LIQUEFACTION ASSESSMENTS BY FIELD-BASED METHODS: FOUNDATION SOILS AT POINSETT SITE 7 DAM SITE IN NORTHEAST ARKANSAS

The Natural Resources Conservation Service (NRCS) is finalizing the design for Poinsett No. 7, a watershed dam to be built approximately 15 miles south of Jonesboro, Arkansas. The site is located within a small valley on the eastern flank of Crowley's Ridge in a high risk area of the New Madrid seismic zone. In 2005 an initial geological investigation raised concerns about liquefaction potential of Quaternary age sediments. This resulted in re-examination of the proposed design. One design considered was excavation of Quaternary deposits with construction of the embankment on more competent Tertiary age sediments. This design required further testing of the foundation materials. Boreholes were not advanced much below the top of the Tertiary age Claiborne Group during the 2005 investigation. Also, the right abutment area was not completely investigated during the initial drilling program due to access problems at that time.

In 2007 a supplemental geotechnical investigation was performed to address these design concerns. This investigation included 13 backhoe trenches and 13 hollow stem auger borings. In the borings, a CME continuous core barrel was advanced to 10 feet, followed by Standard Penetration Tests at 1.5 foot increments until reaching the top of Claiborne deposits. The CME core barrel was then advanced to 10 feet below the top of the Claiborne. Borings and trenches were logged using the Universal Soil Classification System. The shear strength of cohesive materials was measured using a hand-held torvane device, and 35 crumb tests were performed to check for dispersive soils. Dispersive reactions were noted in some Quaternary clay materials.

A deep alluvial channel deposit composed of saturated sand was encountered in Drill Hole 701, which flowed upward into the auger during drilling. This sand deposit and others in the valley alluvium and right abutment area pose a risk of liquefaction during strong seismic events.

With the possibility of removing the saturated sands to provide a dam foundation on the underlying Claiborne clay, there is also the consideration of lignite within the clay. The depth and thickness of lignite can be expected to vary considerably under the dam's footprint. In places lignite would be exposed by excavation, which is troublesome as it has shown a tendency to crumble when exposed in drill cores.