What is it used for?
If you look at the foundation design here you will see that a product called Form-a-drain is used on both sides of the footing. Details on Form-a-drain can be found here . The Form-a-drain is a 10 inch by 1-1/2 inch rectangular hollow cross-section and comes in 12 foot lengths that you connect together to go all round on both sides of the footings. It has slots in it to let the water in and it forms a rectangular pipe to drain the water away. That pipe round the footings also provides a way of venting off any radon gas that might come up from the earth. Another job the Form-a-drain performs is to form the sides of the footings to keep the concrete where you want it while it's setting. Lastly, it is used as a fairly stable reference point to set the height of the concrete slab. The screeding of the slab concrete can be done by using the top edge of the Form-a-drain.
(Click on image to see and download high resolution version)
Here is the drawing version for the internal concrete walls...
Form-a-drain drainage pipe connection
There will be a hole cut in the bottom of the Form-a-drain as it passes over the drainage pipe that was installed previously.
To catch the water, the sloping drainage pipe has upward facing T pieces. To guide the water from the Form-a-drain to the T pieces, short pieces of solid wall 4" drainage pipe cut with a 1.5" slot are used. A cap piece cut with a 1-5/8" deep slot completes the assembly.
With a trowel, dig away the soil round the temporary filter covered perforated pipe (that was used to drain the site while refining the excavation) and remove the perforated pipe. Make sure soil does not fall into the Tee piece.
To determine the length of 4" solid pipe required, use your laser measuring white pipe to the bottom of the Tee piece and subtract the distance from the bottom to the bottom of the inner ridge (4"). The remaining distance to the official footing depth plus 1.5" is the length of pipe required.
Pipe length = Measured from batterboard laser to pipe bottom - 4" - 5' 8.25" + 1.5"
Fabricate and attach the guide pipe assembly
Measure and cut the necessary length of 4" solid pipe. Cut the 4" piping with a slot in the top to guide the water to the T piece. Also cut the top cap as shown in the picture below (use a band saw with a fine tooth blade).
Temporarily assemble the parts. There is no need to glue the pipe and it is best not to as that provides some wiggle adjustment.
With all the upward slotted pipes installed it is possible to place the Form-a-drain (rather than it hitting the temporary perforated drain pipes).
The required lengths can be determined by looking at the foundation drawings. The green line in the drawing below is the outer edge of the outer Form-a-drain and the blue line is the inner edge of the inner Form-a-drain. Note that there is also Form-a-drain used for the footings under the internal concrete walls.
When cutting the lengths of Form-a-drain you need to subtract an amount for the Form-a-drain corner pieces. For the inside bend the amount is 1-3/16" per piece, and for outside of the bend the amount is 2-11/16" per piece. The dimensions required come from your AutoCAD drawings.
The following diagram shows the calculated Form-a-drain lengths.
Using connector pieces, join pieces of Form-a-drain together to make a piece that is longer than required and then measure the overall length and cut to the required length using a sliding miter chop saw. Where possible put the self tapping screws on the perforated side so the heads are not in the way of the EPS lining. Use 6 screws per join. Pre-drill the screw holes with a small diameter drill.
Rest in place over drain pipes
Rest the cut to size Form-a-drain in place over where the drainage pipes with the upward facing T pieces are. Push on the corner joiner pieces. The slits need to face away from from the trench on both the inner and the outer Form-a-drain. The plain side is against where the concrete will be. If the Form-a-drain is slightly bowed then the bowed middle should be down against the soil (and will be shimmed up later). Make sure the bottom of the Form-a-drain is at the right height relative to the batter board strings (5' 8.25" below grade, which in my case is also 5' 8.25" below the batterboard strings). Use wood jigs to keep the Form-a-drain the right distance apart.
Assemble Form-a-drain to make perimeters
Attach the corner pieces to join together the Form-a-drain to make the lower inner and lower outer Form-a-drain perimeters for the bottom of the footing trench. Attach the corner pieces by drilling a small pilot hole through the corner pieces and into the Form-a-drain, then use one self tapper screw per Form-a-drain piece. Only use one screw because it is just temporary to stop it dropping out.
When you are done, you should have a joined rectangle all the way round the outside of the footing and another one all the way round the inside of the footing. If your house is more complicated than just a rectangle then you will also implement a continuous border inside and out, but you will need more connector types and the result will something more complicated than a rectangle.
Get Form-a-drain in right distance apart using jigs
Using wooden spacer jigs is useful to set the outer and inner Form-a-drain the correct distance apart. They are made from 4'11" long 2x4s with 3.5" bits of 2x4 at the ends. The space for the Form-a-drain is 1/6" bigger than the 1.5" of the Form-a-drain.
