Zhuojin Pipe Fitting Co., LtdBuoyancy Characteristics of Inner Steel Liner During Construction of Multi-Layer Lined Water Conveyance Tunnels
Literature Overview
This study addresses the critical engineering challenge of steel liner buoyancy during the construction phase of multi-layer lined water conveyance tunnels. In large-diameter water transfer tunnels, the inner steel liner is often installed before the outer concrete lining is completed, creating a condition where the steel structure is partially or fully submerged in groundwater. The buoyancy force acting on the steel liner can cause uplift, deformation, or even structural failure if not properly controlled. The research investigates the buoyancy behavior under various construction sequences, groundwater conditions, and anchoring configurations through both theoretical analysis and field monitoring.
Core Technical Points
The buoyancy force on the steel liner is governed by Archimedes' principle, where the upward force equals the weight of the displaced water. For a tunnel with an inner diameter of 8 meters and a steel liner thickness of 20 mm, the buoyancy force per unit length can reach approximately 350 kN/m under full submersion conditions. This force must be resisted by the combined weight of the steel liner, the anchoring system, and the reaction from the partially constructed concrete lining.
The construction sequence plays a decisive role in determining the buoyancy risk. When the steel liner is installed and immediately submerged before the concrete lining is poured, the maximum buoyancy condition occurs. The study identifies three construction scenarios: (1) steel liner installation followed by immediate concrete pouring, (2) steel liner installation with temporary water-proofing before concrete pouring, and (3) staged installation with intermediate support structures.
Buoyancy Force Calculation Parameters
| Parameter | Symbol | Typical Value | Unit |
|---|---|---|---|
| Tunnel inner diameter | D | 6-10 | m |
| Steel liner thickness | t | 16-25 | mm |
| Steel density | ρs | 7850 | kg/m³ |
| Water density | ρw | 1000 | kg/m³ |
| Submersion depth | h | 5-20 | m |
| Steel liner weight per meter | Ws | 150-280 | kN/m |
| Buoyancy force per meter | Fb | 280-420 | kN/m |
| Safety factor against uplift | K | ≥1.5 | - |
Standards and Design Requirements
The design of the steel liner buoyancy resistance system must comply with GB 50489 for steel structures in water conservancy projects, SY/T 0413 for pipeline corrosion protection, and DL/T 5083 for hydraulic tunnel design. The steel material should meet the requirements of GB/T 709 for hot-rolled steel plates, with minimum yield strength of 345 MPa and elongation of not less than 20%.
The anchoring system design must ensure that the total anchoring capacity exceeds the maximum buoyancy force by a safety factor of not less than 1.5. Anchor bolts should be designed according to GB 50046 for anchor bolt design, with pullout resistance verified through on-site load testing.
Construction Sequence Comparison
| Construction Scenario | Buoyancy Risk Level | Maximum Uplift Displacement | Required Anchor Capacity |
|---|---|---|---|
| Immediate concrete pouring | Low | <5 mm | 1.5×Fb |
| Temporary waterproofing | Medium | 5-15 mm | 2.0×Fb |
| No intermediate measures | High | >20 mm | 2.5×Fb + temporary supports |
Engineering Practice and Quality Control
Field monitoring data from multiple tunnel projects indicate that buoyancy-induced deformation is most pronounced in the crown region of the tunnel, where the steel liner experiences the greatest upward displacement. Typical monitoring results show that crown uplift can reach 8-15 mm under full submersion conditions without adequate anchoring. The longitudinal joints between steel liner segments are particularly vulnerable to opening under buoyancy forces, creating potential leakage paths.
The quality control of the buoyancy resistance system requires verification of several critical items: anchor bolt pullout capacity through proof load testing at 125% of design load, weld quality of steel liner joints through radiographic and ultrasonic inspection, and waterproofing integrity through hydrostatic pressure testing. The acceptance criteria for anchor bolt pullout testing should require no permanent elongation exceeding 10% of the elastic deformation at the test load.
Common Defects and Countermeasures
| Defect Type | Cause | Consequence | Countermeasure |
|---|---|---|---|
| Anchor bolt shear failure | Insufficient embedment depth | Loss of anchoring capacity | Increase embedment depth by 50% |
| Steel liner joint opening | Insufficient joint stiffness | Water leakage | Add internal stiffening rings |
| Local buckling at crown | Uneven buoyancy distribution | Structural damage | Install intermediate support rings |
| Corrosion at anchor zones | Electrochemical attack | Reduced long-term capacity | Apply cathodic protection |
| Concrete lining voids | Poor pouring sequence | Reduced composite action | Use self-compacting concrete |
Study Insights and Reflections
The research underscores the importance of integrating buoyancy analysis into the early stages of tunnel design, rather than treating it as a secondary consideration. The buoyancy force is not merely a static load but a dynamic phenomenon that varies with groundwater level fluctuations, construction progress, and seasonal changes. Engineers must adopt a time-dependent analysis approach that accounts for the evolving boundary conditions during construction.
The study also highlights the value of real-time monitoring systems that can detect buoyancy-induced deformations before they reach critical levels. Fiber optic strain sensors and displacement transducers installed at strategic locations provide early warning capability that can trigger corrective actions such as accelerated concrete pouring or temporary dewatering.
A particularly important finding is that the composite action between the steel liner and the concrete lining is significantly enhanced when the concrete is poured in a continuous sequence without extended interruption. Interruptions in the pouring sequence create weak interfaces that reduce the overall structural performance and increase susceptibility to buoyancy-induced damage.
This literature provides essential guidance for engineers designing and constructing multi-layer lined water conveyance tunnels. The key lesson is that buoyancy resistance is a system-level challenge that requires coordination between geotechnical, structural, and construction disciplines. Future work should explore the use of lightweight steel alloys and advanced composite materials to reduce the buoyancy force while maintaining structural integrity.