Archive for the ‘How to Build a Retaining Wall’ Category

How to Build a Structural Retaining Wall

               How to build a structural retaining wall is a question you are interested in or you would not be reading this. You have come to the right place. I am the president of Agape Retaining Walls, Inc. and I have more experience in the design and construction of modular block retaining walls than 99.9 % of the people on the planet. I love what I get to do for a living and it is my pleasure to share my knowledge of retaining wall construction with you.


            If you are going to construct a retaining wall the first thing you will need to find out is what the local municipality building regulations are for the construction of retaining walls in your area. This knowledge will let you know if you need a retaining wall building permit and if so what you will need to do to get that retaining wall building permit. After finding out what the building codes are in your local area make sure to follow those codes in the planning and the building process. In St. Louis County, MO the local retaining wall building code calls for a retaining wall permit on any retaining wall 3’ or taller exposed height and any retaining wall over 6’ exposed height needing to be engineered by a qualified retaining wall engineer. If you are planning to build a structural retaining wall you will need to have that retaining wall engineered to insure that your new retaining wall will be built strong enough to last for as long as it is needed. The engineering for your project will take into account your soil types and the type of pressures that will bear upon the structural retaining wall. It is very important to follow the engineering in the construction of your retaining wall. When you deviate from the retaining wall engineering in the construction of your retaining wall, you make yourself the engineer and therefore make yourself responsible for any problems the retaining wall may encounter. For the engineer to design your retaining wall he or she will need to have some key information. The engineer will need to know the type of retaining wall system you are using, how tall the retaining wall is, what it is retaining, and the location of the retaining wall.


            Now that you have all the boring stuff out of the way, you will need to take inventory of the equipment, tools, and products you will need to make your retaining wall construction project go as easy as possible. It is my recommendation that you get at least one or two other people to help you; most 8” tall retaining wall blocks weigh about 85 lbs each, so it is always nice to have another set of hands to help. Also, having a second person can be a matter of safety if there is another person on site and something unexpected happens, having someone there to help you in an emergency will be priceless. Another thing that I would recommend in your retaining wall construction is a mini excavator to excavate your retaining wall footing and the retaining wall dig out as needed for your geo-grid. Mini excavators can be rented as needed for your retaining wall construction. It is my advice to call around to several equipment rental places to make sure you are getting the best price. Also, it is my advice that you do some research on the piece of equipment you wish to rent to make sure that it will be able to do what you need it to in your retaining wall construction. I also would recommend that you test drive the mini excavator and all equipment you plan to operate around the retaining wall to make sure you know how to operate it well before you begin your excavation.  Another piece of equipment that I recommend is a skid loader. Make sure to rent a skid loader large enough to pick up the weight of a full pallet for the type of stone you are planning to use for your retaining wall. A skid loader will come in very handy for moving large quantities of rock and stone into place as needed. This will speed up your retaining wall installation. When operating equipment like a skid loader and mini excavator it is my recommendation that you make sure to wear proper ear protection. It is not a bad idea to wear safety glasses and when hauling large quantities of minus rock to wear a mask or air filter to prevent breathing in fine rock particles that may blow out of the skid loader bucket into your face. Another crucial piece of equipment that you will need to set up a large structural retaining wall is a laser level, tripod, and pole that will be used to make sure all your retaining wall elevations are on the mark. There are many other smaller tools that will be needed during wall construction:

  • a very good level– I prefer the 3 l.f. long Crick level.
  • a heavy rubber mallet to do your footing and straighten the retaining wall
  • a measuring tape 
  • a hammer
  • 3lb sledge hammer
  • a string line
  • a concrete saw with a diamond blade
  • retaining wall adhesive
  • wooden or metal stakes to hold your string
  • marking paint for layout
  • filter fabric to keep the backfill clean and allow water penetration
  • geo-grid
  • retaining wall blocks
  • lots of rock


            Now that you know what you are going to need to start your retaining wall project, let’s get things started. Pick up your retaining wall permits and keep them displayed on site so that the retaining wall inspectors from your local government will know that your retaining wall plans have been approved and that you are doing things by the book. Beginning your retaining wall project without your permits displayed could lead to a stop work order that could be a long delay to your retaining wall project, as well as a fine.


