Gravel, Crushed Stone, Soil and Mulch Delivery in Boulder, CO

Renohill Clay Over Soft Rock

The USDA soil survey maps Renohill silty clay loam, 3 to 9 percent slopes, at the center point of ZIP 80301. The Renohill series makes up 85 percent of the map unit.^[1]

Silty clay loam is the texture, and the series carries a high shrink-swell rating, 7.5 percent. The clay swells after snowmelt and spring storms and shrinks in dry spells, enough to tilt pavers and crack thin concrete over a few seasons.^[1]

The survey lists paralithic bedrock at about 30 inches. Paralithic rock is soft, weathered bedrock that a shovel can scrape but not easily dig. Footings, deep planting holes and drainage trenches can reach it quickly, and water soaking down stops on top of it.^[1]

The survey rates the soil well drained, but it sits in hydrologic group D, with very slow infiltration and high runoff. On a slope of 3 to 9 percent, that means rain and snowmelt run across the surface toward the low side of the lot instead of soaking in.^[1]

At the ZIP's mapped center, the USGS elevation service puts the ground at 5,180 feet above sea level.^[2]

Snow and Cold at the Boulder Station

The NOAA normals station at Boulder, CO, about 5 miles away, averages 92.8 inches of snow a year. Snow on a gravel drive or path melts in the sun, refreezes at night, and works its way into any base that holds water.^[3]

Total precipitation at the station averages 21.2 inches a year, counting snow as melted water. Dry spells between storms let clay shrink and crack, so the ground swings between wet and dry often.^[3]

Dry, sunny days between storms melt snow quickly on south-facing ground and leave it on the shaded side for weeks. Gravel drives that run along a north wall stay frozen longest, so give them the deepest base and the best drainage on the lot.^[3]

The station records a low of 32 F or colder on 138 days in an average year. Frost lift is the main enemy of walks, pavers and gravel drives, and a free-draining stone base is the best defense against it.^[3]

The median last spring freeze arrives on May 6, and the median first fall freeze on October 5. Planting waits until May, and base work fits best in the warm, dry weeks of summer.^[3]

Wet clay that freezes and thaws over and over loses its strength near the surface. That is why gravel laid straight on a bare yard turns rutted each spring, and why the fabric and compacted base under a drive matter more here than in a milder place.^[1],[3]

A Driveway for Clay and Frost

Strip the topsoil and any soft clay, then lay a woven geotextile. On swelling clay that also freezes often, the fabric keeps clay out of the base and spreads the load so the drive moves evenly instead of in humps.^[1],[3]

Compact Road Base or Crushed Concrete in thin lifts. Where bedrock lies at about 30 inches, a deep dig is hard and pointless, so build the base up rather than cutting far down, and crown the surface so water leaves it quickly.^[1]

Top the drive with Driveway Gravel, #57 Crushed Stone or Asphalt Millings. Angular stone and millings both stand up to a plow blade. On group D clay, give the downhill edge a shallow stone-lined ditch so snowmelt runs off instead of soaking back under the drive.^[1],[3]

CDOT names Aggregate Base Course, Class 6, Section 304, Standard Specifications for Road and Bridge Construction 2025 as the base course for state roads.^[4]

Homes and Hillside Yards

The Census counts 12,876 housing units in ZIP 80301, and 34 percent are detached single-family houses. Boulder as a whole has 47,465 housing units, with 39 percent detached.^[5]

The median home in the ZIP was built in 1985, against 1977 for the city. Owners occupy 55 percent of the ZIP's occupied homes.^[5]

On sloping clay, terraces and low walls make level spots for beds and patios. Backfill behind each wall with Drain Rock wrapped in fabric so water from snowmelt drains out at the base instead of freezing and pushing on the wall.^[1]

Small retaining walls on this ground can rest on a compacted Road Base footing set on the firm layer above bedrock. Digging into the paralithic rock itself is rarely needed for a low garden wall.^[1]

Low garden walls built from stacked block or stone are a reasonable weekend project when the base beneath them is level and well packed. Lean each course slightly back into the slope so the wall rests against the soil it holds.^[1]

Snow shoveled off a drive on a sloping lot piles up at the downhill edge and melts there for days. Plan a strip of #57 Crushed Stone or River Rock along that edge, set slightly lower than the drive, so the meltwater has a place to soak in or run off without crossing the drive and freezing overnight.^[1],[3]

Beds in a Short Growing Season

With a median last freeze of May 6 and a median first freeze of October 5 at the Boulder station, the outdoor growing season is short. Raised beds of Garden Soil warm up weeks sooner in spring than heavy clay, which gives seedlings a head start.^[3]

In-ground beds on Renohill silty clay loam benefit from Compost worked in each fall. Organic matter loosens the clay, helps water soak in, and softens the shrink and swell near roots.^[1]

Mulch holds moisture in a dry climate and shields roots from freeze and thaw. Brown Shredded Mulch or Wood Chips spread a few inches deep after the May freeze date protect beds through summer, and a fresh layer in fall keeps roots steady under the snow.^[3]

Stone Dust Screenings or Pea Gravel for a Path

Stone Dust Screenings pack into a firm, smooth path that a shovel can clear without throwing stone around. On a frosty morning they give boots better grip than loose gravel. Lay them over a compacted base and fabric so the clay below does not show through as soft spots.^[3]

Pea Gravel drains fast and never puddles, but it rolls underfoot and scatters under a snow shovel. It works best in garden paths that stay under snow all winter and in play areas, with a solid edge to hold it in.^[1]

For a walk that must stay open all winter, 3/8" Crushed Stone is the middle ground. It drains like gravel, locks together better than round stone, and stays put under a shovel. Edging along both sides keeps it out of the beds when the snow is pushed aside.^[1],[3]