ABSTRACT

In the Great East Japan Earthquake of March 2011, a part of huge amount of earthquake and ‘tsunami’ waste was reused as a foundation material for new seawalls (embankments). The authors hypothesized that by using a reinforcing bar frame structure in the embankment instead of the earthquake waste, the root system of the trees planted on the embankment would entwine with this structure and reinforce the ground. Laboratory and field model tests were conducted for up to four years and confirmed that the tree root system was entwined with reinforcing bars and that the reinforcing bar frame structure reduced the earth pressure.

INTRODUCTION

In the Great East Japan Earthquake that occurred in March 2011, among the enormous amount of earthquake disaster wastes caused by the earthquake and ‘tsunami’. The part of the recycling materials, that were handled appropriately, was reused for the construction of a new tide embankment (filling) (Shuto(1985), Araki(2017)). Pot seedlings of deciduous trees and broad-leaved trees were planted on the tide embankment slope as construction of green with an expectation to act as a salty wind protection forest for a ‘tsunami’ that might come again. Moreover, the effect of their root system was expected to extend and coil around the earthquake disaster wastes, which will reinforce the ground (Sendai-city).

The authors expected that a root system of trees planted in a reinforcing bar frame structure installed in filling instead of earthquake disaster wastes will reinforce the ground by coiling around the reinforcing bar structure and therefore started experiments from 2017. The verification and observation items of the experiments are the following three: 1) If the root system grows up coiling around the reinforcing bar (Ikeya et.al.(2021b)), 2) Degrees of reinforcing effect of the ground by the tree root system (increase in shear strength of soil) (O’Louglin(1974), Wu et.al.(1979), Gray and Megahan(1981), Ikeya and Sugiyama(2020)) and 3) Slope failure prevention effect of the reinforcing bar frame structure. In order to check the above, the first was an Experiment (1). A reinforcing bar frame structure was installed on polyvinyl chloride (PVC) pipes of 19 cm in diameter and 50 cm in height, four kinds of tree pot seedlings were installed and those had been raised for three years from seeds on the top, and observed growth of the nursery stocks and extension of the roots. The second was an Experiment (2). In order to confirm the reinforcing effect of the ground by the tree root system, the measured shear strength and hardness of soil including the root system a year after the nursery stocks were planted in a container. The last was Experiment (3). An earth tank was made with a size of 1.5 m in depth, 1.45 m in height, and 0.5 m in width with composite panel plates of 1.2 cm in thickness. A slope model was subjected to the presence of soil only (decomposed granite soils), reinforcing bar frame structure, and reinforcing bar frame structure with tree pot seedlings. In addition, A slope model was set up outdoors and observed for four years.

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