Deep Water Embedded Rock Interlocking Steel Pipe Pile Cofferdam Design and Construction for Ziyun Bridge

Literature Overview

This paper presents the design and construction technology for a deep water embedded rock interlocking steel pipe pile cofferdam used in the main bridge construction of the Ziyun Bridge in Fengcheng City. The project involves the construction of bridge foundations in deep water conditions with rock-embedded steel pipe piles, presenting significant engineering challenges related to cofferdam stability, pile driving into rock, and dewatering in deep water environments.

The Ziyun Bridge project represents a typical case of bridge construction in challenging river conditions where conventional cofferdam solutions are inadequate. The interlocking steel pipe pile system provides a robust solution that combines the advantages of steel pipe piles (high driving capacity, corrosion resistance, and reusability) with the stability provided by rock anchorage.

Core Technical Content

Cofferdam Design Parameters

Parameter	Design Value	Design Basis
Water depth	15-25 m	River hydrological survey
Pile diameter	800-1000 mm	Structural analysis
Pile spacing	Interlocking (0-50 mm gap)	Seepage control
Pile length above water	8-12 m	Stability and construction access
Embedment depth into rock	3-8 m	Bearing capacity and stability
Cofferdam wall thickness	8-12 mm	Structural analysis
Steel grade	Q345 or Q390	Material availability and performance

Structural Analysis

The cofferdam design must account for several loading conditions:

Hydrostatic pressure: The primary lateral load acting on the cofferdam walls. The pressure distribution is triangular, with maximum pressure at the base of the cofferdam.
Earth pressure: The lateral earth pressure from the surrounding soil, calculated using Rankine or Coulomb earth pressure theory.
Uplift pressure: The upward pressure from groundwater, which must be resisted by the weight of the cofferdam and the embedment depth.
Construction loads: Including crane loads, equipment loads, and material storage loads applied to the cofferdam top.
Seismic loads: Although typically not governing for temporary works, seismic loads may need to be considered for cofferdams in seismically active regions.

Stability Verification

The cofferdam stability is verified against several failure modes:

Overturning stability: The ratio of resisting moments to overturning moments must exceed the required safety factor (typically 1.5-2.0).
Sliding stability: The ratio of resisting shear forces to sliding forces must exceed the required safety factor (typically 1.5-2.0).
Bearing capacity: The bearing pressure at the base of the cofferdam must not exceed the allowable bearing capacity of the foundation soil or rock.
Seepage stability: The seepage gradient at the base of the cofferdam must not exceed the critical gradient to prevent piping failure.

Construction Technology

Pile Driving Process

The pile driving process for rock-embedded steel pipe piles involves several critical steps:

Pre-drilling: In hard rock conditions, pre-drilling may be required to facilitate pile driving. The drill diameter is typically 100-200 mm smaller than the pile outer diameter.
Pile driving: The piles are driven using a hydraulic impact hammer or diesel hammer. The driving energy is controlled to achieve the required embedment depth without damaging the pile.
Interlocking: Adjacent piles are driven in sequence to ensure proper interlocking. The interlocking quality is verified by measuring the gap between adjacent piles.
Rock anchorage: The piles are driven into the rock formation to achieve the required embedment depth, which provides the primary stability for the cofferdam.

Dewatering System

The dewatering system is critical for maintaining a dry working environment inside the cofferdam:

Wellpoint system: For shallow groundwater conditions, wellpoints are installed around the cofferdam perimeter to lower the water table.
Deep well system: For deep groundwater conditions, deep wells are drilled to intercept the groundwater flow and lower the water table to the required level.
Internal dewatering: If the cofferdam is not watertight, internal dewatering pumps are used to remove water that enters through seepage.

Quality Control

Quality control during cofferdam construction includes:

Inspection Item	Method	Acceptance Criteria
Pile driving energy	Hammer sensor	Within specified range
Pile verticality	Theodolite or total station	≤ 1/200 of pile length
Interlocking gap	Visual inspection and measurement	≤ 50 mm
Pile embedment depth	Driving record and measurement	≥ design embedment depth
Cofferdam deformation	Monitoring points	≤ 1/500 of pile length

Engineering Challenges and Solutions

Challenge 1: Pile Driving in Hard Rock

Driving steel pipe piles into hard rock is challenging due to the high resistance and the risk of pile damage. Solutions include:

Pre-drilling: Drilling pilot holes to reduce driving resistance.
Specialized driving equipment: Using high-energy hydraulic hammers designed for rock driving.
Pile tip design: Using reinforced pile tips or driving shoes to prevent pile damage during driving.

Challenge 2: Seepage Control

Maintaining a dry working environment inside the cofferdam is critical for safe construction. Solutions include:

Interlocking quality: Ensuring tight interlocking between adjacent piles to minimize seepage paths.
Cutoff wall: Installing a cutoff wall or grout curtain behind the cofferdam to reduce seepage.
Dewatering system: Implementing a robust dewatering system to lower the water table and remove any seepage water.

Challenge 3: Cofferdam Stability in Deep Water

Deep water conditions impose significant hydrostatic pressures on the cofferdam, requiring careful stability analysis and construction. Solutions include:

Increased embedment depth: Driving piles deeper into the rock to increase stability.
Internal bracing: Installing internal bracing systems to reduce lateral deflection and improve stability.
Staged construction: Constructing the cofferdam in stages, allowing for stability verification at each stage before proceeding.

Study Insights and Reflections

The Ziyun Bridge cofferdam project exemplifies the integration of design analysis, construction technology, and quality control in challenging deep water foundation construction. The success of such projects depends on the careful consideration of all relevant factors, from geological conditions to construction logistics.

I emphasize that the design of temporary works such as cofferdams should receive the same level of engineering rigor as permanent structures. While temporary works are often perceived as less critical, failures in temporary works can have catastrophic consequences for worker safety, project schedule, and overall project cost. The application of modern analysis tools, including finite element modeling and three-dimensional stability analysis, should be standard practice for complex cofferdam designs.

Furthermore, the experience gained from the Ziyun Bridge project can be applied to similar deep water foundation projects, provided that the specific site conditions are carefully evaluated and the design and construction methods are appropriately adapted. The key to success lies in the combination of sound engineering principles, practical construction experience, and rigorous quality control throughout the project lifecycle.