Carnation Construction

01-02 - Foundations - Dig Foundation

My foundation design

Designing foundations needs lots of thought.  It took me a while to get to the foundation design that is shown here...

Foundations 

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 stuctural 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).

North south reference line 

You may also need a copy of the string nearer the bottom of the excavation.

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.

Laser 90 degrees 

Laser setting right angle 

Setting right angle laser along string 

East West reference 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 for 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 for the batterboard assembly.  The "East West" reference string will be used to represent the inner edge of the batterboard that goes along the north wall of the main building rectangle (which in the case of steel stake batterboards is 2 feet out from the edge of the Form-a-drain).

After adding up all the 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.

Foundation plan 

The amount of space added on for the batter board assembly is a variable.  If using a wooden assembly weighted down with rocks it would be about 4 feet, but if using steel stakes and a bit of creativity you can get this number down to close to zero.  Getting it to zero means that the inner edge of the batterboard will be 2 feet out from the outer edge of the Form-a-drain.  You will probably have a number of corners where you need to spend the money on steel stakes in order to get the building tucked into the available space in the excavated hole.

Measuring

To tools are useful.  A 100 foot steel tape measure and a Laser Measurer.

Tape measure 100 feet    Laser Measurer 

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.

Metal stake 3 foot 

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 then 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.

Batterboard North East of main house 

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.

Batterboard corner set in 

Lower north south reference string 

The string seen in the above photo is a copy of the north south reference string.  The eastern edge of the batterboard is set by measuring from that north south reference string.

Now you need to set the rest of the batterboard assemblies.  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

Batter boards 

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 choose 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.  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.

Excavation heights - East West

House foundation depths - North South 

Here's the equivalent drawing I used for the evaluation building...

Excavation cross-section

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.

Self leveling laser

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 underneith.

Batterboard set right height with rocks 

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.

 

Batterboards right height south side 

Site - Batterboards set

Laser measurer target 

Laser measurer on corner mount 

 

You should check that you have accurate 90 degree corners.  This is best done using the 3,4,5 Pythagoras rule (or any multiple thereof, such as 9 feet, 12 feet, and 15 feet).  First put in the corner stakes corresponding to one of the long rectangle sides and tie a string between them.  Then take your best guess for the stake and string that defines one of the short rectangle sides.  Measure 9 feet up the short wall string and mark the point on the string.  Measure 12 feet along the long wall string and mark the string.  Measure the distance between the marks on the two strings.  If you have an accurate 90 degree corner, then the distance between the marks will be 15 feet.  If it isn't, then adjust the location of the short wall stake to make it 15 feet.  After you have all four corner stakes in place then as a final check, measure the lengths of the diagonals across the rectangle.  The diagonals should be the same distance (or at least within a couple of inches of each other).

 

Digging Footing trenches and under slab

Every minute counts

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".  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.  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 (owned and operated by C&S Construction).

Mechanical digger 

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 digging you need done.  Ahead of the backhoe arriving, you need to produce a couple of drawings that just describes the digging that is needed.  You will need to transfer these drawings to the real ground using stakes and spray paint so the digger knows where to dig and how deep.  You will need a "Site Mark-out Plan" and an "Excavation Cross Section".  These recommended drawings are described inline below.

 

Don't disturb soil below excavation level

Whatever method of tree removal is used, it is important not to disturb the soil below the level to which you will excavate to for the foundations.  The same is true of any large rocks you find and decide to remove.  The foundations must be built on undisturbed soil.  If you do accidentally go below this depth then you will need to make the foundations go down to that depth.  In practice you will likely fulfill this requirement by filling the accidental hole with crushed stone (gravel), but even so it is definitely worth avoiding.

 

Marker sticks 

 

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 digger will be inaccurate and you will have a lot of manual digging to do.  If the batter board implementation is complex (for a complex house shape) then you may well need two separate rentals of the digger.

Mark-out the ground

Once the site for the house is level, you will be able to accurately mark out the ground as per the Site Mark-out Plan.  When marking out you will use the rough mark-out of the walls you did during site preparation to guide you and get the house in the right place.

