The ultimate guide to retaining walls

Offering reliable landscaping services to homeowners in British Columbia since 2013

Picture of Natasha Maerz

Natasha Maerz

Owner/Office Manager

This guide is to educate homeowners on all things related to Allan Block retaining walls, such as cost, wood vs concrete, and problems associated with Allan Block retaining walls.

 

At back40landscaping, our goal is to educate homeowners on their potential landscaping projects, and we truly find that honesty is the best policy.

By doing so, we believe that people will be able to make an educated decision for themselves and their family.

We are certified Allan Block wall installers, and everything that we speak about in this guide is per Allan Blocks standard of practice.

allan-block-retaining-wall

Table of Contents

What is a retaining wall?

The simple definition of a retaining wall is that it is a structure that holds, or retains soil behind it.

Retaining walls are designed to restrain soil to a slope that it would not naturally keep to (typically a steep, almost vertical, or vertical slope).

Areas that retaining walls are used in are areas where the landscape needs to be shaped severely, and engineered for a more specific purpose, like hillside farming, or roadway overpasses. A professional engineer should always be brought in when necessary to ensure the integrity of wall. 

Sizes of retaining walls

Retaining walls can be built to suit whatever homeowners need and are looking for and can be built with a variety of materials.Sizes and types of walls range from small landscaped walls that surround a garden, pond, or elevated patio, to enormous soil-retaining projects.

Materials used in a retaining wall build

Retaining walls come in a variety of materials that can be used to build wall systems.  These include concrete blocks, poured concrete, treated timbers, and rocks and boulders.

All of these materials can retain soil, but some will have a shorter lifespan than others.

Other than wood, concrete walls typically come in a variety of colors and shapes those homeowners can choose from, allowing the wall to either blend in with the surrounding area, or act as a focal point to the space.

allan-block-retaining-wall

How are retaining walls made?

Allan Block walls

If you are looking for a wall that is the focal point of your outdoor space, will last a lifetime, and is backed by professional engineers, then an Allan Block concrete wall in our opinion is a great option for your landscape wall. 

While some homeowners like the look of a wood retaining wall, a wall that is built with pressure treated lumber will not last. A wood retaining wall has a life expectancy of 10 years, while a concrete retaining wall is a lifetime product. 

This is because pressure treated lumber is not rated for ground contact, meaning it is meant to stay off of the ground, never touching soil. This is obviously impossible to do when building a retaining wall out of wood, making Allan Block the best material to use for a retaining wall.

Building A Retaining Wall

When it comes to building a retaining wall, the structure and the look of the wall should be homeowner’s main concern.

It might be tempting to want your wall built quick, and as cheap as possible, but cutting corners and not using the proper amount of material will only cause headaches for homeowners in the long run.

When excavating the soil where the retaining wall is going to go, it can be hard not to cheat how much retained soil is getting excavated.

The more retained soil that is excavated, the more gravel that is going to have to be put in its place, and the higher the cost of disposing the soil.

The problem with this is that if you cheat the excavation, and do not add enough gravel, your retaining wall will fail.

Gravel is essential to the base of the wall to provide a solid level of foundation to build the wall on, and will allow water to drain through the gravel, providing proper drainage.

When there is no standing water, there will be no pressure put against the wall, and no frost heave in the winter time.

Other things that aid to the strength of a retaining wall is burying the first row of the retaining wall blocks so that the wall doesn’t slip out, and core filling the back, front, and middle of the wall with gravel. 

Typically, contractors do not put enough gravel behind the wall because more gravel is used, and the cost of labor and material goes up.

To get a retaining wall to look straight, and not have any waves in it, is actually a lot harder to do than it appears.

 If you look at retaining walls in your neighbourhood, you will probably see walls that are not completely straight.

To get a wall that is straight takes quite a bit of time and skill. This is done by string lining the brick, laying each retaining wall block brick one at a time, and making sure that the brick is level from side to side, and front to back.

This process ensures that your wall will be in a straight line.

Designing A Retaining Wall

You might think that designing your wall is as easy as figuring out where the wall is going to go, and what it is going to look like, but there are many things that need to be taken into consideration before your wall is built.

