Dams and Spillways PDF

DAMS AND SPILL WAYS GROUP - 1  Introduction Page 02 Dams and spillways are critical components in water resource management and infrastructure development. Dams serve to store water, control floods, and generate hydroelectric power, while spillways safe...

DAMS AND SPILL WAYS GROUP - 1  Introduction Page 02 Dams and spillways are critical components in water resource management and infrastructure development. Dams serve to store water, control floods, and generate hydroelectric power, while spillways safely release excess water to prevent structural failures. Their design integrates engineering, geology, and hydrology to ensure functionality and safety. These structures are crucial for addressing global challenges like water scarcity, renewable energy demands, and climate resilience, making them highly relevant for sustainable development and disaster risk reduction.  Page 03 Importance and Relevance Water Management: Dams provide water for agriculture, drinking, and industrial uses. Energy Generation: Many dams support hydroelectric power, a renewable energy source. Flood Control: Spillways protect downstream areas from flooding during heavy rainfall or dam overflow. Economic Development: Properly constructed dams support irrigation, navigation, and tourism, boosting local economies. Climate Adaptation: Dams help manage water availability amid increasing climate variability.  Page 04 Issues from Improper Construction 1, Structural Failures: Inadequate geological surveys can lead to foundation instability or seepage, risking collapse. 2, Flood Risks: Poorly designed spillways may fail to manage excessive inflow, causing catastrophic flooding. 3, Environmental Impact: Faulty planning may lead to excessive sedimentation, habitat destruction, and ecosystem imbalance. 4, Economic Loss: Dam failures result in costly repairs, loss of agricultural productivity, and displacement of communities. 5, Human Safety: Neglecting seismic considerations can cause dam failures in earthquake-prone areas, endangering lives.  Page 06 Terminology and Basic Aspects of Dam Construction Crest: The top of the dam. Abutments: The parts of the dam that rest on the side slopes of the valley. Freeboard: The vertical distance between the top of the dam and the water surface of the reservoir. Heel: The upstream end where the dam meets the foundation. Toe: The downstream end of the dam's base. Headwater: The water upstream of the dam. Tailwater: The water downstream from the dam, often influenced by discharge or backwater effects. Gallery: An open space within the dam for machinery, grouting, drainage, and monitoring instrumentsn process involves selecting a dam site based on geological and environmental factors. Subsurface exploration (e.g., drilling) helps evaluate the foundation's suitability, while the presence of faults, joints, and other weak geological features can compromise the dam's stability. Types of Dams and Their Functions 1. CONCRETE DAMS - A concrete dam is a large structure built across rivers, streams, or other bodies of water to store, control, or divert water. Gravity Dams Arch Dams Buttress Dams 2.MASONRY DAMS - acts as an impermeable Types of Dams and Their barrier against leakage of reservoir water. Functions 3. ROCK-FILL DAMS - is a type of dam constructed primarily from rock or gravel, with a relatively small core of impermeable material such as clay or concrete to prevent water from seeping through. The dam's main body is made up of layers of large rocks, which are used to provide stability, weight, and resistance against water pressure. Types of Dams and Their Functions 4. EARTH DAMS - also known as an embankment dam, is a type of dam made primarily of natural materials such as earth, clay, sand, and gravel. It is often used to create reservoirs, control floods, and provide water for various purposes. Earth dams are commonly constructed by compacting these materials in layers to create a stable structure that can withstand the pressure of the water. SPILLWAYS AND GATES 1. NORMAL SPILLWAYS- also called a gravity spillway or overflow spillway, is a crucial feature in dam construction. Its primary purpose is to safely release excess water from a reservoir when water levels rise above the safe operating level, ensuring that the dam doesn't become overwhelmed and potentially fail due to excessive water pressure. 2. PIPE SPILLWAYS- In pipe spillway (Fig. 14.16), the discharge structure is made of a pipe passing through the dam body. The pipe is laid for some lengths of the dam in a dig and cover manner for the low headwater projects. 3. TUNNEL SPILLWAYS- In a tunnel spillway (Fig. 14.17), a tunnel punctures the upper end working SPILLWAYS AND GATES as intake of discharge and the lower end (almost horizontal) discharges the water. It is provided with a gate chamber for free fl ow or fl ow under pressure. 