Our goal at Back 40 Landscaping is to educate our customers so that they are able to make informed buying decisions that work for them and their family.
The purpose for writing this article is to answer all of your questions about retaining wall construction.
Most of this will apply to all types of retaining walls, however this is written with Allan Block retaining walls in mind. As you read through this you may be made aware of many answers to questions you didn’t even think of.
Back 40 Landscaping is located in Abbotsford B.C, Canada and we primarily focus on Allan Block Retaining Walls, Paver Patios, and French Drain installations.
What makes us experts on Allan Block Retaining Walls?
Allow me to introduce myself. I am Sam Maerz, and along with my wife Natasha, we own Back 40 Landscaping. I am a Trained and Certified Level 2 Allan Block wall Installer.
To attain this level of certification is a combination of classroom training, field training, and job site experience, covering all of the most challenging aspects of building a retaining wall.
Allan Block Contractor Training Certification Training Course
The advantages of the Allan Block product line
How Allan Blocks work
Basic Principles of wall design
How Allan Block & Geogrid reinforcement work together
Basic design methodology
Why walls Fail
Wall estimating tools
Proper methods for constructing an Allan Block wall
Hands-on participation building walls with AB products
Written exam
Completion of a wall with min height of 3’, and more than 300 units
Complete an additional 5 walls that meet the following criteria
Walls comply with local building codes
Walls constructed meet industry tolerances
Walls have a min height of 3’, and more than 300 units
Geogrid reinforcement was used on a minimum of 3 of the 5 walls
Stamped drawings from a local engineer on a minimum of 3 of the 5 walls
Walls submitted must contain:
Curves (inside or outside)
Step up or step downs
Slopes above the top of the wall
Terraced wall
Water management detail
Complete additional walls totalling a minimum of 25,000 blocks, or 10 walls with an average of 1000 blocks per wall.
Walls must meet the above requirements
Geogrid reinforcement was used on a minimum of 70% of the submitted walls
Stamped drawings from a local engineer were used on a minimum of 70% of the walls
Each of the following conditions must be covered on 70% of the walls submitted
Curved
Step up/down
Slopes above the wall
Terraced
Water management detail
Letter of reference from a minimum of 2 general contractors/property owners/developers
Our team consists of Sam, Viktor, and Lindon.
We are all Allan Block trained and certified!
Out of all the training and resources Allan Block offers, without a doubt, the most valuable is being able to call Bruce Stickney.
He is a highly knowledgeable veteran of the retaining wall community, and we are lucky to now call him a mentor and friend.
He officially works for Allan Block as their trainer and liaison, and contractors who want to can build their customers the very best Allan Block retaining walls possible.
Retaining wall drainage is a system, or multiple systems, designed to remove water away from behind a retaining wall.
It is can be preventative, like a drainage swale where the ground is graded to move water away from the wall, it can be clean gravel with void spaces so the water freely drains through it and away, or it can be gravel with a drainage pipe.
In most cases, a retaining wall drainage system will be made up at least 1 or multiple methods of removing the water safely away from the retaining wall.
Drainage is crucial to a long-lasting retaining wall for multiple reasons.
The first is to reduce the weight of the soil or ground behind the retaining wall. Wet soil is much heavier, and therefore should be drained to reduce the soil weight as much as possible.
The higher the retaining wall the greater leverage is exerted against it, and the more crucial the drainage system becomes.
Water causes long term damage to any structure, whether that’s erosion, frost heave, or general deterioration to materials or stability of the surrounding soils.
Every kind of retaining wall would benefit from a drainage system, and most certainly ones that are higher than 12” in height, are retaining poor draining clay soil, or have additional weight loads on them such as a slope above the wall, driveway, or other additional weight loads.
In cases where a wood retaining wall is built, water should be removed from behind and underneath the wall, so that the wood does not prematurely rot out, or fall over from an unnecessary weight surcharge.
An Allan Block retaining wall does not degrade in water like wood, but would be subject to frost heave, or expanding water if the water is left to freeze in the wintertime.
As water expands in and around the wall it can cause movement of the blocks, stairs can become uneven, and even become a tripping hazard.
Allan Block walls are designed to handle the effects of rain and snow, and should be built with a drainage system to allow them to handle the weather around them and last for decades.
Allan Block retaining walls should be built with clear gravel.
Clear gravel is defined as having uniform sizes only. ¾” clear only contains ¾” size rocks.
When you use uniformly shaped gravel compaction is easier attained, as there are no smaller rocks to nestle in amongst the larger stones.
A high rate of compaction is achieved simply by placing the rocks on the ground (as long as they are contained).
If you were to use different sizes of gravel, the rocks would have to rotate and move around to find the tightest fit (compacted).
This takes longer to achieve full compaction, is harder to measure as you can’t visually see if its compacted, and puts the wall at risk of settling as it further compacts over the years.
The specifications on the type of approved gravel are very clear to mention that you do not want to use a gravel with fines, or “minus” material.
This type of gravel requires a lot of compaction, and when compacted is very slow to drain water through it.
The fines can plug up any drainage pipes underground, and in general will not drain water away from the wall which is what you want.
Clear gravel is angular in nature, and locks in place. Round rock does not lock together, and is not a suitable gravel to use under or behind your retaining wall.
Clear gravel can be any size between .5” – 1.25”. The most common size used in the Fraser Valley is ¾”. Because ¾” clear gravel is used so much its readily available, and competitively priced.
Drainage zones underneath and behind an Allan Block wall will vary on application. However, a typical or minimum number is 6” of ¾” clear gravel below the wall, and 12” behind the wall.
The typical or minimum amount of gravel to use is a starting point, and can quickly change by specific site conditions. Poor draining onsite soils should prompt you to increase the drainage zone, or any other weight surcharge like a shed, slope above the wall, parked vehicle etc.
