I ISnUV KHOA HOC - CONG NGHE
Xoi cue bo loin nhat co the tai chan tru cau h
cmax
P G S . T S T R A N D I N H NGHIEN
Truing D$i hpc Giao thdng vin tai
T6m tit: Bii bio trinh biy y nghTa v$t ly' cue cdc thdng s6 inh hui^ng (3dn xdi CdC bO trd ciu khdng ngip trong thi nghidm vd trong myc td khai thdc ciu qua sdng cd d/a chit Id dit khdng dinh. khdng chju inh hu^ng cua sdng vi thuy tndu, tu- (36 ndu ra cic yiu ciu cin thidt dd d^t duxrc x6i CdC bd Id'n nhiy (g/ tru trong thi nghidm.
TO' khda. Xdi ci,ic bd, trd ciu, qua trinh xdi. xdi d ciu. sd lidu xdi thuv td.
Abstract: The underlying physics of the parameters affecting the depth of local scour at bridge piers in hydraulic laboratory and the fields based on available data in the literature. The discussion is restricted to the local scour at unsubmerged bridges in straight channels v\/ith beds comprising non- cohessive sediments and no tidal flows, waves. The proposed methodology can be used as guideline for desige of laboratory experiments to achlve the maximum local scour depth.
Key words: local scour, bndge pier, scour process, scour at bridge, bridge scour field measurements.
1. Gidi thieu Chung:
Xdi d ddng sdng dydi cdu g i m 03 loai id xdi do diln biln ty nhien cua dong chay. xdi chung do lam cdu cd dudng din khdng ngap lam thu hep ddng chay so vdl ty nhien vd xdi eye bO ngay tai chdn try, mo hay chdn cdc cdng trinh hudng ddng, bao vO bd do chinh cae edng trinh ndy gdy ra khi cdu bdc qua sdng co dja chat Id dat dd bi xdi. Xdi eyc bO try cau Id sy hg thip cao dO day sdng sdu vd hgp ngay tgi chdn try do ddng nudc tac dung vdo try Idm thay dli cdu true binh thudng cua ddng ehly, lam tang cgc bO dng sudt tilp vd t i c d l dong chay, vupl qua sue can cua hat dat bao chdn try, xdi ddt Ien va diy chung khdi chdn try tgo ra h i xdi eyc bO d try. Xdi tai chdn tru, mo se nguy hilm nhat khi ea 03 logi xdi ndy dong thdl cLing xay ra khi IQ thiet k l thdng qua dudi cau Do vdy van d l dy doan xdi cyc bO ldn nhdt cd the tai chdn try, mo cdu khi IQ thiet k l thdng qua la mOt thdch
thdc khdng nho dli vdi eac nhd nghien cdu, cac ky sy thilt k l cdu dudng trong thiet ke eao dO mdng try eau khdng chf d Vigt Nam md cdn d cdc nude khde Ngoai ra cdc cdng thdc hign thdl hau hit diu tu phdng thi nghigm, song lgi chua eo tieu chuan thong nhlt chung d l thyc hign chinh xac thi nghigm trong phdng. Bdl bdo ndy tgp trung vdo cdc thdng so. dieu kidn cd the gay ndn xdi cgc bO ldn nhlt cd the tai chdn trg cau trong nghidn cdu vd trong thyc hdnh thilt k l cdu dydng.
1. Val trd cua cdc thong sCi fl^n xdi cue bd tru c k :
Xdi cyc bp try cdu Id mOt hi$n typ-ng phdc tgp do nhilu nguydn nhdn, Id sy kit hpp tdc dgng tyang h i gida ddng nydc chay bao quanh try, bun eat d chdn try vdi chinh kich thudc vd hinh dang try trong cdc dilu kign ve ddng sdng vd ddng chiy khde nhau.
Trong han mpt tram ndm qua da cd rdt nhilu cdng trinh nghidn
cuu d cdc gdc dp khde nhau ve xdi eye bO trg cau dup'c cdng bo.
Hdu hit cac nghien cuu thyc hien trong phdng thi nghidm, chl co mOt s l it thi nghiem hien trudng.