Accurately implement strings

Referring to the Site Mark-out Plan you now need to attached the strings that determine the outer position of the footings (shown as red dotted lines on the Site Mark-out Plan).  These strings need to be 20 inches from the centers of the walls.  You don't actually know accurately where the centers of the walls are, so you need to use the figure for the outer footing rectangle that you calculated when you did the Site Mark-out Plan.  The required rectangle is 20 inches bigger all the way round than the rectangle that is the center of the walls.  In the case of my evaluation shed, the length of the outer footing rectangle was 14 feet plus 2 x 20 inches, which is 208 inches, which is 17'4".  The width is 10 feet plus 2 x 20 inches, which is 160 inches, which is 13'4".

The best way to attach strings to the batter boards is by screwing in a wood screw on the top edge of the batter board.  You will actually put the screw in multiple places as you increment towards an exactly square and accurately dimensioned set of strings. Start by accurately positioning an outer footing string for whichever of the long edges of the building you used as your East-West reference on your plot map.  Make the string really tightly stretched.  The batter boards are strong enough to take the tension, so it really just comes down to how much tension the string can take without breaking,  The highest possible tension is important for accuracy.  When the first string is tied taught, position whichever of the short side strings you used as your North-South reference on your plot map.  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).  Initially try to get the 90 degree angle about right by using a good sized framing square. 

Framing Square

You will need to iterate the position of the North-South short wall string to get it right, but the best way to check that you have a fairly accurate right angle between two strings is to measure 9 feet from the string crossover along one string and 12 feet from the string crossover along the other string, and the hypotenuse of the triangle formed by these points should measure 15 feet.  You can of course use any multiple of Pythagoras' 3', 4',  5' rule (the bigger the better).  Note that if you adjust the angle to get it right then the crossover point will also move, so hence the need for iteration.  I find that making temporary marks on strings is best done using pieces of electrical insulation tape.  You will 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,

Now measure the longest outer footing rectangle dimension along the longest string from the string crossover point, and the shortest outer footing rectangle along the short string from the string crossover point, and mark these points using electrical sticky tape round the strings.  These tape markers will allow you to position and tie the other two strings from the batter boards to make the full rectangle that represents where the outside of the footing will be all the way round the building.

Check string positioning and adjust as needed

The most accurate way to check if you have exactly 90 degree angles at the string cross over points is to measure the distances between opposing right-angles of the rectangle.  The distance from the North-West corner to the South-East corner needs to be exactly the same as the distance from the North-East corner to the South-West corner.  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.  Again you will need someone to accurately hold the other end of the tape measure because you cannot hook the tape measure onto the strings without pulling the location of the string crossover point.  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.  The tighter the strings, the less likely it is for the crossover points to be accidently moved when measuring.

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.  You should get the diagonals the same length to within 1/8" and that error is just because it's really hard to measure between string crossover points without accidentally moving the crossover points.

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.  For inner corner walls, the batter boards will be within the house area.

Inner footing strings

It's useful to also tie strings 40 inches in from the outer footing strings.  The 40" is two times the 20" that the footing edge is from the center of the wall.  It's good to attach these strings as it allows you and the digger operator to visualize the width of footing trench you need.  The inner strings are just a guide rather than a formal reference (the outer strings are the formal reference).

Strings will be detached or broken constantly

The reality is that both the outer and inner strings will be in the way when the digger 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 digger 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.  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.

 

Excavate Footings, then under-slab depth

Dig the footings first

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 digger has a level surface to work from.  The digger will have no problem digging the deep footing trenches first.

Digging footing 

Footing corner 

It may also seem strange not to dig the sewer trenches first as you won't be able to get the digger in later to do them, but the reality is that a big digger is not the best choice for digging the sewer trenches as you want narrow sewer trenches and the digger can only dig trenches corresponding to its bucket width (eg 32").

Don't forget about any internal ICF walls in your house design as they also need to have footing trenches.

