Studying the Use of Microbial Induced Calcite Precipitation as a Shallow Stabilization Alternative to Treat Expansive Soils

Expansive soils usually recognized as swell-shrink soils have been a problem for civil infrastructure for a long time. It has been a very common practice to use chemical stabilizers including cement and lime to stabilize expansive soils, especially for lightly loaded structures. However, due to the the detrimental effects of these stabilizers on the environment and several occurrences of premature failures after stabilizing with chemical additives, engineers are in search of sustainable stabilization alternatives. Microbial Induced Calcite Precipitation (MICP) is a promising process, which can improve the properties of expansive soil through calcite precipitation. Previous research has shown promise for the use of MICP in mitigating swelling distresses in expansive soils. There are generally two approaches to apply MICP: Bioaugmentation and Biostimulation. In this research, biostimulation was applied by mixing enrichment and cementation solutions with soils in an effort to develop a new alternative to shallow chemical stabilization. Three soils were selected with varying plasticity for this purpose. Soils were treated by mixing with enrichment and cementation solutions. Enrichment solutions were first added and were allowed to stimulate bacteria for different time periods, termed mellowing periods. At the end of each mellowing period cementation solutions were added to facilitate calcite precipitation. Two protocols were studied for this shallow mixing method of MICP application. In protocol-1, soils were mixed with enrichment solutions at optimum moisture content (OMC) and allowed to stimulate for mellowing periods of 1, 2, 3, and 4 days. Protocol-2 was similar to protocol-1 except for the the initial amount of enrichment solution which was 95% of maximum dry unit weight on the wet-side of standard proctor curve in place of OMC. At the end of each mellowing period, the enrichment solution lost during this time was replaced with cementation solution to reach OMC and soil samples were compacted to untreated maximum dry unit weight. Treatment effectiveness was evaluated with Unconfined Compression Strength test and calcite test. The results indicated that protocol-1 performed better than protocol-2 which indicated that adding higher amounts of enrichment solutions was not beneficial for calcite precipitation and improvement of strength. Following this finding, protocol-2 was discontinued and protocol-1 was chosen for further testing. Five different mellowing periods, three different curing periods and two types of cementation solutions were studied by following protocol-1. Improved test results were observed with the lower concentration of calcium chloride used in the cementation solution. Also, medium to high plastic soils showed improvement in evaluation tests with respect to strength gain, swell reduction, and calcite precipitation. Unconfined Compressive Strength (UCS) value after treatments ranged from 45 to 267 kPa, calcite values ranged from 0% to 1.36% and the Free Swell Indices ranged from 8% to 266%. The maximum change in UCS (284%) was observed for medium plasticity soil C-30

Evaluating Shallow Mixing Protocols as Application Methods for Microbial Induced Calcite Precipitation Targeting Expansive Soil Treatment

Expansive soils, also known as swell-shrink soils, undergo substantial volumetric changes due to moisture fluctuations from seasonal variations. These volumetric changes cause millions of dollars in damages annually. Microbial Induced Calcite Precipitation (MICP) is a promising soil improvement technique, which uses urease producing bacteria to precipitate calcium carbonate. In this study, a stabilization alternative for expansive soils was studied using MICP. Specifically, indigenous bacteria were stimulated by mixing enrichment and cementation solutions with expansive natural soils to precipitate calcium carbonate and make soil stronger and less expansive. This study examined three expansive soils with varying plasticity and mineralogical characteristics. Two protocols for shallow mixing were studied. In Protocol-1, soil samples were mixed with enrichment solutions at optimum moisture content and allowed to mellow for 1, 2, 3, and 4 days. In Protocol-2, soil samples were mixed with enrichment solutions at moisture content corresponding to 95% of maximum dry unit weight on the wet-side of a standard Proctor curve. Moisture was allowed to escape from the mix during the mellowing period under both protocols. Following the mellowing periods, the lost moisture is replaced with cementation solution to reach optimum moisture content, and the soil sample was compacted to its maximum dry unit weight. Unconfined compression strength test was used to evaluate the strength improvements due to treatments. The treatment effectiveness was also evaluated with measurements of calcium carbonate precipitation. The results show promise for this method as an alternative to current shallow stabilization methods. An increase in mellowing period for low and medium plastic soils was determined to be beneficial. The current results also showed that the presence of higher amounts of enrichment solution and addition of less cementation solution is not advantageous for this procedure based on the performance of Protocol-2

Microbial-Facilitated Calcium Carbonate Precipitation as a Shallow Stabilization Alternative for Expansive Soil Treatment

Expansive soils generally recognized as swell-shrink soils have been a problem for civil infrastructure for a long time. Engineers are in search of sustainable stabilization alternatives to counter these problematic soils. Microbial-induced calcium carbonate precipitation (MICP) is a promising biocementation process that can improve the properties of expansive soil through calcium carbonate precipitation. Past research has shown promise for the use of MICP in mitigating swelling distress from expansive soils. In this research, MICP via biostimulation was attempted by mixing enrichment and cementation solutions with soils in an effort to develop a new alternative to shallow chemical stabilization. Three soils with varying clay contents (30%, 40%, and 70%) and plasticity characteristics were selected, and soils were treated by mixing with enrichment solutions followed by cementation solutions. Five different mellowing periods, three different curing periods, and two types of cementation solutions were studied to optimize the method. Treatment effectiveness was evaluated using unconfined compression tests, calcium carbonate tests, and free swell index tests. Results showed that an increase in the mellowing period beyond two days was not beneficial for any of the three soils tested in this research. It was determined that the best improvement was observed at two days of mellowing and seven days of curing