Mdy chgc ndm gan ddy da cd mdt s l danh gia ve xdi eyc bO. thdng qua vigc su dgng s l ligu thi/c t l hign trudng. kit hyp giua s6 lldu hign trydng vdi s l ligu trong phdng dup'c cdng b l , trong 36 phai k l d i n cdc ting quan danh gid cua Chabert va Engeldinger (1956). Laursen va Toch (1956), Laursen (1958. 1962 va 1963), Shen et al (1956,1969).Altunin va ces (1977), Breusers et al (1977), Raudklvi va Sutheriand (1981), Dargahi (1982), Juravlev (1984), Raudklvi (1986) va Melville (1988), Breusers vd Raudklvi (1991), Wallingford (1993), Richardson va Davis (1995). Hoffmans va Verheij (1997), Hamilt (1999) va Austroads (2000). T.O.Nghien (2000), Melville (2008).Cac nghidn cdu deu thing nhlt mot s l cac thdng s l anh hudng den xdi eye bO va dupc vilt d dang
©EteiMi S6 8 nam 20121
KHOA HOC - C O N G NGHE # # # # #
khdng dan vj nhd ly thuyet Pi.
Ly thuylt Pi cho ta quan he gida chilu sdu xdi eye bO tai tru clu he va cac thong s l phy thupc gay ra xdi cyc bO d dgng:
K=f\. Kich thudc try cdu (b, Sh, Al hay a, thdt gian (t));
Ddng chay \u{P^,v,V,h,g), Biin cat day sOng
{ci.J,,P.,K)] (1) o i l vdl try cu t h i thi ba thdng s l cd don vj do dO ddl anh hudng den qua trinh xdi cyc bg Id: Kich cd try (chieu rOng tru b hay dudng kinh try D la tidu bieu); Chilu sdu ddng chay h hay (y); Cd hat bun cat ddy sdng bao try.
Thdng thudng hay xdy dyng quan hd giua chilu sdu xdi cgc bO h,. vdi chilu rOng tru b hay chilu sau ddng chay h. (t cd cac mO hinh quan he giua dudng kinh hat vdi chilu rOng try hay chilu sdu ddng chay. Khi dudng kinh hat bun cat giam thi ban chdt vat ly cua hat d day ddng chay thay ddi. Oil vdi hat cd djjj < 0,7 mm, day cd t h i d dang sdng eat, trong luc hgt cd d^ < 0.1 mm se cd lye dinh kit giua cdc hgt lam tang kha nang chju lyc cua hat, Sy thay doi nay anh hudng din xdi tai try. Neu bd qua sy thay doi eua khoi luong rieng cua nudc P vd cua hat P, thi chilu sdu xdi eye bd tuong doi so vdi ehilu rOng try duac vilt d
<^ng.[' y H V' vb b Vl „, ,,]
-^= j \ — , — • — . — , — , < T „ — , S h , A l b [K b gb u d„ ' b )
(2) Trong bilu thdc (2) tdp hpp trong ngoac tao thanh 03 nhdm cac y l u td lay chilu rOng try b Idm yeu td so sdnh quan trpng.
v l trai cua bleu thdc !a xdi cue bp tuong doi so vdi ehilu rpng try (h^b). thd hign duac ban chat vdt ly eua xdi eyc bO, la hd thing
xody hinh mOng ngua bao quanh try md kich cd Id hdm cua dydng kinh try
C> ve phai' - 04 thdng s l dau iien quan din ddng chay: (vNJ Trang thai chuyin ddng bun cat cua ddng chay din try, gpi Id cydng dO ddng chay; Ty s l (h/b) gpi Id" mdc dO nOng" cua ddng chay; (vVgb) Id s l ale (Euler)- hay quen gpi Id s l Pho rut (Froods) cua try Fr^ ; (vb/v) Id s l Rdy nOn (Reynolds) try
- 02 thdng s l d nhdm thd hai:
(b/d5(,)thl hign dO thd cua hgt bun edt ddy so vdi try; a^ la mdc dO ding diu cua hat
- 03 thdng s l d nhdm thu ba Id: (vt/b) la ty lg phat triln xdi theo thai gian doi vdl tru (thdl gian khOng don vi); Sh vd Al hay (8) Id hinh dang try vd hudng cua ddng chay din try. vdt can giu lai tai tru Tdp hop cdc thdng s l chi ra val trd cua chilu sdu dong chdy dli vdl chilu rOng try "dp ndng ddng chay", nhdm tuang ddi, xoay vd tdn s l cua clu trUc rdi do try tao ra (vVgb,vb/v), toe dd phat trien xdi (vt/b), dang tru vd sy thing hdng so vdi phuang chay cua try). Trong trudng hpp tru Id try ddc l|p khdng bj ngap trong kenh thing, day ddng chay Id cat eung loai, ddng chay khdng bj anh hudng eua thuy tneu, sdng thi cac thdng so ndu trdn se anh
hydng khac nhau din xdi dyyc phdn tich dudi day:
(1) Chilu sdu xdi phy thuOc vd thay doi theo cudng dO ddng chly (hc/b ~v/vc)
Sy thay dli ndy the hien trong nghidn cdu trong phdng thi nghigm cua Chabert va Engeldlnger(1956), Latushenkov (1960). Shen et al (1966), Murtskhulava (1967), Maza Alvarez, (1968), Muromov (1969), Altunin (1972), Juravlev (1977), Ettema (1980), Raudkivi va Ettema (1983), Chlew(1984), Baker, (1986), TO.Nghien (1985- 1988).
