Characteristics of MICP- and EICP-treated sands in simple shear conditions: a benchmarking with the critical state of untreated sand

Microbial- or enzyme-induced calcium carbonate (CaCO₃) precipitation (MICP and EICP) are relatively new ground improvement techniques. In this study, the mechanical behaviour of biotreated (MICP/EICP) and untreated sands was investigated in light of the critical state soil mechanics framework using a series of direct simple shear (DSS) tests. A wide range of calcium carbonate content (C_C), initial void ratio after consolidation (e₀) and effective initial normal stress (⁠$σ′N0$⁠) was considered. The biotreated specimens showed improved shear strength and dilative tendency compared to untreated specimens with similar initial states. The ultimate state (US) for the biotreated sand shifted towards a smaller void ratio (e) than the value of e at the critical state of untreated sand at the same σ′_N in e–log σ′_N space. Compared to untreated sand, a significantly larger US stress ratio was achieved for the biotreated sand, particularly at high C_C and low $σ′N0$⁠. The characteristic features of undrained behaviour, such as instability stress ratio, stress ratio at phase transformation and flow potential showed good relationships with modified initial state parameter, void ratio after biotreatment and C_C. The bonding ratio, (τ/σ′_N)_bond, was used to quantify the interparticle bonding. The peak value of (τ/σ′_N)_bond for the biotreated sand was significantly larger than zero, particularly at high C_C and low $σ′N0$⁠, while the peak (τ/σ′_N)_bond for the untreated sand was negligible. It was also observed that the mobilisation and degradation of calcium carbonate bonds in biotreated sand during DSS shearing are influenced by both C_C and $σ′N0$⁠.