https://doi.org/10.20965/jdr.2019.p1154 Nakazawa, H. et al. Paper: Problems in Earthquake Resistance Evaluation of Gabion Retaining Wall Based on Shake Table Test with Full-Scale Model Hiroshi Nakazawa∗1,†,KazuyaUsukura∗2, Tadashi Hara∗3, Daisuke Suetsugu∗4, Kentaro Kuribayashi∗2, Tsuyoshi Nishi∗5, Shun Kimura∗6, and Shoji Shimomura∗7 ∗1Earthquake Disaster Mitigation Research Division, National Research Institute for Earth Science and Disaster Resilience (NIED) 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan †Corresponding author, E-mail: [email protected] ∗2Eight-Japan Engineering Consultants Inc. (EJEC), Okayama, Japan ∗3Kochi University, Kochi, Japan ∗4University of Miyazaki, Miyazaki, Japan ∗5Construction Project Consultants Inc., Tokyo, Japan ∗6Eight-Japan Engineering Consultants Inc. (EJEC), Tokyo, Japan ∗7Daioh Shinyo Co., Ltd., Kochi, Japan [Received April 5, 2019; accepted August 30, 2019] The earthquake (Mw 7.3) that struck Nepal on Keywords: gabion retaining wall, earthquake-resistant April 25, 2015 caused damage to many civil engi- design, shake table test, trial wedge method, active col- neering and architectural structures. While several lapse angle road gabion retaining walls in mountainous regions in- curred damage, there was very little information that could be used to draw up earthquake countermea- 1. Introduction sures in Nepal, because there have been few construc- tion cases or case studies of gabion structures, nor The earthquake (Mw 7.3) that struck Nepal on April 25, have there been experimental or analytical studies on 2015 caused damage to many civil engineering and archi- their earthquake resistance. Therefore, we conducted tectural structures. In Kathmandu, many of the older, tra- a shake table test using a full-scale gabion retaining ditional brick structures in the historical district collapsed, wall to evaluate earthquake resistance. From the ex- while reinforced concrete (RC) structures, some of which periments, it was found that although gabion retain- were relatively recently built, were also damaged. While ing walls display a flexible structure and deform eas- much of the damage to the new RC structures was likely ily due to the soil pressure of the backfill, they are re- due to inadequate anti-seismic construction, some struc- silient structures that tend to resist collapse. Yet, be- tures remained undamaged because of their earthquake- cause retaining walls are assumed to be rigid bodies in resistant design in accordance with the Nepal National the conventional stability computations used to design Building Code [1], which was adopted recently. Mean- them, the characteristics of gabions as flexible struc- while, much of the damage in the river basins in moun- tures are not taken advantage of. In this study, we pro- tainous regions was induced by rainfall in addition to pose an approach to designing gabion retaining walls the earthquake, including slope collapse and landslides, by comparing the active collapse surface estimated by which blocked the only local community road and pro- the trial wedge method, and the experiment results ob- duced natural dams when mud clogged the rivers. tained from a full-scale model of a vertically-stacked The authors [2] conducted three field surveys, in July wall, which is a structure employed in Nepal that is and November 2015, and in November 2016, of the dam- vulnerable to earthquake damage. When the base of age to the gabion road retaining walls along the Araniko the estimated slip line was raised for the trial wedge Highway in mountainous regions, and have previously re- method, its height was found to be in rough agree- ported the findings. The use of gabions as reported by ment with the depth at which the gabion retaining Nakazawa et al. [3], shown in Fig. 1, indicates that, of wall deformed drastically in the experiment. Thus, we the 115 locations, 49%, or approximately half, were used were able to demonstrate the development of a method as road retaining walls, and 19% as crash barriers. Ta- for evaluating the seismic stability of gabion retaining ble 1 presents a summary of the damage status of the walls that takes into consideration their flexibility by retaining walls and crash barriers that were most often adjusting the base of the trial soil wedge. found in these surveys, where the damage level is divided into the three categories of A, B, and C. Categories A, B, and C indicate no damage, bulging, and collapse, respec- 1154 Journal of Disaster Research Vol.14 No.9, 2019 © Fuji Technology Press Ltd. Creative Commons CC BY-ND: This is an Open Access article distributed under the terms of the Creative Commons Attribution-NoDerivatives 4.0 International License (http://creativecommons.org/licenses/by-nd/4.0/). Problems in Earthquake Resistance Evaluation of Gabion Retaining Wall Based on Shake Table Test with Full-Scale Model Fig. 1. Breakdown of gabion structures along