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My foundation design
Designing foundations needs lots of thought. It took me a while to get to the foundation design that is shown here...
I did a lot of reading of building science papers and a lot of thinking. I then talked through the proposed foundation design with every knowledgeable person I could find. I talked it through with a civil engineering friend, the guy at the local county building department, my local friendly building inspector, the structural engineer, and various building product suppliers. Eventually after lots of iterations I arrived at a design I am happy with.
Accurately mark out house footprint
You should now after the excavation have a completely level flat site. During the excavation you will have uprooted all the stakes you previously put in to show the house outline. Anyway they were probably not very accurate if the land was not flat. You can now mark out the house accurately given the flat surface.
North-South reference
Start by re-doing your "North South" reference string. This should be from the northern perimeter of the excavated flat surface to either due south or to a stake as far south as you can get such that the string is accurately pointing at the object of interest (eg a specific mountain). Note that this initial north-south reference string does not represent anything on the building (eg a wall or whatever). It is purely used to establish the direction you want the building to face.
You may also need a copy of the string nearer the bottom of the excavation. You can set this lower string by hanging plumb-bobs from the upper string.
East-West reference
Next re-establish your "East West" reference. This will be placed at the northern edge of the footing for the main house rectangle. It needs to be exactly at 90 degrees to the "North South" reference string. I find that using a 90 degree dual laser tool is useful for establishing a right angle. You can align one edge with the string at the required point along the string.
On your CAD drawings you should have marked the outside edges of the footings. This should of course also show any building bump-outs on the north side of the building. It is the northern edge bump-outs of the building that determine where the East West reference string will go. These bump-outs need to fit within the excavated level area. Also allow 1 foot for the drainage gravel and another foot to give room for solid soil before getting to the batter board assembly. Allow further distance (say 2 inches) for the batterboard assembly. The "East West" reference string will be used to represent the northern outer edge of the Form-a-drain for the main house rectangle.
After adding up all the bump-out distances that you need to allow for, measure from the northern most edge of the excavated flat area along the north south reference string and that is the point at which the East West reference string will cross at 90 degrees.
The amount of space added on for the batter board assembly is a variable. If using steel stakes and a bit of creativity you can get this number down to close to about 2 inches. Getting it to 2" means that the inner edge of the batterboard will be 2 feet out from the outer edge of the Form-a-drain. Even though steel stakes cost $5 each and you need between 4 and 6 per batterboard, that is still my recommended way of implementing batterboards, particularly when fitting a building into an excavated hole.
Measuring
Two tools are useful. A 100 foot steel tape measure and a Laser Measurer.
Copy north-south reference to be eastern footing edge
The north-south reference string was just used to establish the direction that the building will face. Now you want to establish the east or west wall of the outer house rectangle. If the eastern wall is the most cut into the excavation then choose that wall to become the reference as it is the most difficult to accommodate. You need the outer edge of the Form-a-drain for the east wall to be just over 2 feet in from the excavated bank (to allow for the batterboard etc).
After deciding where the eastern outer edge of the Form-a-drain needs to be, make a copy of the north-south reference string at that position. Do this by measuring the same distance from the north-south reference string at two points. Setting the eastern edge (if that is the most excavated) will ensure that you have the building nicely fitting within the excavation.
The string seen in the above photo is a lower down copy of the north-south reference string. The eastern edge of the batterboard is set by measuring from that north-south reference string.
Batterboard design
The horizontals of the batterboard are made from 2x8 lumber. I cut my own lumber using my SawMill. The stakes are steel stakes because the ground is so solid. Steel stakes also take less space.
It is important that the batterboard assemblies are very strong. A lot depends on the accuracy with which you mark-out the foundations, and that means you don't want the batter boards to move.
The exact positioning of the batter boards does not matter as it will be the strings that you attach to the batter boards that set the accurate position for the foundations. It also does not matter if the stakes go into the ground crookedly, just as long as they do not move around.
