Direct shear box tests on full scale recycled aggregate used in temporary works

The use of recycled aggregate is widely seen as an option in civil engineering projects to meet carbon offset targets. This material is used for a range of applications (e.g. load distribution platforms, sub-bases, general backfill) and can consist of natural aggregates or recycled components. Therefore, its engineering characteristics can vary widely. It is important to characterise accurately the material properties of the aggregate to guarantee safe, economical solutions. The angle of friction is often the main parameter required for geotechnical design and usually obtained by laboratory tests. However, when using standard laboratory equipment there is an upper limit to the size of particle that can be tested, rendering such apparatus inappropriate for obtaining the shear parameters of recycled aggregate (specified size fraction of 0–125 mm). A new unique Giant Shear Box facility, capable of testing samples with particle sizes up to 190 mm dia., has been established at City St George’s, University of London. Series of tests were conducted to determine the strength parameters of the recycled aggregate currently being used for working platforms on HS2 – Euston Site. Three different material specifications were tested, the results indicating that the strength parameters were a function of both aggregate constituents and grading.