Excavation
Dry Well Excavation: Digging the Pit That Soaks Water Away (Oregon)
Cojo
June 19, 2026
6 min read
Dry well excavation in Oregon is the earthwork side of building an underground pit that collects roof and surface water and lets it soak into the ground. You dig the pit, set a gravel-and-fabric fill or a plastic infiltration chamber, run an inlet trench from the downspouts or area drains, and keep proper separation from the foundation and septic. This article covers the dig and install, the actual excavation, while sizing the infiltration and designing the system is the job of the drainage pillar. The big Oregon catch: clay valley soils percolate poorly, so a dry well that drains beautifully in Central Oregon sand can sit full of water in the valley.
A dry well is a buried pit that takes concentrated water, usually from downspouts or yard drains, and disperses it into the surrounding soil instead of letting it pool or run off. The whole thing only works if the surrounding soil can accept the water, which is why where and how you dig it matters so much.
This piece is about execution: digging the pit, setting the media, and connecting the inlet. The sizing math (how big the pit must be for your roof area and soil percolation) and the broader stormwater design belong to the grading and drainage earthwork guide. Get the design from there; get the dig done right here.
The pit is sized to the design, but the dig follows a consistent process:
Depth matters: a dry well that reaches a more permeable layer drains better than one stuck in tight surface clay. But it must stay well above any seasonal high water table, a pit dug into the water table just fills up.
Two common builds:
Either way, clean drain rock and filter fabric are non-negotiable, fines clog a dry well fast. A perforated inlet and an observation or cleanout access make maintenance possible.
The dry well needs water delivered to it. That means a trench from the downspouts or area drains to the pit, sloped so water flows by gravity. The area drain trenching article covers running those collection lines. Where surface water collects faster than it can soak away, a sump pit excavation with a pump is the alternative approach.
Keep the dry well a safe distance from the foundation (you do not want to recharge the soil right against your footings) and well away from the septic drainfield and any wells. Separation is both a function and, often, a code requirement.
This is the honest part. Dry wells live or die on how well the soil drains, and Oregon's soils vary enormously:
Set expectations honestly: in heavy valley clay a dry well may be the wrong tool, and a good contractor will tell you so before digging. There are also DEQ rules on what you are allowed to infiltrate into the ground, clean roof runoff is one thing, contaminated runoff is another.
Cost depends on pit size, the media, and how far the inlet trench runs. Planning ranges only.
| Cost Component | Industry Baseline Range |
|---|---|
| Pit excavation | priced per volume and access |
| Drain rock, delivered | $45 - $110+ per cubic yard |
| Filter fabric | $0.30 - $1.00+ per square foot |
| Infiltration chamber unit | priced per unit |
| Inlet trenching | $8 - $40+ per linear foot |
| Mobilization | $250 - $800+ flat |
The honest cost risk with a dry well is paying to dig one that does not work because the soil will not percolate. In tight valley clay, money spent on a dry well can be money wasted, which is why a percolation check or a contractor's read on the soil up front is worth it.
You do not need a formal engineering report to get a rough read on whether a dry well will work, a basic field check tells you a lot. Dig a test hole where the dry well would go, fill it with water, and watch how fast it drains. If the water disappears in a reasonable time, the soil percolates and a dry well is viable. If it sits there for hours or overnight, the soil is too tight and a dry well will likely disappoint.
In the Willamette Valley, that test hole often just holds water, which is the honest answer that saves you from digging a useless pit. In sandy Central Oregon ground, the same hole drains quickly. Doing this simple check, or having a contractor read the soil, before committing to a dry well is the difference between a drain that works and a hole that stays full all winter. When the soil fails the check, the better move is usually to pipe the water away to daylight or a lower point rather than try to infiltrate it.
A dry well is for clean water, primarily roof runoff from downspouts and clean surface water from area drains. What it should never receive is contaminated water: anything with oil, chemicals, soap, or sediment-heavy runoff from a work area. DEQ has rules on what is allowed to be infiltrated into the ground precisely because a dry well sends water straight into the soil, and groundwater protection matters.
Sediment is also a practical enemy of a dry well. Muddy, silt-laden water clogs the drain rock and shortens the well's life, which is why the inlet should carry relatively clean water and why filter fabric is used to keep surrounding soil out. Keeping the dry well limited to clean roof and surface water protects both the groundwater and the function of the well itself.
A dry well is great earthwork when the soil cooperates and a waste of digging when it does not, so the soil read comes first. We dig the pit, set clean rock and fabric or a chamber, and trench the inlet, and we will tell you honestly if your valley clay calls for a different approach. Our excavation services crew handles dry well installs across Oregon. Request a free estimate, and start with the grading and drainage earthwork guide or the Oregon excavation contractor guide.
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