It even does not matter too much if you want to put one or two of the batter boards further out than normal to for example give room for an access driveway. If you are using corner batterboards (like I typically do) then you can only move them out by about an extra foot or else the legs will not be long enough for the strings in both directions. Typically it is good to stick to having the batterboards between 2-3 feet from where the Form-a-drain edge will be.
You bang in the stakes using a big club hammer. With rocks in the ground, the stakes will probably not go in completely vertical, but that's ok. Being solid in the ground so they don't move is what counts.
Sometimes the batterboard assembly has to be very much set into the side of the excavation. It may be necessary to chip away at the edge of the excavation to get the batterboards in the required place.
You need to set the rest of the batterboard assemblies so that their positions all correspond with the house foundation layout drawing. It is best to do the most difficult ones first, ie the ones that are tight up against the edge of the excavation. In my case I mainly used a convention of the inner edge of the batterboards being 2 feet from the outer edge of the Form-a-drain, which itself is about 8.5" from the outer face of the house wall, so the outer face of the wall is about 2' 8-1/2" in from the inner edge of the batterboard.
Initially it is about getting the batterboard assemblies in the right places as per the plan, rather than setting the height of the batterboards. At this stage, the batterboards are not screwed to the stakes.
Set the height of the batterboards
Once the stakes are securely in the ground, you will then screw horizontal 2x8 boards to the stakes such that the boards are all the same height all the way round the building. They do need to be exactly (better than +/-1/8") the same height because you will want to use the strings as a vertical reference point when implementing the foundations and slab.
The height you chose for the top of the batter boards is not that important (as long as they are all the same). In my case 2' (24") above what I decided was going to be the top of the basement slab level was what I chose. That also happens to make the batterboard strings at reference grade height which is nice. When you have decided on this batterboard height, make sure to add it to your excavation cross-section drawing because you will be using the strings attached to the top of the batter boards as a height reference.
Here's the equivalent drawing I used for the evaluation building...
The easiest way to get the batter boards all exactly the same height is with a self leveling laser level. A good one that self levels and makes a horizontal line is about $150.
Details
are here .
Place the laser on a level platform in the middle of the house area such that the horizontal beam is just above the batterboard height that you have decided to standardize on. Note the distance above, eg 1 inch above. Rotate it to shine in the direction of each batterboard in turn and get the top of the batterboards to all be at that distance (eg 1") below the beam.
Initially it is best to set the batterboard heights using rocks underneath.
Once you are happy with the height then you can screw the stakes to the horizontal lumber.
The batterboard horizontals need to be accurately level as well as accurately the right height. A check worth doing is to move your laser level to one batter board and shine it at another batter board because that will make sure your laser really is working accurately.
Setting the outer edge footing strings
Once the batterboards are all installed in about the right locations and their height has been accurately set then it is time to add the strings that will accurately represent the outer edge of the outer Form-a-drain that forms the footing. The east-west reference string should be about right for the north wall of the main house rectangle, and the copy of the north-south string should be about right for the eastern wall, but there is still an opportunity to make them even more accurate.
Strings are implemented by putting a wood screw in the top edge of the batterboards. Tie small loops in the end of the strings and stretch them between the screws. They need to be as tight as you can possible get them. They want to be just short of the breaking point of the string. You will likely need to tie multiple loops in the end of the string so you can stretch it to the tightest one. The tighter you make the strings the less they will move about and the greater the accuracy you will achieve. Be careful because when strings break you can get a nasty string burn on your arm, leg, or face!
You will likely need to move the screw positions a bit to get everything dimensionally correct and square.
Using string between batterboards may seem a bit low tech, but it really is the most accurate way. It relies on the point of crossing of the strings rather than the absolute position of the batterboards. Avoid the temptation of using glue where the strings cross because accuracy relies on the string's ability to move with things like string age or ambient temperature.
