Retaining Walls: What You Need to Know Before Building One
Retaining Walls; Their Design and Construction
Retaining walls are structures used to provide stability for earth or other materials at their natural slopes. In general, they are used to hold back or support soil banks and water or to maintain difference in the elevation of the ground surface on each of wall sides. Retaining walls can also enhance the appearance and functionality of your landscape by creating terraces, planters, or seating areas.
Retaining Walls; Their Design And Construction
However, building a retaining wall is not a simple task. It requires careful planning, design, and construction to ensure that it can withstand the forces exerted by the retained material and the environment. Some of the common problems that can affect retaining walls are poor drainage, excessive weight of soil, inadequate reinforcement, settlement, sliding, overturning, and cracking.
Fortunately, there are different types of retaining walls that can suit different needs and preferences. Some of the most common types are timber walls, interlocking block walls, stacked stone walls, brick or block walls, and concrete walls. Each type has its own advantages and disadvantages in terms of cost, appearance, durability, maintenance, and installation.
In this article, we will discuss the general principles of retaining wall design, the types of retaining walls and their pros and cons, how to build a retaining wall step by step, how to choose the right retaining wall for your project, and some case studies of retaining wall projects from around the world.
General Principles of Retaining Wall Design
The design of a retaining wall depends on several factors that affect its stability and performance. Some of these factors are:
Soil properties: The type and condition of the soil behind and below the wall influence its strength and deformation. The soil parameters that need to be considered are unit weight, cohesion, friction angle, compressibility, permeability, and shear strength.
Water pressure: The presence of water behind or below the wall increases the lateral pressure on the wall and reduces its frictional resistance. The water pressure can be due to rainfall infiltration, groundwater seepage, or surface runoff. The water pressure can be reduced by providing adequate drainage behind and below the wall.
Surcharge load: The additional load applied on top of the retained soil increases the lateral pressure on the wall. The surcharge load can be due to traffic, buildings, or other structures near the wall. The surcharge load can be accounted for by increasing the thickness or reinforcement of the wall.
Seismic load: The horizontal acceleration caused by an earthquake induces an inertial force on the wall and the retained soil. The seismic load can be calculated by multiplying the seismic coefficient by the weight of the wall and the soil. The seismic load can be resisted by providing adequate reinforcement or anchorage for the wall.
The design of a retaining wall should ensure that it satisfies three criteria: stability against sliding along its base, stability against overturning about its toe, and stability against bearing failure of the foundation soil. The design should also ensure that the wall does not exceed its allowable stress or strain limits under the applied loads.
Types of Retaining Walls and Their Advantages and Disadvantages
There are different types of retaining walls that can be classified according to their material, shape, or mode of action. Some of the most common types are:
Timber walls are made of wooden planks, beams, or logs that are stacked horizontally or vertically and anchored by steel rods or cables. Timber walls are easy and cheap to build and can blend well with the natural landscape. However, timber walls are not very durable and can rot, warp, or decay over time. Timber walls are also susceptible to fire and insect damage. Timber walls are suitable for low-height walls (up to 4 feet) and gentle slopes.
Interlocking block walls
Interlocking block walls are made of concrete or stone blocks that have special shapes and sizes that allow them to fit together without mortar or reinforcement. Interlocking block walls are fast and simple to install and can create various patterns and colors. However, interlocking block walls are not very strong and can shift, crack, or collapse under heavy loads or seismic activity. Interlocking block walls are suitable for moderate-height walls (up to 6 feet) and moderate slopes.
Stacked stone walls
Stacked stone walls are made of natural or artificial stones that are stacked on top of each other without mortar or reinforcement. Stacked stone walls are attractive and durable and can create a rustic or elegant look. However, stacked stone walls are labor-intensive and expensive to build and require skilled craftsmanship. Stacked stone walls are also prone to sliding or toppling over if not properly leveled or supported. Stacked stone walls are suitable for low- to moderate-height walls (up to 6 feet) and gentle to moderate slopes.
Brick or block walls
Brick or block walls are made of brick or concrete blocks that are laid in a regular pattern and bonded with mortar. Brick or block walls are strong and stable and can be painted or plastered to achieve different finishes. However, brick or block walls are also labor-intensive and expensive to build and require a solid foundation and reinforcement. Brick or block walls are also vulnerable to cracking or spalling due to moisture penetration or freeze-thaw cycles. Brick or block walls are suitable for moderate- to high-height walls (up to 10 feet) and moderate to steep slopes.
Concrete walls are made of poured concrete that is reinforced with steel bars or wires. Concrete walls are the strongest and most durable type of retaining wall and can resist high loads and seismic forces. However, concrete walls are also the most difficult and costly to build and require formwork, curing, and finishing. Concrete walls are also plain and unattractive unless they are faced with other materials such as stone, brick, or tile. Concrete walls are suitable for high-height walls (up to 20 feet) and steep slopes.