They ensure the outer edges of the Form-a-drain are exactly 4'8" apart (which is 4'5" between the inside edges). If you want you can also implement a jig to help you get the soil the right height between the Form-a-drain.
At ground level between the Form-A-Drain use pieces of scrap wood (something like 2x4s) cut to a length of 4'5" which is the inside distance between the Form-a-drain. These should be kept in place when putting crushed rock behind the Form-a-drain to stop the Form-a-drain being pushed inwards. Only take them out (sometimes temporarily) when they get in the way in future steps. Using these is more important than using the more complicated jigs described above.
Cut holes in bottom of Form-a-drain
Directly above the drainage pipe T pieces you need to cut holes in the bottom of the Form-a-drain. This can be done using a reciprocal saw and cutting a 1.5" shallow U in the side on the Form-a-drain at the bottom edge. You need to do a check with a plumb bob on the end corners to make sure the Form-a-drain is in the right place.
Drill 1.25" holes for spacer pipes
As you will see later, there are spacer pipes between the upper and lower inner Form-a-drain to allow water to drain. Now (ahead of fitting the filter sock) is a good time to drill the 1.25" holes in the top of the Form-a-drain.
The holes need to be placed such that flushing cleanout water from the top Form-a-drain that is injected via the cleanout 2" pipes enters the bottom Form-a-drain a decent distance from the 2" injector inlet and then flows to a decent distance to where the lower Form-a-drain connects to the 4" sloping drainage pipe. You want this flushing water to cleanout any silt in the Form-a-drain. Basically this means put the drainage spacer pipes about half way along the run of Form-a-drain between the injector positions. You don't want too many drainage spacer pipes or the water will bypass a section of upper Form-a-drain. Figure out the places on your AutoCAD drawing. Note that drainage spacer pipes are only used on the inner Form-a-drain. The red crosses in the following diagram show the positions chosen...
Drill the 1.25" holes with a 1.25" Lenox type Arbored Hole Saw.
Set Form-a-drain locations accurately
Position the lower inner and lower outer Form-a-drain in the exactly right positions. This is done using a Plumb Bob hung down temporarily from the batter board strings. A hook made out of a bit of garden wire allows the Plumb Bob to easily be moved to different places.
Use two Plumb Bobs to set a corner position, one on the north south string and one on the east west string. The Plumb Bob denotes the outer edge of the Form-a-drain and therefore denotes the outer corner (either convex or concave) (rather than the center of the bend where the Form-a-drain corner hole is).
If you cannot get the Form-a-drain edges all to the right locations when you have all the jigs in place that set the distance apart of the Form-a-drain and the Form-a-drain is all the right AutoCAD lengths then you have a problem that needs to be investigated. You need to get the forming to accurately match the AutoCAD drawings. Perhaps you have cut a length of Form-a-drain to the wrong length.
If you find the angles are not 90 degrees then you have a problem with your batterboards and strings. For the squareness of the foundation you are relying on the batterboards being accurately set. If you think there is a possibility that the batterboards have moved then you need to find a way to accurately check the squareness and get the batterboard strings set to their correct positions. This is not easy to do, which is why setting the batterboards and strings accurately during the mark-out phase was so important.
In the above photo, note the liberal use of 4'5" lengths of lumber to stop the Form-A-Drain being pushed inwards (by the crushed rock that is added later).
Looking at the foundation drawing you will see that there is Form-a-drain at the bottom of the footing trench on the inner and outside edges, just inside where the stakes will be banged in. The stakes make sure the bottom Form-a-drain stays in place laterally. You could use steel rebar stakes as it does not matter if they rust away after the concrete pour, but I chose to use corrosion-proof rebar.
As you can see in the foundation drawings, vertical rebar stakes are driven into the ground at the bottom of the footing trench and the lower Form-a-drain is attached to the rebar stakes.
The stakes will be cut off (as needed) at the top of the lower Form-a-drain, so the stake length is 10" plus the length in the ground. The length of stake depends on how hard your soil is, but about a 2 foot stake is a good length. In my case the soil is so hard that I used slightly shorter stakes. If because of a rock you are not able to hammer it in 14" then it will protrude a bit above the top of the lower Form-a-drain. If you have lots of rocks you may have to have several attempts at finding a suitable location along the Form-a-drain where it will go in. It does not matter if it goes in a bit crocked, but try to get the bottom next to the bottom of the Form-a-drain so the bottom Form-a-drain cannot move. There can be a slight gap as the force is all inwards and the wire can span the gap.
