Field Observations of the Build-Up and Dissipation of Residual Pore Water Pressures in Seabed Sands Under the Passage of Storm Waves

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1 Journal of Coastal Rsarch SI ICS 4 (Procdings) Brazil ISSN Fild Obsrvations of th Build-Up and Dissipation of Rsidual Por Watr Prssurs in Sabd Sands Undr th Passag of Storm Wavs S. Sassa, T. Takayama, M. Mizutani and D. Tsujio Port and Airport Rsarch Institut, Yokosuka, , Japan, sassa@ ipc.pari.go.jp Disastr Prvntion Rsarch Institut, Kyoto Univrsity, Uji, , Japan ABSTRACT SASSA, S.; TAKAYAMA, T.; MIZUTANI, M. and TSUJIO, D., 6. Fild obsrvations of th build-up and dissipation of rsidual por watr prssurs in sabd sands undr th passag of storm wavs. Journal of Coastal Rsarch, SI 39 (Procdings of th 8th Intrnational Coastal Symposium), Itajaí, SC, Brazil, ISSN Th purpos of this study was to clarify th natur of instability in a sabd of fin-graind sands undr th action of storm wavs. Th papr dscribs th mthodology and th rsults of systmatic fild masurmnts prformd of th changs in por watr prssurs in th sabd sands at Kouchi port, Japan during th priod of six days involving th passag of Typhoon No.9, 2. It was found that thr occurrd th build-up in th rsidual por watr prssurs at various sand dpths shallowr than 2.m whn th significant wav hights wr gratr than 2.m. Th obsrvd data was furthr xamind by mans of th frquncy spctral analysis and of th tim domain coupld lastoplastic finit-lmnt analysis. Th rsults show that th procss of th sabd instability was closly rlatd to th wav grouping charactristics which manifstd in th fild concrnd. ADDITIONALINDEX WORDS: Brakwatr, instability, liqufaction. Takamatsu Tchnical Offic, Ministry of Land, Infrastructur and Transport Govrnmnt of Japan, Takamatsu, 76-17, Japan INTRODUCTION Wav-inducd instability of sabd soil has bcom an incrasingly important subjct for rsarch in rlation to sdimnt transport and to th stability of submarin piplins and brakwatrs. Liqufaction of sabd soil may b a ky procss rsponsibl for such go-hazards in watrfronts associatd with storm wavs or rapid tidal changs. Th wavinducd liqufaction gnrally hav two forms of mchanisms: namly, cyclic boiling inducd by fluctuating por watr prssurs and liqufaction du to th accumulation of rsidual por watr prssurs. Notably, th liqufaction du to rsidual por watr prssurs xhibits a highly non-linar bhaviour, sinc it stms from th cyclic plasticity (tndncy for contraction undr cyclic loading) natur of sands. Indd, th utilization of cntrifug wav-soil tsting with viscous scaling introducd (SASSA and S EKIGUCHI, 1999), ld to making clar th procss of liqufaction in loos dposits of fin-graind sand undr svr rgular wav Howvr, th difficultis in taking actual prcis masurmnts of th bhaviour of sabd soil undr svr irrgular wav conditions in ral ocans hav bn formidabl. In fact, prvious on-sit obsrvations of th rspons of sabd sands (Z EN t al., 1989; FERTUI and S ILLS, ) wr prformd in th ara of wav braking zons with rlativly shallow watr dpths whr th significant wav hights wr qual to 1m or so at most. Th purpos of this study was to clarify th natur of wavinducd liqufaction of sand bds, on th basis of th systmatic fild masurmnts of watr prssurs abov and por watr prssurs in sabd sands at Kouchi port, Japan undr svr wavs in th watr dpth of 15m. Th prsnt study also invstigatd th influnc of th wav irrgularity or wav grouping charactristics on th procss of th sabd instability. Th papr is organizd as follows. First, w dscrib a mthodology of fild masurmnts prformd of th rspons of sabd sands to high wavs. Thn, th rsults of th continuous obsrvations mad associatd with Typhoon No. 9, 2 will b prsntd and discussd. This will b followd by dscribing th frquncy spctral analysis to look at th rlationship btwn th wav grouping charactristics and th changs in rsidual por watr prssurs. Finally, a discussion will b mad on th mchanism of th sabd instability undr high wavs, by prforming a rang of th tim domain coupld lasto-plastic analysis. FIELD MEASUREMENTS OF THE RESPONSE OF SEABED SANDS TO HIGH WAVES Sit of Obsrvations Th sit of th fild obsrvations was th Higashi Daiichi brakwatr sit at Kouchi port, Japan (Figur 1). Th construction of th brakwatr startd in th yar of 1 and is schduld to continu until 8. W took th masurmnts of por watr prssurs in th sabd soil at th