The taught strings extend beyond the footings (to reach the batter boards) and need to cross each other at exactly a right angle (this is the outer corner of the footing).
Use a 100ft tape measure to set the other wall strings so they correspond with the foundation layout house drawings. By measuring from the reference strings you should be able to preserve the right angle that was previously set using the 90 degree laser tool . Note that sideways force from the lip of a tape measure will move the strings slightly even when they are very tight, so try to avoid and compensate if necessary.
I find that making temporary marks on strings is best done using pieces of electrical insulation tape. You may well need someone to hold the other end of the tape measure because you don't want to accidentally move the strings by hooking a tape measure to them.
Using loops to attach the strings to the wood screws is better than using a tight knot because it allows you to temporarily remove the strings if they get in the way, eg get in the way of the digger. What is actually the most important is that the screws do not get moved.
Refining the 90 degree accuracy
We need the 90 degree corners to be very accurate or else the building will be skewed (and the roof won't fit properly).
If you have an accurate 90 degree corner, then the distance from string crossover to string crossover should be the same across both the diagonals of the rectangle, ie the hypotenuses should be equal. We're looking for better than a 1/8" difference here.
Measuring between string crossover points is hard because even very tight strings have some movement if any side force is applied, eg by the hook of a tape measure. I made assemblies (pictured below) out of plastic card that could be fitted over the string crossovers. On the underside, pairs of 1" squares of plastic card form string width grooves to make sure the strings were accurately in the right place. You just need one assembly that forms a vertical flat target for the laser and one assembly with a vertical edge against which the middle of the laser measurer will be positioned. You use the pair of assemblies to measure one hypotenuse and then you move the pair to the other hypotenuse.
Do fine adjustment of the positions of the string screws in the batterboard top edges to gradually increment until you have the hypotenuses the same length.
Realistically some adjustment will be needed to ensure you have perfect 90 degree angles and you also have exactly the required width and length for the outer footing rectangle. Keep iterating until these requirements have been accurately met. Draw a trapezoid on a piece of paper and note down the lengths of all the sides and both diagonals for each iteration you do, because it is easy to get confused about which direction to move the strings and by how much. Every time you move the screw that the string is hooked on to, try to use it as an opportunity to get the string just a little tighter.
Accuracy is very important
Any error at this stage will cumulatively multiply as you build your house and may even cause the roof not to fit properly, so it really is worth taking the time to get it right.
More complicated shapes
Typically a house foundation is more than just a simple rectangle. In my case I have portico protrusions on the north and south sides of the house and a couple of internal ICF walls. These all need proper footings. The footings for these also need to be marked out using additional batter boards and strings. Use the same techniques as described, but of course do it relative to the main house rectangle.
Here is my foundation layout plan...
Marking out the deck support pillars
In the case of discrete foundation blocks such as are used for the pillars that support concrete decking, it is not very efficient to use batterboards. A better strategy is to implement the wooden squares that are going to be needed later anyway to form the concrete for the pillar foundation. You can then position the squares on the ground to mark where the various holes need to be dug.
The picture below shows the wooden outline used to form a corner block (that helps resist lateral house movement on the downhill side of the slope).
The following shows the site with the discrete foundations marked using their wooden forming frames.
Digging Footing trenches and under slab
It is way too much work to try to dig foundation trenches manually - you need a machine.
A mechanical digger is a big tractor like machine with a big digger arm on the back or front. Sometimes they have big rubber wheels and sometimes they have tracks. They go by various names such as backhoe, trackhoe, excavator, or JCB. You can use a friendly generic name such as "digger", but I tend to say excavator. They are way too expensive to buy your own for building your house, so you will need to rent one, and it will probably come with an experienced operator.
Every minute counts
They are not cheap to rent, so you need to avoid it sitting idle for more than a few minutes. That means you need to be super prepared ahead of it arriving on site. Here is a picture of the one I used on the evaluation building (owned and operated by C&S Construction).
