking of the frame bridge. The jacking force of the jacking pick is used to push the frame bridge forward under the reaction force. In fact, under normal circumstances, each jacking stroke is about 80% of the jacking pick stroke. When a jack-up process is completed, the pick must be returned, and the distribution beam must be put back in its original position, and so on until the frame bridge is in place.
When the bridge is located at the top of the skateboard, the jacking force on both sides of the box should be adjusted in time according to the deviation, so that the bridge can enter the subgrade in strict accordance with the design axis. It is difficult to correct the bridge after it enters the subgrade, because most of the fractals become channels. In the jacking process, the wall earthwork is excavated manually, and other earthwork is excavated by small excavators, loaders and trucks. Excavators are used to dig in the cave, and manual cooperation is provided when necessary. The loader backs up and loads, and the dump truck carries soil. Measures such as excavation should be combined with jack adjustment, so that better results can be obtained. If the bridge body leans to the left, reduce the resistance of the left side wall and the left side, so that the jacking speed of the left side of the bridge body is higher than that of the right side. Similarly, when the bridge leans to the right, the above method is also used for adjustment.
3.3 Control of Jacking of Mass Box Culvert in Soft Soil Foundation The following principles should be mastered in the control of jacking of Mass Box Culvert in Soft Soil Foundation: ① Jacking piles should be set for bored piles at the front end of jacking, and the parameters of pile foundation should be consistent with those of retaining piles. Before the excavation of the working pit, use the wire rope and supporting pile on the other side of the line to pull the anchor. Setting jacking piles can ensure the stability of road subgrade, reduce the jacking distance of soil and help control culvert deviation. (2) Grouting under subgrade can ensure that the front and side of the line will not collapse during jacking construction, and it is also the bearing layer of mass frame structure. Before jacking construction, it is necessary to determine the parameters of each construction, and at the same time, test the grouting effect of the test section, and the construction can only be carried out after the bearing capacity meets the requirements. (3) There is still a period of time between box prefabrication and box jacking construction, so the foundation reinforcement effect affects whether the box sinks. (4) In order to prevent the phenomenon of pricking the head when jacking, the skateboard surface is made in the form of high head and low tail. ⑤ In the process of digging, the outer sides of the two walls can't be hollowed out during jacking, and measurement is very important for box jacking. Therefore, in the process of jacking construction, it is necessary to measure the elevation and left-right deviation, and take scientific measures to adjust it to ensure the smooth positioning of the box.
4 conclusion
According to different jacking forms, appropriate reinforcement measures are determined, and scientific jacking methods are adopted according to the actual situation of the project, which can ensure the smooth and safe implementation of the project. At the same time, this paper provides some reference for the jacking construction of large volume box culvert in soft soil foundation.
;