Rock, Gravel and Decomposed Granite Delivery in Tucson, AZ

A Hard Layer Close to the Surface

At the ZIP's center point, the USDA survey maps the Cave soils and urban land, 0 to 8 percent slopes unit. Urban land, paved or built over, makes up 40 percent of it, and the Cave series accounts for 41 percent. The top mineral layer is a gravelly fine sandy loam.^[1]

The survey records a petrocalcic layer at about 7 inches below the surface. Desert gardeners call it caliche: a band of soil cemented by lime until it is close to rock. Water cannot soak through it, and a shovel bounces off it.^[1]

That shallow hardpan explains a rating that looks odd at first. The Cave soil is classed as well drained, yet its hydrologic group is D, with very slow infiltration and high runoff. The thin topsoil drains, but the cemented layer beneath stops the water, so rain soon runs off or pools on top of the caliche.^[1]

Shrink-swell in the top 100 cm is low, at 1.5 percent, so the soil itself does not heave much as it wets and dries. The mapped geology at the center point is Quaternary surficial deposits, undivided, made of sand, silt and clay.^[1],[2]

Building a Drive Over Caliche

Caliche can be a gift under a driveway. A cemented layer that does not move makes a firm floor for stone, so the job is often more about shaping than digging. Scrape away the loose gravelly soil, keep a gentle slope so water runs off the hardpan and out to the street or a swale, and check for low spots where it could pond.^[1]

Spread Road Base over the scraped ground and compact it in lifts. Its fines fill the hollows in the caliche surface and pack into a smooth, hard bed. Top it with 3/4" Crushed Stone for a crunchy drive, or with Decomposed Granite for a softer, tan surface that blends with the desert around it.^[1]

Where water collects against the hardpan, a shallow trench of Drain Rock along the low edge gives it a path out. With high runoff from a group D soil, an outlet lined with River Rock or Riprap keeps the water from cutting a gully when a storm unloads.^[1]

Where the drive meets the street, the edge takes the most wear. A row of larger stone or a concrete band at the apron keeps tires from pushing gravel into the gutter, and a final pass with the compactor after the first good rain tightens the surface.^[1]

Fill Dirt builds up a low spot before the base goes down, compacted in thin lifts so it does not settle later. Over caliche, keep fill shallow and sloped, since water that reaches the hardpan under it has to flow somewhere.^[1]

Heat, Short Storms and Barely Any Frost

The NOAA station at Tucson WFO, AZ, about 1 mile from the ZIP, averages 11.6 inches of precipitation a year and 0.0 inches of snowfall. Much of that rain can come in brief summer storms that test every slope and drain on a property.^[3]

Frost barely registers here. The station counts just 5 days a year when the low falls to 32 F or below. Its median last freeze date is January 20, and its median first freeze date is December 17.^[3]

With so few freezing nights, frost heave is rarely the thing that ruins a Tucson drive. Washouts, sun and dust are the bigger threats, so pick stone that packs and drains rather than worrying about deep frost protection.^[3]

Summer heat bakes the surface but does little harm to the stone itself. It does dry out a compacted base, so a light watering before rolling helps Road Base or Decomposed Granite bind the first time.^[3]

The ground at the ZIP's center point stands about 2,400 feet above sea level, measured by the USGS elevation service.^[4]

Choosing Stone for a Desert Yard

Decomposed Granite and River Rock are the two staples of a desert yard, and they do different work. Decomposed Granite packs into a walking surface, fills the spaces between plants and lets water soak in where the caliche allows. River Rock stays loose and rounded, and it belongs where water moves: swales, splash pads under downspouts and the low line of a yard.^[1]

Smaller crushed stone, such as 3/8" Crushed Stone or #89 Crushed Stone, gives a crisp, angular cover that stays put on gentle slopes. Pea Gravel is softer underfoot for a seating area, but its round stones drift on any grade.^[1]

Color matters in strong sun. Light stone throws heat back at plants and walls, while darker stone holds warmth into the evening. Mixing sizes, such as large River Rock along a bed edge with Decomposed Granite in the paths, gives a yard texture without adding water use.^[3]

Keep wood mulch out of spots that take the full afternoon sun, where it dries out and breaks down fast. Stone handles that exposure for years with little care beyond a rake.^[3]

Homes Near Downtown

ZIP 85701 holds 3,832 housing units by the Census count. Detached single-family houses make up 35 percent of them, compared with 52 percent across Tucson.^[5]

The median home in the ZIP was built in 1956, older than the citywide median of 1979 for Tucson's 247,823 housing units. Owners occupy 26 percent of the occupied homes in 85701.^[5]

Older courtyard homes and small lots make room for compact projects: a gravel side yard, a Decomposed Granite patio under a mesquite, or a narrow path of Walkway Gravel to a back gate. A few tons of stone can change a small yard completely.^[5]

Planting Where Roots Hit Rock

Roots stop at caliche just as water does. Before planting a tree or a large shrub, break through the hardpan with a digging bar so the hole drains, then backfill with the native soil mixed with Compost. A hole carved into caliche without an outlet turns into a pot with no drain and drowns the roots.^[1]

Raised beds sidestep the problem. Build them on top of the ground and fill them with Garden Soil, which gives vegetables and flowers the depth that the thin native layer cannot. Topsoil works for a lawn patch or to level a bed before planting.^[1]

A top layer of River Rock, Pea Gravel or Decomposed Granite shades the soil and slows evaporation around desert plants. Wood mulch such as Brown Shredded Mulch suits shaded beds near the house, where it keeps roots cooler in summer.^[3]

Planting goes best in the cool months. With the median last freeze date at the Tucson WFO station on January 20, frost-tender plants can go out soon after it, giving roots time to settle before the hottest weeks.^[3]