Influence of smectite content on the deformation behaviour of clay

Abstract

A number of structures are planned to be constructed on or in the Palaeogene clays in Denmark, e.g. the re-development of Aarhus Ø and the construction of the Femern Fixed Link. The Palaeogene clays are generally of very high plasticity, hold low strengths and have high deformation potentials despite being previously compressed by past glaciations. When unloaded the clay adsorbs water and swells profoundly. These geotechnical traits pose a challenge and must be accounted for when designing a structure situated on high plasticity clays. When not considered, the features of the Palaeogene clay may pose a safety concern for the building and settlements may develop over time.Previous studies on Palaeogene clays in Denmark have shown, that the well-known index relations used in Danish geotechnical practice are generally not valid for the in situ behaviour of high plasticity clays. Moreover, several studies illustrates how the vertical yield stress derived from the compression curves of high plasticity clays does not reflect the previously sustained overburden, which normally is the case for clays of low to medium plasticity. The feature have been described as "lack of stress memory".The present thesis investigates the 1D deformation behaviour of high plasticity clays. The effects of the clay mineral smectite on the deformation and swelling potential of the high plasticity clays are of special focus. Based on 1D compression tests the deformation potential of a range of artificial specimens made from mixtures of kaolinite and smectite minerals were investigated. Features typically observed for Danish Palaeogene clays were also observed for the artificial mixtures, e.g. high plasticity and low stiffness, which have both been related to the smectite content in the tested, artificial specimens.Based on a number of tests, conducted following different loading programmes, a deformation model framework is presented which captures the essential features of the high plasticity clays. The model reproduces the compression curves obtained from testing of artificial specimens quite well and have also successfully been applied to reproduce compression curves from oedometer tests on intact natural highly plastic clays presented in literature. Based on the conducted tests it was observed that the lack of stress memory was related to the void ratio increase on a former unloading step, and thus directly affected by the smectite content of the specimen and the extent of unloading. It was also found that the compression potential in normally and overconsolidated conditions is dictated by smectite content but also affected by the length of the stress path followed during testing. III ResuméFlere bygge-og anlaegsprojekter er planlagt opført på de danske palaeogene lere, f.eks. udviklingen af Aarhus Ø og etableringen af den faste forbindelse over Femern Baelt. De palaeogene lere har generelt meget høj plasticitet, lave styrker og høje deformationspotentialer, selvom de tidligere er blevet belastet af isen under forgangne istider. Nå...

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

“…Decrease of curvature was observed though. Since Lodahl's [12] samples were reconstituted and therefore had no structure, destructuration could not occur, which complies with the hypotheses presented by Krogsbøll et al [13] and Gasparre and Coop [15]. Further, these tests indicate that the transition of Cs into Cs,y may be an effect of structure.…”

“…A general linear trend between the two parameters seems to exist, though with some scatter and outliers. Figure 2 shows previously reported correlations, summarised by Paykov and Hawley [11] along with the correlation by Lodahl [12] who studied mixtures of kaolinite and bentonite, thus with controlled mineralogy. After conversion to void ratio (instead of strain) the unloading branches sorted by SC are illustrated in Figure 3.…”

“…Lodahl [12] conducted IL tests on artificial mixtures of kaolinite and bentonite and did not find any effect of consecutive unloading, as suggested by Krogsbøll et al [13] and Rocchi et al [14]. These tests continued to UI as high as 20, but did not transition into a constant Cs.…”

“…Further, these tests indicate that the transition of Cs into Cs,y may be an effect of structure. In Lodahl's [12] artificial samples the SC varied between 0 and 65%. He found that increased SC leads to higher Cs (defined in [12] as the secant value for the entire unloading reloading loop) backing up the trend from Figures 5 and 7.…”

“…In Lodahl's [12] artificial samples the SC varied between 0 and 65%. He found that increased SC leads to higher Cs (defined in [12] as the secant value for the entire unloading reloading loop) backing up the trend from Figures 5 and 7. In Lodahl's [12] tests, the scatter in Cs increased with increasing SC, which may explain the high scatter in the trends for Røsnaes Clay due to a generally high SC.…”