Secant bored pile walls consist of overlapping primary (often referred to as soft) piles and secondary (hard) piles. The primary piles are usually, though not always, un-reinforced. They are typically formed using a cement-bentonite concrete mix for hard/soft walls, or a low strength concrete for hard/semi-hard walls. A structural concrete mix in both primary and secondary piles is used for hard/hard walls. Although not watertight structures these types of walls are ideally suited for use in granular or water bearing strata to control water flows and allow basement excavation to proceed with only limited dewatering.