A pro tip is to consider using gravel much further behind the retaining wall.
On engineered walls you will have to excavate much further behind the wall to place the geogrid.
While you can use soil as backfill on top of the geogrid, you may want to use gravel.
The backfill material has to be compacted, and soil is virtually impossible to adequately compact in a timely manner, or if its wet.
Make sure to plan accordingly prior to the project begins, as this is a huge change to the scope of work in terms of excavation, soil disposal, gravel purchasing, and importing.
Placing drainage gravel under or behind a retaining wall might seem not worth talking about, but considering how much gravel is needed, it quickly becomes a logical challenge to do right.
This will be covered more in greater detail later, but importing enough gravel requires special equipment due to the weight and volume of the gravel required. You can’t simply use a wheelbarrow.
All gravel used under or behind a retaining wall needs to be compacted to prevent future settling, and to strengthen the wall by locking the block and gravel all together.
It’s important to note that the blocks by themselves are not what retains the soil. Even though they weight 70lbs each, they are far too light to hold back the soil.
When you combine the weight of the blocks, and gravel together you double the weight, and now can retain much more soil.
You combine the weight of the blocks together, by compacting the gravel and blocks together. Yes, you run the compactor on top of the blocks, just be sure to use a narrow smaller compactor.
This is an often-overlooked step even by long time retaining wall builders.
Make a minimum of 2 passes on top of the blocks, and extend all the way towards the back of the cut.
All compactors have specific depths of gravel they can compact. Smaller 100-200lb compactors can do about 6-8” of gravel, while larger 1000lb compactors can do up to 24” thick.
Within 3’ of a retaining wall you should use a small 100-200lb compactor. Beyond 3’ you can and should use a larger heavier compactor.
The purpose of a drainage pipe behind the wall is to provide an outlet for the water to go, as well as a dedicated void space that can be cleaned or emptied should the drainage gravel ever get plugged. Just be sure to include clean outs every 50’, and discharge the pipe to daylight.
The 2 most common drainage pipes are “Big-O”, and PVC. Both pipes are 4”, are perforated (slits or holes) to allow the water to enter them, and approved by engineers and municipalities.
Big-O is a slang term for thin walled, corrugated, flexible drain pipe. It’s the lowest cost, easiest to work with, but crushes easily.
When burying with gravel you cannot step on it, and must slowly bury it to prevent crushing. Over time the weight of the gravel, and soil may crush it.
PVC perforated pipe is more commonly used due to its strength, but because its rigid takes much longer to cut, prime and glue any fittings for direction changes.
It’s much more expensive than Big-O, and during the Covid Pandemic was 4x the normal cost, IF you could find it. It’s more readily available as of January 2023, but has doubled in its normal cost.
There is an upgraded pipe called Baughman Tile High Octane. This specialty pipe contains all the very best pros of both other styles, and in B.C is an exclusive pipe of Back 40 Landscaping.
Installing a drainage system, whether that’s just gravel, or gravel and a pipe, is by far the hardest part of building a retaining wall.
Excavation of the “extra” soil, the 12” minimum creates a lot of digging, moving it to the front yard in most cases, trucking, and paying to dispose of it. Then you must buy, truck, and move the same amount of gravel back in its place.
Because of the cost, labor, and knowledge of why it’s important, its very frequently overlooked, skipped, or missed.
I’ll give a quick example to illustrate the labor and cost involved with installing a drainage zone.
Retaining wall 3’ high, 60’ long.
Without a drainage zone:
10.11 yds3 of soil removal
No gravel importing
Total: 10.11 yds3 (61 wheelbarrows, 1 full size dump truck, or 10.5 pick-up trucks)
With a drainage zone:
Cost of pipe, and labor to instal
20.23 yds3 of soil removal
10.11 yds of gravel imported
Total: 30.34 yds3 (182 wheelbarrows, 2.5 full size dump truck, or 30.5 pick-up trucks)
It’s completely understandable that if your using wheelbarrows at some point you’re going to want to cut back or eliminate the drainage zone, especially considering retaining walls are always needed for elevation changes (you’re not wheelbarrowing uphill for long).
We can say with all certainty that a retaining wall built without a drainage system will at some point show signs of failure. Failure is defined as either structural, or aesthetically.
For Allan Block retaining walls structural failure can be caused by excess water weight pushing a retaining wall out, or over.
It can cause the foundation to erode, and the wall to sink or dip in places. Water movement causes erosion, and if left unchecked will cause varying levels of failure.
For a wood retaining wall excess water will present the same weight issues, but as well will cause it to rot out much faster than it normally would if there was adequate drainage.
Allan Block walls can aesthetically fail when water has caused blocks to open up, or separate. This can happen during winter when water freezes and expands. Blocks may go back to normal, or they may permanently stay out of place.
Often time these gaps will then start leaking out dirt and be a constant eye sore. While not structurally deficient, a retaining wall that is moving will not perform as a focal point that it did, or should be.
Straight, true retaining walls are huge focal points. They are the holy grail in landscape design because they have height, depth, and mass.
Maximize your investment by getting all the benefits you deserve. A strong, beautiful lifetime lasting retaining wall.
Retaining walls are shockingly much more complicated than I ever realized before learning how to build them. The Allan Block Engineering manual for instance is 140 pages long, most of it contains mathematical formulas I don’t have the time to understand, IF I ever could! So, for the purposes of this article, I will keep it simple, and talk about the most frequently occurring retaining wall strength issues and solutions.
Curved retaining walls are stronger than similarly built straight walls. That’s not to say that straight walls aren’t strong enough, just know that a curved wall isn’t weaker, it’s stronger.
Knowing curved walls are stronger may not change the design, but it is a good back pocket bit of knowledge when you need to add some extra strength to a wall if your limited in access and can’t otherwise reinforce a wall.
