Gravel, Stone and Soil Delivery in Tracy, CA

Capay clay and why it moves

At the center point of ZIP 95304, the USDA soil survey maps Capay clay, 0 to 1 percent slopes, MLRA 17. Capay is the main series and fills 85 percent of the map unit. The top mineral layer is clay, the survey rates the soil moderately well drained, and it sits in hydrologic group C, where water enters the ground slowly.^[1]

The number that stands out is shrink-swell. In the top meter of soil the survey rates it very high, at 10.7 percent. Clay with a rating like that swells when it soaks up water and shrinks and cracks when it dries. Over a few seasons, that cycle can push a thin layer of gravel down into the cracks and leave soft spots and ruts behind.^[1]

Anyone building on clay of this type should plan around the movement. The steps that help most are a separation fabric over the clay, a thicker base than usual, and good drainage so the ground under the base stays at a steadier moisture level. Each of those comes up again in the driveway plan further down.^[1]

Old river deposits below the soil

The state geologic map puts the center point on a unit called Quaternary alluvium and marine deposits. Its materials are coarse alluvium and fine alluvium, which are sands, silts, clays and gravels laid down by moving water.^[2]

Ground built from layered alluvium can change a lot within a short dig. One hole may hit a sandy lens that drains well, while the next hits dense clay. That is one more reason to dig a test pit before a big project and to look at the layers yourself. The land is close to level, and the center point stands 130 feet above sea level.^[2],[3]

Flat valley land like this has one drawback for drainage work: there is little natural fall to carry water away. A French drain or a swale on a lot near the center point may need to run a long way to reach a low outlet, or end in a dry well filled with Drain Rock. Check the grade with a string line before you dig, so the trench falls the whole way.^[1]

A gravel driveway built for swelling clay

Start a drive on Capay clay by stripping the topsoil and any loose material. Shape the clay subgrade with a slight crown or a cross slope so water runs off it instead of soaking in at the center. Then roll out a woven geotextile across the whole width. The fabric keeps the clay from squeezing up into the stone when it is wet and keeps the stone from sinking into it.^[1]

Over the fabric, a layer of 1-1/2" Crushed Gravel or #4 Crushed Stone spreads the weight of a car or truck over a wider area of clay. Compact that layer, then add Road Base in thin lifts and pack each one hard. A thicker build than you would use on sandy soil pays off here, because the clay below gives less support when it is wet.^[1]

Finish with Driveway Gravel or #57 Crushed Stone on top. Along the low side of the drive, a shallow trench lined with fabric and filled with Drain Rock gives water a path away from the base. Keeping the clay under the drive from getting soaked does more than any single stone choice to stop it from heaving.^[1]

Rain, heat and the freeze dates near Tracy

The NOAA normals station at Tracy Carbona, CA, about 3 miles away, averages 12.7 inches of precipitation a year. That is a dry total, but clay in group C still struggles with it, because rain that falls faster than the ground can take it sits on the surface or runs off.^[1],[4]

That station records 26 days a year when the low drops to 32 F or colder. The median last spring freeze there falls on February 25. The median first fall freeze falls on November 26.^[4]

Frost is a small factor next to the clay. The bigger seasonal swing is between the wet months, when the clay swells, and the long dry months, when it shrinks and opens cracks. A layer of mulch on beds and a well-drained base under hard surfaces both help even out that swing.^[1]

Road Base or open stone on Capay clay

Owners in Tracy often ask whether a drive on clay should use a dense mix or a clean, open stone. Each one handles water in a different way. Road Base is a blend of crushed rock and fines. Packed hard, it forms a tight layer that sheds most rain across its surface, so less water reaches the Capay clay below. That makes it the better wearing base for a drive that sees daily traffic.^[1]

Open stone such as #57 Crushed Stone or #67 Stone has no fines. Water drops straight through it, which is perfect in a drainage trench but a risk on top of slow clay in group C. Water that passes through open stone and reaches the clay can sit there, soften the subgrade and let the stone press down into it. If you want the look of clean stone on top, lay it as a thin wearing layer over packed Road Base and fabric, not as the whole build.^[1]

For a yard that just needs a firm place to park a trailer or a boat, Crushed Concrete or Asphalt Millings over fabric are good options. Crushed Concrete and Asphalt Millings both compact well, and the millings bind more tightly in summer heat. On clay with a very high shrink-swell rating, the fabric under either one matters more than the choice of top material.^[1]

Yards, beds and homes in ZIP 95304

Heavy clay makes a hard home for many plants. Raise the bed instead of digging deep into the clay, and fill it with Garden Soil or a mix of Topsoil and Compost. Cover the surface with Brown Shredded Mulch or Wood Chips to hold moisture through the dry season and to keep the soil from crusting.^[1]

For areas you walk on, a firm path of Stone Dust Screenings over a compacted base drains better than bare clay and stays usable after rain. Pea Gravel and River Rock suit borders and dry creek beds.^[1]

The Census counts 5,410 housing units in ZIP 95304. Detached single-family houses make up 73 percent of them, the median build year is 1999, and owners live in 60 percent of the occupied homes. Across Tracy as a whole there are 30,228 housing units, 81 percent of them detached, with a median build year of 1995.^[5]

For state roads, Caltrans sets its standard for the layer below the pavement as Class 2 Aggregate Base, Section 26-1.02B, Standard Specifications (2026).^[6]