The influence of settlement on flood prevention capability of the floodcontrol wall along the Bund in Shanghai

Transcription

1 IAG6 Paper number 8 The nfluence of selemen on flood prevenon capably of he floodconrol wall along he Bund n Shangha 3 BAO CHN, LIN-D YANG & HUI-FNG CHNG 3 Tong Unversy. (e-mal: chenbao@ong.edu.cn) Tong Unversy. (e-mal: yangldn@ong.edu.cn) Secon of Flood-conrol wall Consrucon and Managemen of Shangha. (e-mal: shsfxq@sh63.ne) Absrac: Based on he measuremen of he flood-conrol wall along he Bund, he nfluence of land subsdence and srucural selemen of he flood-conrol wall on he flood-prevenon capably s suded n hs paper. The man research aspecs nclude calculaon of he equvalen elasc modulus of he man sol layer wh he help of vsco-elasc FM back analyss and predcon of he srucural selemen of floodconrol wall n fuure. The flood prevenon capably of he conrol wall along he Bund was assessed by combnng he predcons of srucural selemen and land subsdence wh he de level sandards made n 984. The resuls of hese sudes can provde a echncal bass for he consrucon and managemen of floodconrol walls n Shangha. Résumé: Basé sur la mesure de assemen du mur conre l'nondaon le long du Bund, l'nfluence d'affassemen de erran e du assemen srucurel de mur conre l'nondaon sur la capacé de prévenon d'nondaon es éudée en ce arcle. Les aspecs prncpaux de recherches ncluen cela, a calculé le module élasque équvalen de la couche de sol prncpale, avec l'ade de l'analyse nverse de FM vsco-élasque, e prévus le assemen srucurel de mur conre l'nondaon dans l'avenr; Combnan les prévsons du assemen srucurel e de l'affasemen de erran, éudé la capacé de prévenon d'nondaon du mur conre l'nondaon le long du Bund selon les normes de nveau de marée faes en 984. Le résula de ces éudes peu êre le souen echnque en faveur de la consrucon e de l'admnsraon de mur conre l'nondaon à Shangha. Keywords: floods, revemen, selemen, land subsdence INTRODUCTION The flood-conol walls (FCW) on boh sdes of Huangpu Rver were desgned and consruced accordng o he waer levels assocaed wh he flood occurrng once n a housand years. The nfluences of earhquakes and he dal varaon was also aken no accoun. The flood-conrol walls have become an mporan barrer for he cy of Shangha avodng poenal flood dsaser from yphoons, sorms, hgh des and floods waers from Ta Lake. For he flood conrol n he urban area, he FCW s one of he lfelne proecs (Yang e al. 997). The sols under he rver banks are very sof clays whch have hgh waer conen, low srengh and evden rheologcal behavour. Under ceran loads he sol deformaon ncreases wh me, and causes selemen, horzonal dsplacemen and declnaon of he FCW. Sgnfcan deformaon may cause ground subsdence behnd he wall. Snce he 99s, he selemen of he FCW has become more serous, especally n he urban area. Measuremen daa show ha here s a par of he FCW more han ~3km long whose oal selemen s abou -3cm, ncludng selemen of he FCW srucure and ha caused by land subsdence. If he level of he FCW canno be mananed, he cy would face he poenal rsk of flood dsaser (Lu 998). Therefore safey predcon for he FCW srucure should nclude consderaon of he me-dependency of he sol deformaon. If he conrollng facors of he rheologcal behavour of he sol can be deermned hen fuure dsplacemen can be predced from he former measuremen.ths wll perm prevenave measures o be aken before falure occurs (Xe and Sun 996). I s herefore mporan o analyse he deformaon based on he dsplacemen monorng and predc he fuure paern of deformaon and he safey of he FCW. BACK-ANALYSIS THORY FOR STTLMNT Vsco-elasc model The hree-elemen vsco-elasc model s a form of seady creep model whch can smulae he nsananeous elasc deformaon and delayed pos-elasc deformaon of sauraed sol.xe and Sun (996) dd some rheologcal ess on hree knds of ypcal sauraed sols n Shangha,.e. muddy clay, dark-green sly clay and brownsh-yellow sly clay. The expermenal curves of deformaon and me showed evden vsco-elascy smlar o ha presened by a hree-elemen vsco-elasc model. Ln e al. (997) analysed he dsplacemen measuremen daa of a road proec along he Bund n Shangha, and concluded ha he me-dependen behavour of an old subgrade sol could be descrbed wh he hree-elemen model. Furhermore, he dsplacemen of he FCW srucure agrees well wh hs The Geologcal Socey of London 6

