Gravel, Crushed Stone and Soil Delivery in Anchorage, AK

Silt, cold and frost heave

The USDA survey maps the ZIP's center point as Cryorthents and Urban land, 0 to 5 percent slopes. Urban land, paved or built over, makes up 44 percent of the map unit. Cryorthents are young, cold-climate soils, and in a city they are often ground that has been moved and reworked.^[1]

A minor soil called Icknuun makes up 5 percent of the unit. It is very poorly drained and sits in hydrologic group D, meaning water infiltrates very slowly and runoff is high.^[1]

Its texture is silt loam, with a low shrink-swell rating of 1.5 percent. Silt does not swell much with water, but it is the soil most prone to frost heave. Fine silt pulls water upward as it freezes, building ice lenses that lift whatever sits on top.^[1]

The ground at the center point stands 40 feet above sea level.^[2]

Urban land in this map unit means streets, lots and buildings, and the soil around them has often been hauled in or reworked during construction. Old fill can be a mix of gravel, silt and debris, so dig a test pit before you plan a base. A pit shows how deep the soft material goes and whether you reach gravel or wet silt.^[1]

Cold nights and a short summer

The NOAA station at Merrill Field, about 1 mile from the ZIP, counts 184 days a year when the low reaches 32 F or colder. That many freezing nights gives frost plenty of chances to work into the ground.^[3]

The median last spring freeze at Merrill Field comes on May 1. The median first fall freeze there comes on September 29.^[3]

Merrill Field averages 14.2 inches of precipitation a year. The amount is modest, but on very poorly drained silt, even modest water lingers, and snowmelt in spring arrives all at once.^[3]

Snow covers the ground for much of the cold months, and its melt in spring comes while the soil below is still frozen. That timing makes spring the hardest season on outdoor work here, because water has nowhere to soak in and runs across the surface until the ground thaws.^[3]

The freeze dates also decide when outdoor work happens. Base work goes best in summer, once the frost is out and the silt has dried, and finishing before fall gives a new base time to settle before freezing nights come back.^[3]

Building a drive on frost-prone ground

A lasting drive in Anchorage depends on a thick layer of clean, coarse stone between the silt and the surface. Dig out soft silt and organic soil, lay a strong woven fabric, and build a deep base. Coarse stone holds little water, so it heaves far less than silt.^[1],[3]

Start with 1-1/2" Crushed Gravel or #4 Crushed Stone for the lower base, which drains freely and bridges soft spots. Cap it with Road Base compacted in thin lifts, then finish with Driveway Gravel or 3/4" Crushed Stone. Crown the surface and give it a clear path for water to leave at the sides.^[1]

In spring, the top of the ground thaws before the frozen layer below it, and water gets trapped in the silt. A drive built thin turns to mud in that period. A thick, well drained base carries the load until the frost is gone.^[1],[3]

Walks and patios follow the same rule as drives on a smaller scale. A paver patio on silt needs a deep layer of coarse, compacted stone under its setting bed of Concrete Sand. Loose surfaces such as Pea Gravel or River Rock forgive frost better than pavers do, since they settle back into place after each thaw.^[1],[3]

Plows and shovels scrape loose stone onto the lawn. Solid edging and a firm surface of Driveway Gravel cut the loss, and a spring rake puts stray stone back where it belongs.^[3]

Draining very wet ground

On very poorly drained ground, give water a planned route. A French drain lined with fabric and filled with Drain Rock, with a perforated pipe at the bottom, carries water from a wet patch to a lower outlet.^[1]

Around a foundation, backfill with clean Drain Rock or #57 Crushed Stone wrapped in fabric, not silt. Silt holds water against a wall and pushes on it when it freezes. Clean stone drains, stays loose and gives frost little to grab.^[1],[3]

Where a low spot stays soggy, raise it with Fill Sand, which drains better than silt and is less prone to frost. Cap it with Topsoil for grass.^[1]

Downspouts need a route on slow-draining ground. Carry roof water away from the house with a buried pipe or a splash pad of River Rock, and send it toward a drain or a lower part of the lot rather than into the silt beside the foundation.^[1],[3]

Raised beds for a short summer

With the median last freeze in early May and the median first freeze in late September, the growing season is short. Raised beds filled with Garden Soil warm faster in spring, drain better and give plants more growing time.^[3]

Do not plant directly into wet silt. Build beds up, fill them with Garden Soil or Topsoil mixed with Compost, and add mulch such as Wood Chips or Brown Shredded Mulch once the soil warms. Mulch also insulates roots during early fall cold snaps.^[1]

Fall is a good time to work Compost into beds. It breaks down over the winter and improves soil structure by spring, which helps heavy, silty ground drain and warm up sooner.^[1],[3]

For a play area, Playground Sand over a layer of Drain Rock dries faster than silt after rain and stays soft underfoot.^[1]

Lawns on silt loam stay soft for a long time after rain or snowmelt. Spreading a few inches of Topsoil mixed with Compost over the silt before seeding gives grass a looser, better draining layer to root into, and it keeps foot traffic from churning the surface into mud.^[1]

Fill Sand is the safer choice whenever a low area needs raising under a walk or a shed. It drains freely and holds far less water than silt, so it heaves less on freezing nights.^[1],[3]

Homes in ZIP 99501 and across Anchorage

The Census Bureau's survey counts 9,076 housing units in ZIP 99501. Detached single-family houses make up 17 percent of them.^[4]

Anchorage as a whole has 120,348 housing units, 47 percent of them detached, and its median build year is 1982.^[4]

In the ZIP, the median home was built in 1974, and owners live in 37 percent of the occupied homes.^[4]

The Alaska Department of Transportation and Public Facilities lists the stone layer under its pavements as Aggregate Base Course in Section 301 of its 2020 Standard Specifications for Highway Construction.^[5]