Drained residual strength of Little Belt Clay

K.K. Sørensen

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Laboratory and Field Testing

ABSTRACT: sidual strength of Little Belt Clay Sørensen, K. K. Geo and Department of Engineering University of Aarhus, Aarhus, Denmark ABSTRACT A series of multiple reversal direct shear tests have been conducted on both precut and intact undisturbed specimens of Little Belt Clay in order to investigate the post peak strain softening behaviour and to determine the drained residual strength properties of the clay. Little Belt Clay is characterised as a high plasticity heavily overconsolidated marine clay of Eocene age. Undisturbed samples were extracted from borings next to the Old Little Belt Bridge of 1935, which crosses the Danish strait Little Belt, in connection to the ongoing ground investigation for the Fehmarnbelt Fixed Link between Denmark and Germany. This paper presents the results together with previously unpublished results from an old series of direct shear test on Little Belt Clay which were conducted in connection to a similar ground investigation for the New Little Belt Bridge of 1970. The results are compared and discussed. RÉSUMÉ Une série de plusieurs tests de cisaillement direct inversé ont été menées sur des échantillons à la fois intactes et aux déformations post-pic, et une argile marine de grande plasticité fortement surconsolidée de l'âge Eocène. Des échantillons intacts ont été extraits de forages proches du vieux pont de Little Belt datant de 1935 qui traverse le détroit danois Little Belt, dans le cadre l'argile de Little Belt qui ont été menées dans le cadre d'une étude de sol similaire pour le nouveau pont de Little Belt datant de 1970. Les résultats sont comparés et discutés. 1 INTRODUCTION Knowledge of the drained residual strength is relevant not only to classical slope stability problems in high plasticity clays in which reactivation of existing natural failure surfaces may occur. It may also be highly relevant in relation to the assessment of the stability and settlement behaviour of heavily loaded shallow foundations situated on top of high plasticity clays. This is particularly true in situations when the foundation is placed on a sloping site. The Old Little Belt Bridge of 1935 which crosses the Danish strait Little Belt is a classic example of a structure with heavily loaded shallow foundations. The bridge piers are founded on high plasticity Little Belt Clay and partly on a sloping seabed. Vertical movement of up to 75 cm have been recorded for the Old Little Belt Bridge up until today, and the bridge piers are presently settling at a rate of approximately 1-3 mm/year (Okkels et al. 2011). Large shear displacements are likely to have occurred in connection to the observed settlements, and hence it is likely that the clay in critical local shear zones below the bridge piers is experiencing strain softening towards a residual strength. However, as discussed by Skempton (1977) the overall stability of slopes that have not undergone previous sliding can in most cases be assessed using a mobilised shear strength which is significantly greater than the residual strength. In cases with progressive failure and reactivation of existing failure surfaces the mobilised shear strength may however approach the residual strength. Skempton (1977) found that 1-2 meters of displacement were required in field cases to reach residual state. Hence, knowledge of the strain softening behaviour and residual strength properties of the Little Belt Clay is essential in understanding the observed settlements and in the assessment of the stability of the bridge piers. In connection to the ground investigation for the New Little Belt Bridge of 1970, a series of direct shear tests on both precut and intact undisturbed specimens of Little Belt Clay were conducted approximately 50 years ago to determine the drained residual strength of Little Belt Clay (test series A, GEO 1964 and 1964b). Tests were carried out on samples that were obtained from both the site of the New Little Belt Bridge of 1970 and the site of the Old Little Belt Bridge of 1935. The two sites are situated approximately 1 km apart. To confirm the previous test results a series of multiple reversal direct shear tests (test series B) has recently been carried out to supplement the old test series. The recent tests were, as previously, conducted on both intact and precut undisturbed specimens of Little Belt clay. Samples were in the recent test series obtained from a boring at the site of the Old Little Belt Bridge of 1935, in connection to the ongoing ground investigation for the Fehmarnbelt Fixed Link (Fermern A/S 2014). The conducted direct shear tests forms part of a larger advanced laboratory testing program which has been carried out to investigate the strength and deformation characteristics of Little Belt Clay. A series of oedometer tests, direct simple shear tests and triaxial tests have also been carried out as part of the testing program (cf. BaneDanmark 2014 and GEO 2014).

RÉSUMÉ: ed residual strength of Little Belt Clay Sørensen, K. K. Geo and Department of Engineering

Access this article:
Canadian Geotechnical Society members can access to this article, along with all other Canadian Geotechnical Conference proceedings, in the Member Area. Conference proceedings are also available in many libraries.

Cite this article:
K.K. Sørensen (2014) Drained residual strength of Little Belt Clay in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper456,author = K.K. Sørensen,title = Drained residual strength of Little Belt Clay ,year = 2014}