Bang in the stakes
You will need to put rebar stakes about every 2 or 3 feet to provide enough retention for the Form-a-drain. The exact spacing and positioning along the Form-a-drain length does not matter. Use the Form-a-drain to guide where the stakes need to be, but make sure the Form-a-drain is in the right location using a plumb bob off the batter board strings. Use the 4'5" wood spacers to help stop the Form-a-drain from moving. Make sure the Form-a-drain does not move when banging in the stakes and if it does then use the plumb bob to get it back to the right location. Also make sure the Form-a-drain is not warped and if it is then compensate for this when putting in the stakes (fixing the Form-a-drain to the stakes should un-warp the Form-a-drain).
Bang the stakes in vertically with a Club Hammer. Realistically the stakes may wander a bit when banged in, but as long as they are within half an inch of the edge of the Form-a-drain then you will be fine. They also may not be exactly vertical due to rocks, but a bit of variation is ok.
Drill holes for the wire
It is easier to drill the holes for the wire before fitting the filter sock. Drill a hole at the bottom of the Form-a-drain (about 3/8" up) in line with where the stake goes into the ground. The other end of the wire on the other side of the stake will go under the Form-a-drain.
You can drill the holes after fitting the sock but you will need to cut the cloth with a craft knife to create a bare area or the drill will wind up the cloth.
Form-a-drain filter sock
For this step you need to pull up the Form-a-drain that has been temporarily placed in the right position. When pulling up a section use the opportunity get rid of any soil that has fallen inside the Form-a-drain.
Over the life of the building you need to avoid the Form-a-drain getting clogged up with soil as it is hard to clean out compared with 4" perforated drain pipe. To avoid soil getting in you need to cover the slots with filter cloth. You need this on the Form-a-drain that goes at the bottom of the footing ditch and on the inner top Form-a-drain. The outer top Form-a-drain is used as a reference rather than for drainage so you don't need or want filter cloth on it.
Here's a picture of a section of 10" Form-a-drain covered in the sock...
Here's what the sock covering looks like at the corners...
Fit screws back into the corner pieces when you have fitted the filter sock. It is not a bad idea to add more than just the one screw to hold the Form-A-Drain more securely.
Where there is an exit drainage connection then you need to cut a slit in the filter cloth.
Put the 4'5" lengths of lumber back in place to stop inwards movement of the Form-A-Drain.
(Note that the 4" perforated white pipe in the photo is typically added at a later step.)
Form-a-drain stake fixing
This applies to both the inner and outer lower Form-a-drain. It helps stop the Form-a-drain being forced into the footing area by the crushed rock. It attaches to the stakes with 16 gauge galvanized wire. The hole will have previously been drilled in the Form-a-drain, but you will need to feel for the hole with your finger under the filter sock.
Before wiring you need to make sure the Form-a-drain is in the right place both longitudinally and laterally to within +/-1/4". You also need to make sure the height is about right. Certainly the Form-a-drain must not be sitting high because it is hard to remove soil and rocks under it once the Form-a-drain is wired to the stakes. Being up to 3/8" low is ok.
The wire end can make a hole in the filter cloth. Twist it on the footing trench side and bend the tail to get it out of the way. Make sure the sharp tail is down into the ground so it does not unnecessarily puncture the lining membranes that will be added later.
The Form-a-drain needs to be laid exactly horizontal (on its edge). This will be what it is if you have done an accurate job with your footing trench excavation, but realistically you need to accurately check it using a self leveling laser set to batterboard height, and make any adjustments needed.
It is good to have a laser target on the far side of the build site to ensure the laser height has not moved.
The Form-a-drain needs to be (eventually) exactly level so that water does not accumulate in it. If necessary, add a few selected shims to get it padded out to the right height and levelness. Pieces of porcelain tile (2"x4") make good padding shims.
If the Form-a-drain is warped then it will have been laid such that the center is lower than the ends. To stop the ends artificially raising up when you put shims under the center then you may need to temporarily place paving slabs on the corners (or on the jigs next to the corners).
It is ok to leave the Form-A-Drain up to 3/8" low at this stage because some further shimming can be done after the upper Form-A-Drain has been installed. You will be able to do it more accurately once the upper Form-A-Drain has been installed because any bowing will have been taken out.
Outside the lower outer Form-a-drain
Round the outside of the footing trench there needs to be a 4" perforated drainage pipe, or as it's sometimes called, a "weeping tile". The outside drainage pipe needs to be installed at this point in the build process so that crushed rock can be placed behind the lower outer Form-a-drain to balance the forces associated with filling the under-slab area with crushed rock.
Drainage 4" pipes will not have filter cloth sock
Many people say that 4" perforated drain pipe should be covered with a sock of filter cloth to keep soil from getting in and blocking it. I prefer to leave off the sock and instead have a good arrangement of cleanouts, down which a long cleanout brush can be inserted and/or a high pressure hose pipe. I worry that over the long life of the house that any filter cloth would itself become blocked and there would be no way to clean it.
