Caliche

Rain carries dissolved calcium into desert ground. Where it stalls and dries, the calcium carbonate glues the soil grains into a solid mass, and that mass is caliche. University of Arizona Cooperative Extension describes two forms: pale lumps under an inch to several inches across, or a continuous layer a few inches to several feet thick. Some crumbles under a shovel; some is close to rock. Not every desert lot has it.

The cost shows up in excavation. Footings have to reach undisturbed bearing soil, utility trenches have to reach depth, and a pool has to come out of the ground. When a cut hits a hard seam, the crew trades the backhoe bucket for a hoe ram or a rock saw, and the day rate climbs while the schedule slips.

The second problem is water. Solid caliche is nearly impermeable, so irrigation and monsoon runoff perch on top of it instead of draining away. Water stands beside a foundation long after a storm, and a tree planted in a hole that stops at the layer drowns slowly. Extension's fix is blunt: dig through the layer, or cut a narrower hole that pierces it, then check that it drops four inches in four hours.

For example, Maricopa County wants three backhoe-dug test holes at least 12 feet deep on a five-acre Apache Junction parcel before it sets a soil absorption rate for septic. Hit refusal partway down, which caliche will cause, and the county tells you to stop and call. The usable depth shrinks and the drain field gets bigger.

Caliche is not expansive soil, and the two get confused constantly. Expansive clay swells with water and shrinks as it dries, so it moves under a slab. Caliche stays put. It is cemented, which makes it decent to build on and miserable to dig. Your soil report calls out both, and the perc test shows whether a hard layer will drive your septic design.