For the main house I needed a full sized excavator because I was dealing with very hard rocky ground and a much bigger area. The pictured one is from Dan Crocker Construction (who I used for the main house).
Do very targeted excavation drawings
You will have done detailed drawings of your house floor plan and your foundation design, but these drawings have lots of unnecessary information that does not pertain to the marking-out and the digging you need done. Ahead of the excavator arriving, you need to produce a couple of drawings that just describes the specific digging that is needed. You will need to transfer these drawings to the real ground using wooden stakes and spray paint so the excavator knows where to dig and how deep. You will need a nice simple "Excavation Plan" and a simple "Excavation Cross Section" so it is easy to explain to the excavator operator what's needed.
Do the excavation drawings accurately in your CAD package. Add a layer that shows where the excavation cuts should be and then turn off all the other layers. Take a JPEG of the drawing and then color in the different heights.
A few useful numbers
For external walls
From outside of concrete to the outside of the outer Form-a-drain is
1'4".
From outside of
concrete to the outside of the inner Form-a-drain is 3'4".
From outside of concrete to center of vertical rebar is 5-3/4" (both 8"
and 12" cavity walls).
For internal walls
From outside of concrete to the outside of the outer Form-a-drain is 2'0".
From outside of concrete to the outside of the inner Form-a-drain is
2'8".
From outside of
concrete to center of vertical rebar is 4".
Where there is a drainage pipe round the outside then add 6" of extra
excavation width.
Where there is not a pipe (eg on the inside) then
just add 1" of extra excavation width.
Note that the Form-a-drain strings on the batterboards are different for internal walls compared with external walls. That means there will sometimes be two outer strings and two inner strings on one batterboard.
Excavation batterboard strings
During the excavation for the footings etc you are not interested in the position of the Form-a-drain. You will remove the Form-a-drain strings and just have strings that tell the excavator where to dig. You will not remove the screws that the Form-a-drain strings attach to so it will be easy to put them back later. You will add the excavation strings using the Form-a-drain screws as the reference.
Your CAD drawn excavation plan will tell you where the excavation strings should go, but typically they are 6" further out than the outer Form-a-drain string position, and 4'9" inwards from the outer Form-a-drain string position.
Strings will be detached or broken constantly
The reality is that both the outer and inner strings will be in the way when the excavator starts digging. What's important is that the wood screws in the top of the batter boards stay in place. You can unhook the strings from the screws when necessary and when strings get broken you can implement new strings. To help the excavator operator you can use spray paint on the ground and/or temporary stakes, that you place by being guided by the strings before you unhook them. If the excavator operator is color blind then it is best to use white spray paint. Once in a while you can re-hook the strings to make sure the digging is proceeding well and re-implement temporary stakes as necessary.
Don't dig until properly marked out
The actual foundation digging (ie the footing trenches and the under-slab level) must not be done until the batter boards and batter board strings are accurately set in place or else the digging with the excavator will be inaccurate and you will have a lot of manual digging to do to finish off (and correct).
Note that the underslab area will also be excavated down as part of the same dig operation that digs the footing trenches. It is best done in one dig process so as to make it possible to get the excavator positioned where it needs to be and keep it level. Typically a length of footing trench is dug and then as the excavator backs out of that area it will take the underslab area down to its required lesser depth.
Excavate Footings, then under-slab depth
Dig the drainage ditches
The thin ditches for the drainage pipes are the lowest part of the excavation. You will need to fit the excavator with the narrowest possible bucket. Ideally the bucket would be only 6" wide, but it is difficult to find an excavator company with a bucket that narrow on a big excavator. You do need a big excavator if (like me) you have big rocks.
The drainage ditches have a slope of 1/4" per foot.
Assuming the ditches are narrow, it should be possible to drive the excavator over them without them collapsing.