            Next, study the retaining wall engineering to make sure you have a very good working knowledge of what the engineer wants you to do in the retaining wall construction. Keep referring back to the retaining wall engineering to make sure you are doing everything to plan. Then measure out the exact location of the retaining wall. You can hire a company to mark your property lines or even the exact location where your  retaining wall will be located. If you choose to figure out the location of the retaining wall yourself make sure that you use the scale on the engineering. Take note of the depth of your retaining wall block along with the rate of step back per row installed. Include these measurements in your retaining wall lay out to make sure you have the proper location marked at the bottom of your wall adjusted as needed to make sure the top of wall ends up where you want it. After you have the location of your wall marked out you will need to shoot the elevation from the place where you want the top of your retaining wall to end up when the retaining wall is complete. Take your laser level (You can rent a laser level from most places that rent equipment) and shoot the elevation for the top of your retaining wall. Make sure the place where you shoot this elevation is a landmark that will not change. You will always have this landmark to go back to as a reference point. When you have that number on the bean pole transfer the height of the new retaining wall onto the bean pole. So, when you are going down in elevation you will go up on the bean pole and when you are going up in your elevation you will move the receiver down on the bean pole. So if the retaining wall you are building starts 10’ lower than the top elevation from your landmark elevation and you are toning for final top of wall grade you will go up 10’ in elevation on the bean pole with the receiver to get a tone for the bottom of your retaining wall. In this case however, you will also bury at least 10” of retaining wall below the finish grade and you will need at least 6” for footing rock. So in the case of this 10’ retaining wall you are building you would tone the top of wall elevation, raise the receiver 10’ on the bean pole for the height of the retaining wall, then raise the receiver another 10”  on the bean pole for the buried stone, then raise it another 6” for the footing rock. This will give you the depth that you need to be on your dig out. Now have your helper hold the beanpole and receiver while you follow the paint marks you have made for the wall dig out. When you are at the proper depth the receiver will tone. After you get the footing dug out and everything tones across your footing rock trench you will need to take a smooth shovel and go through the trench for the footing and remove any loose dirt. If you have a soils tech to check the soils type now would be a good time to have them come out to make sure the soil you are building on will be able to bear the load of your new wall. Sometimes the retaining wall engineer will want to see the soils at this point. 


            Now you are ready for the footing rock. In the past we would always use 1” minus compacted in 3”-4” lifts with a plate compactor or a 6” lift if we were using a Sheep’s Foot compactor. There is nothing wrong with still installing your footing rock with that method, but over the last 3-5 years we have abandoned the minus rock and have switched our footing rock to 1” clean. It is my recommendation that your footing rock be 1” clean rock inside a trench with filter fabric used on the walls of the trench to keep dirt out of the clean rock. The clean rock is more expensive but has several clear advantages over minus rock. One advantage when you use clean rock is that compaction is not the same issue. You can use 6” clean rock in your footing and tighten it up with a plate compactor and you will not get settlement.  Secondly, along those same lines, minus rock has to have a certain amount of moisture for optimum compaction.  If you have to much moisture it will pump and be unstable and  if you do not have enough moisture it will not compact correctly. 1” clean rock removes these potential problem and has another advantage in that it is not susceptible to erosion in the same way as minus rock is. Clean rock will allow for water to  flow through it and will not allow for hydrostatic pressure to build up. 

Now that you have decided on 1” clean rock for your footing, and the footing trench is dug out, you need to set the beanpole for top of your footing rock so that your footing rock will be right on allowing your retaining wall base course or footers to be installed more easily and with more speed. To set your footing rock right on just lower the receiver 6” on the beanpole. You should also pay close attention to the beanpole to make sure all your measurements are correct since some beanpoles are in feet and inches and others are in tenths. After you have your top of rock right on you will need to hop in that skid loader and start bringing in rock for the retaining wall base. Your helper can hold the beanpole and guide you as to how the rock looks in the trench. Some people prefer to have the person holding the beanpole to keep toning the trench as you fill in the rock and after rake it all out perfect then run over it several times with a plate compactor to make sure all the rock is nice and tight. If for some reason you do not have a helper to tone the rock as you move down the trench you can tone the sides of the trench yourself and mark the exact elevation with paint. This will give you a bench mark to guide you on the amount of rock you need as you install your footing rock. After all your footing rock is installed go back through and rake it to the exact elevation and tighten it up with the plate compactor. Now you are ready to start the base course of retaining wall blocks .


