8.3 Design of retaining walls (gravity, cantilever, and reinforced earth walls)
5 min read•august 16, 2024
Retaining walls are crucial structures in geotechnical engineering, holding back soil and water. This section covers various types, from gravity walls to reinforced earth systems, each suited for different applications and soil conditions.
Understanding lateral earth pressures is key to designing stable retaining walls. We'll explore active, passive, and at-rest pressure states, as well as how factors like surcharge loads and water pressure impact wall design and .
Retaining Wall Types and Applications
Gravity and Cantilever Walls
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Gravity retaining walls use their own weight to resist lateral earth pressures suitable for low to medium height applications
Cantilever retaining walls leverage backfill soil weight for stability efficient for medium to tall wall heights
Use reinforced structure
More material-efficient than gravity walls for taller heights
Counterfort retaining walls incorporate vertical concrete ribs to enhance stability ideal for very tall applications
Ribs act as additional support against bending and shear forces
Buttress retaining walls feature protruding supports on exposed face increasing resistance against overturning for tall structures
Buttresses typically spaced at regular intervals along wall length
Reinforced Earth and Sheet Pile Walls
Reinforced earth walls create composite structure using tensile reinforcement elements within backfill soil suitable for various heights and soil conditions
Reinforcement elements include (geotextiles, geogrids, metal strips)
Can accommodate significant vertical and lateral loads
Sheet pile walls consist of slender walls driven into ground commonly used in temporary excavations or waterfront structures
Materials include (steel, vinyl, concrete)
Effective in areas with high water tables or soft soils
Soldier pile and lagging walls combine vertical piles with horizontal lagging often employed in urban environments
Suitable for both temporary and permanent earth retention
Allow for staged construction in confined spaces
Lateral Earth Pressures on Walls
Earth Pressure States and Theories
Lateral exerts horizontal force on wall structure influenced by soil properties, wall movement, and loading conditions
develops when wall moves away from soil representing minimum pressure state
Typically used in design for yielding walls (cantilever, gravity)
occurs when wall moves towards soil representing maximum pressure state
Often utilized in resisting forces at wall base
At-rest earth pressure exists with no wall movement commonly applied in design of rigid, unyielding structures (basement walls)
Rankine's theory and Coulomb's theory provide fundamental methods for calculating lateral earth pressures
Rankine assumes no , simplifying calculations
Coulomb accounts for wall friction, more accurate for some scenarios
Additional Pressure Considerations
Surcharge loads contribute additional lateral pressures accounted for in wall design
Examples include (adjacent structures, traffic loads, construction equipment)
Water pressure and seepage forces significantly increase lateral pressures requiring careful consideration
Proper drainage design essential to mitigate hydrostatic pressure
Seepage analysis may be necessary in areas with high groundwater table
Gravity Wall Design and Stability
Stability Analysis
design checks for overturning, sliding, and modes ensuring overall stability
against overturning calculated by comparing resisting moment to overturning moment
Resisting moment from wall weight
Overturning moment from lateral earth pressure
Sliding stability evaluated by comparing frictional resistance along base to horizontal component of lateral earth pressure
May include passive resistance at toe if applicable
Includes wall weight and vertical component of earth pressure
Eccentricity of resultant force at base must be within acceptable limits preventing excessive stress concentrations
Typically limited to middle third of base width
Design Considerations and Drainage
Wall proportions iteratively adjusted to meet stability requirements while optimizing material usage
Base width typically 0.5 to 0.7 times wall height
Stem thickness varies based on height and material (concrete, masonry)
Proper drainage systems essential to prevent hydrostatic pressure buildup behind wall
Weep holes allow water to pass through wall face
Drainage layers (gravel, geotextiles) facilitate water movement to base of wall
Cantilever Wall Analysis and Design
Structural Components and Analysis
Cantilever retaining walls consist of vertical stem and base slab utilizing backfill soil weight over heel for stability
Stem designed as cantilever beam to resist bending moments and shear forces from lateral earth pressures
Moment increases cubically with height
Shear force increases quadratically with height
Base slab divided into toe and heel sections designed for different loading conditions
Toe experiences upward soil pressure and downward wall weight
Heel subjected to downward soil and wall weight with upward soil reaction
Reinforcement Design
Flexural reinforcement in stem concentrated on tension (soil) side with distribution steel on opposite face
Main reinforcement typically vertical bars
Horizontal bars provide temperature and shrinkage control
Shear reinforcement may be required near stem-base junction where high shear stresses occur
Stirrups or bent bars used to resist diagonal tension
Temperature and shrinkage reinforcement provided to control cracking in both stem and base slab
Typically minimum of 0.2% of gross concrete area
Reinforced Earth Walls: Principles and Design
Reinforcement Mechanics and Stability
Reinforced earth walls incorporate tensile reinforcement elements within backfill soil creating composite, gravity-type structure
Reinforcement elements transfer tensile stresses from soil to facing elements through friction or mechanical interlock
Friction developed along length of reinforcement
Mechanical interlock achieved through apertures in geogrids or ribs on strips
Internal stability analysis checks for reinforcement pullout and tensile rupture failure modes
Pullout resistance depends on overburden pressure and soil-reinforcement interaction
Tensile rupture considers long-term strength of reinforcement material
External stability considerations include sliding, overturning, bearing capacity, and global stability similar to conventional retaining walls
Design Elements and Drainage
Spacing, length, and strength of reinforcement layers designed based on required tensile resistance at different depths
Typically denser spacing near top of wall where lateral pressures are highest
Length of reinforcement usually 0.7 to 1.0 times wall height
Facing elements can be precast concrete panels, modular blocks, or wrapped each with specific connection details to reinforcement
Concrete panels provide durable, aesthetic finish
Modular blocks allow for easy construction and curved wall layouts
Drainage design crucial in reinforced earth walls to prevent water pressure buildup and ensure long-term performance
Granular drainage layers behind and within reinforced soil zone
Collector pipes at base of wall to remove water from system
Key Terms to Review (26)
AASHTO Guidelines: The AASHTO Guidelines refer to the standards and best practices established by the American Association of State Highway and Transportation Officials for the design, construction, and maintenance of transportation infrastructure. These guidelines play a crucial role in ensuring safety, durability, and efficiency in projects such as retaining walls and chemical stabilization techniques. They provide engineers with a framework for evaluating materials, methods, and structural integrity, promoting consistent quality across various projects.
