Tree Root Inspired Foundations ERC Participants Project Goals Project Motivation & Role in Strategic Plan

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1 Tree Root Inspired Foundations ERC Participants CBBG Faculty Dan Wilson, UC Davis Jason DeJong, UC Davis Alejandro Martinez, UC Davis Graduate Students Matt Burrall, UC Davis Undergraduate Students (REU) Kyle Doughty, UC Davis Project Goals The overarching goal of this project is to study and gain inspiration from natural biological anchorage systems, such as tree root systems, in order to create practical bio-inspired foundation/anchorage systems that are more resilient, efficient, and/or sustainable relative to conventional linear pile/anchor/nail systems utilized in practice. Achieving this goal will occur through collaboration with related ERC projects at Georgia Tech and NMSU as well as through collaboration with researchers at the University of Dundee. At UC Davis the project includes the following steps: (1) review and formulation of a bio-inspired engineering design research approach, (2) review of tree root literature in the plant physiology field, (3) design and execution of a full scale tree pullout test program of trees with different root architectures, (4) extraction of key observations/principles from the field test program and abstraction to conceptual anchorage designs, (5) DEM modeling of concepts to understand and quantify relative contributions of different mechanisms, and (6) centrifuge modeling of prototypical bio-inspired anchorage designs. Year 1 initiation of the project consisted of a survey across steps 1, 2, 3, and 6. Year 2 progress has included completion of step 1, producing a draft report of step 2, and initiation of step 3. Year 3 is expected to consist of completion of steps, 2, 3, and 4, and will target beginning step 5. Project Motivation & Role in Strategic Plan Deep foundation and anchorage systems used in industry are comprised, almost solely, of linear elements with a constant cross-section geometry. This functional form has remained the same for more than a century, primarily due to the materials available and the ease of installation. However, a comparison of some basic foundation systems against natural foundation/anchorage systems, tree root structures for example, reveal that the biological analog can perform up to 20x more efficiently under lateral or tension load when normalized to the material volume. Knowledge and understanding of what aspects of the natural, biological systems most contribute to this dramatic improvement in performance is currently unknown. The motivation for the current project is to identify and transfer concepts and principles from natural systems in order to realize similar performance gains in built foundation/anchorage systems.

2 Research into natural biological analogs to foundation/anchorage systems in order to gain bioinspiration and develop more efficient, sustainable, and/or resilient infrastructure directly aligns with the bio-inspired theme of the ERC and the Infrastructure Construction thrust. This project, in combination with the others mentioned above at partner universities, comprise one theme within the ERC that is exploring the less studied bio-inspired discovery approach. Any research aspect that involves foreign collaborations, especially indicating the length of time US faculty or students spent abroad conducting their work, and vice versa, and the value added of that work to the student s/faculty work. Collaboration with Prof. Jonathan Knappett and colleagues at the University of Dundee, Scotland, is in place. Prof. Knappett has a project funded by the Forestry Department that is focused on understanding how the root structures of trees on slopes adjacent to roadways help reinforce and increase the stability of constructed and natural slopes. While the goal of their project is not directly bio-inspired, the underlying work of how root systems provide geotechnical reinforcement to soils is relevant. Achievements in previous years As stated above, the launch of the project in Year 1 was designed to be a relatively high-level survey to explore, discover, and learn what research approach might be appropriate, how readily could performance gains in capacity or stiffness be realized, and what might a bio-inspired foundation/anchorage system look like. Thus, Year 1 focused on learning the literature on bioinspired design and formulating a research approach for our research group, beginning a literature review in the biology / tree physiology research fields on tree root systems, performing a single full-scale tree pullout test, and designing and testing early conceptual bio-inspired foundation design in the small centrifuge at UC Davis. Achievements in the past year Year 2 activities have primarily focused on the following three areas. Development of Bio-Inspired Design Approach Bio-inspired engineering design is relatively young, with the bulk of literature discussing best practices, methods, and approaches for performing research in the field and being published in the past decade. Currently there is no firm consensus on best approaches and fundamental differences exist within the community regarding the extent to which nature itself represents an optimized system or solutions; camps of researchers differ widely on this issue. Review and distillation of the literature in Year 1 culminated in our research group articulating the research approach that we have decided to implement. This approach aligns with past work by Prof. Goel which articulated problem or solution driven research/development processes. In our view, neither is appropriate alone; rather, both problem and solution domain knowledge and experts should be engaged in a collaborative, iterative design process. For our project both engineering and plant science expertise is necessary. Further, we have adopted the position that solutions in nature likely do not represent completely optimized, ideal solutions. Instead, considering the natural constraints (e.g. limited water, sun light, nutrients, growth season), it is more likely that natural systems represent innovative, multi-functional, minimum energy systems that have developed a close-to-optimal solution to provide sufficient performance. This perspective has

