Mutual Effect of Fibre and Geogrid on the Strength and Deformation Properties of Sand in Triaxial Compression Test

Sand is a typical backfilling material for geotechnical engineering structures. However, in many cases, its bearing capacity is insufficient, or its deformation is excessive, and therefore a reinforcing method is necessary. There are generally two reinforcing methods, i.e., i) mixing with fibre; and ii) reinforced with geogrids in layers. Most of the past studies have been performed to investigate effect of these two methods separately. This study interests on studying the strength and deformation properties of a sand mixed with fibre and reinforced with geogrid layers by conducting a series of consolidated-drained triaxial compression (CDTC) tests. CDTC test samples were fully saturated by a double-vacuuming technique and consolidated by applying partial vacuuming of either 30, 60 or 85 kPa from the sample’s inside. There are four sample types: i) unreinforced sand; ii) sand mixed with fibre; iii) geogrid-reinforced sand; and iv) geogrid-reinforced sand mixed with fibre. Fibre contents were varied from 0 to 1% by dry mass. Three layers of reinforcing geogrid were used. Test results were analysed for compressive strength, internal friction angle, secant modulus (E’₅₀) and average Poisson’s ratio (n₅₀), and then they were compared. It was found that mixing sand with fibre results in: i) more lateral expansion upon applying the vertical compression; and ii) increase of cohesion and internal friction. Henceforth, when reinforcing the fibre-mixed sand with geogrid layers, the followings are resulted: i) increase of the tensile force mobilised in geogrid which thus increases the confining stress; and ii) increase of compressive strength of geogrid-reinforced sand by the increasing internal friction angle. Therefore, reinforcing sand by mixing it with fibre and reinforcing it with geogrid layers mutually can improve its strength and deformation properties better than using only one of them alone.