**2. Literature review**

Over the past decade, the potential for microbially induced calcite precipitation (MICP), or simply bio-cement, to improve soil and rock responses has been extensively studied by petroleum, geological, and civil engineers [4, 5]. Recently, studies were undertaken to understand the geomechanical parameters of granular soils using microbes in biochemical process, which produce bio-cement in the subsurface [1, 6, 7]. It has been suggested [8] that these reactions simulate the natural geochemical processes that transform sand into sandstone. However, the MICP process is rapid and produces a precipitate with soft and powdery crystals, whereas natural limestone forms slowly and creates a very hard precipitate [6]. Most of this research has focused on the use of ureolytic bacteria, which have been shown to be capable, with the addition of urea and reagents, of producing calcite that binds to soil particles [6–13].

The general trends of cementation effects on granular material are increases in strength and stiffness, which increase with the amount of cementing material, although this may vary greatly depending on the amount of cementing material used. It has been noted that the effectiveness of cement depends on the density, the effect of cement being greater at lower densities [14–17]. Many studies on artificially cemented soils have shown that cementation significantly increases the initial tangent modulus of a soil and monitoring the stiffness can be a useful method of tracking the amount of cementation [18, 19]. A range of cementing agents have been investigated including ordinary Portland cement (OPC), gypsum, sodium silicates, and calcium carbonate [14–18] to understand the influence of cementation and to simulate materials used in ground improvement work. Generally, the geomechanical responses of bio-cemented granular soil are similar to any other artificially cemented granular soil [7].

Although the cementing effect is more significant in loose sand, it is found that more bio-cement is needed to achieve the strength of dense sand when applied in loose sand [13]. It has also been suggested that the growth of calcite crystals at points of contact between sand grains has a significant influence on UCS strength [20]. As bacteria and nutrients are pumped through sand continuing calcite precipitation, it can lead to a large proportion of the voids being filled, and high

**111**

**Table 1.**

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works*

from small-scale experiments, and even at larger-scale (100 m3

increased from 35.3 to 39.6° and the cohesion from 0 to 93 kPa [25].

A range of applications for bio-cement have been suggested. A recent example is a feasibility study carried using bio-cement for slope stabilization by means of a surficial treatment where a layer of hard stratum was obtained on the subsurface with UCS strength of 420 kPa after 10 days of treatment [7, 26]. However, this method of treatment is not extensively applied in the field of ground engineering.

Microbes, chemical substrate, and reagents have been used to produce biocemented specimens. This process uses bacteria to catalyze the urea hydrolysis reaction that precipitates calcite. The ureolytic bacteria known as *B. megaterium* (strain ATCC 14581) were used. To produce sufficient bacteria for the cemented samples, a KWIK-STIK (produced by Microbiologics®) containing the microorganism strain was cultured in batches using liquid medium. The growth medium (refer to **Table 1**) in liquid form was prepared in advance and placed in the incubator at 30°C for 24 hours using a 50 ml beaker. Importantly, this bacterium is nonpathogenic and

Fixed quantity of clean, dried sand was placed in a mixing bowl. Then the required amount of urea powder and calcium chloride powder was added based on a percentage of the sand weight free from moisture. The nutrient masses (urea and calcium chloride) ranged from 5 to 20% of the sand weight free from moisture. Additional water was added to facilitate mixing to give a water mass of about 10% of the mass of the dry ingredients. The ingredients were then thoroughly mixed for

Nutrient broth 3 g Urea 20 g NH4Cl 10 g NaHCO3 2.12 g CaCl2 2.8 g

UCS can be obtained. For example, strengths of up to 30 MPa [20] were obtained

to 12 MPa [21] have been reported. Other results show maximum compressive strengths obtained for bio-cemented sand of about 14 MPa [22]. As much as UCS strength increase can be related with individual soil particle strength, factors like roundness, size, and shape too may reduce the strength [20]. An increase in shear strength of 35, 50% and more than 100% is observed for round coarse particles, coarse angular particles, and round fine particles, respectively [23]. According to Al Qabany and Soga [24], for the same amount of precipitated calcite, the greatest UCS strength was obtained when the concentration of the solution was low. These results were obtained over a range of different initial relative densities. Cheng et al. [17] also reported that it is possible to get higher UCS strengths at the initial phase when sample is low or partially saturated. The different soils used and different preparation procedures have resulted in a wide range of parameters to describe the bio-cemented soils. For example, clean Ottawa sand treated with microbes produced calcite in the range of 0–4%. During the treatment, the angle of friction

) strengths of up

*DOI: http://dx.doi.org/10.5772/intechopen.88159*

**3. Materials and methods**

poses no harm to humans.

**Ingredients (L<sup>−</sup><sup>1</sup>**

*Typical liquid medium or broth.*

**)**

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works DOI: http://dx.doi.org/10.5772/intechopen.88159*

UCS can be obtained. For example, strengths of up to 30 MPa [20] were obtained from small-scale experiments, and even at larger-scale (100 m3 ) strengths of up to 12 MPa [21] have been reported. Other results show maximum compressive strengths obtained for bio-cemented sand of about 14 MPa [22]. As much as UCS strength increase can be related with individual soil particle strength, factors like roundness, size, and shape too may reduce the strength [20]. An increase in shear strength of 35, 50% and more than 100% is observed for round coarse particles, coarse angular particles, and round fine particles, respectively [23]. According to Al Qabany and Soga [24], for the same amount of precipitated calcite, the greatest UCS strength was obtained when the concentration of the solution was low. These results were obtained over a range of different initial relative densities. Cheng et al. [17] also reported that it is possible to get higher UCS strengths at the initial phase when sample is low or partially saturated. The different soils used and different preparation procedures have resulted in a wide range of parameters to describe the bio-cemented soils. For example, clean Ottawa sand treated with microbes produced calcite in the range of 0–4%. During the treatment, the angle of friction increased from 35.3 to 39.6° and the cohesion from 0 to 93 kPa [25].

A range of applications for bio-cement have been suggested. A recent example is a feasibility study carried using bio-cement for slope stabilization by means of a surficial treatment where a layer of hard stratum was obtained on the subsurface with UCS strength of 420 kPa after 10 days of treatment [7, 26]. However, this method of treatment is not extensively applied in the field of ground engineering.
