Impact Of Regional Settlement On High Speed Railroad Foundation And Bridge Construction And Coping Strategies

1. Causes of regional subsidence Regional subsidence involves a large area, is not easy to detect in the early stage, and is difficult to attract attention. It is a slow-changing geological disaster. The causes of regional subsidence are as follows.
(I) Regional tectonic activity The crust has been in a subsidence zone for a long time, and the ground has been sinking continuously, especially in areas near rivers. The filling materials are mostly river alluvial deposits with high water content, or affected by earthquake activities, and the new structure is not fully developed, causing the ground to continue to sink.
(II) Large ground load The gradual density and high-rise of urban buildings have increased the ground load and become the main factor of regional subsidence. Under the building load, the additional stress of the soil increases, causing the bearing layer soil layer to deform and settle instantly. For example, in a group of buildings, the base is superimposed with stress, and the deformation of the foundations of adjacent buildings is superimposed, causing large-scale ground subsidence.
(III) Soil consolidation settlement Under constant pressure, the soil layer settles over time. The consolidation pressure within 20 to 30 m of the soil layer is relatively small, which is underconsolidated soil. Combined with the self-weight pressure, it continues to consolidate and compact, causing natural settlement. This settlement has nothing to do with groundwater exploitation settlement, but it should also be taken seriously.
(IV) Underground mining settlement Under regional settlement, the exploitation of groundwater or oil and gas is one of the important reasons for causing settlement. When extracting oil and gas, the extraction of oil will reduce the pore hydraulic pressure of sandstone. Under the self-gravity, the unconsolidated part of the sandstone will continue to consolidate, causing the section to sink. When water is injected into the oil layer, the settlement will be reduced, causing some sections to rise. Groundwater exploitation will destroy the original formation stress, reduce the pore water pressure, increase the stress of soil particles, cause the aquifer to deform, and cause ground settlement.

2. Impact of regional settlement on high-speed railway subgrade and bridge construction Regional settlement is a widely distributed geological disaster. It is difficult to detect the occurrence process. After long-term accumulation, it will cause serious harm, aggravate or induce other disasters, and have adverse effects on people's lives and urban development. Regional settlement has the characteristics of progressiveness and slow change. It has a great impact on linear projects such as high-speed railway subgrades and bridges, mainly affecting the design elevation and line slope, and even destroying buildings and making it impossible to carry out normal construction.
(I) Dynamic change of elevation Under the influence of regional settlement, the elevation of the leveling points set along the project will change dynamically. Relevant units are required to do a good job in settlement monitoring, regularly measure and map the leveling points, and must be measured together with settlement points outside the region, increasing the workload of ground settlement measurement.
(II) Impact on line slope Under regional settlement, if uneven settlement occurs, it will affect the slope of the high-speed railway line, and the slope change will gradually increase over time. According to the monitoring data of the leveling points along the high-speed railway, the railway road surface is wavy due to the influence of long-term regional settlement, which is not conducive to the operation of high-speed railways and will increase operating costs. Under regional settlement, the longitudinal section adjusts the track surface elevation according to the measured elevations of bridges, culverts, roadbeds, etc.; the longitudinal section should reduce the thickness of the roadbed and the height of the landing road to ensure the connection with the existing track surface.
(III) The roadbed will be deformed and damaged under the influence of vehicle load and nature. The regional settlement damages the foundation because the embankment sinks, causing vertical displacement on the roadbed surface, and the road surface damage will affect the line operation. For the simply supported structure of the bridge, the regional settlement will affect the bridge structure and the bridge force. If the pier settlement is at the same slope, it will not affect the structural force; if the settlement is at different slopes, it will increase the structural force and reduce the stability of the project.

