MORPHOLOGY AND CHARACTERIZATION OF SOILS FORMED IN MARLBORO CLAY REGOLITH

Abstract

The Marlboro Clay is a late Paleocene/early Eocene geologic formation that outcrops in significant portions of Prince George's County, Maryland. The formation exhibits slope instability and is unsuitable for most building structures. In this study, we describe and characterize the mineralogical and shrink-swell properties of soils formed in Marlboro Clay regolith. Samples were collected from 2 pits, 7 deep cores, and 38 1-m surface auger borings, five of which were collected at the same site as the deep cores. Potential volume change, particle size, and pH value were analyzed on all samples. X-ray diffraction scans were performed on selected samples. Mineralogical analysis indicated that the clay fractions of surficial samples were from approximately 10 to 50% smectite. However, the swell index values resulting from the potential volume change analysis were generally moderate for soil samples (most samples were less than 18,000 kg m -2 ). Deep cores generally had greater swell index values (mean, 21,205 kg m -2 ) than did surficial samples, indicating that pedogenesis reduces the shrink-swell capacity of these soils. Air-dried pH values were less than 4.0 in many samples, with a minimum of 2.7, indicating sufficient infilling of sulfide-bearing glauconitic sediments from underlying and overlying formations to cause sulfuricization upon oxidation. The pedogenesis of Marlboro Clay materials reduces their shrink-swell potential, although these soils may still be problematic for slope instability and construction because of low permeability.