# Evaluating Biosedimentation for Strength Improvement in Acidic Soil
## 1 Introduction  
Marine clay soil is a type of soil that results from the weathering and erosion of limestone or dolomite, which contains a high content of magnesium, calcium and iron. This soil has high water content and soluble salt that makes it difficult for concrete to set and is porous. They regularly plunge to huge depths of more than 30 m, as shown in Figure 1[1]. Due to its high-water content [2] and a high amount of organic materials that can exceed 10.0% of the total dry weight [3], it has low undrained shear strength. Additionally, marine clay soils are a thixotropic substance, meaning that when the water content increases, the thixotropy strength ratio increases [4]. Moreover, acidic clay soils with pH levels less than 4.0 show a considerable loss in shear strength [5]. As a result, significant settlingoccurs both before and after construction [2; 6]. Thus, several treatment approaches using lime [7], basalt fibers [8] and biomineralization [9] have been emphasized and oriented toward the enhancement of \"environmental soil treatment\" that is ecologically friendly and resource-efficient over previous procedures [10]. In this respect, biomineralization treatment is one of the numerous environmentally friendly and cost-effective soil improvement strategies that may be used to reinforce weak soils without requiring their replacement or the deployment of a special foundation system [7; 11]. Microbially induced carbonate precipitation (MICP) was studied to treat weak and loose soils, including a 7-m sand column [10], an upscaling sand treatment beneath footing [12] and a 15-cm clay specimen [13]. For MICP, species of the Bacillus family have the ability to precipitate calcium carbonate (CaCO3) by the hydrolysis of urea (CO(NH2)2) into ammonium (NH4+) and carbonate (CO32-) ions [14; 15]. They result in an increase in the pH level of their cell contents due to the precipitation ability of calcium carbonate (CaCO3) in the surrounding microenvironment [16]. The following are the reaction equations [17; 18]:  
\[\text{CO(NH}_{2})_{2}+\text{H}_{2}\text{O}\to 2\text{NH}_{3}+\text{CO}_{2} \tag{1}\]  
Ammonia (NH3) and dissolved inorganic carbon are generated in large amounts by the urease enzyme. Meanwhile, the pH rises, and the production of ammonia (NH3) and dissolved inorganic carbon continues until the urea (CO(NH2)2) reduces dramatically:  
\[2\text{NH}_{3}+2\text{H}_{2}\text{O}\to 2\text{NH}_{4}{}^{+}+2\text{OH} \tag{2}\]  
\[2\text{OH}^{-}+\text{CO}_{2}\rightleftharpoons\text{HCO}_{3}{}^{-}+\text{OH}^{- }\rightleftharpoons\text{CO}_{3}{}^{-}+\text{H}_{2}\text{O} \tag{3}\]  
Alkalinity accumulates with the hydrolysis reaction within bacterial cells:  
\[\text{Cell-Ca}^{2+-}+\text{CO}_{3}{}^{-}\rightarrow\text{Cell-CaCO}_{3} \tag{4}\]  
Sporosarcina pasteurii, also known as _Bacillus pasteurii_[18], is a bacillus family bacterium that produces urease enzymes intracellularly. _S. pasteurii_ is an aerobic endosperming soil bacterium with a diameter of 0.5-1.2 \(\upmu\)m and a length of 1.3-4.0 \(\upmu\)m [19]. It can also survive in extremely alkaline (pH 10) circumstances, making it a relevant agent source for the MICP phenomena [20; 21]. Sporosarcoma pasteurii, on the other hand, has a peak activity in urease enzyme synthesis in pH ranges from 8 to 9 [22; 23; 24], while precipitation requires a pH of 9 [25]. Sporosarcoma pasteurii also possesses a mechanical pressure resistance of 0.50 J/cm\({}^{3}\)[26]. In general, the precipitation rate of calcium carbonates (CaCO3) during the MICP phenomenon is influenced by the bacterial type, cell concentrations, the pH value of the surrounding microenvironment, calcium concentration, urea concentration, temperature and humidity [27; 28]. Soil treatment with the MICP phenomenon increased the strength by up to 570 kPa for sand soil [10] and 92 kPa for clay soil [29], with the carbonate concentration up to  
Figure 1: Typical isoline showing marine clay’s total thickness [1]. 60 kg/m\({}^{3}\) of soil specimen [10].