Building permits are required for retaining walls that are above 4’, and you may need a professional engineer to do the retaining wall design in order to get a building permit.

When determining the height of the wall, a detailed understanding of site elevations and grade changes are needed to determine what the height of the wall should be. Remember, a basic retaining wall that is four feet tall and fifteen feet long has to support up to 20 tons of retained soil pressure.

With every additional foot of height, the pressure of the retained soil increases substantially.

This is why it is important to know what you are in for ahead of the build, so you can make sure that your wall is structurally sound, and will last.

Are Retaining Walls Environmentally Friendly?

Allan Block is an environmentally conscious company that strives to be a leader in the future of green building products.

Building with more environmentally friendly products will reduce the negative impact we have on the world we live in.

Unlike treated wood retaining walls, Allan Block concrete retaining wall block has no toxic chemicals to harm the earth.

 

Because Allan Block retaining walls last longer than wood retaining walls, there is less impact on the environment when building the wall, due to only having to use machines and materials once while building.

allan-block-retaining-wall

Do retaining walls need drainage?

Every retaining wall needs drainage. The slightly more complicated answer though…is what kind?

Most wooden retaining walls are not built with drainage, but they should be. For a wooden wall you want to limit the amount of water behind the wall for 2 reasons.

The first reason would be to slow down the rate at which the wood will rot. Constant moisture is not good for wood long-term. The 2nd reason is water is heavy, and reducing the weight of the soil pushing on the wall will help to make your wall last longer, and stay straight while doing so. The worst place for wet heavy soil is at the top of the wall is this is where it has the most leverage to overturn the wall.

For a concrete retaining wall, specifically Allan block water effects it differently. Unlike wood concrete is almost impervious to moisture. For all intents purposes water will not affect concrete… So why is water so bad for it?

Well for the same reason as the wood, water is heavy and will put a huge unnecessary surcharge on the wall. But unlike wood this concrete retaining wall will have to resist this pressure for 50-100 years. Maybe even longer. The added lifespan of the concrete means you also have to take into account the wear and tear of mother nature year after year.

Water if not drained away, will also create problems during freezing temperatures as the water freezes it expands and will cause the wall to move or shift. This may not structurally affect the wall, but it won’t look straight anymore. This reduces the aesthetic look of your focal point.

Water present around a concrete retaining wall will also encourage root growth from nearby trees or shrubs, all of which can upheave or cause a wall to bow outwards.

These types of aesthetic blemishes may not be a big problem for a wooden retaining wall if it’s being replaced frequently, but for a Allan Block retaining wall that will last a lifetime an early blemish should be avoided!

Fortunately, the Allan Block contractor training program teaches us contractors how to avoid these problems, and how to build strong structurally sound retaining walls.

How much do retaining walls cost?

A Typical Retaining Wall Will Cost Between $10,000 And $50,000

For A 25’ X 3’ Allan Block Retaining Wall Expect to Pay Between $12,000 – $15,000

For this price you can expect your driveway, flower beds, walkways and lawn protected or remediated afterwards from heavy equipment, a structural retaining wall built to last a lifetime, and of course a fully insured, licensed and reliable construction company handling your project.

What Is Typically Included When Building a Retaining Wall?

  • Creating an excavation ticket with BC1CALL
  • Protecting concrete areas and lawns
  • Excavation of the site
  • Hauling/disposal dump truck & trailer of soil
  • Hauling and importing gravel to build site
  • Building the gravel wall foundation
  • Construction of the retaining wall
  • Proper Drainage system tied into existing source
  • Backfilling and compaction behind wall
  • Site clean-up, lawn repair if damaged
  •  

What Could Drive the Price of a Retaining Wall Higher?