How to Build a Retaining Wall
The process of building a retaining wall varies depending on the type of wall, but in general, it involves the following steps:
Site preparation: The site where the wall will be built should be cleared of vegetation, debris, and loose soil. The site should also be marked with stakes and strings to indicate the location, alignment, and height of the wall.
Excavation: The soil behind and below the wall should be excavated to create a level base for the wall foundation. The excavated soil should be stockpiled for later use as backfill.
Foundation: The foundation of the wall should be constructed according to the design specifications. The foundation can be made of concrete, gravel, sand, or compacted soil depending on the type of wall. The foundation should be level, stable, and well-drained.
Drainage: The drainage system behind the wall should be installed to prevent water accumulation and pressure behind the wall. The drainage system can consist of perforated pipes, gravel, filter fabric, or weep holes depending on the type of wall.
Backfill: The soil behind the wall should be backfilled in layers of 6 to 12 inches and compacted with a tamper or a roller. The backfill should be slightly sloped away from the wall to facilitate drainage.
How to Choose the Right Retaining Wall for Your Project
Choosing the right retaining wall for your project depends on several factors that you need to consider before making a decision. Some of these factors are:
Budget: The cost of building a retaining wall varies depending on the type of material, size, design, and labor involved. You should compare the initial and long-term costs of different types of walls and choose the one that fits your budget and expectations.
Aesthetics: The appearance of the retaining wall can affect the overall look and feel of your landscape. You should choose a type of wall that matches or complements the style and color of your house and surroundings. You can also add features such as plants, lights, or ornaments to enhance the beauty of the wall.
Durability: The durability of the retaining wall depends on the quality of the material, construction, and maintenance. You should choose a type of wall that can withstand the environmental conditions and wear and tear over time. You should also follow the proper maintenance procedures to extend the life span of the wall.
Maintenance: The maintenance of the retaining wall involves inspecting, cleaning, repairing, and replacing parts of the wall as needed. You should choose a type of wall that requires minimal or easy maintenance to save time and money. You should also avoid using harsh chemicals or pressure washers that can damage the wall.
Height: The height of the retaining wall determines its stability and strength. You should choose a type of wall that can support the height of the retained soil without collapsing or tilting. You should also check the local building codes and regulations for the maximum allowable height of the wall.
Slope: The slope of the retained soil affects the pressure and movement of the wall. You should choose a type of wall that can resist the lateral forces and prevent sliding or settlement. You should also consider adding reinforcement or drainage to improve the stability of the wall.
By considering these factors, you can choose the right retaining wall for your project that meets your needs and preferences.
Case Studies of Retaining Wall Projects
To illustrate how retaining walls can be designed and constructed in different situations and contexts, here are some examples of successful retaining wall projects from around the world.
The Great Wall of China
The Great Wall of China is one of the most famous and longest retaining walls in history. It was built over several centuries by different dynasties to protect China from invaders. The wall stretches for about 13,000 miles across mountains, valleys, and deserts. The wall is made of various materials such as stone, brick, wood, earth, and lime. The wall has different sections that vary in height, width, shape, and design. The wall also has features such as watchtowers, gates, bridges, and fortifications. The Great Wall of China is a remarkable engineering feat and a cultural heritage site.
The Hoover Dam
The Hoover Dam is a massive concrete gravity-arch dam that spans the Colorado River between Nevada and Arizona. It was built in the 1930s to provide hydroelectric power, water supply, and flood control for the region. The dam is 726 feet high, 1,244 feet long, and 660 feet thick at its base. The dam holds back Lake Mead, which is one of the largest man-made reservoirs in the world. The dam is reinforced with steel bars and cables and has a complex drainage system to prevent water seepage and pressure. The Hoover Dam is a marvel of modern engineering and a national landmark.
The Machu Picchu Terraces
The Machu Picchu Terraces
The Machu Picchu Terraces are stone retaining walls that form part of the ancient Inca city of Machu Picchu in Peru. They were built in the 15th century to create flat areas for agriculture, housing, and religious ceremonies on a steep mountain slope. The terraces are about 10 feet high and vary in width from 20 to 50 feet. They are made of granite blocks that are fitted together without mortar or reinforcement. They have drainage channels and irrigation systems to prevent erosion and flooding. The Machu Picchu Terraces are a testament to the ingenuity and skill of the Inca civilization and a UNESCO World Heritage Site.
The Berlin Wall
The Berlin Wall was a concrete barrier that divided the city of Berlin into East and West from 1961 to 1989. It was built by the communist regime of East Germany to prevent its citizens from fleeing to the democratic West. The wall was 12 feet high, 96 miles long, and had barbed wire, electric fences, watchtowers, and checkpoints along its length. The wall also had a "death strip" that was mined and patrolled by guards who had orders to shoot anyone who tried to cross. The wall was a symbol of the Cold War and a source of oppression and suffering for many people. The wall was demolished in 1989 after the fall of communism and the reunification of Germany.