4. GLORY HOLE (OR SHAFT) SPILLWAYS- The glory spillway (Fig. 14.18) is in the form of a vertical or steeply inclined shaft for water to fl ow freely or under pressure. Its head structure may be gated or ungated  5. SIDE CHANNEL SPILLWAYS- is a type of spillway used in dam construction where the water flows from the reservoir through a separate channel located at the side of the main dam structure. OUTLET WORKS- A portion of the reservoir water may be diverted through a tunnel or conduit for the purpose of supply to irrigation canals, water supply schemes, or hydroelectric plants for power generation. This water diversion structure is termed the outlet. SPILLWAYS AND GATES GATES OF DIFFERENT TYPES AND THEIR FUNCTIONS 1. PLAIN GATES- These are commonly known as planks and work by translation movement along Gates are mobile structures that facilitate closing slots in piers across the spillway crest. It may be of or opening passes for water to fl ow down. They lift or slide type, as shown in Fig. 14.21(b). can be opened completely or partially to allow controlled discharge. 2. RADIAL GATES- These operate by rotary turning on hinges, see Fig. 14.22 Geomorphology and Dam Design Dams are designed to meet strict safety standards while minimizing costs. Factors like river valley shape, foundation materials, and availability of construction resources are crucial in determining the optimal dam design. Factors Affecting Dam Construction River Valley Foundation Materials Wide, shallow valleys are well-suited The depth to bedrock and material Availability of suitable construction for embankment dams, while deep, properties of the foundation are critical materials like rock, soil, and cement narrow valleys favor concrete dams. for structural integrity. can impact feasibility and cost. Geological Evaluation for Dam Sites 1 Site Assessment Thorough geological surveys identify potential issues like fault lines, weak rock formations, or unstable slopes. Material Testing In-depth analysis of soil, rock, and other construction 2 materials ensures they meet design specifications. Monitoring Ongoing monitoring and instrumentation track the dam's structural performance and integrity over time. 3 CAUSATIVE FACTORS OF DAM DISASTERS Geological Causes Other Causes -Weak or Porous Foundation Rocks -Faulty Spillway Design -Laminated or Weak rocks -Drainage Issues -Faulted or Cavernous Foundation -Natural Calamities -Landslide and Slope Failure -Poor Construction Materials Preliminary Investigation and Selection of a Dam Site Page 07 Purpose: Identify geological weaknesses to ensure feasibility and minimize costs. Key Tasks: - Map geological features (1:1000 scale). Use remote sensing and satellite imagery for site analysis. Study faults, shear zones, slide scars, and slope characteristics. Compare multiple site options based on geological and engineering factors. Detailed Investigation of a Dam Site for Design purposes Purpose: Provide surface and subsurface data for dam design and cost estimation. Key Tasks: - Create detailed maps (1:100–1:500 scale) of lithology and structures. Conduct trenching, drilling, and core logging to assess rock types, weak zones, and permeability. Perform lab tests for density, porosity, strength, and load-bearing capacity. Analyze faults, folds, and cavities for stability and leakage risks. Source of Building Materials for Different Types of Dams Insights from Engineering Geology Building materials are a critical component in the construction of dams, determining their safety, durability, and cost-efficiency. This section emphasizes identifying and sourcing appropriate materials for various types of dams Building Materials for Concrete Dams aggregate materials Building materials are a critical component in the construction of dams, determining their safety, durability, and cost-efficiency. This section emphasizes identifying and sourcing appropriate materials for various types of dams  Building Materials for Dams Concrete and Cement Rock and Earth Fill Earth Dams Use clayey soils and sand for impermeable Use sand, pebbles, gravel, and crushed Require durable rock-fill materials, cores, mixed to achieve the required rock as aggregates typically sourced from nearby quarries. properties. Testing: Crushing strength, impact resistance, shape, and size. In practice, concrete used in the construction of a dam may have more than one type of mix. Table 14.3 provides a list of different mixes (weight of aggregates and cement) used in the Umiam concrete dam in Meghalaya for one cubic metre of concrete. Building Materials for Concrete Dams Boulders and Rock Fragments for Masonry Dams Masonry structures