Basic Backhoe Operation

WVBill · Apr 3, 2007

OK, groundbreaking on the Quilting Studio for my wife is set for next Saturday. It's going to be a basic 24' x 24' stick-built building set on a footing/concrete-block stemwall foundation. The building site is on a slight slope - the right rear corner is 30" lower than the front left corner. I need to set my footings 30" minimum below finished grade per code here.

So I'm renting a Bobcat 331G excavator with a 24" bucket to dig the trench for the foundation. The trench will be 36" deep at the front left corner and 6" deep at the back right corner (I'll use the spoils to bring the grade up to the minimum 30" after the foundation is built).

So I need some hints/tips on how to dig the bottom of the trench level all the way around.

Thanks - and yes, I will post pictures.

WVBill

SnoFalls · Apr 3, 2007

I'd level out a 30'x30' area first. It's easier to cut/fill/compact now then after the foundation is up. Also, give some thought to how runoff from the upslope corner will be diverted around the building pad.

Then you can mark out for the foundation and have a level area to measure depth from.

EddieWalker · Apr 3, 2007

Are you sure you can set part of your walls into six into the ground and build up around it with spoils to achieve your 30 inch requirement? There are several reasons for the depth of a footing, and side walls are a big part of it.

I think you might be creating long term problems with the 6 inch footings, but I could easily be missing something here.

Two things to digging a "flat bottom" are to know the curl of your bucket. Dig with the teeth facing towards you and back, then lift the spoils. Don't dig down and curl up. You will end up with a bottom that is all over the place. This is one of those things that is very difficult to do. Flat bottoms are required for sewer lines, water mains and of course, footings.

To keep it level, you can either mark the hoe stick and bucket with tape to know how deep you are, or have a helper. I suggest you have a helper. Using the hoestick with indicators isn't easy. Some guys have a natural ability to stay level just by feel and eyeballing it. I don't. It's a skill that I'll probably never aquire, so I use markings and double check with my lazer level. If my wife or somebody else is around, I get them to take measurements.

If you dont' have a laser level, you can do it with a framing level. I've taped them to boards to measure larger areas. It does the job, but takes a little more effort.

Go slow.

Eddie

john_bud · Apr 3, 2007

I'd think that the advice Snofalls gave is real good. Get the work site up to the correct grade, level and compacted in 3-4" layers so it won't settle. Even with repeated rounds with a compactor, it will settle down over time. Spoils will settle more than a good gravel/sand mix. Around here the best stuff if what the counties use for road bases and is called county mix.

It's like painting a car or a house. The difference between a rotten job and a brilliant job are the time and effort taken in preparation. Get the grade level, get the fill compacted, if you have time let it settle then compact it again. Then start digging. Getting a flat level area 30" down from a level flat top is much easier than from a sloping uneven top area!

jb

EddieWalker · Apr 3, 2007

I was gonna agree with SnoFalls too, but WVBill isn't pouring a level slab, he's puring kneewalls to build a pier and beam floor. The grade isn't important to this type of construction. Drainage is, but the slope of the land isn't. There really isn't any advantage to leveling it out that I can see for what he's going to build.

What am I missing?

Eddie

WVBill · Apr 3, 2007

Eddie's right, I'm pouring a 12" wide X 8" deep footer on which I will build a concrete block wall to minumum 8" above finish grade.

Here's my understanding:
Prescriptive IRC 2003 Code requirement is that the footer sits on "undisturbed soil" cleared of vegetative matter and that the bottom of the footer be below the local frost line. In my area that's 30" below the finished grade. Inside the foundation will be a crawl space clear of vegetative matter and a minimum of 18" below the bottom of the floor joists. There's no requirement to backfill inside the foundation - or even that it be level. There will be no interior slab - just a plastic sheet vapor barrier.

I plan to use the spoils from the trench and getting the interior 18" below the joists to raise the exterior finish grade a minimum of 30" above the bottom of the footers on the low side(s).

WVBill

john_bud · Apr 4, 2007

EddieWalker said:
I was gonna agree with SnoFalls too, but WVBill isn't pouring a level slab, he's puring kneewalls to build a pier and beam floor. The grade isn't important to this type of construction. Drainage is, but the slope of the land isn't. There really isn't any advantage to leveling it out that I can see for what he's going to build.

What am I missing?

Eddie

Gotcha. I was picturing slab on grade.

jb

MossRoad · Apr 4, 2007

First, I'd pound stakes in the ground all the way around the trench, a few feet inside the excavation.

Next, I'd mark a level line on all the stakes. It really doesn't matter on the height for now, as long as they are all level to each other. Put a nail at the bottom of each level mark, leaving it out enough to support the end of a level.

Then I'd determine how far above the finished bottom of the trench the level mark is, and make a stake that long.

From that point on, all you have to do to check the level of the trench as you go is put a level on any of the stake mark nails and rest it on top of your depth stake. Very simple and very fast.

WVBill · Apr 5, 2007

Thanks, MossRoad and Eddie

MossRoad, your level reference line will really help. The overall slope is gentle enough that it's not easy to see without a reference.

Eddie, thanks for the technique hints - I'll take it slow and easy. Fortunately, the bottom of a footer doesn't need to be "dead" flat - and only needs to be level to within 1 in 10.

Sure wish the weather had stayed in the 70s and 80s like it was two days ago. For my "big dig" Saturday it's forecast to be in the 40's with snow showers.

WVBill

tlbuser · Apr 5, 2007

Three things that should help.

1) transit or laser level
2) torpedo level
3) marking or striping paint

Use the striping paint to layout the dig path, it will be visible between the cab and bucket to help keep you in a straight line. Strings and sticks are nice, but have a tendency to be in the wrong place at the wrong time.

The torpedo level (or other short level) is for the machine's left to right and front to back level. If the machine isn't level , the bucket isn't level, the hole isn't level, etc.

Set the transit up out of the machine's path. Then use a "story pole" for depth. (piece of pvc, stick, etc, with a bullseye marked on it at whatever depth you need) quick, easy and accurate