PVC drainage connection approval memo, Apr 1985

NYC 9/11 Public Portal Document

Therefore, Johns-Manville suggests that Loads imposed on buried conduits have, the maximum long term load be in past practice, been calculated by determined by the prism toad for design. using the Marston toad formula. For 14 trench loads, Marston has a formula for WiOttiililSiBliifclO rigid-pipe and another formula for 12 Oeao toad 120 Ib.fcu. flexible.pipe. It is important to recognize that under identical conditions of bury, Where: Iio H20 tew toad Pv = Pressure at the top of the pipe due u. *■ tmpaci / / the soil load generated on a flexible to the weight of the soil (Ib/ft^) r 8 conduit is less than the load generated w s Soil unit weight (Ib/fH) ffl o 6 3 Rz on a rigid conduit. The comparative load on a rigid conduit versus the load on a flexible conduit is expressed as the ratio H = Depth from top of pipe to top of ground (ft) O o 4^ (T1 Total toad Bvs ♦ dead of trench width to the flexible pipe O.D, Note: To convert prism load (Ib/ft^) to S’ 2 Ib/iinear ft, multiply by the O.D. of the X - . By definition, a flexible conduit is one pipe in font, or: 500 1000 1500 2000 which Wil! deflect before reaching failurn. W - "L Pv vertical soil pressure Marston’s formulas for, soil loads; ' Live loads Figure 1. , Rigid pipe W C, ,v B/ ’ Live loads imposed on buried conduits Note: To cenverTvertical soil pressure to riex.blo pipe W Cd w Be Ba from traffic,must also be considered in a load on pipe — pounds per lineal foot . design and become more important at multiply by O.D. of pipe in ft. Where; shallow depths. The combination of soli W = Load on pipe (Ib/lin ft)" toad and live load must be added fLive toad applied on assumed area of Cd = Load coefficient together to design for the maximum load 36" X 40". w = Soil unit weight (Ib/fC) as shown in curves above. Bd = Ditch width (ft) Be = O.D. of pipe (ft) The soil load and live load must be At shallow depths of cover ■— 3 feet and added to determine the total external less, flexible conduits can deflect and . p-ism :.03d load on a buried conduit. This combined - rebound under dynamic loading Loads imposed on buried conduits have load should be used for design. Figure 1 conditions if the trench width is not been calculated by using the Marston illustrates the magnitude of soil and live sufficiently bridged. Unless special ■ load formulas for rigid and flexible pipe. loads separately and also charts the precautions are taken to bridge the .1 However, it has been determined that magnitude of the combined or total .trench in shallow installations, the , the Marston formula for flexible pipe may loads. The curves in Figure 1 apply only breaking up of flexible.road surfaces not determine the maximum long-term for H20 highway loading and a s©il may result. Therefore, for shallow load — the “Prism load” formula is more weight of 120 ib/cu ft. installations under flexible road surfaces accurate. The "Prism toad" is the weight • (less than 3 feet), J-M recommends of the column of soil directly above the □ass i:*,iBateriat be used in the pipe pipe. Thus, precautions in keeping ths zone,and up to the road elevation. This - Trench narrow are unnecessary for the ; recommendation is-not meant to conflict flexible pipe installation. The important with the design engineer's specifications , thing is to compact the haunching and his specifications will govern. rnatc'ia! from thv p;pe out to the undisturbed trench wails. *See page 9 for definition of Class I.

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