Gravel, Crushed Stone and Soil Delivery in Portland, OR

Steep ground over a fragipan

The USDA survey maps the center point of ZIP 97201 as the Goble-Urban land complex on 15 to 30 percent slopes. That is steep ground, the kind where a wheelbarrow wants to run away and soil creeps downhill with every wet season. Goble soil makes up 50 percent of the map unit, and paving and buildings cover 40 percent.^[1]

Goble's top mineral layer is a silt loam. The survey rates it well drained but places it in hydrologic group C, so rain enters it slowly and much of it runs across the surface on a slope. Shrink-swell is low, at 1.5 percent in the top 100 cm, so the soil does not heave much as it dries.^[1]

Paving and buildings already cover a large share of the map unit, so many hillside yards are cut-and-fill benches shaped long ago. Check the outer edge of any bench for loose fill before loading it with a patio or a parked car.^[1]

A fragipan begins about 38 inches below the surface at the center point. A fragipan is a dense, brittle layer that roots and water barely penetrate. Rain that soaks through the silt above can stop at it and then travel sideways along the slope, coming out lower down as a wet patch or a seep.^[1]

On a hillside, that sideways flow is what pushes on retaining walls and softens the toe of a slope. Any wall over knee height needs drainage behind it: a column of #57 Crushed Stone or Drain Rock against the back of the wall, a perforated pipe at its base, and an outlet that lets the water escape.^[1]

The same layer limits deep planting. Trees and large shrubs do best in wide, shallow holes backfilled with native soil blended with Compost, so roots spread across the top of the silt instead of hitting the pan.^[1]

Wet spots that appear partway down a slope after a few days of rain are a sign of water riding on a dense layer. A curtain drain, which is a gravel trench across the slope just uphill of the wet area, can catch that water and lead it away to a safe outlet.^[1]

Rain through most of the year

The NOAA normals station at Portland KGW-TV, OR, about 1 mile away, averages 44.1 inches of precipitation and 3.6 inches of snowfall a year. Most of that comes as rain over the cool months, which keeps silt soils damp for long stretches.^[2]

That station records 18 days a year with a low of 32 F or colder. The median last spring freeze arrives on February 25, and the median first fall freeze on December 1. Frost is a light concern for a base here. Saturation is the heavier one, since a wet silt subgrade loses strength under load.^[2]

Plan outdoor building for the drier months when the silt can be graded and packed without smearing. Mulch and erosion cover should go on before the wet season begins, so bare soil is never exposed to a winter of rain.^[1],[2]

Retaining walls and terraces

Terracing turns a slope of 15 to 30 percent into usable flat space. Each terrace wall needs a firm footing of packed Road Base or 3/4" Crushed Stone, a drainage zone of #57 Crushed Stone behind it, and fabric between that stone and the silt so fines do not clog it.^[1]

Where a full wall is not needed, Riprap set into a bank slows runoff and holds soil in place. Large, angular pieces bed into the slope and lock together, and planted pockets between them fill in over time.^[1]

Bare slopes should never stay bare through a wet season. A thick layer of Wood Chips or Brown Shredded Mulch on a planted bank shields the silt from rain and stops small rills from starting.^[2]

Driveways that climb

A gravel drive on a slope fights gravity and water together. Cut it with a slight cross slope toward the uphill side, where a ditch lined with River Rock or Drain Rock can catch runoff, rather than letting water run down the middle of the drive.^[1]

Over silt loam, the base needs fabric beneath it and Road Base packed in thin lifts. On steep sections, a finish of Driveway Gravel or 3/4" Crushed Stone grips better than round stone, which rolls under spinning tires. Water bars, shallow diagonal ridges across the drive, turn running water off the surface before it gathers speed.^[1]

Snow is rare at the KGW-TV station, but an icy morning on a steep gravel drive is a real hazard. Angular stone with some fines gives better footing than smooth gravel, and a pile of 3/8" Crushed Stone kept by the garage makes a quick traction fix for the steepest stretch.^[2]

For short, very steep pads, some owners choose Asphalt Millings, which bind into a firm surface once rolled and warmed. Crushed Concrete makes a strong recycled base under either finish.^[1]

Garden beds and mulch on a slope

Silt loam that stays wet through the rainy months suits raised beds well. A frame filled with Garden Soil drains faster than the ground around it and warms up earlier in spring. In-ground beds benefit from Compost worked in each year, which opens the silt and helps it shed extra water.^[1],[2]

Mild winters let plenty of plants keep growing through the cool season. With a median last spring freeze of February 25 at the KGW-TV station, early crops and new shrubs can go in sooner than in colder parts of the state. Mulch new plantings right away so winter rain does not crust the surface.^[2]

On a slope of 15 to 30 percent, the choice of mulch matters. Wood Chips are coarse, irregular pieces that knit together and resist sliding, so they hold on a steep bank and break down slowly into the soil. Brown Shredded Mulch is finer and looks neater in a bed, but on a steep face it can wash downhill in a heavy storm.^[1]

Hillside lots can use Wood Chips on the steep banks and paths, and shredded mulch on the flatter beds near the house. Either way, a layer thick enough to hide the soil is what protects silt from the force of falling rain.^[2]

Housing and the Oregon base spec

The Census counts 11,069 housing units in ZIP 97201. Detached single-family houses make up 13 percent of the total, and the median home was built in 1987. Owners live in 27 percent of the occupied homes.^[3]

Portland city has 307,719 housing units, and 52 percent of them are detached houses. Its median build year is 1966.^[3]

The Oregon Department of Transportation names the crushed layer under its pavements Dense-Graded Base Aggregate, Section 00641, in the Oregon Standard Specifications for Construction (2024).^[4]