Concrete for Rigid Inclusions—Potential Risks and Their Mitigation Strategies

Rigid inclusions are used for ground improvement works and are usually designed as unreinforced low strength concrete columns with typical diameters ranging from 270 to 600 mm and depths up to 25 m. The installation methods of rigid inclusions often involve full-displacement drilling methods, including (but not limited to) Controlled Modulus Columns (CMC) and Controlled Stiffness Columns (CSC), or non-displacement techniques like Continuous Flight Auger (CFA) Columns. Both kinds of methods utilise pumped and pressurised in-situ concrete for the construction of rigid inclusions. Alternatively, vibro-displacement (VD) methods are also a proven technique for the installation of rigid inclusions, but VD do not use pressurized concrete. Ground improvement using cast-in-place rigid inclusions often requires the use of low to medium strength concrete to achieve project-specific design criteria. Concrete mix designs need to allow sufficient workability and stability requirements for the fresh concrete to be able to withstand external pressures from pumping and placement. This is important for minimising the risk of creating integrity issues or defective columns. Also, the workability life and the early strength development of the concrete mix need to be considered to allow for high quality rigid inclusions as well as smooth and efficient construction. This paper discusses the importance of adequate workability of the fresh concrete and, when fresh concrete performance is inadequate, outlines the effects of unsuitable concrete mix designs on the quality and integrity of rigid inclusions. The paper also highlights selected construction-related aspects—for instance, insufficient concrete placement and site traffic—that may affect the quality and integrity of rigid inclusions.