New Mexico Geological Society Annual Spring Meeting — Abstracts

We present the method used and the resulting 1:750,000 scale map of collapsible soil susceptibility for New Mexico, the first time such a map has been generated in the western United States. Collapsible, or hydrocompactive soils are sedimentary deposits that under combined wetting and loading will subside significantly (up to 30% strains). Composed of water-sensitive-clay-rich sediments, these deposits present a common but difficult to map hazard across the desert Southwest, including New Mexico. However, because compaction of collapsible soils usually happens after structures have been built on them, their incidents are not well reported nor are they readily apparent from surface morphology. Maps of hydrocompaction susceptibility generally are done at scales greater than 1:24,000, allowing for detailed geotechnical sampling to take place combined with geologic mapping. This was not tenable at the scale of New Mexico. To overcome the lack of observable morphology, the paucity of reported incidents and the lack of statewide geotechnical data, we compiled a map of known but geotechnically-unconfirmed locales and then developed a multi-proxy-based spatially weighted average (i.e., an overlay method) of susceptibilities. Proxies used in this study are surficial deposit maps of depositional style, age, geomorphic setting, sediment provenance, and deposit texture; NRCS gSSURGO maps of both soil texture of the bottom 1/3 of the pedon, and the soil taxonomic classification to the great group level; a Köppen-Geiger climate zone map; NLCD land-use maps; and reported static water levels from the NMOSE Water Rights Reporting System. Not all proxies are available throughout the state, meaning that the number of proxies available at a location affected the reliability of the estimated susceptibility. Each proxy was iteratively assigned a weight, or quality factor, and a range of susceptibilities between zero (not susceptible) and four (extremely susceptible) depending on the proxy value. We assessed the effectiveness of each proxy by comparing the frequency of occurrence in and within 500-m of mapped hydrocompactive locales; at this point, we iterated on both the proxy quality and proxy susceptibility. Then, 500-m resolution rasters of quality-weighted susceptibility (total susceptibilities), average quality and the number of proxies used were generated. Breaks in raw total susceptibilities were assessed by examination of the histograms in and near (within 500-m) mapped hydrocompactive soils: < 0.5 was not susceptible, 0.5-1.5 was low susceptibility, 1.5 to 2.5 was moderate susceptibility, 2.5 to 3 was highly susceptibility, and > 3 was extremely susceptible. The new 500-m resolution, 1:750,000 scale maps of total hydrocompaction susceptibility, average quality of estimated susceptibility, and number of proxies used should be used together as a planning aid to assess the susceptibility of a region to hydrocompactive soils. They are not, however, substitutes for detailed, location specific geotechnical or geohazards analyses.