when is double wall casing need? 800mm double wall casing

when is double wall casing need? 800mm double wall casing

Double-wall casing is selected when the structural integrity of the borehole or the casing itself is at risk. While single-wall casing is lighter and cheaper, it lacks the rigidity required for the most demanding geotechnical environments.

You generally need double-wall casing in the following four scenarios:

1. High-Torque and High-Pressure Conditions

When drilling deep into dense soils or rock, the friction against the outside of the casing becomes immense.

Torque Resistance: Double-wall casing has a much higher section modulus. It can withstand the massive twisting force (torque) of a rotary rig or casing oscillator without "corkscrewing" or buckling.

Side Friction: In deep bores (often exceeding 20–30 meters), the "grip" of the soil on the casing is so strong that a single-wall pipe might tear or collapse during extraction.

2. Use of Casing Oscillators or Rotators

If the project requires a hydraulic casing oscillator, double-wall is almost always mandatory.

The oscillator grips the casing with massive hydraulic pressure to twist and push/pull it.

A single-wall casing would likely be crushed or severely dented by the oscillator’s clamping collar, whereas the double-wall structure provides the necessary "crush resistance" to maintain its shape.

3. Unstable or Water-Bearing Soils (Hydrostatic Pressure)

In coastal areas or sites with high water tables, the external pressure from water and "soupy" soil can be significant.

Collapse Prevention: As you pump out the soil from inside the casing, the external pressure tries to collapse the tube inward. The reinforced ribs between the inner and outer shells of a double-wall pipe act as a bridge, preventing this "implosion."

Watertight Seals: Double-wall joints (like the Bauer-type screw joints) are designed to be more watertight, keeping the borehole dry for a clean concrete pour.

4. Working with Large Diameters and "Hammer Grabs"

When using a hammer grab (a heavy mechanical jaw dropped into the hole to remove soil), there is a high risk of the tool striking the inside of the casing.

Internal Impact: In a single-wall casing, a strike from a heavy grab can cause a "bulge" or dent that protrudes outward. This makes it extremely difficult to continue drilling or to extract the casing later.

The Buffer Zone: In a double-wall system, the inner wall protects the outer structural wall. Even if the inner wall takes a minor dent, the outer shell remains perfectly circular, ensuring the casing can still be pulled out of the ground successfully.

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