Dig the footings
It may seem strange to dig the footing trenches first given that they are deeper than other parts of the excavation such as the floor slab level, but this is necessary so the excavator has a level surface to work from. The excavator will have no problem digging the deep footing trenches before the slab area. In practice the excavator will dig a section of footing and then dig down to under-slab level as they move away from that area.
Don't forget about any internal concrete walls in your house design as they also need to have footing trenches.
In my design, the depth of the footing trenches is 5' 5-1/2" (65.5") below grade (which in my main house case is 3' 8-1/2" (44.5") below the height to which the site was cleared and 3' 5-1/2" (41.5") below top of slab) and that also happens to be 5' 5-1/2" (65.5") below where I put the batter board strings (because I made the batterboard height be at reference grade, ie 2'0" (24") above top of slab).
The width of the footing trenches needs to be 4'8" in from the outer footing marker strings and an additional 6 inches outside of the outer footing marker strings for the weeping tile drainage pipe. That 6" corresponds to a 4" perforated pipe surrounded by 1" of crushed rock on each side. You also need one inch of clearance on the inside. That means the footing trench will be a total of 4'8" + 6" + 1" = 5'3" wide at the bottom.
With all the digging, it is worth finishing off the last couple of inches by hand as this will achieve the best accuracy, so get the excavator operator to subtract 2 inches from the depth on the drawings when they are digging.
In practice, the cut away earth verticals round the outside will be a steep angle rather than actually vertical, but that's ok. Even so, try to keep them as vertical as possible as that undisturbed soil is useful for preventing lateral movement of the building in an earthquake.
Also under-slab area
The excavator will be used to dig the rest of the under-house area down to the under-slab depth. Even though this is done after the footings, the reality is that the excavator will work across the house area digging a bit of the footing trench and then digging down to under-slab depth in that area before moving on to do a bit more of the footing trench. Operating an excavator requires lots of skill and an important part of this skill is avoiding snookering yourself so you can't now get access to something you need to dig. Make sure the excavator operator knows all the details of what you want for the trench depth and location and the under-slab depth and they will figure out the best digging sequence to achieve the right final result.
As per the ground markers and Excavation drawings
Prior to the dig and as needed during the dig, you will have transferred the batter board string positions to the ground using (white) spray paint and some temporary marker sticks.
You will continuously check depths throughout the digging process by measuring down from the batter board string height. Remember that it is very important not to ever go deeper than the depths shown on the drawing because you need to build the foundations on undisturbed soil.
If you do accidentally go too deep in a particular small place, eg to remove a large rock, then it will be necessary to fill the area with well tampered crushed rock. Never just add soil and try to compact it into the hole as it will not work properly and you house may sink (unevenly). It is safest to have the excavator stop digging about 2 inches less than on the drawing and finish off the last 2 inches by hand.
Dig pipe trench from the building
Used for sewer, drainage, water, electricity, and data
Before the excavator leaves your site you will need a sewer and drainage pipe trench dug from the building to the septic system and for the drainage water to a drain area well away from the building. You may well also choose to put other utilities such as electricity and data etc in the same pipe trench. It's certainly not a bad idea to have some spare electrical conduit in there. All these details should be on your plot drawing.
Must be proper slope
The pipe ditch with the sewer pipe must be sloped downwards at a uniform angle of 1/4" per foot (+/-1/8"). This slope also works well for a drainage pipe. If you cannot maintain this proper slope then you will need to route the sewer pipe to a collection tank and then pump it from there to the septic system.
The starting point for the top of the pipe trench slope is the depth at which the drainage pipes go under the footing Form-a-drain at the furthest side of the building. You need to maintain the drainage slope under the building, so that determines the depth of the drainage ditch on the side nearest the drain away. It is the drainage pipe that determines the depth of the trench from the house because the sewer pipe actually exits through the wall rather than through the slab. Mark the depths on your foundation excavation plan.
Photos of main house excavation
Evaluation Shed dig photos
The following photos were taken while implementing the evaluation building.
Access drive to site...