are constructed using large boulders of river deposits. When boulders of the requisite size and quality are not available in river beds, these are obtained in large fragments or slabs by quarrying rocky hill faces and are used in masonry works after chiselling into blocks. The main considerations in masonry dam construction are availability and suitability of the rocks conforming to the specifi cation of the ISI Building Materials for Concrete Dams Fill Materials for Rock-fill Dams Compared to masonry dams, rock-fi ll dams are more favoured in modern time as they are more economical. A rock-fi ll dam uses the entire deposit in a river or terrace as fi ll materials constituted of boulders, pebbles, gravels, sand, and clay. The rock fragments quarried from nearby rocky hill slopes are also used. In the mountainous terrains, the river valleys, hill slopes, and terraces contain plenty of construction material for rock-fi ll dams, and if the dam is proposed in the valley, the haulage charge will also be less. However, the overall quality of the bulk material will conform to the design requirement. Building Materials for Concrete Dams Sandy and Clayey Material for Earth Dams Sand, silt, and clay, the decomposed products of rocks, are available in plenty in most parts of the projects, especially in the reservoir basins. The decayed rock of hill slopes especially in areas composed of sedimentary rocks are generally devoid of pebbles and boulders and are found to have good quality material for use in earth dams. Both pervious and semi-pervious types of materials are necessary for the earth dams Building Materials for Concrete Dams Sandy and Clayey Material for Earth Dams Sand, silt, and clay, the decomposed products of rocks, are available in plenty in most parts of the projects, especially in the reservoir basins. The decayed rock of hill slopes especially in areas composed of sedimentary rocks are generally devoid of pebbles and boulders and are found to have good quality material for use in earth dams. Both pervious and semi-pervious types of materials are necessary for the earth dams Rip-rap for Dam Slope Purpose Protection Rip-rap protects dam slopes from erosion caused by nature and wave action. Material Selection Hard, durable rocks are preferred, sourced from riverbeds, terraces, or quarries. Compressive Strength Desirable compressive strength is 350 kg/cm2, achieved by various rock types. Alternative Material Concrete blocks can be used as rip- rap if durable rocks are unavailable. Impervious Core Materials for a Dam The selection of suitable materials for rock-fill and earth dams is crucial for their structural integrity and performance. Ideal Material Composition A well-graded mixture of sand, gravel, and fines with highly plastic tough clay, exhibiting a plasticity index greater than 20, is ideal for the core of a rock-fill dam. The shell can consist of crushed soft rocks blended with clayey earth. Alternative Materials When sufficient quantities of suitable earth materials are unavailable, alternative materials such as crushed soft rocks can be utilized, as demonstrated by the Beas project in Himachal Pradesh. Beas Project Example In the Beas project, 50% crushed sandy-rock was mixed with 50% clay-shale to create a core material that satisfied the design requirements. River Diversion and Construction Work is a common practice when building infrastructure near or across rivers. It involves temporarily rerouting the river's flow to allow for construction activities within the normally submerged area. This can be necessary for building bridges, dams, levees, or other structures that require a dry worksite. Common methods for river diversion: Cofferdams: These are temporary walls built around the construction area to hold back the water. Tunnels: Water is diverted through tunnels or pipes that bypass the construction site. Channels: A new channel is dug to carry the river's water around the construction area. Methods of Diverting River Methods of diverting a river typically involve constructing structures like diversion dams, weirs, canals, culverts, or pipes which raise the water level at a specific point, allowing the water to be redirected to a designated area, often through a separate channel, away from its natural flow path; this can be done for various purposes like irrigation, flood control, or construction projects. Key methods of river diversion: Diversion dams: The most common method, where a dam is built across the river to raise the water level and divert a portion of the flow into a canal or channel. Weirs: Similar to dams but smaller, used to divert smaller amounts of water. Canals: Man-made channels constructed alongside the river to carry diverted water to a desired location. Culverts: