Influence of Dry Beach on the Dynamic Stability of Ash Storage Field: A Numerical Analysis

Abstract

Ash storage sites are a commonly used method of disposing fly ash, a byproduct of coal combustion, in China today, and when it accumulates to a certain height, serious geological hazards may occur as a result of seismic activity. In this study, an in situ standard penetration test was carried out on a constructed ash storage site in Northwest China to evaluate the potential for liquefaction of alluvial fly ash within the site, and the results show that dynamic liquefaction can occur within a newly constructed three-stage subdam. A numerical analysis of the influence of dry beach length on the dynamic response of the primary dams and subdams and an assessment of the extent of dynamic liquefaction in the ash storage field were carried out using the Wenchuan seismic waves as input ground motion. Numerical results prove that the acceleration within the ash storage field is relatively low in the original breccias layer and gradually increases with height, with the peak acceleration occurring in the vicinity of the third subdam and a decreasing trend from the subdams towards the ash storage field. As the length of the dry beach increases, the Peak accelerations in the ash storage area occur near the third subdams at larger dry beach length. Meanwhile, the acceleration in the ash storage area close to the surface gradually increases, and, significantly, the range where higher accelerations occur also becomes larger. The maximum horizontal displacements at different dry beach lengths occur at the crest of the third subdam and in the adjacent ash storage area. As the length of the dry beach increases, the maximum horizontal displacements show a certain decrease, but they occur progressively further away from the third subdam, so that, under dynamic action, the dams become safer. The extent of liquefaction decreases at larger dry beach length and extends further away from the third subdam into the ash storage area. It is, therefore, recommended that the length of the dry beach should not be less than 150 m for this ash storage site.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

China is a large coal-consuming country, producing hundreds of millions of tons of fly ash every year due to power generation, most of which come from the fly ash power boilers of large and medium thermal power plants. As a byproduct of coal combustion, fly ash is treated in a different way from other industrial waste residues due to the special nature of its chemical composition, and appropriate treatment methods should be chosen in order to minimize the environmental pollution and maximize the waste utilization [1]. Two main methods of handling fly ash both at home and abroad are integrated utilization and storage in ash storage sites. Especially in recent years, the use of fly ash to improve the engineering properties of materials in industries such as civil engineering and road construction has become an effective way to resource fly ash [2–4]. Although the utilization rate of fly ash is increasing every year, the overall utilization rate is low, and storage in ash storage sites is still the mainstay. This approach also faces several environmental pollution problems [5], e.g., air pollution caused by the wind-driven escape of bare fly ash from ash storage sites into the atmosphere and pollution of the surrounding soil and underground water bodies after leaching of harmful chemicals from fly ash [6–8]. However, more importantly, there is also the risk of dam failure when fly ash accumulates to a certain height in the ash storage site [9]. In addition to destabilization due to heavy rainfall, severe geological hazards can be induced by seismic motion.

Extensive studies have been accumulated on the dynamic properties of fly ash in ash storage sites. Li et al. [10] analyzed the stress-strain characteristics of fly ash under consolidated-drained and consolidated-undrained conditions by means of a triaxial vibration test and found that relative density greatly influences the area of the liquefaction zone. Based on the dynamic effective stress analysis of the dam, Wang and Wang [11] determined the safety factor for sliding stability, and there is a large difference in the acceleration distribution coefficient along the dam height and slope, and the base frequency of the dam decreases as the dam softens. Hu [12] found, from the liquefaction and dynamic stability analysis of ash dams, that the dynamic instability will occur even without liquefaction zones in the ash dam during the seismic motion. Zhou et al. [13] found that the maximum pore water pressure was more sensitive to liquefaction than that occurring near the top of the dam and that these areas were more sensitive to liquefaction. Liu et al. [14] studied the dynamic properties of fly ash and silty clay on road base and concluded that the dynamic properties of fly ash were better than those of silty clay after three freeze-thaw cycles. Dong [15] found that the dynamic stress-strain relationship for soil-fly ash mixture can be approximated by the hyperbolic function, and the more the soil mixed, the higher the dynamic shear stress corresponding to the same shear strain. Wang [16] observed that dynamic strain of fly ash-treated soil grows at higher dynamic stress or at higher confining pressures, while the dynamic strength increases at higher contents of fly ash. Wang et al. [17] studied the constitutive model of improved loess by fly ash. Zhang et al. [18] investigated the dynamic performance of fly ash in view of the dynamic liquefication and softening of dynamic modulus. Li et al. [19] carried out dynamic torsional shear tests on the dynamic behavior of fly ash. Wei et al. [20] studied the dynamic properties of fly ash considering the influence of temperature. The above scholars have carried out valuable researches on the dynamic properties of fly ash. However, little research has been done on fly ash in ash storage areas, and because of its high accumulation, its safety cannot be neglected in case of earthquakes.

In order to study the dynamic stability of the ash storage field, field tests were carried out, and the liquefaction potential of the ash storage field was analyzed by means of standard penetration tests. Numerical analysis was used to investigate the dynamic response of the primary dam and subdams at different dry beach lengths, including acceleration, horizontal displacement, and the extent of dynamic liquefaction using a pore pressure level-based approach. The results of the study provide a basis for the evaluation of the dynamic safety of existing ash storage sites.

