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<br /> Guidelines for Depth of Borings <br /> Area for Boring Depth <br /> Investigation <br /> tructural Foundation The depth of borings depends upon the soil profile and the type of feasible foundation. <br /> Proceed as follows: (1) If no preliminary soN information is avaNabie, start with one or two <br /> deep borings to bedrock or to a depth equal to the width of the stnx~ure. (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. bek~nr any organic soils, muds, artifidal fiN, or compressible layer, <br /> 2. suffdently deep for establishing the bottom elevation of foundation (footings, piles, or <br /> caissons); and <br /> 3. suffidently deep for checking the possibility of a weaker sal, at a greater depth, which <br /> may settle under the sustained load. <br /> Stnx~ure with Extend to a depth where increase in vertical stress for combined foundations is less than <br /> e dosety spaced 1096 of effective overtwrden stress. Generally all borings should extend to no less than 30 <br /> outings. feet below lowest part ~ the foundation unless rock is encountered at shaHovVer depth. <br /> slated rigid foundation. Extend to a depth where increase in vertical stress decreases to1096 of bearing pressure. <br /> Generally all borings should extend to no less than 30 feet below lowest part of the <br /> foundatice unless rock is encountered at shallower depth. <br /> ighways and airfields; Minimum depth of borings is 5 ft below bottom of structure, but should extend below <br /> pe lines. organic soil, muds, artifdal fill, or compressible layers such as soft days and s<'Its. <br /> taming walls. 1. Bekriv organic soil, muds, artificial fill, or compressible layer; <br /> 2. deeper than possible surface of sliding; and <br /> 3. deeper than wrdth of the base of waN. <br /> lope stability. Extend to an elevation bekn~v 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 sal, muds, artfidal fiN, or compressible layer, <br /> 2. deeper than possible surface of sliding; and <br /> 3. depth between'/, to 1 times base width of narrow cuts. <br /> Where cut is above ground water in stable materials, depth of 4 to 8 feet bekriv base may <br /> suffice. Where base is bebw groundwater, determine extent of pervious strata below <br /> base. <br /> mtiankments. Extend to: <br /> 1. bekwv organic soil, muds, artifiaal fill, or compressible layer, <br /> 2. deeper than possible surface of slid'uig; 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 ojl <br /> <br />