Presented by Assist. Lecturer Israa Abdul-Jalil Majeed Department of Civil Engineering – Al-Mustaqbal University Introduction Hydraulic structures are vital engineering projects that play a fundamental role in managing water resources, regulating water flow, and meeting the requirements of irrigation, energy, and flood protection. These structures include dams, pumping stations, intake and outlet structures, regulators, barrages, and aqueducts. Foundation design is one of the most important engineering aspects in constructing these structures, given the conditions to which they are exposed: continuous water action, hydraulic uplift pressure, seepage through the soil, and the potential for scour and fluctuations in groundwater level. Hence the importance of geotechnical engineering in assessing the properties of soil and rock and determining the appropriate foundation type to ensure the structure’s stability and long-term efficiency. First: The Nature of Foundations in Hydraulic Structures The foundations used in hydraulic structures vary depending on the type of structure and the geological and hydraulic conditions of the site. In general, they can be classified into two main types: 1. Shallow Foundations: These are used when the soil layers near the ground surface have adequate bearing capacity and can carry the structure’s loads without unacceptable settlement. Examples include mat (raft) foundations and concrete footings, which can be used in some pumping stations and small hydraulic structures. 2. Deep Foundations: These are used when the surface layers are weak or unstable, or when the nature of the loads requires transferring them to deeper, stronger layers. They include reinforced concrete piles and steel piles, and are an important option for some structures built on soft soils or at sites exposed to scour. The choice of foundation type depends on the results of the geotechnical investigations, the applied loads, the depth of the bearing layers, and the groundwater conditions. Second: Geotechnical Investigations and Their Role in Design The geotechnical design process begins with comprehensive field and laboratory investigations of the site, aiming to form an accurate picture of the soil, rock, and hydrological conditions surrounding the structure. These investigations include: Field tests: such as drilling boreholes and conducting Standard Penetration Tests (SPT), and Cone Penetration Tests (CPT) where suitable for the site. Laboratory tests: to determine grain-size distribution, Atterberg limits, density, moisture content, and shear, compressibility, and permeability parameters. Determining the groundwater level: and studying possible changes in it resulting from the operation of the structure or changes in surface water levels. Assessing rock layers: in terms of degree of weathering, fractures, rock strength, and rock mass properties. The results of these tests are used to determine the required design parameters, such as shear strength, bearing capacity, expected settlement, and the permeability coefficient. Third: The Most Important Geotechnical Factors Affecting Foundation Design Hydraulic structures have additional factors that make their foundation design more complex than that of many conventional structures. The most important are: 1. Bearing Capacity: It must be verified that the soil can carry the vertical and horizontal loads from the structure and the water without shear failure or excessive deformation. Incorrect estimation of bearing capacity may lead to loss of foundation stability. 2. Settlement and Deformation: Foundations may settle as a result of soil layers compressing under sustained loads. Studying differential settlement is especially important in hydraulic structures, because differences in settlement between parts of the structure may lead to cracking and affect the structure’s safety and the performance of its components. 3. Hydraulic Uplift Pressure: Uplift pressure resulting from water seeping beneath foundations is one of the most important geotechnical challenges, as it can reduce the structure’s effective weight and its resistance to sliding or overturning. Therefore, the distribution of water pressures must be studied, and drainage and control systems must be designed. 4. Seepage Through Soil: Water passing through the soil or beneath the foundation may increase pore pressures and produce high hydraulic gradients, which may cause soil particles to be washed away and lead to internal erosion or piping. These risks can be reduced by using cutoff walls, filters, drainage systems, and cement grouting when needed. 5. Scour: Scour occurs due to high-velocity water movement around the foundations of structures, particularly aqueducts and piers near river channels. Loss of the soil surrounding the foundation may reduce its lateral resistance and affect its stability. Therefore, the design scour depth must be estimated and its effect taken into account when determining the foundation level. Fourth: The Basic Stages of Geotechnical Foundation Design The geotechnical design process for hydraulic structures can be organized into a set of integrated stages: Site study: Determining the geological, hydrological, and engineering conditions of the site, and carrying out the necessary field and laboratory investigations. Determining the applied loads: These include the structure’s weight, water pressure, operational loads, uplift forces, and earthquake loads where necessary. Selecting the foundation type: Determining the suitability of shallow or deep foundations according to the properties of the soil and rock and the nature of the loads. Verifying stability: Performing analyses of bearing capacity, settlement, sliding, overturning, and overall stability according to the nature of the structure and the design loading cases. Seepage and water pressure analysis: Studying water movement through the soil and beneath the foundation, evaluating uplift pressures and hydraulic gradients, and determining drainage and seepage control requirements. Assessing the effect of scour: Determining the potential scour depth and ensuring the foundation remains stable and safe after it occurs. Establishing treatment and monitoring procedures: Determining the need for soil improvement, grouting, or cutoff walls, and adopting instruments to monitor settlement, pore pressures, and seepage during operation. Fifth: The Importance of Soil Improvement in Hydraulic Structures At some sites, the natural soil properties may not be sufficient to meet design requirements, which calls for soil improvement techniques before the foundations are constructed. The most prominent of these techniques are: compaction, lime or cement stabilization, cement grouting, stone columns, cutoff walls, and drainage systems. The choice of treatment method depends on the soil type, its degree of weakness, the depth of the affected layers, and the nature of the structure, with the need to verify the effectiveness of the treatment through appropriate tests and investigations. Conclusion Designing the foundations of hydraulic structures is not limited to determining the soil’s bearing capacity and choosing the foundation dimensions; it requires an integrated understanding of the interaction among soil, water, and the structure, taking into account the effects of seepage, uplift pressure, scour, settlement, and overall stability. Geotechnical engineering plays a pivotal role in achieving this integration through accurate investigations, scientific analyses, and the selection of appropriate design solutions. Consequently, the success of hydraulic structures and the sustainability of their performance depend largely on the efficiency of the foundation design and its compatibility with the geological and hydraulic conditions of the site. References U.S. Bureau of Reclamation (2012). Design Standards No. 13: Embankment Dams – Chapter 12: Foundation and Earth Materials Investigation. U.S. Bureau of Reclamation (2014). Design Standards No. 13: Embankment Dams – Chapter 8: Seepage. U.S. Army Corps of Engineers (2004). EM 1110-2-2300: General Design and Construction Considerations for Earth and Rock-Fill Dams. Environment Agency (2021). Scour at Bridges and Other Hydraulic Structures Manual, Second Edition. U.S. Army Corps of Engineers (2007). EM 1110-2-6053: Earthquake Design and Evaluation of Concrete Hydraulic Structures.