Natural polymers (biopolymers) are discussed as environmentally friendly and sustainable grouting chemicals. This paper presents guidelines for selecting potentially useful biopolymers for strengthening cohesionless soil. Agar and six modified starches were identified for further study over a range of concentrations (1–4% agar and 0.5–1% starch). Experimental results demonstrated the compatibility of agar and starch. Depending on the biopolymer concentration, the unconfined compressive strength of the sand treated with agar and starch biopolymers ranged from 158 to 487 kPa. Triaxial compression tests over a range of confining pressures also indicated that the biopolymers effectively increased the cohesion intercept and stiffness of the treated sand.

The deformation and stress profiles of passive arching in a coarse sand and two rubberized sand backfills were investigated using a trapdoor apparatus. The trapdoor apparatus was instrumented with high-speed imaging equipment and a series of pressure sensors. The images of the deformation process in the backfills were analysed using the digital image correlation technique. The effect of a local surcharge on the deformation and stress profiles was also examined. It was observed that the rubber inclusions helped reduce the deformation of the backfills. Passive arching moduli and stress variations between the backfills examined are compared.

The mechanical performance of microfine ground-granulated blast-furnace slag grouts for improving sandy soils was evaluated using unconfined compressive strength, drained triaxial compression and Brazilian tensile strength tests of grouted sand specimens. Two activator types, namely sodium hydroxide and rapid-hardening cement (RHC), and two biopolymer additives, namely xanthan gum (XG) and diutan gum (DG), were investigated for slag-based grouts prepared at various water–binder (w/b) ratios. The bench-scale investigation examined the cured strength properties of grouted sand specimens. It was observed that the RHC activator with modest biopolymer additive and smaller w/b ratio resulted in markedly higher strength gains. In terms of activator type, compared to sodium hydroxide, the RHC resulted in significantly better performance, mobilising increased cohesion in specimens tested in drained triaxial compression. When used together with slag grouts, XG and DG mostly showed comparable strength improvements, but DG proved more effective at lower dosage in increasing tensile strength of grouted sand.