In my design, the depth of the footing trenches is 65.5 inches below grade (which is the height to which the site was cleared), and that is 85.5 inches below where I put the batter board strings.

The width of the footing trenches needs to be 40 inches in from the outer footing marker strings and an additional 12 inches outside of the outer footing marker strings for the weeping tile drainage pipe.  You also need one inch of clearance on the inside.  That means the footing trench will be a total of 47 inches 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 digger 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 digger will be used to dig the rest of the under-house area down to the under-slab depth.  Even though this done after the footings, the reality is that the digger 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 a digger 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 digger 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.

Footings half done 

In my design, the depth of the under-slab level is 43.5 inches below the the cleared site level (which is "grade" height) and that is 63.5 inches below where I put the batter board strings.

As per the ground markers and Excavation Cross-Section drawing

Prior to the dig and as needed during the dig, you will have transferred the batter board string positions to the ground using spray paint and some temporary marker sticks.  This is the info that the digger operator needs from the Site Mark-out Plan.

Site mark-out plan 

Here's the version of the plan for the main house...

House foundation plan 

For the depths, it is best to just give the digger operator the Excavation Cross-Section drawing.  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 place then it will be necessary to use more concrete to fill it.  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 digger stop digging about 2 inches less than on the drawing and finish off the last 2 inches by hand.

Excavation cross-section 

Here's the equivalent (and more complex) version for the main house...

House foundation depths 

Implement mid-way alcoves for bracing stakes

When there is a long length of foundation wall it is not sufficient to rely on the horizontal lumber bracing to hold the outer wall of the foundation straight and in the right location.  It is necessary to bag in a 2x4 stake half way along and to brace from this to the horizontal lumber bracing on the foundation wall.  In order to make room for the vertical stakes, it is necessary to make some alcoves beyond the 12" extra needed for the drainage pipes.  An extra 6" will suffice, with an alcove width of 6".  Alcoves are needed about half way along a straight section of foundation if the length is greater than about 20 feet.

Dig pipe trench from the building

Used for sewer, drainage, water, electricity, and data

Before the digger 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 the other utilities such as water, electricity, and data etc in the same pipe trench.  Details on implementing a pipe trench can be found here .

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 sewer and drainage pipes exit under the footing.  They will exit with the sewer pipe above the drainage pipe but you can gradually transition to them being side by side if you want as long as the sewer pipe gradient is properly maintained and the drainage pipe has some sort of downward slope.

It is very important to get the digger operator to dig the trench with the proper slope.  It needs to be right and you won't want to finish the digging by hand as it's hard work.

Evaluation Shed dig photos

Access drive to site...

Make access driveway 

Footing trench seen from bottom of trench...

Digger from ditch 

Part way through the dig.  Some of the footing trenching has been done and some of the under-slab level...

Footings half done 

Excavation viewed from the pipe ditch...

Excavations from ditch 

Digger has finished doing its work...

Shed digging done 

Shed excavation from above 

Shed excavation from above 

Batter boards 

 

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 1 inch short of all the figures on the drawings and finish things off by hand digging.  You need to dig down to 65.5" below grade (which in my case was 85.5 inches below the batter board string reference).  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 less than 65.5" deep.

Transferring points to the ground

The batter boards are on the cleared site height, but you are now working down a hole.  You need to be able to relate the positions where the batter board strings cross to the ground in the hole so you know exactly where the footing trenches should be.  This is done using Plumb Bobs.  It's good to use 8 oz ones because they are heavy enough to work but not too heavy that they will cause the batter board strings to sag excessively.

Plumb Bob 

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.

Glue Gun

Then tie vertical strings from the string cross-over points with a plumb bob attached on the end of the string just above the ground in the hole.

Plumb bob hanging down 

Strings between batter boards 

Allow for string droop

The weight of the plumb bobs will cause the batter board strings to droop a bit and that will mess up your height reference.  The easiest way to re-establish the reference height is to double up on all the batter board strings.  One set of strings is used to attach the plumb bobs and a second set attached to the same batter board screws will not have any weight on them so will be at the correct reference height.

