Concrete related construction risks for rigid inclusions

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 and controlled stiffness columns, or non-displacement techniques like continuous flight auger 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 pressurised 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. This paper discusses the importance of adequate workability of fresh concrete and outlines some potential effects of unsuitable concrete mix designs and placement on the quality and integrity, including column necking and cracking.