Double-Wall Casing: An Upgraded Solution for Challenging Ground Conditions

1. The Development History of Segmental Casing: From Full Casing to Segmental Casing

The development of segmental casing is, in essence, the result of foundation construction technology continuously adapting to increasingly complex ground conditions, greater drilling depths, and the need for more flexible and mobile equipment. Segmental casing did not emerge as a completely new product overnight. Instead, it gradually evolved from the traditional full casing construction system.

In early pile foundation construction, the primary purpose of casing was clear: to support the borehole wall during drilling and prevent borehole collapse, necking, and instability caused by groundwater. This was particularly important in loose sand, gravel, backfilled soil, and groundwater-rich formations, where relying solely on slurry stabilization was often insufficient to maintain borehole stability. As a result, steel casing gradually became an important method of borehole support.

1.1 Origin: The Development of Full Casing Drilling Rigs

In the 1950s, the French company BENOTO was among the first to systematically apply full casing construction equipment to pile foundation projects. This gradually led to the development of what became widely known as the Full Casing Method, also referred to as the Benoto Method.

The basic principle of this construction method is to continuously support the borehole wall with steel casing while drilling, using drilling tools inside the casing to remove soil or break rock. As drilling progresses downward, the casing advances simultaneously, keeping the newly formed borehole wall under casing protection at all times.

Compared with conventional methods that rely solely on slurry for borehole stabilization, the greatest advantage of the full casing method is its stronger control over borehole stability. In loose formations that are prone to collapse, the casing provides direct mechanical support. In areas with abundant groundwater or where slurry stabilization is less effective, it can also significantly improve borehole stability.

For this reason, full casing technology provided significant engineering value for early pile foundation projects in difficult ground conditions.

1.2 Evolution: From Long, Continuous Casings to Segmental Casing

However, as pile foundation projects developed toward deeper boreholes, larger diameters, and more complex working environments, long, continuous casings gradually revealed several practical limitations.

First, the longer the casing, the more difficult it becomes to manufacture, transport, and lift. If a single casing section needs to reach 10 meters or even several tens of meters in length, transportation becomes more difficult, while installation and lifting require larger cranes and more working space on site.

Second, during actual construction, casing needs to advance gradually as the borehole becomes deeper. When a single long casing is used, it becomes difficult to adjust the casing length flexibly according to the actual borehole depth.

As a result, engineers gradually developed traditional one-piece long casings into segmental casing.

Segmental casing typically consists of multiple casing sections with standardized lengths, such as 2 m, 3 m, or 4 m per section. During construction, casing sections are added one by one according to the actual drilling depth, with adjacent sections connected using bolted joints.

Although this appears to be a relatively simple structural modification, it brings several significant engineering advantages:

Easier transportation: Shorter casing sections are easier to load, transport, and move around the jobsite.

More flexible lifting: The lower weight of each individual section reduces the lifting requirements for the crane.

Greater flexibility in drilling depth: Casing sections can be added incrementally according to the actual borehole depth.

Easier manufacturing and maintenance: Standardized casing sections are easier to mass-produce, replace, and repair.

Better compatibility with rotary drilling rigs: Segmental casing can be integrated with the drilling, tool extraction, and casing installation sequence of rotary drilling rigs, providing greater flexibility during construction.

From this perspective, segmental casing is not a rejection of the full casing concept. Rather, it represents a modular and more practical engineering improvement of the traditional approach.

1.3 Adoption and Maturation: Integration with Rotary Drilling Methods

With the development of rotary drilling rig technology, the application of segmental casing expanded further.

Rotary drilling rigs offer high mobility, fast drilling speeds, and convenient tool changes, making them suitable for pile holes with different diameters and depths. However, in soft soil, sand, cobble and gravel layers, backfilled ground, and groundwater-rich formations, simply drilling rapidly with rotary drilling tools cannot completely solve borehole stability problems.

As a result, segmental casing gradually became an important auxiliary technology for rotary drilling operations. During construction, casing sections can be installed progressively according to ground conditions, allowing the casing to advance together with the borehole. This creates an effective combination between the high drilling efficiency of the rotary drilling rig and the borehole support capability of the casing.

The technology was subsequently introduced and further developed in countries such as Japan, Germany, and the United Kingdom. China began introducing related construction technologies in the 1970s and gradually started developing related equipment domestically in the mid-1990s.

As equipment, casing joints, and construction techniques continued to mature, segmental casing evolved from a simple “borehole support tool for preventing collapse” into a more complete technology system for construction in complex ground conditions.

2. Why Was Double-Wall Casing Developed?

Although segmental casing solved many of the transportation, lifting, installation, and construction flexibility issues associated with traditional long casings, the development of pile foundation projects toward greater depths, larger diameters, and more complex formations introduced new limitations related to the structural capacity of single-wall casing.

This is particularly evident in deep-hole construction, hard interlayers, cobble and gravel formations, highly weathered rock, and alternating soft-hard formations. Under these conditions, casing is no longer required simply to provide borehole wall support. It must also withstand axial forces, lateral earth pressure, bending loads, and torque generated during rotation by both the surrounding ground and construction equipment.

This means that the casing itself must have greater structural stiffness while maintaining good alignment and stability throughout the drilling process.