Doi vdl xdi nudc trong (ddy cat trudc tru nai khdng h\ anh hudng cua try khdng ehuyin ddng (v <
v^), chieu sau xdi cuc bO d cat cOng loai (o <1,3 din 1.5) tang gdn nhu tuyen tinh vdi tie do, dat gid trj ldn nhdt tgi tie dO tdi han V^ (hay tdc dp khdng xdi V^), Khi tie dd ddng chay vuot qua toe dO tdi han (xdi cue bO chuyin sang xdi nudc due), xdi cyc bg trudc hit giam sau do tang trd lal dat dfnh thu hai thap hon dfnh thd nhat tuong ung vdi trang thdi ddy phdng (dang chuyin tilp) eua day long ddn, dinh nay gpi la dfnh cua xdi nudc due,Sdng cat hinh thdnh khi 1.4v^ < v < 2 8v^; bun cat chuyin ddng khdng ngdng v>2 8v^ (Altunin). Su thay ddi chilu sdu xdi Id kit qua cua cd
XOI uiroc tioiig Xoi uiroc fliic Uieii bieii xdi tlieo cu6ii,a ft^ dbua duty
Hinhl.
Diln bien xoi theo VA/^(hayVA/J, (MELVILLE)
I So 8 nam 2012
0t§gM KHOA HOC - CONG NGHE
hgt vd dO die eua hlnh dgng sdng eat day tgi tic dO ddng chay cy the nhu kit qua thi nghidm cua (Chi ddn tinh xdi try clu cua Lidn Xd trudc ddy, 1962; Chee, 1982, Chlew/, 1984; Melville, 1984;
Raudklvi, 1986;TD,Nghldn 1985- 1988. Melville vd Coleman, 2000;
Sheppard, 2005.2010), Sdng edt cdng die vd cdng cao thi chilu sdu XOI eye bO cdng gidm, Khi sdng cdt ngypc hinh thdnh nd se ldm cho ddng chdy tidu tin ndng lupng nhilu han d tie dO ldn hon. vl vdy md chilu sdu xdi eye bO se lgi bj gidm di. DO ldn (mdc dO dao dOng) cua chilu sdu xdi cgc bO do hinh dgng ddy gdy ra ed the xdp xi mOt nua bidn dO sdng eat ddy (Shen et al, 1966;
Chee. 1982;Chiew, 1984), D l i vdi cdt khdng deu hat (o
> 1,3 din 1,5)se ed sy thd hda mdt h i xdi Idm giam xdi cue bd vi ndng luong ddng chay chf du de tach vd day mdt vdi loai hgt nhd nhat djnh khdng du day cac hat ldn, chidu sdu xdi Idn nhat duac gpi la dfnh thd hda tuong dng vdl tie dO V^ vd dinh nay Id dfnh nudc xoi due. Sy thd hda diln ra khi V <
VgVd xdi dat gidi han tuong dng.
Tic dO vupl qud toe dO thd hda (V
> VJ hien tuong thd hda se bj xda va xdi nudc due tilp tuc diln ra.
Dfnh xdi nudc dye xuat hidn d giai doan chuyen tilp (day phdng) khl tat ca cdc hat cdt d day khdng diu hat cung chuyin dOng. Tai dfnh xdi, chieu sdu xdi eye bO tuong tu nhy eat dong nhlt. khi hai logi ndy cd cung dudng kinh hat trung binh d^ Hinh 2 chi ra anh hudng cua cd hat ddt va toe dO tuong doi cua ddng chay den xdi cyc bd tuong doi. MOt vai so lieu thuc do khl sdng edt hinh thanh vupl qud dudng cong thi nghigm, Chieu sdu xdi hemax phan tan, khong phy thuOe vdo cd hay khdng hinh thanh sdng
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] Hinh 3.
Anh hudng cua cdp ph6i hgt vd t i c dO tuang ddi d i n xdi tuong d l i t u s l li$u thyc flo.
cdt day ngy y chi ra xu t h i trong phdng thi nghigm khdng giai thich dypc qud trinh xdi thyc t l , sy phdn tdn ed t h i do cdp phoi hat,ehieu sau ddng ehdy tgo ra.
