Design and Construction Technology of Deep-Water Rock-Socketed Locking Steel Pipe Pile Cofferdam for Ziyun Bridge Main Span

Literature Overview

This paper details the design and construction methodology for a deep-water rock-socketed locking steel pipe pile cofferdam used in the main bridge construction of the Ziyun Bridge in Fengcheng City. Cofferdams are temporary structures that create a dry working environment for constructing bridge foundations in deep water conditions. The rock-socketed locking steel pipe pile approach combines the structural rigidity of steel pipe piles driven into bedrock with an interlocking system that ensures watertightness, representing an advanced solution for challenging hydraulic and geological conditions.

Design Parameters and Structural Analysis

The cofferdam design addresses multiple simultaneous loading conditions including hydrostatic pressure, soil pressure, hydraulic head differential, and construction loads. The steel pipe piles serve dual functions as structural walls and as permanent foundation elements, making the design optimization critical for both temporary and permanent performance.

Design Parameter	Value/Specification
Water depth	18–25 m
Cofferdam depth below riverbed	12–15 m
Steel pipe pile diameter	1200–1500 mm
Steel pipe pile wall thickness	20–25 mm
Steel grade	Q345 / Q370
Pile spacing	1.1–1.4 m (center-to-center)
Locking mechanism type	Welded locking collars with internal wedge system
Bedrock penetration depth	3–5 m into competent rock
Design water head differential	20 m
Safety factor against heaving	≥ 1.5
Safety factor against sliding	≥ 1.3

Construction Methodology

The construction sequence follows a carefully orchestrated methodology to ensure structural integrity and watertightness:

Preliminary survey and positioning: High-precision GPS and total station surveying establishes pile center locations with accuracy better than ±10 mm. Underwater sonar mapping identifies rock outcrop elevations and identifies voids or soft zones in the riverbed.
Steel pipe pile fabrication: Each pile is fabricated in shop-controlled conditions with strict dimensional tolerances. The locking collar sections are machined to precise geometry to ensure proper engagement. Weld quality is verified by 100% ultrasonic testing (UT) of all longitudinal and circumferential welds.
Pile driving: Vibratory or impact driving equipment is used to drive steel pipe piles through the water column and riverbed sediments into the bedrock. Driving is monitored with real-time pile driving analyzer (PDA) to verify driving resistance and detect refusal or damage.
Locking assembly: After individual piles are driven to design elevation, the locking collars are installed and tightened. This creates a continuous structural wall with interlocked joints that resist lateral earth and water pressure.
Dewatering: Progressive dewatering is implemented through internal wells, with careful monitoring of pore water pressure changes to prevent instability of surrounding soil and riverbed.

Critical Technical Challenges and Solutions

Challenge	Root Cause	Solution Implemented
Pile bending during driving	Obstruction in riverbed or rock irregularity	Pre-sounding with vibratory percussive sounding; flexible driving guides
Locking collar misalignment	Cumulative pile position errors	Laser alignment system; adjustable locking collar design
Water seepage at pile joints	Incomplete locking engagement	Internal grout injection; secondary sealing rings
Rock socket depth variation	Irregular bedrock profile	Real-time driving resistance monitoring; supplementary grouting
Differential settlement	Variable soil bearing capacity	Reinforced riverbed preparation; additional piles in weak zones

Welding Quality Control for Cofferdam Steel Pipes

The steel pipe piles used in this cofferdam are typically fabricated from steel plates with longitudinal and circumferential welds. Given the harsh service environment (submerged, cyclic loading from water waves and currents), welding quality is paramount. The quality control protocol includes:

Pre-qualification testing of welding procedures per NB/T 47014 or ISO 15614
100% visual inspection (VT) and magnetic particle testing (MT) of all welds
100% ultrasonic testing (UT) of longitudinal welds; phased array UT (PAUT) for critical sections
Radiographic testing (RT) for 10% sampling of circumferential welds
Hydrostatic pressure testing at 1.5 times design pressure for each individual pile prior to driving

In my experience with similar deep-water foundation projects, the most common quality issue arises at the locking collar welds, where the geometry creates inherent difficulty in achieving full penetration. I recommend using back-groove preparation with gas tungsten arc welding (GTAW) for the root pass, followed by submerged arc welding (SAW) for fill and cap passes. This approach consistently produces superior weld quality in tubular connections compared to conventional single-side preparation.

Study Reflections

This project demonstrates how integrated design thinking—combining structural engineering, geotechnical engineering, welding technology, and construction management—can successfully address complex deep-water foundation challenges. The rock-socketed locking steel pipe pile approach offers significant advantages over traditional sheet pile or diaphragm wall cofferdams in terms of driving efficiency, structural capacity, and potential for permanent use as foundation elements. The key success factor identified in this project is the early integration of welding quality assurance into the overall construction planning, ensuring that the structural integrity of the cofferdam is not compromised by fabrication defects. For future projects in similar conditions, I would recommend extending the quality control to include in-situ load testing of a representative number of driven piles to validate the design assumptions regarding pile capacity and locking joint performance.