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Guidelines for Depth of Borings <br />Area for ~ Boring <br />Investigation <br />Foundation _ The depth of borings depends upon the soil profile and the. type. of feasible foundation. <br />Proceed as follows: (1) If no preliminary soil information is~available, start with one or two <br />deep borings to bedrock or to a depth equal to the width of the structure. (2) Analyze the <br />boring results and determine the number and depths of additional borings. <br />Borings should be carried to a depth: <br />1. below any organic soils, muds, art~dal fill, or compressible layer, <br />2. suffidently deep for establishing the bottom elevation of foundation (footings, piles, or <br />caissonsj; and <br />3. sufficiently deep for checking .the possibility of a weaker soil, at a greater depth, which <br />may settle under the sustained toad. <br />Structure with Extend to a depth where increase in vertical stress for combined foundations is less than <br />rte closely spaced 1096 of effective overburden stress. Generally all borings should extend to no less than 30 <br />!s- feet below lowest part of the foundation unless rodk is encountered at shallower depth.. <br />d rigid foundation. Extend to a depth where increase in vertical stress decreases #0.90'/0 of bearing .pressure. <br />Generally alt borings should extend to no less than 30 feet below kwvest part of the <br />foundation unless rods is encountered at shallower depth. <br />ways and airfields; Minimum depth of borings is 5 ft below bottom of structure, but should extend below <br />lines- organic soil, muds, art~dal fill, or compressible layers such as soft days and silts. <br />fining walls. 1. Below organic soil, muds, artficial fill, or compressible layer, <br />2. deeper than possible surface of sliding; and <br />3. deeper than width ~of the base of waN. <br />stability. Extend to an elevation below active or potential failure surface and into hard stratum, or to <br />a depth for which failure is likely because of geometry of cross section. <br />Extend to: <br />1. below organic soil, mudk, .artificial fiN, or compressible layer, <br />2. deeper than possible surface of sliding; and <br />3. depth between 3/. #01 times base width of nan'ow cuts. <br />' Where kart is above ground water in stable materials, depth of 4 to 8 feet below base may <br />suffice. Where base is below groundwater, determine extent of pervious strata below <br />base. <br />Extend to: <br />1. below organic soil, mudk, artificial fill,. or compressible layer, <br />2. deeper than possible surface of sliding; and <br />3. depth between'/ to 1'/. times horizontal length of side slope in relatively homogeneous <br />foundation. <br />Where soft strata encountered, borings should reach hard materials. <br />Page 2 of 2 <br />