location markd by No.1 in Figur 1. Watr dpth was qual to 15m thr. Th soils concrnd wr fin-graind sands with th man grain diamtr D5 falling in th rang of.15-.mm. Th upprmost sand layr as thick as 2m was a soft layr whr th soil N-valus wr lss than tn. In conjunction with th prsnt obsrvations of th fr sabd rspons at th No.1 sit, w startd th continuous obsrvations of th rspons of th foundation soil blow rubl mounds, wav-dissipating blocks and a caisson at th location markd by No.2 in Figur 1, from August of 3. Th purpos of th invstigation is to lucidat th mchanism of th sttlmnt bhaviour of brakwatrs in th sabd soil. Th rsults of th invstigation will b rportd lswhr in th nar futur. Mthod of Masurmnts Por watr prssur snsors wr installd at fiv diffrnt sand dpths of.67m,1.m,1.33m,1.67m and 2.m blow th sabd surfac, and a watr prssur snsor was installd at th dpth.5m abov th sabd for masuring th wav-inducd fluid prssur fluctuations. Th masurd data signals wr Figur 1. Sit of th fild obsrvations at Kouchi port, Japan.

2 Fild Obsrvations of Rsidual Por Watr Prssurs in Sabd Sands 411 (a) 18 - Figur 2. Path of Typhoon No.9, 2. transfrrd through multi-cord cabls laid ovr th sabd, and rcordd in th data loggr in th obsrvatory room constructd on a caisson. Th ON/OFF for masurmnts was mad by radio control using a portabl phon. Th ntir systm mad it possibl to tak continuous masurmnts during th priod of 6 days with a sampling rat of 5Hz undr svr wav conditions whr th significant wav hights bcam gratr than 2.m. For th installation of th por watr prssur snsors in th sabd soil, w took considration to fix th snsors in th soil so as to obtain quantitativ masurmnts, without disturbing th sabd soils thr.th systm of th installation of th snsors mployd was as follows: A stl cag covr (3m 3m) was usd to hang fiv diffrnt snsors unitd with fiv thin bars ( =25mm) from th top of th covr. Th covr itslf was supportd in th sabd soil with ight stl pils ( =8mm). A watr prssur snsor was installd on th top of th covr usd. Mthod of DataAnalysis Th obsrvd data wr analyzd as follows: a) grouping of th data for 3 minuts ach, b) data charactrization by moving avrag mthod, c) calculation of th paramtrs prtaining to th sabd instability. Not hr that th procdur b) is faturd by dividing th obsrvd data of watr prssurs and por watr prssurs into thr componnts ach as dscribd blow. Th obsrvd data of watr prssurs p wr dividd into th wav componnt p, th long-wav componnt pl and th tid componnt p. Th data p rprsnts th componnt with (a) 3 - (b) t t Tim: ŠÔ Fs s Figur 3. Masurd profils of (a) watr prssur p abov th sabd surfac and (b) por watr prssur p at soil dpth of.67m. - (b) Tim: s Figur 4. Masurd profils of (a) fluctuating componnt p and (b) rsidual componnt p. th priods longr than min., th data p dos th componnt with th priods shortr than 4s. Th data pl rprsnts th componnt p-p -pt. Th obsrvd data of por watr prssurs p wr dividd into th watr prssur long-priod componnts ( pt+ pl), th fluctuating componnt p and th rsidual componnt p. Th data p rprsnts th componnt with th priods shortr than 4s. Th data p rprsnts th componnt p- p -pt-pl. Th paramtrs calculatd from th obsrvd data wr 1) th fluctuating por prssur amplitud ratios p / p and 2) th rsidual por prssur ratios p / v at fiv diffrnt soil dpths. Hr, th symbol v rprsnts th initial vrtical ffctiv strss in th soil horizon concrnd. RESULTS OF THE OBSERVATIONS In association with Typhoon No. 9 2, w prformd continuous obsrvations during th priod of six days from 11:3a.m. on th 24th of July 2 to :3a.m. on th 3th of July 2. Typhoon No.9 progrssd northwstward from to 27 July, as shown in Figur 2. Th lowst cntral atmosphric prssur of this Typhoon was qual to 9hPa. Whn th Typhoon approachd to th Kouchi port, th cntral atmosphric prssur bcam 975hPa. Th masurd profils of fluid prssur fluctuation p just abov th sabd and of por watr prssur fluctuation p at th soil dpth of.67m ar shown in Figur 3. Th data was takn from 6: to 6:3a.m. on th 25th of July. It is sn that both of p and p varid significantly around 15kPa. Th associatd profils of th fluctuating componnt p and th rsidual componnt p ar shown in Figur 4. On can s that th man lvl of th fluctuation of p is qual to zro. Th rsidual componnt was obsrvd to hav accumulatd Figur 5. Masurd tim historis of wav-inducd fluid prssur fluctuations p just abov th sabd surfac.

