Publication Date

12-2018

Date of Final Oral Examination (Defense)

11-12-2018

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Civil Engineering

Department

Civil Engineering

Major Advisor

Bhaskar Chittoori, Ph.D.

Advisor

Malcolm Burbank, Ph.D.

Advisor

Debakanta Mishra, Ph.D.

Abstract

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.

DOI

10.18122/td/1491/boisestate

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