2 IAG6 Paper number 8 model. In order o predc he dsplacemen and safey of he FCW n he fuure, he hree-elemen model wll be adoped n hs paper for esablshng he predcon mehod by opmum back analyss. Fgure shows a hree-elemen vsco-elasc model ha s composed of one sprng elemen n seres wh a Kelvn model (one sprng elemen conneced parallel wh a Newon elemen). When σ=σ s a consan, he creep equaon can be wren as: ε σ σ η σ ( ) = + ( e ) = Here, and are elasc modul, η s he coeffcen of vscosy and = + ( e η ) s he equvalen elasc modulus, and The lnear vsco-elasc problem can be smplfed and solved wh he correspondng heory. A any me, here s an equvalen elasc modulus, whch can be used o descrbe he comprehensve effec of, and η, so as o smplfy he wo-dmensonal vsco-elasc plane-sran problem no a lnear problem. The equvalen elasc modulus decreases wh me, and ends owards a consan value (Fgure ). = + ( ) σ ε e ε η ε v σ Fgure. Three-elemen vsco-elac model + Fgure. Relaonshp beween equvalen elasc modulus and me of hree-elemen vsco-elac model The behavour of sol layer agrees well wh ha presened by he hree-elemen model. When he sress boundary and dranage condons are kep unchanged, a = he basc equaon of FM s : [ K ]{} u = { F} Here, he expresson of he elemens n [ K ] s smlar o ha of [ K ], he dfference s a = usng he equvalen elasc modulus nsead of. The parameers of he hree-elemen model can be ganed from back analyss mehod usng measuremen nformaon from he se.

3 IAG6 Paper number 8 Prncples of back-analyss The opmum back analyss heory provdes he conceps of he opmum soluon and dscussed he unqueness of he soluon. In hs heory, o have an opmum soluon he basc condon s ha he obecve funcon of he requred parameers should be a convex funcon. However, for mos acual complex problems, s dffcul o udge f he obecve funcon s convex or no. In order o avod hs problem, he followng mehod was used o esablsh he obecve funcon for he back analyss of wo-dmensonal vsco-elasc problem (Yang e al. ). For any one pon of measuremen, he varey of dsplacemen wh me can be descrbed as a dsplacemen seres { u, u,! u k } accordng o he me seres {,,! k }, and here exss a seres of equvalen elasc modulus {,,! k }. The a me can be calculaed wh back analyss of elasc problem. In order o avod he nfluence of oo much dfference beween quanes and uns on he resuls of back analyss, he obecve funcon can be wren as: k n u F( ) = mn w( ) ( =,n; =,k ) * u = = * Here, u and u are he calculaed dsplacemen and measured dsplacemen respecvely a he me of ; n s he oal number of he measuremen pons; k s he group number of he daa seres; w s he wegh consan of dfferen measuremen nformaon, normally w =. Because u changes wh parameer. When F s s mnmum value, he relevan FM o solve he equvalen elasc modulus, so F s he funcon of he s he opmum soluon of he back analyss. Usng a me, he conrol equaon s he formula (4). Usually he obecve funcon F s a non-lnear funcon of parameer, and canno be solved wh normal heorecal mehods bu can be acheved by mahemac programmng. In hs paper, he smplex algorhm s adoped for he opmum back analyss. PRDICTION OF FCW S STTLMNT FM mesh The numercal analyss of he FCW srucure s regarded as a plane-sran problem. In vew of he nfluence of he waer n he rver and he loads on roads and pavemens along he bank, he compued doman s seleced as 8m long (m on bank, and 6m for he nsde of FCW), and 63.5m (5m for he nsde of FCW) hgh. The nal condons are dvded no he sress condon and he boundary condon. The nal sress condon s calculaed from he densy of he sol layers. The nal boundary condons are: he lef and rgh laeral boundares are suppored wh horzonal bearng rods, he boom boundary wh vercal bearng rods, and he upper surface s z free boundary under ceran known loads. The normal waer able of he rver s.m and he ground waer able under he bank sde s.m 平均水位. 5 m 63.5 m 8 m Fgure 3. FM mesh for selemen analyss of flood-conrol wall The FM mesh s shown n Fgure 3. The mesh has 88 nodes and 83 elemens. There are 73 quadrangle elemens for smulang he sol layers, asphalc pavemen and subgrade wh geoexle; beam elemens for PHC ppes and he casson of he FCW. 3

