Zhuojin Pipe Fitting Co., LtdBearing Capacity Calculation of CFT Columns in Cavity Pile Method Underground Station Construction
Literature Overview
Liu Li and Gao Xincai from Beijing Municipal Engineering Design & Research Institute (2019) address a critical gap in design standards concerning the bearing capacity calculation of concrete-filled steel tube (CFT) columns during the construction phase of "cavity pile method" underground railway stations. The study, supported by the Beijing Science and Technology Plan, focuses on the Beijing Metro Line 16 stations where CFT central columns have their bottom ends embedded into pile foundations. The research establishes that under specific geological and construction conditions, the pile foundation constraint at the column base can be simplified to a fixed-end condition.
Core Technical Points
The "cavity pile method" involves constructing underground stations by excavating cavities between pre-installed piles, with CFT columns serving as central load-bearing elements. The critical engineering question is whether the column base embedded in the pile foundation should be modeled as a pinned connection, a fixed connection, or an intermediate condition. The authors reference high-pile-cap pile foundation vertical bearing capacity calculation methods from railway bridge and building pile foundations to develop their numerical model.
| Parameter | Condition | Simplification |
|---|---|---|
| Pile embedding length | Meets code requirements | Fixed-end constraint valid |
| Surrounding soil | Silty clay and pebble strata | Sufficient lateral confinement |
| Initial eccentricity | Typical design values | Moderate reduction in bearing capacity |
| Column-pile interface | Direct embedding | Friction and bearing resistance developed |
The study's conclusion that the base can be simplified to a fixed-end constraint is practically significant because it allows engineers to use more favorable boundary conditions in structural analysis, potentially reducing the required column cross-section area and steel consumption. However, this simplification must be applied with caution, as it depends on the specific geological conditions and construction details.
Engineering Practice Implications
From a steel pipe fabrication and welding perspective, CFT columns require careful attention to several quality aspects:
- Pipe material verification: The steel pipe should conform to GB/T 8162 (seamless) or GB/T 3091 (welded) with specified yield strength grades (typically Q235B, Q345B, or Q355B). Chemical composition and mechanical property verification through sampling per ASTM A370 or GB/T 228 is mandatory.
- Welding of column splices: Field-welded connections between CFT column segments require qualification procedures in accordance with NB/T 47014 or ISO 9606. The weld metal selection must ensure matching or slightly higher yield strength than the parent steel, with adequate impact toughness at the minimum service temperature.
- Concrete filling quality: The concrete filling process must ensure complete compaction without voids. This typically requires the use of self-compacting concrete or mechanical vibration during filling. The quality of the steel-concrete interface is critical for the composite action, and any gaps or voids will significantly reduce the actual bearing capacity below the calculated value.
- Embedding depth verification: The actual embedding length of the column base into the pile foundation must be verified during construction through measurement records and, if possible, through post-construction pull-out testing.
Key Questions and Reflections
The most important contribution of this paper is the formalization of the boundary condition assumption that has been applied informally in practice. By providing a numerical basis for the fixed-end simplification, the authors enable more defensible design calculations. However, engineers should recognize that the validity of this simplification is conditional on specific geological and construction parameters. In cases where the surrounding soil is softer (e.g., saturated sand or soft clay), or where the embedding length is insufficient, the actual boundary condition may be closer to a pinned or semi-rigid connection, which would require more conservative design assumptions. The paper's approach of borrowing from railway bridge and building pile foundation methodologies is methodologically sound and demonstrates the value of interdisciplinary knowledge transfer in solving specialized engineering problems.