Soft clayey soils were problematic soils with less bearing capacity and were highly susceptible to excessive settlements. Due to the increasing population and living standards, the scarcity of land with the growing construction activities needs to stabilise the clay deposits for construction activities. The present study focuses on studying the influence of waste foundry sand and bio-enzyme additives on improving the engineering properties of clay soil collected from the local region. The foundry sand was the waste generated in huge quantities from an iron-casting unit, and it should be utilised as a soil stabilisation additive as an eco-friendly solution. In addition, the bio-enzyme serves as a binder to hold the clay particles and improves the cohesion strength. The basic experimental tests were conducted for both the microscopic analysis and geotechnical characterisation of soft clay soil. The present study also examines the effect of curing time changes from 0 to 28 days on the sustainability of strength improvement of clay soils treated with foundry sand and bio enzyme. The UCS tests were carried out on both untreated and treated clay specimens prepared at the maximum dry density corresponding to their optimum water content. Direct shear tests were also carried out to examine the increase in the angle of internal friction of clay soil with the addition of foundry sand. The experimental test results confirm that the internal friction angle of clay soil was significantly improved by adding waste foundry sand, and the bio-enzyme improves cohesion strength. The study found that the shear strength of clay soil increased with the increase of additives up to the specific dosage and then decreased. The shear strength of treated clay soil was increased with curing time, indicating that bio-enzyme takes place in the chemical reaction in the presence of water content in clay soil.