【Daily Discussion】Hydropower Engineering - 20200715

Based on the different water-permeability characteristics of the geotechnical materials, dam foundation seepage can be divided into three main types: ① Pore-based seepage: Seepage that occurs through the pores in sand and gravel, typically presenting as a uniform flow; the amount of seepage depends mainly on the particle size composition of the soil and its permeability coefficient. ②Fissure leakage: Leakage that occurs through joints and fissures in rocks. When there are many cracks that intersect with each other, the seepage is approximately uniform; when the cracks are unevenly developed or irregular, the seepage often takes on a pulsating pattern. ③Tubular leakage: Leakage that occurs through cavities in soluble rocks such as limestone and dolomite; the amount of leakage depends on the size and number of these cavities. These three types can exist simultaneously in different locations within the dam foundation, or they can appear separately, depending mainly on the distribution of rocks and soils as well as the geological structure of the dam foundation.

1. If the foundation is not cleaned thoroughly or not cleaned at all, the reservoir will start leaking shortly after it is built. 2. The cut-off trench was not considered in the design, or its dimensions did not meet the requirements, resulting in it being damaged during use.

1. If the foundation is not cleaned thoroughly or not cleaned at all, the reservoir will start leaking shortly after it is built. 2. The catch basin was not taken into account in the design, or its dimensions do not meet the requirements; as a result, the catch basin gets damaged during use

①Porous leakage: Leakage that occurs through the pores in sand and gravel, usually in a uniform flow; the amount of leakage depends primarily on the particle size composition of the soil and its permeability coefficient. ②Fissure leakage: Leakage that occurs through joints and fissures in rocks. When there are many cracks that intersect with each other, the seepage is approximately uniform; when the cracks are unevenly developed or irregular, the seepage often takes on a pulsating pattern. ③Tubular leakage: Leakage that occurs through cavities in soluble rocks such as limestone and dolomite; the amount of leakage depends on the size and number of these cavities. These three types can exist simultaneously in different locations within the dam foundation, or they can appear separately, depending mainly on the distribution of rocks and soils as well as the geological structure of the dam foundation.

①Porous leakage: Leakage that occurs through the pores in sand and gravel, usually in a uniform flow; the amount of leakage depends primarily on the particle size composition of the soil and its permeability coefficient. ②Fissure leakage: Leakage that occurs through joints and fissures in rocks. When there are many cracks that intersect with each other, the seepage is approximately uniform; when the cracks are unevenly developed or irregular, the seepage often takes on a pulsating pattern. ③Tubular leakage: Leakage that occurs through cavities in soluble rocks such as limestone and dolomite; the amount of leakage depends on the size and number of these cavities. These three types can exist simultaneously in different locations within the dam foundation, or they can appear separately, depending mainly on the distribution of rocks and soils as well as the geological structure of the dam foundation.

Porous leakage, fissural leakage, tubular leakage.

①Porous leakage: Leakage that occurs through the pores in sand and gravel, usually in a uniform flow; the amount of leakage depends primarily on the particle size composition of the soil and its permeability coefficient. ②Fissure leakage: Leakage that occurs through joints and fissures in rocks. When there are many cracks that intersect with each other, the seepage is approximately uniform; when the cracks are unevenly developed or irregular, the seepage often takes on a pulsating pattern. ③Tubular leakage: Leakage that occurs through cavities in soluble rocks such as limestone and dolomite; the amount of leakage depends on the size and number of these cavities. These three types can exist simultaneously in different locations within the dam foundation, or they can appear separately, depending mainly on the distribution of rocks and soils as well as the geological structure of the dam foundation.

Depending on the location where dam leakage occurs, it can be classified into three types: leakage within the dam body, leakage at the dam foundation, and leakage around the dam. The causes of each type of leakage are analyzed as follows. 2.1 Main causes of dam leakage: During the process of raising and thickening the dam, it undergoes numerous expansions. Improper handling at the junctions between the new and old dam sections, poor quality of the soil and rock materials used for construction, high moisture content in these materials, as well as an excessive amount of boulders and debris in the fill material, all contribute to the risk of dam leakage. If the dam raises the water level stored behind it, the impermeable layer of the dam structure will be subjected to greater water pressure, which may cause the impermeable wall to lose its function and pose a risk of dam failure. 2.2 Main causes of foundation leakage Foundation leakage in earth-rock dams is usually caused by the low strength of the dam foundation; therefore, the degree of integration between the foundation and the dam’s impermeable structure is crucial for the safe operation of the dam. For example, in some reservoirs, the overburden layer on the foundation is very thick, or the foundation consists of permeable rock zones, such as unconsolidated or poorly consolidated sand and gravel layers, igneous rocks, basalt, as well as karst permeability zones. Due to the failure to investigate the geological conditions during the reservoir construction process, or the lack of reinforcement measures in accordance with construction specifications despite such investigations, safety hazards gradually emerged after many years of operation. This resulted in severe leakage at the dam foundation. 2.3 Main causes of seepage around the dam. Seepage around the dam, also known as abutment seepage, refers to the phenomenon whereby, after a reservoir is filled with water, the water seeps downstream along the pores, fissures, karst caves, faults, and other geological features in the rocks on both sides of the dam, due to the difference in water head between the upstream and downstream areas. The main reason is that the geological conditions of the slopes at the dam ends are poor; for example, there are permeable sand and gravel layers beneath the slopes on both sides of the earthfill dam ； After excavation or erosion of the bank slope by wind and waves, the impermeable covering on the upstream bank slope was damaged ； Improper treatment of the impermeable barrier at the junction between the dam end and the bank slope, such as exposure of the impermeable barrier, actually exacerbates seepage around the dam ； Sometimes, construction is not carried out in accordance with design requirements, resulting in quality defects and creating leakage pathways.

Based on the different water-permeability characteristics of the geotechnical materials, dam foundation seepage can be divided into three main types: ① Pore-based seepage: Seepage that occurs through the pores in sand and gravel, typically presenting as a uniform flow; the amount of seepage depends mainly on the particle size composition of the soil and its permeability coefficient. ②Fissure leakage: Leakage that occurs through joints and fissures in rocks. When there are many cracks that intersect with each other, the seepage is approximately uniform; when the cracks are unevenly developed or irregular, the seepage often takes on a pulsating pattern. ③Tubular leakage: Leakage that occurs through cavities in soluble rocks such as limestone and dolomite; the amount of leakage depends on the size and number of these cavities. These three types can exist simultaneously in different locations within the dam foundation, or they can appear separately, depending mainly on the distribution of rocks and soils as well as the geological structure of the dam foundation.

1. If the foundation is not cleaned thoroughly or not cleaned at all, the reservoir will start leaking shortly after it is built. 2. The catch basin was not taken into account in the design, or its dimensions do not meet the requirements; as a result, the catch basin gets damaged during use