Exploratory Investigation of Bio-inspired Flexible Calcite Precipitation for Soil Improvement

民用基础设施(如公路、铁路、桥梁和建筑物)通常建在软弱和/或松散的土壤上，需要改善以抵抗施加的荷载，包括由自然灾害(例如地震荷载)产生的荷载。通常，这些土壤是使用不环保的能源密集型材料和技术进行改良的。这些问题将在未来25年加剧，因为民用基础设施预计将大幅扩张，以适应预计将增长30%的世界人口。为了解决这些问题，必须开发可持续和有弹性的材料和建筑技术。一种这样的土壤改良方法依赖于利用土壤中的本地细菌(微生物)的生物中介过程，最近的研究重点是微生物诱导的碳酸盐沉淀(MICP)过程。对比未处理和MICP处理的土体的响应，文献中的数据表明，土壤的抗剪强度可以提高高达500%。然而，MICP技术的应用遇到了挑战和困难，包括：(1)脆性胶结键在小应变下断裂；(2)氨(一种有毒废物)的产生；(3)微生物对小于细砂的土壤孔隙的渗透有限。这些挑战和困难(A)将MICP法的应用局限于特定的土壤颗粒大小，(B)降低了土壤的剪切强度和模数，这限制了MICP法在减轻液化和基础支撑等岩土应用中的好处。这笔赠款通过探索开发一种具有弹性和可持续的新方法来促进科学进步，以利用自然发生在海绵中的生物启发过程来改善土壤特性。这种方法提供了强大和灵活的土壤颗粒结合，并可用于更细的土壤。拟议的概念具有变革性，它加速了生物地质土壤改良技术的发展，推动了生物灵感岩土工程领域的进一步创新发展，并加强了跨学科合作。该项目还将对本科生和研究生进行培训和教育，特别是将培养具有传统岩土工程教育中通常不采用的生物岩土工程技术和程序的知识和能力的新研究人员；并教育普通公众、K-12教师和学生、大学生和实习工程师。该项目的目标是探索生物灵感柔性方解石(BIFC)沉淀的概念，以改善土壤的粘结(胶结)、延展性、硬度和强度。这项研究的重点是使用硅酸盐-α酶，在没有微生物的情况下，在土壤中诱导灵活的方解石沉淀。初步实验证明，硅酸盐酶可在实验室中沉淀仿生柔性方解石(BIFC)，并证实硅酸盐酶含量为10%~16%的BIFC在承受大于20%的应变时具有非凡的柔韧性，没有断裂迹象，剪切阻力是自然形成的方解石的9倍。因此，假设在没有微生物的情况下，使用硅酸盐-α酶在土壤中产生BIFC沉淀物将消除小应变时胶结键的断裂，并提高土壤在剪切时的力学性质(延性、刚度和强度)，从而避免地震荷载后的额外愈合处理。拟议的工艺还避免了MICP法产生的铵；不需要地下刺激或增加微生物，潜在地降低了工艺成本；并可能扩大可以处理的土壤颗粒大小的范围。为了实现该项目的目标，这项探索性研究将集中于：(1)利用小试反应器和发酵罐优化硅酸盐-α酶和生物多样性碳的生产，并使用高清晰度光学显微镜和扫描电子显微镜(SEM)对生物多样性碳进行表征；(2)利用原子力显微镜(AFM)悬臂梁测试和颗粒尺寸测试表征沉淀生物多样性碳的微观力学性能；以及(3)通过三轴试验研究生物多样性碳处理后的砂土和粉土的力学性能。

Civil infrastructures (e.g., highways, railroads, bridges, and buildings) are commonly constructed on weak and/or loose soils that require improvement to resist applied loads including those generated by natural hazards (e.g., earthquake loading). Often such soils are improved using energy-intensive materials and techniques that are not environment-friendly. These issues will be exacerbated over the next 25 years, as civil infrastructure is expected to expand greatly to accommodate an anticipated 30% growth in the world's population. To address these issues, sustainable and resilient materials and construction techniques must be developed. One such approach for soil improvement relies on bio-mediated processes that use indigenous bacteria (microbes) in the soil, with recent research focusing on the microbial induced carbonate precipitation (MICP) process. When comparing the responses of untreated and MICP-treated soils, the data available in the literature show that soil shear strength can increase by up to 500%. However, applications of the MICP technique have encountered challenges and difficulties including: (1) breakage of the brittle cementation bonds at small strains; (2) the generation of ammonium (a toxic waste product); and (3) limited penetration of microbes through the pores of soils smaller than fine sands. These challenges and difficulties (a) limit the MICP application to specific soil particle sizes and (b) reduce the shear strength and modulus of soils, which limits the benefits of the MICP technique in geotechnical applications such as liquefaction mitigation and foundation support. This grant promotes the progress of science by exploring the development of a new resilient and sustainable method to improve soil properties using a bio-inspired process that naturally occurs in sea sponges. This approach provides strong and flexible bonding of soil particles and can be used in finer soils. The proposed concept is transformative, and it accelerates the development of biogeotechnical soil improvement techniques, motivates further innovative developments in the field of bio-inspired geotechnical engineering, and enhances cross-disciplinary collaboration. The project also enables the training and education of undergraduate and graduate students, and in particular, will develop new researchers who are knowledgeable and competent in the techniques and procedures employed in biogeotechnical engineering that are not usually employed in traditional geotechnical engineering education; and educate the general public, K-12 teachers and students, university students, and practicing engineers. The goal of this project is to explore the concept of bio-inspired flexible calcite (BiFC) precipitation to improve the bonding (cementation), ductility, stiffness, and strength of soils. This research focuses on the use of the silicatein-alpha enzyme, without microbes, to induce flexible calcite precipitation in soils. Preliminary experiments demonstrated that the silicatein-á enzyme can precipitate bio-inspired flexible calcite (BiFC) in the laboratory and confirmed that BiFC with 10% to 16% silicatein á enzyme content has exceptional flexibility with no sign of breakage when subjected to strains greater than 20% with shear resistance that is 9 times greater than that of naturally-formed calcite. Therefore, it is hypothesized that using silicatein-alpha enzyme, without microbes, to produce BiFC precipitate in soils will eliminate the breakage of cementation bonds at small strains and enhance the mechanical properties of soils upon shearing (ductility, stiffness and strength), avoiding the need for additional healing treatments after earthquake loading. The proposed process also avoids the generation of ammonium that occurs with MICP; does not require subsurface stimulation or augmentation of microbes potentially reducing the cost of the process; and may extend the range of soil particle sizes that could be treated. To achieve the goal of the project, this exploratory research will focus on: (1) optimizing the production of silicatein-alpha enzyme and BiFC using bench-scale reactors and fermenters and characterizing BiFC using high-definition optical microscopy and scanning electron microscopy (SEM); (2) characterizing the micro scale mechanical properties of precipitated BiFC using Atomic Force Microscopy (AFM) cantilever beam tests and particle-scale tests; and (3) investigating the mechanical properties of BiFC treated sand and silt using triaxial testing.