Engineering a Retaining Wall (2023)

Sure, retaining walls look like simple stacked stone, block, or timber. But in fact, they're carefully engineered systems that wage an ongoing battle with gravity. They restrain tons of saturated soil that would otherwise slump and slide away from a foundation or damage the surrounding landscape.

These handsome barriers also make inviting spots to sit, and can increase usable yard space by terracing sloped properties, something that is increasingly important as flat home sites become ever more scarce in many regions.

Along with sloped landscapes where water runoff causes hillside erosion, ideal locations for a retaining wall system include spots downhill from soil fault lines and where the downhill side of a foundation is losing supporting soil or its uphill side is under pressure from sliding soil.

If your property needs a retaining wall, or if the one you have is failing, follow our guide on how to build a retaining wall or hire a pro. We also review the four most common types below: timber, interlocking blocks, stacked stone, brick or block, and concrete.

Common Problems: Drainage, Weight of Soil

Although retaining walls are simple structures, a casual check around your neighborhood will reveal lots of existing walls that are bulging, cracked, or leaning. That's because most residential retaining walls have poor drainage, and many aren't built to handle the hillside they're supposed to hold back.

Even small retaining walls have to contain enormous loads. A 4-foot-high, 15-foot-long wall could be holding back as much as 20 tons of saturated soil. Double the wall height to 8 feet, and you would need a wall that's eight times stronger to do the same job.

With forces like these in play, you should limit your retaining wall efforts to walls under 4 feet tall (3 feet for mortarless stone). If you need a taller wall, consider step-terracing the lot with two walls half as big, or call in a landscape architect or structural engineer for the design work (have the architect or engineer inspect the site thoroughly) and experienced builders for the installation.

Retaining Wall Landscaping Cost

If you have your retaining wall built, figure about $15 per square face foot for a timber wall, $20 for an interlocking-block system or poured concrete, and $25 for a natural-stone wall. Preparing a troublesome site—one that includes clay soil or a natural spring, for example—can raise costs substantially. Add 10 percent or so if you hire a landscape architect or engineer. But shop around; some landscape firms do the design work for free if they do the installation.

Engineering a Retaining Wall (1) Photo by Carolyn Bates

How to Build a Retaining Wall

Poor drainage resulting in saturated soil and frost heaving is the main cause of failure. That's why all strong retaining walls begin with landscape fabric, backfill, and 4-inch perforated drainpipe.

How deep should the footing be for a retaining wall?

The depth you need to excavate depends on frost depth as well as the wall and soil type. Mortared or concrete walls in heavy-frost areas require footings dug below the frost line. Nonmortared walls should be built on a gravel-filled trench dug below frost line. If you live where it doesn't freeze and your soil drains well, you may be able to just scrape away topsoil to form a base for nonmortared walls.

Before adding gravel, lay down enough landscape fabric to contain the new gravel. Form the fabric into a large C shape, with the open mouth of the C facing downhill. The fabric should wrap around and create a border between the gravel and topsoil to keep sediment from clogging the gravel and drainpipe.

Backfilling basics

Replace native soil with 3/4-minus gravel (no stones under 3/4 inch in diameter) or "bank-run" gravel (washed stones 1/4 inch to 6 inches in diameter). Shovel at least a 4-inch layer of gravel onto the landscape fabric. Grade this layer so it slopes 1 inch for every 4 feet, allowing water to drain away. Then lay in 4-inch perforated PVC drainpipe at the base of the wall and cover it with gravel.

Shovel in backfill as you build the wall, one tier at a time. Don't add all the backfill at the end—it won't compact thoroughly. Tamp down the gravel as you go with a heavy hand tamper. Behind the top tier of the wall, add 6 inches of topsoil and lightly compact it.

Battering and deadmen-tieback system

All retaining walls should lean into the hill 1 inch for every 12 inches of height. Timber walls 4 feet or higher should be tied to the hillside with "deadmen" anchors (6-foot-long, T-shaped tiebacks buried in the hillside) attached to the wall every 8 feet, extending 6 feet back to a 2-foot-wide T-bar.

Deadmen are not included in some interlocking-block systems if the design allows backfill to secure the blocks individually in place. Still others require geo-grid, weblike tiebacks that get buried in the backfill. Check the manufacturer's literature.

(Video) Retaining Walls Explained | Types, Forces, Failure and Reinforcement

A final heads-up on masonry walls—concrete blocks chip and crack easily. Carefully inspect the blocks upon delivery, and don't be shy about returning damaged blocks for credit.

Engineering a Retaining Wall (2) Illustration by Trevor Johnston/Paul Perrault

Types of Retaining Walls

Concrete

Upside: Strong. Well-designed and properly drained and backfilled, concrete walls rarely fail.

