Monitoring progressive cementation of sand by biogrouting through time-lapse shear-wave seismics

Carefully designed time-lapse shear-wave seismic measurements were carried out during a large-scale biogrouting experiment in sand. The results point out the possibility for a reliable, quantitative monitoring in time and space of the effect of biogrouting in the shallow subsoil. High-resolution time-lapse shear-wave seismic can uniquely define the evolution of cementation and stiffening of the sand during progressive grouting, which is otherwise difficult to monitor. The seismically estimated distribution of in-situ stiffness as a function of distance from the injection well matches quite well with the stiffness distribution independently derived by empirical analyses of lab test data on samples. Surface seismic measurements using high-frequency shear waves do have the potential to resolve the grouting induced change in soil stiffness as shallow as the first few meters of the top soil. One significant conclusion is that not only velocity but also the amplitude of the shear waves can be a powerful indicator of the state of CaCO3 cementation, and can shed light on the underlying physical mechanism.