On the use of biocementation to treat collapsible soils

Abstract

The effectiveness of biocementation to treat the collapse potential of a well graded sand is investigated. The novelty on the application of this treatment for this purpose is that the analysis is done considering structural changes experienced by the soil during the treatment and not only in service. Oedometer tests were performed on treated and untreated soil samples, both in saturated and unsaturated states, to study structural changes due to loading. The presence of biocement bond was confirmed by indirect tensile tests, and other complementary tests were performed to detect the presence of biocement and evaluate the efficiency of the treatment to reduce the collapse potential. Saturated permeability measurements, collapse tests and pore size distribution from mercury intrusion porosimetry tests confirmed the collapsible nature of the soil and that the treatment by bio-cementation reduces collapse potential. However, the treatment was effective not because of the presence of the biocement providing bonds to a metastable soil structure, but mainly because it requires the full saturation of the soil, therefore causing early collapse. Therefore, further collapse on rewetting of the treated soil is basically the second collapse, and for this reason the amplitude of the deformation observed reduces and the treatment is effective. Nevertheless, the physical bonds provided by the biocement precipitated were evident on the results of oedometer and indirect tensile tests, providing additional strength to the soil particles, and therefore some maintaining structure after the treatment. Therefore, the biocementation treatment to reduce the collapse potential is the junction of the treatments pre-wetting the soil, and those providing the reinforcement of the soil with binder, in this case with biological nature.