An added bonus is when curved walls are not out of place, they look way better than a straight wall.
Unfortunately though there is significant extra work on cutting the caps and making them fit good.
A normal straight wall doesn’t require any cuts expect for the last cap, where as a curved wall may need two cuts per block. That’s about 5-10 min per cap in total to fit and place them well.
Gardner’s rejoice! Plants can add strength to a retaining wall.
How to plant near an Allan Block retaining wall:
In most cases you will have 11” deep of soil to plant in (the rest is gravel)
Choose plants that can survive and thrive in 11” or less of soil depth
Be cognizant of excess watering on top of a retaining wall
Plant so that at full maturity the canopy of the plant is not encroaching on the wall
Choose plants that are recommended by a local horticulture who can see a photo of the retaining wall and measurements
Choose plants that do not have an aggressive/damaging root structure
Almost all trees are damaging to retaining walls if planted to close. Ensure at full maturity the canopy of the tree stays a safe distance away from the front or top of a retaining wall.
If you have an existing tree to work around it may not be possible to avoid it completely so using extra gravel in the area can help to “air prune” the roots, and cause the tree to avoid the wall.
Trees are very damaging and in the planning stage of the wall building suitable plans should be made to ensure the retaining wall is not damaged or effected in the future.
Retaining wall block sizes makes a difference in terms of strength and staying power. Allan Block retaining walls are segmented block walls, meaning lots of individual blocks make up a greater structure.
The fewer joints, or independent blocks you use the stronger, and more uniform the wall is.
It is best practice to use the largest block available in a product range for the base course, then on consecutive courses, patterns, or smaller combinations of blocks can be used.
A classic size Allan Block is 18” wide. This is the size of the base course blocks, and usually the blocks that make up the rest of the wall.
There is a half width block available and these are used frequently by homeowners because they are much lighter.
Professionally though they can be used when you want to make a tighter radius curve. Being narrower than the full blocks they curve much tighter. A combination of the large and half blocks can be used, however strictly using the half blocks in most cases is not recommended by professionals.
During general construction it’s advised not to use tiny cuts. Instead cut down several surrounding blocks to ensure the smaller block is as big as possible.
The bigger the cut block is, the more surface area it has to sit straight, level, and not visually stand out as a tiny cut.
Stairs should be planned out as best as possible to avoid small blocks. This is especially important, as stairs are actually individual walls (they don’t have a 2nd row to hold it in place).
Without a doubt the most common issue we face when working on retaining walls is space issues.
This means if its awkward, or we can’t get our equipment in its not going to be as strong of a wall.
This is usually noticed on staircases where equipment wasn’t able to bring in lots of gravel, or compaction had to be done by hand.
Common wall issues are found in corners, stairs, remote areas where equipment couldn’t reach, or anywhere else the wall isn’t built to the same standards as the easily accessed areas.
It’s not impossible by any means to build strong walls with limited access, it just takes longer, and costs more.
Be aware of this, and discuss this in the planning stages, as the contractor will need you to provide the financial assistance so they can build you the wall you deserve and are expecting
Financial discussions can be awkward, and for some reason a little taboo. While we can’t change that, we can at least point out some main points to consider, and be aware of.
Financially stable companies are easier to deal with and are the safer choice. They will be more reliable, trustworthy, and upfront.
You can expect to get what you pay for, and have your project completed as per your agreement with your contractor.
In the old days a lot of contracting work was completed before payments were ever made. This has changed over the years for some companies as material prices have increased.
Your deposit payment with Back 40 Landscaping pays directly for your material. Your material is never charged to an account for payment “later”, it is paid for in cash. This includes all material, trucking, and every other aspect of our business. We are a debt free company.
Your subsequent payments pay for the remainder of your materials, and the labor to complete your project.
As long as you keep up with the scheduled payments outlined in our terms of service, your project continues to progress until completion.
This simple approach is designed to ensure your project can be completed to your expectations, and allows us to focus on building you the best retaining wall we can.
Depending on who you ask, you will get different answers on the financial state of any company.
Ask the customer, and you’ll hear they charge to much, ask the owner, and they don’t charge enough, ask the accountant and they will say they should double their rates, or ask the banker and they will have never heard of them.
Perspective is the key here, as the numbers are all the same for these 4 different opinions.
In simplest terms, the business needs to be very aware of their overhead, expenses, and labor. They should double and triple check this before pricing any work.
The service industry is not a commodity, meaning it’s impossible to compare apples to apples.
People, skill, equipment are all variable.
If one company charges $110 / labor hr, and another charges $120, who’s the better value? One has a higher price, while the more expensive one could have an extra piece of equipment that completed the job much faster.
Add in other variables like work ethic, experience, skills, training, methods of construction, equipment breakdowns, weather, etc. and you should start to see it’s impossible to compare the service industry like a commodity.
A pro tip is not to get hyper focused on price. Overall cost is much more important.
This includes:
Communication
Time onsite (how fast you can get your yard back)
Personalities of Installers
Equipment and tools
Training and Certification
Experience / Skill
Google Reviews
For us at Back 40 Landscaping it is a tactical business decision to purchase the equipment and tools we have. For us it’s all about doing the project better, and more efficiently.
In today’s job market skilled craftsman are hard to find, and more importantly to retain and keep happy.
Homeowners do not want a craftsman running a wheelbarrow and blowing their back out pushing it up hill all day.
So, we have tools and equipment to make our jobs easier so we can focus on building the best retaining walls we can for you.
It takes a lot of specialty tools and equipment to build a concrete retaining wall, so in most cases we would suggest making sure the contractor you work with owns that equipment.
Ideally this equipment is owned outright so you know they will have access to it anytime they should use it on your project.
