The Use of Test Pits to Investigate Subsurface Fracturing and Glacial Stratigraphy in Tills and Other Unconsolidated Materials

Joints and fractures, common in Ohio glacial tills, often influence shallow ground water flow paths and rates. Environmental site investigations in glacial till and lacustrine sediments should include determination of the glacial stratigraphy and evaluation of the presence, extent, and density of subsurface fractures. The test pit is one approach to directly assess fracturing and stratigraphy. The design and construction of deep test pits is examined in this research report, which includes an extensive literature review and case studies from three test pit sites in Ohio. A generic design is recommended that may be used for 1-meter, 2-meter, 3-meter, or 4-meter deep test pits. Scaled drawings are included. OHIO J SCI 100 (3/4): 100-106, 2000 INTRODUCTION Joints and fractures are common in Ohio's unconsolidated subsurface materials, including glacial tills and lake plain sediments (White 1982). These features can extend from the soil structural units into the lower geologic strata, acting as conduits for ground water and contaminant flow from shallow to deep systems (Kirkaldie 1988; Kirkaldie and Talbot 1992). Older glacial deposits such as Illinoian tills typically have higher hydraulic conductivities than younger deposits such as Wisconsinan tills. This is due to greater fracturing and greater leaching of soluble minerals from the matrix. The clepositional environment also has implications on extent of fracturing. Lodgement tills typically have more shear stress fracture networks than ablation tills or glaciolacustrine tills which typically exhibit polygonal desiccation fracturing. Characterizations based on primary porosity will often provide erroneous conclusions if the secondary porosity is controlling ground water flow due to fractures, joints, and other macropores. Therefore, environmental investigations of sites containing fine-grained unconsolidated materials should use methods that are designed to determine the local stratigraphy and to check for the presence and extent of fracturing on a site-specific basis. Knowledge of the stratigraphy including depositional and post-depositional history can greatly aid in predicting the hydraulic properties of a site, as demonstrated by Melvin and others (1992) and Simpkins and Bradbury (1992). One site investigation method that is cost-effective and relatively easy to implement is the use of test pits. Such pits also allow the investigator to identify other hydraulically conductive pathways such as sand lenses and paleosols. These features are common along the ice margins where there were repeated minor glacial advances and retreats. Shallow test pits are commonly cited in the soils literature. The USDA's Soil Survey Manual (Soil Survey 'Manuscript received 21 July 1999 and in revised form 22 February 2000 (#99-25). Division Staff 1993) describes such pits for the detailed study of soil pedons, and recommends that the pit expose a vertical face approximately 1.0 m in width and usually 2.0 m or less in depth. The USDA manual also recommends that horizontal sections of each soil layer be excavated to expose structural units and patterns. Deeper pits have been used by researchers in Denmark (Klint and Fredericia 1998; McKay and others 1999) and Canada (McKay and others 1993; McKay and Fredericia 1995) to study geologic materials underlying the soil layers. In one case, freshly excavated benches in an active landfill were used to map the geology and fracturing to depths of up to 18 m (McKay and Fredericia 1995). Test pits have also been used in the United Kingdom to characterize potential landfill sites. Gray (1996) reported excavating 26 test pits, each 2.0 to 5.0 m deep, into fissured glacial till in Norfolk, England. Croxford (1996) reported using 57 test pits laid out in a grid pattern across a site in Scotland that was composed of peat, boulder clay (till), and fractured flagstone bedrock. Remedial investigators of coal gasification sites in northeast England included the excavation and sampling of numerous test pits up to 4.5 m deep stating that "considerable benefit is gained from the use of trial pits which are relatively cheap to cany out and provide the investigator with an excellent visual appraisal of the site" (Forth and Beaumont 1996). Test pit investigations are often superior to mapping of natural exposures, that is, stream cuts or pre-existing excavations such as road cuts and quarries. The advantages of using a test pit include the flexibility of choosing the location and depth of the excavation, and that the test pit provides a fresh exposure. A fresh exposure is helpful to avoid the confounding effects of weathering, erosion, oxidation, and vegetation. MATERIALS AND METHODS The methodology begins with clearly defining the objectives of the field investigation before designing the test pit and fracture mapping procedures. For example, at a site where there is a very thick sequence of clayrich glacial deposits (20-40 m or more), the primary OHIO JOURNAL OF SCIl'NCK A. I). CHRISTY, L A. McFARLANI), AND 1). CARKY 101 concern may he lateral migration of water and contaminants towards nearby streams, ditches, or agricultural drainage tiles. In this situation, investigators may be primarily interested in sand lenses at any depth and fractures in the shallower weathered and oxidized zone. At a site where the clay-rich deposits are relatively thin (<1() m) and/or overlie a prolific aquifer, the main concern will likely be downward flow and contaminant migration. In this case, investigators will be interested in identifying the presence of deep, possibly widely spaced, fractures. This situation would favor the excavation of not one, but several test pits, each with limited mapping of the weathered zone and more intensive mapping of the deeper benches. The number of pits, focus ol the field analysis, and the extensiveness of the mapping effort will be dependent upon the overall goals of the investigation and available resources.