Retaining Walls

Retaining wall construction is probably one of the more serious types of work one might do.  Retaining walls typically hold up the earth and other nearby structures.  Should the wall be built incorrectly, it could have major consequences and liability issues to deal with.  Especially if property or people are harmed from Retaining wall block failure events.  Fortunately, we will be covering how to build a retaining wall, retaining wall ideas, when you should use an engineer, Geogrid, and retaining wall maintenance.  

Retaining Wall Ideas & Types

Natural Stone Retaining Walls can be built in a dry stack process(no mortar) or with mortar.  Dry stack walls are meant for shorter and decorative applications and not typically anything structural.  Stone and mortar walls can be built for taller and structural applications while being decorative.

Retaining wall ideas are pictured below as well as some info on different types.  The main types of walls are

  • Block Retaining Wall- Hollow and solid

  • Poured Concrete Wall

  • Timber Retaining Walls

In our opinion, interlocking retaining wall blocks are a better option in most instances.  So we will talk about those first.

       Retaining wall blocks are made in a controlled environment as opposed to poured concrete which can be compromised by uncontrol able factors in the field or work site.  

      Hollow Blocks as the name implies have an open core that gets filled with drainage gravel as the wall is constructed.  The hollow core makes them lighter and able to be 6" to 8" tall

       Solid Blocks are completely solid and most feature grooves or holes to install interlocking pins or glue to lock the retaining wall blocks together.    

Poured concrete walls are good for tight spots, large projects, and everything in between.  The downside to poured concrete is, it is going to crack.  No matter what you do it is going to crack, that doesn't mean it is going to fail.  That's just what happens with poured concrete.  

Timber walls are great for shorter walls and decorative garden walls.  They can last but not in taller applications or abnormally wet conditions.  Timber walls provide a nice rustic look but are not good for true retaining wall situations.

How to build a Retaining Wall

How much do I excavate?

To know how much you need to excavate, you'll need to know the dimensions of your retaining wall block.  for this example, we will assume our block is 18 inches long, 8 inches tall, and 12 inches wide.  Retaining wall footers should be at least

  • 6 inches wider than the face of the retaining wall block

  • 18 inches behind the back of the block

  • 6 inch thick bed of gravel- at least

  • Half of the base layer block needs to be below grade, if not more

For our example our footer would need to excavate a 36 inch wide trench (18" + 6" + 12" = 36") and whatever the length of the wall.  Now the depth:

  • half the block 8 in. / 2in. =4in.

  • 6" of gravel

Minimum depth of footer 10inches and you should always start from the lowest point and build up.

Base Preparation
Retaining Wall Footer

     The most important part of any project is the base or foundation.  Without a solid one, your project is sure to fail.  Characteristics of a good base are:

  • 6 inch deep or more bed of gravel (411's or 57's angular)

  • footer is 6inches wider than the face of the block and 18 inches behind the block

  • undisturbed/compacted native soil

  • geotextile fabric -  keeps gravel from mixing with soil- Optional but HIGHLY recommended - cheap insurance

  • Level base layer of block- VERY IMPORTANT these blocks need to be level front to back ---  side to side --- and to each other get this right and the rest is downhill

  • backfill gravel- washed rounded 57 stone, not pea gravel,

Geo Textile-why you should use it!

Geo-textile is a fabric like material the improves the integrity of the base soils while keeping the soil separated from the base gravel.  The material lets water flow through as well.  The geo-textile should be laid on top of the base soil and extend up the backfill zone acting as a filter.  Cut into strips where geo grid is needed to fit through-this will help keep the drain gravel clean.

Geo-grid

What is geo-grid?  It is a synthetic woven mesh material that gets laid in between the layers of retaining wall blocks.  This helps anchor the wall to the retaining wall zoneGeo-grid is the key to retaining wall install and is required in certain conditions like

  • walls over 4ft tall and over

  • some locations are 6ft tall and over

  • retaining a heavy load

In our opinion, if your retaining wall is going to be over 20 to 24 inches tall you should use geogrid.  It is relatively inexpensive when compared to the cost of the project and what it would cost to repair or replace

Setting Base Layer

Starting at your lowest point, work your way to the higher grade.  To be clear your footer should be nearly level. 

  1. The top layer of gravel will need to be perfectly level as you will be setting your block on it.  This is where your dead blow hammer will come in handy.  They are filled with sand and are exceptional at persuading the retaining wall blocks to level. 

  2. Use your torpedo level to check the retaining wall blocks levelness from front to back and left to right, as well as the previously laid block

  3. After you have a few laid uses your longer levels to make sure the blocks are all level to each other. making sure each block is touching the level- raise or lower blocks accordingly by removing or adding gravel

  4. If your wall is going into a hillside and you reach a point where the base layer of the block is going to be buried completely below grade- you can step up your footer- see picture for further explanation

  5. If your wall is fairly level continue building and checking levelness and straightness periodically- if it is a curved wall step back and look at the big picture and make sure it feels and looks right- sometimes plans to practice don't always work and adjustments need to be made​

  6. Once the first layer is completed install a drain pipe with a cloth covering or other filtering function to prevent soil from clogging the drain pipe

  7. Drainpipe should be daylighted away from wall base and at least 10ft away from the retaining wall- in some instances, a face drain is needed to allow water to drain from behind the wall

Stacking, Backfilling, Geo-grid
Engineer- What you need to know

Engineers are required to design retaining walls if they are 4ft or taller and in some areas 6ft or taller.  Best to play it safe and contact the local building department where the project would occur.  Engineers only design the wall, while we as the contractors come in and put their design to practice.  Often the engineer will make site visits during the project to make sure his designs are being followed.

Some benefits of engineers:

  • Peace of mind your retaining wall is designed as best as it could be

  • Another professional inspecting our work as it progresses 

Engineers are a bit of an added expense though; ranging from $1000 to $2000 or more and the average being around $2000

  1. After completing the first layer, your ready to add backfill gravel- you could also proceed to the next layer and then add gravel but we prefer to add gravel first.  This helps lock the retaining wall blocks into place.

  2. Backfill Gravel- make sure to fill hollow retaining wall blocks with gravel---fill the 18-inch cavity behind the blocks with drain gravel--- compact every layer of block laid for best results

  3. When stacking the next layer on be sure the previous layer is clean from gravel and chips by sweeping or gently blowing with a leaf blower- any unevenness will be seen at the end of the project

  4. If using geo-grid- install after you have backfilled drain gravel flush with the block--- make sure geo-grid is laid all the way to the face of the block but not exposed.

  5. Engineer drawings should be followed exactly as laid out-

  6. Continue stacking and backfilling until the last layer- depending on the retaining wall location you may want a couple of inches to be soil instead of gravel- use a filter fabric before installing the soil to help prevent soil mixing with the gravel backfill

  7. Now for capstones

Capstones and Cutting

Straight retaining walls are easier to install the capstones and cut them to fit.  Curved retaining walls are a bit tougher as there are more cuts and at various angles.  Typically straight walls have straight cuts which are easier, especially if you have a table wet saw.  Speed square, framing square, and other scribing tools are going to be helpful here as well.  Check out the video for marking and cutting pavers

Once you have cut and double-checked they all fit together nicely, get ready to glue them down with your favorite landscape , concrete, or construction adhesive- all weather is a must

Cutting Capstones.jpg