Project: Root System Evaluation for Multi-Function Foundation Bio-Inspiration

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1 Project: Root System Evaluation for Multi-Function Foundation Bio-Inspiration ERC Team Members CBBG Faculty David Frost, GT Jeannette Yen, GT Graduate Students Seth Mallett Rodrigo Borela Undergraduate Students Caroline Colbert (REU) Project Goal The overarching goal of the proposed research is to provide insight into the performance of multifunction root systems which can lead to the development of novel construction configurations and techniques for the enhancement of common infrastructure systems, such as deep pile foundations and retaining structure anchorage systems. The project's role in support of the strategic plan This project aims to elucidate the anchorage and failure mechanisms of roots of varying architecture subjected to typical plant loading situations, including pullout extension (herbivores), compression (self-weight), and lateral (wind/earthquake) forces and use this insight to propose and develop new anchorage systems. Analogous synthetic root structures, instead of living roots, will be designed and utilized based on simplified optimal root architectures revealed in experimental studies. Test protocols and an associated taxonomy to characterize the behavior of typical root configurations on their own (no soil embedment) will be developed and then related to those of both model experiment results and discrete element method simulations of root-soil systems under various loading conditions. Fundamental Research, Education, or Technology Advancement Barriers Index tests will be developed to evaluate the response of various root systems under extension and compression loading conditions. Using an X-ray Computed Tomography (CT) system, soil samples with buried root structures will be imaged at various time increments during the loading procedure. Observation of the failure mechanisms will allow for a conceptual model to be formulated to predict the system capacity under the different loading scenarios for the various root architectures. The role of the root shape and surface characteristics will also be evaluated. Tests on real root systems as a function of time will also be constructed to provide insight into the evolution of capacity growth with time. The experiments on the analogue and real root systems will provide insight into the extent of the soil engagement in each of the cases. Experimental studies will be complemented by Discrete Element Model simulations of root-particulate systems.

2 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. A collaboration was established in year 1 of this project between Georgia Tech and researchers at the Port and Airport Research Institute (PARI) in Okosuka, Japan. This very productive collaboration enabled extensive X-ray CT testing to be performed during pull-out-tests on 3-D printed root analogues. A more recently established collaboration between Georgia Tech researchers and research from L3SR at Universite Grenoble Alpes, France is aimed at increasing international collaborations focused on studying root-inspired anchorage systems, particularly with respect to continuum based numerical simulations of root-soil anchorage systems. A third collaboration was established this year also with ENPC in Paris, France to focus on DEM numerical modeling of root anchorage systems. PhD student Seth Mallett spent 3 months at PARI in Summer 2016 supported on an NSF EAPSI Fellowship and 3 months in Summer 2017 at ENPC supported on an NSF IRES (International Research Experience for Students Fellowship. He also spent 2 weeks at L3SR working with researchers there on numerical modeling issues. Mr. Rodrigo Borela Valente attended a one week Winter School on computational micro geo-mechanics at L3SR in January 2017 supported by funds from the Higginbotham Chair. Achievements in previous years To begin to better understand the relationship between tree roots and the structural capacity of the roots as a foundation system, Year 1 studies involved both experimental and numerical simulations using root analogues. The analogues being used included un-branched and branched cables that incorporate many of the characteristics of tap roots as well as 3D printed roots that incorporate characteristics of fibrous roots. A selection of root analogues are shown in Figure 1. Figure 1: Selection of tap and fibrous root analogues. Results from experimental pull-out tests conducted on un-branched and branched tap roots with and without friction (achieved by coating outside of root fibres with sand) show significant differences in terms of both peak and large displacement root resistance. Typical results are shown in Figure 2 and indicate that branched tap roots reach a larger peak than un-branched ones and also exhibit significantly less post-peak reduction in resistance. Further, roots with textured surfaces (higher friction) typically indicate higher pull-out resistance compared to their smooth-surfaced (lower friction) counterpart as seen in Figure 2. Results from DEM numerical simulations of compression loading of fibrous root analogues further demonstrated differences between typical human-constructed and nature-constructed foundation systems. Figure 3 shows the force versus displacement response for different root analogues (straight, zig-zag, 15 degree splay and 30 degree splay). It can be seen that the common human