  • Limited space that doesn’t allow heavy equipment or material stockpiling
  • Any demolition of an old wall (wood retaining wall or concrete retaining wall), fencing, or landscape
  • Structural support design for fencing installed near any part of the wall
  • Working around detailed landscaping that needs to be fixed afterwards
  • Protecting surfaces in excess of 100’ (laying down plywood to drive on)
  • If preventing damage is not practical then thorough remediation after project is complete
  • Buried and unexpected finds underground (although rare, its worthy of a mention)
  • Soil conditions: Geogrid may be required for walls higher than 13” in clay soils
  • Length or complication of tying proper drainage system into appropriate source (storm drain, sump basin etc.)

What Could Drive the Cost of A Retaining Wall Down?

  • Wide open site allowing bigger or more efficient equipment
  • No existing retaining wall requiring demolition
  • No fencing in or around retaining wall
  • No detailed landscape in front or behind wall (flower beds, walkways) 
  • Not needing surface protection
  • Wall being built prior to finished outdoor space landscape, and not having to tidy up from wall construction
  • No excavation surprises
  • Soil conditions: Water permeable soil that reduces the need for Geogrid, or as much gravel as normal
  • A nearby proper drainage system that can accept the water from behind the wall

For A 75’ X 6’ High Allan Block Retaining Wall Expect to Pay Between $45,000 – $49,000 Including Permits and Engineering

For The Permit, And Typical Engineering Expect to Pay Between $3600-$5000

allan-block-retaining-wall

The last substantial difference in a small retaining wall vs a larger one is the drainage system.

A wall that requires reinforcement such as Geogrid automatically requires a 2nd system called the heel drain.

This drain captures water further away from the wall, and lowers the surcharge on the wall.

There are a lot of variables when designing and building a concrete retaining wall. In every case their job is to hold back a lot of weight for a lifetime. 

allan-block-retaining-wall-with-flower-beds

Allan block retaining walls vs. wooden retaining walls

While both wood retaining walls and concrete retaining walls do the same thing, retain soil, there seems to be a debate on what retaining wall product is the best. 

1/3 of our inquiries from homeowners looking to build or replace a retaining wall ask us what are the main differences between a wood retaining wall vs. a concrete Allan Block retaining wall.

So, What Is the Price Difference, The Longevity, And the Pros And Cons Of Wood Retaining Walls Vs. Concrete Retaining Walls?

Retaining walls are not only built to be a focal point, but can protect the surrounding areas of your property. If they fail, there could be major damage done to homeowner’s outdoor space.

It is important for anyone looking to get a retaining wall built to understand and compare wood retaining walls and concrete retaining walls so that they are able to make an educated decision. 

Ground Rules – Wood Vs. Concrete Allan Block Comparison

Retaining Wall Size

Our project today will be for a 75’ long by 3’ (36”) retaining wall.

Building code permits, professional Engineering, and Geogrid reinforcement will not be required.

Building A Retaining Wall

On any project without permits and professional engineering the building practices will vary. The best practices are a higher initial investment, but like all high-quality builds last substantially longer.

wood-retaining-wall

Wood Retaining Walls

Wood is a natural resource commodity and being such does not have a company behind it offering building advice, warranty work, or ANY resources.

Most pressure treated lumber is not rated for ground contact. Meaning it’s supposed to be off the ground, and never touching soil. It really isn’t suitable for a retaining wall.

Typically, you won’t find ground contact lumber at Home Depot, Lowes, Rona or any other local big box store.

Allan Block Retaining Walls

On the other hand, Allan retaining wall blocks are a manufactured concrete product where every block is identical, and engineered to last a lifetime or longer. There are hundreds of resources available covering all scenarios. Backed by Engineers, scientific research, and evidence.

Wood is less expensive, and will last approximately 20-30 years.

Allan retaining wall blocks are a larger initial investment and will last a lifetime.

However, both walls will fail much sooner than expected if not properly installed.

The Majority Of Retaining Walls Fail Because Of A Lack Of A Proper Drainage System Behind The Wall

A proper drainage system is composed of a minimum of 16” of clear draining gravel directly behind the wall, and a drainpipe at the bottom of the wall moving the water away.

The Wall Itself Whether That’s Wood Or Concrete Is Not What Holds The Weight Back.

