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<br />. <br /> <br />Chapter 5 <br />System Stability <br /> <br />+. <br /> <br />5-1. Modes of Failure <br /> <br />The Joads exerted on wall/soil system lend 10 produce a <br />variely of polential failure modes. These failure modes <br />!he evaluation of !he loads on !he system, and selectio~ <br />of certain system parametelll 10 prevent failure are dis- <br />cussed in lhis chapter. <br /> <br />a. Deep-seated failure. A polential roIaIional fail- <br />ure of an entire soil mass conlaining an anchored or <br />cantilever wall is illuslrllled in Figure 5-1. This poten- <br />tial failure is independenl of Ihe slructural characteristics <br />~f Ihe wall and/or anchor. The adequacy of Ihe system <br />(I.e. factor of safety) againSllhis mode of failure should <br />be ~sed by Ihe geotechnical engineer Ihrough con- <br />ve~Ual analyses. for slope Slabilily (EM 1110-2-1902). <br />This type of failure cannot be remedied by increasing <br />!he deplh of penetration nor by repositioning !he anchor. <br />The only recourse when !his type of failure is antici- <br />~ is to change !he geometty of retained material or <br />unprove Ihe soil strengths. <br /> <br />. <br /> <br />b. Rotational failure due ta inadequate pile pelle- <br />tratioll, Latetal soil and/or water pressures exerted on <br />!he wall lend to cause rigid body rolation of a cantilever <br />or an~horec:J wall as illustrated in Figure 5-2. This type <br />of failure IS prevented by adequate penetration of Ihe <br />piling ~ a cantilever wall or by a proper combination of <br />penelraUon and anchor position for an anchored wall. <br /> <br />~'. .Other failure modes. Failure of !he system may <br />be IDIlialed by overstressing of Ihe sheet piling and/or <br />anchor componenls as illuslraled in Figures 503 and 54 <br />Design of !he anchorage 10 preclude Ihe failure depicted <br />in Figure 54a is discussed later in lhis chapter. Design <br />?f Ihe Slructural componenls of !he system is discussed <br />In Chapler 6. <br /> <br />. <br /> <br />5-2, Design for Rotational Stability <br /> <br />a; AssumptiollS. RoIational slabilily of a cantilever <br />wall is governed by Ihe deplh of penetration .of !he <br />piling or by a combination of penetration and anchor <br />~tion f~ ~ anchored wall. Because of Ihe complex- <br />11y of beliaVlor of !he waIVsoil system. a number of: <br />simplifying assumptions are employed in Ihe classical <br />design techniques. Foremosl of these assumptions is <br />thai Ihe defonnationsof !he system are sufficient to <br />produce limiting active and passive earth pressures at <br />any poinl on !he waIlIsoil inlelface. In !he design of !he <br /> <br />EM 111~2.2504 <br />31 Mar 94 <br /> <br />anchored wall, Ihe anchor is assumed to prevent any <br />IaIeraI motion at !he anchor elevation. OIlIer assump- <br />tions are discussed in Ihe following patagtaphs. <br /> <br />b. Preliminary data. The following preliminary <br />infcmtation must be established before design of Ihe <br />system can commence. <br /> <br />(1) Elevation at !he top of Ihe sheet piling. <br /> <br />(2) The ground surface profile extending 10 a inini- <br />mum distance of 10 times Ihe exposed heighl of Ihe <br />wall on eilher side. . <br /> <br />. (3) The soil profile on each side of Ihe wall includ- <br />ing location and slope of subsurface layer boundaries, <br />strenglh parametelll (angle of internal friction .. <br />cohesive slrenglh c. angle of wall friction II. and <br />wall/soil adhesion) and unil weighl for each layer 10 a <br />deplh below Ihe dredge line nol less !ban five times Ihe <br />exposed heighl of Ihe wall on each side. <br /> <br />(4) Water elevation' on each side of Ihe wall and <br />seepage characteristics. <br /> <br />(5) Magniludes and locations of surface surcharge <br />loads. <br /> <br />(6) Magniludes and locations of external loads <br />applied directly 10 Ihe wall. <br /> <br />c. Load cases, The Joads applied 10 a wall f1ucluale <br />during ilS service life, Consequently. several loading <br />conditions must be defmed within Ihe .conlexl of !he <br />~ function of !he wall. As a minimum, a cooper- <br />ative effort among structural. geotechnical, and hydrau- <br />lic engineers should identify Ihe load cases oullined 10 <br />be considered in Ihe design. <br /> <br />(1) Usual conditions. The loads. associated wilh lhis <br />condition are Ihose mllSl frequently experienced by Ihe <br />system in performing ilS primary fimction Ihroughoul ils . <br />service life. The loads may be of a long-term sustained <br />nalUre or of an intermillent, but repetitive, naIure. The. <br />fundamenlal design of !he system should be optimizild <br />for these loads. Conservative factors of safety should <br />be employed for !his condition, <br /> <br />(2) Unusual conditions. Construction and/or main- <br />tenance operations may produce loads of infrequenl <br />occurrence and are short duration which exceed Ihose of <br />Ihe ~ condition. Wherever possible, Ihe sequence of <br />operauons should be specified to liinil !he magnitudes <br /> <br />5-1 <br />