3 implemented approach of using a straight shaft pile yields the lowest resistance while the large splay pile yields the largest resistance. Figure 2: Pull-out resistance tests on tap root analogues. Figure 3: Close-up of setup (a) pile, (b) zigzag, (c) 15 degree and (d) 30 degree load test simulations. (e) Results of load tests. Force chain maps for (a) pile, (b) zigzag, (c) 15 degree and (d) 30 degree load test simulations. Achievements in past year The pullout tests conducted in year 1 were completed with an extensive pullout testing program performed in an X-ray CT facility. The experimental set-up is shown below in Figure 4. Experiments performed using a range of root analog configurations including those with 3, 4 and 6 branches of different lengths at different orientations. A vertical section through one specimen along with the a horizontal CT scan showing concentric shear zones induced during the rot pullout test are shown in Figure 5.

4 Figure 4. Root pullout chamber in X-ray CT scanner Figure 5. Vertical and horizontal sections through root analog in CT scanner. Key project accomplishments in past year included: Completed preliminary characterization experiments on various root systems grown in lab including quantification of root topology and volume measurements. Conducted extensive program of pull-out tests on various root analogues using range of sands (sizes, angularity). Root analogues included 3-D printed roots as well as unbraided rope systems. Performed extensive program of compression/tension loading on 3-D printed root analogues to develop simple index test for root characterization. Performed 2-D DEM simulations of various root analogues in compression to demonstrate potential for non-straight (conventional) foundation elements to provide higher resistance. Initiated extensive pullout testing program on real root systems to develop understanding of evolution of root-soil anchorage mechanisms with time.

5 Extensive analysis of time-lapse X-ray CT measurements to determine failure surfaces generated by root analogues with varying number of branches, length of branches, orientation of branches. Summary of other relevant work being conducted within and outside of the ERC and how this project is different Complementary research is being undertaken at UCD where the initial focus is on the study of full-scale root systems. In particular, natural root systems, such as trees, are providing the context for new bio-inspired foundation and anchorage systems. This is occurring through innovative capacity evaluation of full scale trees with different root topologies. Similarly, centrifuge testing of selected root analogues is scheduled and will be valuable to the further interpretation of the studies summarized herein. Plans for the next year These research tasks conducted during the next year of the project are expected to lead to a clear understanding of the interaction mechanics that occur between root analogues and soils and provide the basis for the develop of a rational theory which could be transferred to the design of a next generation of pile and anchor foundation systems that are bio-inspired and can yield a higher certainty of performance at a lower cost than current systems. 1. Complete lab pullout tests on 3-D printed root analogues including varying moisture and surcharge conditions. 2. Complete X-ray CT experiments including additional experiments on root analogues in compression. 3. Performed additional 2-D DEM simulations of various root analogues in extension to complement X-Ray CT experiments and demonstrate potential for non-straight (conventional) root inspired anchorage elements to provide higher capacities. 4. Perform analyses to identify optimal foundation/anchor configuration to yield highest capacities. 5. Begin development of concepts for installation of non-conventional foundation and anchor elements as critical -cursor to moving project from Fundamental Knowledge to Enabling Technology plane. Expected milestones and deliverables for the project The successful completion of this project is expected to lead to a number of contributions as follows: Quantification of the extent of bulb contributing to root anchorage capacity in both tension and compression; Development of new sensing systems to capture large scale response; Improved understanding of root-soil interaction mechanisms; Development of appropriate root analogues for model studies (bench scale, centrifuge and numerical). Member company benefits Longer-term benefits of the project will be the development of next-generation multi-function selfadaptive anchorage systems that yield sustainable, resilient geotechnical systems. If relevant, commercialization impacts or course implementation information None advanced at this stage of project.