Seismic performance and numerical simulation of earth-fill dam with geosynthetic clay liner in shaking table test

Abstract

In this paper, shaking table tests were carried out on both a small-scale and a full-scale earth-fill dams with geosynthetic clay liners to examine their seismic performance. The behavior of these fully instrumented earth-fill dams when subjected to seismic loading was also simulated by numerical analysis. Firstly, in the small-scale shaking table test, no failure was observed along the geosynthetic clay liner when the earth-fill dam was subjected to seismic motion. Numerical analysis confirmed that the behavior of the model earth-fill dam was unaffected by the geosynthetic clay liner. Secondly, a comparative shaking table test was carried out on full-scale earth-fill dams, one with a sloping core zone and another with a geosynthetic clay liner. Both model dams showed similar acceleration response and deformation behavior. It should be mentioned that the acceleration response increased gradually toward the top of the dam, and the deformation, after shaking, was relatively large near the foot of the slope. These observations were successfully simulated by the numerical analysis.

Introduction

There are approximately 200,000 small earth-fill dams (i.e., reservoirs) in Japan. However, about 70% of these dams were constructed over 150 years ago, before modern design standards and compaction techniques were established (Ministry of Agriculture, Forestry and Fisheries, 2017). Furthermore, there are many small earth-fill dams where the fill material and construction method are unknown. Since the seismic performance of these dams cannot be assumed, there are concerns that they have been or will be damaged by seismic activity. Accordingly, it is considered necessary to prevent water leakage and improve seismic performance for these old earth-fill dams which have insufficient earthquake resistance.

A large number of small earth-fill dams have been severely damaged during previous earthquakes in Japan. Almost all those damaged were constructed before the implementation of modern seismic design standards (Ministry of Agriculture, Forestry and Fisheries, 2009). Tani (1996, 2000) investigated the features of dams damaged in past earthquakes, including the 1995 Hyogo-ken Nanbu Earthquake, and classified the types of damage and the damage rate. The survey results indicated that large earth-fill dams with heights above 15 m constructed using current dam designs exhibit high earthquake resistance and are sufficiently safe (Tani, 2000).

In 2011, the Tohoku Earthquake occurred. Though its epicenter was in the Pacific Ocean, the earthquake severely damaged numerous small earth-fill dams in Japan (Fukushima Prefecture, 2011). In Fukushima prefecture, a total of 745 out of 3730 earth-fill dams were damaged by sliding failure of upstream or downstream slopes, crest settlement, and cracks to the longitudinal section at the crown (Hori et al., 2012). Among them, three dams collapsed completely. In particular, the 18.5 m high Fujinuma earth-fill dam, which was completed in 1949 to supply irrigation water, collapsed, resulting in casualties (see in Fig. 1). Several researchers have investigated the condition and features of each damaged dam (Hori et al., 2012; Tanaka et al., 2012; Mohri et al., 2014). None were well-compacted or waterproof. It was reported that the Fujinuma dam had a low degree of compaction, which brought about a deterioration of undrained shear strength under high water levels by applying stronger cyclic loadings (Tanaka et al., 2012). One of the lessons of the 2011 earthquake was the importance of fill material compaction in the design of earth structures (Tatsuoka et al., 2017).

Japan expects a strong earthquake to occur along the Nankai Trough within the next 30 years. Improving the earthquake resistance of earth-fill dams is paramount. To this end, the Ministry of Agriculture and the Waters Ministry undertake aseismicity inspections of earth-fill dams nationwide (Suzuki, 2015). Boring surveys, sounding tests, and laboratory tests on fill materials are performed to evaluate the sliding failure stability, residual displacement, and liquefaction risk. If deemed necessary, remedial works are carried out to improve waterproofing and earthquake resistance.

A widely-used method for improving small earth-fill dams is to build a sloping core zone with low-permeability clay on the upstream side of the dam (Ministry of Agriculture, Forestry, and Fisheries in Japan, 2015). Recently, because of a shortage of low-permeability clay and logistics making bringing a large amount of soil to a site impractical (Oda et al., 2015), geosynthetic clay liners, which boast outstanding waterproofing abilities, have been used to prevent water leakage and improve dams’ earthquake resistance (see in Fig. 2). However, geotechnical engineers are concerned about the potential for a sliding-type failure at the interface between the soft clay liner and the fill material.

It is therefore necessary to research the seismic performance of small earth-fill dams with geosynthetic clay liners. Sasaki et al. (2015) carried out direct shear tests under low confining pressures to find out the frictional interaction between a geosynthetic clay liner and the typical soil in an embankment. The shear strength of the geosynthetic clay liner and the interface between the woven or non-woven geotextile of the geosynthetic clay liner and soil were evaluated, respectively. Other researchers have undertaken shaking table tests (Koyama et al., 2014; Jeong et al., 2016a). Their research did not corroborate the concerns about a sliding-type failure at the interface between the liner and the fill material. However, because the shaking table tests were qualitative studies on small-scale models, the results were not conclusive. Thus, full-scale shaking table tests were carried out on 3 m high earth-fill dams with either a sloping core zone or a geosynthetic clay liner to examine the seismic behavior of both (Sawada et al., 2016; Oda et al., 2016) with prior numerical analysis (Jeong et al., 2016b). Large longitudinal cracks had developed at the crest of the dam with the geosynthetic clay liner. Nonetheless, after subjection to seismic, the amount of residual settlement in each dam was similar and no water leakage was observed in either dams. Therefore, earth-fill dams with geosynthetic clay liners are not considered to display inferior seismic performance compared to dams with sloping core zones (Sawada et al., 2018). Another full-scale shaking table test was carried out on earth-fill dams with geosynthetic clay liners, comparing the installation shapes: one was installed straight, the other was installed in a staircase shape with overlapping joints (Sawada et al., 2019a, Sawada et al., 2019b).

Based on the previous research, this paper first examined the sliding-type failure and seismic behavior of a small earth-fill dam with a geosynthetic clay liner in a small-scale shaking table test, as well as by numerical analysis. Secondly, a comparative shaking table test were carried out on a full-scale earth-fill dam with a sloping core zone and another with a geosynthetic clay liner in order to examine their seismic performance. In addition, the behavior of these fully instrumented earth-fill dams was simulated by numerical analysis.