Pipes or underground channels used to divert water under a barrier like a road or embankment. Pumping systems: In some cases, pumps can be used to lift water from a river and transport it to another location. Foundation Preparation refers to the process of clearing, leveling, and compacting the ground at a construction site to create Key aspects of foundation preparation include: a stable base for building a foundation, which typically involves removing debris, excavating to the required Site clearing: Removing trees, vegetation, rocks, and depth, and compacting the soil to ensure the structure other debris from the construction area. can properly support the weight of the building above it. Excavation: Digging out the soil to reach the desired depth for the foundation. Grading and leveling: Shaping the excavated area to create a flat and even surface. Soil compaction: Using specialized machinery to compact the soil, increasing its load-bearing capacity. Soil testing: Analyzing the soil properties to determine if additional stabilization measures are needed. Drainage considerations: Implementing drainage systems to prevent water accumulation around the foundation. Construction Approaches for Different Types of Dams When constructing different types of dams, the primary construction approaches vary based on the dam design, with gravity dams utilizing large blocks of concrete, arch dams relying on curved concrete structures to transfer water pressure to the valley walls, and embankment dams being built by compacting layers of earth or rockfill materials, each requiring specific techniques for excavation, placement, and jointing depending on the site conditions and desired function of the dam. Main Dam Types and Construction Approaches: Gravity Dams: Construction Method: Large blocks of concrete are poured in sections, with joints carefully designed to manage thermal stresses and ensure watertight seals. Key Considerations: Proper foundation preparation, precise placement of concrete, and monitoring of thermal expansion and contraction. Arch Dams: Construction Method: Concrete is poured in curved sections, utilizing the natural shape of the valley to distribute water pressure laterally onto the abutments. Key Considerations: Precise alignment of the arch curvature, strong foundation rock quality, and managing the forces acting on the abutments. Embankment Dams (Earth-Fill and Rock-Fill): Construction Method: Layers of compacted earth or rockfill are placed in a controlled manner, often with a central impervious core to prevent water seepage. Key Considerations: Proper soil selection and compaction, managing water control during construction, and ensuring stability of the slopes. Buttress Dams: Construction Method: A variation of gravity dams with a series of buttresses supporting the dam face, allowing for reduced concrete volume while maintaining stability. Key Considerations: Designing the buttress geometry to effectively transfer loads. Other Considerations: Cofferdams: Temporary structures used to divert water flow during dam construction, allowing access to the excavation area. Diversion Structures: Channels or tunnels built to redirect water flow around the construction site. Grouting: Injection of grout to seal cracks or improve the foundation stability. Monitoring and Instrumentation: Sensors and monitoring systems to track dam behavior during construction and operation. POST-CONSTRUCTION WORK Engineering geology plays a crucial role in the construction and post-construction phases of dams. It involves understanding the geological conditions of the dam site, selecting suitable materials, and addressing potential problems that may arise during and after construction. This presentation will delve into the importance of engineering geology in dam projects, highlighting key aspects of pre-construction investigations, construction supervision, and post-construction monitoring. Post-Construction Challenges: Landslides and Earthquakes Seepage Reservoir filling can trigger landslides or Seepage is a common post-construction earthquakes that affect the dam or its problem. Tracer studies and grouting are often appurtenant structures. In-depth geological employed to address seepage paths and seal study is crucial for identifying and addressing leaks. these risks. Scouring Drainage Issues Scouring or cavity formation at the dam's Blockage of drainage holes or malfunctioning downstream part due to spilled water is a common grout curtains can lead to uplift pressure, problem. The Sikasar project in Madhya Pradesh requiring immediate attention and remedial experienced scouring due to soft shale and quartzite measures. in the spill channel foundation, requiring a flexible sausage training wall to stop the scouring.