The Dead Sea Retaining Wall
The Dead Sea Retaining Wall is a steel and concrete structure that surrounds the Dead Sea Works, a mineral extraction plant in Israel. It was built in the 1990s to prevent the collapse of the plant due to the receding water level of the Dead Sea. The wall is 40 feet high, 6 miles long, and has a thickness of 3 feet. The wall is anchored by steel piles that reach up to 130 feet deep into the ground. The wall also has pumps and pipes that transfer water from the Dead Sea to the plant. The wall is a vital protection for the plant that produces potash, bromine, and other valuable chemicals.
Retaining walls are useful and versatile structures that can enhance the stability and appearance of your landscape. However, they also require careful design and construction to ensure that they can perform well and last long. There are different types of retaining walls that have different advantages and disadvantages depending on your needs and preferences. You should consider factors such as budget, aesthetics, durability, maintenance, height, and slope when choosing a retaining wall for your project. You should also follow the proper steps and guidelines when building a retaining wall or hire a professional to do it for you. By doing so, you can enjoy the benefits of a retaining wall that meets your expectations and requirements.
What is the best type of retaining wall?
There is no definitive answer to this question as different types of retaining walls have different pros and cons depending on your situation and preferences. However, some general factors that can help you choose the best type of retaining wall are:
The height of the wall: Higher walls require stronger and more durable materials such as concrete or brick.
The slope of the soil: Steeper slopes require more stable and resistant materials such as stone or interlocking blocks.
The appearance of the wall: If you want a more natural or decorative look, you can opt for materials such as timber or stacked stone.
The cost of the wall: If you have a limited budget, you can choose cheaper and easier materials such as timber or interlocking blocks.
The maintenance of the wall: If you want a low-maintenance wall, you can choose materials that do not rot, crack, or fade such as concrete or brick.
How do I calculate how much material I need for a retaining wall?
To calculate how much material you need for a retaining wall, you need to know the dimensions of the wall (length, height, and thickness) and the type of material you are using (unit weight and volume). You can use this formula to estimate the amount of material you need:
Material = Length x Height x Thickness x Unit Weight / Volume
For example, if you want to build a 10-foot-long, 4-foot-high, and 1-foot-thick concrete wall, and the unit weight and volume of concrete are 150 pounds per cubic foot and 27 cubic feet per cubic yard, respectively, you can calculate the amount of material you need as follows:
Material = 10 x 4 x 1 x 150 / 27
Material = 222.22 pounds or 0.11 cubic yards of concrete
How do I install drainage behind a retaining wall?
To install drainage behind a retaining wall, you need to follow these steps:
Place a perforated pipe along the base of the wall behind the first course of blocks or stones. The pipe should have a slight slope away from the wall and toward an outlet or a dry well.
Cover the pipe with gravel or crushed stone to create a drainage layer. The gravel or stone should fill the space between the wall and the pipe and extend about 6 inches above the pipe.
Wrap the gravel or stone with filter fabric to prevent soil from clogging the pipe and the drainage layer. The fabric should cover the entire drainage layer and overlap at the joints.
Backfill the soil behind the filter fabric in layers of 6 to 12 inches and compact it with a tamper or a roller. The soil should be slightly sloped away from the wall to facilitate drainage.
Repeat these steps for each course of blocks or stones until you reach the top of the wall.
How do I repair a cracked or leaning retaining wall?
To repair a cracked or leaning retaining wall, you need to identify the cause and extent of the damage and apply the appropriate solution. Some of the common causes and solutions are:
Cracks due to settlement or shrinkage: You can fill small cracks with mortar or epoxy and seal them with waterproof paint. You can also inject grout or foam into larger cracks to stabilize them.
Cracks due to water pressure or frost heave: You can install drainage behind the wall to reduce water pressure and prevent freezing and thawing cycles. You can also add weep holes or vents to allow water to escape from behind the wall.
Leaning due to sliding or overturning: You can reinforce the wall with steel rods or cables that are anchored into the ground or into a concrete footing. You can also add counterweights or buttresses to balance the lateral forces on the wall.
If the damage is too severe or beyond your ability to repair, you should consult a professional engineer or contractor for advice and assistance.
How long does a retaining wall last?
The lifespan of a retaining wall depends on several factors such as the type of material, quality of construction, environmental conditions, and maintenance. In general, retaining walls can last from 20 to 100 years or more depending on these factors. Some of the approximate lifespans of different types of retaining walls are:
Timber walls: 20 to 40 years
Interlocking block walls: 25 to 50 years
Stacked stone walls: 50 to 100 years
Brick or block walls: 50 to 100 years
Concrete walls: 75 to 100 years