**1. Introduction**

Various types of dams have been increasingly constructed all over the world for irrigation, flood controlling, power generation, environment protection, etc. [1]. Normally, most dams are preferentially built on rock foundations where seepage control is not a very difficult task. However, with the exploitation and exhaust of natural satisfactory dam sites, many new dams have to be constructed on thick overburden layers, as better sites are not available and removal of the existing overburden is technically or economically unfeasible. This adverse situation is often encountered along many hydropower-rich rivers in the southwest and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

northwest regions of China [2, 3]. When such thick overburden foundation layers can neither be avoided nor removed, a rockfill dam is often a priority due to its excellent adaptability to such geological conditions. In recent years, more than 50 high rockfill dams, including earth core rockfill dams (ECRDs), asphalt core rockfill dams (ACRDs) and concrete faced rockfill dams (CFRDs), have been constructed in China, as selectively listed in **Table 1**. Challenges can be seen from both the height of these dams and the thickness of the overburden layers.

wall, if used. The inhomogeneity of foundation materials can result in differential and incompatible deformation within the dam and may ultimately lead to threatening cracks.

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

http://dx.doi.org/10.5772/intechopen.78547

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**b.** *Permeability and erosion resistance of the overburden layers*. One of the most important functions that should be achieved in dam engineering is the ability to control the seepage within the foundation. Designing an impervious system for the dam foundation depends, to a large extent, on the permeability and erosion resistance of the involved strata and the

**c.** *Liquefaction potential of the underlying fine layers*. Earthquake is one of the most disastrous natural events that dams are expected to experience. Cyclic shearing by earthquakes can cause excessive pore water pressure to build up in fully saturated sandy soils, leading to a decrease, or even loss, of their shear strength. As a result, uncontrollable deformation can occur in both the foundation and the dam itself and may result in the worst-case scenario

**d.** *Connection techniques for the impervious systems of the dam and its foundation*. An effective impervious system means not only successful control of the seepage through the dam and its foundation but also satisfactory performance of the connection points between different impervious components. These points are usually places where parts with different rigidity levels meet and joint, and are therefore vulnerable to cracks and concentrated leakage.

The main challenge in constructing a rockfill dam on thick overburden layers is the design and successful construction of an impervious system for the foundation, accounting for the distribution of the underlying soil and rock layers as well as their physical and engineering properties. In this chapter, the authors review several high rockfill dams built on thick overburden layers in China in order to provide a reference for similar cases that might be encountered in the future. The chapter starts with general descriptions of some frequently implemented geological and geotechnical investigation techniques. Next, seepage control techniques used in some selected cases are introduced. Attention is also paid to the connection techniques for impervious systems used in different kinds of rockfill dams and to the widely adopted foundation reinforcement measures in engineering practice. Directions that deserve

Thick overburden layers generally refer to quaternary materials deposited over river beds, including boulders, cobble, gravel, sand, silt and clay constituents. Mixtures of these complex overburden materials are often much more compressible and permeable than an intact rock foundation. Adequate geological and geotechnical investigations on the distribution, thickness and other relevant properties of the soil strata are necessary for the design of impermeable systems and for the preparation of required foundation treatments during dam planning stages. In particular, weak layers, such as sand lenses, soft clays and collapsible loess, should be revealed in these investigations and then properly treated to eliminate safety risks to both

available foundation treatment equipment and techniques.

further research and development are presented.

the foundation itself and to the overlying dam.

**2. Geological and geotechnical investigations**

of a dam breach.

Technical problems requiring special attention in the design and construction of rockfill dams over thick overburden layers include, but are not limited to, the following aspects:

**a.** *Shear strength and deformability of load-bearing layers*. The shear strength of underlying foundation layers influences the overall stability of the dam, while the deformability of these layers controls not only the deformation of the dam but also the deflection of the cutoff


Note: ECRD = earth core rockfill dam; ACRD = Asphalt core rockfill dam; CFRD = concrete faced rockfill dam; *H*max = maximum height of dam; *T*max = maximum thickness of overburden; '/' means the dam is still under construction and has not been finished.

