تست های بارگذاری فونداسیون سطحی بزرگ مقیاس در خاک رس نرم Large-scale shallow foundation load tests on soft clay – At the National Field Testing Facility (NFTF), Ballina, NSW, Australia

1. Introduction This paper presents the results of four large-scale shallow foundation tests on soft estuarine clay carried out at the Australian National Field Testing Facility (NFTF), near Ballina, NSW, established as part of the activities of the Centre of Excellence for Geotechnical Science and Engineering (CGSE). Two rigid square pad foundations were loaded concentrically to failure in an unconsolidated and undrained condition (UU1 and UU2). Another two foundations were loaded with 30% and 50% of the measured UU failure load and allowed to fully consolidate before being brought to failure under undrained vertical concentric loads (CU1 and CU2). In this paper, the foundation response is analysed in conjunction with high quality site investigation data. An international shallow foundation prediction exercise was run in conjunction with these field tests to assess the current state of practice for predicting the undrained load-settlement response and ultimate capacity of shallow foundations. The outcomes of the prediction exercise are presented in a companion paper in this special issue [1]. Results of large-scale foundation tests on soft clay under vertical centric loading are sparse in the literature, and most notably include those carried out at the UK’s national soft soil test site, in Bothkennar, Scotland [2,3]. The Bothkennar site was extensively characterised as part of the research initiative [4]. The philosophy of the NFTF, at which the tests presented in this paper were carried out, was based on the Bothkennar exemplar. Similarities also exist in terms of the site conditions and foundation geometry. The Bothkennar site comprised a weathered crust to around 2 m depth below ground level overlying clay and silt that were deposited under marine or estuarine conditions with the summer water table 0.9 m below ground level [2]. The undrained shear strength within the clay/silt layer was also similar at the two sites. At Bothkennar, five rigid square pad foundations, of the same shape and similar size were tested over a period of 11 years enabling insights into the effect of long term preloading on foundation bearing capacity. However, some notable differences preclude direct comparison of outcomes from the Bothkennar tests and those from the NFTF. For example, the Bothkennar foundations are cast within the crust layer rather than at the top of the underlying estuarine clay layer as at the NFTF and the rate of loading to failure of the foundations at Bothkennar was two orders of magnitude slower than those at the NFTF, while the consolidation period between sets of tests was an order of magnitude greater. The experimental approach adopted for the Bothkennar field tests was drawn from earlier work of field tests on clays [5–9]. The shallow foundation construction details and observed performance, as presented in this paper, are freely available in digital form through www.geocalcs.com/datamap [10]. The site data and results from all the laboratory and in situ testing are also available through the online platform.