Line the outside with geo-fabric
The geo-fabric goes from the bottom of the lower outer Form-a-drain, under the perforated drain pipe, and then up the outside of the excavation. The first strip runs in parallel with the Form-a-drain so is only wide enough to go part way up the excavation. The actual width of geofabric does not matter as long as the width is greater than 4 feet. The Form-a-drain goes up the perforated side of the Form-a-drain to its top. More geo-fabric will be added later before it comes to adding more crushed rock.
Bed of drain rock
Prior to installing the 4" perforated pipe, lay down a thin layer of 1.25 inch crushed rock without fines for it to lie on. Near the drainage outlets the bed should be about half an inch, but it should very gradually get thicker as it goes back in order to give the pipe a slight slope. Something like 1/16" per foot is sufficient. Certainly you should check that there are no places where the gradient is the wrong way.
If you buy crushed rock that has not been washed after the crushing then you will need to wash it yourself (which is hard work).
Install the drainage pipe
The drainage pipe round the outside of the footings connects into the connectors previously installed in the drainage pipe that runs across the building area and away. The perimeter pipes join to the T pieces of the drainage pipe at both the exit point and at the far side of the building.
The job of the perforated 4" PVC pipe is to catch the bulk of the water before it even gets to the Form-A-Drain. Some water will get through to the Form-A-Drain because in some places the 4" pipe is slightly higher than the bottom of the Form-A-Drain in order to provide a slight downward slope to the pipe.
There are cleanout Wye fittings that connect to vertical 4" pipe. Pairs of these are at the east and west sides as well as on the north west of the north portico and south east of the south portico.
There are also cleanout pairs in the north west and south east corners.
Regular (tight turn) bend right angle pieces are used at the building corners. Using these regular tight turns keeps the pipe close to the Form-a-drain .
Note that you can glue together the piping ahead of the geofabric and bed of crushed rock as it is easy to lift up the pipe after it has been glued.
The pipe needs to go round the outside of the portico deck support pillars because the portico deck pillars are right up against the Form-a-drain.
Put geofabric on inside Form-a-drain
This is so that crushed rock can be put on the inside at the same time as it is added on the outside so that the forces can be balanced.
Use 4' wide geofabric.
Add crushed rock to height of pipe (inside and outside)
Add crushed rock on top of the peripheral drainage pipe. Do not at this stage add too much crushed rock or it will cause the Form-A-Drain to bend inwards. Fold the geofabric over the top of the crushed rock to keep soil from dropping into it.
When adding crushed rock to the outside you also need to add crushed rock to the same height behind the inner lower Form-a-drain in order to try to balance the forces.
Inner top Form-a-drain
Join together pieces of the right cut and joined length to make a perimeter that is the same dimensions as the inner lower Form-a-drain. It's best to assemble it by resting it on the lower perimeter of Form-a-drain.
It is best to put the Form-a-drain the other way up relative to the lower Form-A-Drain such that any bowing of the Form-A-Drain is in the opposite direction. The wired spacer pipes will ensure that the Form-A-Drain is accurately held the correct distance apart and thus any bowing will be taken out in the completed assembly.
This is the third perimeter of Form-a-drain, and like the previous two is also used for drainage, so it needs the filter cloth sock and its slots face outwards from the footing trench. The gap between the bottom and top Form-a-drain should be exactly 9-1/4". It will attach to the spacer pipes with 16 gauge galvanized wire top and bottom.
Implement pipes from inner top Form-a-drain to bottom inner Form-a-drain
The 9-1/4" long 1.25" bore pipes strategically spaced around the perimeter are to allow water collected in the top Form-a-drain to drain to the bottom Form-a-drain so it will then flow to the 4" drainage pipes and away. The 1.25" holes in the top of the lower Form-a-drain will have been drilled in a previous step. Look through the filter sock for where the holes are and cut away the filter sock in that place.
The pipes are made using 9-1/4" lengths of 1.25" PVC water pipe into which internal collar pieces made from 1" PVC water pipe are glued. The 1" pipe as it protrudes 1/8" out of the 1.25" pipe will fit into a 1.25" hole drilled in the Form-a-drain. A small amount of filing is necessary to increase the 1.25" hole enough to fit the pipe.
The pipes act as convenient spacers to get the right distance between the upper and lower Form-a-drain. The number of pipes that do water drainage is small, so it is necessary to implement a large number of dummy pipes that act purely as spacers. These are just 9-1/4" lengths of 1.25" pipe but without the holes in the Form-a-drain or the inner collar pieces of 1" pipe.
Use them with a 4 feet gap between them (4' 1-1/2" on center), starting with about a 1 foot gap from a corner. Use a piece of 4' wide cut EPS as a spacing guide. Note that real world 4' wide EPS may well be about 3' 11-7/8" which is why it is best to use a piece of your actual EPS rather than measuring the distance.