Effective retaining wall systems are a combined effort between the wall, and the gravel, and in some cases even further back if Geo Grid or Deadman’s are used. The combination MASS of the wall, gravel and soil is what resists the weight surcharge, and prevent the wall from failing and overturning.

Therefore, if a heavier concrete wall like Allan Block is calling for specific amounts of gravel, and drainage to build a structural retaining wall…

A lighter product like wood should also be built to at least the same standards if not better.

wood-retaining-wall

Site Preparation for A Retaining Wall

Both projects below will be prepared the same way. Temporary plywood roads to protect driveway, lawns, and any other valuable existing landscaping from excavation equipment.

Excavation Of a Retaining Wall

The same size area will be excavated for both the Wood retaining wall and Concrete retaining wall. Including hauling and disposal of the same amount of soil.

Cubic Yard Calculation Including Base And 16” Behind Retaining Wall

42” X 75’ X 16” = 12.96 YD3

Soil expands by 1.3-1.4 times in volume when unearthed so this produces 17 YD3 of soil to excavate, load, haul, and dispose of.

Gravel For a Retaining Wall

Wood retaining walls and Concrete retaining walls both need an adequate gravel base and a proper drainage area behind the wall.

Gravel Base of a Retaining Wall

Gravel is essential for the base to provide a solid level foundation to build upon. It drains water away to eliminate soil erosion, decomposition of the wall material, and to remove the standing water (proper drainage) below the wall system to prevent frost heave in the winter time.

Gravel Behind the Wall of A Retaining Wall

We already covered this above but it’s worth another mention because of how important it is, and how often it’s not done.

Excavating and removing soil behind your wall, and replacing with gravel will add significant costs to the project, but must be done if you expect it to last for longer than 5-10 years.

WELL OVER HALF of all retaining walls are in a state of failure, built unlevel, crooked, have gaps between the Ties/Blocks that open up more every year

Drainage Pipe Behind the Retaining Wall

A proper drainage system consists of one 4” pipe behind the base of the wall called the Toe Drain. Larger walls require 2 pipes. On the far end of the wall there should be a clean out cap for future maintenance, and the low end of the pipe should be tied into an appropriate drainage system, such as city storm drain, inground concrete sump, perimeter drain system, or ditch.

Other Details for A Retaining Wall Build

Both the Wood retaining wall and Concrete wall are laser levelled for grade accuracy, and installed with a string line for straightness. This helps the retaining wall blocks be completely straight. 

Both walls will have a 6-12° setback to aid in structural integrity.

The Wood retaining wall will be installed with 5 nails / 10’.

Allan Block Caps glued down with PL Concrete Glue 

 Summary Of the Ground Rules

  1. Size 75’ long 3’ high
  2. Built to best practices based on Allan Block’s Engineering and Scientific Evidence
  3. Wood RATED FOR GROUND CONTACT – 20-30 yr. durability
  4. Allan concrete block products – Lifetime durability
  5. Surface protection – Driveways, lawns, finished landscapes etc.
  6. Excavation – 17 Cubic yards of soil hauled and disposed of
  7. Gravel – 13 cubic yards of ¾” clear drain rock for Base and Drainage behind the wall
  8. 4” Drainage Pipe tied into appropriate discharge area
  9. Installed level, straight, consistent setback
  10. Wood – nailed, Concrete – glued
  11. Site cleaned up, lawn repaired if needed, driveway and street cleaned
allan-block-retaining-wall

What’s better, wood or Allan Block?

    • Recommendation of the company you are hiring
    • Suitability for your property
    • Design, color, and style options
    • Cost of initial investment and maintenance
    • Durability
    • New retaining walls tend to be Allan block / Concrete option
    • Replacement of wood is 50% wood, 50% Allan Block upgrade

Recommendation Of The Company You Are Hiring To Build Your Retaining Wall

For ALMOST every project we recommend a Concrete Allan Block wall or SIMILAIR concrete, natural stone wall.

Why choose concrete for my retaining wall?