Downside: Bare concrete isn’t particularly attractive. It can be veneered with masonry, or special forms can be used that embed decorative designs in the finished wall. Also, if a wall fails, patching may not be possible and removal is costly. Walls over a few feet high should be formed and poured by a pro unless you’ve had experience with vertical pours.

Cost: Around $16 to $20 per square face foot installed.

Remember:

Follow all rules for landscape fabric, drainage, and backfill. The footing should be below frost depth or on well-drained gravel that reaches this level. Use 3/4-inch ply and 2-by-4 bracing to form the wall. And install #4 rebar wired in 12-inch grids for added strength. Use mechanical vibration or strike the forms with a rubber mallet every 6 inches when concrete is wet for a smooth finished face.

Timber Walls

Upside: Only moderately challenging to build by yourself up to 4 feet high. If an engineer has designed the wall, located the deadmen, and specified the backfill and drainage, you can install an even taller wall yourself.

Downside: Not as long-lived as masonry. Making square cuts is challenging. Also, components are heavy and hard to manage alone. Plan on about three days to build a wall 4 feet tall by 15 feet long.

Cost: $10 to $15 per square face foot installed, depending on your region—higher if extensive excavation, soil prep, and backfilling are needed.

Remember:

Use 8-foot-long, 6x6-inch pressure-treated wood designated "For Ground Contact," and have all materials delivered. Follow all rules for landscape fabric, drainage and backfill. All timber walls require deadmen every 4 feet at midwall height or higher. Pin the first tier of timbers to the ground with #4 rebar.

Interlocking Concrete Block

Upside: Also called segmented retaining walls, interlocking-block systems from Keystone, Risi, Rockwood, Tensar, Versa-Lok, and others are mortar-free and easy to assemble. Units are small and modular, so walls can taper, turn, wrap, and curve. Available in many textures, shapes, and colors, these engineered systems, which can be used for walls up to 20 feet high, rely on several techniques including:

  • Keyed, battered design (block shapes key into one another and are stacked so they lean into the hillside)
  • Backfill trap (block shapes allow backfill to be shoveled into the block webbing, trapping each block individually)
  • Geo-grid webs (block maker supplies geo-grid plastic-net tiebacks that attach to the block and are buried 5 feet in the hillside at specific heights).

Downside: You can't mix and match manufacturer's systems. Block systems that use metal pins to tie blocks together can be a challenge to line up exactly.

Cost: About $12 to $20 per square face foot installed, depending on block configuration and site. More expensive systems tend to be stronger and stack higher.

Remember:

(Video) 8. Retaining Walls

Arrange before delivery from the masonry yard where materials will be stockpiled in your yard and if the forklift used to off-load the truck will fit through backyard gate, etc. Follow all rules for landscape fabric, drainage, and backfill. Use manufacturer's calculators to determine how many blocks, pins and tiebacks you'll need. When stacking a row of blocks, sweep off each layer; small pebbles can disrupt the pattern. Cap walls with flat units or stone held down with silicone caulk.

Stone, Brick, or Cinder Block

Upside: For a stone retaining wall, a handsome rustic appeal. Collecting stones on site and doing the work yourself can also save money. Brick provides a more formal look. Cinder block is inexpensive and can be reinforced with steel and concrete.

Downside: Stone-wall masonry is harder than it appears. Fitting the stone is exacting work and making mortar joints look natural requires experience (nonmortared stone walls don't offer much holding power). Brick masonry also requires skill to hit the visual standard all of us are used to. Cinder block has to be faced with stucco, brick, or stone or overgrown with plantings to make it attractive.

Cost: About $10 to $12 for cinder block; for brick and stone, around $20 to $25 per square face foot (double that figure for a two-sided wall).

Remember:

Follow all rules for landscape fabric, drainage, and backfill. A mortared wall needs a footing and a drainage system that will defeat frost heaving. A dry, nonmortared wall allows water to seep through, relieving pressure behind the wall naturally.

Safeguarding Against Three Common Failures

Retaining walls usually fail slowly. Common problems can often be fixed if you act quickly. You can also protect a new wall in the building process by safeguarding it against the three most common failures:

Blowout Failure

What happens: A load is added within 3 feet of the top of the wall. The wall leans out at the top and eventually tips over

What to do: Tell your landscape architect or engineer if a car or shed will be placed near the wall. The pro should then beef up the footer and increase the number of tiebacks or deadmen to add strength. Adding retrofit tiebacks is expensive and requires excavation, partial dismantling, and reengineering the wall.

