Bio-Enzymatic Treatment of Expansive Black Cotton Soil Using TerraZyme and Fly Ash: Quantifying the Strength–Swelling Trade-off

Abstract

Black Cotton Soil (BCS), prevalent across central India, poses severe geotechnical challenges owing to its high plasticity, low bearing capacity, and pronounced swelling behaviour. Fly ash and bio-enzyme stabilizers have each been shown to improve expansive soils, but the two have not previously been optimised sequentially on the same highly expansive soil, and the interaction between them has not been quantified. This study reports a three-phase laboratory programme on a CH black cotton soil (LL = 61%, PI = 38%, FSI = 66.67%). Phase 1 optimised the TerraZyme (TZ) dosage on untreated BCS; Phase 2 added Class-F fly ash (FA) at 10–40% by dry weight to the TZ-optimised soil; Phase 3 measured swelling magnitude and swelling pressure on the three key mixes. The replicate scheme is stated in full in Section 3.6: one specimen per mix per condition for UCS and CBR, and two for the swelling tests. Phase 1 identified 67 ml/m³ as the optimum TerraZyme dosage, raising Day-0 UCS from 117.44 to 838.24 kN/m² (7.1×), Day-7 UCS from 134.38 to 1205.43 kN/m² (9.0×), soaked CBR from 3.28% to 13.87% (4.2×) and unsoaked CBR from 13.42% to 16.24% (+21%), while reducing the plasticity index from 38% to 23%. Phase 2 identified 20% Class-F fly ash as the best-performing addition on strength and plasticity criteria, and Phase 3 confirmed that this mix reduced swelling pressure from 2.40 to 1.35 kg/cm² (−43.8%) and swelling magnitude by 50%. However, the fly ash addition was strongly antagonistic to bearing capacity: relative to the TerraZyme-only optimum the Day-7 UCS fell by 70%, and both the soaked (1.82–2.55%) and the unsoaked (6.37–9.12%) CBR of every TZ + FA mix fell below the corresponding values of the untreated soil. The combined mix is therefore not recommended as a load-bearing pavement subgrade. TerraZyme alone at 67 ml/m³ is the appropriate treatment where bearing capacity governs, whereas the TZ + 20% FA combination is suited only to applications in which volume stability governs and the treated layer is not required to carry traffic loading, such as expansive-soil cover or cushion layers, canal linings and non-structural fills. Quantifying and mechanistically explaining this strength–swelling antagonism, rather than recommending the combined mix, is the principal contribution of this study.