I will discuss here the ins and outs of building a landscape retaining walls of various materials. In many areas, a building permit is required for retaining walls so check with your local building department before you start the work. In my Township only walls over 48″ high require a permit but best to check in any case.
There are many, many types of retainingwalls quotes Canberra materials available today. Fieldstone, railroad ties, pressure-treated ties, brick, inter-locking masonry block that resembles stone, brick, and so on. The different types require slightly different methods of installation but all have some of the same basic concepts. You must start with good solid soil underneath. Do not attempt to build on top of topsoil, wet loam or other soft and pliable ground material. Your wall will simply move with the earth and self destruct over time. If you are dealing with topsoil or soft ground, it will be necessary to excavate and remove this soil to a foot or two below ground level, placement of a good sub-base gravel or item material, compaction and then build your wall on top of that. A good sub-base will provide you with years of good looking retaining walls with very little maintenance work. In my area, a wall-less than 4′ high is considered a landscaping item. Over 4′ in height, the building departments want a permit and drawings. CHECK with them before you build your wall. Multiple levels of retaining walls can create great garden spaces. Seating areas and flower gardens can be created in this way as well.
The wheelbarrow, sledgehammer, tape measure, pencils, string line, level, 60 penny spikes, 3 /4″ electric drill, wood bits, extension cords, 7 1/4″ electric saw, cheap carbide blades, pointed and square shovels. You will also need extra loose suitable material to backfill your wall as you go. 3/4″ gravel works the best. Several 2′ pieces of either #4 or #5 rebar cut 24″ long. Pin bars, picks, electric demo hammers all can be great assets in making the job easier.
WOOD TIE WALLS
Whether you use 6 x 6 pressure treated ties, railroad ties or other dimensional lumber, they all start with at least one tie below finished grade. Placing one tie below grade will help anchor the bottom of the wall and keep it from sliding out once the backfill material is placed behind it. The first or base tie is the most critical. It should be level, align nicely with the others for the length of the wall and be secured firmly. Once you have set the bottom or base tie in place., using your 3/4″ drill with an appropriate sized wood bit for the rebar you have, drill a hole at least every 4 feet along the length of the ties starting at one end and always ending with a hole at both ends. Using your sledgehammer, drive a piece of the 2′ rebar down into the earth until it is level with the top of the tie. This will securely hold the first tie in place. Now place the second level of ties upon the first. Make sure you stagger the joints so no two end joints line up vertically. This is important for wall strength. I also want you to set the 2nd tie back from the front of the bottom tie about 1/4-3/8 inches. This is called “battering”. As each subsequent tie is added, each one will set back the same amount causing a slight backward slope of the face of the wall. In effect, it is leaning the wall back into the earth behind it. Once the second row is in place, using your drill, drill pilot holes of sufficient size to allow the installation of your 60 penny spikes. DO NOT try to drive these spikes through a pressure treated tie. You will only bend the spike or crack the tie. Removal of these spikes when they are half driven into the wood is almost impossible. They are also fairly expensive so you don’t want to waste them. Starting at one end, space the spikes 3-4 feet apart, always ending with one on each end of each tie. Once again, as the backfill is placed behind the wall, the pressure will be trying to topple your wall forward. This built-in “lean” will keep your wall nice and solid. Once you have the second tie in place, go ahead and backfill behind the first two ties and in front of the bottom tie. Take some time to clean up and rocks or other debris that remains underfoot. This will make a safer work area as you proceed upward with the building of your wall.
Depending on the finished height of your wall, you may have to install what are called “dead-men” every few feet along the length of the wall, every few courses. These are a piece of the tie turned at a 90-degree angle to the wall with a cross piece nailed on the anchor as an additional anchor behind the wall. A piece 3′ long piece with a 2′ cross end piece in the shape of a T, placed behind the wall with the end of the long leg nailed to the wall between ties is what is required. When backfilled, this becomes a sold anchoring device for the wall. The pressure now not only has to try and push your battered ties over forward but has to pull the anchor out of the solid ground. If done correctly, this is not likely to happen. The outside face of the wall will appear to have a six-inch piece of wood every few feet but this is, of course, the butt end of the anchor piece.
As you proceed upward with each course of your ties, you will backfill each course, lightly tamping the stone to prevent excessive settlement and remove any air pockets in the earth behind the wall caused by the excavation. DO NOT be tempted to throw large rocks behind the wall to make the backfilling work go quicker. Frost’s action will push on the large stones and could cause your wall to fail. 3/4″ gravel drains quickly, keeping water that can freeze/thaw away from the back of your wall and small gravel presents a lesser area for the frost to push upon. If your working in a normally wet area, you may want to install a perforated PVC drain pipe behind the first tie taking the end(s) to daylight. If you do use a drainage pipe, cover the pipe with a filter fabric material to prevent silt from washing into the pipe and plugging it. Simply place a small of amount of stone over the pipe, place the fabric over the stone and continue with your ties and backfill gravel as described above.