4 IAG6 Paper number 8 Loads The pavemen srucure s also analysed as a plane-sran problem wh a wdh of m. The surface load on he FCW casson, pavemen and non-raffc lane s seleced as 3.5kN/m. The raffc load s modelled as a unform equvalen load of.79kn/m The buldngs along he Bund are far from he FCW, her foundaons are que deep and he consoldaon of he sol layer s consdered o be almos complee, so he nfluence of hese buldng on he FCW s selemen s no aken no accoun. Parameers Asphalc pavemen For he old asphalc pavemen, he elasc modulus s aken as =9MPa, Posson s rao ν=.3; whle for he new asphalc pavemen, =7MPa and ν=.3. Subgrade maeral For he subgrade maeral conssng of he fly ash fll, gravel and geoexle, s elasc modulus s aken o be =8.85Mpa and Posson s rao ν=.3. Sol The sol parameers from he geologcal nvesgaon are shown n Table. The parameers forhe sol layers whch manly nfluence he selemen of he FCW, wll be calculaed wh back analyss. Table. Parameers of sols under road subgrade Sol name Thckness (m) γ (kn.m -3 ) lasc modulus (MPa) ν K c (kpa) φ ( o ) Muddy clay Fll Grey muddy clay Grey clay Grey sly clay Dark green sly clay Yellow fne sand Asphalc pavemen / Subgrade maeral / Foundaon and srucure The foundaon and srucure of he FCW are manly composed of ube ppes (4 rows of PHC ppes φ 3mm), reanng ppes ( mm) and renforced concree casson. The man maeral parameers of he FCW are shown n Table. Table. Maeral parameers of foundaon and srucure of FCW Srucure ype Sze (m) Spacng (m) γ (kn.m -3 ) Area (m ) momen of nera (m 4 ) (MPa) PHC ppe φ Reanng ppe.35.5 Closely Ppe of slab Boom longudnal beam slab.35 enre 3 4 Column Top beam plae enre

5 IAG6 Paper number 8 Selemen predcon of FCW srucure The measuremen pons were se along he FCW, and he selemens of he FCW srucure monored. Based on he measured selemen and he resuls of he back analyss, he srucural selemens of he FCW n he fuure are predced. The measured selemen and he calculaed selemen are shown n Table 3, and selemen curves wh me are shown n Fgure 4. Table 3. Measured and calculaed srucural selemens Ouer measured pon Inner measured pon Dae (y-m-d) Measured selemen Calculaed selemen Measured selemen Calculaed selemen Fnal measured and calculaed selemens of FCW Dae (Y-M-D) Fgure 4. Increase of calculaed srucural selemen of FCW wh me //3 4//4 4//4 4//5 34//5 44//6 Dae (Y -M -D ) Fgure 5. Relaonshp beween equvalen elasc modulus and me The decrease of he equvalen elasc modulus wh me s shown n Fgure 5. The parameers of he hree-elemen model were derved accordngly: =. 6 kpa, =3. 3 kpa, η =.45 7 kpa.d 5