Renting some tools or equipment is ok, as long as it’s rarely needed, and readily available. For instance, we own 5 compactors, but once or twice a year we rent a different size than we own.
On a daily basis a lot of equipment doesn’t get used, until it does. Then you need to own it, and have it sitting idle ready to go to work.
Another bonus to owning is you know your equipment, and there is no learning curve.
Don’t tell my wife, but a lot of tools aren’t actually as advertised, and you don’t learn this until you use them in a wide variety of situations.
You must trust your contractor, and your contractor must trust you. Both parties have important roles to play in the successful completion of any project.
If you plan on hiring a contractor to “work” for you, you will have problems, instead working together.
Here is a quick breakdown of the client and contractors’ roles
Client:
Communicate openly and honestly the entire time
Provide input during the design phase
Be available to answer questions when things come up (they always do)
If you have an issue at any time communicate it immediately
Take care of the finances, and pay on time
Contractor
Communicate openly and honestly the entire time
Provide input and guidance during the design
Be proactive with as much information as you can
If you have an issue at any time communicate it immediately
Do at a minimum what you agreed to, and if possible, exceed expectations
Respect the finances, pay off all materials, labor, and stay in business for your customer
Trust your contractor to either protect surfaces or repair them afterwards.
A great example of this is your lawn. Heavy equipment running over your yard for days or weeks will rut it up over time.
Your choice would be for us to instal temporary plywood roads. Currently we can run about 400’ on plywood roads. Or, we could repair the lawn afterwards.
Usually, the decision comes down to the surface, and what is most economical. Hard surfaces like driveways, sidewalks are almost always protected, or avoided all together if possible.
While covering every issue ahead of time is impossible, there are common issues when it comes to building retaining walls.
On our site consultation we go over the major components such as parking, material staging, equipment parking and access, and where our portable toilet should go. Please openly communicate all questions, concerns, and requests at any time.
If we are excavating during the dry season, the space we are working in will be dusty. There are a few months near the end of summer where we usually have several inches of very fine dust similar to icing sugar.
This gets stirred up, and will blow several houses down the road. It can get all over your home, and even inside a little bit. Keep doors and windows closed during these times.
We can wet the areas to suppress the dust, but midday we usually can’t get enough water down fast enough before it dries.
Cutting blocks with a gas-powered saw is noisy, and there is no way around it. We work during normal hours, and try to keep the noise down early in the morning, but on some days the saw may need to start up at 7 am.
Communicate any noise restrictions, and we can work around them no problem.
Most of our equipment runs on diesel engines. The noise isn’t too bad inside the home, but outside may be as loud as a lawn mower, although maybe not as annoying of a sound.
Our equipment runs ¼- ½ the time we are onsite. During the beginning stage when we are excavating it will run all day, while other stages it may run very little.
Change orders are changes to the contract, that you agree upon prior to the work being done.
This can be optional add Ons, or project changes due to unforeseen site conditions.
The strangest example we can think of was when we uncovered buried drywall from the 1970’s when excavating for a retaining wall in 2021.
There were additional disposal costs for the contaminated soils.
Change orders are documented and paid for prior to the work being completed. Live signatures may be possible if you are home, or we can email you a DocuSign for completing at work. E-Transfers, cheques, cash or credit cards are accepted.
To ensure a successful, low stress experience, we need to preplan your project. The timeline, material choices, colors, etc. are all components of a successfully preplanned project.
While most of this is behind the scenes for us, you have an input role of communicating your expectations, and concerns.
Building you a focal point retaining wall doesn’t just happen without a plan and your input!
Trust your instinctive emotional response to what you are learning about retaining wall construction. If a contractor is not a good fit, neither of you want to work together. That’s ok!
If you have a concern, you should be able to discuss it, and confirm if it’s actually a concern, or just a misunderstanding.
In person is most effective, while phone calls are most efficient. Texting should be avoided on major issues as it causes more confusion due to lack of tonality.
Google Reviews are a great way to vet a company, and learn what it’s like to work with them from a customer’s perspective.
Reviews will vary from just a star rating, to a rating, photo’s, and a long write up. If you read them well you should get an InSite into the character of the team.
Did they go the extra mile somewhere, or work late to stay on a timeline? Customers notice this, and usually mention it if they had a great experience.
When you hire Back 40 Landscaping, you will be asked to leave an honest, and accurate review.
Retaining walls don’t just get stacked up. Surprisingly the blocks are not actually all the exact same sizes, so while building the wall small touches and fixes are constantly being done. This is why some stairs, corners, or long walls look slightly off.
They may not be structural issues, but in order to be a focal point, they need to look good.
A good work ethic, and integrity is essential to ensuring a great wall is built.
Training and Installer Certification from Allan Block ensures the builder has the knowledge to build the wall.
When you hire a company, you should know who is doing the work.
Are they trained, certified, and experienced? At Back 40 Landscaping Sam, Viktor, and Lindon will be the ones building your wall.
As the foreman in the field, I am fortunate to work alongside both Viktor, and Lindon. I trust them with my life and family. Now this may seem like an overstatement, but as a retired structural firefighter I’m used to either trusting people completely or distancing myself from them.
Everyone on our team has proved time again they can be relied upon, and are of utmost character. We simply do not hire, or work with questionable people.
The weather plays a huge role in our field work. Most of it can be worked through, but it certainly presents a challenge.
During the rainy season we tend to work more off of plywood roads so we don’t bury our equipment in the mud, while during the dusty conditions in August we soak the ground to reduce the dust.
Compaction is a great real life trust issue. If it’s not done right your retaining wall will sink, or settle. This is apparent even on large commercial projects where multiple oversights failed.
During the winter months if the ground is frozen you cannot compact. The ground just won’t respond to compaction equipment.