  • They look significantly better. Day after day for decades you will look at your wall. While it’s true they do serve a very valuable structural component they also look impressive when built level and straight. They add a tremendous depth to your landscaping, and will be a focal point in your outdoor living space for a lifetime.
  • They are available in a huge array of sizes, colors and shapes, sure to meet even the most detailed designers’ dreams.
  • Structurally they are stronger and backed by Allan Block Resources
  • Less stress – Knowing you will not have to replace or construct another wall can be a huge weight off of you and your families’ shoulders.
  • Our world and lives are too busy as it is. Its more efficient to do your retaining wall project once, and not have to revisit it again.
  • Total investment – In the long run a Concrete Allan Block wall will be much more cost effective.
  • Higher resale value IF your realtor/home inspector know their true value/cost

 Why choose wood for my retaining wall?

  • When the client likes the look of a Wooden Retaining Wall
  • Existing wooden retaining walls on the property you want to match
  • Older home that may be extensively remodeled in the near future
  • Initial investment is a concern
  • Replacing an existing wooden retaining wall that in your opinion has served you well, and you are comfortable in knowing what you are going to be getting

 

Lean Towards an Allan Block Retaining Walls When

  • Water will be involved – Lakeshore, High water table area, Base of long hill
  • Clay soil – Heavy clay soil requires stronger retaining wall
  • Vehicle traffic will be frequent, close, and unusually heavy (Commercial vehicle, Contractor’s trailer, RV, etc.)
  • Heavily shaded area – concrete can handle being damp its entire life.
  • Limited excavation room behind the wall. Allan blocks can be installed with as little as 12” behind the wall
wood-retaining-wall

Lean towards a Wooden Retaining Wall When

  • Moisture in and around the wood can be limited
  • Sandy/rocky fast draining soils
  • Shorter in height – for a tall wall, Allan Blocks would be the way to go
  • Limited finished landscaping around the wall, such as flower beds or walkways as replacement in the future will cause damage to the landscaping, or increase the cost to protect and work around it.
  • Large excavation room behind the wall for installing Deadman Anchors (4-10’)

Design, Color, And Style Options for Your Retaining Wall

Allan Block walls come in an array of sizes, and colors, and can be cut into curved architectural pleasing designs.

While there are some limitations, their installation is almost free of physical restrictions in terms of design, and layout.

Wooden retaining walls are somewhat limited in terms of colors. Yes, you can stain and paint them, but it is not very common due to the in-ground moisture levels. The paint or stain will require more maintenance and upkeep than expected.

Cost Of Initial Investment and Maintenance of Retaining Walls

Wooden retaining walls have a lower initial cost, and are the smart investment if the retaining wall is not going to be around for ever. Plans to redevelop or change the outdoor space and landscaping in the near future make a wood retaining wall a smart financial decision.

However, if a wall is likely to be needed for the next few decades it may be foolish to save money upfront and pay double in the long run. These decisions should be weighed heavily if there is no clear winner in the beginning of your buying process.

Your home’s value should reflect the investment in the landscaping. More home inspectors are starting to realize that significant oversights to the state of a home’s infrastructure are not tolerable to home buyers.

Allan Block retaining walls have a higher initial cost and win in almost every category, including overall lowest price.

Maintenance of Wood retaining walls vs. Allan Block retaining wall is a wash, literally. Washing your walls to keep them looking their best is the majority of the required maintenance.

The first few years after a retaining wall construction you will want to keep an eye on the topsoil directly behind the wall. You want the soil a little higher than the wall to allow any rainfall to run over the top and away vs. sit on top and create a weight surcharge at the point it has the greatest leverage to overturn a wall.

The drainage pipe will require virtually no maintenance. There should be a clean out cap though in case you want to flush the pipe out with a garden hose every 3-5 years to get rid of any sediment buildup.\

Durability Of Wood Retaining Walls And Concrete Retaining Walls

When both Wood retaining walls and Allan Block retaining walls are installed the same as already discussed, you can expect a lifetime from your concrete retaining wall, and 20-30 yrs. from a wooden one.

What Happens with Allan Block Walls When They Are Not Installed Properly?

