Microbial or enzyme-induced calcium carbonate precipitation (MICP/EICP) are relatively new ground improvement technique. In this study, the mechanical behaviour of biotreated (MICP/ EICP) and untreated sands were investigated in light of the critical state soil mechanics framework using a series of direct simple shear (DSS) tests. A wide range of CaCO3 content (CC), 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 for the biotreated sand shifted towards a smaller void ratio (e) than 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 ultimate state stress ratio was achieved for the biotreated sand, particularly at high CC 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 CC. 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 CC and low σ′N0, while the peak (τ/σ′N)bond for the untreated sand was negligible. It is also observed that the mobilisation and degradation of CaCO3 bonds in biotreated sand during DSS shearing are influenced by both CC and σ′N0.