Biologic improvement of a sandy soil using single and mixed cultures: A comparison study

Because of global environmental problems due to desert expansion and dust storms as well as constructional requirements, looking for more efficient and comprehensive methods to stabilize dune sands and weak grounds seems to be an essential necessity. MICP (Microbial-induced CaCO3 precipitation) is an innovative technique in which bacterial activities within soil mass are harnessed to modify the physical and mechanical properties of soils. This method produces calcium carbonate precipitation in the soil pores by fracturing urea in the presence of calcium ions. An important factor in achieving uniform calcite deposition (and hence consistent enhancement of geotechnical properties) throughout the treated soil mass is the protocol adopted for injecting the reagents of ureolytic bacteria, urea and calcium. In this research study, an urease microorganism (i.e. B. pasteurii) was prepared at single and mixed cultures (with B. subtilis) in the laboratory and injected into dune sand samples. After required and appropriate curing time, the samples were subjected under unconfined compression, direct shear and falling-head permeability tests. The test results showed significant strength improvement and reduction of permeability of the treated samples in comparison with those of the untreated soil, while the performance of the mixed culture was much better than that of the single culture in the stabilization outcome. More specifically, the unconfined strength of the soil samples treated with the mixed culture was 1.2 times that of samples treated with the single culture. The amount of permeability reduction in the mixed medium was also 34% more than that obtained with the single culture. The research results verified the capability of the application of biological treatment, particularly with the mixed culture, on the dune sand which may be regarded as a potential technique to control desert expansion, and dust storms through the stabilization of the weak grounds.