1.1 Types of Embankment Dam
In dam engineering, many dam types, such as the arch dam, gravity dam, arch-gravity dam, barrage, and embankment dam, are used, but the embankment dam is the most important type. This is because the majority of dams around the world are embankment dams. Embankment dams are mainly made from compacted earth. There are two main types; rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like gravity dams made from concrete.
Rock-fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth utilized often contains a high percentage of large particles, hence the term rock-fill. The impervious zone may be on the upstream face and made of masonry, concrete, plastic membrane, steel sheet piles, timber, or other materials. The impervious zone may also be within the embankment, in which case it is referred to as a core. In instances where clay is often utilized as the impervious material, the dam is referred to as a composite dam. To prevent internal erosion of clay into the rock-fill due to seepage forces, the core is separated using filters. The filters are specifically graded soils designed to prevent the migration of fine grain soil particles. When suitable material is at hand, transportation is minimized leading to cost savings during construction. Rock-fill dams are resistant to damage from earthquakes. However, inadequate quality control during construction can lead to poor compaction and sand in the embankment, which can lead to liquefaction of the rock-fill during an earthquake. Liquefaction potential can be reduced by keeping susceptible material from being saturated, and by providing adequate compaction during construction.
A concrete-face rock-fill dam has concrete slabs on its upstream face. This design offers the concrete slab as an impervious wall to prevent leakage and a structure that will resist uplift pressure. In addition, the concrete-face rock-fill dam design is flexible for topography, faster to construct, and less costly than earth-fill dams. The concrete-face rock-fill dam originated during the California Gold Rush in the 1860s when miners constructed rock-fill timber-face dams for sluice operations. The timber was later replaced by concrete as the design was applied to irrigation and power schemes. As concrete-face rock-fill dam designs grew in height during the 1960s, the fill was compacted and the slab's horizontal and vertical joints were replaced with improved vertical joints. In the last few decades, the design has become popular. Currently, the tallest concrete-face rock-fill dam in the world is the 233 m tall Shuibuya Dam in China, which was completed in 2008.
Earth-fill dams, also called earthen, rolled-earth, or simply earth dams, are constructed as a simple embankment of well-compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a locally plentiful shell with a watertight clay core. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. An outdated method of zoned earth dam construction utilized a hydraulic fill to produce a watertight core. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. An interesting type of temporary earth dam occasionally used in high latitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it.
A third type of embankment dam is built with an asphalt concrete core. The majority of such dams are built with rock and/or gravel as the main filling material. Almost 100 dams of this design have now been built worldwide since the first such dam was completed in 1962. All asphalt-concrete core dams built so far have an excellent performance record. The type of asphalt used is a visco-elasto-plastic material that can adjust to the movements and deformations imposed on the embankment as a whole, and to the settlements in the foundations. The flexible properties of the asphalt make such dams especially suited to earthquake regions.
In this book, the rock-fill dam with a soil core is called the earth-rock fill dam. There are usually two types of soil core. One is the vertical core, and the other is the sideling core. The problem of hydraulic fracturing in the soil core of the earth-rock fill dam is focused on in this book.
According to statistics analysis from the Chinese National Committee on Large Dams (CHINCOLD) and the International Commission on Large Dams (ICOLD), by the end of 2005 the number of the dams higher than 100 m worldwide was 851, and the number in China was 130. By the end of 2008, the number of the dams higher than 100 m in China was up to 142. In the 142 dams above 100 m in China, the number of embankment dams was 69. And according to the Bulletin of First National Census for Water given by Ministry of Water Resources, P. R. China and National Bureau of Statistics, P. R. China (2013), by the end of 2011, the number of reservoirs in China totaled 98 002, with a combined storage capacity of 932.312 billion m3. Among these reservoirs, 97 246 were completed, with a total storage capacity of 810.410 billion m3, and 756 were under construction, with a total storage capacity of 121.902 billion m3.
Hydropower is a renewable energy source where power is derived from the energy of water moving from higher to lower elevations. It is a proven, mature, predictable, and price competitive technology. Hydropower has the best conversion efficiency of all known energy sources (about 90% efficiency, water to wire). It also has the highest energy payback ratio. The total worldwide technically feasible potential for hydropower generation is 14 368 TWh per year with a corresponding estimated total capacity potential of 3838 GW (IJHD, 2005); five times the current installed capacity. Undeveloped capacity ranges from about 70% in Europe and North America to 95% in Africa indicating large opportunities for hydropower development worldwide. China, Canada, Brazil, and the US together account for over 46% of the production (TWh) of electricity in the world and are also the four largest in terms of installed capacity (GW) (IEA, 2008). According to the work of Tortajada (2008), in China the gross theoretical hydropower potential was 6083 TWh per year, the technically feasible hydropower potential 2474 TWh per year, the economically feasible hydropower potential 1753 TWh per year, and the planned hydro capacity 49–65 GW. About 75% of the existing 45 000 large dams in the world were built for the purpose of irrigation, flood control, navigation, and urban water supply schemes. Only 25% of large reservoirs are used for hydropower alone or in combination with other uses, as multipurpose reservoirs.
1.2 Hydraulic Fracturing
In China, 17 earth-rock fill dams higher than 100 m have been constructed, and more than 24 are to be constructed. Most of them are located in Western China where water resources are very abundant. Among these high earth-rock fill dams, some are higher than 200 m, such as the Nuozhadu Dam (261.5 m in height) on the Lancang (Mekong) River in the Yunnan Province, the Shuangjiangkou Dam (322 m in height), and the Changhe Dam (240 m in height) on the Dadu River in the Sichuan Province in the southwest of China. It is well known that cracks frequently occur in the soil core of the earth-rock fill dam. The cracks are believed to be the result of stress arching action and/or hydraulic fracturing in the soil core (Zhu and Wang, 2004). Care must be taken to prevent such cracking and the engineers must decide whether the cracks are likely to extend and become serious, whether they are stable and can be backfilled, or will self-heal.
There have been a number of well-studied cases where dams have failed or been damaged by concentrated leaks for no apparent reason. In some of these experiences, investigators concluded that differential settlement cracks were probable causes, even though no cracks were seen on the surface. In these examples, it was not determined whether the cracks were open before the reservoir filled or whether they might have opened afterward.
In a number of the histories, a concentrated leak appeared abruptly at the downstream side of the dam after the reservoir was filled, perhaps several hours or several days later. This indicated that no large open cracks existed before the reservoir was raised. This is one piece of evidence for the conclusion that, under certain conditions, the reservoir water pressure acting on the upstream face of the dam can cause existing closed cracks to open or can create new ones.
As the embankment is deformed by differential settlement, the minor principa...