Work from the corners inwards (which leaves an arbitrary spacing in the middle). Wire the pipe to the Form-a-drain top and bottom. You will need to cut away a 2" section of filter cloth sock at the spacer pipe positions and then drill holes in the Form-A-Drain for the wire.
Use 16 awg galvanized wire to fix the top Form-a-drain to the white pipe using small drilled holes. Also use wire at the bottom. Because the Form-A-Drain is the opposite way up, the pipe with the wire top and bottom will take out any warp in the Form-a-drain. You may need to weight it down to allow you to wire it.
Use a plumb-bob from the batterboard strings to get the upper inner Form-a-drain in the right location. Use a piece of EPS to get the pipe properly vertical so you know where to drill the hole in the upper Form-A-Drain.
Cut the filter sock on the upper Form-A-Drain to allow the hole to be drilled. Twist the wire with pliers to make it fully tight.
After the upper inner Form-a-drain has been installed on top of the pipes (some dummy and some water drainage), this is what it looks like...
Inner Form-a-drain exact height
Now that any bowing of the Form-a-drain has been taken out, use a self leveling laser on the top of the upper inner Form-a-drain to make sure it is exactly at the right height all the way round. The distance from the top of the inner Form-a-drain to the laser reference (grade level, which is also the top of the batterboards) needs to be 3'3". Use porcelain tile shims under the inner lower Form-a-drain to increase the height to get the top of the inner upper Form-a-drain to the right height.
After the the inner Form-a-drain is set at the right height using the shims then you can fill the space between the shims with some non-organic filtered soil. This will stop the Form-a-drain from sagging down.
Inner Form-a-drain strengthening
Rebar pipe strengthening
Rebar (I prefer composite rebar) is used to prevent the Form-a-drain from unduly bulging inwards with the pressure of the crushed rock. Cut pieces of rebar to a length of 2' 4.25". They are positioned such that the top is 1" below the top of the upper inner Form-A-Drain. Wire them to the Form-a-drain at the rebar top and at the pipe top, pipe middle, and pipe bottom.
Wire side to side between inner upper Form-A-Drains
Put 16 gauge galvanized wire side to side across the earth of the slab area between the top of the rebar where it is wired to the inner upper Form-a-drain . Use two parallel wires to between each opposite rebar pair. The wires ensure that the Form-a-drain will not bulge out when the crushed rock is added to the slab area. The wires will stay in place, but it won't matter if they rust away after the concrete is poured. Measure that the distances between the Form-a-drain are correct and tension the wires to ensure the correct dimensions. Tension them by twisting a 6" length of rebar in the middle between the two wires and twist it.
(Note that the photos below show crushed rock over the slab area that is typically added later.)
There needs to be wire between all opposite stakes including the pairs near the corners.
Cleanout and radon venting
In a high radon area to allow radon gas to vent out and to provide a way to route flushing water into the inner Form-a-drain to clean it out, a 4" diameter white PVC pipe will attach to the top inner Form-a-drain. In a high radon area the 4" pipe ideally needs to come up through the slab (because radon gas is lighter than air). The footings and slab form an umbrella that will catch any radon gas and this needs to be provided with a path upwards from the underslab area so it can escape.
In my area there is not actually a radon gas problem, and there is no building code requirement, so I can do without a radon vent. I still however want a vent of sorts, but I decided to make it only 2" diameter and route it under the footing rather than have it come through the slab (so I avoid having a hole through the waterproof membrane).
For my main house foundation there are actually 5 potential radon collection areas because the underslab area within the footings (including the internal concrete wall footings) forms three umbrella-like compartments and then there are two porticos. If I ever needed to flush out gas from under the footing (very unlikely in my area) then I would blow in air on one side and vacuum it out from the other side. I could potentially also use water to flush everything out.
The vent pipes are above where the drainage pipes are (the pipes share a trench).
The 6 mil polyethylene sheet that is directly above, level with the top edge of the top inner Form-a-drain, will trap any radon gas and route it into the upper inner Form-a-drain and if needed be blown out via the vent pipe.
Vent pipe implementation
Drill a 2" hole in the upper inner Form-a-drain near the top. The outside of the pipe fitting needs to end up being 1" below the top of the Form-a-drain.
Fit the 2" white PVC pipe using a threaded fitting and then a 90 degree bend. Use PVC glue.
Inner Form-a-drain filler strips
The 1.5" EPS is cut to 9-1/4" high for the inner Form-a-drain. Fit the 9-1/4" strips of polystyrene sheet between the inner Form-a-drain. Some trimming is needed when you have bulges due to Form-a-drain joins.
