Building Pad Prep on Cochise County Soils

A building pad is the least visible thing on a finished property and the only one you cannot go back and fix. Once there is a structure on it, a pad that settles is a foundation problem, and a foundation problem is a demolition-and-rebuild problem rather than a repair.

It is also, for that reason, the part of the job people are most tempted to economise on, because on the day it is finished a good pad and a bad pad look identical.

Siting it, before shaping it

Where the pad goes is decided by more constraints than most people expect, and they interact:

• Setbacks from the property line, which means you need the corners found first.
• Separation from the septic and the well, and those two have their own 100-foot separation from each other. Site the well first, then the septic, then the building — the well location is the most constrained by geology and rig access.
• Out of the drainage. Not just out of the wash, but off the path water takes when the wash is over capacity.
• Out of any mapped flood zone, and clear of any earth fissure with a real setback.
• Solar orientation. Long axis roughly east–west with the glazing facing south means winter sun comes in and summer sun does not, which is worth real money in cooling load over twenty years.
• Slightly high, not low. A pad in the lowest part of a parcel is a pad that collects.
• Access for a concrete truck, which is heavier and less forgiving than anything else that will come.

Also worth checking before you shape anything: what your soil map says is under it. NRCS Web Soil Survey gives the depth to a restrictive layer for your exact parcel, and if that layer is shallow it may be cheaper to move the pad thirty yards than to break through it.

The pad itself

Strip first

Topsoil and organics come off the pad footprint entirely. Organic material decomposes and compresses; building on it is building on something that is going to get smaller. Stockpile it — it is the material you want for landscaping later, and it is the only topsoil you have.

Cut and fill

You are shaping natural ground into a level or gently sloping surface. The important principle: cut is better than fill. Undisturbed native ground that has been sitting there for ten thousand years is more predictable than anything you place, however well you place it. Where you have a choice, put the structure on cut.

Where fill is unavoidable, it goes in lifts — four to six inches at a time, each one compacted before the next goes on. A three-foot fill dumped and rolled once on top is compacted for the top six inches and loose underneath. That is the classic failure, and it does not show up until there is a house on it.

Compaction, which is measured

This is the part where a contractor's word is not the point.

Building codes typically require compaction to 90–95 percent of maximum density, verified by testing. That is a laboratory-referenced number produced by a technician with a nuclear density gauge or equivalent, not a judgement made by looking at the ground.

The reason it is a number rather than an opinion: soil compacts differently depending on moisture content, and there is an optimum. Too dry and the particles will not rearrange; too wet and water occupies the space you are trying to close. Hitting optimum moisture in a place with twelve inches of annual rainfall and single-digit summer humidity means you are often adding water to the fill and working it, which is not intuitive to anyone who has not done it.

Drainage, which is the whole point

A pad is not just a flat spot. It is a shaped surface whose job is to keep water away from the building.

The pad surface should be near level for construction — around one percent for drainage, up to about five percent before it becomes awkward to build on.

The ground around the structure should fall away at a minimum of five percent, and ten is better — roughly six inches of drop in the first ten feet. This is the single most effective thing you can do for a foundation, and it is routinely lost during construction when the pad gets churned up and never re-graded.

Swales on the uphill sides, to intercept sheet flow before it reaches the pad and route it around. On a pad that is cut into a slope this is not optional.

Somewhere for it to go. Water you intercept has to be released somewhere that will not erode and — per the county's clearing ordinance — somewhere that does not alter existing drainage patterns or cause property damage off site.

Design all of this for a monsoon cell, not for the annual average. Fifty-seven percent of the year's rain at Pearce–Sunsites arrives between July and September, in bursts.

What caliche changes

A competent caliche layer is excellent bearing material — the difficulty is getting through it, not standing on it. So the calculation goes both ways.

If the layer is at the depth you were going to cut to anyway, that is good news: you have a hard, stable base and you may be able to found directly on it. If it is shallower than your cut, you are into a ripper or a breaker. If it is a foot below your finished level, the thing to check is drainage — because water in the fill above it has nowhere to go and will move sideways, toward whatever you built.

And caliche you excavate is caliche you can use. It compacts hard and it makes good driveway surfacing, so the material coming out of the pad frequently goes straight into the access road, which saves a haul in both directions.

Pads for things other than houses

Container homes and shipping containers concentrate their entire load into four corner castings. That is a point-load problem, not a distributed one — piers or footings at the corners on properly compacted bearing, and level, because a container racked out of level does not close its doors.

Sheds and outbuildings. Note that Cochise County exempts one-story detached accessory structures used as tool or storage sheds from permitting where the floor area is not greater than 200 square feet and there are no utilities. Both conditions, not either.

Water tank pads need continuous uniform support under the entire base, not blocks — a full 5,000 gallon tank is over twenty tons on a small footprint, cycling on and off as it fills and empties.

Ground-mount solar footings are their own excavation job, and in caliche a non-trivial one.

Sequencing

The order that avoids doing work twice:

1. Corners found and pad staked.
2. Driveway and access built, so everything arrives on something.
3. Clearing, only to the extent needed.
4. Strip and stockpile topsoil.
5. Rough grade and get the site drainage working, before the pad exists.
6. Cut and fill the pad in lifts, compacting each.
7. Compaction testing.
8. Trenching for water, conduit and propane, while there is still open ground.
9. Final grade around the pad, including the fall away from the structure.
10. Erosion control on everything disturbed.

Step nine has a habit of getting skipped, because by then there is a building and everyone has moved on. Re-grading the fall away from the structure after construction is the last cheap thing you can do for a foundation, and it is worth going back for.

What to ask for in a quote

A pad quote should tell you the finished elevation and slope, the lift thickness for any fill, whether compaction testing is included and to what standard, and what happens to the stripped topsoil. If it is a price and a square footage with none of that, it is not a specification — and the parts that are missing are exactly the parts you cannot inspect afterwards.

If you are at the stage of picking where the house goes, that conversation is worth having before anything is staked. Send us the parcel number and what you are planning to put on it.