3 412 Sassa t al. Figur 6. Masurd profils of por watr prssurs diffrnt soil dpths. p at fiv by.5kpa in avrag, xhibiting variations with svral cycls. Th pak valu of p was qual to.7kpa for th duration concrnd. Th rprsntativ faturs of th obsrvd data will b dscribd blow. Wav-Inducd Prssur Fluctuations on th Sabd Th masurd tim historis of th wav-inducd fluid prssur fluctuations p just abov th sabd surfac ar shown in Figur 5. In this figur, th symbols 2pmax and 2p dnot th maximum and avragd crst-to-trough amplituds of p during th priod of 3 minuts ach in th cours of th six days obsrvations. At th start of th obsrvations, t =, th sa was not calm alrady with 2pmax = 15kPa. Th wavs bcam svrr with tim and rachd a stat with 2pmax = 39kPa at t = 1.3days. This valu corrsponds to th wav hight of 4.2m or so. Wavs thn bcam mildr with tim. In fact, for t 4days, th maximum amplitud 2pmax dcrasd to th valu as low as 5kPa, corrsponding to th wav hight of.6m or so. Phas Lag of Por Watr Prssur Fluctuations Th masurd profils of por watr prssurs p at fiv diffrnt soil dpths ar shown in Figur 6. It is sn that th wavform at ach dpth fluctuatd in th priod of 14s or so. No significant diffrnc was obsrvd in th phas of th wavforms at th fiv soil dpths, indicating that th sabd soil was ssntially in a saturatd stat at th tim of th obsrvations. Dcay with Dpth of Por Watr Prssur Fluctuations Th masurd tim historis of th fluctuating componnt ratios p / p at fiv diffrnt soil dpths ar shown in Figur 7. Figur 7. Masurd tim historis of p soil dpths. / p at fiv diffrnt Figur 8. Masurd tim historis of rsidual por watr prssurs p at fiv diffrnt soil dpths. It is sn that th p / p -ratio dcrasd with incrasing dpth blow th sabd surfac throughout th ntir priod of th obsrvations. In fact, th p / p -ratio bcam as low as.85 at th soil dpth z= -2.m. It may b instructiv hr to not that with th aid of th poro-lasticity thory (M ADSEN, 1978; Y AMAMOTO t al., 1978), th thortical p / p -ratio at z = - 2.m was qual to.92 undr th wavs concrnd. This may indicat that th sabd soil did not bhav purly lastically as dscribd blow. Rsidual Por Watr Prssurs Th masurd tim historis of th rsidual componnts of th por watr prssurs p at fiv diffrnt soil dpths ar shown in Figur 8. Th maximum and man valus plottd in this figur corrspond to th pak and avragd valus rspctivly for ach masurd group of 3 minuts during th priod of six days of th obsrvations. It is sn that thr occurrd th rsidual por watr prssurs p at all th soil dpths (z = -.67m, -1.m, -1.33m, -1.67m, -2.m) during th priod btwn t = and t = 2 days. Indd, at z = -.67m, th p -valu was qual to.5kpa alrady at th start of th obsrvations, t =, and built up with incrasing wav hights, raching a pak of.8kpa or so at t = 1.1 days. It thn startd to dcras with tim, ralizing a stat of full dissipation of p for t 2 days. Essntially th sam pattrns of changs in p wr

