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10 0.0 1.0 2.0 3.0 4.0 5.0 0.0 0.5 1.0 1.5 2.0 2.5 (a) % Total Sulfate % E ttr in gi te Halaquepts Ildefonso Berthoud Bloom clay Dwyer sand Stochiometry 0.0 1.0 2.0 3.0 4.0 5.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (b) % Total Sulfate % E ttr in gi te Halaquepts Ildefonso Berthoud Bloom clay Dwyer sand Stochiometry Figure 1. Comparison of ettringite concentrations in soil (a) Predicted using phase diagram approach and (b) Measured using a Differential Scanning Calorimeter. Figure 1 shows three important results: (1) the threshold sulfate content for the five soils is different and, since all other factors are constant, threshold levels must depend on mineralogical differences of the soils; (2) the threshold levels determined based on phase diagram/mass-balance calculations and the DSC measurements are in reasonable agreement with each other and are less than the stoichiometrically determined maximum possible values; and (3) the threshold levels are in reasonable agreement with the levels defined in the literature as problematic levels (33). RISK IDENTIFICATION Risk assessment along the alignment of the proposed roadway is a key factor in deciding the fate of a project. Risk identification prior to soil exploration can help develop effective soil exploration and sampling techniques, define the scale of testing needed, and help decide the controls required during construction. Sulfate concentrations in soil exhibit high spatial heterogeneity and hence selecting locations to perform sulfate testing is critical. Identification of problem areas depends on the availability of techniques that can characterize important soil properties in a time effective manner. Although the sulfate concentrations in soil can vary for many reasons, identification of problematic locations can be achieved by careful consideration of certain selected features along the alignment. These are discussed in the following paragraphs. Geology and Mineralogy Soil characteristics can influence the potential of a selected soil to precipitate ettringite or thaumasite. Clay-rich soils generally have higher salinities, carbonate, sulfate (as measured in gypsum), and water-holding capacities than other soils, all of which are important components for the formation of ettringite/thaumasite. The probability of finding sulfate sources, like gypsum, in soil varies among soil formations and can be identified using geologic maps of the location. Soil formations that overlap the alignment can be identified by overlaying the alignment of the project on appropriate geological atlas maps. These maps also contain information regarding mineralogical composition of the soil formations. Details of parent rock formation provide insight into the mineralogy of the bedrock and soils in a given location. Potential sulfate-bearing minerals can be identified from these maps and incorporated into design considerations for the location. This information is to a degree compiled in County Soil Survey
11 Reports, published and updated by United States Department of Agriculture and the National Resources Conservation Service (NRCS), and can be used as a source by which to assess the potential of a selected soil to develop sulfate-induced distress (35). Initial identification of areas with potentially high concentrations of the chemical constituents of ettringite can be made using existing soil geospatial databases and using geologic and topographic maps of the location. The geospatial data can be used to identify important soil characteristics including mineralogy, clay content, and carbonate and sulfate levels which can be used to identify potential âhot spotsâ along the alignment. These hot spots can vary in shape and size from as little as 10-25 m in diameter to extended sulfate seams. GIS maps are an effective source of information by which to identify the locations with high sulfate concentrations. Like soil survey reports, decisions should not be based solely on the information acquired from these maps, but rather such information should be used as a preliminary identification of potential problematic areas. Laboratory-based sulfate testing must be performed to confirm any level of reconnaissance before soil treatment. Climatic Characteristics and Drainage Features Climatic conditions can also influence movement of ions in soils. In dry arid areas, sulfate deposits are likely to be found in near-surface environments due to evaporation processes, which leave previously dissolved sulfate ions in the soil. In wet and humid areas, water infiltration can carry sulfate ions into deeper strata which can be transferred back to the surface due to capillary action. In rolling terrains, significant sulfate concentrations may accumulate in low-lying areas due to surface runoff creating sulfate hot spots in the soil. Moving water also acts as a medium for ion migration into the stabilized layers to form a continuous source for limiting reagents in the system (2, 25). Copious amounts of water are needed to form these hydrous minerals. Water may be provided by fluctuation of the water table in the location, by surface infiltration, or by capillary suction (4, 28). Surface runoff and rainfall can also be the source of water needed for ettringite formation. Water can gain access to sulfate minerals in soil through seepage, through surface cracks and openings, or through permeable layers in the pavement section. Low-lying areas can accumulate water which increases the risk for sulfate heave in the location. Rolling terrain or slopes along the corridor can promote water accumulation along the pavement section. Sulfate crystals precipitating in soils during dry seasons can be washed along the slopes or through desiccation cracks in clays into stabilized layers along with rain water. A rise in the water table during wet periods can dissolve sulfates in the soil or transport sulfate ions from underlying parent rocks. Soil Classification Soil type has a strong impact on soil conductivity, and by implication, on sulfate concentration. Hence, soil classification can also be used in accessing the potential for ettringite formation in soils. Soil matrix suction can provide a potential to draw water from the underlying water table. This uplift of water can transfer the dissolved sulfate ions in to the pavement layers. Swift movement of water in the pavement section carries these dissolved sulfates to streams or other areas of low hydraulic potential. But, when the soil has high capillarity and low hydraulic conductivity, water cannot flow readily through these soils and the dissolved sulfate ions are not easily transported through the pavement sections. Therefore, soils with low hydraulic