S l liOu thyc t l trong Hinh 3 d l i vdi 04 loai cdp phoi hgt ddt ddy sdng cho thly eac dilm khd phdn tdn, t i c dO xdi Idn nhdt cd t h i rai vdo xdi nydc trong hay xdi nudc dye tuang ung vdi t i c dO khd khde nhau vd cdp phli khde nhau chdng td quan hg xdi vdi toe dO khdng phy thuOc vao cdp phoi hgt, dong thdi cdeh phdn chia hign thdi thdnh xdi nudc trong va xdi nudc dgc doi vdi ddt khdng deu hgt edn chua thda ddng.Xdl cyc bp Idn nhat h.^,^ khdng phy thuOe vao hinh dgng ddy d dgng
sdng cdt hay khdng cd sdng c^, Nlu ddng chay din tn,i bj thd hoa trong luc h i xdi khOng thd hda thi xdi sdu han so vdi eat diu hat trong cung dilu kidn; Nlu ho xoi eung thd hda tht chieu sdu xdi cue bO se nhd han d l i vdi cdt khong d i u hgt cho ea hai logi xdi (x^
nudc trong va xdi nudc clue).
Hinh 3 cdn eho thay kit qua till nghiem chua t h i giai thich difOC thyc te, cd the tinh khdng diu cua hgt ddt, chieu sdu ddng chayvS tinh khdng I n dinh cua hd Vn6i^
xoay mdng ngya la chia khda cho sy phdn tan cua cae so lieu 3o thuc t l .
(2) Anh hudng eua "dO ndng dOng chay", h/b
mim So 8 nam 201ZI
KHOA HOC-CONG NGHE ^J
DO ndng the hien chieu sau tuong ddi cua ddng chay trudc xoi so vdi chieu rOng trg -(h/b), Trudng hp'p ddng chay ndng chieu sau xdi cyc bd ty Id thuan vdl chieu sdu vd coi nhy doc lap vdl chieu rpng try, Doi vdi try cd ty s l h/b Idn. chilu sdu ldn. try COI nhu try nho, chilu sdu xdi eye bO tang ty id vdl chilu rOng try va cd t h i coi nhu ddc idp vdl chilu sau vl cudng do xoay mdng ngya va ddng xudng lien quan chat che vdi chieu rdng try. Oil vdl try cd ty s l d pham vi ehuyin tilp (trung binh). chilu sau xdi cyc bO phu thuOc vdo ca chilu sdu ddng chay vd chilu rOng tru, Khuynh hudng chieu sau xdi thay dli theo ba xu thd dupc chi ra d Hinh 4.
(Meiville,2008').
Sd dyng kit qua thi nghlem Melville va Coleman (2000) kiln nghj:
Tru "rdng" nlu b/h>5(hay h/
b< 0,2) thi h^- h; Try " hep" nlu b/h<0,7(hay h/b>1,43) thi h^- b ; Try trung gian giua try "rOng" vd try "hep"nlu 0.7<b/h<5
(hay 0,2< h/b<1,43) thi h^
(hb)"'^. Richard va May (nudc Anh)cho bidt xdi eye bd khdng tang khi b/h>4,45(hay h/b<0,22) va xdi Idn nhdt tgi v/vc = 0,8 Phuang trinh eua Breusers (sd dung d Hd Lan) xdc djnh try
"hgp" nlu b/h<1,0 thi h,.-kb; tru
"rdng" nlu b/h>1,0 thi h^~kh.
Johnson vd Peggy A. (1999) xem try la tru "rOng" neu h/b<0,8 vd Fr<0,8, nlu ea hai dieu kidn nay thoa mdn thi xdi glam 5%; neu h/
b=0,5 va Fr=0,5 thi xdi giam 19%.
Muromov (1969) eho bilt neu b/
h>1,0 thi xdi nudc trong Idn han xdi nudc due; nguoc Igi nlu b/
h<1,0 thi xdi nydc trong nhd han xdi nydc due. Mac diJ quan niem v l "dd ndng ddng chay" ehua dwac thing nhat, song ta cd t h i giai thich cac khuynh hudng nay
"\i'mg tuffn^ tac''xoay mat va xoay day
b=cliieii rpng try
lU«.i
-0.2
h/b -1 4 tru"roug" tru truug biuh tru "hep"
Hinh 4.
