13 Design of Revetments, Seawalls and Bulkheads Forces & Earth Pressures

Transcription

1 13 Desgn of Revetments, Sewlls nd Bulkheds Forces & Erth ressures Ref: Shore rotecton Mnul, USACE, 1984 EM , Desgn of Revetments, Sewlls nd Bulkheds, USACE, 1995 Brekwters, Jettes, Bulkheds nd Sewlls, le Buck, 199 Steel Sheet lng Desgn Mnul, Non Steel Cororton rncles of Geotechncl Engneerng, Brj M. Ds, 1994 ort Engneerng, er Bruun, 1981 Tocs Forces nd ressures of concern for Vertcl Bulkhed Structures Lterl Erth ressure Forces Forces nd Condtons of Concern for Vertcl Bulkhed Structures The stblty of vertcl bulkheds, rtculrly sheet-le structures, requres consderton of overturnng nd stblzng forces. Sttc forces: ctve sol nd wter ressures from the bckfll wter nd ssve sol ressures on the sewrd sde nchor forces (when lcble Dynmc forces: wve cton nd seege flow wthn the sol. (ve mcts ncrese sol ressure n the bckfll nd requre lrger resstng ssve erth ressures nd nchor forces to ensure stblty berthng nd moorng forces (when lcble. Unlke the desgn of vertcl brekwters where wve forces re the mn consderton wth resect to structurl stblty, the stblty of wter front retnng wlls s mnly concerned wth bck sde erth ressure nd bove ground surchrge. A ressure dgrm on vertcl retnng wll s shown n Fgure 1. Desgn Low ter condton crtcl concerns: ctve erth ressure ssve erth ressure resdue wter ressure surchrge scour Desgn Hgh ter condton crtcl concerns: overtong wve mct lodng on structurl comonents

2 Fgure 1, ressure dgrm on vertcl erth retenton wll Lterl Erth ressure Forces The ntensty of horzontl erth ressure exerted on vertcl wll cn be nlyzed n smlr mnner s the wter ressure excet tht the ressure dgrm s modfed due to: the resence of sol lodng sol resstnce. The degree nd mnner of sher strength moblzton deends on the reltve movement of the wll gnst the surroundng sol. Rnkne edge A Rnkne edge γ γ γ = 45º - φ/ Actve Sol ressure γ = 45º + φ/ ssve Sol ressure Fgure, Actve nd ssve Sol ressure Referrng to Fg., f the wll moves wy from the embnkment wedge of sol wll eventully serte from the embnkment.

3 At the threshold of serton ths segment mntns ts equlbrum through the norml forces from the wll nd the embnkment the sher stresses between the segment nd the embnkment nd the sher stress between the segment nd the wll. Under ths condton, the sher of the sol s fully moblzed gnst sldng. The horzontl ressure exerted on the wll under ths stte s known s ctve ressure, A. The ctve ressure s relted to the norml stress by the followng equton: A = where A s the ctve ressure coeffcent nd σ z s the effectve norml stress t elevton z. For strght wedge flure surfce cn be obtned smly from the geometry s, where φ s the sol nternl frcton ngle. = tn σz o ( 45 - φ/ If the sol s not cohesonless, the ctve ressure cn be estmted by the followng modfed equton: = A σ z - c where c s the sol coheson or the undrned sol sher strength. Thus, sol coheson cts lke reducton of sol overburden or surchrge. Smlrly, when wll moves towrds the embnkment, the flure wedge wll be such tht the sher strength wll be fully moblzed to resst the movement, thus ncresng the ressure on the wll. The erth ressure t ths stte s known s ssve ressure,. For cohesonless sol t s gven by: = where s the ssve ressure coeffcent gven by, = tn σz o ( 45 + φ/ Agn, f the sol s not cohesonless s modfed s, = σ z + c Here the sol coheson cts lke n overburden. The effectve norml stress, σ z, s generlly tken s the vertcl stress t elevton z due to the effectve sol weght,.e., σz = γ z + σ where γ s the effectve secfc weght of sol nd σ s s the dded stress due to surchrge. γ vres wth sol roertes, wter content nd degree of comcton nd cn be comuted by: γ = V 1+ w = G γ 1+e s w

4 where = weght of sol dry sol secfc weght, bout V = volume of sol = wter secfc weght. e = vod rto = V w /V s. w = w / s = wter content. G = secfc grvty of dry sol = γ s /γ w. γ s = roxmtely 165 lb/ft 3, or.65 ton/m 3. γ w = 6.4 lb/ft 3, or 1.0 ton/m 3. For submerged sol, ts effectve secfc weght s G -1 γ s = γw 1+e For the more generl cse of sloed wll gnst sloed surfce wth dfferent lyers of sols, the ctve nd ssve ressures re functon of the followng vrbles: where A = f( φ, β, α, δ,c,h, γ = f( φ, β, α, δ,c,h, γ φ = nternl sol frcton ngle n lyer. α = bulkhed ngle. δ = frcton ngle between sol nd wll. β = surchrge ngle. C = sol coheson strength (undrned sher strength. h = sol lyer thckness. γ = effectve secfc weght n lyer I. Sloed ll Mult-lyer Condton Smle Mult-lyer Condton β ζ 1 ζ h 1 1 ζ h Ruture Angle, ζ = 90 - (45 - φ/ α Fgure 1, Mult-lyer condtons

5 For cohesonless sol, the followng equtons gve the ctve nd ssve sol ressures. = wcosα Σ γ h + α - β cosα nd = ( cos α δ+ α cos ( φ - α sn( φ - β sn( φ + δ 1+ δ α - β = wcosα Σ γ h + α - β cosα = ( cos α cos ( φ - α sn( φ + β sn( φ - δ δ+ α 1- δ α - β The ctve nd ssve surfce cn be constructed by the followng r of equtons, cot ( ζ -β = - tn( φ + δ+ + sec( φ + δ+ δ sn( φ + δ sn( φ -β for ctve ressure wth ζ beng the ngle of the ctve ressure surfce wth resect to the horzontl, nd cot ( ξ -β = tn( φ - δ - β + sec( φ - δ - β δ sn( φ - δ sn( φ ± β for ssve ressure wth ξ beng the ngle of the ssve ressure surfce wth resect to the horzontl.