12 conductivity, high capillarity, and high suction properties can create sulfate reservoirs in subgrade sections. Evaporation/transpiration can also remove water from these soils, leaving behind residual crystalline sulfate formations in the soil. These properties are typically associated with clays and shales. Once these problematic soil types along the project alignment are identified, conductivity measurements, detailed later in the report, can effectively be used to target these sulfate reservoirs. It is also important to note that soil texture is not the controlling factor in determining soil conductivities. High sulfate pockets can also exist in gravely, sandy, and silty soils. Topography and Spatial Variability Topographic slope influences hydrologic processes, including overland flow and subsurface flow and therefore has a strong influence on residual sulfate concentrations in soils. The major sources of sulfur in soil are evaporates like gypsum (CaSO4) or sodium or potassium sulfates or from SO42- that is the product of the oxidation process of pyrite (30). Topography influences the transport of these relatively nonreactive solute sulfates along a gravity gradient (downhill). Slopes shaped by erosion can transport sulfate ions to locations far from parent source and into pavement sections that might intercept these flow channels. A rolling topography favors the process and the risk due to sulfate heave is increased when these soils occur in areas that are dissected by stream erosion. Sulfate accumulation typically occurs in low-lying areas and near dry stream channels since evaporation/transpiration processes are likely to favor the accumulation of sulfates near the stream channel in the drier months due to the proximity of the groundwater table to the land surface in these areas. Therefore, topography, through its influence on hydrology, is likely to have a strong influence on the redistribution of sulfates along the landscape. The influence of topography on sulfate accumulation should be identified and considered during soil investigations. Among all the chemical constituents that comprise ettringite, SO42- ions are likely to have the highest spatial variation due to variation in source rocks and as a result of their hydrologic mobility. The relative amount of sulfate in a soilâs parent material can vary considerably among soil formations. Sulfate is also fairly mobile in the environment because of its relatively weak adsorption to soil minerals and due to solubility of gypsum and other evaporates in near-surface environments. Two hydrologic processes account for the mobility of sulfates and their accumulation in low-lying areas: surface/subsurface runoff and the upward migration of water from a shallow water-table aquifer through capillary action. In both cases, sulfate accumulates as the water evaporates and dissolved salts precipitate. Sulfate concentrations are generally higher in subsurface layers where the processes of moisture infiltration and evaporation and transpiration reach a state of general equilibrium and deposit a higher concentration of sulfates at a specific depth within the pedological profile of the soil. Because the NRCS soil survey reports provide in-depth pedological profile descriptions, these documents are an excellent source by which to identify where sulfates accumulate in the pedological profile and the spatial distribution of sulfates along the alignment. Since the pedological profile descriptions extend to a depth of several feet, they generally also provide an excellent source of data to determine whether or not cut and fill operations or whether fill sources may run the risk of containing high sulfate levels.
13 Visual Inspection Field evaluation for sulfates is critical and should be performed if any project location carries the risk for sulfate heave based on the criteria discussed above. Visual inspection should be performed along these selected locations of the highway and for all potential borrow sources. Size distribution of gypsum crystals can vary from visible crystals to microscopic crystalline phases in soil (Figure 2). Figure 2. Variability in size of gypsum crystals found in soil. Solubility of gypsum is dependent on particle size and surface area of crystals (36). Smaller particle size provides higher surface area which translates to faster dissolution of minerals when in contact with water. Therefore, fine-grained gypsum, if present, can dissolve faster and release ions faster when compared to coarse-grained fractions in soil. Well-formed gypsum crystals can be easily identified during visual inspections, whereas fine gypsum crystals can be detected as white powdery efflorescence on the surface, especially during dry seasons. The efflorescence is due to precipitates left behind after evaporation of ground water. Soil Investigation Identifying sulfate levels in soil is a critically important step. Soil investigations, prior to mix design, can assist engineers in selecting the effective type and levels of additives to be used and to choose appropriate techniques to be followed during stabilization. Sulfate level quantification can also be used to make a rough estimate of the quantity of ettringite that can form in soils based on stoichiometric calculations. Although this is a good technique for risk assessment, the extent of expansion of stabilized layers depends on whether or not the ettringite formed can be accommodated within the soil matrix. Moreover, ettringite precipitation and stability depends on geochemical controls including pH, activities of participating ions in solution, temperature, and activities of dissolved CO2 and H2O (3, 10, 37). Hence, based on these factors, it must be remembered that the estimated ettringite concentrations based on stoichiometric calculations are only approximate. In some cases this estimate must be validated by actual testing of the volumetric expansion potential of the treated soil. But, even when swell testing is done, such as âone dimensionalâ swell testing (ASTM D 4546) or the âthree dimensionalâ swell testing advocated by Petry (38), one must remember that the field 5 mm 2.54 cm
