Effect of Precipitation Pattern of Enzyme Induced Calcite on Mechanical Properties of Treated Sand

Abstract

The biocementation of sand through calcite precipitation is a promising approach that has been investigated for nearly 10 years as an attempt to find effective solutions to ground improvement challenges due to sustainability considerations. The literature reports significant variations of strength of treated soils for a given amount of precipitated CaCO3, which suggests that not only the amount of precipitated CaCO3 but also the precipitation pattern can also contribute to the strength of treated soils. Thus, in order to understand the reasons why different precipitation patterns can possibly lead to varying strength outcomes, the monitoring of precipitation characteristics of CaCO3 at pore scale might be a useful approach. This study aims to evaluate the effect of precipitation pattern on unconfined compressive strength of sand treated via enzyme induced calcite precipitation (EICP) technique. The micro-scale characteristics of CaCO3 (spatial distribution, type, size and amount) at 3 different precipitation conditions (EICP, skim milk-added EICP and EICP on calcite nanoparticle coated medium) was investigated through test tube, glass slide and microfluidic chip experiments. The outcomes of this observation were then linked to the unconfined compressive strength (UCS) of sands treated under each of these conditions via column tests. The densely packed, homogeneous and small sized calcite oriented-crystallization pattern promoted by the nanoparticle-coated condition leaded to lower UCS values (maximum of 0.15 MPa at 5% CaCO3 content) while the sparsely distributed, large sized vaterite oriented-crystallization yielded higher values (maximum of 1.4 MPa at 3.9% CaCO3 content). The precipitation pattern of CaCO3 was found to greatly influence the UCS of EICP treated sand.