4 Fild Obsrvations of Rsidual Por Watr Prssurs in Sabd Sands Start of obsrvation ó' v p / max pmax / ' Procss A Procss B Figur 9. Masurd p v vrsus max rlationship at soil dpth of.67m. obsrvd to occur at th othr soil dpths (z = -1.m, -1.33m, m, -2.m). It should b notd hr that th obsrvd magnituds of p dscribd abov corrspond to th initial vrtical ffctiv strss ' v of th soil layr as thick as mm at most. This implis that th upprmost layr of mm thicknss of th sabd sands was in a liqufid stat undr th svrst wav For th purpos of looking at th influnc of th svrity of wav loading on th tndncy for liqufaction, w plottd in Figur 9 th masurd variation of th rsidual por prssur ratio p v at th soil dpth of.67m against th cyclic strss ratio max = k pmax, that is an indx for th svrity of wav loading on th sabd soil. Hr k is th wav numbr and ' is th submrgd unit wight of th soil. Not that th condition whr p v =1. stands for th stat of complt liqufaction at th soil horizon concrnd. It is sn in Figur 9 that th obsrvd p / ' ratio dvlopd with incrasing, and E v max rachd a pak valu of.1 at max.7 (dnotd by procss A). Thn, th p v ratio wnt down with dcrasing max and rachd zro at max.2 (dnotd by procss B). Notably, th masurd prformanc xhibitd a hystrsis whr th obsrvd curv for th dissipation procss B plottd blow that for th build-up procss A. This may suggst that th tndncy for contraction of th sands dcrasd aftr th soil had xprincd th maximum wav hights undr th wavs concrnd. Th masurd magnitud of p v against max, such as p v=.1 at max=.7, was found to b consistnt with th rsults of th cntrifug progrssiv-wav tsts on th loos dposits of fin-graind sand with viscous scaling introducd (SASSA and S EKIGUCHI, 1999). Figur 11. Comparison of frquncy spctrums for nvlop and for p. FREQUENCY SPECTRAL ANALYSIS Th obsrvd prformanc dscribd abov showd that thr occurrd th rsidual por watr prssurs in th sabd of fin-graind sands undr svr wavs whn th wav hights bcam gratr than 2.m thr. It is intrsting to not that th obsrvd rsidual por watr prssurs consistd of th sum of th componnt which changd gradually with tim in th ordr of thousands wav cycls, and of th componnt which varid in th ordr of svral wav cycls (s Figur 8). For th purpos of xamining th rlationship btwn th variations of rsidual por watr prssurs and th wav grouping charactristics, a frquncy spctral analysis was prformd. W considrd th nvlop of th amplituds of wav-inducd fluid prssur fluctuations p as rprsnting th wav grouping charactristics. An xampl nvlop of p is shown in Figur, togthr with th corrsponding p -data. On can s that th calculatd nvlop rprsnts th obsrvd pattrn of variation in th p -amplituds. Th frquncy spctrums obtaind for th p - nvlop and for th associatd rsidual por watr prssur p at th soil dpth of.67m ar compard in Figur 11. Th figur shows that th form of th frquncy spctrum is similar with ach othr, suggsting that th variation of th rsidual por watr prssurs is closly rlatd to th wav grouping charactristics in trms of th spctrum. COUPLED ELASTOPLASTIC FINITE- ELEMENT ANALYSIS This sction dscribs th rsults of a rang of th tim domain coupld lastoplastic finit-lmnt analysis. Th analysis procdur can ralistically dscrib th cyclic plasticity of fin-graind sands and th partial drainag bhaviour undr wavs (SASSA and S EKIGUCHI, 1). For th slction of th matrial paramtrs for th prsnt plan-strain analysis, w rfrrd to th soil constitutiv paramtrs for Lighton Buzzard fin-graind sand ( D5 =.15mm) as dscribd in Sassa and Skiguchi (1). Th cofficint of latral arth prssur at rst K was assumd to b qual to.75, th Darcy cofficint of prmability k for th sabd sands D p - Figur. Envlop of p as a wav grouping charactristics. Figur 12. Comparison btwn th obsrvd tim history of rsidual por watr prssur and prdictd rsults from th tim domain coupld lastoplastic finit-lmnt analysis.

