Experiences with post-construction retesting of engineered clay fills

A considerable amount of disputation, both legal and informal, arises where compacted clayey fills are retested some time after completion of the works and is predicated on two assumptions. Firstly, that once compacted, clayey fills remain unchanged thereafter and secondly that the results of post-construction retesting are more credible than the results of control testing carried out at the time of construction. The authors have been exposed to a number of cases, including legal proceedings, where earthworks having apparently been properly carried out and reputably tested during construction were retested some time after completion and assessed to be below specification. The simple conclusion often drawn by owners and their experts in such instances is that the earthworks were inadequately carried out at the time of construction. However, in the authors’ view there has developed a body of factual evidence which does not support that simple conclusion. This evidence has arisen from a variety of sources involving actual construction works where multiple testing and/or retesting by a range of reputable authorities often under conditions that were less than ideal. Nevertheless they have provided a series of experiences or case histories to which geotechnical engineers, earthworks contractors, lawyers and owners should have regard and from which valuable insights and lessons may be drawn. This paper deals with the factual aspects of seven cases which, in the authors’ view, challenge the simple conclusion based on the assumptions of essentially inert compacted clayey fills and the primacy of retests over tests during placement.