GEOTECHNICAL BEHAVIOURAL STUDIES ON LOW ORGANIC SOIL TREATED WITH CHITOSAN AND CASEIN BIOPOLYMERS

Abstract

The ground improvement techniques have evolved since their inception and is slowly making a paradigm shift to the use of novel materials. Biopolymers are materials derived from natural sources such as plants, microorganisms, and animal waste products and are therefore non polluting, biodegradable, and sustainable. The current study investigated the influence of two hydrophobic biopolymers, namely chitosan (Dch) and casein (Dca) in enhancing the geotechnical properties such as strength, compressibility, and durability under alternate wetting and drying (w-d) cycles of a low organic soil. The effect of dosage (Dch and Dca), and curing period (CP) of 7, 14, 28, 60, and 90 days on the efficacy of biopolymer-treated soil was evaluated. The inclusion of the biopolymers led to the development of stable aggregated and flocculated structure at selected dosages (0.5%, 1%, 2%, and 4%). The Unconfined Compressive Strength (UCS) attained maximum values of 4.39 MPa and 3.13 MPa for chitosan and casein treated soils respectively. The curing period facilitated the formation of fibers leading to higher stability of soil-biopolymer mixes. Under alternate w-d cycles, the dosages (Dch and Dca) of 1% and 2% sustained up to 5 w-d cycles after completing CP of 60 and 90 days. The unsoaked CBR values attained for untreated and biopolymer-treated soil samples were greater than 5% for all dosages (Dch and Dca) and CP. However, the soaked CBR witnessed a decline at 4% (Dch and Dca) due to the presence of higher interaggregate voids. The optimized dosages (Dch and Dca) of 1% and 2% were adopted for conducting CU tests owing to the higher stability imparted to soil-biopolymer mixes. These dosages were fixed based on the experimental findings from UCS, unsoaked and soaked CBR, and durability tests. The highest effective cohesion (c’) and effective friction angle (ϕ’) were attained as 179.6 kPa and 21.57⁰ respectively, at 2% Dca and 60-day CP which can be attributed to the dense fibrous structure in casein-soil mixes. In general, the primary compression index (Cc) and secondary compression index (Cα) values reduced with an increase in dosage due to the filler characteristics of biopolymers causing a reduction in soil volume. At higher consolidation pressures, the primary compression index for each dosage increased due to the disruption of stable structures. However, the interaction of biopolymers with organic content led to an inconsistency for Cα with an increase in the consolidation pressure. The hydraulic conductivity (K) of chitosan treated soils iii Geotechnical Behavioural Studies on Low Organic Soils Treated with Chitosan and Casein Biopolymers NIT Warangal Synopsis exhibited higher values compared to untreated soil; whereas, casein-treated soils exhibited a reduction at all dosages and consolidation pressures. The correlation between independent variables such as Dch, Dca, CP, and consolidation pressure with dependent variables such as the strength and consolidation parameters was established through multivariate non-linear regression models with R2>0.95 for untreated and biopolymer-treated soils. Validation studies established the applicability of these equations for soil with organic content in the lower range of 10-20%. The feasibility of using biopolymer-treated soils for various civil engineering applications was evaluated by conducting a Target Reliability-Based Design Optimization (TRBDO) approach. The uncertainties associated with the variation in the independent parameters such as Dch, Dca, CP, and consolidation pressure on the dependent parameters such as UCS, soaked CBR, and hydraulic conductivity (K) were duly addressed by conducting a reliability analysis. The required UCS, soaked CBR and hydraulic conductivity (K) values for satisfying the safety criteria of each application was denoted as UCSmin, CBRmin, and Kmax. While assessing the suitability of biopolymer-treated soil as an embankment material, it was observed that a target reliability index of more than 3.0 was attained at a minimum Dch and Dca of 2.25% and 1.85% respectively, for 7 day cured samples at COV of UCS min of 10%. As a potential subgrade material for low-volume roads, the 28-day cured chitosan and casein samples attained a reliability index of 3 at dosages of Dch and Dca of 0.7% and 1.25% respectively at COV of CBRmin of 20%. Casein outperformed chitosan in attaining higher factor of safety and reliability index against hydraulic conductivity failure of liner material. The casein amended samples attained the target reliability index of 3 at all dosages and consolidation pressures at COV of Kmax of 20%. The evaluation of embodied carbon emissions (in tonnes) was based on the emission factor to produce 1 MWh of electricity which is obtained as 0.82, which was adopted from the CO2 baseline database for the Indian power sector. Initially, the different manufacturing stages were identified for each biopolymer and the energy consumed in kWh was calculated for each stage. The embodied carbon emissions of chitosan and casein were obtained as 0.00793 and 0.0476 tonnes/tonne of chitosan and casein, respectively. The current study demonstrated the suitability of biopolymer-treated low organic soil for various civil engineering applications such as embankment, subgrade or liner, and also aids the practicing engineers to adopt appropriate dosages and mixing criteria to be adopted in the field.