Araniko Highway (adapted from [3]). Category A refers to retaining walls judged to be undamaged in the damage survey, B to those that were partially deformed or bulged out, and C to those that had collapsed. The broken lines and arrows in photos B and C indicate the deformed or collapsed range of the retaining wall and its direction. Table 1. Summary of survey results of retaining walls and crash barriers (adapted from [3]). Investigation results Ways to lay gabions Main stone forms packed in gabions Others Angular Loose Structure Number of Angular or Damage Vertical Stepwise Total steps and structures type investigated trimmed for Roundish type∗ type type of gabions roundish partly places infill work mixture including A 21 11 10 2–5 18 2 1 3 Retaining walls B 27 11 16 2–5 23 1 3 2 C 9 3 6 2–7 6 1 2 2 A 8 – – 1–3 2 3 3 2 Crash barrier B 8 – – 1–2 1 5 2 7 C 6 – – 1–2 2 2 2 1 *) A: No major damage; B: Partial deformation; C: Collapse tively. Among the road retaining walls studied in this pa- quake countermeasures in Nepal. Therefore, following per, vertically-stacked and stepped types each constituted the above-mentioned survey, we conducted a shake ta- roughly half of category A, while the stepped structures ble test using a full-scale gabion retaining wall in order exceeded the vertically-stacked structures in categories B to evaluate their earthquake resistance employed in Nepal and C. Although vertically-stacked construction is rarely that tended to become damaged in the recent earthquake, used at Japanese construction sites due to its instability as as well as a structural form for the purpose of proposing compared to the stepped type, it is characteristic of Nepal future earthquake-resistant structures. From this series of and is often used. experiments, it was found that, although gabion retain- It is essential that reconstruction is executed rapidly af- ing walls display a flexible structure and deform easily ter an earthquake to recover residents’ former daily lives due to the soil pressure of the backfill, they are resilient and economic activities, and it is desirable for the recon- structures that tend to resist collapse [4]. Yet, because struction results to serve as permanent structures and not retaining walls are assumed to be rigid bodies in the con- be used just for temporary relief purposes. There is a rel- ventional stability computations that are used to design ative scarcity in Japan of construction examples or case retaining walls, the advantages of gabions and other flex- studies of gabion structures that serve as permanent fa- ible structures are not fully used. cilities, and of experimental or analytical studies on their Thus, in this paper, Section 2 reviews previous stud- earthquake resistance, with the result that there is very lit- ies and investigations, Section 3 outlines the experimental tle information that could be useful for drawing up earth- results necessary for the present study, referring to the re- Journal of Disaster Research Vol.14 No.9, 2019 1155 Nakazawa, H. et al. sults of the shake table test conducted previously by the tion of the virtual back surface for inverted T- and L-type authors, and Section 4 examines the applicability of the retaining walls, and proposed an improved trial wedge trial wedge method to gabion retaining walls, which is the method, assuming that two slip lines are generated in the objective of this study. As a first step toward practical de- backfill. sign, we apply the trial wedge method to estimate the ac- The above brief survey indicates that retaining walls tive collapse surface in a vertically-stacked gabion retain- have conventionally been treated as rigid bodies, without ing wall reported in a previous full-scale experiment, and any consideration for the deformation of the retaining wall we then propose an approach to, and discuss problems re- itself. In those cases where they are treated as flexible lated to ultimately establishing a design methodology for structures, stability is ensured by adopting a leaning-type gabion retaining walls. structure, while few studies exist that examine a flexible gabion structure as a vertically-stacked wall. 2. Previous Studies and Design/Construction Examples 3. Shake Table Test Using Full-Scale Retaining Wall Model Conventional studies of the seismic stability of retain- ing walls generally examine concrete gravity, inverted 3.1. Outline of Experiment T-shape, L-shape, leaning, and reinforced earth retain- The full-scale model experiment referred to in this ing walls. For instance, Watanabe et al. [5] conducted study was conducted by Nakazawa et al. [3] to examine a model shake test using retaining walls of various types, the seismic resistance performance and dynamic behavior and systematically evaluated their seismic stability, soil during earthquakes of gabion retaining walls, and it has pressures, and phase characteristics during earthquakes.