Rammed-earth walls used to be elementary. When the ancient building method was revived in the U.S. almost 40 years ago, mineral soils were typically scooped from the job site, moistened, and compacted one thin layer at a time inside simple forms resting on concrete footings. Portland cement was sometimes mixed in to add strength and durability, and concrete columns and bond beams often supported the roof. In the raw, the economical walls seemed to beg for a sod roof.

These days, though, you're more likely to find rammed earth serving as functional art in upscale architecture, with a hefty price tag to match. A striking example is the Walnut Farm Retreat, a sprawling net-zero contemporary taking shape about a mile from the San Andreas Fault near Hollister, Calif. Designed by architect Jonathan Feldman, the...

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