Wet-Soil Failure

What happens: Soil behind the wall gets saturated, causing hydrostatic water pressure and weight to topple the wall.

What to do: Replace native soil behind the wall with 3/4-minus or bank-run gravel for 2 feet. Line the inside base of the wall with 4-inch perforated tile drain on a gravel bed that slopes 1 inch for every 4 feet of run to carry water to daylight or a dry well. Topsoil should take up only the top 6 inches behind the wall.

Frost-Heave Failure

What happens: Retaining wall lacks proper drainage or a footer. Soil becomes saturated and freezes, heaving upward and breaking the wall apart.

What to do: Walls should rest on 3/4-minus or bank-run gravel, with the footer or wall base buried beneath the frost line (6 to 48 inches, depending on region). For deep frost, use concrete block rather than retaining wall to ground level, then build the retaining wall on that. Well-drained gravel behind and beneath the wall can substantially diminish frost heaving.

Where To Find Retaining Wall Services:

Hickson Inc.

1955 Lake Park Dr., Suite 250

(Video) Why Retaining Walls Collapse

Smyrna, GA 30080

www.hickson.com

770-801-6600

Keystone Retaining Wall Systems

4444 West 78th Street

Bloomington, MN 55435

www.keystonewalls.com

800-747-8971

Osmose Wood Preserving

1016 Everee Inn Rd., Box O

Griffin GA 30224-0249

www.osmose.com

770-228-8434

Risi Stone Systems

8500 Leslie St., Suite 390

(Video) Failing Retaining Wall Inspection

Thornhill, ON L3T 7P1 Canada

www.risistone.com

800-626-9255

Rockwood Retaining Walls, Inc.

7200 N. Highway 63

Rochester, MN 55906

http://rockwoodwalls.com

800-535-2375

Tensar Earth Technologies

5775-B Glenridge Dr., Lakeside Center, Suite 450

Atlanta, GA 30328

www.tensarcorp.com

800-836-7271

Versa-Lok Retaining Wall Systems

6348 Highway 36, Suite 1

(Video) What is a retaining wall? I Geotechnical Engineering I TGC Ask Andrew EP 1

Oakdale, MN 55128

www.versa-lok.com

800-770-4525

FAQs

Do I need a structural engineer for a retaining wall? ›

Retaining walls are needed to retain soils at different levels where there is insufficient space to allow stable slopes. Retaining walls greater than 1m high should be designed by a civil or structural engineer who is familiar with site and ground conditions.

What are the factors to be considered when designing a retaining wall? ›

What should I consider when designing a retaining wall? When designing a retaining wall, there are seven factors to consider: materials, type of wall, design or on-site placement, drainage, foundations, cant or batter, and anchors or “deadmen”.

What are the 3 type of forces acting on a retaining wall? ›

There are various types of loads and forces acting on retaining wall, which are: Lateral earth pressure. Surcharge loads. Axial loads.

How much is a structural engineer for retaining wall? ›

Applying for planning permission

As a baseline, structural engineers tend to charge around £50 to £90 per hour for their services.

At what height does a retaining wall need to be engineered? ›

What is the Wall's Height? Most governments will require a building permit and design from a licensed engineer if a wall is taller than 4 feet (measured from the bottom of the first block to the top of the final block).

How thick should my retaining wall be? ›

It should be at least 215mm thick and bonded or made of two separate brick skins tied together. This should be enough in most cases with minimal water pressure or where the ground level difference is less than a metre.

What is the tension side of a retaining wall? ›

The reinforcement is provided on the tension side that is vertically behind the steam and horizontally below the heel and the toe slab. They are the most commonly used retaining walls and are much lighter than gravity walls, therefore need lighter foundations.

Does retaining wall need columns? ›

When considering Retaining wall with greater heights, introduction of columns can give a better economy. This paper conducts a thorough analysis and design of Cantilever Retaining Wall (CRW) and Column Cantilever Retaining Wall (CCRW) for a span of 20 m and height from 2.5 m to 5 m, along with its footing.

Why is retaining wall tapered? ›

Retaining walls with Tapered retaining wall thickness are commonly used where high tall walls are required. Such high soil retaining capacities are almost impossible to achieve with regular constant width walls unless the thickness of the wall is made extremely large through the entire height.

Can I use wood for a retaining wall? ›

Wood can be a budget friendly solution for a DIY retaining wall. However, wood is subject to fungal rot, termites, and water damage. When following all of the best recommendations, a wood retaining wall can last up to 40 years. Without proper techniques, a wood retaining wall can fail in as soon as 5 years.

Where is the most pressure on a retaining wall? ›

In addition to the friction at the base, most retaining walls rely on the passive pressure at the front of the wall to prevent sliding problems.

