1.1. Background and Objective
It is difficult to secure grounds in Korea due to increasing costs of raw material led by the depletion of natural materials and the expansion of national key industry resulting from industrial development. Therefore, soft ground not previously utilized as construction sites increasingly attracts attention to efficiently use land. Soft ground, coastal dredged, and reclaimed grounds are used for construction sites as a base ground. Therefore, significant attention is now given to the method for improving soft ground that contains loose sandy soil or silt, not typically considered as a construction site before.
Among other methods, in civil engineering projects, the grouting method has been used for reinforcing, repairing or waterproofing dams, slopes, embankments, dredged and reclaimed areas. Now, it is applied to almost every construction site, some examples being construction of expressways, airfields, express railways, undersea facilities, ports, tunnels, and subways. This implies the grouting method is now essential in construction sites.
For the grouting method employed in construction sites of Korea, 1–3 types of liquid waterglass-based chemicals, urethane, or high-pressure jet grouting are employed. However, most of them focus on enhancing ground strength, and do not suggest specific solutions for environmental issues, including discharged CO2 and groundwater pollution due to cement and chemical liquid used as a raw material in grouting. Accordingly, a significant amount of funds is invested into developing new methods in the field of ground engineering in relation to the method of improving soft ground. There is a great need for a method that addresses the environmental issues in the process of improving soft ground.
The grouting method has had some issues caused by grout materials, and researchers have studied how to address the problems. The method generally employed in grouting with chemical liquid is the Labile Waterglass (LW) method which is effective for grouting into the gravel and sandy (larger than 0.6 mm) layers, but not effective for grouting into fine sand (i.e.
, smaller than 0.6 mm) layers (Ahn [1
]). The major material used in the LW method is cement, of which disadvantages include gelation and low levels of permeation. However, recent development of materials, cement crushing and classification technology includes new cement-based grouts, for example, micro cement, quick-hardening cement (Kim et al.
]), and plastic cement (Kim et al.
]). Their anticipated effect is improved cement gelation and permeation (Kim et al.
]). Advantages of the LW method are to increase water resistance and adhesion between the soils and waterglass-based grouts. The fast flow of ground water can wash out the chemical liquids used in the grouting. Therefore, for the LW method, it is reported that raising the chemical liquid concentration, and the injection rate and speed, with shorter gel time, if it is applied to running water ground, enhances its efficiency.
The Korea Cement Association states that the volume of cement produced in Korea in 2006 is about 48 million tons, which is seventh place for cement production in the world. To produce one ton of cement, about 0.9 tons of CO2
are discharged. If a material that can replace 1% of 48 million tons of cement were developed, about 480 thousand tons of cement would be saved (Korea Cement Association [5
]). This results in reducing the cost of approximately 13 billion won/year ($30 per ton of CO2
for the carbon credit). Korea is a participant in reducing CO2
emission in accordance with the Kyoto Protocol 2015. It is thus necessary to develop environment-friendly materials to replace cement or reduce the use of cement in the ground improvement field where cement is a major material to contribute to the green growth policy, for example, combating global warming. It is also necessary to address the issues of increasing raw material costs and lack of construction materials by developing new materials.
For this reason, in this study, Bio Grouting Material consisting of CaCO3 was produced by biochemical reaction of microorganism Sporosarcina Pasteurii (KCTC 3558) and Calcium Chloride, and was made with a form of powder-like cement. The following ten mixing ratios used for the conventional LW method and bio grouting method were prepared: OPC (Ordinary Portland Cement, hereinafter referred to as “OPC”) 100%; Micro (Micro Cement, hereinafter referred to as “Micro”) 100%; Bio (Bio Grouting Material, hereinafter to as “Bio”) 100%; OPC 100% + sodium silicate No. 3; Micro 100% + sodium silicate No. 3; Bio 100% + sodium silicate No. 3; OPC 100%+Bio 30%; OPC 70% + Bio 30%; Micro 100% + Bio 30%; and Micro 70% + Bio 30%. Then, the uniaxial compression strengths of these homogel mixtures were measured on days 1, 3, 7 and 28 of curing, and compared.
1.2. Previous Studies
Various studies about cementation of soil using microorganisms have been conducted over the last decade. Among numerous microorganisms present in the soil, many researchers chose Sporosarcina Pasteurii
in particular to study cementation of soft grounds using the Bio grouting material produced through their biochemical reactions (Kim [6
]; Park [7
]; Kim et al.
]; Park and Kim [10
]; Jeon [12
]; Mitchell and Santamarina [13
]; Dejong et al.
The method, which enhances the strength of soil by mixing the Bio grouting material into loose sandy soil is named the Microbial Calcite Precipitation (MCP) method. The MCP method has been recognized as an environment-friendly method that can improve the strength of loose sandy grounds or improve soft grounds by having cement permeate the gaps between particles. In addition, a variety of studies for improving the grounds has been conducted using Biopolymer, organic materials, plant extracts and others, to fundamentally reduce the cement (Kim et al.
]; Chang and Cho [16
It is confirmed that the MCP method improves the strengths of pure sandy grounds. However, a limitation exists in that the Bio grouting material needs to be injected several times in order to precipitate into loose or soft grounds and thus it is hard to apply in the field (Dejong et al.
]; Soon et al.
Therefore, in order to ensure a more efficient and practical application of the MCP method, the eco-friendly microbial cementation method, the Bio grouting method, was proposed by combining the injection technology for soft ground treatment and the MCP method (Park and Kim [11
]; Kim et al.
