Pier and Post Foundation Holes: Digging Footings (Oregon)

Quick Verdict

Digging pier foundation holes in Oregon means getting each hole to the right diameter and depth, down to competent bearing soil, then keeping it open and dry long enough to pour. An auger on a skid steer or excavator drills most holes fast; hand-digging fits tight or small jobs. Bell-bottom piers flare at the base for more bearing. The depth is set by where firm soil starts and, in cold zones, by frost depth. The Oregon catch is wet clay: holes slough and fill with water in winter, so timing and technique matter. Always get the holes inspected before any concrete goes in.

Where Pier and Post Foundations Are Used

A pier or post foundation carries a structure on individual points rather than a continuous footing. Instead of a full perimeter wall, the load comes down through posts or columns into concrete piers in the ground. They show up under:

• Decks and covered porches.
• Shops, pole barns, and outbuildings, often as part of pole barn pad excavation.
• Cabins and small additions.
• Manufactured and modular homes, set on pier supports.

These are some of the most common residential foundation jobs, and they fall squarely under foundation excavation in Oregon. The earthwork is the holes themselves, dug to spec so the piers bear properly.

Auger vs Hand-Dig

There are two ways to make the hole, and the choice is about access and volume.

Method	Best For	Notes
Auger (machine)	Most jobs, many holes, deeper piers	Skid-steer or excavator-mounted auger drills clean, consistent holes fast
Hand-dig / hand auger	Tight access, few holes, under decks	Slow but fits where a machine cannot reach

A machine auger is the default when access allows, since it produces uniform holes quickly. Hand-digging takes over in cramped backyards, under low decks, or where only a handful of holes are needed and bringing a machine in is not worth it.

Bell-Bottom Piers and Diameter

A straight-shaft pier is a simple cylinder. A bell-bottom pier flares wider at the base, like an upside-down funnel, to spread the load over more soil and add uplift resistance. Bells are used where bearing soil is marginal or loads are higher, and they require a special belling tool or careful hand work at the base.

Diameter is set by the load each pier carries and the bearing capacity of the soil. Bigger loads or softer soil mean wider holes or bells. An engineer or the building plans specify the diameter and depth; the excavation matches that spec exactly.

Depth: Getting to Bearing and Below Frost

Two things set how deep a pier hole goes:

• Depth to competent bearing soil. The pier must reach firm, load-bearing material, not soft topsoil or loose fill. On sites with deep soft layers, the hole goes deeper or a different system is considered.
• Frost depth in cold zones. East of the Cascades and in higher elevations, the bottom of the footing must sit below the frost line so freeze-thaw cannot heave it. Frost depth varies by location and is set by local code. West-side valley sites have shallow frost; cold-zone sites are deeper.

The plans or the local building department give the required minimum depth. The excavation hits that depth and confirms the soil at the bottom is firm enough to bear.

The Oregon Catch: Caving and Water in Wet Clay

This is where Oregon makes pier holes tricky. In the wet season, freshly dug holes in clay do two frustrating things: the walls slough inward and the bottom fills with water. A hole dug in the morning can be partly collapsed and full of water by afternoon.

Strategies that help:

• Dig and pour quickly so holes do not sit open overnight in the rain.
• Use forms or sonotubes to hold the shape and keep soil out of the concrete.
• Dewater the bottom before placing concrete; you cannot pour into standing water and get a sound pier.
• Schedule for drier conditions where the project allows.

In Central Oregon, the opposite problem appears: basalt and hard rock east of the Cascades may require a rock auger or hammering to reach depth. Same goal, different obstacle. Where soils are deep and soft or very wet, it is worth weighing helical piles vs excavation, since screwed-in piles can sidestep the caving-hole problem entirely.

Current Market Reality

Cost is usually quoted per hole and depends on diameter, depth, soil, and access. A few shallow holes in firm dry ground are cheap; many deep, belled holes in wet clay or rock cost much more.

Industry Baseline Range: pier holes commonly run $100 - $600+ per hole for the excavation, with deeper, belled, or rock holes and difficult access running higher; mobilization and a minimum callout often apply to small jobs. These are industry baseline ranges for planning only -- actual pricing depends on site conditions, soil, access, depth, haul-off, and current market conditions. Get a site-specific quote. Costs run higher when holes must be deep, belled, drilled through rock, or dewatered before pour.

Inspection Before the Pour

On permitted work, the holes are inspected before any concrete is placed. The inspector confirms the depth, diameter, soil at the bottom, and any reinforcement match the approved plans. Pouring before inspection risks having to dig it back out, so the sequence is dig, inspect, then pour. Building this step into the schedule keeps the job moving and code-compliant.

The Bottom Line

Pier foundation holes come down to the right diameter and depth into firm bearing soil, kept open and dry long enough to pour and inspect. Oregon's wet clay slough and rock east of the Cascades are the obstacles, so technique and timing matter. Cojo is CCB licensed and insured and works statewide across Oregon and the I-5 corridor, drilling pier holes to spec for decks, shops, and additions. See our excavation services, read the full Oregon excavation contractor guide, and request a free estimate.