Capacity of driven precast prestressed concrete hexagonal piles: a Winnipeg region geotechnical study

ABSTRACT: Driven precast prestressed concrete hexagonal piles have been used to support industrial, commercial, and residential buildings in Winnipeg and the surrounding area since 1961. Pile design was originally governed by the National Building Code of Canada which stipulated the use of various methods, including static load tests, the Hiley dynamic pile driving formulae, and local experience. Governance of PPCH pile design was superseded in 1964 by the Metropolitan Corporation of Winnipeg By-law 711 which introduced maximum design capacities for specified pile sizes installed under ideal soil conditions. These capacities eventually became known locally as the “historical values” and provide the basis for the current state of local practice for geotechnical design of this pile type. The historical values were based on driving the piles to practical refusal in very dense silt till (hardpan). However, the historical capacities may not be appropriate where hardpan is absent and where sand inclusions under high porewater pressure may be present. The By-law was succeeded by the Manitoba Building Code in 1977 which omitted the historical values, although they are still commonly used today. This paper investigates the capacity of PPCH piles driven to practical refusal (estimated from dynamic pile load testing results) at three sites with varying till conditions to quantify potential differences between measured nominal capacities and those which would be derived from historical (allowable) capacities. The results demonstrate that there may be significant variability in the nominal capacity of PPCH piles driven to practical refusal in till indicating that the historical capacities may not account for this variability.