  • Visually distracting if crooked or unlevel
  • Will overturn if significant weight surcharge on top of the wall is not part of design
  • Will show signs of movement between blocks, or will overturn if water is not allowed to drain from behind the wall (No proper drainage system)
failing-allan-block-retaining-wall
failing-wood-retaining-wall

What Happens with wooden retaining Walls When They Are Not Installed Properly?

  • Visually distracting if crooked or unlevel
  • Will show signs of separation and localized pushout/blowout if load behind exceed retaining strength
  • Water left unaddressed will dramatically shorten a wood retaining walls life by rotting out the lower sections of the wall

 New Retaining Walls Tend to Be Allan Block / Concrete Option

For the most part when a site is having its first retaining wall installed its usually a concrete Allan Block retaining wall.

This is in large part because of the benefits already discussed.

In most cases the client wants to do it “right” from the start, and wants it to last as long as everything else they are building.

Replacement Of Wood Is 50% Wood, 50% Allan Block Upgrade

Furthermore, when a wood retaining wall is due for replacement it seems like half the time, they are replaced with a concrete retaining wall, and the other half of the time they choose wood again.

Anecdotally this suggests 75% of the time Concrete Retaining Walls are the clear choice, and 25% of the time Wood Retaining Walls are chosen.

However,

100% of the time when wooden retaining walls need replacing, our clients wish the builder installed a Concrete retaining wall right from the start.

Cost Of a Wooden Retaining Wall In 2022

The pricing you are about to read is based on everything above. It assumes the site has easy access for excavators, and skid steers, and is within 300’ of truck access located in Abbotsford B.C.

How Much Does A 75’ X 3’ Wood Retaining Wall Cost?

Ground contact rated wood dimensions 3 9/16” X 10’

Expect to pay $23,912.00*

Ground contact rated wood dimensions 4 1/8” X 10’

Expect to pay $23,982.00*

Ground contact rated wood dimensions 6” X 10’

Expect to pay $25,316.00*

How Much Does A 75’ X 3’ Allan Block Retaining Wall Cost?

Concrete Allan Block with matching concrete capstones:

Expect to pay $27,260.00*

*Tight site access, other site challenges and applicable GST can affect the price.

allan-block-retaining-wall

What Are Maintenance Requirements of a Retaining Wall?

Allan Block walls have relatively little yearly maintenance, however as with any excavated area some settling is likely to take place over the first year or two.

It will be minor if properly compacted when the area is backfilled.

Should there be any unusual or excessive settling these areas should be corrected. Low areas will allow water to pool, and could make the problem worse over time.

Correcting grade issues early will ensure you get a lifetime of performance from your Allan Block wall.

After construction It is important to finish the landscaping around your wall.

Vegetation, whether that’s grass, or plants, is a great way to control erosion, direct water where you want it, and generally sturdy up the soil surrounding your wall.

Weeds or vegetation growing in your wall are possibilities, and if caught early are easy to pull before they establish. Otherwise spraying them with a roundup or selective herbicide just like your lawn, will be effective if done annually.

Annual spring inspection of the wall and surrounding area should alert you to anything out of the ordinary.

If a wall is not properly installed, or an adequate drainage system was not installed, you may have some movement in the wall.

Early recognition or hiring a qualified contractor can correct or prevent this from happening in the first place.

What are the pros and cons of a retaining wall?

Pros Of a Retaining Wall

When retaining walls are built correctly:

  • They prevent soil (retained soil) from eroding and coming into your yard
  • Keep surrounding structures (like flower beds)  safe from elements that could damage them
  • Prevent flooding in your landscape (proper drainage) 
  • Great decorative feature for any yard (Walkways can be put in in front of the wall system) 

Cons Of a Retaining Wall

  • The retaining wall can fail causing damage to property
  • Can be costly to redo the retaining wall, causing a headache and possibly financial strain
  • If you have a wood retaining wall the lifespan of the wall is 10+ years, meaning eventually it will need to be replaced

Keep in mind that these cons are a possibility if the wall is not built correctly, what type of wall material is used, the height of the wall, whether or not a professional engineer provided plans, and if proper drainage was added. 

allan-block-wall-with-fence

Problems with retaining walls

 Allan Block Walls Are Usually Not Built Correctly

This is the FIRST problem with retaining walls. THEY ARE NOT BUILT CORRECTLY. How can we be sure that retaining walls are not built correctly? Just take a look at any neighbourhood and you will see dozens of walls on the verge of collapsing, or that have already collapsed. 