Chilu s^u x6i thay (36i theo "do ndng ddng chay"
h/b
nhd vao sy tuang tac giua xoay mgt nudc vd xoay mdng ngua d chdn try (Hinh 5a va Hinh 5b) do ddng chay tac dOng vao try tgo ra.
Hai loai xoay nay ngupc chilu nhau, xoay mat nudc anh hudng tdi xody ddy, hay xoay day bj anh hudng bdi xoay mat thi chilu sdu xdi cyc bd phy thudc ca vao chilu rdng tru vd chilu sau, Khl hai xoay nay ddc lap thi chieu sau xdi eyc bO dOc ldp vdi chilu sdu ddng chay. Chilu sdu cdng giam, val trd cua xoay mat cdng tdng, khong che xoay day lam eho xoay day giam cudng dp, giam han kha nang tdch hat ddt bao chdn try lam cho xdi cyc bd glam Oil vdl try "rdng", ddng chay ndng, try trong ddy cat. do xody mdng ngya kem phat trien nen hinh dang day eat tien vao try va hinh thdnh dang day trong h i xdi, lam cho hinh dang ddy cat anh hudng true tilp tdi chilu sdu xdi cue bg, de tao ra xdi cyc bd ddl hdi phai
cd thdi gian dai. Gial thich neu tren dya vao kit qua thi nghiem cua Chabert va Engeldinger (1956), Laursen va Toch (1956), Laursen (1963), Hancu (1971), Bonasoundas (1973), Basak (1975). Breusers et al (1977), lain va Fischer (1979), Ettma (1980), Chee (1982),- Chiew (1984), Raudklvi (1986).Klt qua cua eac thi nghlem cho thdy anh hudng cua "dO nOng dong chay"la hdm cua h/b va b/d50. Odi vdi b/
d50=50 anh hyang nay duac bd qua khi h/bs2,6 va gldi hgn cudi la h/b=4,3 . Ndl chung bdng kit qua thi nghidm ,cdc nhd nghien cdu deu thong nhdt khi cudng dd ddng chay khong ddl thi chilu sdu xdi tang theo chilu sdu ddng chay, song nlu chidu sau tilp tuc tang thi xdi cue bp lai ddc lap vdi chilu sdu ddng chly h/b>4, Melville va Sutherland (1988) xdy dung he s l Kh la dudng bao kit qua thuc nghiem phan anh anh hudng
I S6 8 nam 2012
mm
# # # # # KHOA HOC - CONG N G H g
cua chieu sdu ddng chdy d i n xdi, song d u d n g cong ndy khdng thda man so lidu do t h y e t l , hay so lidu t h y c t l khdng tudn theo khuynh h u d n g md thi nghi$m dgt d u o c (Hinh 6). S l lidu d Hinh 6 cho thdy trong t h y c t l xdi tu'ong doi cd xu t h i tdng theo c h i l u sdu t y o n g d l i , d y d n g n h y cd s y t y dieu chinh cdc y l u t l ddng chdy trong d i l u kidn dOng l y c hpc cua ddng n y d c vd bijn cdt.
(3) Anh h y d n g cua s l Euler vd s l Reynolds try
Ettema et al (1988) s u dyng s l lidu thi nghidm chi ra h / b tdng theo vVgb; Thdng s l ndy Id cdn t h i l t d l md ta bien thidn ndng l u p n g eua ddng bao try, Id thdng s l t h i hign ty s l cOt n u d c t i e dO tai d i l m tdch ddng (cOt n u d c tdch ddng) vdl e h i l u rdng try. Trg edng nhd thi gradient cang Idn; eung t r u d n g dOng chay thi trg nhd se cho ty s l xdi h / b it h o n try Idn;
khdng phu h o p cho tinh xdi d t r u d c chdn try. laroslavtsev(1956) Id n g u d l dau tien dp d y n g dp l y e d y tai mgt try (v^/g), xdc dinh hg so toe dp cua ddng d i n try Kv=f(v^/gb) de neu ra xdi t u o n g doi ( h / b ) tang theo (vVgb). Quan he ndy cQng d u o c Bogomolov vd ces (1975) chi ra bdng thi nghidm.
Krkil vd ccs(2012, dj hpelOWA) bdng k i t qua thi nghigm trong cung dieu kign ddng c h l y xdi n u d c trong (uVu*c=0.8) vdl e h i l u sau ddng chay h=1000mm, t i e dO ddng c h l y v=0.46m/s, ty s l h/b= 2 . 4 6 - 15,63 eho thay xdi t u o n g d l i ( h / b ) giam khi c h i l u r i n g try b tdng.
