Internal Compression Of Fill Material Originating From Bringelly Shale

A rule of thumb handed down from senior to junior geotechnical engineers in Australia is that internal compression of embankment fill is 0.1% of embankment height and it has been widely used in performance specifications for major infrastructure projects around Australia. It is not clear where this rule of thumb originated from or on what basis it was developed. In Western Sydney this rule of thumb has been adequate for many years because the scale of earthworks in terms of fill thickness has been relatively minor. Large scale earthworks have started to occur over the past decade or so as more significant road and rail development has occurred. Recent experience with higher fills constructed in Western Sydney shows that internal compression strain rate can be greater than 0.1% per log cycle of time. Embankments to about 10 m height on a rail infrastructure projects on Western Sydney were constructed from Bringelly Shale. The fill materials and compaction were compliant with the relevant engineering standards. Comparison of topographic survey about 5 years after construction with design profiles indicated that they had settled between 0.05 and 0.25 m, subject to construction tolerances, and the internal compression strain rate varied between 0.5% and 6.3% per log cycle of time, adopting 1 year as the starting time of post construction settlement. Anecdotal evidence from road embankments of up to 14 m have identified similar magnitudes of settlement response from fill formed of the same Bringelly Shale materials. These values are much higher than the rule of thumb of 0.1% per log cycle of time. Though Bringelly Shale-based fill material has shown such a significant settlement issue, to the authors􏰜 knowledge, there are no references found in the technical literature that provides some guidance on assessing the internal compression of Bringelly Shale based fill material. Therefore, a series of laboratory tests have been conducted to understand the settlement behaviour of compacted Bringelly Shale. Compaction tests, particle size distribution, Atterberg limits, and small and large-scale compression tests of compacted fill material have been conducted. This paper summarises some of the findings of the laboratory tests and authors’ view on the performance of Bringelly Shale fill material and future studies.