As an important component of urban underground space development, underground passages are widely used in infrastructure construction such as urban roads, rail transit, and municipal pipelines. During their construction process, they are susceptible to various factors such as geological conditions, construction techniques, and surrounding environment, resulting in settlement phenomena. If the settlement of underground passages exceeds the allowable range of regulations, it may cause cracks and deformations in the passage structure, affect traffic safety, and may also damage surrounding underground pipelines, buildings, and road facilities, causing safety hazards and economic losses. Therefore, the scientific application of settlement control technology measures in underground passage construction is the core support for ensuring construction safety, structural stability, and surrounding environmental safety. Based on industry standards and engineering practices such as the "Construction and Acceptance Specification for Urban Underground Passage Engineering" (CJJ1-2008) and the "Construction Quality Acceptance Standard for Underground Railway Engineering" (GB50299-2018), settlement control technical measures in underground passage construction are carried out around three core links: pre construction preparation, construction process control, and post monitoring and disposal. Each measure is coordinated and connected to build a comprehensive and full process settlement control system, ensuring that settlement is controlled within the allowable range of design and specifications.
The settlement control of underground passage construction follows the core principles of "prevention first, precise control, dynamic adjustment, and collaborative protection". The core goal is to effectively control soil disturbance during the construction process through systematic technical measures, reduce settlement amount and rate, avoid uneven settlement, ensure the safety and stability of underground passage structure and surrounding environment, and balance construction efficiency and engineering economy. The selection of technical measures should be based on the depth of the underground passage, geological conditions (soft soil foundation, sandy soil foundation, rock foundation, etc.), and the distribution of surrounding buildings and pipelines. Targeted optimization plans should be implemented to avoid blind application and ensure that the measures are scientifically feasible and effective.
Pre construction preparation is the foundation of settlement control, and the core is to avoid settlement hazards from the source through geological survey, scheme optimization, equipment debugging and other measures, laying the foundation for settlement control during the construction process. The focus covers three key aspects: geological survey, scheme design, and equipment and material control.
Geological survey is a prerequisite for settlement control, which requires a comprehensive understanding of the geological conditions, hydrogeological conditions, and distribution of underground pipelines and buildings in the construction area. Through drilling surveys, in-situ testing, and other methods, key parameters such as soil distribution, bearing capacity, compressibility, and groundwater depth can be identified to identify settlement risk points (such as soft soil interlayers, underground cavities, and areas with dense pipelines). For areas with high settlement risks such as soft soil foundations, it is necessary to focus on investigating indicators such as consolidation coefficient and sensitivity of soil layers, in order to provide accurate basis for the formulation of settlement control technical measures. The survey results need to form a detailed report, clarifying the key areas for settlement control and targeted measures, to ensure that the scheme design is in line with the actual engineering situation.
The scheme design should be combined with geological survey results, optimize the construction process of underground passages, and prioritize the use of construction methods that cause less disturbance to the soil to reduce settlement caused by soil deformation during the construction process. Common construction methods include open excavation method and underground excavation method (shield tunneling method, shallow buried underground excavation method, etc.). For soft soil foundation areas, minimally invasive construction techniques such as shield tunneling method and shallow buried underground excavation method are preferred. Open excavation construction requires optimization of slope gradient and support scheme to reduce disturbance of surrounding soil caused by soil excavation. At the same time, the settlement control indicators need to be clearly defined in the plan. Based on the type of underground channel structure and the sensitivity of the surrounding environment, the allowable settlement amount and settlement rate should be determined. Generally, the allowable settlement amount of the main structure of the underground channel should not exceed 30mm, and the allowable settlement amount of the surrounding buildings should not exceed 20mm. The settlement rate should be controlled within 2mm/d. In addition, targeted foundation treatment measures need to be developed in the plan to reinforce weak foundations in advance, improve the bearing capacity of the foundation from the source, and reduce settlement.
Equipment and material control must ensure that the equipment and materials used in construction comply with regulatory requirements, providing assurance for settlement control. Construction equipment (such as shield tunneling machines, excavators, support equipment, etc.) need to be comprehensively debugged to ensure stable equipment performance and avoid construction interruption and soil disturbance caused by equipment failure; Supporting materials (such as steel sheet piles, anchor rods, shotcrete, etc.) need to be sampled and retested to ensure that their strength and stiffness meet the design requirements, and to avoid support failure and settlement caused by unqualified supporting materials. At the same time, it is necessary to prepare the necessary instruments and equipment for settlement monitoring (such as settlement meters, inclinometers, etc.) and calibrate them to ensure accurate and reliable monitoring data.
Construction process control is the core link of settlement control. The core is to optimize construction technology, strengthen support and protection, control soil disturbance, adjust construction parameters in real time, ensure that settlement is controlled within the allowable range, and focus on four aspects: foundation treatment, excavation construction, support construction, and precipitation control.
Foundation treatment is the key to controlling settlement. For areas with insufficient bearing capacity such as soft soil foundation and backfilled soil foundation, targeted reinforcement measures need to be taken to enhance the bearing capacity of the foundation and reduce its settlement. Common foundation treatment methods include replacement cushion layer method, compaction reinforcement method, CFG pile reinforcement method, grouting reinforcement method, etc. The method of replacing the cushion layer uses graded sand, gravel, lime soil and other materials to replace the original weak soil layer, compact it layer by layer, and ensure that the bearing capacity of the cushion layer meets the design requirements; The compaction reinforcement method uses mechanical compaction to improve the compactness of the soil and reduce its compression deformation; The CFG pile reinforcement method involves implanting CFG piles to form a composite foundation with the soil, thereby enhancing the bearing capacity of the foundation; The grouting reinforcement method injects grout into the soil, fills the soil pores, enhances soil adhesion, and reduces settlement. After the foundation treatment is completed, a bearing capacity test must be carried out, and only after passing the test can the next process be carried out.