Depending upon what you want to see when you are finished with the wall, you may either carry the gravel right up to the finished grade level with the lawn behind the wall or if you want grass or plantings along the wall, you must decide when to stop back filling with the stone. Planting flowers behind the wall is fine but avoid the temptation to use any plants or shrubs with deep root systems. The roots will act upon the wall, pushing against it from behind. If you are going to place topsoil behind the wall for plantings, install a filter fabric at least two ties down from the top of the wall to prevent the soil from silting down into the gravel below.
PRE-CAST MASONRY RETAINING WALLS
Additional tools required:
In addition to the tool list given above, you will need a gas powered 12″ cutoff saw with masonry blades. Blades wear out quickly so get several of them. A chipping chisel, a brick set and 5 pound lump hammer are also required.
There are several pre-cast concrete masonry units on the market today that are made for building retaining walls. Visit various retailers to see the different costs per piece and availability in your area. Basically the prep work is the same for a wood tie wall. You want to first course of block to sit below the finished grade as an anchor. A solid sub-base of stone, stone dust or concrete is required and must be tamped into place before starting to avoid settlement of the wall. This base must be as level as possible as an out-of-level start will show badly as the wall goes higher and can cause wall failure. Make sure the row is nice and straight or the curve is a smooth gradual one.
Most if not all of these pre-cast units have a lip and groove configuration that allows you to sit one piece upon the other automatically battering the wall as you go. No guesswork. Some also have pins that are pushed down into the block below, further holding the individual pieces in place. All of these walls require a backfill of stone. Many also have large holes in the block which must be filled with stone further aiding in the drainage of water from behind the wall. If these walls are kept less than 4′ in height, most do not require a dead man assembly. Ask your salesperson if they carry a pre-made dead man. It will save lots of work.
ANCHORING YOUR WALL:
Pre-cast blocks require a mesh netting called “Geo-Grid” to be installed on the top of the first course and usually every two or three courses thereafter. This netting comes in rolls and can be cut with a utility knife. Rule of thumb says the netting projects back into the hillside the same distance the wall is tall. Four-foot wall, 4-foot wide netting but many walls require engineering and may require additional netting. The netting is placed on top of the first course within 1″ of the front edge of the blocks below. The 2nd course of blocks is then set. After the 2nd course is complete and connector pins installed between the blocks, the netting is the stretched backwards into the hillside leaving no wrinkles in the fabric. A layer of good soil/stone is then placed on top of the netting and compacted into place up to the top of the 2nd course. The netting will hold the wall in place by the sheer weight of the soil you placed behind it. This process continues up to the height of your wall.
All the manufacturers I have seen can provide a finished cap piece for their walls. This provides a neat clean appearance to your finished walls. If the wall is low enough, it also provides a great place to sit and check out the garden behind. Top caps are installed using an adhesive and caulking gun. Adhesive generally cannot be installed below 40 degrees. Check the adhesive tube for instructions for use.
All blocks come in straight pieces. You may remove some of the rear of the block to make an inside curve or space the backs apart to make an outside curve. In any case, the fronts are always held tightly together. (NOTE: On an inside curve, the geo-grid must NOT overlap itself. This can cause the blocks to slide on the double layer and cause the wall to fail. Cut away the portion that overlaps on one of the layers).
There are many brands and types of masonry wall units available and all vary slightly in their installation methods. Make sure you read your instructions before you start. Some brands use plastic pins to anchor each course to the one below, some use steel dowels. Most require filling of the units with clean gravel. Take your time. The wall must last forever.
Drainage pipe installed in stone behind wall.
It is a good idea and may be required by your building department to install drainage piping at the base of your wall to carry away surface and ground water. Freezing water can push a wall over or badly damage it. Perforated plastic pipe can be used and today flexible plastic pipe comes in rolls making it very easy to do curved walls and have less or no joints.
Also available is fabric covered piping. If you do not use the pipe with fabric already on it, after the plain pipe is installed and stone placed on top of it, you must install filter fabric to prevent silt from blocking the holes in the pipe over time. Make sure the ends of the pipe exit to daylight to allow free flowing of any ground water. As your wall progress’s, it is required that the first 1 foot space behind the wall is filled with clean stone to allow water to travel down behind the wall to the drainage pipe. The soil behind that portion can be existing soil as long as it can be compacted properly. The stone is brought up to approximately 1 foot below finished grade and then topsoil can be placed for the last foot. This prevents surface water from entering behind the wall. If your wall is quite long you may have to install additional holes through your wall for additional drainage exits using plastic tee fittings. Notch the wall block carefully and place the tee in the slot you cut. The next course of block will hold the tee fitting in place.
In my area, walls above 30″ require a handrail to prevent anyone from walking over the edge of the wall from above. If your wall is going to have a sidewalk on top, a handrail is a must. Check with your local building department to see if one is needed. There are special details required to install a handrail behind the walls. Check with your supplier.
Before you start your project, surf the internet a little using “images” in your search engine to see the hundreds of styles of walls design ideas, etc. that are available. Maybe it will give you a good idea for your project.