]; Paassen et al.
]; Wiffin et al.
]; Whiffin [21
Park and Kim [11
] confirmed the effects of cementation and grouting of loose sandy grounds with the Bio grouting material, produced through biochemical reactions of microbes. A grouting test was conducted by producing a two-solution, one-step grouting device by injecting a mixture of 2000 mL microbial solution and 2000 mL calcium chloride solution using the device into a specimen with the diameter (D) of 5 cm and the height (H) of 10 cm, under conditions similar to the field. The test result showed the cementation range of approximately 5.4 cm and the cementation strength of 150 kPa in the drainage condition, confirming the potential of Bio grouting.
The bio grouting test was conducted using the two-solution one-step grouting device for a small specimen with the size of (D) 5 cm × (H) 10 cm, which was only a basic study that confirmed the grouting potential of the Bio grouting material. Therefore, it is necessary to analyze the soil behaviors according to the mixing ratio of Bio grouting material under different ground conditions for medium-sized ground specimens, and analyze the schematic field applicability by conducting the LW method and the Bio grouting method, which are the representatives of chemical grouting methods.
Internationally, studies using microorganisms have been actively conducted in Europe, the USA and Japan, and currently “bio grouting” is commonly used as a term for the technique that combines the MCP method and the grouting method.
In addition to studies on bio grouting, several studies on cement sandy or soft grounds with the Bio precipitating into the gaps between sand particles, by injecting microorganisms to the surface of the sand particles where calcium ions were concentrated, were conducted (Scholl et al.
]; Torkzaban et al.
Representative previous studies that apply the bio grouting method include the following.
] made a cylindrical specimen with PVC with the volume of 6 L and the dimension of 66 m (diameter D) × 5 m (length L), and filled it with Itterbeck sand (D60/D10, the grain-diameter ratio corresponding to 60% and 10% of the soil grain size distribution curve) with the same relative density (DR). After 6 L of microbial solution was injected to the specimen, 6 L of calcium chloride solution (0.05 M) was injected in order to settle down the microorganisms evenly throughout the entire sand column. Then, 1 M of urea and 9 L of calcium chloride solution was injected repeatedly until urease reaction stopped. Afterwards, the sand column was left for 24 h so that the urease could react. Then, the concentration of the discharge solution was measured using a turbid meter. Both the microbial solution and the calcium chloride solution were injected freely over the length of 5 m a low pressure (approximately 7 m/day).
As a result of the experiment, a uniaxial compression strength value of 100 kg/cm2 was obtained using the strength transformation formula for the non-destructive testing ultrasonic velocity. However, as this is a uniaxial compression strength based on estimation, not obtained through quantitative analysis, it is not highly reliable. In addition, as a result of periodically injecting the microbial solution and the calcium chloride solution, the precipitation of calcium carbonate was created throughout the 5 m sand column, mostly around the inlet section and proportionally reduced depending on the distance from the inlet. This confirms that the calcium carbonate precipitation occurs when the microbial solution and the calcium chloride solution are mixed and the strength varies with the amount of calcium carbonate precipitation.
Paassen et al.
] carried out an experiment to investigate the field applicability by filling a container box (0.9 m (width B) × 1.1 m (length L) × and 1 m (height H)) with the same relative density. Onto the inner sides of the container box, drainage screens were installed and at its bottom, a drainage device was installed. Using this container, the grouting experiments were conducted with Mass river sand (D60/D10 = 1.6) and IItterbeck sand (D60/D10 = 1.64).
Mass river sand was filled into the container box through free fall using a forklift at the same height in order to maintain the same relative density (DR) inside the container box. Then, 100 L of the microbial solution and the calcium chloride solution (0.5 M) were injected from the center to the sides of the box at a constant injection pressure of 50 L/h. Over a period of 50 days, 3500 L of the reaction solution was injected in eigth installments, and 200 M of the calcium carbonate (20 kg/m3) was precipitated at an efficiency of approximately 12%. The experiment confirmed that calcium carbonate was precipitated along with the bottom edges and the inner sides of the container.
According to Hamdan [24
], chemical reactions of MICP involved in bacterial ureolysis produce undesirable and potentially toxic end products: ammonia (NH3
(g)) and ammonium (NH4+
). He stated that it is unclear whether or not a substantial portion of ammonium can be converted to nitrate in comparison to the amount produced, since nitrifying bacteria are limited by the lack of dissolved oxygen and may also experience severe inhibition at elevated ammonia concentrations, reported by Anthonisen et al.
] and Antoniou et al.
]. In addition, several studies have suggested that ammonium ions produced by the ureolysis reaction may exchange with radionuclide and metal contaminants sorbed to subsurface minerals, and this may enhance the availability of these contaminants for co-precipitation into calcite (Colwell [27
]; Fujita et al.
]; Fujita et al.
]; Mitchell and Ferris [30
]). Hamdan [24
] pointed out that although the ion-exchange concept may be theoretically feasible, the studies have not done bench-scale or field-scale tests to verify the idea that ammonium, a water-soluble polyatomic ion, may sorb to minerals in exchange for radionuclide and metal contaminants. Although a number of studies on the MCP have been done, little attention has been given to treat ammonia (NH3
(g)) and ammonium (NH4+
), requiring further study regarding this topic.
In order to develop such a bio grouting method, an engineering assessment of the grouting materials needs to be conducted first and, in this study, the engineering assessment of OPC, Micro, and Bio was conducted to apply the bio grouting to the field.