Double wall casing, also known as dual-tube casing or double-walled pipe, is a type of casing system used in drilling operations to provide stability and support to the borehole. It consists of two concentric pipes, with the inner pipe known as the "drive pipe" and the outer pipe as the "casing."
Here are some key features and uses of double wall casing:
Construction: The double wall casing is typically made of steel and consists of two pipes that are joined together. The inner drive pipe is usually smaller in diameter, while the outer casing pipe has a larger diameter. The two pipes are connected at the bottom and top, creating a continuous annular space between them.
Stability and Support: The double wall casing provides increased stability and support to the borehole during drilling operations. It helps prevent the collapse of unstable formations, such as loose or water-bearing soils, and maintains the integrity of the borehole.
Soil and Fluid Containment: The annular space between the drive pipe and casing allows for the circulation of drilling fluids, such as drilling mud, which helps to remove cuttings and maintain pressure control during drilling. It also acts as a barrier, preventing the intrusion of soil or fluids into the borehole.
Grouting: In some cases, the annular space between the drive pipe and casing may be filled with grout, a cementitious material, to enhance the integrity and load-bearing capacity of the drilled shaft. Grouting helps to improve the bond between the casing and the surrounding soil or rock, providing additional support.
Extraction of Drive Pipe: After the drilling and grouting process is completed, the drive pipe can be extracted from the double wall casing. This allows for the installation of reinforcement, such as steel rebar, and the subsequent pouring of concrete to form a permanent drilled shaft.
Double wall casing is commonly used in various drilling applications, including bored piling, drilled shafts, and deep foundation construction. It is particularly beneficial in challenging soil or rock conditions where stability and support are crucial. The use of double wall casing helps to ensure the integrity of the borehole and enhances the overall performance and load-carrying capacity of the drilled foundation.