Footings multiple strings 

 

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 12" to allow for the 4" perforated drainage pipe.  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 thoroughly 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 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.

 

Mark-out sewer and internal weeping tile trenches

As per Site Mark-out Plan

The required locations of the sewer and internal weeping tile drainage pipes are marked on the Site Mark-out Plan (in the drawing below, it's the joggled line in the center).  Note that my evaluation shed does not actually need a sewer pipe, but I went through the motions because the job of my evaluation shed was to test out the various building techniques prior to building the main house.

Site mark-out plan 

Here's the version of the plan for the main house...

House foundation plan 

The trenches for the sewer and drainage pipes as they exit under the footing are the deepest part of the whole excavation.  In the diagram below you will see that they are 80" below grade level as they pass under the footings, which is in my case 100" below the height I used for the batter board strings (the strings are the vertical reference).  At the furthest point from the exit point, the drainage ditch will be 6" deeper than the footing depth.

Excavation cross-section 

Here's the equivalent (and more complex) version for the main house...

House foundation depths 

Note that the actual depth that the pipes exit at on a particular house design is determined by what is necessary to achieve the necessary slope.  See below for details on the required slope.

Narrow as possible

With strings and simple wooden stakes you need to mark-out on the under-slab excavation area where the sewer and weeping tile trenches need to be.  The trenches should be as narrow as possible because you don't want to disturb more of the soil 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 sewer ditches about 8" wide.  This allows for a 4" sewer pipe and a 4" perforated drainage pipe on top of each other with 2 inches of drain rock either side.  In the places where the trenches pass under the footings you will need to widen the trench to about 12 inches so that the drainage pipe can be offset to connect upwards with the Form-a-drain .

Standard angles

You should try to do it to closely match the locations of the drainage and sewer 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 sewer 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 sewage flow won't be as good and building code requires more clean-outs..

 

Dig sewer and internal weeping tile trenches

Drainage pipe depth

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" below the footing excavation depth (which is 91.5" below the batter board string height in my evaluation shed case).  The drainage pipe will slope gradually down to the height that it exits under the footing.  The ideal slope is 1/4" per foot, but it needs to be at least 1/8" per foot if it is going to correspond with the sewer pipe slope.

Sewer pipe must be correct gradient

For a sewer pipe you need to achieve the right gradient to get proper sewage flow.  The gradient needs to be 1/4" per foot and this is required by building code (the minimum of 1/8" per foot).  The trench for the sewer stack that is furthest from the exit point will be the shallowest trench.  The trenches will gradually get deeper (at a rate of 1/4" per foot) as you get towards the exit point for the house site.  Obviously the exit point needs to be the lowest point (ie the deepest trench) so that the sewage flows there.  The furthest sewer stack is the highest point.  The trench needs to be deep enough to be properly under the slab and insulation, but with the deep design of footings, that is unlikely to be a problem.  The sewer pipes will be at the required 1/4" per foot slope and then will travel up vertically through the slab.

Sewer pipe will be mounted on top of drainage pipe

Both the drainage pipe and the sewer pipe are in the same ditch and have the same slope.  As such it makes sense to mount the 4" sewer pipe directly on top of the 4" drainage pipe.  Having them on different planes makes it possible to do Y connections for both.  You will actually need to space them apart slightly to allow for the greater diameter of the couplings and a good way to space them is to use some 1/2" polystyrene sheet cut into 4" strips.  Here's the section of the foundation design drawing that shows the pipes...

Foundation sewer and weeping tile pipes 

Even though one on top of the other is the preferred way (to keep the ditch narrow), where the pipes go under the footings you need to offset them by 4" so that the drain pipe can collect water from the Form-a-drain above.

  

Video of Evaluation Shed dig

There's a video of digging the footings here .

Shed digger film

 

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 hopefully 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 big 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 12 inches all the way round on the outside of the 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 sewer and 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.

Swept footing ditch 

A few extras

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 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 assessable 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.