Progressive Consolidation Settlement of Naturally Deposited Clayey Soil Under Embankment Loading

Abstract

In a naturally deposited soil foundation subject to embankment loading, a delayed settlement can often be observed after the embankment's completion, sometimes also showing acceleration over time. Frequently in such cases the soil exhibits an increase rather than a dissipation of excess pore pressure in some of its parts, a phenomenon which cannot be explained by conventional elasto-plastic consolidation theory. In this paper the possible mechanism for this kind of delayed settlement is investigated numerically using a soil-water coupled elasto-plastic computation under plane strain conditions. It is assumed that the soil section contains a medium dense sand layer and highly structured clay layers. A Super /subloading Yield Surface Cam-clay model is used to describe the elasto-plastic behavior of both the sand and the clay with respect to soil structure, overconsolidation and anisotropy. It is found that the delayed settlement behavior occurs under a certain constant embankment load, and persists over a period of 40 years, with increases in the settlement rate accompanied by both dissipation and a rise in excess pore pressure. The cause of this is the softening that co-occurs with the plastic compression of the soil skeleton. In other words, consolidation settlement can be considered as an example of "progressive consolidation with decay of structure". Some typical characteristics of this delayed settlement behavior are also numerically examined with reference to the height and weight of the embankment, and to soil improvements using sand drains. When the embankment is much lower, the foundation does not undergo delayed consolidation, and when it is higher, the foundation becomes subject to circular slip failure. Soil improvement with sand drains can effectively shorten the length of time up to final settlement.

Prediction of Settlement in Natural Deposited Clay Ground with Risk of Large Residual Settlement Due to Embankment Loading

“…15, the dissipation of pore pressure is delayed. As a result, large settlement continues to occur at a languid pace over a long period time in these layers (Noda et al, 2005). Figure 16 shows the deformation of the ground surface immediately after construction of the embankment and at the end of consolidation.…”

Ground Improvement of Intermediate Reclaimed Land by Compaction Through Cavity Expansion of Sand Piles

“…0 ＝44.4 kPa. The material constants and initial conditions of the crushed mudstone specimens outlined in Table 1 are shown in Table 3 together with those of typical sand (Nakano et al, 2005) and clay (Noda et al, 2005c). The calculations were performed using the same conˆning pressure p?…”

Proposal of a Simple Method for Assessing the Susceptibility of Naturally Deposited Clay Grounds to Large Long-Term Settlement due to Embankment Loading

“…This type of delayed consolidation settlement with acceleration of settlement is actually what has been observed at the Kanda site mentioned at the beginning of this paper. Further details and the mechanism of the above phenomenon have been described in other works by various authors (for example, Noda, et al, 2005).…”

“…In previous research studies (for example, Noda et al, 2005), the authors have shown that structure dominates the mechanical characteristics of naturally deposited clays even within the skeleton structure and that the delayed consolidation settlement behavior leading to large residual settlement is particular to clays with a high degree of structure. In the current study, special attention was paid to the degree of evolution of structure in the skeleton structure of the clays and the manner in which the structures change in order to investigate how these factors aŠect the sensitivity and compression index ratios.…”

“…Using the soil-water coupledˆnite deformation analysis program GEOASIA (Asaoka et al, 1994;Asaoka and Noda, 2007;Noda et al, 2008) with the SYS Cam-clay model (Asaoka et al, 2002), which is an elasto-plastic constitutive model capable of describing the action of the soil skeleton structure (structure, overconsolidation and anisotropy), the authors have clariˆed that large delayed and major consolidation settlements leading to large residual settlements similar to that mentioned at the Kanda site is caused by the decay of structure of highly structured naturally deposited clay (Asaoka et al, 2000;Noda et al, 2005). In other words, when highly-structured clay in-situ is subjected to large stress increases greater than the consolidation yield stress, a large amount of compression occurs because of the signiˆcant structural degradation.…”