3 since guided the focus and distillation of content present in our literature review and in the design of the field scale test program that is now underway. The design approach was published and presented at the ASCE GeoFrontiers 2017 as A Bio- Inspired Perspective for Geotechnical Engineering Innovation. Graduate student Matthew Burrall presented the paper at the conference. Figure 1. Graduate student Matthew Burrall presenting at ASCE GeoFrontiers 2017 conference. Review and Synthesis of Tree Root Literature in Biology/Plant Science Literature A literature review of over 250 articles/reports was performed with guidance and collaboration from faculty in the Plant Science Department. Distillation, in the context of our project focus, into the following sub-topics helped organize the content, though inevitably many inter-relations exist: (1) Individual root pullout behavior, (2) Root system pullout behavior, (3) Developmental processes and individual root architecture, (4) Defining root architecture, (5) Modeling of mechanical anchoring, and (6) Effect of environmental perturbations on root anchorage. This led to identification of relevant mechanisms/processes that exist at different length scales, including: structural root function varies with diameter with smaller roots primarily contributing to tensile capacity; root growth occurs primarily by radial expansion, which also results in soil densification; root tortuosity and branching to form nonlinear root structures has a first order effect on capacity generation; the tensile yield stress and root density can be incorporated into soil strength estimates; root cross-sectional shape change (such as elongation) is directly linked to structural loading conditions; root architecture variability inter- and intra-species is largely due to a direct dependence on surrounding environmental conditions; and the total anchorage capacity of a tree is largely dictated by the weight of the soil-root plate, the soil-root tensile capacity, the tensile resistance of the windward roots, and the bending resistance of the leeward hinge. The results of this study have guided development of the full-scale field testing program underway now and seeded ideas for root-inspired foundation/anchorage designs.

4 a) b) c) Figure 2. (a) change in pullout capacity as a function of branching (Mickovski et al. 2007), (b) extreme example of root elongation due to structural loading ( and (c) components contributing to lateral pullout capacity. Design and Execution of Full-Scale Tree Pullout Test Program in UC Davis Orchards A unique full-scale field test program is currently underway at UC Davis. Through the development of our bio-inspired research approach and the literature review it became evident that (1) close study of the biological system (i.e. tree root systems) was important and (2) tree root systems vary tremendously with the surrounding environmental conditions. Collaboration with the Plant Science Department has provided access to an existing fruit orchard that is used for graduate teaching and research. This orchard, in which rows of six different fruit trees are planted yearly, provides access to a variety of different trees of the same age and under identical environmental conditions. Further, in current farming practices trees are typically grown by grafting a specific fruit species on to a root stock. In this orchard six different root stocks are used, and three of them vary substantially with respect to root architecture. The three root stocks that represent the range of architecture variations present are Myrobalan, Lovell, and Marianna. The research program now underway is testing three year old trees. Three tests on each of the three different root stocks will be performed. A 5 ton gantry crane positioned over the tree provides displacement controlled uplift to gradually fail the trees in tension after the upper tree section has been cut off. Measurements during testing include axial load-displacement, surface displacement measurement to map the failure surface, surface accelerometer array to spatially monitor subsurface root failure in time, and photogrammetry to later reconstruct 3-D images of failure progression. After complete failure, the root mass and attached soil mass is weighed. Photogrammetry is then used for reconstruction of the root bulb architecture. Air-spade excavation will occur for a subset of the trees to exhume the surround root architecture remaining in the ground after testing of all trees is completed. Root samples will be obtained and tested in the laboratory to characterize the tensile root capacity. Mini-cone penetration testing and soil samplings are being performed adjacent to select trees as well. The testing program is currently underway, with the load testing to be completed by end of September. Analysis will continue through much of Year 3. Figure 3 below shows an initial image during test setup.

5 Figure 3. Picture showing initial setup for field testing. Summary of other relevant work being conducted within and outside of the ERC and how this project is different. Related projects on bio-inspired root foundation systems are being performed at Georgia Tech (Frost) and at NMSU (Newtson). We have held several collaboration meetings during Year 2. Plans for the next year Year 3 activities will focus on the following: Complete report on literature review, highlighting concepts and ideas relevant to project goals. Execute full scale tree pullout test program of trees with different root architectures. Extract key observations/principles from field test program and abstract these ideas to conceptual anchorage designs. Begin DEM modeling of concepts to understand and quantify relative contribution of different identified mechanisms. Expected milestones and deliverables for the project Milestones and deliverables will include: Complete report on literature review. Completion and data report of field scale testing. Identification of conceptual ideas for bio-inspired foundation/anchorage systems. Member company benefits The development of more efficient foundation and anchorage solutions has the potential to increase performance efficiency, resulting in less material usage and fewer elements installed.

6 Several of the current companies as well as others being courted to join the ERC work in the foundation/anchorage/retaining industries have expressed strong interest in the research program. If relevant, commercialization impacts or course implementation information None to date.