Design Principles of Weirs on Permeable Foundation | Civil Engineering Optional Notes for UPSC PDF Download

Design Principles of Weirs on Permeable Foundation

• Floor Thickness: The floor of a weir on a permeable foundation is subject to uplift pressure from the flow of water underneath. To counteract this uplift pressure, it is necessary to provide a suitable thickness of the floor at different points. The thickness is calculated based on the expected uplift pressure, the weight of the floor material, and safety factors. A thicker floor is generally required at points where the uplift pressure is higher, such as near the upstream cutoff.
• Exit Gradient Control: When water flows under the weir floor, it creates a hydraulic gradient. If this gradient is too steep, it can cause piping (erosion of soil particles) and eventual failure of the structure. To control the exit gradient, a sheet pile is driven to a suitable depth at the downstream end of the floor. The depth of this sheet pile is determined by calculating the maximum allowable exit gradient and the seepage path length required to achieve it.
• Sheet Piles: Scour holes can form on either the upstream or downstream side of the weir due to turbulent flow and erosion. These scour holes can undermine the foundation and cause the weir to fail by slipping or sliding of the subsoil into the holes due to lateral earth pressures. To prevent this, sheet piles are installed at both the upstream and downstream ends of the floor, extending to the expected depth of scour.
• Pressure Distribution: The pressures under the downstream floor increase as the depth of the downstream sheet pile increases. This is because a deeper sheet pile creates a longer seepage path, resulting in a higher uplift pressure at the downstream end of the floor. The upstream sheet pile has little effect on reducing these pressures due to the generally larger spacing between the upstream and downstream sheet piles.
• Intermediate Sheet Pile: An intermediate sheet pile is installed between the upstream and downstream sheet piles as a second line of defense. If either the upstream or downstream sheet pile fails, the intermediate pile can help prevent the complete failure of the structure. However, it does not significantly alter the pressure distribution or prevent undermining on its own.
• Depth of Sheet Piles: The depth of all sheet piles (upstream, downstream, and intermediate) is generally determined by the expected depth of scour during normal flood conditions. This depth is calculated using empirical formulas, such as Lacey's formula, which relates the scour depth to the flow characteristics and sediment properties.
• Downstream Sheet Pile Depth: In addition to reaching the expected scour depth, the depth of the downstream sheet pile is also determined to provide a safe exit gradient in conjunction with the length of the floor. A deeper sheet pile increases the seepage path length, reducing the exit gradient.
• Intermediate Sheet Pile Location: Commonly, a single deep intermediate sheet pile is installed under the crest of the weir, as this is a critical location for potential failure. However, multiple intermediate sheet piles can also be used, such as one upstream of the crest and another downstream under the toes of the glacis (sloping apron).
• Downstream Floor Level: The level of the downstream floor is set such that the hydraulic jump (a sudden rise in water level due to a change in flow regime) occurs on the sloping glacis itself, even under the worst flow conditions. This ensures that the energy dissipation occurs in a controlled manner on the glacis, preventing excessive erosion or damage to the downstream area.
• Glacis Slope: The slope of the glacis (sloping apron) is designed to be between 1 in 3 (18.4°) and 1 in 5 (11.3°). These slopes are considered suitable for achieving economy in construction while also providing maximum energy dissipation through the formation of a hydraulic jump and turbulent flow.
• Downstream Floor Length: The length of the downstream horizontal impervious floor should be at least 5 times the height of the hydraulic jump. This is because the disturbance caused by the standing wave (hydraulic jump) dies out at a distance of approximately 5 times the jump height. This length ensures that the flow downstream of the jump is stable and uniform.
• Floor Construction: The impervious floor should be laid as a single, solid mass to effectively counteract uplift pressures. If the floor is laid in layers and these layers separate, full uplift pressure can be transmitted through the cracks, potentially leading to failure. Alternatively, steel reinforcement can be used to tie the layers together, making them inseparable and preventing the transmission of full uplift pressure through any cracks.
• Abutments and Wing Walls: The abutments and wing walls of the weir are designed to withstand various types of soil and water pressures, including dry soil pressure, saturated soil pressure, and hydraulic pressure acting in conjunction with saturated soil pressure. These pressures are calculated based on established soil mechanics and hydraulics principles.
• Foundation Levels: The foundations of abutments and flank walls between any pair of sheet pile lines should extend down to the level of the bottom of those sheet piles. Similarly, the foundations of the upstream and downstream return walls should reach the levels of the bottom of the corresponding sheet pile line. Additionally, the return walls should be extended back into the bank by a distance equal to twice the depth of the corresponding sheet pile below the floor level. This ensures that the foundations are adequately supported and not undermined by scour or seepage.
• Upstream Protection: At the upstream end of the impervious floor, additional protection is provided by placing concrete blocks over loose stone for a length equal to the depth of scour below the river bed. This protection is typically 1.2 m thick and consists of concrete blocks over 60 cm deep graded stones. This upstream protection helps prevent erosion and scouring of the river bed in front of the weir.
• Downstream Protection: On the downstream side, after the impervious floor, an inverted filter is provided. This filter varies in length from 1.5 to 2 times the depth of scour below the river bed. The inverted filter is generally 60 cm thick and is protected by 1 m to 1.2 m deep concrete blocks. This downstream protection helps dissipate energy, prevent erosion, and control seepage downstream of the weir.
• Maintenance: The upstream and downstream bed protection is designed to be flexible, allowing it to adjust to slight subsidence or settlement. However, this loose protection is not intended to cover the scoured face in the same way as a launching apron (a rigid, sloping apron that extends into the scour hole). As a result, this loose protection requires constant maintenance to ensure its effectiveness.
• Guide Banks: Guide banks are provided upstream and downstream of the weir or barrage to direct the flow through the waterway. These banks help control the flow pattern, prevent excessive turbulence and erosion, and ensure that the water passes through the intended channel or waterway.