6 IAG6 Paper number 8 The long-erm equvalen elasc modulus s =3kPa, and wh hs parameer he fnal selemen of he FCW was predced o be more han 3mm. Predcon of land subsdence along he bund Shangha Geologcal Invesgaon Insue has predced he land subsdence along he Bund base on he observaon daa from 96 o 998. The land subsdence ncremens near he Bund from 994 when he FCW was reconsruced s shown n Table 4. Table 4. Land subsdence predcon of along he Bund from 3 Tme Iem Land subsdence VOLUTION OF TH FLOOD PRVNTION CAPABILITY OF TH FCW ALONG TH BUND The oal selemen of he FCW can be obaned by combnng he predced srucural selemen of he FCW and he land subsdence near he Bund from 994, and These resuls ndcaed a reducon of he flood prevenon capably. The fuure flood prevenon capably of he FCW along he Bund for he housand year flood levels for Shangha approved n 984, are shown n Table 5). Table 5. voluon of he flood prevenon capably of flood-conrol wall Year 3 5 Flood prevenon level of 984 sandard (m) Acual level of FCW (m) Hegh shorage of FCW (m) From Table 5 s can be seen ha he presen hegh of he FCW wll no sasfy he requremen of flood prevenon, because he FCW wll be.44m,.89m and.3m lower han he flood waer level for a housand year reurn even occurrng, 3 and 5 respecvely. Ths represens a poenal rsk for he urban cy of Shangha, and ndcaes ha measures should be aken mmedaely o rase he hegh of FCW o compensae for he predced fuure hegh loss caused by he land subsdence and srucural selemen. CONCLUSION The background of hs paper s he reconsrucon of he flood-conrol wall along he Bund n Shangha. Based on he selemen measuremen of he flood-conrol wall, he srucural selemen of he wall was predced by means of a vsco-elasc FM Back Analyss. A wo-dmensonal FM analyss was used o deermne he sol layers ha have he maor nfluence on he he selemen, and hen derved he equvalen elasc modulus of hs layer wh FM Back Analyss, so as o predc he developmen of he selemen n he fuure. The nfluence of srucural selemen and land subsdence on he floodprevenon capably of flood-conrol wall was hen examned by combnng he selemen wh subsdence o deermne he flood-prevenon capably of he flood-conrol wall a presen and n he fuure agans he de levels of he once n a housand year flood. The resuls presened n hs paper are pernen o he consrucon and echncal managemen of he flood-conrol wall n Shangha. Correspondng auhor: Dr Bao Chen, Tong Unversy, 39, Spng Road, Shangha, 9, Chna. Tel: mal: chenbao@ong.edu.cn. RFRNCS LIN, X.G Sudy on he geoexle behavour and he seconde phase road proec along he Bund, Thess of Maser, Tong Unversy (n Chnese). LIU, Y Deermnaon of he hegh of rver flood dkes n Chna should consder land subsdence. Shangha Geology, 67, 6- (n Chnese). XI, N., SUN, J Rheologcal behavors of Shangha clays. Journal of Tong Unversy, 996, 4(3), (n Chnese). YANG, L.D., QIU, S.H., & YANG, Z.X.. Predcon on dsplacemen and sably of frame bracng srucure of foundaon p. Rock and Sol Mechancs, (3), 67-7 (n Chnese). YANG, L.D., XU, R.Q., & CHN, B Research on he deformaon of flood-conrol wall n Shangha. Chnese Journal of Geoechncal ngneerng, 9(3), (n Chnese). 6