You must have faith and trust your retaining wall builder will work around, or address this issue. Watching the forecast, and planning ahead, or switching to another task will ensure you compact when the ground is workable. Trust, its important!
Gluing your retaining walls caps down is a critical step to ensure your wall looks its best, and last for many years before it needs any maintenance.
Gluing can’t be done if there is a layer of ice on your retaining wall blocks and caps.
There are 2 types of glue that are designed for dry weather, or rain/cold weather.
Many days we have had to postpone gluing caps in the winter due to freezing conditions. It can be inconvenient, but it’s the right decision so you do not have to call us back to reglue caps in a few months.
The physical stress of building a retaining wall can be enough without adding weather conditions. The rain, heat, and cold can make a physically demanding project that much harder.
Long days outside lifting 70lb blocks (1000’s of them), takes a special person who is hard working, smart, and dedicated to building you a quality wall.
Fortunately, when you hire us, we have a team ready to get to work.
Building efficiency benefits everyone involved. The sooner we can build your retaining wall, the lower the cost for you, and more projects we can complete in a year. Becoming efficient however is the hard part.
A few years ago when we were addressing this we realized that specialty equipment really does pay for itself. We have 2 diesel powered track dumpers that can handle 3000 lbs each.
We are able to swivel the hopper, and dump gravel exactly where we need it. This not only saves time, but in at least 50% of our jobs I don’t know how you could get that much gravel behind our walls.
Another great example of increasing efficiency is our screeding bars system. This produces a laser perfect gravel bed for basing an Allan Block. Its fast, and exact.
Did you notice how in both of these examples not only was time saved, but quality went up?
Cutting corners or cheating an install is not being more efficient. Instead, you need to think outside the box sometime to become more efficient, and do quality work.
In 2023 pricing should be on every website on the internet. There is no excuse not to. We have access to enough technology to get accurate “ballpark” pricing. Exact pricing is determined onsite, but getting range to with 10% or so is very common.
Please check any of our Services pages, or read any of our cost articles. We aim to give you an idea of what your project could cost, anytime you want to read!
If you haven’t noticed by now, we are very passionate about educating homeowners.
Natasha writes the vast majority of our articles, but from time to time I get to sit down, and pour out what I hope is useful, helpful content that helps you make an informed buying decision.
In full transparency, we most likely want to work with you, but if you choose to hire someone else to build your retaining wall that’s ok.
We will both feel much better knowing you had access to every piece of information about retaining walls, and made an educated decision.
If you only catch a little from this article hear this
Retaining walls needs a lot of drainage
Hire a trained and certified builder
Make sure you can TRUST them!
Add Ons to me are fun extra stuff we get to install to bring your retaining wall project to life.
They ideally are planned beforehand, and not added halfway through the project as a way of upselling you on something. They are optional, and entirely up to you.
The essential add Ons to a retaining wall project will vary, and some or many of these may not be relevant to your specific project.
Fence post Anchors, Sleeve-It
If you are planning to have a fence installed behind your retaining wall, we need to install anchors for it while we are constructing the wall.
The drainage gravel behind your retaining wall is part of the “structure” that holds the soil back.
The taller the retaining wall, the more important this zone behind the wall is. Disturbing this zone can cause structural issues.
When retaining walls are reinforced with geogrid (a web like netting) it physically blocks or prevents someone from digging a hole for a fence post. Preplanning is essential when having a fence installed behind the wall.
Installing a fence on top of the concrete Allan Block caps is not recommended.
The caps are 4” thick and while strong themselves are fastened by adhesive. The leverage a fence will excerpt on the caps will cause the glue to break free, and the fence to come down.
The larger and more solid the fence is the quicker the wind will affect it, and cause failure.
Any required site services should be considered before constructing a permanent retaining wall. Pipes, conduit, and gas can all be routed under or behind your retaining wall if planned for in advance.
Even if you don’t need this presently, you may want to add conduit behind the retaining wall should you ever want to add lighting or power plugs down the road.
Upon completion your retaining wall is going to look great! It will be a bold new addition, structurally sound, and your welcome to stop here.
If, however you want to make your new brick wall blend in with nature we are going to have to add some plants, rocks, and mulch to finish the landscaping in front and behind your retaining wall.
As this is essential in my mind we can go off of your ideas, or ours. The next level of plantings is to have this professionally designed. I’ll cover that in the next section.
You really should…
We can get a little excited now, as we dream about how you want to bring your retaining wall to life. Again, to recap, this is optional and as the title suggests I think you really should!
A well thought out, designed, and drawn landscaping design will completely transform the look of your yard.
It costs about $1000-2000 and provides you the comfort knowing the right plants are chosen, planted in the right lighting condition, and they all work together.
This ensures that you start the clock immediately on establishing your landscape. It saves you the time of lost plants, future work and helps you get to your finish line quicker, and easier.
Landscape lighting is the final brush stroke on an otherwise beautiful space.
The pinnacle of landscape design is elevation, depth, color, curves, and lighting. This is because in the evening as the summer light fades your able to highlight plants, and trees in a very warm way.
Block and path lights highlight all the work you have had done, and guide your eye to future points you want to stand out, as well as illuminate your paths and show off that depth in your yard.
Hand rails
Hand railings incorporated into to an Allan Block staircase may be a consideration for you.
In a residential setting they are not very common, but in a commercial application they are much more common. There may be a municipal requirement, however in our experience it has not been enforced.
Railings
Guard railings should be considered when there is a fall potential of greater than 24”.
The requirement is usually dependant on if the space above the wall where the fall could take place from is accessible or not. If it is accessible than this should be discussed. Again, it may be required by the city in which the retaining wall is built, but to date we have not had it enforced.
Fences
Fences or privacy screens on top of a retaining wall can really enclose the space, and make you much more comfortable outside.
