**3. Materials and methods**

*Sandy Materials in Civil Engineering - Usage and Management*

The discussion in this chapter concentrates on:

sponding amount of calcite/cement

predict the success of bio-cementation.

specimens

**2. Literature review**

soil particles [6–13].

cemented granular soil [7].

cemented specimens

is beneficial for cementing, and comparisons are made between mixed samples in this study and other studies where injection has been used to explore this hypothesis. The small strain shear stiffness has been used as an indicator to evaluate the success of bio-cementation [4], and shear wave velocity was monitored in this study to explore the influences of cement level and stress on the data and the ability to

i.The effect of preparation and curing time on cementation level and corre-

ii.A comparison of UCS strengths of bio-cemented and gypsum-cemented

iii.The effects of confining stress of soil on the geomechanical behavior of bio-

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

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

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

iv.The influence of calcite content on the strength and stiffness

**110**

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 poses no harm to humans.

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


**Table 1.** *Typical liquid medium or broth.* 1 min before being placed in cylindrical molds. To prepare uniform and reproducible samples with a consistent density, the mixture was divided into five portions before being filled in the molds and gently tamped each time.

The bio-cemented samples have been compared with gypsum-cemented samples which have been prepared by combining the dry ingredients (sand and unhydrated gypsum) followed by mixing with water and placing in a mold similarly to the bio-cemented samples.

The preparation technique produced cylindrical samples with 55 x 110 mm in dimensions. After extraction and curing, the samples were either placed directly in a compression machine to perform UCS tests or in a fully computerized triaxial testing apparatus to perform geomechanical tests with elevated confining stresses, which was also fitted with bender elements to monitor the secondary (shear) wave pulse. Once the UCS and triaxial shearing tests were completed, bio-cemented samples were extracted and analyzed to determine the amount and distribution of the calcite precipitated. Further details of methods and procedures are provided by Duraisamy [2].
