A study on the beginning of secondary compression of soils

Abstract

The time-compression curve obtained from a laboratory one-dimensional consolidation test generally consists of three parts, namely initial compression, primary consolidation, and secondary compression. While the initial compression occurs immediately after the application of a load increment, the primary consolidation and the secondary compression are time-dependent. It is not known whether both primary consolidation and secondary compression occur concurrently or if the secondary compression begins after the end of primary consolidation, as assumed in general practice. This paper examines this aspect based on the pore water pressure measured at the base of the sample during consolidation. It was observed that the beginning of secondary compression strongly depends on the load increment ratio (LIR). For the inorganic soils tested, the secondary compression begins practically after the end of primary consolidation. When a small LIR of 0.2 is adopted, the secondary compression starts as early as about 55% of the primary consolidation is over. For the organic soil tested, the secondary compression begins much earlier, compared to the inorganic soils even at a higher load increment ratio. The secondary compression-time relationship can be well represented by a hyperbolic model. The method adopted in this article is capable of separating the primary consolidation and secondary compression from time-compression data once the pore water pressure at the base of the sample is known.