An Empirical Index for Texas Counties Peer-Reviewed Article
Expansive-Soil Geohazards:
Author James L. Gooding, MEM-3070
Abstract A simple index for the relative likelihood of expansive-soil damage, including effects of soil plasticity and drought-flood
cycles, is formulated for Texas counties from publicly-available, online data for soil engineering properties and rainfall time-series. A dimensionless scale on which index values rise from zero toward or beyond 1.0 indicates the counties most likely to experience expansive-soil effects. For the 39 most populous Texas counties, the range of index values is 0.02-1.40 although 18 counties cluster within the range of 0.80-1.20. The index is proposed as a quick screening tool to inform deci- sions about whether more detailed field or laboratory studies are advisable for expansive-soil geohazards.
Keywords: Geohazard, expansive soil, hazard index, soil expansion
Expansive-Soil Geohazards and Environmental Variables Shrink-swell forces related to expansive soils are well-
known geohazards encountered when building foundations and roadways in Texas (for example, Wise and Hudson, 1971; Hudak, 1998). Engineering studies have demonstrated that the expansive forces also depend upon repeated cycles of wet- ting and drying (see, for example, Nowamooz and Masrouri, 2008). Therefore, the key variables include the widely rec- ognized soil engineering properties of liquid limit (LL) and plasticity index (PI) along with some measure of the magnitude of cyclical precipitation swings. However, to date, no standard variable has been established for quantifying precipitation cycles in the context of shrink-swell processes.
Elegant formulations for shrink-swell potential have been developed after meticulous laboratory analyses of soil min- eralogy and cation-exchange capacity (Thomas and others, 2000). Even so, such detailed information is not available for most areas and therefore does not facilitate rapid screen- ing of expansive-soil hazards. Risk management plans for geohazards are most effectively addressed in an iterative manner where preliminary screening is used to determine the need for, and to prioritize, more detailed studies before investment decisions are made either to avoid development of an asset or to invest resources in mitigations to reduce asset risk. Accordingly, a method to use widely available geoscientific data to quickly screen the likely expansive-soil damage potential would add value to geohazard assessments.
An Empirical Index Calibrated for Texas Texas is subject both to major droughts
and floods, resulting in environments of cyclical drying and wetting of naturally clay-rich soils across diverse landscapes. During 2011-2015, Texas experienced a widespread and intense drought (NDMC, 2019) that led to renewed attention to road damage caused by expansive soils (TXDOT, 2012). Road repair costs during the drought are at least partly reflected in the road maintenance expenditures by counties (TXDOT, 2013). Those expen- diture data are publicly available and therefore offer an opportunity to test correlations of damage costs with soil- environment variables. (Alternative damage metrics, such as building foun- dation repair costs, are not available in authoritative public databases.)
Road maintenance costs during the
most intense period of the 2011-2015 Texas drought were tested as a depen-
Figure 1. Road Maintenance Costs Correlated with Soil Plasticity Index and Rainfall
www.aipg.org
dent damage cost (DC) prospectively driven by independent geoscientific
Jul.Aug.Sep 2019 • TPG 9
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