Applicability of Soybean Crude Urease-Calcite Precipitation (SCU-CP) Method for Improvement of Expansive Soil

Expansive soil is one of the problematic soils that can swell and shrink, causing damage to buildings. Expansive soil needs improvement before being used for buildings. The typical method for expansive soil improvement is mixing soil with additives such as cement, lime, and fly ash. However, mixing additives like cement, lime, and fly ash is not easy to distribute evenly and negatively impacts the environment. Soybean Crude Urease-Calcite Precipitation (SCU-CP) is a soil improvement method currently being developed. SCU-CP is easier to apply in the field and more environmentally friendly than the common method. SCU-CP method was proven to impact sandy soil and peat soil significantly, but SCU-CP’s impact on expansive soil still needs to be discovered. This study aims to know the effects of SCU-CP on soil swelling parameters and determine the optimum reagent concentration for expansive soil improvement using SCU-CP. This study used three reagent concentrations (1.0 mol/L, 1.5 mol/L, and 2.0 mol/L) and one control sample (without SCU-CP). The swell percentage decreased by 2 – 3% after SCU-CP treatment, but adding reagent concentration did not significantly reduce the swell ratio. Swell pressure did not decrease after SCU-CP treatment. Swell pressure was only reduced by 7.92 kPa with 1.0 mol/L reagent of SCU-CP, while the other concentration swell pressure tended to be increased. This study’s optimum reagent concentration for expansive soil improvement is 1.0 mol/L.


Introduction
Expansive soil is clay soil that can shrink and swell depending on the soil water content.Expansive soil will swell if the water content is high, and then the soil will shrink when it loses water content [1,2].The Montmorillonite mineral causes expansive soil swell and shrink behavior because Montmorillonite minerals have a negative charge, absorbing much water and filling soil voids [2].Expansive soil behavior makes it problematic because it makes the soil unstable and has a low-bearing capacity soil.Some problems caused by expansive soil are massive cracks in buildings, building foundation upsurges, and bumpy roads [3,4].These problems can cause losses both in terms of cost and feasibility because the built infrastructure cannot be used optimally.Hence, if expansive soil will be used for building, it needs soil improvement to improve the physical properties of the soil [1].Soil improvement will reduce the swell potential and increase the bearing capacity of expansive soil.A common method for stabilizing expansive soils is mixing soil with additives such as cement, lime, and fly ash.Mixing these additives is intended to increase the soil's bearing capacity and reduce the soil's plasticity so that the soil's potential to swell and shrink is reduced [5].Based on studies that have been done, additive material has a positive effect on expansive soil-bearing capacity enhancement.Expansive soil's compressive strength increases along with the addition of cement content, and fly ash addition can reduce plasticity, swell pressure, and swell potential significantly [6,7].However, this method still has some areas for improvement, such as how to apply it in a field and its environmental effect.A homogeneous mixture is challenging to obtain when using additives such as cement and fly ash because the distribution is uneven when mixing it [8].Moreover, this method can increase soil pH and pollute the environment.Soil permeability will also decrease because the chemicals used have high viscosity and harden quickly [9].Based on these matters, a more effective and environmentally friendly soil improvement method is needed.Soybean Crude Urease-Calcite Precipitation (SCU-CP) is a developing soil improvement method.SCU-CP method can improve expansive soil because SCU-CP is 1249 (2023) 012036 IOP Publishing doi:10.1088/1755-1315/1249/1/012036 2 easier to apply and more environmentally friendly than the common method using additives.SCU-CP is applied to the soil by pouring it without injecting or mixing it.SCU-CP principal uses urease enzyme from soybean as a catalyst in urea breakdown so that calcite formation will be faster in urea and calcium chloride reaction.Formed calcite will precipitate in the soil, bond the soil particles, limit soil particle movement, and repair the soil properties [10].SCU-SP method has been used for sandy soil and peat soil, which both give a positive effect after SCU-CP treatment.Based on the study done by Pratama et al., the strength of sandy soil increased by more than 100% after the application of SCU-CP with a soybean concentration of 20 g/L and several injections [11], while the result of the study by Meisnnehr et al., the strength of sand soil reached 168 kPa at a soybean concentration of 60 g/L [12].Based on the study by Ramadhan and Putra, peat soil treated with SCU-CP experienced an increase in strength of 38-48% when compared to untreated soil [13], while the result of the study by Pratama et al., the shear strength of peat soil increased with the optimum solution composition, namely one mol/L reagent and 15 g/L soybean [14].Based on the results of these studies, the potential of SCU-CP is proven to be effective in increasing the strength of sand and peat soils.Based on the results of these studies, the possibility of SCU-CP is proven to be effective in increasing the strength of sand and peat soils.However, the potential of SCU-CP as a soil stabilization method in other soil types, such as expansive soil, still needs to be discovered, so further research is required.In this study, the application of SCU-CP was carried out on expansive soils.The applicability of SCU-CP in expansive soil was evaluated based on the soil swelling parameters from the one-dimensional swelling potential test for cohesive soils, namely heave and swell pressure treated by varying concentrations of reagents, to determine the optimum composition for expansive soil stabilization.