Active Earth Pressure: Active earth pressure is the lateral pressure exerted by soil on a retaining structure when the soil is allowed to expand, often due to wall movement away from the soil. This condition typically occurs when the wall moves outward or when there is an increase in soil volume, leading to a reduction in stress against the wall. Understanding this concept is crucial for designing various types of retaining structures, analyzing soil behavior, and ensuring stability.
Bearing capacity failure: Bearing capacity failure occurs when the soil beneath a structure cannot support the loads applied to it, leading to a failure in structural stability. This type of failure is critical in construction and geotechnical engineering, as it can cause excessive settlement or even collapse of structures. Understanding bearing capacity is essential for the design of various structures, ensuring that they can withstand expected loads without compromising safety.
Cantilever Wall: A cantilever wall is a type of retaining wall that uses a cantilever design to hold back soil and other materials, relying on its own weight and structural design to resist lateral earth pressures. This type of wall consists of a vertical stem and a horizontal base slab, which work together to provide stability against forces acting on the wall, particularly in conditions where soil loads and water pressure must be countered effectively.
Concrete: Concrete is a composite material made from a mixture of cement, water, aggregates (like sand and gravel), and sometimes additives to enhance its properties. It serves as a fundamental building block in construction due to its strength, durability, and versatility, making it particularly valuable in the design and construction of retaining walls, including gravity, cantilever, and reinforced earth walls.
Dead load: Dead load refers to the permanent static load that a structure must support, which includes the weight of the building materials, fixtures, and any other components that are fixed in place. Understanding dead load is crucial in the design and analysis of structures, as it influences stability and structural integrity. This concept is especially important in the context of retaining walls and shallow foundations, where the dead load affects how these structures manage gravitational forces and resist potential failures.
Drainage control: Drainage control refers to the methods and systems employed to manage the flow of water through soil and rock materials to maintain stability and prevent erosion or failure in geotechnical structures. It plays a vital role in various engineering applications, such as retaining walls, slope stability, and the prevention of slope failures by ensuring proper water management.
Earth Pressure: Earth pressure is the force exerted by soil on a structure, such as a retaining wall, due to the weight of the soil and the influence of external factors like water and surcharge loads. Understanding earth pressure is crucial for designing structures that can withstand these forces without failing or deforming, especially in the context of retaining walls, which hold back soil and prevent landslides or collapses.
Eurocode: The Eurocode is a set of European standards for the design of buildings and civil engineering structures, ensuring safety, durability, and sustainability. It provides a unified framework for structural design, which includes guidelines for various materials and structural systems, promoting consistency across Europe. The Eurocode is especially important in the context of retaining walls as it outlines specific design principles for gravity, cantilever, and reinforced earth walls, enabling engineers to apply standardized practices.
Factor of Safety: The factor of safety is a measure used in engineering to provide a safety margin in design, ensuring that structures can withstand loads greater than the maximum expected load. It is defined as the ratio of the strength of a material or system to the actual applied load, indicating how much stronger a system is than what it needs to be for safe operation. This concept is crucial in various engineering fields, including geotechnical engineering, where it plays a vital role in assessing the stability of structures and soil conditions.
Geosynthetics: Geosynthetics are synthetic materials used in geotechnical engineering to improve the performance of soil and rock structures. They include products like geotextiles, geomembranes, geogrids, and geonets, which serve various purposes such as separation, filtration, drainage, reinforcement, and containment in construction projects. By providing enhanced stability and efficiency, geosynthetics play a crucial role in the design and construction of retaining walls.