            If you are using a standard 8” building block for your wall, move the receiver on the beanpole down 8” and this will give you the top of block elevation you need for your footing stone. It is very important to laser tone the first footing stone to make sure you are setting it just right. If you do not laser tone the first base stone you install it could be a little too high in which case you will catch yourself needing to add rock all the way across your  base course or if it is to low you will be digging rock out all the way across your base course. Both of these options are not good and will cost you a lot of time and work. It is much better to make sure that the top of that first base stone is toned perfect on every side. Once you have that tone you will not need to laser tone the other base stones since you will level each stone as you go across. However, since you have the laser level and it is set for the right elevation and you do not do this every day it may not be a bad idea to check every 4-5 stones just to make sure you are putting the stones in level. The base row will take the most time to install and it is so important to get everything just perfect. As you are placing the base stones in place it is a very good idea to have someone placing the stones in place for you while you set them. Before you begin setting the base stone, set up your string line across the straightaway to guide your stone placement to make sure you are installing the base course straight. The best way to set up your string line is to put a stake in on both sides of the straight of way then tie the string to one side and pull the string across to the other stake. Adjust this line to be about a half inch above the top face of the stone. Now you have a straight line to guide you as you install the base course to your retaining wall. To set a base stone you will need to take your level and place it across the top face of the stone making sure it is in the middle of the bubble on your level. Next check the stone face to tail on the top and make sure it is level that way. The best way to level the stone most of the time is to have the stone just a tiny bit too high and tap it down into place using your rubber mallet . After you have your first stone in place and you have toned it to make sure the elevation is correct you will use this stone as a reference point to level the next stone. Take your level and place it across the stone that you just set and the stone you are setting. Make sure the new base stone you are setting is level both side to side and face to front with the stone that has already been leveled. You will continue this process.  As you are doing your base retaining wall course you will have the person or people helping you begin to stack stone across the base stones.


            The first stone stacked on each new course of stone needs to be measured to make sure it is in the center over the two base stones below it. For example, with a retaining wall block that has a face 18” wide you will measure to make sure the stone stacked on the top of the base stones is exactly 9” on each stone. When this measurement is correct on a wall that is running straight you will keep your bonds lined up. My advice at this point is to stack 3-4 rows of stone behind the person doing the footing but always making sure to stay at least two stones back so they do not lose the ability to go back and check for level. After the footing on the straight-away is completed and the wall is stacked across 1-2 rows depending on where the retaining wall engineering calls for you place your first geo-grid. now it is time to use your string to straighten each row of retaining wall block making sure that your wall is straight. The best way I have found to straighten a row of retaining wall blocks is to have one person on one side holding the string with a level or piece of wood wrapped around it to keep the string from touching the retaining wall block in any way. Have another person on the other side doing the same thing. If you only have two people tie the one side of the string to the grade stake and wrap the other side around the level or wood to keep the string from touching while the person on the other side pulls the string tight. Now that the string is set up you take your rubber mallet and go to the end of the row of retaining wall blocks you are straightening and knock out the block on the end as far as it will go. Make sure it is straight but has been pushed as far forward as it will go. After this go down to the other end of that row and do the same thing with that end block. Now your two end retaining wall blocks are out as far as they will go you look down the string and block by block knock all the stone out till the face of the retaining wall stone is in line with the string. Repeat this for every row of stone that has been stacked. Next, after the base stone is installed the rows are stacked and straightened, you will need to start backfilling behind the retaining wall blocks you have already stacked.


