Cyclic liquefaction resistance of MICP- and EICP-treated sand in simple shear conditions: a benchmarking with the critical state of untreated sand

In the present study, the undrained cyclic behaviour of biotreated sands using microbial and enzyme-induced carbonate precipitation was investigated for a wide range of initial void ratio after consolidation ( e₀ e 0 ), initial effective normal stress ( σ_N₀ σ N 0 ′ ) and calcium carbonate content ( C C ) under direct simple shear (DSS) testing conditions. The critical state soil mechanics framework for untreated sand was first established using a series of drained and undrained (constant volume) tests, which served as a benchmark for evaluating the undrained cyclic liquefaction behaviour of untreated and biotreated sands. The results indicated that the modified initial state parameter ( ψ_m_0 ψ m 0 ) in DSS condition showed a good correlation with instability states and phase transformation under monotonic shearing. In undrained cyclic DSS loading condition, samples displayed cyclic mobility indicated by an abrupt accumulation of large strain or σ_N₀ σ N 0 ′ transiently reaching zero or a sudden build-up of excess pore water pressure. The linkage between static and cyclic liquefaction was established for untreated and biotreated sand specimens based on the equivalence of characteristic soil states. The number of cycles before liquefaction ( N L ) for the biotreated sand specimens was mainly controlled by the cyclic stress ratio, e₀ e 0 , σ_N_0 σ N 0 ′ and C C . For a similar initial state prior to undrained cyclic loading, the biotreated specimens required a larger N L compared to the untreated sand. The cyclic resistance ratio at N L = 15 (CRR 15 ) increased with decreasing ψ_m_0 ψ m 0 for the untreated sand, while the CRR 15 for biotreated sand increased with increasing C C and decreasing σ_N₀ σ N 0 ′ .