Gravity wall: A gravity wall is a type of retaining wall that relies on its own weight to resist the lateral pressure exerted by soil or other materials behind it. These walls are typically made of heavy materials such as concrete or masonry and are designed to prevent soil movement by resisting gravitational forces. Their design involves considerations of earth pressure states and theories, ensuring stability under various conditions, including seismic events.
Live load: Live load refers to the variable weight or force that a structure must support due to occupancy or use, such as people, furniture, vehicles, and other movable objects. This load changes over time and is distinct from dead loads, which are constant and include the weight of the structure itself. Understanding live loads is crucial in ensuring that structures, including retaining walls and shallow foundations, can safely support the dynamic forces they encounter throughout their lifespan.
Overturning failure: Overturning failure refers to a structural failure mode in which a retaining wall or other similar structure rotates around its base due to excessive lateral earth pressures, leading to a loss of stability. This type of failure is critical to consider during the design of retaining walls, as it can result from the forces exerted by the soil and any additional loads above or behind the wall, requiring careful analysis and consideration in engineering design.
Passive Earth Pressure: Passive earth pressure refers to the lateral force exerted by soil on a retaining structure when the structure moves away from the soil, such as when it is pushed or tilted. This pressure develops due to the soil’s resistance to deformation and plays a crucial role in the design and stability of various types of retaining walls. Understanding passive earth pressure is essential for calculating the forces acting on walls and ensuring they remain stable against soil movements.
Reinforced Earth Wall: A reinforced earth wall is a type of retaining structure that uses layers of compacted soil and reinforcing elements, such as geogrids or steel strips, to support vertical loads and resist lateral earth pressures. This system provides a stable, flexible wall that can accommodate settlement and deformation, making it ideal for various applications in geotechnical engineering.
Sliding: Sliding refers to the movement of soil or rock mass down a slope due to gravitational forces exceeding the resisting forces that hold the material in place. This phenomenon is crucial for understanding how retaining walls function and how they can be designed to prevent such failures. When assessing the stability of structures like retaining walls, it's important to evaluate the potential for sliding, as this can significantly impact their overall safety and effectiveness.
Sliding failure: Sliding failure is a type of slope failure that occurs when a mass of soil or rock moves downward along a distinct slip surface due to the forces acting on it exceeding its resistance to movement. This phenomenon is critical when designing retaining structures, as understanding how sliding failure can occur helps engineers ensure stability against lateral earth pressures and potential sliding movements.
Soil Cohesion: Soil cohesion refers to the internal attraction between soil particles that helps them stick together, which is vital for understanding the strength and stability of soil in various engineering applications. This property plays a significant role in determining how soil behaves under stress and how it interacts with structures such as retaining walls, contributing to overall soil stability and pressure distribution in different scenarios.
Soil Friction Angle: The soil friction angle is a measure of the resistance to shear stress in soil, reflecting how effectively soil particles interlock and resist sliding against one another. It plays a crucial role in geotechnical engineering, particularly in assessing the stability of slopes, foundations, and retaining structures. A higher friction angle indicates better resistance to shear forces, which is vital for the safe design of various structures.
Stability: Stability refers to the ability of a structure, like a retaining wall, to maintain its position and resist forces that could cause it to fail or collapse. This concept is crucial in engineering because it ensures that structures can withstand environmental pressures, such as soil loads and water forces, without leading to deformation or failure over time.
Steel reinforcement: Steel reinforcement refers to the use of steel bars or mesh embedded within concrete to enhance its tensile strength and structural integrity. This process is essential in construction, especially for structures like retaining walls, where the combination of concrete and steel provides greater stability against lateral earth pressures and other loads.
Subsurface conditions: Subsurface conditions refer to the physical, chemical, and biological properties of the soil and rock layers beneath the Earth's surface. These conditions are crucial for understanding how structures will interact with the ground, particularly in engineering applications such as retaining walls, where factors like soil strength, moisture content, and layering affect design and stability.
Surcharge load: A surcharge load is an additional load applied to the surface of the ground, typically resulting from structures, fill materials, or other constructions placed above the natural soil. This load can significantly influence the stress distribution within the soil, impacting the design and stability of structures such as retaining walls. Understanding how surcharge loads affect soil behavior is crucial for predicting earth pressure states and ensuring adequate support against sliding or overturning.
Wall friction: Wall friction refers to the resistance that develops between the wall of a structure, such as a retaining wall, and the soil or other materials it supports. This friction plays a crucial role in the stability and overall design of structures by affecting the lateral earth pressures acting on the wall. Understanding wall friction helps engineers determine how much force a wall must resist, which is essential for ensuring that structures can safely hold back soil and prevent failures.
Wall thickness: Wall thickness refers to the measurement of the width of a retaining wall from its inner face to its outer face. This dimension plays a crucial role in determining the structural integrity and stability of the wall, influencing factors such as load distribution, resistance to lateral earth pressure, and overall durability. A well-designed wall thickness ensures that the wall can safely support the soil and other loads it encounters, while also affecting construction materials and methods.