Footing trench seen from bottom of trench...
Part way through the dig. Some of the footing trenching has been done and some of the under-slab level...
Excavation viewed from the pipe ditch...
Digger has finished doing its work...
Finishing off the dig accurately by hand
Need good accuracy
It is important to get your excavation as close as you possibly can to your drawings to avoid cumulative errors later. The best way to do this is have the digger stop 2 inches short of all the figures on the drawings and finish things off by hand digging. It is ok to accidentally go down an extra inch as a bit of extra concrete will fill in the extra, but around the edges of the trench (where the foundation forming goes) it must not be lower than the figure in the drawings.
Transferring points to the ground
The strings between the batterboards accurately give the outer edge position of the Form-a-drain and accurately give a reference height. What we need is to transfer the position of the Form-a-drain to the earth at the bottom of the footing trench. If we were to hang Plumb Bobs from the strings then the weight would pull the strings down and mess up the height reference. The solution is to fit two strings to everywhere that there is currently one string. The string without the Plumb Bobs will be the height reference.
It's good to use 8 oz Plumb Bobs because they are heavy enough to work but not too heavy that they will cause the batter board strings to sag excessively.
The first step in attaching the plumb bobs is to glue the batter board string cross-over points so they don't move around. Hot melt glue works well for that. This is only done for the set of strings designated for Plumb Bobs hanging. Gluing hurts the position accuracy but is necessary when hanging Plumb Bobs at the crossover points.
Tie vertical strings from the glued string cross-over points with a plumb bob attached on the end of the string just above the ground in the trench. You will need to be able to sometimes vary the length of the vertical string to the Plumb Bobs to tie the string with that in mind.
Excavate width beyond string locations
The batter board strings mark the inner and outer locations of the 40" wide footings. Round the outside, the excavation needs to be an extra 8" to allow for the 4" perforated drainage pipe and crushed rock. Round the inside, the excavation needs to be an extra 1" to allow enough space for the stakes that hold the Form-a-drain. These extra width areas need to be excavated to the same depth as the footings.
Compact the soil
Hire a compactor (if required)
Depending on the soil characteristics, the bottom of the excavated footing trench may need to be compacted, particularly in areas where large rocks were removed. (Do not try to compact the under slab area because it will likely cause the edges to cave into the footing trench.) The best way to do the compaction if it is needed is to hire a compactor machine for a day. You need one that is 5HP or more.
The subgrade needs be compacted to 95% relative compaction. Move compactor over every square foot of soil until the soil is firm and smooth looking. It is possible to have the soil tested to measure the compaction.
For my soil type compaction was not necessary as it was well compacted by a glacier during the last couple of ice ages and I was careful during excavation not to disturb the soil at the bottom of the footing trench and under the slab area.
Mark-out internal weeping tile trenches
As per Site Mark-out Plan
The required locations of the internal weeping tile drainage pipes are marked on the foundation site plan.
The thing that determines the depth of the drainage pipes is the need to drain away any water that has collected in the bottom inner Form-a-drain. That means the 4" drainage pipe needs to pass immediately under the Form-a-drain on the furthest side. That means the ditch needs to be 6.5" below the footing excavation depth at the farthest away point.
The trenches for the drainage pipes as they exit under the footing are the deepest part of the whole excavation. It is necessary to keep them sloped at an angle of 1/4" per foot.
The length of the drainage pipe from the furthest point to the exit point is a worst case of 72 feet. With the 1/4" per foot slope that means the ditch at the exit point needs to be about 18 inches deeper than at the furthest point. At the exit point the drainage ditch will be 18" + 6.5" = 24.5" (2' 0-1/2") below the footing depth. That is 65.5" + 24.5" = 90" (7' 6") below the batterboard string reference height.