Ideally this should be planned prior to construction so structural components can be added when building your Allan Block retaining wall.
There may be additional add Ons that are invented, or thought of over time. Your unique space, and design ideas can make a big difference in your overall landscaping.
We suggest you spend enough time making sure the design is considered, and not just have a retaining wall built.
Retaining walls completely transform spaces when they incorporate add Ons.
Base
The foundation of your Allan Block retaining wall is as you can imagine, critical.
When excavating you must dig down to undisturbed soil. If over excavating has to be done for site services, or other reasons soil cannot be added back to raise the grade. Instead you need to add compactable gravel, until you reach the desired elevation.
Soil preparation for a retaining wall
For a typical retaining wall, will need to excavate 10”-12” of soil. This provides enough room for a minimum of 6” of gravel, 4” of buried block.
Once excavated the ground will be flat, and clean of all organic matter. Remove any big rocks so a compactor can rest completely on the soil. Compact the area as much as it requires.
In some cases, the soil can be flat, while other times you may want a slope to aid in the drainage of the “trench”. Instal any drainage pipe directly on the soil, and at the back of the trench making sure it won’t be under the block wall.
Once the soil is clean, compacted, and the drainage pipe if required is in begin to import the angular ¾” clean gravel. If you have a small compactor import the gravel in small layers and compact.
The bigger your compactor,the deeper the gravel can be before you compact. This is called compacting in lifts. 6-8” lifts are normal for small compactor under 200lbs. a minimum of 2 passes is required.
You should use water to keep the dust down, and to lubricate the rocks as they rotate to lock together.
We call the ¾” chunky gravel the subbase, and the last layer of gravel we call the base.
The base is a thin (1”-1.5”) layer of ¼” clear gravel that is much easier, and better to perfectly level.
You can do this by hand, but using a laser level, and a screed bar setup produce PERFECT results.
With a perfectly prepared base you should be able to base each block in about 5 min, while a bigger rock sub base will take longer.
The size and depth will vary a lot depending on site conditions. If building on poor soil you will need a much deeper, and wider base.
Typically this is about 6” minimum of gravel, and about 30” wide. This gives you room for 12” of drainage gravel behind the wall, 12” of room for the Allan Block itself, and 6” of working room in front of the wall.
The size of this Base trench will affect the quality, lifespan, and cost of the retaining wall.
The bigger the base, the stronger your wall will be (within reason of course).
The majority of your retaining wall design will come from your ideas, and your contractor’s vision.
Allan Block retaining walls have their own unique requirements ,so you will have to rely almost exclusively on your contractor’s knowledge. Their knowledge will either make or break your project.
An example of this would be you need a drainage zone behind your retaining wall, so you cannot put the wall right at the property line, it needs 12” of space behind it on your property.
Another example would be if you’re replacing a wood retaining wall. The wood may be 4” wide, while an Allan block is 12” deep. The physical dimensions, are different than wood and this must be taken into account for.
There are solutions to every scenario though, and this is what makes Allan Block the most versatile retaining wall block.
Pro tip: Caps are not always required. Plants or grasses can overspill the front of a wall, and create a very natural look.
Engineers play a vital role in designing what we call reinforced retaining walls.
Reinforced walls are those that include upgrades to make a normal “gravity” wall stronger. This includes Geogrid, no fines concrete, or other less commonly used methods.
While Engineers understand the physics and mathematics very well, there is an element to building walls that one must learn from building walls.
Ideally your builder will work with the engineer to improve the design right from the beginning, and throughout the project.
A city building permit, along with an engineered set of drawings, are legally on retaining walls over 4’ in height in all of the municipalities in the lower mainland.
On top of the cost for the drawings, and city permit, expect to pay for 3-6 site visits by the engineer, and “memo’s” written for WorkSafeBC stating the safety concerns, and safety guidelines on site.
Usually, the city will do 2-3 site visits to inspect the property prior to construction, a plumbing inspection for the drainage system, and a final inspection confirming the wall was built as per the drawings.
Surveyors are responsible for confirming property lines, and can also provide the elevations of the property.
This is useful for determining the exact heights needed to be retained.
Property lines must always be established prior to the building of any retaining wall. Allan Block recommends this is the property owners’ responsibility.
Every retaining wall will be subject to what we call a “field fit”. This refers to an onsite reality check, when reality meets a drawing on the paper.
This is unavoidable, and common on walls that have access issues, extreme elevations, or just for basic design improvements.
Usually as you build you come across a challenge, and then you have to decide which of 2 or 3 choices are optimal.
Every problem most likely has several solutions all which have pros and cons!
We are fortunate to have a great relationship with Bruce Stickney who is our technical rep from Allan Block.
He is quick to answer calls, jump on a video call so he can see what we see, or come out to the site.
Every retaining wall project we work on Bruce has input before, during or after. He constantly challenges us to be better, and provides coaching after every project. He’s a true mentor.
Cutting Allan Blocks and their caps is always required. At a minimum you will need to cut blocks and caps to length, and frequently you need to customize blocks or caps to make them fit tightly, especially when building staircases.
To cut blocks we use a gas-powered Stihl TS420 saw with a 14” concrete blade. This is a hand held saw that is highly portable, and quick.
To avoid breathing the silica dust that this saw produces when cutting concrete, wear a tight fitting N95 dust mask, and or cut with water if temperatures allow.
If it is freezing, they do make a 2-shop vac setup that can capture the dust before its airborne.
For the caps, free hand cutting isn’t ideal as its impossible to make a perfectly square and plumb cut. If you’re just cutting one end to shorten a cap freehand is acceptable, but if you’re doing 45-degree mitres, or cuts that will be visible you should use a table saw.