Almost every contractor builds their retaining wall differently. The methods that they use to build their retaining walls are based on speed, ease, cost, and general practices.  

Let’s say that the retaining wall that is being built for your outdoor space is made out of Allan Block. Allan Block concrete retaining wall blocks have one method, with specific site variations, that need to be used when building a retaining wall. 

If the commercial installation manual is calling for reinforcement, it needs to be done regardless of what the site looks like.

If A Retaining Wall Is Not Built Correctly What Happens?

Retaining walls that are not built correctly have varying levels of failure, from minor separation between the blocks, to catastrophic failure where large portions of the wall fall over.

Allan block retaining walls are a mortarless system, that interlock in a dry stacking pattern. They are flexible in nature, and unlike a solid concrete structure will not form cracks.

However poor retaining wall design, installation method, or any other cost saving method will result in failure on some level.

 Anything outside of Allan Blocks allowable movement for a flexible structure is a failure.

When a homeowner is spending 10s of thousands of dollars there should be a strict standard for quality.

Retaining walls are a big investment when built to best practices. Not only should they do their job of holding back hundreds of thousands of pounds of force (retained soil), they should be straight, true, and an aesthetically pleasing center piece in your outdoor space and landscaping backdrop.

 

Cheating The Excavation Area – Limiting the MASS Of Your Wall

Hardscape Retaining Walls require a tremendous amount of excavation and in most cases soil disposal.

Geogrid is used to combine the weight of the blocks, gravel and soil behind the wall.

This large amount of mass is what is actually resisting the load on the wall. It should almost NEVER be just the retaining wall blocks holding the weight load.

The weight load could be as simple as soil, a parking spot, or a road. Each scenario requires a different amount of load bearing capacity.

In general, 60% of the wall height is used to determine how far back needs to be excavated to install the Geogrid.

The picture shows Geogrid locking all of the mass of the wall, BLOCKS, GRAVEL and SOIL together.

geogrid-retaining-wall

Unsuitable Backfill Material – What to Backfill a Retaining Wall With?

This has a direct correlation to the previous problem (excavating and soil disposal). The majority of the soil that is hauled offsite needs to be replaced with Gravel.

This is why so many contractors cheat the excavation zone, and use UNSUITABLE BACKFILL MATERIAL. (Like using soil). 

If 2 loads of soil are hauled out, and 2 loads of gravel are hauled in, this quadruples the backfill material costs when compared to not removing soil at all which is by far the most common INCORRECT method.

When your comparing proposals this will be very apparent in the pricing.

We tend to take retaining walls for granted, and expect them to stand forever because they are made out of concrete.

Concrete is a great building product, but if we expect them to last, we need to do what we can to ensure they don’t fail. ALMOST EVERY failed retaining wall is because of the lack of gravel behind the wall.

Allan Blocks requires a minimum of 16” of clear draining rock, OR 12” of No Fines Concrete directly behind the wall, AND prefers the infill soil area shown in this picture to be a compactable material, such as ¾” Minus or road-base.

You will almost never find these 2 types of gravel behind your wall without hiring a qualified contractor.

If your wall is not built using proper backfill material you will have a problem with your retaining wall.

No Drainage System – Does My Retaining Wall Need a Drain?

We all know how much water can affect the weight of something. Multiply that over the length of your wall, for several feet behind your wall, for its entire life, it’s no wonder why you should want a proper drainage system behind your wall.

If you have a small wall, and Geogrid is NOT required than you really only need 1 pipe. This is called the Toe drain in the photo.

If you have a larger wall and Geogrid is required than you will need two drains, the Toe drain, and the Heel drain.

retaining-wall-drain

Common problems with retaining wall drainage are that there either no drains at all, pipes are not protected from migration of fine materials (think clay soil plugging up the pipe), or the drains don’t drain to a good location.

