Laserfiche WebLink
<br />I <br /> <br />I <br />I <br /> <br />EM 1110-2--2902 <br />3 Mar 1969 <br /> <br />I <br />I <br /> <br />mobilizing more of the relieving fill pressure. The proportions should be selected carefully and <br />the tangent-length-to-radius ratio will usually be between 0.5 and 1.0. The conduit design should <br />cover a range of possible loading conditions from initial or construction condition to the long <br />time condition. Here also, a geologist or soils engineer should be consulted before final determi- <br />nation of the base shape of a conduit. <br />The "horseshoe" section (see plate 1) is generally less economical than the oblong and is <br />therefore not often used. Its stress distribution is not as desirable as that of the circular or <br />oblong section, and shear stirrups may be requ ired in the base. It may be practicable, however, <br />for some foundation conditions where the fill height is low. <br /> <br />I <br /> <br />I <br /> <br />4. Loads. <br /> <br />a. Groundwater and 8'Urcharge water. Beca use of the ratio of vertical to horizontal pressure, <br />the most severe loading condition will generally occur when the reservoir is empty and the <br />soil is in a natural drained condition. However, the following loads occur where there is ground- <br />water and/or surcharge water: <br /> <br />(1) Vertical pressure due to the weight of the natural drained soil above the groundwater <br />surface, the weight of the submerged soil below the groundwater surface, and the weight of the <br />projected volume of water above the conduit inclu ding any surcharge water above the fill surface. <br /> <br />(2) Horizontal pressure from the lateral earth pressure obtained by using soil weights for <br />the appropriate moisture conditions and full hydrostatic pressure. <br /> <br />b. Water pressure inside the conduit. Internal water pressure should be considered but will <br />seldom govern the design for the usual type of outlet works. However, internal pressures must be <br />analyzed for pressure conduits for interior drainage in local protection projects. <br /> <br />c. Distributim of concentrated live loads. Because of varying soil conditions the designer <br />can expect only reasonably approximate results in computing pressures resulting from con- <br />centrated surface loads. However, the Boussinesq method is presented here because of the sim- <br />plicity of the equations and because they are widely accepted. The following equation for the <br />vertical pressure, w" at depth, Ho, due to a poi nt load at the surface has been developed from <br />formulas by Boussinesq and is considered satisfactory for most soil conditions: <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3P,Ho' <br />Wr = 2...D/l.s <br /> <br />_ __ __ __ __ __ __ __ _ _ _ (1)' <br /> <br />A sketch and the method of computing D, are given on plate 2 (based on figure 10 of <br />reference (11)). Where construction loads are p laced on fills of more than about 8 feet, increased <br />allowable stress should be permitted. <br />For relatively high fills, the above formula will give reasonably accurate results for highway <br />and railroad wheel loads and the loads on relatively small footings. However, where the con- <br />duit is near the surface or where the contact area of the applied load is large, these loads must <br />be divided into units for a more accurate analysis. The use of influence charts as developed by <br />Newmark in reference (9) will be helpful in computing the stress due to loads on relatively <br />large and irregular areas. <br />The horizontal pressure due to a concentrated surface load should be computed by convert- <br />ing the vertical load into equivalent backfill height and adding this value to H in equations 4, <br />7, 12 or 13. <br /> <br />.Note. For nomenclature, see symbols at end of test. <br /> <br />2 <br />