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Proceedings of the 4th Asian Regional Conference on Geosynthetics757

June 17 - 20, 2008 Shanghai, China

DESIGN OF EARTH DAMS ALLOWING TEMPORARY OVERTOPPING BASED ON

HYDRAULIC FAILURE EXPERIMENTS AND FLOOD ANALYSIS

K. Matsushima1, Y. Mohri2, S. Yamazaki3, T. Hori4, M. Ariyoshi5 and F. Tatsuoka6

ABSTRACT: Every year a great number of small earth dams󰀂for agricultural irrigation are seriously damaged or totally fail because of flood overflow exceeding the drainage capacity of a spillway. To stabilize downstream slopes of earth fill dams against overflow, a new stabilization technology using geosynthetic soil bags with extended tails (GSET) was developed. GSET functions as a spillway for temporary flooding while it is designed to have a high seismic stability. A series of hydraulic failure experiments were performed on a 3.5 m-high and 2.3 m-wide full-scale GSET model with a downstream slope of 1:1.2 (V: H). The overflow was stepwise increased. Based on the test results, a flood stability design for GSET spillways allowing temporary overflow is developed. A design working example is presented based on an actual flood record. It is shown that the remedy work of an existing earth dam by means of a GSET-spillway is effective in particular with small earth dams with short-peak flooding.

KEYWORDS: overtopping, soil bag, flooding, spillway, remediation work

INTRODUCTION WingSoil bag Tail There are approximately 210,000 reservoirs with earth dams constructed for agricultural irrigation that are Soil bags stacked Overflowlower than 15 m in height across Japan. Approximately inclined downward 20,000 of these earth dams have deteriorated and need at the backNormalurgent but cost-effective repair. Moreover, every year a Tail great number of dams are seriously damaged or even totally fail due to flood overflow exceeding the drainage Fig. 1 Basic Spillway made of geosynthetic soil bags capacity of a spillway and earthquakes, as typically seen with extended tails (GSET) allowing temporary in Photo 1. To substantially increase the stability of the flooding (Matsushima et al., 2005a&b) slopes of such small earth dams against overflow and seismic loads, Matsushima et al. (2006a) proposed dam with geosynthetic reinforcement (Fig. 1). This protecting the slope by stacking soil bags inclined system, called geosynthetic soil bags with extended tails downward at the back and anchored to the inside of earth (GSET) spillway, is designed to properly function during

emergency temporary flooding (Mohri et al. 2005; Downstream Sliding to the Overtopping side upstream side Matsushima et al. 2005a&b, 2006a & b, 2007a).

A soil bag system has a self-confining mechanism, in

Downstream which the tensile force is mobilized in the soil bag sheet slope associated with compressive deformation of the backfill Cracks on the top of Upstream the upstream slope (Fig. 2: e.g., Matsuoka et al. 2001; Tatsuoka, 2004; Eroded part of the embankment Lohani et al. 2006). This mechanism increases the confining pressure in the backfill, resulting in an Photo 1 Failure of small earth dams: a) by the Niigata-chuetsu earthquake in 2004 in Kawaguchi town; and b) increase in the strength of the backfill. Therefore, large

totally collapsed by heavy rainfall during Tokage tensile force is developed in the geosynthetic soil bag by

typhoon No. 200423 in Awaji islandpre-compaction. It is considered that an stacked soil bag

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Researcher, Dept. of Geotechnical and Hydraulic Engrg., National Institute for Rural Engineering, JAPAN. Email: kenichim@affrc.go.jp Chief of laboratory of Soil Mechanics, Dept. of Geotechnical and Hydraulic Engrg., National Institute for Rural Engineering, JAPAN. Email: ymohri@affrc.go.jp 3

Researcher, Mitsui Chemicals Industrial Products, Ltd., JAPAN. Email: yama820@affrc.go.jp 4

Researcher, Dept. of Geotechnical and Hydraulic Engrg., National Institute for Rural Engineering, JAPAN. Email: thori@affrc.go.jp 5

Researcher, Dept. of Geotechnical and Hydraulic Engrg., National Institute for Rural Engineering, JAPAN. Email: ariyoshi@affrc.go.jp 6

Professor, Dept. of Civil Engineering, Tokyo University of Science, JAPAN. Email: tatsuoka@rs.noda.tus.ac.jp