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U.S. OEPAATMENi OE AGRICULTURE • PAG~ 2 OF 2 NATURAL RESOURCES CONSERVATION SERVICE 04/26/00 SDTL FEATURES Endnote -- SOIL FEATURES this report gives estimates of various sail features. The estimates are used in land use planning that involves engineering considerations. Depth to bedrock is given if bedrock is within a depth of 5 feet. The depth is based on many soil borings and on observations during soil mapping. the rock is either "Soft" or 'Hard". If the rock is "Soft" or fractured, excavations can be made with trenching machines, backhoes, or small rippers. if the rack is "Hard" or massive, blasting or special equipment generally is needed for ezcavatimn. Cemented pans are cemented or indurated subsurface layers within a depth of 5 feet. Such pan; cause difficulty in excavation. Pans are classified as "chin" or "chick". A "Thin" pan is less than 3 inches thick if continuously indurated or less than IR inches thick if discontinuous or fractured. Excavations can be made by trenching machines, backhoes, or small rippers. A "chick" pan is more than 3 inches thick if continuously indurated or more than l8 inches thick if discontinuous or fractured. Such a pan is so thick or massive that blasting or special equipment is needed in excavation. Subsidence is the settlement of organic soils or of saturated mineral soils of very low density. Subsidence results from either desiccation and shrinkage or oxidation of organic material, or both, following drainage. Subsidence takes place gradually, usually aver a period of several years. This report shows the expected initial subsidence, which usually is a result of drainage, and total subsidence, which usually is a result of oxidation. Not shown in the report is subsidence caused by an imposed surface load or by the withdrawal of ground water throughout an extensive area as a result of lowering the water table. Potential frost action is the likelihood of upward or lateral expansion of the sell caused by the formation of segregated ice lenses (trust heave) and the subsequent collapse of the sail and loss of strength on thawing. Frost action occurs when moisture moves into the freezing zone mf the soil. Temperature, texture, density, permeability, content of organic matter, and depth to the water table are the most important factors considered in evaluating the potential for frost action. It is assumed that the soil is not insulated by vegetation or snox and is not artificially drained. Silty and highly structured clayey sails that have a high water table in winter are the most susceptible to frost action. Well drained, very gravelly, or very sandy soils are the least susceptible. Frost heave and low soil strength during thaxing cause damage mainly to pavements and other rigid structures. Risk of corrosion pertains to potential sail-induced electrochemical or chemical action that dissolves or xeakens uncoated steel or concrete. the rate of corrosion of uncoated steel is related to such factors as soil moisture, particle-size distribution, acidity, and electrical conductivity of the soil. the rate of corrosion of concrete is based mainly on the sulfate and sodium content, texture, moisture content, and acidity of the soil. Special site examination and design may be needed if the combination of factors creates a severe corrosion environment. The steel installations that intersect soil boundaries or soil layers is more susceptible to corrosion than steel in installations that are entirely within one kind of soil or within one soil layer. Far uncoated steel, the risk of corrosion, expressed as "low", "Moderate", or "High", is based an soil drainage class, total acidity, electrical resistivity near field capacity, and electrical conductivity of the saturation extract. For concrete, the risk of corrosion is also expressed as "Low", "Moderate", or "High". It is based on soil texture, acidity, and amount of sulfates in the saturation extract.