Once a site foundation has been graded to within an acceptable tolerance, the footing layout and excavation can begin. Footings are the building equivalent of snow shoes – they carry the foundation, and spread the load out so the building doesn’t sink. Footings are typically made of concrete, often reinforced with rebar to control cracking, and in the event of cracking to prevent separation that would lead to foundation cracks and settlement that would spread throughout the structure.
In the case of a structure that will have a block foundation set upon the footing, the line of block must be centered upon the footing, and the footing must have projection, extra material that extends beyond the block on both sides. The number of inches of required projection will vary by local building code, and around here it is 4″. So, we would move the dig line, 4″ out beyond the line of the wall. In the case of my shop, I won’t have a block foundation, as the “I ” beam columns that support the roof and walls will bolt directly to the slab. However, I do still need footers, to provide increased concrete depth for weight bearing under the columns and to key the slab into the ground all around the perimeter.
As the shop layout is a simple rectangle, no surveyor was needed. We used a 100′ tape measure to find the wall lengths, and set pins at the corners. We measured the diagonals to ensure they were equal, which in turn, assures us that the rectangle is square. It has (4) 90 degree corners. We used nylon string to connect the 4 corners, and used marking paint to transfer to the ground as seen above. In truth we need to be pretty close here, but setting the form boards later is when we must be exact.
My weapon of choice for the dig is an aging Takeuchi TB68 excavator. The machine weighs in just over 15K lbs, and has a maximum digging depth of about 14 feet. We won’t need to dig more than about a foot here, since we are going to add 4″ of compacted gravel under the slab, and will form up with 8″ high boards around the footer perimeter, so this should move along fairly quickly. The end result will be a 4″ thick slab placement, with 20″ deep footers.
Since I wanted to preserve as many trees as possible, I didn’t clear much space beyond the side walls of the shop, so there is no room to place the excavated soils outside the footings, and we don’t want to pile it inside since we are going to be placing gravel next. So, I have the dump truck stay about 10 feet behind the machine, and dump each scoop into the truck as we go. Perhaps 1/3 to 1/2 of the soil can be reused to backfill up to the slab, as it will project 8″ above grade. We will want to keep the soil sloping away to avoid any water issues during heavy rain.
We set up a rotating laser level on my surveyor’s tripod, and use it with a detector and marking stick. We first find the lowest corner on the site, which will have the highest number reading on the stick, and then move the detector up the stick 12″ so that when we dig down a foot, it will show on grade again. I start digging, and Leslie checks the depth as we go, pointing out what is high, low, and on grade.
Running an excavator takes a lot of eye and hand co-ordination, and the abilty to co-ordinate multiple movements together to achieve the desired result. Your left hand works the main boom by pushing and pulling. Moving your left hand side to side swings the whole turret. A push or pull with your right hand moves the stick, or dipper depending upon your terminology, while moving side to side curls or uncurls the bucket. Multiple machine function movements may be accomplished with diagonal hand movements of varying amounts.
Now the trouble is that the geometry of it all changes as you move the bucket toward the machine while trying to dig a straight line of a controlled depth. As you pull your right hand back, and the stick the bucket is attached to rotates toward you and approaches a line perpendicular with the ground, it effectively grows longer, digging deeper unless compensated for by raising the boom with your left hand a proportionate amount. As the bucket is curled, it’s contribution to the effective length changes with the arc, which must be compensated for by adjustment of the boom or stick. As you can see, conceptually managing these three variables isn’t hard, it just demands practice to do so smoothly. As an infrequent operator, it takes me about 10 feet worth of trench to get back into it. That first 10 feet is characterized by a scalloped floor as I fail to properly compensate for the arc of the bucket. Were it not months between digging sessions, I’m sure it would be smooth from the first scoop of the day to the last. As it stands, a little inevitable hand work with the ole’ shovel, and all is well.