Numerical Simulation Analysis of Prefabricated Steel Tube and Larssen Steel Sheet Pile Cofferdam Structure

Engineering Background and Research Objective

Cofferdam structures are essential temporary works in hydraulic and marine engineering, providing a dry working environment for foundation construction in water-bearing ground. The traditional cofferdam construction using driven Larssen steel sheet piles and steel tube pile walls is a well-established practice, but the prefabricated assembly approach offers advantages in construction speed, reusability, and quality control. This research conducts numerical simulation analysis of a prefabricated cofferdam structure combining steel tubes and Larssen steel sheet piles, providing insight into the structural behavior under various loading conditions and informing the design and construction methodology.

The prefabricated approach involves assembling the steel tube segments and sheet pile sections in a controlled environment before installation, which reduces on-site welding operations and improves weld quality. This is particularly significant for engineers familiar with the challenges of underwater welding and the quality control difficulties associated with field-welded cofferdam structures.

Structural Configuration and Simulation Methodology

The cofferdam structure studied consists of prefabricated steel tube pile segments connected to Larssen steel sheet piles through lock-and-bolt or welded connections, forming a closed perimeter that retains water and soil. The numerical simulation employs a three-dimensional finite element model that captures the soil-structure interaction, the water pressure loading, and the excavation-induced stress changes.

The finite element model uses shell elements for the steel tube and sheet pile walls, beam elements for the bracing system, and solid elements for the surrounding soil. The soil is modeled using the Mohr-Coulomb or Cam-Clay constitutive model, calibrated with geotechnical investigation data. The water pressure is applied as a hydrostatic load on the external face of the cofferdam walls, with the groundwater level fluctuating during the construction sequence.

The simulation captures the following construction stages:

1. Installation of the steel tube piles and sheet piles to the designed depth
2. Application of internal bracing to resist lateral earth and water pressure
3. Excavation inside the cofferdam to the foundation level
4. Construction of the foundation structure inside the cofferdam
5. Dewatering and backfilling after foundation completion

Structural Response and Key Findings

The numerical analysis reveals several important structural behaviors that are critical for design and construction planning:

Lateral displacement: The maximum lateral displacement of the cofferdam walls occurs at mid-depth during the final excavation stage, reaching values of 0.3-0.5% of the wall height. The prefabricated steel tube piles exhibit lower displacement compared to Larssen sheet piles due to their higher bending stiffness, with displacement ratios of approximately 1:1.5 (tube to sheet pile) for identical wall heights.

Bending moment distribution: The bending moment in the steel tube piles shows a characteristic shape with maximum positive moment near the excavation level and maximum negative moment at the bracing level. The moment values are approximately 20-30% higher than those predicted by the classical beam-on-elastic-foundation model, indicating that the three-dimensional interaction effects are significant and should be accounted for in design.

Connection performance: The prefabricated connections between steel tube segments and sheet piles are the critical details governing the overall structural integrity. The simulation shows that the connecting plates experience high stress concentrations at the bolt holes, with stress ratios approaching 1.0 (yield) under extreme loading conditions. This highlights the importance of proper connection design and the need for oversized bolt holes to accommodate installation tolerances.

Soil settlement: The excavation inside the cofferdam induces settlement of the surrounding ground surface, with maximum settlement occurring at a distance of 0.5-1.0 times the cofferdam depth from the wall face. The settlement values range from 15 to 40 mm for typical conditions, which may be unacceptable in areas with sensitive adjacent structures.

Welding and Prefabrication Quality Control

The prefabrication approach shifts the critical welding operations from the field to the factory, where quality control is significantly easier to implement. However, the following welding considerations remain critical:

1. Segment connection welds: The steel tube segments are connected by full-penetration groove welds at the factory. These welds must be qualified per ISO 15614-1 or ASME IX, with procedure qualification tests demonstrating adequate toughness at the minimum service temperature. The HAZ should be tested by Charpy V-notch impact testing to verify minimum absorbed energy of 27 J at the service temperature
2. Connecting plate welds: The plates connecting steel tubes to sheet piles are welded on-site, often in challenging conditions. The weld design should incorporate a full-penetration groove weld on the tube side and a fillet weld on the sheet pile side. The weld size should be determined by the expected shear and tensile loads, with a minimum throat thickness of 6 mm for typical connection plates
3. Bolted connection details: Where bolted connections are used instead of welding, the bolt grade (typically 8.8 or 10.9) and the number of bolts must be verified for the design loads. The bolt holes should be reamed to ensure proper fit, and the bolt tightening torque should be verified by torque wrench testing
4. Corrosion protection welds: The weld details should be designed to avoid crevices where water can accumulate, which would accelerate corrosion. The weld profile should be smooth and continuous, with no undercut exceeding 0.5 mm depth

The prefabrication approach also requires careful attention to dimensional tolerances. The steel tube segments should be fabricated to ISO 1307 tolerances for straightness and roundness, and the connecting plates should be machined to ensure proper alignment during assembly. The accumulated tolerance from multiple prefabricated segments should be calculated and verified to be within the installation tolerance limits.

Practical Implications and Recommendations

The numerical simulation provides valuable guidance for the design and construction of prefabricated cofferdam structures. The key recommendations include:

• The steel tube pile diameter should be selected to achieve a bending stiffness (EI) that is at least 1.5 times that of the adjacent sheet pile section to ensure uniform load distribution
• The connecting plates should be designed for a stress ratio not exceeding 0.8 of the yield strength under the most severe load combination, providing adequate margin for construction tolerances and unforeseen loading
• The internal bracing should be installed before excavation proceeds beyond 1.5 m below the previous bracing level, to limit the unsupported wall height
• The cofferdam walls should be monitored during construction using inclinometers, displacement gauges, and strain gauges, with alert levels set at 60% of the predicted maximum values

The prefabricated approach offers significant advantages in terms of construction quality, schedule efficiency, and worker safety, but requires careful planning of the fabrication sequence, transportation logistics, and installation methodology. Engineers should ensure that the prefabrication facility has the capability to produce segments to the required tolerances and that the welding procedures are qualified for the specific material grades and joint configurations used in the design. The numerical simulation results should be validated with field measurements during the first few construction stages, and the model should be updated based on the actual soil properties and structural response observed in the field.