Materials
The materials used in this study are expansive soil taken from Bekasi District, reagents consisting of Urea (CO(NH2)2) and Calcium Chloride Dihydrate (CaCl2.H2O) laboratory grade (EMSURE ®), soybean powder (0.037 -0.3 mm), and distilled water as a solvent in SCU-CP solution.The characteristics of expansive soil used in this study were investigated by conducting soil index properties tests and Proctor standard compaction tests.Soil index properties tests were conducted based on SNI 1964:2008 for specific gravity [15], SNI 1967:2008and SNI 1966:2008 for Atterberg limits [16,17], and SNI 6371:2015 for grain size analysis [18].Table 1 shows the result of soil index properties tests.Based on the grain size analysis using USCS soil classification, the soil in this study is an inorganic clay with high plasticity because the soil that passes sieve No. 200 (0.075 mm) is more than 50%, and the liquid limit is more than 50 [18].The soil used in this study has very high swell potential based on the plasticity index because the plasticity index is more than 35 [19].Proctor standard compaction test conducted based on SNI 1742:2008 [20].Based on Proctor standard compaction test result, the soil used in this study has an optimum moisture content (OMC) of 24.13% and a maximum dry density (MDD) of 1.37 g/cm 3

Preparation of SCU-CP Soulution
The SCU-CP solution is made by mixing filtered soybean with a reagent solution of Urea and Calcium Chloride.Each of the materials used for SCU-CP is mixed in a specific concentration.Putra et al. stated that calcite precipitation reached stable and optimum conditions at 20 -40 g/L soybean concentration with 1.0 -1.5 mol/L reagent [21].Based on that statement, the soybean concentration used for this study is 20 g/L with a variation of reagent concentration, viz.1.0 mol/L, 1.5 mol/L, and 2.0 mol/L.Theoretically, the higher the reagent concentration, the more calcite is formed.The reaction of calcite formation in the SCU-CP method is shown in Equation ( 1) - (3).
The procedure for making SCU-CP solution refers to a study by Loebis and Putra [22].The untreated samples were soil samples that were not treated with SCU-CP solution and were only given water, while the treated samples were treated with SCU-CP.The experimental conditions of this study are presented in Table 2, with UT as an untreated sample and TE as a treated sample.UT and TE samples were prepared under MDD conditions by adding water/SCU-CP solution in the amount of OMC obtained in the Proctor standard test.The scheme for making soil samples is presented in Figure 2. Before testing the swell potential, the UT samples were not given a curing time, while the TE samples were given a curing time.SCU-CP effect can be evaluated after 3 days of curing, but to have a better result on soil strength, curing time used for treated sample in this study is 7 days [24].

SCU-CP Impact on Soil Swell Parameters
Soil swell potential is evaluated based on its swell percentage (heave) and swell pressure.Swell percentage (heave) compares the sample's initial height/void ratio and sample height/void ratio after swelling.The average swelling percentage obtained for untreated samples and samples with 1.0 mol/L, 1.5 mol/L, and 2.0 mol/L reagents were respectively 6.53%, 4.45%, 4.53%, and 3.13%.The average time to reach the end of the primary swell of untreated samples and samples with 1.0 mol/L, 1.5 mol/L, and 2.0 mol/L reagents, respectively, were 168, 257, 124, and 357 minutes.Based on the degree of swell classification by Seed et al., the sample without SCU-CP treatment is soil with a high degree of swelling because the swell percentage is around 5 -25%, while samples with SCU-CP treatment are soils with a moderate degree of swelling because the swell ratio is approximately 1.5 -5% [25].The comparison of swell percentage and the time to reach the end of the primary swell is summarized in Figure 3. Applying SCU-CP can reduce the swell ratio by 2-3% and increase the time to reach the primary swell up to 213%.In this study, a decrease in percent swelling and an increase in time to get the end of primary swelling on expansive soils are signs of calcite formation in the soil.In principle, calcite from SCU-CP solution formed in the soil is the same as adding lime to the soil.Several chemical reactions in the water between the lime and the soil minerals can explain the role of lime in reducing soil swell.The chemical reactions can be divided into three main categories: cation exchange, flocculation-agglomeration, and pozzolanic reactions.The results of these reactions can form bonds that prevent swelling in the soil [26].Figure 4 depicts the process of calcite to reduce soil swell potential.During the pozzolan reaction process, Calcium cations gradually react with Alumina and Silica to produce a cementitious material which causes soil solidification, thereby reducing swelling potential [26,27].Swelling pressure is the pressure that is generated when the soil swells.Swelling pressure can also be interpreted as the pressure to prevent the soil from swelling [28].Swelling pressure is obtained by drawing a line parallel to the right from the starting point to intersect the test result curve as in Figure 5.  F shows that the SCU-CP method does not significantly reduce swell pressure.In Hasan et al.'s research, the swell pressure of expansive soil decreased from 80 kPa to 7 kPa after being given bagasse ash and lime [28].In the research of Khemissa and Mahamedi, the swell pressure of expansive soil decreased along with the addition of cement and lime content [30].When compared with other studies related to expansive soil improvement using lime, the results of this study are contradictory because the swell pressure only decreased by 7.92 kPa when SCU-CP was applied with a reagent concentration of 1.0 mol/L, whereas at concentrations of 1.5 and 2.0 mol/L, the expansion pressure tends to increase.The increase in swelling pressure occurred due to the inconsistent sample water content in each sample even though it had been made under MDD conditions where the water content of the sample with reagent concentrations was 0.0 mol/L, 1.0 mol/L, 1.5 mol/L, and 2.0 mol/L respectively are 26.2%,30.5%, 25.0%, and 29.50%.Inconsistent sample moisture content affects the results obtained because expansive soil's swelling potential and pressure decrease when the initial moisture content increases.Conversely, swelling increases when the water content decreases [31].The swelling potential and pressure reduction as the water content increases occur because a low initial moisture content results in a low degree of soil saturation.Wet granules tend to absorb more water than when the initial degree of saturation is high [32].

