Quick Verdict
An infiltration trench is a gravel-filled excavation that captures stormwater and lets it soak into the surrounding soil, while a drywell is a deeper pit or chamber doing the same job in a smaller footprint. Both move roof and surface runoff into the ground instead of piping it to the street. In Oregon they only work where the native soil actually drains, which is the catch: heavy Willamette Valley clay soaks slowly and often needs a bigger system or a different approach. Getting the sizing, depth, and soil right up front is what separates a dry yard from a swamp.
How Infiltration Trenches and Drywells Work
Both systems share one idea: store water temporarily, then let it percolate into the soil. An infiltration trench is long and shallow, wrapped in filter fabric and filled with clean drain rock, often with a perforated pipe running through it. A drywell is more vertical, either a rock-filled pit or a manufactured chamber, and it takes concentrated flows like a downspout or a low spot.
The excavation is the heart of the job:
- Dig to a depth that reaches free-draining soil and stays above the water table
- Line the excavation with non-woven geotextile fabric to keep soil fines out
- Backfill with clean, washed drain rock, not native spoil
- Tie in inlets, overflow, and a cleanout so it can be maintained
Because they rely on soil, these systems are a design problem before they are a digging problem. The Oregon excavation contractor guide covers how soil and water shape every dig.
Oregon Soil Is the Deciding Factor
Infiltration lives on percolation rate, and Oregon soils vary enormously. Sandy and gravelly ground drains fast and makes an ideal home for a drywell. Dense Willamette Valley silty clay drains slowly, so a small drywell just fills and backs up during a real storm. A percolation or infiltration test tells you what the ground will actually accept, and a good design sizes the system to that number plus a safety factor.
Water table matters as much as soil. In low-lying areas the winter groundwater can rise to within a foot or two of the surface, which drowns a drywell and leaves it with nowhere to send water. If your ground will not infiltrate, a surface system like yard drainage and catch basin excavation that carries water to daylight is often the better fix.
Trench vs. Drywell: Which Fits
The two options solve slightly different problems. A simple comparison:
| Feature | Infiltration trench | Drywell |
|---|---|---|
| Shape | Long and shallow | Deep and compact |
| Best for | Spread-out or linear runoff | Concentrated downspout flow |
| Footprint | Larger surface area | Smaller footprint |
| Soil needs | Draining soil over its length | Draining soil at depth |
What It Costs to Install
Cost tracks the size of the system, the digging depth, how much clean rock it takes, and whether you hit water or hardpan.
Industry Baseline Range: an infiltration trench or French-style drain commonly runs $15 to $120+ per linear foot, and crushed or drain rock delivered runs $45 to $110+ per cubic yard. A trenching machine or excavator with operator runs $150 to $350+ per hour, and disposal of native spoil runs $75 to $300+ per load.
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.
Current Market Reality
Real costs often run 2 to 3 times a clean baseline when the ground does not cooperate. Slow-draining clay forces a much larger system, a high water table can rule out infiltration entirely, and unmarked utilities or rock in the trench line slow the dig. A small residential system also carries a $500 to $1,500+ minimum callout.
Testing the Soil Before You Dig
The single most important step in an infiltration project happens before any excavation: finding out whether the ground will actually accept water, and how fast. This is not a guess you want to make from the surface, because two lots on the same street can drain very differently depending on their soil layers and depth to groundwater.
An infiltration or percolation test digs a test hole to the proposed depth, saturates it, then measures how quickly the water level drops. That rate, expressed as inches per hour, tells the designer how much water the soil can swallow, which in turn sets the size of the system. Fast, sandy ground might absorb water quickly and need only a modest drywell. Slow Willamette Valley clay might infiltrate so slowly that a properly sized system would have to be enormous, at which point a different approach usually wins.
Reading the Results Honestly
The test protects you from the most common infiltration failure, which is a system that looked fine on a dry summer day and then backed up in the first real winter storm. A few things a good design accounts for:
- The seasonal high water table, not just the summer level, since winter groundwater can drown a drywell
- A safety factor above the measured rate, because soils clog and slow over time
- Pretreatment, such as a catch basin or filter, to keep silt from sealing the soil
- An overflow path for storms bigger than the system was sized for
Skipping the test to save a few hundred dollars is a false economy. An undersized or misplaced system either fails outright or has to be dug up and redone, which costs far more than testing would have. The test turns infiltration from a hopeful experiment into an engineered solution sized to your actual ground.
The Bottom Line
An infiltration trench or drywell is an elegant way to manage stormwater on the right soil, and a false economy on the wrong one. The smart move is to test the ground, size the system honestly, and build it with clean rock and fabric so it keeps working. If you are fighting standing water or a downspout with nowhere to go, our team can tell you whether infiltration fits your lot. See our excavation services or request a free estimate.