Percolation & Soil Tests Before Drainage Work in Oregon

The Test That Picks the Right Drain

Before you spend money on a drainage system, there's one question worth answering: does your soil actually let water soak in? The answer drives every design decision that follows. A solution that depends on water absorbing into the ground — a dry well, an infiltration trench, a rain garden — only works if the soil underneath drains. In a lot of Oregon, especially the heavy clay of the Willamette Valley, it doesn't.

A percolation test, or "perc test," is the simple field check that settles it. It measures how fast water drops in a test hole, which tells you whether to design a system that infiltrates water into the soil or one that conveys water away to a different outlet. Getting this wrong is expensive: a perfectly built dry well in tight clay just becomes a hole full of water. This guide walks through what a perc test is, how to run a basic one, and how the result shapes your drainage plan. For the bigger picture, see property & site drainage in Oregon.

What a Perc Test Measures

A percolation test measures the rate at which water moves down through saturated soil — usually expressed as the number of minutes it takes the water level to drop one inch, or as inches per hour. That single number sorts soil into broad categories:

• Fast (sandy, gravelly soils): Water drops quickly. Infiltration-based systems work well.
• Moderate (loams): Reasonable drainage. Most systems are viable.
• Slow (clay, silty clay): Water barely moves. Infiltration systems struggle or fail; you need conveyance instead.

Oregon's geography produces a wide range. Valley floors and many residential lots sit on dense clay that percolates very slowly. Coastal and some upland areas vary. Even two corners of the same yard can test differently, which is exactly why a field test beats a guess.

A Simple Yard Perc Test You Can Run

For drainage planning — as opposed to a formal septic-system perc test, which has strict regulated procedures — you can run a basic field test yourself to get a feel for your soil:

1. Dig a hole roughly 12 inches deep and 6 to 12 inches wide, in the area where you're considering a drainage feature.
2. Pre-soak it. Fill the hole with water and let it drain completely once. This saturates the surrounding soil so you measure true percolation, not just dry soil absorbing the first fill. In clay, pre-soaking matters a lot.
3. Refill and measure. Fill the hole again, mark the starting water level, and time how far it drops over an hour. Note the inches-per-hour rate.
4. Repeat in a few spots if your yard varies, and ideally test during the wet season when soil is already moist — that's the honest condition your drain will face.

If water is still sitting in the hole hours later, you have slow-draining soil and an infiltration system is the wrong approach. If it drains within a reasonable time, infiltration becomes an option worth sizing.

What the Result Tells You to Build

The perc rate points you toward a category of solution:

If your soil drains well (fast perc): You can use infiltration-based systems that put water back into the ground on-site. A dry well collects water and lets it soak away. Rain gardens and infiltration trenches become viable. A French drain that discharges to soil also performs better.

If your soil drains slowly (slow perc — common in Oregon clay): Infiltration won't keep up. The water has nowhere to go fast enough, so you switch to conveyance — moving water away to a real outlet rather than trying to soak it in. That means surface solutions like graded swales, dry creek beds, and area drains, plus solid pipe carrying water to a daylight outlet or storm connection. This is the core reason deep French drains often underperform in valley clay, a point we cover in the yard drainage cost guide.

Why Clay Changes Everything

Clay-heavy soil is the defining drainage challenge across much of Oregon. Clay particles are tiny and pack tightly, leaving little pore space for water to move through. The result is soil that holds water at the surface and resists letting it sink in — exactly the opposite of what infiltration systems need.

In clay, the instinct to dig a deep gravel trench and expect water to vanish into the ground simply doesn't pan out. The water reaches the bottom of the trench and stalls because the surrounding clay won't accept it. That's why drainage design in clay leans on getting water off the surface and away — shallow, surface-first solutions that intercept water and carry it to an outlet, rather than deep systems that bank on infiltration. A perc test makes this concrete: you see, in your own yard, that the water isn't going down, so the design goes outward instead.

Beyond Perc: Reading Your Soil

A perc test pairs well with a quick look at what's actually in the ground. When a contractor digs an evaluation hole, they note:

• Soil texture — sandy, loamy, silty, or clay-heavy — by feel and appearance.
• Mottling or gray colors, which signal a seasonally high water table where water sits part of the year.
• A hardpan layer, a dense compacted band that can perch water above it even if deeper soil drains.
• Depth to groundwater, especially relevant on valley floors and near wetlands.

These observations refine the plan. A high seasonal water table, for instance, can defeat an infiltration system even in soil that perc-tests okay during a dry spell — which is why testing in the wet season is so valuable.

When to Get a Professional Evaluation

A DIY perc test is great for orientation, but larger or higher-stakes projects justify a professional soil and site evaluation. That's true when you're planning a sizable infiltration system, designing stormwater for a new build, dealing with a chronically wet lot, or any situation where guessing wrong means redoing the work. A contractor can dig proper test pits, read the soil profile, account for the seasonal water table, and design to what the ground will actually do.

Our excavation services include site evaluation as part of drainage design, so the system we install matches your soil rather than fighting it. Treat the DIY guidance here as a starting point — a site-specific assessment is always the reliable basis for a real installation.