What causes retaining walls to fail? ›

The number one cause of retaining wall failure is poor drainage. If too much water gets absorbed into the soil behind the wall, the hydrostatic pressure can push on the wall causing it to bow out or crumble.

How do you determine the stability of a retaining wall? ›

Requirement: Check stability against overturning and sliding, and check soil bearing capacity.
  1. Check overturning stability. Active earth coefficient: Ka = tan (45-f/2)2=0.361. ...
  2. Check soil bearing capacity: Total weight of retaining wall: W=1500+900+4600+150+800 = 7950 lbs. ...
  3. Check sliding stability.

How much does a structural engineers report cost? ›

You can expect to pay around £100 an hour for a structural engineer in 2022.

Does a retaining wall need building regulations? ›

Independent, freestanding retaining walls may not require building regulation approval; however, any structures must be structurally sound and well maintained.

Do I need a structural engineer? ›

Essentially, any project that alters the structure of the building or requires building control approval will require the expertise of a structural engineer.

Can a civil engineer design a retaining wall? ›

The site civil engineer is also typically responsible for site layout and grading plans, including slopes and retaining wall locations.

Can a landscape architect design retaining walls? ›

A landscape designer is an expert at designing hardscape features such as patios, retaining walls, and walkways. They are also skilled at plant selection and building structures such as fences, arbors, and pergolas.

Do all retaining walls need footings? ›

Every retaining wall needs a footing, but what type depends on the design, material, height and weight of the wall. Large masonry retaining walls built out of rigid materials like concrete, cinder blocks, cement or brick need a concrete footing. Solid masonry walls are stiff and unable to flex because they'll crack.

What is the strongest type of retaining wall? ›

Concrete and Masonry Retaining Walls

Poured concrete is the strongest and most durable choice for retaining walls. It may also be carved and formed to look like mortared stone depending on your taste.

What is the best material to use for a retaining wall? ›

Retaining walls can be made from wood, bricks, natural stones or concrete blocks. For DIYers, it's best to use concrete retaining wall blocks, which can be interlocking and are heavy enough to stay in place without cement or other adhesive. Interlocking blocks fit together and add extra security to the wall.

What is the best block for a retaining wall? ›

The Allan Block system is widely accepted as a reliable retaining wall block product by contractors and engineers.

Do I need drainage behind retaining wall? ›

Retaining wall drainage is critical. It ensures water does not collect behind the wall, causing it to fail. A quality drainage system collects and redirects rainwater away from the wall. It decreases pressure on the soil around the foundation and within the wall itself, reducing erosion and settlement.

Do I need a concrete footing for a retaining wall? ›

You should always build a garden wall on a solid foundation of a trench filled with concrete.

How deep should a footing be for a 4 foot retaining wall? ›

The general rule of thumb is to bury about one-eighth of the height of the wall. For example, if your wall will be three feet (36 inches) tall, the first course of blocks should start five inches below soil level. The gravel base should start three inches below this.

What two forces does a retaining wall have to resist? ›

Gravity Retaining Walls predominately resist lateral earth pressure (from soil backfill known as active soil) as well as the gravity weight of the structure.

How much force can a retaining wall hold? ›

The exact amount that a wall can hold back depends on its design. However, a well-constructed paver retaining wall that is 4 feet high and 15 feet long could hold back up to 20 tons of weight.

What is toe and heel in retaining wall? ›

A typical retaining wall has four main components: the Stem is the vertical member holding the backfill, the Toe is the portion of the footing at the front of the wall, the Heel is the portion of the footing at the backfill side, and the Shear Key projects down under the footing.

How do you build a reinforced retaining wall? ›

How to Build a Reinforced Retaining Wall Using Geogrid - YouTube

What is a structural retaining wall? ›

A retaining wall is a structure designed and constructed to resist the lateral pressure of soil, when there is a desired change in ground elevation that exceeds the angle of repose of the soil. The allowable height of walls is stipulated in the Zoning Code.

What are the main failure types of a retaining walls? ›

Types of failure of a Retaining Wall
  • Failure by Overturning. Overturning of Wall. In this failure mode, the toe will act as centre of rotation and the wall would deform something like in the above image. ...
  • Failure by Sliding. Sliding of Retaining Wall. ...
  • Failure by Bearing Pressure. Bearing Failure of Wall.
29 May 2021

How far can a wall lean without falling? ›

Acceptable Limits for Leaning or Bulging Walls

Generally speaking you should be concerned with anything more than 25mm of distortion as it lowers the stability of the wall. There is a general rule known as the V3 rule, which asks that you consider the walls centre of gravity.