The excavation construction must strictly follow the principle of "layered excavation, segmented advancement, and support before excavation", control the excavation speed, depth, and sequence, and reduce soil disturbance. The open excavation method requires layered excavation, with each layer's excavation depth controlled between 1.5-2.0m. After excavation, timely support should be provided to avoid long-term exposure of the soil, which may cause collapse and settlement; The underground excavation method requires controlling the excavation speed, and the shield tunneling speed is generally controlled at 3-5cm/min. The shallow buried underground excavation method requires segmented excavation, with each segment length controlled at 5-10m. After excavation, concrete support should be sprayed in a timely manner to seal the surrounding rock and reduce soil deformation. During the excavation process, it is necessary to avoid over excavation and under excavation. Over excavation can cause stress release in the soil, leading to settlement, while under excavation can affect the support effect. It is necessary to clean up the over excavation part in a timely manner to ensure that the excavation size meets the design requirements.
Support construction needs to be carried out in coordination with excavation construction to ensure timely and effective support, resist soil pressure, and reduce settlement. The open excavation method commonly uses steel sheet piles, row piles, underground continuous walls and other forms of support. Steel sheet piles and row piles need to be driven into the design depth to ensure firm support. The underground continuous wall needs to ensure wall thickness and strength, with tight joint sealing to prevent soil leakage and deformation; The underground excavation method commonly uses a combination of shotcrete, anchor rods, and steel arches for joint support. The thickness of shotcrete should be controlled between 100-150mm, anchor rods should be implanted at the design depth, and the spacing between steel arches should be controlled between 0.8-1.2m to ensure the overall stability of the support system. After the completion of the support construction, it is necessary to regularly check the deformation and displacement of the support structure, adjust the support parameters in a timely manner, and avoid settlement caused by support failure.
Precipitation control is an important measure to avoid settlement caused by groundwater. During the construction of underground passages, excessive groundwater can lead to an increase in soil moisture content and a decrease in bearing capacity, causing settlement. It is necessary to select appropriate precipitation methods based on hydrogeological conditions to control the groundwater level. Common precipitation methods include light well point precipitation, deep well precipitation, and open drainage through water collection. Light well point precipitation is suitable for areas with shallow groundwater levels and low soil permeability, deep well precipitation is suitable for areas with deep groundwater levels and high soil permeability, and open drainage through water collection is suitable for areas with low groundwater flow. During the precipitation process, it is necessary to strictly control the rate of groundwater level drop to avoid soil consolidation and settlement caused by rapid groundwater level drop. The rate of groundwater level drop is generally controlled at 0.5-1.0m/d, and the settlement of surrounding buildings and underground pipelines should be monitored to adjust the precipitation plan in a timely manner.
Post monitoring and disposal are important guarantees for settlement control. The core is to monitor the settlement situation in real time through regular monitoring, discover settlement hazards in a timely manner, take targeted disposal measures, and ensure settlement stability. The focus covers settlement monitoring, hazard disposal, and post maintenance.
Settlement monitoring requires the establishment of a comprehensive monitoring system, with monitoring points set up in the main structure of underground passages, surrounding buildings, underground pipelines, and construction areas. The layout of monitoring points should be uniform and reasonable, covering areas at risk of settlement. The monitoring content includes settlement amount, settlement rate, horizontal displacement, etc. The monitoring frequency is adjusted according to the construction stage. During the excavation construction stage, it is monitored every 12-24 hours, and after the construction is completed, it is monitored every 3-7 days until the settlement is stable. Monitoring data needs to be promptly sorted and analyzed, settlement curves should be drawn, and settlement patterns should be mastered. If the settlement amount or rate exceeds the allowable range, construction should be stopped immediately, the cause should be investigated, and disposal measures should be taken.
The disposal of hidden dangers requires targeted rectification measures to be taken for hidden dangers such as excessive settlement and uneven settlement discovered through monitoring. If the settlement exceeds the standard due to insufficient bearing capacity of the foundation, additional foundation reinforcement measures need to be taken; If settlement occurs due to support failure, it is necessary to reinforce the support structure and adjust the support parameters; If settlement is caused by improper groundwater control, it is necessary to optimize the precipitation plan and control the groundwater level. For the potential settlement hazards of surrounding buildings and underground pipelines, measures such as reinforcement and replacement should be taken to reduce the impact of settlement and ensure the safety of the surrounding environment.
In the later maintenance stage, it is necessary to strengthen the maintenance of the underground passage structure and surrounding environment, regularly inspect the cracks and deformations of the structure, and timely repair damaged parts; Regularly clean the drainage system to avoid soil softening caused by groundwater accumulation and settlement; Regularly inspect the foundation reinforcement area to ensure stable reinforcement effect. Meanwhile, it is necessary to continuously monitor the settlement situation until it is completely stable, ensuring the long-term safe operation of the underground passage.
The implementation of settlement control technology measures in underground passage construction needs to be carried out throughout the entire construction process, with coordinated control at all stages, strict adherence to industry standards, and optimization of technical solutions based on actual engineering conditions. With the application of intelligent monitoring technology and new foundation reinforcement materials, the level of refinement and standardization of settlement control continues to improve. Through scientific pre preparation, standardized construction control, and precise post monitoring and disposal, the settlement of underground passage construction can be effectively controlled, avoiding settlement hazards, ensuring construction safety, structural stability, and surrounding environmental safety, and providing strong support for high-quality construction of urban underground passages.
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