**Table 1.** Basic information of typical rockfill dams built on overburden layers in China.

wall, if used. The inhomogeneity of foundation materials can result in differential and incompatible deformation within the dam and may ultimately lead to threatening cracks.

northwest regions of China [2, 3]. When such thick overburden foundation layers can neither be avoided nor removed, a rockfill dam is often a priority due to its excellent adaptability to such geological conditions. In recent years, more than 50 high rockfill dams, including earth core rockfill dams (ECRDs), asphalt core rockfill dams (ACRDs) and concrete faced rockfill dams (CFRDs), have been constructed in China, as selectively listed in **Table 1**. Challenges can be seen from both the height of these dams and the thickness of the overburden layers.

Technical problems requiring special attention in the design and construction of rockfill dams

**a.** *Shear strength and deformability of load-bearing layers*. The shear strength of underlying foundation layers influences the overall stability of the dam, while the deformability of these layers controls not only the deformation of the dam but also the deflection of the cutoff

**No. Name Type Year** *H***max (m)** *T***max (m) Cutoff wall References**

Note: ECRD = earth core rockfill dam; ACRD = Asphalt core rockfill dam; CFRD = concrete faced rockfill dam; *H*max = maximum height of dam; *T*max = maximum thickness of overburden; '/' means the dam is still under construction

**Table 1.** Basic information of typical rockfill dams built on overburden layers in China.

and has not been finished.

2 Dam Engineering

 Shiziping ECRD 2010 136 102 1.2 × 1 101.8 [4] Xiaolangdi ECRD 2000 160 80 1.2 × 1 82.0 [5] Pubugou ECRD 2010 186 75 1.2 × 2 78.0 [6] Changheba ECRD / 240 79 1.4 × 1 & 1.2 × 1 50.0 [7] Maoergai ECRD 2011 147 57 1.4 × 1 52.0 [8] Shuiniujia ECRD 2006 108 30 1.2 × 1 32.0 [9] Luding ECRD 2011 84 148 1.0 × 1 110.0 [10] Qiaoqi ECRD 2006 125.5 72 1.2 × 1 70.5 [11] Xiabandi ACRD 2009 78 148 1.0 × 1 85.0 [12] Badi ACRD / 97 120 1.0 × 1 105.0 [13] Huangjinping ACRD 2015 85.5 134 1.2 × 1 113.8 [14] Yele ACRD 2005 124.5 420 (1.0–1.2) × 1 154.5 [15] Xieka CFRD 2014 108.2 100 1.2 × 1 86.0 [16] Nalan CFRD 2006 109 24 0.8 × 1 18.0 [17] Miaojiaba CFRD 2011 111 48 1.2 × 1 41.5 [9] Jiudianxia CFRD 2008 136.5 56 1.2 × 1 30.0 [18] Aertash CFRD / 164.8 94 1.2 × 1 90.0 [19] Chahanwusu CFRD 2008 110 47 1.2 × 1 41.8 [3] Duonuo CFRD 2012 112.5 40 1.2 × 1 35.0 [20] Laodukou CFRD 2009 96.8 30 0.8 × 1 29.6 [21]

**Width(m) Height (m)**

over thick overburden layers include, but are not limited to, the following aspects:


The main challenge in constructing a rockfill dam on thick overburden layers is the design and successful construction of an impervious system for the foundation, accounting for the distribution of the underlying soil and rock layers as well as their physical and engineering properties. In this chapter, the authors review several high rockfill dams built on thick overburden layers in China in order to provide a reference for similar cases that might be encountered in the future. The chapter starts with general descriptions of some frequently implemented geological and geotechnical investigation techniques. Next, seepage control techniques used in some selected cases are introduced. Attention is also paid to the connection techniques for impervious systems used in different kinds of rockfill dams and to the widely adopted foundation reinforcement measures in engineering practice. Directions that deserve further research and development are presented.