Between the inner Form-a-drain it is necessary to prevent the backfill drain rock and the filler polystyrene from falling out ahead of having the footing lining done. There are two ways of doing this dependent on the amount of space that there happens to be between the Form-A-Drain and the excavation soil. Where there happens to be enough space then glue an 11" tall piece of 2" EPS behind the 9.25" 1.5" EPS, ie use the same method as with the outer Form-A-Drain.
Sometimes where the excavation earth is tight you will not be able to pre-stick the sheets together and you will need to glue in place and use some creativity to get it to fit. Where there is room, use 2" EPS for the backing piece, but use 1.5" or 1" in places where there is less room.
Where there is not sufficient space then it is necessary to fix some thin plastic sheet between the upper and lower inner Form-a-drain on the inside of the footing trench. It should be a 1 foot wide strip cut from a 8'x4' sheet. Suitable sheeting is about 0.060" thick PVC (available from Lowes). Use self tapping screws on the top and bottom edges to attach the PVC sheet to the Form-a-drain.
It is necessary to remove any 2x4 spacer jigs that use U's on the lower Form-a-drain and replace them with straight bits of 2x4 that are 4'5" long.
(Typically the EPS between the outer Form-a-drain shown in the photo is added in a later step.)
Fill under-slab area
Get crushed rock delivered
The best crushed rock to use depends on what's available in your area, but in my area it is "1.25 inch crushed rock without fines". This has jagged edges so locks together to form a good stable foundation base. Because the bits of crushed rock are a decent size it allows water to drain through.
Typically you need to order aggregate by the truck load. That means you get something like 15 tons at a time of your chosen aggregate. The trucks will dump it on your site in a place of your choosing in a big pile. Note that aggregate trucks are big so you will need to provide plenty of access for them to get to the place where you want it dumped.
Depending on the rock you get you may have to put it through a sieve to get rid of rock powder and dirt. Typically you will fill buckets with the clean crushed rock and carry to wherever it is required.
Fill the under-slab pipe ditches with non-organic soil
If not already done, fill the pipe ditches right up to the height of the under-slab excavation. Make sure the pipes are well supported and well surrounded. As needed use a manual tamper tool to get it well compacted.
Put geo-fabric over the under-slab area
Drape it down the gully between the edge of the under-slab earth mound and the Form-a-drain.
If needed you can add some non-organic soil behind the geo-fabric to fill in any big voids as you add crushed rock on the Form-a-drain side.
Add wood bracing between upper Form-A-Drain
These are 4'5" lengths of scrap lumber. The wood width does need to be wide enough to handle the height difference between the upper inner and upper outer Form-A-Drain. Using 2x6 lumber is ideal.
Make sure you still have 4'5" lengths of lumber between the lower Form-a-drain at footing trench earth level to stop it being pushed inwards.
Fill behind the inner Form-a-drain with crushed rock
Completely fill with "1.25 inch crushed rock without fines" up the inch or so gap between the inner Form-a-drain and the center soil mound that is under the slab. You will need to trickle it down there. Do not over force it or compact it as that may make the Form-a-drain bulge out. The crushed rock will be retained by the top and bottom inner Form-a-drain and the polystyrene.
It is ok to use crushed rock as the slots in the Form-a-drain are thin and the forces on the Form-a-drain are small so it won't be punctured (and it wouldn't matter even if it was punctured). Crushed rock is the best choice because it interlocks and is more structural than drain rock, so it will help resist lateral forces in an earthquake.
Put 3" of crushed rock over slab area
Add crushed rock
You will need to make yourself a ramp and carry buckets of crushed rock.
The crushed rock should come up to 1" below the top of the inner Form-a-drain.
Compact the crushed rock
Even though the inner Form-a-drain is in place and back filled with drain rock, you still need to go easy with the compactor near the footing trench edges because you don't want to move the Form-a-drain from its correct location.
Use a long straight length of lumber resting on the top edge of the upper inner Form-a-drain to get the crushed rock the correct height right across the slab area.
Deck column forming
Even though the next expected step in the forming of the foundations would be to fit the outer top Form-a-drain, it is actually better to first do the forming for the deck columns that are near the building perimeter. It is partly because it is easier to access them before fitting the upper top Form-a-drain, and partly because you will want to buttress the Form-a-drain to the deck column forming to help hold the Form-a-drain in the right location and stop it flapping about.
The house is surrounded by a big wrap-around concrete deck at the same level as the main level floor. The columns that hold up this deck have their own square footings on undisturbed soil.
Outer top Form-a-drain
This is the forth and last perimeter of Form-a-drain. Its outer top edge is used as the reference for the foundation. The top edge is the height of the top of the concrete floor slab. Its outer face corresponds exactly to the green line on the foundation drawing that was used to layout the position of the building.
