Impact of biopolymer type and concentration on stabilization of excavated subgrade soil

This study evaluates the potential of three biopolymers – sodium lignosulfonate (SLS), chitosan (CH), and guar gum (GG) – for stabilizing excavated sandy subgrade soil from construction sites in Qatar. The mechanical performance of treated and untreated soils was evaluated using unconfined compressive strength (UCS) testing after a 7-day dry curing period and a subsequent 2-cycle wet-dry testing. Two SLS grades were examined: low-molecular-weight analytical grade (SLS1) and high-molecular-weight commercial grade (SLS2). SLS2 significantly improved soil strength, with 1 wt% treatment yielding a UCS of 7.3 MPa, compared to 3 MPa for untreated soil. Hybrid treatment of 1 wt% SLS2 and 0.2 wt% GG achieved the highest UCS of 7.7 MPa under dry curing and 5.1 MPa after wet-dry cycling. While GG improved moisture resistance, its high viscosity adversely affected the material's workability. CH and SLS1-CH hybrids showed limited improvement in strength, likely due to poor soil compatibility and gypsum dissolution under acidic conditions. The superior performance of SLS2 was inferred from viscosity analysis, which indicated a higher molecular weight than SLS1, as evidenced by its higher solution viscosity at comparable concentrations. These findings demonstrate that high-molecular-weight SLS, when combined with GG, offers an effective and sustainable solution for enhancing the strength and early-stage resistance of excavated soils used in subgrade construction.