Double-Wall Casing: Structure, Functions, Types and Selection Guide

What Is Double-Wall Casing?

Double-wall casing is a type of casing used for borehole support in foundation piling. It consists of two steel pipe walls, an inner wall and an outer wall, connected by spiral ribs, reinforcing ribs, or other structural elements to form a composite steel pipe with a hollow interlayer.​

It is primarily used in foundation engineering projects such as rotary bored piles and full casing construction. During drilling, it supports the borehole walls, prevents soil collapse, and provides stable borehole conditions for drilling and subsequent concrete pouring.​

I. Basic Structure of Double-Wall Casing

• Outer Wall: In contact with the surrounding soil, it withstands lateral earth pressure and external forces generated during construction.​

• Inner Wall: Forms the internal passage of the casing, providing space for drilling tools and drilling operations.​

• Internal Ribs: Connect the inner and outer walls, support the two steel pipes, and improve overall stiffness and resistance to deformation.​

• Casing Joint: Connects individual casing sections and transfers torque, axial compression, and extraction forces.​

II. Main Functions of Double-Wall Casing

Function	Description
Borehole Support	Prevents borehole collapse in loose soil, sandy formations, and groundwater conditions.
Improved Structural Stiffness	The double-wall structure and internal ribs work together to enhance resistance to deformation.
Load Transfer	Withstands construction loads generated during rotation, pressing, and extraction.
Adaptability to Complex Ground Conditions	Improves casing reliability in unstable formations.
Reusability	With proper design and maintenance, the casing can be reused for multiple pile holes.

III. Double-Wall Casing vs. Single-Wall Casing

Comparison	Single-Wall Casing	Double-Wall Casing
Structure	Single steel pipe wall	Inner and outer steel walls connected by structural elements
Stiffness	Mainly depends on the thickness of the single wall	Provided by the combined action of the inner wall, outer wall, and internal ribs
Weight	Generally lighter at the same outer diameter	Generally heavier and more complex
Manufacturing Cost	Relatively low	Generally higher
Typical Applications	General borehole support conditions	Applications requiring higher stiffness, deformation resistance, and load-bearing capacity

IV. Why Does Double-Wall Casing Require Internal Ribs?

Double-wall casing is not simply made by placing one steel pipe inside another. The internal ribs between the two walls serve several important functions:​

• Maintain the structural spacing between the inner and outer walls.​

• Limit local deformation of both walls.​

• Transfer construction loads between the inner and outer walls.​

• Improve the overall structural stability of the casing.​

Double-wall casing can be classified into different structural types according to the number, cross-sectional shape, and arrangement of the internal ribs, as well as their resulting structural performance and manufacturing characteristics.​

The three common types discussed here are multi-rib spiral, square-rib circular, and single-rib spiral structures.​

V. Differences Between the Three Double-Wall Casing Structures

Comparison	Multi-Rib Spiral	Square-Rib Circular	Single-Rib Spiral
Structural Design	Multiple spiral ribs or reinforcing ribs arranged between the inner and outer walls	Square-section reinforcing ribs arranged circumferentially or spirally, depending on the manufacturer's design	A single continuous spiral rib connecting the inner and outer walls
Main Features	More ribs and more support points	Regular rib cross-section and relatively rigid connections	Relatively simple structure with continuous spiral reinforcement
Stiffness and Deformation Resistance	Generally high, depending on rib height, thickness, and spacing	Square rib sections can improve local support stiffness	Depends on rib dimensions, pitch, and welding quality
Torsional Resistance	Multiple ribs can improve overall torsional stiffness	Depends on the connection between the square ribs and the two walls	Continuous spiral reinforcement can transfer torque, subject to design parameters
Weight and Cost	Generally higher steel consumption and welding requirements	Depends on rib dimensions and arrangement	Relatively simple structure with potential for lower manufacturing cost
Typical Applications	Large-diameter, deep-pile, and high-load construction	Applications requiring local support and structural stiffness	General construction loads where weight and cost are important considerations

Key Differences: Multi-rib spiral structures emphasize multiple support points and overall reinforcement; square-rib circular structures focus on rib cross-section and connection design; single-rib spiral structures provide continuous reinforcement through a relatively simplified design.​

However, torsional resistance cannot be determined by the structural name alone. The performance of double-wall casing also depends on thethickness of the inner and outer walls, rib height and thickness, spiral pitch, weld quality, and casing joint design.​

Double-wall casing must be capable of transferring torque, axial compression, and extraction forces generated by the piling equipment.​

VI. How to Select Double-Wall Casing for Different Construction Conditions

1) Multi-Rib Spiral

This structure is suitable for applications where overall casing stiffness and deformation resistance are key considerations, such as:​

• Large-diameter and deep-pile construction.​

• Casing installation involving high torque or rotary pressing.​

• Complex ground conditions with significant lateral earth pressure.​

• Applications requiring high extraction forces and repeated casing use.​

Its main advantage is enhanced structural reinforcement. However, casing weight, manufacturing cost, and the welding quality between the ribs and the inner and outer walls should also be considered.​

2) Square-Rib Circular

If "square-rib circular" refers to double-wall casing with square-section reinforcing ribs arranged circumferentially or spirally, its potential advantages include a regular rib cross-section and good local support capacity.​

Key factors to consider include:​

• Square rib dimensions and spacing.​

• Welded connections between the ribs and the inner and outer walls.​

• Uniformity of support between the two walls.​

• Local stress concentration under torsional loads.​

Whether this structure is superior to a multi-rib spiral design cannot be determined by cross-sectional shape alone. The number of ribs, effective cross-sectional area, and overall structural design must also be compared.​

3) Single-Rib Spiral

This structure typically uses one continuous spiral rib to connect the inner and outer walls, resulting in a relatively simple design.​

Potential advantages include:​

• Simple structure and relatively straightforward manufacturing.​

• Potential to reduce casing weight and manufacturing cost.​

• A continuous spiral connection that provides a load-transfer path.​

However, if the rib cross-section is too small, the spiral pitch is too large, or the welding quality is insufficient, local deformation or stress concentration at the connections may occur under high torque and axial loads.​

VII. Key Parameters for Casing Selection

Parameter	Impact on Construction Performance
Inner and Outer Wall Thickness	Determines resistance to compression, bending, and local deformation
Rib Cross-Sectional Area	Affects support between the walls and load transfer
Spiral Rib Spacing / Pitch	Influences support density and local deformation
Number of Reinforcing Ribs	Affects structural arrangement and load distribution
Weld Design and Quality	Determines the reliability of torque, compression, and extraction force transfer
Casing Joints	Affect torque and axial load transfer along the casing string
Casing Diameter and Length	Influence overall stiffness, weight, and construction loads

VIII. Practical Selection Recommendations

• Large-diameter piles, deep piles, hard rock, or high-torque construction: Prioritize comparison of multi-rib spiral structures with higher reinforcement capacity.​

• Applications requiring a balance between stiffness and structural regularity: Evaluate the rib cross-section, welding method, and support spacing of square-rib circular structures.​

• General borehole support applications where weight and cost are important: Consider single-rib spiral structures, provided that the actual construction loads are properly calculated.​These recommendations provide a structural basis for selection rather than a fixed performance ranking. In full casing construction, the casing must withstand various loads, including rotation, pressing, and extraction. The final selection should therefore be based on the structural drawings and load-bearing calculations for the specific casing model.