Liquefaction risk, before you commission the geotech.

When saturated loose sandy soils are shaken hard, pore pressure spikes and the soil temporarily behaves like a liquid. Buildings on top settle, tilt, or slide laterally toward lower ground. The 1989 Loma Prieta earthquake liquefaction of San Francisco's Marina District — sand fill placed for the 1915 Panama-Pacific Expo — is the textbook case. Property loss from liquefaction typically exceeds the ground shaking itself in the affected neighborhoods.

Three data sources tell you: (1) State-published liquefaction hazard maps — California, Utah, Washington, and Oregon have detailed statewide maps. (2) USGS Q-fault and NEHRP site-class data — nationwide coverage, coarser resolution. (3) USDA soil series — young alluvial and marine deposits are the classic red flag. Buildability queries all three at run time and returns a preliminary susceptibility read in about 20 seconds. A commissioned geotechnical report is the binding answer.

A residential-scale geotechnical investigation with 2–3 borings runs $3,500–$8,000 typically, with the deeper end reflecting more borings and lab testing (SPT, atterberg limits, gradation). Commercial projects run $10,000–$50,000+ depending on scope. Buildability's live check is not a substitute — but it tells you whether to bother commissioning one, before you spend the money.

Three practical strategies. (1) Deep foundations — driven concrete or steel piles, or drilled shafts, that transfer load to competent stratum below the liquefiable layer. Adds $30,000–$120,000+ to a residential build. (2) Ground improvement — stone columns, deep dynamic compaction, or vibro-densification that stiffens the loose soil in place. $15,000–$60,000 typical residential. (3) Walk away — the honest answer when the site's severe and the deal math doesn't support the foundation premium.

Yes, and it's usually already priced in for buildable sites in mapped zones — comparable sales reflect the required foundation premium. The value hit is worst on parcels that were bought before disclosure requirements and are now hard to insure or hard to sell without a geotech report in hand. In California, natural-hazard disclosure statements have to name liquefaction risk since 1998.

A specific liquefaction failure mode where the liquefied layer flows downslope, dragging structures with it. Bay margins, riverbanks, and lake edges are the classic settings. It's not just settlement — the ground moves horizontally toward the free face, and buildings crack, tilt, or slide off their foundations. Where lateral spreading is possible, deep piles alone are not enough; the design has to resist the lateral drag.

No. Ground shaking is the direct effect of seismic waves — every parcel in a seismic zone experiences it. Liquefaction is a soil-specific response that happens only where the shaking, the water table, and the soil type all line up. A parcel with high PGA on competent bedrock has strong shaking risk and low liquefaction risk. A parcel with moderate PGA on young bay mud has moderate shaking risk and severe liquefaction risk. They price different fixes.

In California, the Natural Hazard Disclosure Statement covers state-mapped liquefaction zones and must be delivered to residential buyers. Elsewhere, requirements vary by state. Commercial and institutional lenders routinely order Phase I environmental and geotechnical work regardless of disclosure requirements; residential lending typically does not, which is why serious buyers verify independently.

Yes — all 50 states. USGS seismic data and USDA soils cover every U.S. address. State-mapped liquefaction zones are queried where available (CA, UT, WA, OR most comprehensively); the base soil + PGA read works everywhere else.