The best method is to connect directly to your existing drainage system or storm connection.

Having this water pool in your yard somewhere will cause you long term problems.

Redirect the water away from your wall, and get it off your property or at least to an acceptable designed area.

Remember water freezes in the wintertime, and will cause movement in your wall if it’s not drained away

Filter Fabric – Do Retaining Walls Need Filter Fabric?

Without a doubt the most misinformed topic with retaining walls is filter fabric.

Filter fabric does just like its name suggests, it filters something.

There are 2 main types:

The first is a fabric that claims to allow water to pass through, and the 2nd is a tested fabric that actually does.

It is not uncommon for a filter fabric to claim to have a flow rate of 100 gal/min/ft. when they may not even allow 5 gal/min/ft.

In a typical scenario, filter fabric will only be used at the top section of the wall to filter the 11” of soil placed on top of the wall.

This keeps soil from migrating down through the clear drain rock directly behind the wall, and equally as important, it allows any water to pass through into the drainage zone.

In the diagram you will notice the fabric on the 2nd row from the top extending back past the midway point of the backfill point.

reinforced-retaining-wall

How High Can Retaining Walls Be Built? 

The simple is you can build a retaining wall that is less than 4’ without a permit or engineering, and almost as high as needed with professional engineering and a city permit.

There are some caveats though! To fully understand how high you can build a retaining wall, we need to understand  3 main factors.

The 3 main factors are city building requirements, the type of soil on site, and the retaining wall block manufacture specifications.

Most cities in the Fraser Valley allow walls under 3.9’ to be built without a permit or engineering.

If the wall is over 3.9’  a building permit and engineering are required. Some cities also have a bylaw stating that you can’t build a large wall if it doesn’t fit with the neighbourhood.

The 2nd factor is the onsite soil type. Soils that are heavy clay retain water, and are very heavy.

This may mean that structural precautions and overbuilding may be a really good idea, even on a smaller wall under 4’ in height.

You may want to build a wall to an engineered standard even if it is not required.

The 3rd factor is closely related to the site soils. Manufactures like Allan block have a chart that says the wall height for common applications.

These wall heights can be as low as 13” tall when they start to require additional structural components.

To summarize, there are a lot of influences and factors that set the maximum wall height.

Some retaining walls can be simple and built to a height of 4’, while others will need some structural support for walls that are only 13” tall.

For every retaining wall you should at least consider what forces will be applied to this wall, as it will need to stand up to them for a lifetime.

What questions should I be asking potential contractors?

 

When Hiring a Contractor to Build Your Retaining Wall, Homeowners Should Be Sure to Ask Them the Following:

 

  1. What courses have you taken related to retaining wall builds?
  2. What material do you plan on building with? (Wood retaining wall or concrete retaining wall)
  3. How much soil are you going to be hauling out of my yard?
  4. How much gravel are you using, and will you be filling the back of the wall? (Reinforced retaining wall) 
  5. Will you be burying the first row of the retaining wall blocks?
  6. How do you ensure that my retaining wall and caps are straight?
  7. What machines do you use?
  8. What is the timeline for this type of project? (You do not want a contractor who thinks that they can complete a wall in a day or two. This is a red flag that they are wanting to be in and out as quickly as possible, and will cut corners to do so.)
  9. If needed, will you hire a professional engineer to draw up the plans? 

 

When having any project done at your place, be sure to do your research, and learn best practices for the job you are wanting to have completed. By being educated, you can find a contractor that is a great fit for you and your family, and enjoy the beauty of your yard for years to come.

allan-block-retaining-wall

When having any project done at your place, be sure to do your research, and learn best practices for the job you are wanting to have completed. By being educated, you can find a contractor that is a great fit for you and your family, and enjoy the beauty of your yard for years to come.

 

At back40landscaping we build our Retaining wall systems with methods and materials that will last homeowners decades.

We hope this guide has given you greater insight into what to expect with a retaining wall installation, and has answered most of your questions regarding Retaining wall systems.

Thank you for taking the time to read this guide. We look forward to helping you with your Retaining wall installation!