Wet brick saws are popular as they have been around for a long time, and are kind of affordable. The newest saw is a 16” dry cutting saw that eliminates 99.5% of silica dust, and Is OSHA / WorkSafeBC compliant.
Factory edges have a 45-degree bevel to them. When you cut a block, you will lose this factory edge. Choose which angle you will most view the retaining wall from, and make your cut on the far side of the block. This will keep your eye from seeing the cut end most of the time.
Plan your retaining wall so that you don’t need to make cuts close to corners, stairs, or other focal points.
This will keep your “center” clean. Adding a block or two and then having the cut moves it a little further away from the area you want it to look its best.
Do your best to avoid small blocks or cuts. It’s a lot more work, but instead of a 4” block, cut 3” off of the blocks on either side. This will turn your 4” block into a 10” block and it will blend in much better.
What we do is cut the blocks on either side down so it maintains a 4” overlap on the seam, and then cut the middle block.
Blocks should overlap at a minimum ¼, which is about 4.5”. To accomplish this the 2nd row usually offsets the first row by half a block.
Typically, a full block is just cut in half, as its quick, and onsite. If the budget and time allow you may want to consider using a factory half block. This preserves all of the clean ends. It looks a little better, but is more inventory, and work to coordinate when ordering.
Half high blocks are great for adding 4” to the wall height vs the normal 8” every row. They also are great for doing step down as they draw out the step downs.
It looks more natural, and it softens the normal 8” step down.
Corners are available in regular 12” long, or professional 18” long. They are the standard 8” high, and are faced on 2 sides. These are used for “outside” corners.
12” Allan Block corners are best used for placing sideways to finish a wall off, or when building step up/downs. Corner step down drop the wall 8” every 12” and is a taller step down compared to the half high’s.
The pro 18” corners are what you would want when building outside corners for staircases.
You don’t have to use the bigger ones, but they do tie the two walls together stronger, and provide 36” of “stitching” the two walls together vs. the smaller only provides 24” of “stitching”.
Pro corners provide a 33% stronger outside corner.
Allan Block retaining walls can be built using a single sized block, or a combination up to 4 different sizes.
Most common is the single size large block, but I personally love the Ashlar patter which uses all 4 sizes.
Allan Block pattern walls have their own unique challenges when building as they are built 2 or 3 rows at a time.
In theory they are built the same, but they do require a lot more preplanning to have enough of all the sizes onsite, as well as a careful eye to not break the patter. They require a little more “fitting” to ensure the seams don’t line up with previous rows. This is especially an issue on curves and when ending a row. Account for at least 30% more time when building pattern walls well.
Pattern walls are just as strong as non-pattern walls. The first row of all walls should be made with the large 17.75” block. Then on subsequent rows you can built whatever pattern you like. There is standard 2-row patterns, and 3 row patterns.
This combination allows you to build whatever number of rows wall you need to.
Yes, the 2-row and 3-row patters can be mixed. If you are building a 6-row wall there is no preference if the 3-row pattern is on the top or bottom.
I.E
base row, 2-row pattern, then 3-row pattern, caps to finish,
or
base row, 3-row pattern, then 2-row pattern, caps to finish
Allan Block is a name for a collection of 3 styles of retaining wall blocks.
The most recognized is the AB Classic collection.
AB Classic collection *Most popular
The AB Collection gives a smooth fluid finish to any outdoor living space.
Enjoy the beauty and durability of this collection’s classic cut stone look that adds distinguished style to any landscape.
AB Aztec Collection *NEW
The AB Aztec Collection offers a smooth, molded face look and style inspired by old Incan walls to emulate a centuries old feel.
AB Aztec is the same dependable system just offering a new look that can be integrated in with the AB and AB Europa Collections.
AB Metropolitan Collection *NEW
The AB Metropolitan Collection offers a mid-century modern style with the same dependable Allan Block system in this brand-new smooth face look.
With clean, sharp lines and minimal aesthetics, this collection focuses on functionality.
Due to the nature of concrete, blocks are not exactly the same size. When building a retaining wall, we do run into this and adjust as needed.
It is very normal for corner blocks to be a bit shorter than their full-size counter parts.
All concrete products from every brand are like this.
Allan Block collection block materials
All of the AB collections are a dry-stack without mortar or footings block system. There is no consumer noticeable difference in strength. They are all hollow core that must be filled with clear angular stone for drainage and strength. The same drainage system should be installed behind each of the different block styles in the Allan Block Collection.
Excavation is the process of digging soil. It can be by hand, or with an excavator. In BC you must call Bc1Call 3 days prior to digging.
Excavation for a small retaining wall 50’ x 4’ will produce a “cut” about 30” wide, and 5’ high. This will be about 30yds or 3 dump trucks that needs to be moved to the road, and then trucked away to a disposal site.
For larger engineered retaining walls, we need to excavate 12” below the surrounding grade, and back at least 60% of the wall height. This means for a 10’ high wall, 6’ behind it has to be excavated, and cut out to make room for the Block, gravel, and geogrid.
On a wall 100’ long, 10’ high this produces about 350 yds3 of soil, or 29 full size dump trucks.
Regardless of how much soil has to be moved on your retaining wall project, its most likely more than your expecting, and definitely more than anyone should attempt with a wheelbarrow.
Excavating and moving as much soil to the front yard as we have to requires small heavy construction equipment.
In the large example above, it works about to about 1400 trips across the yard to move the soil and gravel in and out. Add in another few hundred trips for the blocks and cleanup you can easily be around 2000 trips in and out.
No lawn will stand up to 50 trips, let alone 2000.
To avoid damage, we can put down a plywood road to drive on, or repair the lawn afterwards.
On small project using plywood is the obvious choice, while on larger projects repairing afterwards is usually the smart choice. This is especially true if you would like a new lawn afterwards!
