Abstract

Secondary compression can be an important source of settlement in artificial fills, even when these fills are well constructed. In some cases, especially when the fill thickness is greater than about 15 m, the resulting long-term settlements can adversely impact the performance of structures and infrastructure, and thus may necessitate special preventive design provisions. Yet, this source of settlement is often mistakenly overlooked. Secondary compression can be even more problematic when the fill is poorly constructed. Backfills of former open-pit mines are examples of practical projects where assessments of long-term secondary compression settlement are necessary, especially when these backfills are deep and/or not properly engineered. Laboratory assessments of secondary compression in these materials are inherently problematic and become impossible when the fill contains large particles or has other complicating characteristics. However, this problem is an excellent opportunity to apply the observational method where the coefficient of secondary compression, Cαε, is assessed in-situ using settlement monument data. This Cαε value is then used to forecast future settlements, which typically continue for decades, and thus provides essential information for the site-specific design of structures and infrastructure. However, the experimental and analytical processes for conducting these evaluations are more difficult than might be expected, and missteps can lead to significant errors in the computed future settlements. Some of these difficulties are due to limitations in our knowledge of the underlying physical processes and in the analytical models used to describe them. Methods of collecting the required field data and conducting these settlement evaluations are discussed based on experience with deep fills in California as well as published data from elsewhere.