Wall retention
Managing the forces that shape and impact our landscapes is where wall retention comes into play. Wall retention, a key component of geotechnical engineering, focuses on the construction of structures to resist lateral pressure of soil when there is a desired change in ground elevation.
What is wall retention: definition, types and importance in civil engineering
Wall retention, another way of referring to retaining walls, is a structure that holds the soil behind it, and contains it. It can be made of different materials and reinforced using geosynthetics.. These structures are crucial for managing earth pressure across a variety of slopes, and vertical civil construction requirements such as bridges, roads, embankments, etc.
Importance of wall retention in civil engineering
Wall retention is important in civil engineering as it makes sloped areas functional. In addition to providing necessary support to the soil, retaining walls also reduce the surface runoff by reducing sharper gradients
Soil stability
Wall retention prevents the erosion of soil on steep slopes or near construction sites. Without proper wall retention, soil, depending on its type, loses its cohesion, leading to landslides. Retaining walls absorb and distribute the lateral pressure, thus stopping the structure from tilting, falling or collapsing under the pressure of materials such as aggregate or rocks.
Foundation support
Wall retention stops the downward force of the soil from affecting the foundation of any infrastructure. By reinforcing the soil underneath, retaining walls prevents excessive load displacement.
Water drainage
A well designed wall retention lets water pass without exerting additional pressure, through weep holes and gravel backfills. This avoids hydrostatic pressure buildup which weakens the wall structure. In areas where groundwater rises, wall retention directs water away and stops the saturation of the soil, preserving the surrounding landscapes.
What are the types of retaining walls?
Retaining walls can be classified based on the types of functions they achieve. Some are created to resist lateral pressure, while others are used for load resistance, helping hydrostatic pressure in cases of saturated soil, and other civil construction challenges.
Gravity walls
Constructed from concrete, stone, or brick, gravity retaining walls hold back soil through their weight alone, making them suitable for heights up to 3 meters. These walls are commonly used in highway construction and waterfront structures. Gravity walls hinder soil movement and support embankments by enhancing stability against lateral earth pressures.
Cantilever walls
Cantilever retaining walls are structures that utilize a vertical stem and a horizontal base slab to resist soil pressure. This features a backward-inclined stem that effectively transfers the load of the retained soil to the base. Cantilever walls are made of reinforced concrete and are suitable for necessary elevation changes.
Counterfort walls
Counterfort walls are reinforced concrete slabs that are attached perpendicular and spaced at regular intervals along the length of the wall. This type provides extra strength by transferring the lateral pressure from soil to the wall’s base. This reduces the bending moment and the amount of material needed in the main wall structure.
Gabion walls
Gabion walls consist of wire mesh boxes filled with rocks or other materials, being a flexible solution for erosion control and slope stabilization. Their design promotes drainage and allows for vegetation growth, thus making them environmentally compatible while providing structural stability.
Sheet pile walls
Sheet pile walls consist of thin interlocking sheets made from materials such as steel, vinyl, or wood, driven into the ground to form a continuous barrier. These are known for erosion control and retain materials without taking up much surface area. All these features make them versatile in landscaping and civil engineering projects.
Anchored walls
Anchored walls involve drilling anchors into the earth behind the wall, which are then connected to the wall itself. This withstand higher lateral pressures than conventional walls providing additional support and stability. Anchored walls are useful in applications where space is limited, and soil retention is required, making them apt for steep slopes and heavy loads.
What materials are used in wall retention?
Concrete
Concrete is a material of choice for wall retention due to its strength, flexibility, and resistance to environmental factors. Reinforced concrete, precast concrete, stamped concrete are some that are used because of its resistance to weathering, erosion and chemical attacks. It further has high compressive strength and can be cast in many shapes and dimensions.
Steel
Steel reinforces the concrete, providing its tensile strength and avoiding cracking under the pressure applications. Galvanised steel, stainless steel and corten steel are some of the best choices. Steel sheet piles offer protection from both soil and water pressures making it an appropriate solution for seismically active locales.
Composite materials
Composite materials combine the strength of concrete and the ductility of exogenous materials such as polymers and geosynthetics. Fiber reinforced polymer(FRP) and geopolymer concrete are widely used as it is lightweight compared with traditional materials. This aspect simplifies installation and handling. It is also more robust to aging, environmental stresses, as well as wear and tear. FRPs are, in addition, corrosion-resistant, so they are appropriate for long-term use.
Geosynthetics
Geosynthetics, such as geogrids and geotextiles, used with other materials like aggregates reinforce the soil and enhance the structural stability of retaining walls. These materials distribute loads more evenly through the soil, reducing the infiltration of water and reducing potential wall pressure.
