Stress dependency and unloading-induced swelling behaviour of a high plasticity overconsolidated clay of Paleogene origin

Abstract

High plasticity clay of Paleogene origin in Denmark, possess unique deformational behaviour with significant stress dependent stiffness in oedometric loading/unloading. This is challenging settlement/heave predictions. The behaviour has been so far associated with high smectite content and complex geological loading history. Burial followed by erosion and then glaciotectonic activity, has resulted in fissures and slickensides, along with a high level of overconsolidation. Several currently planned and existing Danish construction projects encounter problems with such clays. Specifically, the immersed tunnel project connecting Denmark and Germany: “Fehmarn Fixed Link”, for which an extensive laboratory campaign of IL and CRS oedometer tests, has been carried out. These tests have been re-analysed, to obtain a better description of the swelling behaviour of Paleogene clays, due to the net unloading state expected for this immersed tunnel. The development of the swelling index during unloading, although non-linear, tends to reach a constant value at sufficient stress reduction. This constant value as well as the average swelling index increases with the stress at which unloading starts and increased smectite content. These trends are confirmed by an additional CRS test.

Mode-based characterisation of swell deformations in a high-plasticity Paleogene clay

“…Bjerrum (1967) observed a change from behaviour A to B when remoulding samples, while Mesri et al (1978) reported behaviour A for remoulded samples. Cui et al (2013) and Nguyen et al (2014) defined the term C sp for the plateau C s and observed that C sp increases with the maximum stress applied before unloading, σ' v-max , which is also seen in the data by Takahashi et al (2005), Gasparre and Coop (2008) and Kinslev et al (2019). However, neither Mesri et al (1978) nor Powell et al (2012) found any such dependency.…”

“…Several other authors only observed initial increasing C s onto a plateau (Takahashi et al 2005;Gasparre and Coop 2008;Powell et al 2012;Cui et al 2013;Nguyen et al 2014;Grønbech et al 2016;Kinslev et al 2019), displayed as behaviour B in Figure 1. Bjerrum (1967) observed a change from behaviour A to B when remoulding samples, while Mesri et al (1978) reported behaviour A for remoulded samples.…”

“…When comparing unloading branches between different stress levels, the analysis of the development of e with σ' v is not as straightforward as in compression since there is no unique reference line (like the ICL, NCL or SCL for compression). The comparison of swelling responses across different tests is therefore performed by plotting the change in void ratio since unloading, de, with the unloading index, UI, (ratio between σ' v-max and current stress, as defined by Kinslev et al 2019). Kinslev et al (2019) described how this normalisation collects the unloading branches at a common origin while retaining their shape from the compression plane.…”

“…The comparison of swelling responses across different tests is therefore performed by plotting the change in void ratio since unloading, de, with the unloading index, UI, (ratio between σ' v-max and current stress, as defined by Kinslev et al 2019). Kinslev et al (2019) described how this normalisation collects the unloading branches at a common origin while retaining their shape from the compression plane.…”

“…London Clay, the Flanders deposits, the Danish Paleogene clays, and Bearpaw Shale are the most extensively studied formations of these clays (Takahashi et al 2005;Gasparre and Coop 2008;Powell et al 2012;Cui et al 2013;Nguyen et al 2014;Grønbech et al 2016;Kinslev et al 2019). They are generally stiff and dense with low permeability caused by the compression from previously overlying Neogene deposits and glaciation.…”