Since the reform and development, the number of highway construction in China has been increasing, and rich experience has been accumulated in highway construction. Through the scientific analysis, summary and induction of these experiences, it has laid a good foundation for the study of lateral uneven settlement of highway subgrade in China. On this basis, expressway experts and scholars in China have made useful explorations on this issue and achieved many practical results. Some of the technical achievements have reached the international advanced level, such as the application of geogrid and geocell technology in preventing lateral uneven settlement of subgrade, and the application in subgrade widening or splicing engineering in soft soil foundation area, which has also achieved good engineering application results.
1, Cause Analysis of Lateral Uneven Settlement of Subgrade
The occurrence of lateral differential settlement of subgrade is the result of comprehensive action of many factors. Among them, the internal cause lies in the foundation and subgrade itself; The external causes are vehicles, groundwater and self-weight.
1. 1 Influence of foundation on lateral uneven settlement of subgrade Improper treatment of embankment foundation: trees are cut down, roots are removed, topsoil treatment is not thorough or the compactness of subgrade base is not enough, and once impurities rot and deteriorate after embankment is formed, the subgrade is soft and uneven settlement. In the section with cross slope greater than 1: 1.5, the foundation was not dug into steps as required before embankment filling, and the filler was poorly combined with the foundation. Under the load, the filler is easy to lose stability and slide along the slope, resulting in uneven lateral settlement. Special foundation section: when the subgrade is built in soft soil section, the mechanical properties of soft soil layer are poor. Under the action of additional stress, consolidation settlement, secondary consolidation settlement and lateral plastic extrusion will occur, resulting in obvious settlement deformation.
1.2 uneven lateral settlement of subgrade caused by subgrade itself uneven filling of embankment: it is quite difficult for us to effectively control filling and grading in subgrade construction. If the filler is mixed with some inferior soil, such as humus soil and planting soil, etc. , or the filler contains unbroken chunks, or the specifications of the stones are different, and the internal gap of the gravel is so large that obvious lateral settlement will occur locally in a specific time. Insufficient compaction of subgrade fill: Due to insufficient compaction, lateral uneven settlement and deformation often occur in filled subgrade, and longitudinal cracks appear on both sides of subgrade. The main reasons for the insufficient compactness of subgrade soil are as follows. Uneven settlement of half-filled and half-excavated parts: Because the sedimentation coefficient of the fill is different from the sedimentation coefficient of the excavation, under the action of traffic load, with the passage of time, the settlement difference between the fill and the excavation is getting bigger and bigger, and uneven settlement of the soil foundation is easy to occur at the joint, resulting in longitudinal cracks in the subgrade.
1.3 The lateral differential settlement of subgrade caused by hydro-climate affects the lateral differential settlement of subgrade, and climate is also an important factor causing the lateral differential settlement of subgrade. Excessive rainfall, flood, freezing, snow or excessive temperature difference may cause uneven lateral settlement of high embankment. Influence of groundwater on lateral uneven settlement of subgrade: Under the alternating action of groundwater, the water content in subgrade soil changes repeatedly. The weight of soil fluctuates in a certain range, and more importantly, the negative pore water pressure caused by capillary tension can reach a considerable value. Coupled with the softening and lubrication of water, it may cause lateral settlement and deformation of subgrade. Groundwater dynamics in subgrade or foundation has great influence on differential settlement of subgrade. There are three main sources of groundwater recharge in embankment and its foundation, namely, lateral recharge of groundwater, lateral recharge of rainfall and lateral recharge of surface water. Their dynamic changes and erosion affect the effective stress distribution, structural characteristics and strength of soil, which leads to uneven lateral settlement of subgrade.
1.4 due to construction reasons, the filling sequence is improper, the filling is not layered in the whole width range, the filling thickness does not meet the requirements, the quality of the filler does not meet the requirements, the water stability of the filler is poor, and the fillers with different properties are mixed, resulting in uneven settlement due to the differences in compressibility and water resistance of different soils. The moisture content of subgrade filler is not strictly controlled, and there is no large-scale leveling and rolling equipment, which makes the degree of compaction fail to meet the requirements. During the construction, no attention was paid to drainage. In rainy days, there is serious water accumulation in subgrade, so it is impossible to drain water by itself. Part of the accumulated water is immersed in the subgrade to form a water bag. If the construction is carried out in sunny days, there will be hidden dangers if the water is not excluded and the water content is not controlled, and the filling will continue.
2. Main countermeasures for lateral differential settlement of subgrade
2. 1 Do a good job in geological exploration and investigation in design: We need to investigate the terrain and hydrogeological conditions that the route passes through in detail, and different general sections need specific design data. Ensure the minimum filling height of subgrade: The minimum filling height of subgrade must ensure that its stability will not be reduced due to the influence of surface water, groundwater, capillary water and frost heaving. According to the requirements of subgrade design code, the minimum filling height of subgrade is determined according to the dry and wet types of soil foundation and the height of capillary water level. Earth excavation subgrade needs to be filled with gravel not less than 60cm; 30cm gravel cushion should be set for subgrade excavation, and blind ditches should be added for sections with poor lateral drainage. Improve the comprehensive drainage design of subgrade: In the design of highway engineering at or above the county level, we must follow the principles of adjusting measures to local conditions, overall consideration and comprehensive consideration, and carry out the longitudinal and transverse drainage design of subgrade to avoid long-term flooding on both sides of subgrade, reduce the bearing capacity of subgrade and cause settlement and deformation.
2.2 Optimization of construction organization design in construction: The key link to ensure the quality of subgrade construction is to arrange its sequence reasonably, allocate personnel and equipment in the construction stage, and understand the relationship between subgrade and structure. Prepare well before construction: carefully consult the design documents before construction to make clear the filling, excavation, geology and deployment of each section. In the key survey of key areas, if errors are found in the design documents, they should be reported to the owner in time and reasonable measures should be taken to solve them. Quality control: when subgrade is not filled, construction lofting should be carried out according to design, semi-permanent temporary leveling points and coordinate points should be established, and corresponding records should be made. In order to ensure that the width of subgrade meets the design requirements, the lofting of subgrade slope toe must be correct. Within the scope of subgrade slope angle, plastic rolling should be carried out and weeds should be removed. Drainage for subgrade construction: In the process of subgrade construction, in order to make the subgrade dry and hard, it should meet the design requirements. First, drainage works and temporary drainage facilities near the construction site should be done well. Maintenance after construction: Different types of protection should be constructed on slopes and based on stability. For the mortar and concrete needed for the protection project, mechanical mixing should be adopted and post-construction maintenance should be done well.
3. Concluding remarks
Highway engineering diseases caused by uneven lateral settlement of subgrade are one of the common quality problems in highway engineering. In this section, the causes of common quality defects are carefully studied and analyzed, and the key points, quality standards and corresponding countermeasures and measures of governance work are put forward for design, construction and supervision units. This is of great practical significance to the implementation of the whole process quality management. In highway engineering construction, we should aim at achieving quality objectives, focus on improving quality awareness and strengthening quality measures, take strict management as the means, strengthen responsibility system as the key, and take comprehensive treatment of common quality problems as the breakthrough to ensure engineering quality. The key points and measures of comprehensive management of engineering quality should be revised and improved step by step, and finally become institutionalized, standardized and standardized operating procedures.
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