S l Rdy ndn(Reynolds) trg Reb dup'c Roper, Shen vd Schneider (1967) s d dyng de ddnh gid xdi try. song ehinh Shen vd ces (1969) lgi k i l n nghj xdi t y o n g d l i h / b - (vVgb). Vide phdn tich s l ligu thi nghigm trong phdng cd R e b ^ l 60000, so lieu thi nghidm
Hlnh 5a. H$ thing ddng chdy bao quanh try
Hinh 5b. H$ t h i n g tdch ddng chdy bao quanh try
. - . - ; • . , • • . •
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Hlnh 6.
Anh hudng cOa chilu sdu tuang d l i d i n xdi tuang d l i theo s6 li$u thyc do so sdnh vdi dudng cong k i t qua thi nghi$m cua Melville vd Sutherland (1968) (NguIn: T Paul Teng. P.E.2005)
trong sdng cd 160000<Reb
£400000 vd s l lidu hid t r y d n g c d Reb>1200000 eua Altunin (1975) cho thay cac d u d n g quan hd g i u a h / b vdl Reb khdng trung nhau.
khdng cung xu t h i . d u d n g k i t qua thi nghigm gdn n h y vuOng gdc v d l d y d n g k i t qua t h y c do, Monti (1994) bdng k i t qua thi nghigm d l i vdi try trdn cho t h l y vdi eye bO dOc ldp v d i s l Reb try khi Reb>
7000. C d t h i ndi ridng s l Rdy ndn try Reb hinh n h u khOng t r y c t i l p anh h y d n g d i n x d i , song Igi anh h y d n g d i n t i n s l tdch b u t xoay (n) ra sau try. trong do d d so Strouhal(nb/v) = 0.2 d l i v d i try trdn. D o i v d i try cd cCing toe dO ddng d i n V, tdn s l b d t xoay ty lg nghjch v d i kich e d try, do d d try nhd se tgo ra dO b d t xody l d n . Altunin (1975) k i t hg-p s l Reb tn^
vdi s l Reh eua ddng c h l y d l d u a ra cdng t h d c xdi t y o n g d l i d l i v d i
ddng chay he / h .
(4) A n h h u d n g cua dO thd h^t bun cdt (Hinh 7)
S l lidu cua thi nghigm ty 1$
nhd trong phdng cho t h l y cat diu hgt khdng anh h y d n g d i n dii^u sau xdi eye bO, t r d t r y d n g hppt}
Id c u a hgt so v d i tru Id Idn. Khl d < 0.074 m m thi vai trd cua lyc d i n h se quan trpng. Theo Ettema (1980) khi ty s l b / d „ < 8 thi x6i chf xdy ra dpc mgt ben tn^ vd do vdy xdi c y c bO giam Khi hgt th6 khdng quan sdy thay xody dgng mdng n g y a , xdi ehf do vai trd cua ddng t h y c i p dpc thdn try. bldg,^
130 d d n g t h d c a p tao ra d i e trong h i xdi, 30 £ b/dgj,<130 xdi chu ylu do d d n g t h u c d p va rat it hgt 6 day h i xdi, 8< b/d^^OO xdi ehf do ddng t h d d p , bld^^<8 khdng t h ^ xdi (Michael W. Horst .2004). S6 lldu trong phdng cho t h l y ehilu
mmsi
So 8 nam 2012 |KHOA HOC - CONG NGHE ggMgff
sau xdi chju anh h u d n g cua hat khl b/dg^ < 50, ngodi phgm vi nay xdi c y c bO khdng glam ma hdu n h u khong thay doi (Chiew va Melville ,1987; Breusers vd Raudklvi, 1991) Sheppard et al (2004) ehi ra glam e h i l u sdu xdi khi b/dgj, tang bang k i t qua thi nghiem doi v d l try trdn cd d u d n g kinh (0.114; 0,305; vd 0.914 m) trong cat deu hgt cd d u d n g kinh la (0.22; 0,80, 2,90 mm). Cdc thi nghigm cd ty so b/d^^ d i n 4155.
So lieu eua Sheppard cho thay xdi t u o n g doi Idn n h l t khl b/d^^ = 46, cdc ty s l b/djj, <46 vd > 46 deu lam cho xdi t u a n g doi giam trong cung dieu kien v/v^ vd h/b. Nghien c u u eua Lee vd Sturm (2009) eho thay xdi glam khi b/d^j, > 25. Can e d vdo s l lieu cua cac nghien c d u cd t h i thdy anh h u d n g cua c d hgt d u o c bd qua n l u 25< b/dgj,<130,
(5) Ddi vdl cat khdng deu hat T h i nghiem xdi n u d e trong eua Ettema (1976. 1980). doi v d i n u d c dye cua Chiew (1984), vd Baker (1986). K i t qua cua cac thi nghiem cho khuynh h u d n g thay d l i (h/b~v/v,.)trong b i l u d l Hinh 8. S y thay ddi eua e h i l u sau xdi d l i vdi hat khdng deu.