How do you backfill a retaining wall? ›

Steps For How to Backfill a Retaining Wall

Lay your base of compacted native soil (about three inches deep). Tamp the soil to ensure that it is secure and firm. Fill the next six to twelve inches with aggregate or gravel. Tamp the gravel or aggregate to ensure a sturdy base.

What is a Deadman retaining wall? ›

In construction, a deadman is a relatively heavy weight, typically a mass concrete block used to provide support or resistance to a load. These blocks are usually embedded firmly in soil; however, some blocks may simply rest on the ground surface.

How deep should a post hole be for a retaining wall? ›

The post holes into which the retaining wall posts will be placed shall be 450mm diameter with minimum 100mm concrete cover below the post. Footing depth is typically equal to the height of the retaining wall. For this example, therefore, the post hole is 1200mm.

What is the cheapest retaining wall? ›

What Is the Cheapest Type of Retaining Wall? The cheapest type of retaining wall is poured concrete. Prices start at $4.30 per square foot for poured concrete, $5.65 for interlocking concrete block, $6.15 for pressure-treated pine, and about $11 for stone.

Does a retaining wall need building regulations? ›

Independent, freestanding retaining walls may not require building regulation approval; however, any structures must be structurally sound and well maintained.

Are retaining walls structural? ›

A retaining wall is a structure designed to uphold the soil behind it. This allows for changes in the grade of a plot and flat areas to be used for building structures or backyard features. Several factors must be considered in order to build the proper retaining wall for your property.

Do Civil engineers design retaining walls? ›

The site civil engineer is also typically responsible for site layout and grading plans, including slopes and retaining wall locations.

What is the Australian standard for retaining walls? ›

The Australian Standard, AS4678 – Earth Retaining Structures deals specifically with walls that are: Greater than 800mm and less than 15m in height. 70° or more to the horizontal (thereby excluding revetment structures with gentler slopes).

Do I need a concrete footing for a retaining wall? ›

You should always build a garden wall on a solid foundation of a trench filled with concrete.

Can I build a retaining wall myself? ›

Choose a DIY-friendly building material. Retaining walls can be made from wood, bricks, natural stones or concrete blocks. For DIYers, it's best to use concrete retaining wall blocks, which can be interlocking and are heavy enough to stay in place without cement or other adhesive.

Do you have to put drainage behind a retaining wall? ›

Every retaining wall should include drainage stone behind the wall. Though it is a good idea to install a drainage pipe on all walls, there are certain situations where a perforated drain pipe is absolutely necessary.

Do retaining walls need rebar? ›

Retaining walls must be stronger than freestanding walls. Insert rebar in the footing when you pour it; this should be done at every three blocks or at intervals specified by your local codes.

What is the maximum height of a retaining wall? ›

Ten feet is usually the max for how tall a retaining wall can be, although we recommend staying under that. For width, make sure the base of your retaining wall measures at least one-third of its overall height.

How do you stop a retaining wall from leaning? ›

Wall anchors can be a long lasting solution to strengthen retaining walls and actually pull them back as close to their original position as possible. Wall anchors can be used to shore up bowed or leaning basement walls, but the same solution can be used in your retaining walls.

How long does it take to build a retaining wall? ›

Plan on about three days to build a wall 4 feet tall by 15 feet long. Cost: $10 to $15 per square face foot installed, depending on your region—higher if extensive excavation, soil prep, and backfilling are needed.

What is considered a surcharge on a retaining wall? ›

A “surcharge” on a retaining wall is any load in addition to level grade, within that area defined by a 45 degree angle from the bottom of the footing to level grade.

What is an engineered wall? ›

Today's engineered wall systems are usually designed to provide more insulation, a tighter building envelope and a stronger wall than a stick-framed building. These products and methods can save money and the environment.

How far apart should retaining walls be? ›

As a rule of thumb, the minimum distance between segmental retaining wall terraces (D) for each wall to act independently must be at least equal to twice the height of the lower wall (D > 2H1).

How thick are retaining walls? ›

Base width = 1/2 to 1/3 of the height of the wall. Base thickness = 1/8 of the height of the wall but not less than 12 inches. Stem thickness = 6 inches + ¼ inch for each foot of wall height.

How do you build a concrete retaining wall in Australia? ›

How to Build a Concrete Block Retaining Wall
  1. Contact Your Local Council. ...
  2. Locate Your Wall. ...
  3. Add 100mm Road base. ...
  4. Lay The Retaining Wall Blocks. ...
  5. Install Agricultural Drain. ...
  6. Add Additional Courses. ...
  7. Add Capping blocks. ...
  8. Backfill.
17 Oct 2015

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