This led to the gradual development of double-wall casing. Simply put, it represents a structural reinforcement and functional upgrade based on traditional single-wall segmental casing. Its development has primarily focused on two key requirements: greater rigidity and more precise guidance.

2.1 Dealing with Complex Deep Formations: The Need for Greater Rigidity

In shallow construction, single-wall casing can usually meet basic borehole support requirements. However, as the casing extends deeper, the earth pressure, frictional resistance, axial forces, and torque applied by the drilling equipment can increase significantly.

In dense sand, cobble and gravel formations, or hard interlayers, casing does not simply advance under its own weight. Construction equipment often needs to apply both axial pressure and rotational force to gradually drive the casing through high-resistance formations.

Under these conditions, the casing must withstand complex combined loads. If the wall thickness, cross-sectional stiffness, or joint strength of a single-wall casing is insufficient, several problems may occur:

Local deformation or ovalization of the casing;

Bending of the casing under load;

Significant stress concentration at casing joints;

Torsional deformation during rotary operations;

Difficulty advancing the casing, potentially affecting subsequent drilling operations.

Moreover, the longer the casing, the more important structural stiffness becomes. During deep-hole construction, if the casing develops even a certain degree of bending or lateral displacement, the deviation can accumulate as the depth increases, ultimately affecting borehole position and verticality.

Double-wall casing was developed partly to address this problem.

By adding an inner cylindrical wall to the conventional casing and creating a combined cross-section between the inner and outer walls, the overall bending stiffness, torsional resistance, and deformation resistance of the casing can be significantly improved.

In other words, a double-wall casing is not simply a matter of “adding another steel tube.” It changes the structural characteristics of the casing cross-section, allowing it to withstand more complex construction loads.

Greater torsional resistance also enables the casing to transmit rotational forces and torque from the construction equipment more reliably, reducing the risk of significant torsional deformation of the casing itself.

Therefore, for deep-hole, large-diameter, high-friction, and complex hard-layer construction, double-wall casing generally provides a greater structural reserve than conventional single-wall casing.

2.2 Ensuring Borehole Quality: The Need for More Precise Guidance

During rotary drilling, the drill stem and drilling tools move vertically and rotate inside the casing. When the borehole is relatively shallow and the formation is uniform, the drilling tools can generally maintain a relatively stable trajectory.

However, as the borehole becomes deeper, the drill stem and drilling tools may experience a certain degree of deviation. This is particularly common in alternating soft and hard formations, inclined rock layers, or localized hard interlayers, where drilling resistance is uneven and lateral displacement is more likely to occur.

If there is insufficiently stable guidance between the casing and the drilling tools, the tools may rub against the inner wall of the casing. This can increase drilling resistance and equipment wear, while also changing the trajectory of the drilling tools and ultimately affecting the borehole diameter, position, and verticality.

The inner cylindrical wall of double-wall casing plays an important role here.

It can provide a more stable internal guidance space, giving the drilling tools clearer geometric constraints as they move inside the casing. Particularly in deep-hole construction, this guidance function can help reduce excessive tool oscillation and uncontrolled lateral deviation.

From a construction perspective, this relationship can be understood as follows:

The outer casing improves overall structural stability, while the inner wall further improves the operating and guidance conditions for the drilling tools.

Together, these two structural elements enable the casing to function not only as a “borehole protection layer”, but also as a stable guiding structure during drilling.

2.3 Why Are “Rigidity + Guidance” Both Essential?

If the casing has strong load-bearing capacity but undergoes significant bending or displacement in a deep borehole, it may still fail to provide stable guidance.

Conversely, if the casing provides good guidance but has insufficient stiffness, deformation caused by high earth pressure, frictional resistance, and torque can also compromise drilling accuracy.

Therefore, construction in complex ground conditions places a combined requirement on the casing:

It must be both stable and accurately aligned.

The double-wall structure is optimized around this requirement. The inner and outer walls increase the overall cross-sectional stiffness and torsional resistance, while the inner cylindrical wall improves the guidance conditions for drilling tools during operation.

As a result, the casing can better adapt to deep-hole construction and complex ground conditions.

2.4 From a “Borehole Support Tool” to a “Structural and Guidance System”

The development of double-wall casing can therefore be viewed as an important stage in the evolution of casing technology from a single-function component toward a multifunctional system.

Early full casing technology primarily addressed one question:

“How can we prevent the borehole wall from collapsing?”

Segmental casing addressed a further question:

“How can casing be made easier to transport, install, and advance section by section?”

Double-wall casing takes the technology one step further:

“How can casing maintain sufficient rigidity, torsional resistance, and guidance stability in deep holes and complex ground conditions?”

Therefore, double-wall casing should not simply be understood as a conventional single-wall casing with “an additional layer of steel.” Instead, it should be regarded as a structural upgrade developed specifically for deep-hole, high-load, and complex-ground construction requirements.Its core value lies in using an inner-and-outer double-wall structure to achieve improved overall performance in terms of rigidity, bending resistance, torsional resistance, deformation resistance, and guidance stability, providing a more reliable foundation for increasingly demanding full-casing drilling operations.

As the leading supplier of casing, Drillmaster produces full range of casing, including steel casings, single-wall segmental casing, double-wall segmental casings and all accessories. Welcome to contact us if any needs, we are glad to prepare all-in-one solution for you.