Thd hda xay ra d ddng chay d i n try va trong h i xdi. Khl v/v^. tang cao. ddng chay euon theo tdt ca cac hat thi cat khdng ddu it anh h u d n g t d i xdi; song d m u c dO v d a phai thi vai trd dp khdng dong d i u (thdng qua o ) lam glam dang k l c h i l u sdu xdi. C h i l u sau xdi giam khl tang ojd^^, n l u ag/dgo> 0.3 thi thd hda ma hd xdi xuat hien lam giam xdi (Ettema,1976).
Trong phdng thi nghiem ty Id b/dgQ =800, sdng cdt hinh thanh se giam xdi khi 100< b/dg^ <800 Tong t h u e t l sdng cat khdng hinh thanh khi b/d5(j<900 ma tap trung hinh thanh tai b / d „ =1000 va d u d i gia trj xdi Idn nhat d l i v d i hinh thdnh sdng cat, trong luc ( h / b ) m a x
= l l < c :
Z2^S>JZM.„.,
Hinh 7.
w
0 5
Hf so cap phoi h^t Xlgniii
Hinh 9.
Anh hudng cua hS s6 cdp phli hat den xdi tuang d6i theo so li$u thyc tS
doi vdl hinh thanh sdng cat tai b/dj_Q=4000 v u o t qua phgm vi sdng cat hinh thanh T h y c te d Hinh 9 cho thay xdi t u o n g doi khl sdng cat hinh thanh ldn hon khi khdng cd sdng eat doi vdi he sd cdp p h l i hgt a <=2.5. xdi t u o n g ddi giam khi o >2,5,Cdu hdi d u p c neu ra d l i v d i mOt l u p n g nhd s l ligu khi sdng cat hinh thdnh & hd s l a^
ldn. B i l u do cho thay s u p h d c tap cua t h y c t l ma thi nghiem trong phdng ehua glai thich d u o c dong t h d i khdng t h i so sanh thi nghigm v d i t h u e t l .
(6) Anh h u d n g eua hinh dgng try vdn d l hinh dang try anh h u d n g d i n xdi doi v d l try d o n cd chieu rOng hay d u d n g kinh khOng dOi theo chieu sau ddng chay da duioc nghidn c d u kha chi t i l t , trong dd phai k l d i n cae nghien c d u cua: Tison (1940),
Laursen vd Toch (1956), Chabert va Engeldinger (1956). Garde (1961), Varzehotis (1960), Larras (1963), Venkatadri et al (1965), Yaroslavtsev (1960), Dletz (1972), Neil (1973)va Richardson vd Davis (1995),Gia tri trung binh cua hg s l hinh dang tru Sh hay Ksh cho trong bang 1
(7) Anh h u d n g cua h u d n g ddng chdy d i n tru
Trong t h y c hanh anh h u d n g nay dang k l khi h u d n g ddng chay lech vdl tryc dpc qua try v u p t qua 5 " , gdc Igch tang se lam anh h u d n g dang ke d i n xdi thOng qua he so nghieng eua ddng chdy hay KO d bang 2 doi vdi try khdng phai try trdn.