Narrow as possible
With strings and simple wooden stakes you need to mark-out on the under-slab excavation area where the drainage trenches need to be. The trenches should be as narrow as possible because you don't want to disturb more of the soil in the slab area than is absolutely necessary. That's why you were not able to do these trenches with the digger as a digger can only dig a trench to be the width of its bucket (say 32"). You should make the drainage ditches about 8" wide. This allows for a 4" perforated drainage pipe with 2 inches of drain rock all around it.
Standard angles
You should try to do it to closely match the locations of the drainage pipes with your drawing, but an occasional bit of adjustment if there is a big rock in the way is ok. It is important, both in your trench design and in the actual mark-out to stick to angles that correspond to available pipe bend pieces. Using 45 degrees is a good choice, but 22.5 degrees is also available if needed. You can also get 60 degrees and 90 degrees, but anything tighter than 45 degrees should be avoided as the flow won't be as good.
Sewer pipe
Typically a sewer pipe from the sewer stacks is also routed under the slab and comes up through the slab. I opted not to have the sewer pipes come through the slab as it is a potential leak point. Instead I route the sewer pipe through the wall just above the slab and route it through the basement under a raised floor.
Video of Evaluation Shed dig
There's a video of digging the footings here .
Dig holes for concrete corner blocks
On the two downhill corners
These are the two big blocks of concrete in the downhill corners (south-west and south-east corners in my case) that will help stop the house moving laterally down the slope in an earthquake. If your house is not on a slope then you can do without the corner blocks. This concrete is outside the polystyrene insulated envelope, so the concrete blocks will be at outside earth temperature (45 degrees F in my area).
Deep as possible away from footings
The holes will be deeper as they go away from the footing. The bottom of the holes should slope away from the footings with a 2:1 force line. Go as deep as possible, particularly at the deepest point under where in my case the decking support column goes. Need to take every opportunity (eg removed rock) to get the concrete well pinned into the solid soil as deep as possible.
Useful dump site
Concrete trucks will not want to wait around while you work on a problem with your ICF walls. These corner block holes will be a useful place to dump concrete if you run into problems while pouring the ICF walls.
Finishing off the dig
The digging is pretty much at an end (big sigh of relief as it's hard work), It is however worth doing a bit of checking and finishing off.
Check list
Measure all your excavation depths in all places. Make sure the trenches are the right widths and in the right places relative to the batter board strings. Have you allowed 8 inches all the way round on the outside of the Form-a-drain marking plumb bobs for the outer weeping tile pipe? Have you allowed an inch clearance on the inside of the inner footing strings? Make sure there is not a single place where the footing trench is not deep enough or not quite wide enough. Even one sticking up rock will stop you placing the forming for the footing (the Form-a-drain). It is ok if the ditch is slightly deeper than 65.5" below grade because the concrete will fill up the difference, but it cannot be less than 65.5" under any place that the Form-a-drain will go.
Make sure your drainage pipe ditches are cut across the footing trenches. Note that the drain pipe ditches are lower than the bottom of the footing trench.
Make sure there is room for the vertical drain pipe clean-outs to come up outside the footing (on the opposite side of the building from the exit point).
Make sure the drainage pipe ditch continues on from the exit point on the building and maintains the proper 1/4" per foot slope.
Sweep the ditches
As a final step, sweep the ditches with a broom to get out the last of the loose soil.
A few extras
If appropriate, dig the trench or trenches (or at least the beginning of the trench or trenches) to bring electricity, water, data, and anything else you need to the wall behind where your services will be received in the house. It's good to do these excavations now so the digging does not disturb the intricate forming of the footings. It may even be that you need to route some of these services under the building, possibly in the same trench used for the drainage pipes.
For these various services it is best to route them through the north wall of the building near where you plan to have your services area inside the building. It's best to come through the wall rather than through the slab as the ground water table pressure will be a bit less and it will be more accessible to fix any problems. Even though it is through the wall, the water pipe does still need to be below grade and below the frost line. It's good to put the electrical feed at a similar height to keep it hidden out of the way.