This reference piece of Form-a-drain is not used to collect water so does not need or want to have filter cloth. It is installed with the slots facing inwards into the footing trench.
Even though there are no water drainage pipes needed from the outer top Form-a-drain, it is still necessary to implement dummy pipes as spacers. In the case of the outer Form-a-drain the spacer pipes need to be 1' 10-1/4" long. You need to cut 1.25" PVC water pipe to lengths of 1' 10-1/4". You need enough to be able to have spacers about every 4 feet and on either side of corners. Drill hole pairs at each end for the wire.
The gap space between outer top Form-a-drain and the bottom outer Form-a-drain is 1' 10-1/4". Start by attaching spacer pipes of length 1' 10-1/4" to the top of the lower outer Form-A-Drain. Use 16 gauge galvanized wire through holes to hold the Form-a-drain to the spacer pipes. Use a scrap piece of 4' wide EPS to set the distance between the spacer pipes so that the amount of cutting of the EPS you will later have to do is less.
Join and cut Form-a-drain to the required lengths as shown on your AutoCAD drawing. Wire it to the top of the spacer pipe. Fit Form-a-drain corners.
Outer Form-a-drain strengthening rebar
The main aim is to reduce the amount that the top of the outer lower Form-A-Drain is bent inwards due to the force of crushed rock behind it. The rebar length is 3' 1" for the outer Form-A-Drain. Use 16awg galvanized wire through extra drilled holes in the Form-A-Drain. Also wire it top and bottom and middle to the white spacer pipes.
You may need to dig away or wiggle away any crushed rock that stops the rebar going all the way down to the bottom of the lower Form-A-Drain.
Also wire it at the top of the rebar. The twin holes are 5-3/4" from the top of the Form-A-Drain.
Add additional geofabric up the excavation earth
Previously the geofabric was folded over the crushed rock to keep it clean. Now put it vertically up against the soil. From now on you need to be careful that soil does not fall down into the crushed rock.
Join additional geofabric to the existing geofabric with a stapled folded seam and nail through them both to the earth. Even in the case of the non-slope side of the building, the added geofabric should be wide enough to extend up the excavation and later provide enough left over to fold over the backfill crushed rock.
Stop the top falling down using some paving slabs or other creative means.
Polystyrene between outer Form-a-drain
Between the upper and lower Form-a-drain you need some 1.5" thick polystyrene sheet. It needs to be cut into strips that are 1' 10-1/4" wide for the outer Form-a-drain. It is worth getting these widths right so that the polystyrene is a good interference fit between the Form-a-drain. You need to edge it in there by pushing it with the palm of your hand. You measure the inside distance between the spacer pipes and where necessary cut the polystyrene strips to that length.
Expanded Polystyrene Sheet (EPS)
The EPS between the outer Form-a-drain is held in place (from the inward pressure of the crushed rock backfill) by having a wider sheet of 2" EPS on the outside of the Form-a-drain. This extra EPS provides important insulation for the footing.
As you will see on the foundation drawing the additional 2" EPS sheeting is placed on the outside of the outer Form-a-drain . It overlaps the bottom of the top outer Form-a-drain by 1" (so that it is under the lumber bracing) and overlaps the lower outer Form-a-drain by 3/4" (to provide a mechanical stop, yet not cover the drainage holes). The overall width of the outer EPS is 24" (which is conveniently half a 4' wide sheet). At the bottom you may need to cut it and/or groove it to avoid the rebar stakes.
Note for the future: After the concrete pour for the foundations, the last 9" up to the top of the Form-a-drain will have extra 2" EPS sheet added, but this cannot be now because the lumber bracing (added in a future step) needs to be removed before this can happen.
Use a Hotwire cutting bench to cut the 1.5" polystyrene sheet to 1' 10-1/4" wide strips that are 4' long. Also cut some 2" thick EPS sheet to be 2' wide. Use foam adhesive to glue two sheets together. At the bottom the overlap is 3/4" and at the top 1". Weight the assembly down while the foam adhesive is drying.
Fit the polystyrene assemblies between the outer Form-a-drain.
Where there is a rebar stake in the way, cut away the EPS as necessary.
You can use a hotwire knife, but actually I usually prefer to use a 9mm long blade craft knife.
Some trimming is necessary where there are Form-a-drain joins.
At the corners, cut lengths of EPS assembly to fit. Use foam gap filler to glue the edges.
Outer Form-A-Drain height
When you have installed the Form-a-drain all the way round the building, check for levelness and height using a self leveling laser level and a length of white 2" PVC pipe with a felt pen marker line. If any slight adjustment to the levelness of the upper Form-a-drain is needed then this should be done by slight padding under the lower Form-a-drain with porcelain tile shims.
At this stage of accurate height setting it is good to have a laser target on the far side of the build site to ensure the laser height has not moved.
