Liner/casing is one type of formwork. It gives the desired shape to green concrete. Generally, it is made of steel, circular in shape.
Incase of cast-in-situ pile , there are two types of liner-
- Temporary liner: This type of casing is removed after the casting of concrete is complete (before start of final setting).
- Permanent liner:- This type of casing is concreted with the concrete and remains there for Lifetime of Pile.
Steel liners are used in the construction of bored cast in situ RCC piles especially in waterborne transport infrastructure. The Embedment length of the liner is the length between dredge level/bed levels to the termination level of the liner. Sufficient embedment length for steel liner ensures the quality of the pile. The minimum embedment depth of liner is calculated as per IS 2911-2010, PArt-1 Section 2; using the same procedure as given for piles.
Steel liners play an important role in the construction of cast in situ piles, especially in marine conditions. Temporary steel liners are used to stabilize the sides of the borehole during construction. In port and harbour structures liners become a permanent part of the foundation, which requires minimum load carrying capacity to withstand the construction loads. Liner is used to maintaining the clear cover between the main reinforcement and concrete surface during the stages of construction. It is used for bored cast in situ pile that extends through very soft soils, such as marsh deposits, to reach an underlying stratum that is more stable. In such cases, the liner is used to prevent the outward bulging of the fluid concrete into the surrounding soft soils. If a bulge forms at an elevation corresponding to an extremely soft stratum, there can be a risk of defects in the concrete due to a neck in the pile above the bulge, or deformation of the reinforcement cage.
In construction practice, there is no standardized method to fix the minimum embedment length of the liner. The embedment length of the liner depends on the stiffness of the steel liner and soil properties.
In cohesion-less soil, penetration resistance or N values are used to determine the modulus of subgrade reaction of the soil sample, which is helpful to determine the embedment depth of steel liner in sandy soil.
In clayey soil and rocky strata, the embedment depth of liners is based on the undrained shear strength of the soil, moment of inertia of the liner and Young’s modulus of the liner material. The unconfined compressive strength is assumed as two times the undrained shear strength.