Hard surfaces like driveway are limited in use so we don’t cause any damage to them.
We will however need to stage our porta potty, pallets of materials, and the equipment. No matter how much space we have access to we can quickly use it all up.
We avoid putting soil and gravel on driveways whenever possible.
It’s very hard on decorative concrete to scrap the dirt back up when we load dump trucks. This does however cause an issue. It’s not technically legal to stockpile material on city roads, especially overnight. While not ideal it sometimes is the lesser bad choice.
Coning off materials on the road, and doing a good gas-powered broom cleanup every night helps keep the inconvenience in check. So far!
Plants or trees that are in the excavation zone will usually need to be removed, however if they are important and worth working around, we certainly can.
Running tracked equipment over ground with roots usually doesn’t cause to much damage to trees. It is a possibility though, and something you should be aware of it.
Retaining walls can be built with limited tools, but just like anything if you want to do really high-quality work you need to invest in the proper tools.
The right tools and equipment produce high quality work, helps us give you your yard back faster, and is much easier on your wall builders mentally and physically.
We have found that systems, and process have really helped our young guys pick up the craft quickly, and enabled everyone on our team to do every aspect of building a retaining wall.
We do the bulk of our earth moving with our excavators, track dumpers, and compact track loader. On the street is our dump truck and trailer.
On large projects we subcontract out the trucking to a company that have a fleet of dump trucks.
Specific equipment for Efficiency
Back in the day, or, a few years ago, we used wheelbarrows to move materials that our mini excavator dug up.
It required a lot of labor and wheelbarrows to keep up. In an effort to improve this aspect of moving soil, we found the Wacker Neuson DT10 Track dumper.
Its 31” wide, and carries as much as our big skid steer.
It may look like a tonka toy, but I assure you this thing moves some serious material. After using it for a few months we realized we could never be without it. Seeing as how you can’t rent it locally, we purchased a 2nd unit for redundancy first, and to double our capacity.
One thing I have never understood about equipment is why a company would run old equipment.
We are retaining wall builders, not mechanics, so uptime, and reliability is important to us.
Timeline are tight, and having a piece of equipment run when we need it is nice. Of course, breakdowns are going to happen, but let’s do what we can do reduce them!
There is a pretty big difference when comparing owning your equipment vs. renting equipment to build a retaining wall.
Generally speaking, I’m ok with renting, especially if you rarely use it, or it’s a big-ticket item.
For building retaining walls though I would prefer to own everything we need to use.
A lot of the tools you need sit idle for long periods of time, until you need them. When you need them, you don’t want to have to go get it, or even play the can I get away without it game.
Having all the right tools and equipment only goes so far unless you own it outright.
Back 40 Landscaping is a debt free company, with full access to all of our tools and equipment whenever we need them to build you the very best retaining wall.
Our little unsung hero that we take for granted is our dual slope laser level. This thing is accurate out to 2500’, and makes short work of excavating to the correct depth with our excavator receiver.
Being a dual slope laser, we can set an exact grade % in 2 directions.
This allows us to excavate to water will flow to one end of a retaining wall, AND drain towards the back of the trench where the drain pipe is.
Basing the retaining walls first course is time consuming and difficult to get perfect unless you have a lot of experience.
By using a laser lever, and a screed bar system we are able to grade the gravel to an exact elevation. This not only produces perfect conditions for the installer basing, it makes it so anyone with an eye for detail can actually base on their first day.
It’s great for moral, and allows us to deliver a high-quality retaining wall for you.
A common misconception is that a retaining wall is built simply by stacking blocks. For a good percentage of a wall that’s true, but there is a lot of hand work when building stairs, and details usually on both ends of the wall.
We carry all of the stone masonry hand tools you would expect cold chisels, hammers, levels, as well as rubber, nylon, and sledgehammers.
Our specialty retaining wall tools would be the cutting and coring drills, gas powered brick saw, and our table top brick saw.
Working outdoors we need to protect our tools, and keep your space clean, and tidy. The majority of our tools live in Milwaukee Packout kits, so they can stay dry while not in use.
Our 2nd biggest packout bin is our Emergency medical kit. Fully equipped to handle the most severe injuries onsite, and prepare for rapid transport to the hospital.
Other items in the kit are aimed at preventing injuries, and protection; gloves, earplugs, dust masks, and face shields.
Health and Safety of our staff and clients is taken serious here at Back 40 Landscaping. Safety is all of our responsibility, so if you see something, say something. It’s as much your job site as ours.
As you will notice we have some equipment with poor visibility, and excavations that are tripping hazards.
Please be careful when moving around the site. If possible, please gets someone eye contact when approaching equipment.
If there are any serious safety concerns, we will take appropriate action to restrict access.
Your neighbours will be minorly inconvenience. Less on a small project, but as we all use the street small delays, dust, and noise will occur.
Prior to starting every project, we knock on all your surrounding neighbours’ doors, and give them our direct line should we be able to anything that helps them out. We will make their experience with us as great as we can.
Everyone on our team is safety conscious and trained to protect themselves at all times. Safety is everyone’s responsibility and we look out for each other. We are covered by WorkSafeBC and our premiums are paid.
Cutting concrete produces silica dust which is harmful if exposed to it. All clients and unprotected employees are to stay aware from the area if any dust is present.
To avoid being exposed to harmful silica dust when cutting retaining wall blocks, we cut with water, and dry vacuums.
This reduces the exposure potential, and satisfies WorkSafeBC.
All employees have access to PPE (gloves, hearing protection, dust masks, face shields), and first aid equipment.
If you have made it to the end of this document thank you! We truly hope that this article has answered all of your questions about retaining walls, and how Back 40 Landscaping's retaining wall build process works.
As always, if there is a question you have that was not included in this document please reach out to us via email, text, or phone call.