The distance from the top of the outer upper Form-a-drain to grade should be 2'2" which in my case is also 2'2" below the batter board strings.
It goes without saying that you need to set this very accurately. The top needs to be 2'2" at all points.
Bracing outer upper Form-a-drain
Horizontal lumber bracing
The outer upper Form-a-drain is susceptible to bending and warping and generally flapping around. It needs to be stiffened up so that it stays straight. This is done by attaching 2x6 lumber round the outside. The actual dimension of lumber I use from my sawmill after edge trimming on a table saw is 1.5" x 5". The 2x6s are attached using 2x4 on the edge to form an L. The 2x4 has hole pairs every 6".
You will need to make them in sections as the 2x6 lumber is not anything like as long as a length of Form-a-drain. Use the longest lengths of 2x6 as you can easily obtain. Choose only nice straight lumber. The 2x6 is attached to the lower edge of the 2x4. The top holes in the 2x4 are for screwing into the Form-a-drain. The 2x4 piece stops short of the end of the 2x6 in order to provide clearace for the Form-a-drain corner joins.
The L-shaped lumber assembly should be installed 1.75" below the top of the Form-a-drain so it is clear of the top reference edge (even if an extra buttressing piece of 2-by is added on top). I use a 1.75" spacer wooden block to set the height.
Use 2.5" wood screws about every 6" to screw from the 2x4s into the Form-A-Drain. Be careful not over tighten the screws as this would strip the plastic of the Form-a-drain..
At the corners use 45 degree points as it provides clearance from the excavation earth.
Connect pieces of long 2x6 using joiner pieces of about 2 foot length (I use 2'2" as I can get two pieces from the 4'6" lumber that conveniently comes from my sawmill).
Also use overlapping pieces of 2x6 at the corners. The same design of joiners can be used on both outside and inside corners.
Ensure the Form-a-drain is exactly straight as you attach the joining pieces. This can be done by looking along the length to ensure straightness and more importantly using plumb-bobs from the batterboard strings.
Bracing outer Form-A-Drain
To give something solid to attach bracing, it is good to use the batterboard steel stakes. Lumber pieces should be attached to the stakes near the ground so any force does not bend the stakes (which would upset the batterboard location). Use screws through the stake holes and also put lumber on both sides of the stakes and screw together.
You can creatively make use of any nearby structures such as a work shed.
Where there are not suitable batterboard stakes or structures then you will need to add extra stakes.
Where there are nearby columns you can brace to them. Attach the bracing to the bottom-most column stem square as this is the most firmly sited.
Crushed rock behind outer Form-a-drain
Necessary to balance forces
This is crushed rock over the 4" perforated drain pipe that goes round the outside. You should fill with enough crushed rock at this stage to come up to the top of the EPS. The reason not to go above the EPS is that you need to be able to add more EPS later. That height of crushed rock is sufficient to constrain the wet concrete pressure.
You need to be careful and watchful that this does not cause the outer Form-A-Drain to move inwards. To avoid inward movement, do not ram the crushed rock, just pour it in with no gaps. Make sure you have 4'5" lengths of lumber between the lower Form-a-drain to stop it being pushed inwards. You may also add 4'5" lengths of lumber between the upper Form-a-drain as an added precaution.
Where there are big voids you can fill the other side of the geofabric with non-organic soil. Particularly in the case of wet soil, use extra 4'5" bracing planks between the top Form-A-Drains to ensure no inward movement. With the extra bracing in place, wet down the soil with a hose to help it consolidate and set into place (rather than waiting until a rain downpour does it in a less controlled way).
Crushed rock washing
Depending on the type of crushed rock you have obtained, it may be necessary to filter and wash it to remove the rock powder and dirt. This is a lot of work so it is better to try to buy pre-washed filtered crushed rock.
Crushed rock delivery
Carry the crushed rock in buckets and pour it in. When access is tight because the excavation wall is close it sometimes is necessary to trickle it in slowly.
In the case of portico columns, some of the crushed rock will need to be added later because for the moment you need to allow access for fitting the column rebar.
Manicure footing trench soil
This is pretty much the final opportunity to make sure the earth along the footing area is the right height with no high or low spots.
You need to check there are no soil high points and certainly no sharp high points. If there are indents where rocks were removed then fill them with either bag mixed concrete or if you have soil that sets like concrete (like I do) then you can use some filtered non-organic soil. Adding some type-S mortar powder to some filtered less good non-organic soil also works well.
Even though it should have previously been done, it is worth checking the soil at the bottom of the footings is properly compacted. If you have been careful not to over excavate and if you are lucky with the soil type then it may not be necessary to use a compactor on the soil. It wants to be 95% or better relative compaction and it is possible to have it tested if you are unsure about how compacted it is.