5 414 Sassa t al. indicat that th accumulation of rsidual por watr prssurs has a mor profound influnc on th stability of th fingraind sabd sands than th fluctuating por watr prssurs occurring in th phas of th troughs undr wavs. CONCLUSIONS v Figur 13. Variation of with tim as prdictd from th tim domain coupld lastoplastic finit-lmnt analysis, suggsting th mchanism of th sabd instability. ( D5 =.15-.mm) was st to b.2mm/s with rfrnc to th kd-valu of.15mm/s for Lighton Buzzard sand. Th soil rgion st for analysis, having a width of 9m and a dpth of 4m, was dividd into a total of 15 quadrilatral finit lmnts. Wav loading was facilitatd by imposing th wavinducd prssur fluctuations dfind as p~ = p (t) sin(kx- t) on th soil surfac. Hr p (t) rprsnts th masurd tim historis of th nvlop of th wav-inducd prssur fluctuations dscribd abov. Th wav priod of T= 14s and th wav lngth of L = 16m wr usd for th calculations of k=2 /L and =2 /T. Th prdictd tim history of th rsidual por watr prssur at th soil dpth of 2.m is shown togthr with th corrsponding obsrvd rsult in Figur 12. It is sn that th prdictd tim history rproducs rasonably wll th obsrvd on for rsidual por watr prssur. With th abov-dscribd rsult in mind, w prformd furthr analysis, with th aim of clarifying th mchanism of th sabd instability undr svrr wav Spcifically, w considrd th wav loading 1.5 tims as svr as that imposd for th prcding analysis. Th prdictd rsult is shown in th form of Figur 13. In this figur, th way in which th vrtical ffctiv strss v at th soil dpth of 2.m has varid aftr 31 hours of wav loading is plottd. Not that th initial valu of v thr was qual to 19kPa. It is sn in Figur 13 that th v -valu of th soil has dcrasd significantly to th valu as low as kpa or so from th initial valu of 19kPa, owing to th accumulation of rsidual por watr prssurs thr. On can also obsrv that th v valu dcrass in th phas of wav troughs, in contrast to th incras of v in th phas of wav crsts, undr th svr irrgular wav Ovrall, th prdictd prformancs dscribd abov Fild Obsrvations of Rsidual Por Watr Prssurs in Sabd Sands Th papr has prsntd th rsults of th systmatic fild masurmnts prformd for th changs of watr/por watr prssurs in th sabd of fin-graind sands at Kouchi port, Japan undr th passag of Typhoon No.9, during th priod of 6 days from 11:3a.m. on th 24th of July to :3a.m. on th 3th of July 2. Th principal findings obtaind from th prsnt obsrvations may b summarizd as follows. (a) Th obsrvations wr abl to captur for th first tim th procsss of th build-up and dissipation of rsidual por watr prssurs in a sabd of fin-graind sands undr svr wav (b) Th obsrvd magnituds of th rsidual por watr prssurs at fiv diffrnt soil dpths implid that th upprmost sand layr as thick as mm at most was in a liqufid stat undr th svrst wav conditions whr th wav hights rachd 4.m or largr. (c) Th rsults of th frquncy spctral analysis of th obsrvd data show that thr was a clos link btwn th variations of th rsidual por watr prssurs and th wav grouping charactristics which manifstd in th fild. (d) A rang of th tim domain coupld lastoplastic finitlmnt analyss wr prformd by taking account of th obsrvd tim historis of th wav grouping charactristics. Th rsults indicat that th accumulation of rsidual por watr prssurs plays a pivotal rol in causing th instability of th loosly dpositd sabd of fin-graind sands undr svr storm wavs. LITERATURE CITED FERGUI, O. and SILLS, G.C.;. Sabd masurmnts on a modl brakwatr in Porthcawl: Univrsity of Oxford, Dpartmnt of Enginring Rport, Sp. MADSEN, O.S.; Wav-inducd por prssurs and ffctiv strss in a porous bd. Géotchniqu, 28(4), SASSA, S. and SEKIGUCHI, H., Wav-inducd liqufaction of bds of sand in a cntrifug. Géotchniqu, 49(5), SASSA, S. and SEKIGUCHI, H., 1. Analysis of wav-inducd liqufaction of sand bds. Géotchniqu, 51( 2), YAMAMOTO, T.; KONING, H.L.; SELLMEIJER, H. and HIJUM, E., On th rspons of a poro-lastic bd to watr wavs. J.Fluid Mch., 87, ZEN, K.; YAMAZAKI, H. and SATO, Y., Liqufaction of sabd undr wavs: Rport of th Port and Harbor Rsarch Institut, 28(3), pp (in Japans).