Crushed Brick Waste–Soil Mixtures for Embankment Applications: A Bimsoil Characterization Using Large-Scale Multistage Shear Tests and Technical Applications

Crushed brick waste (CBW) is a major fraction of construction and demolition waste, yet design-oriented shear strength data for its reuse in earthworks remain scarce. This study examines CBW–soil mixtures within the block-in-matrix soil (Bimsoil) framework and quantifies their shear strength at large scale. The block-to-matrix contrast was checked through elastic modulus, friction angle, and unconfined compressive strength ratios. Large-scale multistage direct shear tests with a double shear-plane configuration were performed on one specimen per mixture, with the volumetric block proportion (VBP) ranging from 0% to 67%, and the failure point of every stage was defined with a single strain-based criterion. The friction angle increased from 21.3° to 38.3° with VBP, following a significant linear trend (R2 = 0.94), whereas the apparent cohesion dropped from 29.1 kPa for the natural soil to 10.6–18.5 kPa for the mixtures, without a significant trend among them. For VBP ≥ 48%, the envelopes intersect that of the natural soil at 46–52 kPa, within the tested range, so the natural soil governs at lower and the mixtures at higher normal stresses. In illustrative 6 m high gabion- and geogrid-reinforced soil walls, VBP of 67% reduced the gabion weight and geogrid area by 44.4% and 59.1% and by 28–45% and 19–61% over the examined wall heights and surcharges. These results provide a material-specific basis for selecting CBW content in embankments and retaining-wall backfills.