Prefabricated Diaphragm Wall Vertical Joint Steel Pipe Interlocking Rigid Joint Bending and Shear Test Study

Literature Overview

This paper presents experimental and analytical investigations on the mechanical performance of steel pipe interlocking rigid joints used in prefabricated diaphragm walls. The study focuses on the bending resistance and shear capacity of the vertical joints where steel pipe interlocking elements connect adjacent prefabricated wall panels. The research addresses a critical engineering challenge in deep foundation and excavation support systems, where the integrity of vertical joints determines the overall structural performance of the diaphragm wall under lateral earth and water pressures.

Experimental Setup and Test Methodology

The experimental programme involves fabricating test specimens that represent the steel pipe interlocking rigid joint configuration used in prefabricated diaphragm walls. The specimens are subjected to cyclic bending and monotonic shear loading to evaluate their mechanical response under conditions simulating service loads and seismic events. The test matrix covers different steel pipe diameters, wall thicknesses, and joint geometries to establish design guidelines for various engineering applications.

Test Parameter	Variation Range	Test Purpose
Steel pipe outer diameter	89 mm – 219 mm	Size effect evaluation
Wall thickness	6 mm – 12 mm	Thickness influence study
Joint overlap length	150 mm – 400 mm	Interlock geometry effect
Loading type	Cyclic bending / Monotonic shear	Load case simulation
Loading rate	0.5 mm/min – 2.0 mm/min	Rate sensitivity check

The specimens are instrumented with strain gauges, displacement transducers, and load cells to capture the full load-deformation behaviour. The test results reveal the typical failure modes, including local buckling of the steel pipe interlocking elements, weld fracture at the connection points, and plastic hinge formation in the joint region.

Key Findings and Technical Analysis

The bending capacity of the steel pipe interlocking rigid joint is primarily governed by the plastic moment resistance of the interlocking pipe elements and the weld strength at the connection interfaces. The study shows that the joint bending capacity increases with the steel pipe diameter and wall thickness, following a nonlinear relationship that accounts for local buckling effects at higher loads. The interlock overlap length has a significant influence on the shear capacity, with longer overlap lengths providing greater shear transfer resistance through increased bearing area and weld length.

The cyclic bending tests reveal that the joint exhibits satisfactory ductility and energy dissipation capacity, with stable hysteresis loops observed under moderate displacement amplitudes. However, at larger displacement levels, degradation of the load-carrying capacity is observed due to cumulative damage in the weld regions and progressive local buckling of the interlocking elements. The shear test results indicate that the joint can sustain substantial shear loads, with failure typically occurring through shear fracture of the interlocking pipe walls or weld rupture.

Welding Quality and Defect Analysis

The welding quality of the steel pipe interlocking joint is a critical factor determining the overall joint performance. The following defect analysis and countermeasures are identified:

Defect Type	Root Cause	Countermeasure
Weld undercut	Excessive arc voltage or travel speed	Reduce voltage, optimise travel speed
Porosity	Moisture in shielding gas or base metal	Dry flux, improve gas flow
Incomplete fusion	Insufficient heat input	Increase current, reduce travel speed
Cold cracking	High carbon equivalent of base metal	Preheat to 100–150°C, post-weld stress relief
Distortion	Asymmetric welding sequence	Balanced welding sequence, back-step welding

The carbon equivalent of the steel pipe material (typically Q235 or Q345 grade) must be carefully controlled to prevent cold cracking in the weld and heat-affected zone. For Q345 grade steel with a carbon equivalent exceeding 0.45%, preheating to 100–150°C is recommended, and the interpass temperature should be maintained between 150°C and 250°C. Post-weld stress relief treatment at 550–620°C for 2 hours per 25 mm of thickness is advisable for critical applications.

Engineering Application and Design Recommendations

The prefabricated diaphragm wall system with steel pipe interlocking rigid joints offers significant advantages in construction efficiency and quality control compared to in-situ cast diaphragm walls. The prefabrication approach allows for factory-controlled welding quality, which is particularly beneficial for ensuring consistent joint performance. However, engineers must carefully consider the transport and handling of prefabricated panels, as improper handling can introduce geometric deviations that affect the joint fit-up and welding quality.

Design recommendations include: selecting steel pipe interlocking elements with a minimum diameter of 114 mm and wall thickness of 8 mm for typical excavation depths up to 15 m; ensuring weld penetration of at least 60% of the pipe wall thickness; and providing adequate stiffening plates at the joint connection regions to prevent local buckling. The study confirms that the steel pipe interlocking rigid joint can serve as a reliable structural connection in prefabricated diaphragm walls when designed and fabricated in accordance with the recommended parameters.