2. Field Tests

2.1. Overview of the Ash Storage Site

The ash storage site is located in the Gobi Desert at the northern edge of the Yinchuan Basin, where the Baolan Railway and National Highway 110 pass by, adjacent to the Yellow River in the east and overlooking the Helan Mountains in the west. The ash storage site adopts a stage-by-stage damming scheme, with a permeable primary dam constructed by local Gobi sand and gravel material, and the later stage adopting the upstream method of raising the dam with compacted ash. The maximum height of the primary dam is approximately 22 m. The first subdam is approximately 4.0 m. Currently, the ash site has a secondary subdam that was raised at the end of 2007 with a height of approximately 5 m, a crest elevation of 1124.30 m, and a restricted ash storage elevation of 1123.30 m. In 2010, a third subdam was raised with a height of 5 m, an upstream and downstream slope of 1:3.0, and a width of 4 m at the top of the subdam. The crest elevation and the limiting ash storage elevation are 1129.30 m and 1128.30 m, respectively, while, for the ash field drainage system, one reinforced concrete shaft-canal drainage channel was used, including two shafts with an inner diameter of 3.0 m and a height of 27.0 m and 23.0 m, respectively. The internal diameter and total length of the culvert drainage channel are 1.6 m and 600 m, respectively. Geomembranes were used for the whole ash storage field to prevent infiltration of ash and polluted groundwater, and the thickness of soil over the impermeable membrane is 0.3 m. Figure 1 presents the schematic diagram and photo of the ash storage field.

[figure omitted; refer to PDF]

In addition, we also plotted the maximum horizontal displacements at five different positions of the ash storage site with different dry beach lengths, as shown in the horizontal displacement histogram for each location of the storage site in Figure 10. It is clear that the maximum horizontal displacement at the five different sites occurs within the ash storage field, followed by the top of the third subdam, the top of the second subdam, the top of the first subdam, and the minimum value at the top of the primary dam. The horizontal displacements at the primary dam do not vary much with different beach lengths. For the rest of the ash storage site, the maximum horizontal displacement decreases as the beach length increases.

[figure omitted; refer to PDF]

Figure 14 illustrates the time-course curves of horizontal displacements ($L_d$ = 150 m) for the third subdam and the storage field for three different input earthquakes. For a given ash storage site, the time-course curve of the horizontal displacement can be seen to exhibit significant fluctuations, which are closely related to the temporal variation of the input ground acceleration. The peak horizontal displacement at the selected site increases significantly with increasing peak acceleration of the input ground acceleration, which can be clearly seen in Figure 15. In addition, of the five selected ash storage positions, the peak horizontal displacement within the ash storage field is the highest, which is due to the relatively weaker mechanical properties of the alluvial fill material within the storage field compared to the compacted soils of the subdams and primary dams. Another phenomenon is that peak horizontal displacement in the third subdam is slightly lower than that in the ash storage field monitoring position, but it is significantly higher than that in the remaining subdams and primary dams. As the filling height decreases, the peak horizontal displacement decreases progressively, and this phenomenon is observed for all three input earthquakes considered. The higher the peak acceleration of the input ground acceleration is, the more pronounced this phenomenon becomes.

[figures omitted; refer to PDF]

[figure omitted; refer to PDF]

Figure 16 presents the contour of the effective stress distribution and the range of the liquefied area in an ash storage field with 150 m dry beach length under three input earthquakes. It is noted that the effective stress distribution, the area enclosed by the liquefied area, and its distance from the three-stage subdam do not change significantly at the two input earthquakes of 0.10 g and 0.20 g, but at a peak acceleration of 0.30 g in the input earthquake, the liquefied area increases significantly, even through the center of the storage field. This also means that, for an ash storage site with a dry beach length of 150 m, the first two types of input ground shaking considered do not have a significant effect on the liquefaction range in the ash storage site and that the liquefaction range changes significantly as the input acceleration of the ground acceleration increases. Other measures should be taken to maintain the dynamic stability of the dam and the ash storage field, such as the improvement of drainage facilities.

[figures omitted; refer to PDF]

5. Conclusions

This study carried out a dynamic response analysis of an ash storage site in northwest China, using the Wenchuan seismic waves as input ground shaking, and investigated the effect of dry beach length on the horizontal displacement, peak acceleration, and liquefaction range of the site. The main conclusions are as follows.

(1) In situ standard penetration tests at all levels of subdams, primary dams, and alluvial ash in the ash storage field have shown that areas of liquefaction do not occur at all levels of subdams but may occur in the alluvial ash storage area and should be dealt with, for example, by strengthening drainage facilities.

(2) The acceleration in the ash storage field is relatively low in the breccias layer but increases with height, with the peak acceleration occurring in the vicinity of the third subdam and tending to decrease from the subdams towards the alluvial ash storage area. At a larger dry beach length, the peak acceleration occurs at similar locations within the ash storage area but increases with dry beach length.

(3) The maximum horizontal displacements of the different dry beach lengths occur at the crest of the third subdam and in the adjacent ash storage area. At a larger dry beach length, the maximum horizontal displacement decreases but occurs progressively further away from the third subdam, so that, under dynamic forces, the dam becomes safer. The extent of liquefaction also decreases and extends further away from the third subdam and into the ash storage field. It is, therefore, recommended that the dry beach length should not be less than 150 m for this ash storage site.

Acknowledgments

The research described in this paper was financially supported by the National Natural Science Foundation of China (Grant nos. 51608442 and 51778528), the Basic Research Program of Natural Science of Shaanxi Province (Grant No. 2019JLM-56), and the Research Fund of the State Key Laboratory of Eco-hydraulics in Northwest Arid Region, Xi’an University of Technology (Grant No. 2019KJCXTD-12). These supports are greatly appreciated.

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