Design considerations for wall retention
Soil properties
Soil behavior, such as load-bearing capacity, compressibility, and shear strength assess how these properties vary in conditions like moisture content and compaction. Soil cohesion, friction angle, permeability, and strength are influenced by grain size distribution and water content. Hence, this is pivotal for guaranteeing wall stability.
Wall materials
Selecting the right type of retaining wall is crucial. Common types include gravity walls, cantilever walls, and anchored walls, each for different conditions and load requirements. The choice should align with the project’s specific needs, taking into account factors such as height, soil conditions, and the presence of surcharge loads.
Seismic and dynamic loads
These loads include the weight of the soil, any additional structures, and dynamic forces such as wind or seismic activity. Ignoring these factors result in the wall being unable to support the required loads, ultimately leading to failure.
Effective drainage solutions
Without adequate drainage, water accumulates behind the wall, leading to hydrostatic pressure. Subsequently, this leads to bulging, cracking, or even collapse. Proper solutions include the installation of weep holes with water escape from behind the wall. Also, use drainage pipes that channel water away from the structure. Engineers also employ French drains or sub-surface drainage networks to manage water flow properly.
Construction techniques depending on the wall type
Construction techniques for wall retention vary depending on the wall type, materials used, and specific project requirements.
Gravity wall construction involves stacking the materials like concrete or brick without additional reinforcement, making it relatively straightforward but labor-intensive.
Cantilever and counterfort walls utilize reinforced concrete, which incorporates steel bars for strength and flexibility. The construction process begins with trench excavation for the foundation, followed by installing formwork to shape the wall.
Anchored retaining walls stabilize soil using anchors or tiebacks. Key design considerations include wall height, soil properties, groundwater conditions, and lateral forces. Proper tensioning of the tieback cables is crucial for counteracting soil pressure.
Sheet pile walls are driven into the ground to create a continuous barrier that resists soil or water pressure. The selection of appropriate installation methods, such as vibratory or impact hammers ensure wall integrity during construction.
Importance of using geosynthetics in wall retention
In wall retention systems, geosynthetics contribute to structural stability through mitigation of settlement and improvement of drainage of engineered earth structures.
Geogrids: Geogrids reinforce soil by increasing its soil shear strength. By spreading loads more evenly, it creates a tensile stress in the soil mass that resists deformation
Geotextiles: Geotextiles block the migration of the soil particles while allowing the water to pass through, maintaining the stability around the wall. Filtration property reduces clogging leading to erosion control.
Geocells: The three dimensional honeycomb like structures improve soil confinement and prevent the lateral movement of the soil. Geocells also hinder the buildup of pore water pressure behind the wall.
Geocomposites: Geo composites that are made of geotextiles, geo grids, and geonets serve as reinforcements, filters, and drainage in constructing retaining walls.
Common failures
Overturning
Overturning occurs when the wall retention rotates about its base due to excessive lateral pressure from the retained soil. This happens when the resisting moment is insufficient compared to the overturning moment.
Sliding
Sliding takes place when lateral earth pressure exceeds the wall’s resisting forces. The magnitude of the pressure depends on the type of soil and its cohesion, the wall and the design of the wall.
Settlement
This refers to the downward movement of the wall’s foundation or the surrounding soil due to the weight of the wall, loads, or changes in soil conditions. Settlement causes titling, cracking, or even failure of the wall.
Water pressure
One of the primary indicators of poor drainage is the appearance of visible cracks on the wall surface, which signal hydrostatic pressure buildup behind the wall. Additionally, the wall may start to lean or tilt due to the increased pressure from accumulated water, threatening the wall’s stability.
Challenges and limitations
Structural failures
The risk of structural failure occurs due to factors such as inadequate design, construction errors, and unforeseen external pressures. Failures like deep shear failure arises when weak soil layers exist beneath the wall, further complicating stability considerations
Maintenance and inspection
Over time, retaining wall structures develop cracks, bulges, or water pooling, necessitating proactive measures to prevent deterioration. The maintenance of drainage systems avoid hydrostatic pressure buildup behind the walls.
Economical considerations
High initial construction costs, especially for large-scale projects, may deter investment. Furthermore, ongoing maintenance requirements lead to overall lifecycle costs of these structures. In some cases, the economic factors overshadow necessary design considerations, causing suboptimal solutions compromising structural integrity.
Design complexity
The complexity involved in designing effective retaining walls is another limitation. Engineering professionals must consider numerous variables, including soil properties, anticipated loads, and environmental conditions. Inadequate understanding or miscalculations related to these factors lead to failures.
By thoughtfully considering material selection, coordination with other landscape elements, and low-maintenance landscaping strategies, wall retention seamlessly blends into their natural surroundings while meeting structural requirements.