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Microbially induced calcite precipitation (MICP) is a sustainable biological 11 ground improvement technique that is capable of altering and improving soil mechanical and 12 geotechnical engineering properties. In this paper, laboratory studies were carried out to 13 examine the effects of some key environmental parameters on ureolytic MICP mediated soils, 14 including the impact of urease concentrations, temperature, rainwater flushing, oil 15 contamination and freeze-thaw cycling. The results indicate that effective crystals 16 precipitation pattern can be obtained at low urease activity and ambient temperature, resulting 17 in high improvement in soil unconfined compressive strength (UCS). The microstructural 18 images of such crystals showed agglomerated large clusters filling the gaps between the soil 19 grains, leading to effective crystals formation. The rainwater flushing was detrimental to the 20 bio-cementation process. The results also indicate that although traditional MICP treatment 21 by the two-phase injection method did not succeed in treatment of oil contaminated soils, and 22 the proposed premixing of bio-flocs with soil can significantly improve UCS and stiffness of 23 oil contaminated soils. Finally, MICP treated soils showed a high durability to the freeze-24 thaw erosion, which is attributed to the inter-particle contact points and bridging of crystals 25 formation. 26 2 Author Keywords: Microbially induced calcite precipitation (MICP); bio-cemented sand; 27 ground improvement; microstructural analysis; unconfined compressive strength (UCS). 28 74 deposition of calcite crystals in the contact points between the sand grains, forming effective 75 bridges that contribute to the shear strength improvement of bio-cemented soil.76 77The calcite precipitation pattern is another factor that greatly influences the target application 78 of bio-cementation in the field as it influences the flow properties of porous media, which 79 may lead to treatment homogeneity by shaping the preferential flow path according to the 80 5 size, shape, and structure of the pore throats affected by the accumulation of calcite crystals 81 (Al Qabany et al. 2012). The calcite precipitation pattern also affects the load transfer 82 mechanism between the soil particles, through the area of contact points developed by the 83 precipitated calcite, leading to variation in strength and stiffness of bio-cemented soils (Ismail 84 et al. 2002b). 85 86 This paper aims to investigate the impact of some key environmental parameters on the 87 efficacy of MICP for its in-situ implementation and optimization process. The parameters 88 studied include the effects of urease enzyme concentration, degree of temperature, rainwater 89 flushing, oil contamination, and freeze-thaw weathering in winter alpine regions. The impact 90 of these parameters on the performance and effectiveness of MICP technique was related to 91 both the shear strength improvement of bio-treated soil and CaCO3 precipitation pattern at the 92 micro-scale level. In t...

9Limited research has been reported on strength improvement of bio-cemented soils in relation 10 to crystal patterns of microbial induced calcite precipitation. In this study, sand samples were 11 treated under the co-effect of different bacterial culture (BC) and cementation solution (CS) 12 concentrations, to evaluate the optimum BC and CS combination that yields the highest soil 13 strength. It was found that for lower CS condition (0.25 M), higher BC produced stronger 14 samples, whereas for higher CS condition (0.5 M or 1 M), lower BC was more dominant in 15 improving the soil strength. This can be attributed to the effectively precipitated CaCO3 16 crystals, which were in rhombohedral shape and large size, and were concentrated at the soil 17 pore throat rather than deposited on the individual sand grain surface. This finding was 18 confirmed with the scanning electron microscopy (SEM) analysis. The strength and 19 permeability of the optimized bio-cemented samples were also compared with sand samples 20 treated with ordinary Portland cement (OPC). The optimized bio-cemented sand provided 21 higher strength and permeability than those obtained from the samples treated with similar 22 content of OPC at curing period of 28 days. 23 24 29cementing agent, binding soil particles together inside the soil matrix (Cheng, et al., 2013). 30 Another potential of microbial grouting is through the concept of bio-clogging, as a result of 31 the agglomeration of the CaCO3 crystals in the soil pore throats, thus, controlling the water 32 flow in the porous media to reduce the hydraulic conductivity of the bio-clogged soils 33 (Ivanov and Chu, 2008). 34 35 The mechanical performance of MICP stabilized soils largely depends on the microstructure 36 of the precipitated CaCO3 crystals, which are affected by various chemical, environmental 37 and physical parameters. Studies by Chu, et al. (2013), Al Qabany, et al. (2012), Mortensen, 38 et al. (2011) and Martinez, et al. (2013) discussed the treatment conditions to achieve an 39overall MICP treatment process efficiency in terms of the uniformity of CaCO3 precipitation. 40It was suggested that the use of lower concentrations of cementation solution (CS) and 41 bacterial culture (BC) would result in better distribution of CaCO3 precipitation, particularly 42 at lower cementation levels. Cheng, et al. (2013) demonstrated that the bio-cemented soils 43 treated at partially saturated conditions resulted in higher strengths than those achieved under 44 full saturation conditions, due to the more CaCO3 crystals precipitated at the contact points 45 between the sand grains. In other words, alteration of the CaCO3 crystals precipitation 46 patterns can have significant effect on the mechanical response of bio-cemented soils. 47 48 3 Earlier studies found in the literature concluded that the alteration of the cementation solution 49 concentrations (in terms of different combinations or the equimolar amount of urea and 50 calcium) would result in uniformly distributed CaCO3 crys...

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