Dynamic Response Analysis of CO2 Phase-Change Fracturing in Subway Station Construction

Literature Overview

This study examines the dynamic response behavior of surrounding rock and structures during carbon dioxide phase-change fracturing operations in subway station construction environments. The technique leverages the rapid phase transition of liquid CO2 to gas to generate high-pressure gas expansion forces that fracture rock without detonation, offering a safer alternative to conventional explosives in urban underground construction where vibration control is critical. The paper presents both numerical simulation results and field monitoring data to characterize the stress wave propagation, displacement response, and structural integrity implications of this method.

Core Technical Content and Key Parameters

The CO2 phase-change fracturing process involves injecting liquid CO2 into pre-drilled boreholes, then triggering rapid expansion through an ignition mechanism. The phase transition generates pressures exceeding 300 MPa within the borehole, creating radial and tangential stress waves that propagate through the surrounding rock mass. The study focuses on how these stress waves interact with existing subway station structural elements, including tunnel lining, waterproof layers, and adjacent utility infrastructure.

The dynamic response analysis employs finite element methods coupled with discrete element modeling to simulate the coupled rock-fracture-structure interaction. Key findings indicate that the peak particle velocity at subway station structures decreases rapidly with distance from the fracturing face, following an inverse power-law relationship. At distances beyond 15 meters, the vibration velocity typically drops below 15 mm/s, satisfying most urban construction vibration control standards.

Structural Safety Assessment Methodology

The study adopts a multi-criteria evaluation framework for assessing structural safety during CO2 phase-change fracturing operations. The methodology integrates three analytical approaches: time-domain dynamic analysis for transient response prediction, frequency-domain analysis for resonance identification, and cumulative damage assessment for long-term structural health evaluation.

The time-domain analysis captures the transient stress-strain response of tunnel lining elements subjected to sequential fracturing events. The frequency-domain approach identifies whether the dominant frequencies of the stress waves coincide with the natural frequencies of the station structure, which would indicate potential resonance amplification. Cumulative damage assessment evaluates whether repeated fracturing operations over the construction period could lead to progressive fatigue damage in structural members.

From an engineering practice perspective, the study recommends implementing real-time vibration monitoring at critical structural locations during fracturing operations. The monitoring system should include geophones positioned at the tunnel invert, springing line, and crown, providing immediate feedback on whether vibration levels remain within acceptable thresholds. If measured values approach the alarm threshold of 15 mm/s, the fracturing sequence should be modified to reduce energy input or increase stand-off distance.

Engineering Practice Implications and Reflections

The findings of this study have direct relevance to cladding and bimetal product manufacturing engineers who work in urban underground environments where existing infrastructure must be protected. When fabricating or installing bimetal pressure vessels or clad plate assemblies in proximity to subway stations, understanding the dynamic loading environment is essential for proper design and installation planning.

A critical insight from this literature is that the CO2 phase-change method, while inherently lower energy than conventional explosives, still produces dynamic loading that requires careful management. The study demonstrates that proper borehole spacing, controlled sequence intervals, and adequate stand-off distances can maintain vibration levels well within safety limits. However, in geologically complex conditions with heterogeneous rock mass properties, the stress wave attenuation behavior may deviate significantly from predictions, necessitating conservative design margins.

For engineers involved in pressure vessel fabrication near urban infrastructure, this study reinforces the importance of site-specific dynamic analysis before commencing any energy-intensive operations. The principle of understanding dynamic response propagation is directly applicable to evaluating the effects of hydrostatic testing, impact testing, or thermal processing operations that may generate transient loads affecting nearby sensitive structures.

The study concludes that CO2 phase-change fracturing is a viable and controllable method for rock excavation in urban subway station construction when implemented with proper engineering controls, monitoring protocols, and conservative design margins. Future research should focus on long-term monitoring of structures subjected to prolonged fracturing operations to validate the cumulative damage predictions and refine the safety assessment methodologies presented in this work.