COLLAPSIBLE SOILS IN THE RIO GRANDE VALLEY OF CENTRAL NEW MEXICO

Collapsible soils are common throughout the Rio Grande Valley in central New Mexico. Significant losses have occurred for those who unknowingly build on collapsible soils as well as significant cost increases have been incurred by those who are aware of the hazard and take remedial measures to develop the site.

New Mexico is undergoing dramatic urban development and expansion of it cities and communities beyond the Rio Grande Valley terraces and flood plain onto the alluvial fans from the mountains. The alluvial fans upon which urban development is occurring also happens to be the environment of collapsible soils.

The arid climate seldom wets the surface soils to a depth of a more than a few centimeters. Intense very brief thunderstorms occurring during the summer monsoon season are greatly intensified the uplift of mountain ranges. Resultant flash floods carry large sediment loads from the sparsely vegetated mountain foothills onto the alluvial fans. The highly porous structure of the mudflow or debris flow remains intact and is often further buried. Cementation by ubiquitous calcium carbonate and cohesion of clay and silt minerals to sand grains provides considerable strength to the soil when dry.

During urban development, arroyos are intercepted by flood control structures or canals to protect homes. Roads and home sites are graded into the landscapes exposing cuts up to several meters in depth. Irrigation systems are installed to support landscaping. Wells and septic systems are utilized if the reach of municipal water and sewage services is exceeded. Water entering the soil through irrigation and waste disposal exceeds that than realized during the formation of the native landforms and soils several fold. Rainfall runoff which seldom occurred before development is concentrated by impermeable surfaces such as roofs, driveways, and streets and drains into landscaping, roadside ditches and low spots permeating several tens of meters in the coarse textured soils.

The introduction of water reduces the soil strength and induces collapsible which is transferred to the soil surface with devastating effect upon structures. Up to one meter or more collapse has been observed. Creation of awareness and recognition of the expression of the hazard and remediation measures are important in preventing future losses.