            It is my advice to backfill all the way up to the top of the row of stone right below where the geo-grid is to be placed. So if the geo-grid is called for on the top of the third row in the retaining wall engineering then you will fill behind the retaining wall with 1” clean rock all the way back to the end of the geo-grid. So on a retaining wall that is 10’ exposed height of wall more than likely the geo-grid will be 7 l.f. The rock must be backfilled to the end of the geo-grid zone. On the dirt bank or behind the clean rock you will need to put filter fabric to separate the dirt backfill from the 1” clean rock.

 As you are building the retaining wall up you will keep repeating these steps over and over. You will need to install footing stepping up into the hillside as your new retaining wall returns into the hillside on each end.  If you are able to backfill from the top of wall without driving directly behind it or backfill from the front of the wall it is my recommendation to use 1” clean rock all the way up to 1’-2’ from the top of retaining wall. After the rock backfill is complete, put filter fabric over the rock to separate the clean rock in your retaining wall from the dirt that will be installed on the top of wall to top off the wall. Before we get to that however, make sure to install all your retaining wall cap on the retaining wall.


We like to glue our retaining wall cap and then straighten the cap using a string line. Get your cap on the wall close to being straight. Make sure you put your cap on so that they straddle the block evenly. Make sure that the first cap you put on the wall is half over one block and half over the other block. I hate it when I see retaining walls where the contractor or the homeowner installs the cap lined up with the row of retaining wall blocks below it. That does not look as nice and it is not as strong. Make sure the glue you use is a retaining wall glue. Also, if you decide to put your glue down under the cap it works best to put the glue on the back of the stone in case the retaining wall cap ever needs to be taken off for some reason at a later date. If you put the glue on the back of the retaining wall block it allows the cap to be removed more easily in an emergency but does not allow the cap to move or come off. Also, it is my recommendation that if you put the glue under the cap and then straighten the cap with a string line that you move fast before the glue sets up. In the summer time when things are hot and dry the glue will be starting to set up inside of 10-15 minutes tops. In some places you will need to saw cut the retaining wall cap with a diamond blade saw. There are many different ways to approach this so I would say to look at the recommended installation instructions from the manufacturer of the specific retaining wall system you are installing. All closed face retaining wall systems will need to at least cut some cap on the end or on the corners of your retaining wall. It is pretty simple but gets much more complicated when you are cutting in a radius around a curved wall. After the cap is glued and straightened and the needed saw cuts are made lay some filter fabric behind the retaining wall block on the top of your wall.


             If you have used 1”clean rock you will need to put filter fabric down over the clean rock to keep the dirt out of the rock. If you used 1” minus rock to backfill your retaining wall you will only have to put the filter fabric behind the top rows of retaining wall blocks that will have dirt backfill. You should have plenty of dirt still on site. As a matter of fact, one of the unforeseen problems many times is there is so much dirt generated during dig out that you may not have enough room to stockpile it. Many times we will need to haul dirt away during the excavation of the retaining wall.  There are trucking companies you can hire to take dirt away although you will need to load them with your skid loader and at times that can be kind of tricky. It is my recommendation if you are bring in a trucking company to haul dirt away, check them out with the BBB and ask for some references. Also, establish the way you will pay and the rate you will be charged. Will they charge you a set fee by the load or will they charge by the hour? One last thing about the hauling company; make sure that the truck they send out to you will not be so tall that your skid has a hard time loading it. Back to the dirt backfill.  To top off your retaining wall you will start digging into the pile of dirt that you have on site. If you have no dirt you will need to have dirt brought in.  Make sure to use fill dirt for the low parts since it is much cheaper than top soil. Anyway, you will almost definitely have more dirt than you know what to do with and start digging into that mound of dirt with your skid loader and start dumping it bucket by bucket into the area to be filled. Be sure to put your bucket down to smooth it out as you go, tracking in the dirt and compacting it with your skid-loader. Be very careful working behind the wall; do not let your wheels or tracks get too close to the wall because there is a real danger of pushing the top of your retaining wall out with the skid loader if you get too close. After the bulk of the dirt has been tracked in and smoothed out you are very close to being finished. A good clean up, some sod or seed and it will not be too long until you will be enjoying your new yard and the great accomplishment of finishing such a large retaining wall project yourself.