(8) Anh h u d n g eiJa thanh bdn mang thi nghiem
I S6 8 nam 2012
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8ggg KHOA HOC - CONG NGHE
D e tranh anh h u d n g cua thanh mang thi nghigm, ty s l giua e h i l u rOng mdng B v d l c h i l u rOng try b can v y p t q u d 10 (B/
b> 10) (Laursen vd Toch, 1956;
Chiew vd Melville. 1987).Ty s l B/
hs 3 se lam cho thdnh bdn dnh h y d n g d i n phdn p h l i t i c dO.ddng c h l y thdnh ddng 3 c h i l u (Graf vd Altlnakar,1998),
(9) O i l u kidn cdn t h i l t d l dat d u p c XOI cyc bO ldn nhdt d chdn try trdn d bang 3
3. U\ luan
Cdn c d v d o cdc k i t qua nghidn c d u hidn thdl cd t h i rut ra: (1) C h i l u rOng try b. hlnh dang try S h , e h i l u sdu ddng e h l y h. t i c dO ddng chay v. tinh chat dat ddy sOng la cdc bien quan trpng xac dinh vdi eye bO he - T h y c t l m i l b i l n nay anh h u d n g d i n xdi rdt p h u c tap vi tac d y n g t u o n g h i g i u a chung va t u d i l u chfnh trong d i l u kign dOng l y e cua ddng n u d c vd bun cat d t h i t h i n g n h l t khl cac d i l u kiOn thay d l i dien ra.(2) Can phai nghidn c d u s u t u o n g tdc tiem I n g i u a cdc b i l n hay thdng so a n h h y d n g d i n xdi eyc bO-(3) Vai trd cua tinh chat bun cdt ddy rdt quan trong trong d y dodn xdi, (4) k i t qua t h i nghidm c h y a t h i gial thieh d y a e t h y c t l , cd the tinh khdng d e u cua hat ddt, e h i l u sdu ddng e h l y vd tinh khdng I n djnh cua hd thong xody mdng n g y a la chia khda eho s y phdn tan cua cdc s l lieu d o t h y e t l . Mdc du vdy vigc t i l n hdnh t h i nghigm cd ty lg l d n la rat edn t h i l t d l nghidn c d u cdc khia cgnh vdt ly khdc nhau cua cac b i l n d i n xdi e y c bp. n h l t la c o c h l xdi. (5) X d i c y c bO l d n nhdt doi v d i try d a n c a n thda m a n dieu kIgn d bang 3 •
Bdng 1 . Hg s d hinh dang try trung b i n h Ksh (Garde va Kothyari 1998).
Hinh dgng try Try tr6n
H l n h h 9 t i J § u ( 2 ' 1 , 3 1,4 1) Hlnh Elip (2,1,3 1) Hlnh ki4u Jukovsky (4 1 ,5 1) Try nhon dau c6 goc CT dmh
15(0) 60(0) 90(0) 120(0)
" 150(0)
Ksh 1,0 1 1 +1,26 1,0,0.86
1 0, 0,8 0 45 0,76 0 88 0 94 1 0 B i n g 2. H$ s 6 Al hay KB c h o trg c i u (Richardson v a Davis,2001)
l/b 4 8 12
8(0) 0
1 0 1 0 1 0
15 1 5 2 0 2 5
30 2 0 2 75
3.5 45 2 3 3 3 4.3
90 2 6 3 9 5 0 Bdng 3. D i l u kig dat h^^,,trong thi nghiem d d i v d i ddt khdng dinh
Yeu to anh hudng Cap phdi hgt Cudng dp ddng chdy Thcinh ben C d h a t
Chidu sau ddng chay N u d c vd try Thdl gian xdi cdn b i n g
Dieu kiOcdn thiet o g £ 1.3 + 1.5 0 , 9 s v / v c s 1.0-1.3 B/ba 10vSB/h>3.5-i-5
(1)d„>0 7mm. (2) 2 5 < b / d „ <130 tr^nh thd hda b/d50=50,h/b>4 3;b/d50 >50 thi nen giam h/b Reb >7000
Grimaldi(2005),t>2 106b/v,Ahc.24gio s0.05b/3 TAI LIEU THAM KHAQ
1 Tran Dinh Nghien "Nghien cuu xoi cyc t)0 tiv cdu qua song" LuSn dn tien sy.truang eHGTVT2000
2. A.Tafarojnonjz et al "Required conditions to achieve the maximum local scour at a circular pier'XXXII Convegno Nazionale di Idraulica e Costruzioni Idraulich Palermo,14-17setlenbre 2010
3. FOOT Scour Manual March 2010
4. Garde and Kothary "Scour around bridge piers" PINSA 64,A,No.4,July 1988
5. Kirkil et al "Simulitude of coherent turbulent structure in flume studies of bridge scour' The Univ.of Iowa. NCHRP 24-20,US TRB
6. Lee "Physical modeling of locaj scour around complex brirge pires"
Ph D. Dissertation May, 2006
7 Melville'The physics of locaj scour at bridge piers" 4th Inter Confer.on scour and erosion 2008
8, Sheppard and Miller "Live-bed local scour experiments" J.Hydr.Engrg.
Vol,132, No 7 July 1,2006
9, T,D,Nghi6n "Laboratory investigation of scour reduction near bridge pier by delta-wing like passive device" IVI.tech.Oepartt.Civil Engrg UT Kanpur.January. 1988
10, TPaul Teng,P.E."Field observations and evaluations of streambed scour at bridge" Publication No.FHWA-RD-03-052, May 2005
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S l 8 nam 20121