Optimum Reagent Concentration for Expansive Soil Improvement with SCU-CP Method
The optimum composition of the SCU-CP solution in this study was determined based on the smallest swelling potential and pressure.Based on the test results, the smallest swelling potential is owned by samples with 2.0 mol/L reagent.However, the swell potential when adding 2.0 mol/L reagent is not much different from samples with 1.0 mol/L reagent because an increase in reagent concentration at the same soybean concentration results in a lower calcite precipitation ratio [33].Sample with 1.0 mol/L reagent owns the smallest swelling pressure.Based on the potential and swelling pressure results, the optimum SCU-CP reagent concentration to improve expansive soil in this study was 1.0 mol/L.

Conclusions
Applying the SCU-CP method can reduce the potential for swelling of expansive soils by 2-3%, so the degree of swell changes from high to medium.However, adding reagent concentration did not significantly reduce the swell pressure because it could only reduce 7.92 kPa at a 1.0 mol/L concentration.In contrast, the swelling pressure increased at 1.5 mol/L and two mol/L reagent concentrations.Increasing the concentration of reagents results in low effectiveness in reducing expansive soil expansion.However, the optimum reagent concentration that can be applied to improve expansive soil development parameters with the SCU-CP method is 1.0 mol/L.Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) tests are needed to validate calcite formation in the soil and determine the calcite crystal phase that forms in the soil.The potential of SCU-CP to reduce soil swelling can be increased by increasing the concentration of soybeans at higher reagent concentrations so that the hydrolysis rate of soybeans increases and can increase the amount of calcite formed.
Figure depictsthe procedure of SCU-CP solution-making.SCU-CP solution was prepared by dissolving soybeans, Urea, and CaCl2 with distilled water in different glasses, and then the soybean solution was filtered through a sieve No. 400 (0.037 mm).After the soybean solution was filtered, the Urea and CaCl2 solution were mixed, added to the filtered soybean solution, and stirred again.The required reagent (R) and soybeans are calculated using the OMC Proctor test results.

Figure 1 .
Figure 1.Procedure for making SCU-CP solution2.3.Swell Potential TestSoil swell potential test refers to SNI 6424:2008 about the method of the potential for swell or settlement of one dimension of cohesive soil test[23].The soil swelling potential test determines the onedimensional swell or shrinkage value in relatively undisturbed or compacted cohesive soils.Heave and swelling pressure indicate the swell potential in the soil sample.The modified method A was used in this study because method B and method C required field pressure data (overburden pressure) which was not obtained in this study.The soil swells potential test was carried out under two conditions: untreated and treated.The soil swells potential test was carried out in untreated and treated conditions.The untreated samples were soil samples that were not treated with SCU-CP solution and were only given water, while the treated samples were treated with SCU-CP.The experimental conditions of this study are presented in Table2, with UT as an untreated sample and TE as a treated sample.

Figure 2 .
Figure 2. Procedure for making soil swell potential test sample

Figure 3 .
Figure 3. Schematic diagram of calcite process on reducing swell potential

Figure 4 .
Figure 4. SCU-CP effect on soil swell potential The vertical load used for this test are 98.1 kPa, 196.1 kPa, and 392.3 kP.average swelling pressure for untreated sample and treated samples with 1.0 mol/L, 1.5 mol/L, and 2.0 mol/L reagents, respectively, is 131.61 kPa, 123.69 kPa, 144.54 kPa, and 153.86 kPa.All samples reach their initial condition when loaded with 196.1 kPa load.Based on the classification of swell potential by Garcia-Iturbe, all samples in this study have a low swelling potential because the swelling pressure is less than 196.1 kPa [29].

Figure 5 .
Figure 5. Schematic diagram of swell pressure evaluation

Figure 6 .
Figure 6.SCU-CP effect on soil swell pressure