Engineering geology of the Socorro area, New Mexico
Gary D. Johnpeer and Hamil, Brenton, M.
The silts, sands, and clays of the floodplain are the materials on which approximately one-half of the City of Socorro is situated. These poorly-cohesive materials, combined with shallow, fluctuating ground- water conditions, contribute to foundation instability and drainage problems and are highly susceptible to liquifaction during seismic shaking. The alluvial fans bordering the floodplain cause fewer engineering problems because of their coarse, well-graded consistency and drainage characteristics. In places, caliche development prevents water infiltration and contributes to flash-flood potential at lower elevations. Mud- flow deposits may be susceptible to collapse when wetted, causing foundation instability. Few engineering geology problems have been reported for the rock outcrops west of Socorro. Locally, landslide blocks undergo creep, causing disruption of utility lines. Potential exists for rock fall or slope failure in response to earthquakes or explosions.
Surficial deposits are utilized locally for a variety of construction purposes with many of the finer-grained, clay-rich floodplain deposits being used in adobe brick. The coarser sediments are used as aggregate in road and foundation fill. Aggregate is now obtained from alluvial deposits on the west bank of the river and perlite is mined on the southeastern flank of the Socorro Mountains.
Ground water occurs both in the floodplain units of the Rio Grande and in the Sierra Ladrones Formation. Depth to water in the floodplain is approximately 3 to 15 m and in the Sierra Ladrones Formation the depth to water ranges from 10 to more than 50 m below the surface. Socorro obtains a year-round supply of water from the constant flow of three springs issuing from bedrock at the base of the Socorro Mountains, and from five wells drilled through the alluvial-fan gravels into axial-river sands of the Sierra Ladrones Formation. Chemical analyses indicate reasonably constant, acceptable quality for these sources.
Socorro has historically experienced moderate seismicity levels with estimated Richter magnitudes as high as 6.0. Since 1906 activity has been infrequent and moderate.The volcanic hazard for Socorro is appreciable. Several small, shallow intrusive bodies imply a potential for proximal volcanism.
Potential for flooding of areas within the city exists, mainly along major arroyos and upstream slopes of levees, roads, and railroad embankments. A diversion dam at San Acacia reduces the risk of flooding on the east side of town, but low-lying areas remain at some risk of inundation.
Subsidence and earth fissures caused by groundwater withdrawal have recently been reported near Deming, New Mexico. Groundwater pumping and the San Marcial crack (of unknown origin) indicate a potential for fissures in the Socorro area. Most geotechnical investigations in the area consist of routine soil borings, trenching, and standard soil-engineering tests. Geotechnical investigation and appropriate foundation design can accommodate the local unstable soil conditions.
- Johnpeer, Gary D.; Hamil, Brenton, M., 1983, Engineering geology of the Socorro area, New Mexico, in: Socorro region II, Chapin, C. E., New Mexico Geological Society, Guidebook, 34th Field Conference, pp. 339-344.