3. Construction strategies for dealing with regional settlement in high-speed railway subgrade and bridge construction
(I) Clarify treatment principles In the construction of high-speed railway subgrade and bridge, we must pay attention to the settlement problem. If there is a large or uneven settlement of the roadbed, it will threaten the safety of train operation. In foundation treatment, the following three principles must be followed. First, control the roadbed settlement. Soft soil has low strength and large compression. When building high-speed railway bridges on soft soil foundation, foundation treatment must be carried out to avoid excessive roadbed settlement, causing roadbed diseases and affecting train operation. Second, resist uneven settlement. In particular, the settlement of the funnel edge area must be reinforced to resist uneven settlement. Third, ensure economy. Combined with geological conditions, choose a foundation treatment method that is convenient to construct and technically feasible to save investment on the basis of ensuring project safety.
(II) Foundation reinforcement treatment
1. Drainage consolidation method In foundation treatment, drainage consolidation method is a method with simple construction and low cost. In specific applications, the treatment depth must be controlled at 15 m to avoid increasing the difficulty of dissipating pore water pressure. Considering the above factors, the specific construction is as follows.
① Vacuum preloading. Arrange a pumping pipe in the sand cushion layer to extract air, form a negative pressure to remove moisture from the soft soil layer, and set up a sand well and a plastic drainage belt to enhance the preloading effect.
② Load preloading. Use a pressure exceeding the permanent load of the building to preload the foundation to eliminate settlement as much as possible.
③ Combined loading and vacuum preloading. Superimpose the calculated load, which is suitable for miscellaneous fill, silt, soft clay, etc. First, extract the gas in the foundation to create a vacuum effect, so that negative pressure is generated in the foundation. After exceeding 80 kPa, preload the roadbed fill with piled load to consolidate the foundation and improve the soil consolidation degree.
④ Reduce groundwater. It is suitable for projects with low excavation depth and fine sand foundation to extract groundwater and consolidate the soil layer.
2. Composite foundation method The high-speed railway foundation treatment has strict requirements on ground settlement. For soft soil areas along the line, if foundation settlement occurs, the composite foundation method must be adopted and CFG piles must be used for reinforcement. CFG piles are composed of sand, gravel, fly ash, etc., and form a composite foundation with the cushion layer and the soil between piles, so that the soil between piles bears the load. Through the action of the piles, the load is transferred to the deep soil, thereby improving the bearing capacity of the composite foundation. This method does not require reinforcement, and the admixture uses fly ash, etc., which can effectively reduce expenses and has high economic and social benefits.
3. Use geocells and prestressed pipe piles In foundation treatment, geocells and prestressed pipe piles together constitute horizontal loads and vertical supports, increase the shear strength of the foundation and improve the deformation modulus. Prestressed pipe piles have large load-bearing capacity and small settlement, and can be tensioned on geocells, thereby generating tensile stress in the geocells, laterally constraining the soil, and increasing the shear strength of the composite soil. Geocells have high elasticity and generate vertical forces after being stressed, which can offset some concentrated loads and avoid shear damage to the foundation. The soil between the piles is squeezed by the upper load and adheres to the vicinity of the pipe piles, enhancing the friction between the pipe wall and the soil; geocells are roadbed cushions, which can reduce roadbed settlement and improve the stability of the foundation. Geocells and prestressed pipe piles work together to consolidate the foundation as quickly as possible and control lateral deformation and settlement.
(III) Treatment of long and large roadbeds Long and large roadbeds must ensure vertical stiffness to eliminate ground settlement and avoid coupling with post-construction buildings to affect the roadbed. In long and large roadbeds, the jet grouting method can be adopted, which has the advantages of fast construction speed and simple equipment, and is suitable for artificial fill, silt, etc. In the jet grouting pile reinforcement, after the slurry is prepared, the drill drills the hole, the grouting pipe, nozzle, etc. are placed at the grouting position, and the slurry is quickly sprayed using a high-pressure jet device, causing direct damage to the soil. During grouting, the drill bit is raised while stirring to fully mix the slurry with the soil to form a columnar consolidation body to reinforce the foundation. During this process, it is necessary to control the injection material, speed up the grouting pipe, increase the grouting volume, etc., test the reinforcement strength in advance, and ensure that the compressive strength is between 1 and 20 MPa.

4. Comprehensive prevention and control measures for regional subsidence
(I) Control groundwater exploitation Groundwater exploitation is an important factor in regional subsidence. Prohibiting or controlling groundwater exploitation can curb ground subsidence. First, limit groundwater exploitation and rationally allocate the exploitation of groundwater levels; second, close the wells along the line. Even shallow groundwater exploitation will affect the high-speed railway subgrade structure, especially if the wells are within 200 m from the high-speed railway line, which must be controlled.
(II) Automated roadbed detection High-speed railway subgrade bridges are more sensitive to subsidence. In order to avoid subsidence after construction, subsidence monitoring must be strengthened to ensure the safety of high-speed railway operations. During the operation stage, it is difficult for high-speed railways to adopt conventional monitoring methods. In order to ensure real-time testing and driving safety, monitoring points must be arranged along the line. In special areas, diversified points are arranged to achieve automatic monitoring. As for existing roadbed monitoring, the data collection and transmission system should be integrated to establish a monitoring platform based on distributed processing technology, sensor technology, and communication technology. Real-time monitoring and data transmission of high-speed railway subgrade bridges, drawing of subgrade settlement curves, and good prediction work are carried out to ensure that settlement beyond the safe range can be immediately discovered and handled.
(III) Grouting reinforcement treatment During the high-speed railway operation stage, in order to reduce the impact of subgrade settlement, the existing settlement sections must be reinforced. In the past, drainage consolidation methods were mostly adopted, but they occupied a large area, and padding was filled after the track was removed, which made construction difficult and increased cost expenditure. For this reason, the settlement treatment in the operation stage can be carried out by grouting reinforcement. Using high-pressure conveying equipment, mud is configured and poured into the stratum. After the slurry solidifies, the foundation reinforcement can be achieved. When reinforcing the foundation, it is necessary to control the reinforced soil to ensure the stability of the reinforced subgrade, and it is also necessary to do a good job in grouting material selection and grouting pressure control.