Target Reliability-Based Design of Embankments Using Biopolymer-Modified Cohesive Soil

Abstract

This study evaluates the unconfined compressive strength (UCS) behavior of biopolymer-modified cohesive soil exposed to natural field conditions. Two biopolymers, xanthan gum (G1) and guar gum (G2), at dosages of 0.5, 1, 2, and 4%, are considered for the study. The performance of biopolymers under natural curing simulates the field conditions better. To assess the stability of biopolymer-modified soil, UCS specimens were subjected to daily variations in temperature and relative humidity, for curing periods of 7, 14, 28, 45, and 60 days. UCS test results indicate that the dehydration caused due to natural curing resulted in a significant increase in UCS, owing to the crosslinking of individual soil particles with hardened biopolymer for both G1 and G2. The effects of dosages of G1 and G2, as well as curing period, on UCS are represented by multivariate regression equations based on experimental data collected after the employed curing periods. The influence of the curing period on UCS of untreated cohesive soils is also provided using nonlinear regression equations. The dosages of G1 and G2 required to stabilize the cohesive soils for curing periods of 7 and 28 days for the construction of embankments are computed using deterministic design and target reliability-based design optimization (TRBDO). The results obtained using these approaches reveal that the factor of safety and reliability index against UCS failure of embankment increases significantly with the increase in G1 content from 0.5% to 4%. On the contrary, when the cohesive soils are blended with G2 with a dosage of 0.5% to 2.9%, the factor of safety and reliability index continued to increase significantly. However, the addition of G2 beyond 2.9% reduces the factor of safety and reliability index considerably. The results indicate that the optimum dosage of G2 is 2.9%. The TRBDO provides a reasonable and systematic procedure for the optimum design of embankments for biopolymer-blended cohesive soils.