36(1), 149-159 Tap chi CAC KHOA HOC Vfe TRAI B A T
/ NGHIEN Cl/U Sir BIEN DOI AP LlJC NlTOC LO RONG CUA D A T LOAI SET BAO HOA NlTCJC CHIU TAI TRONG D Q N G CHU KY DON VA DA PHLTONG TRONG DIEU KIEN KHONG THOAT NlTdC
TRAN THANH NHAN, DO QUANG THIEN Email: [email protected]
Trudng Dgi hoe Khoa hge Hue N g a y n h a n b d i : 6 - 8 - 2 0 1 3
l . M d d d u
Dudi tdc ddng ciia tdi trgng dgng, dat lo^i set (ke ca dat set ydu) cd siic khang dgng tdt ban dat loai cdt [4] nen cdc nghidn ciiu ve tinh chat ddng hge ciia dit ndn chii ydu tap trung trdn ddt cat [14], Tuy nhidn, kdt qud quan trie thyc te cho thay lyc cit trugt dgng cua sdng S (sdng ngang) trong dgng dat gdy tang nhanh dp luc nudc Id rong vd lam gidm siic khdng cdt ciia dit ndn. Hon nifa, sy phan tdn ciia dp lyc nudc Id rong sau ddng ddt, chang han nhu sau ddng ddt tai Mexico City nam 1985 [12] hay ddng ddt Hyogo-ken Nanbu nam 1995 [5]
gay ra hidn tugng lim manh, liin lech vd gay mdt dn dinh nin vd mdng cdc cdng trinh xdy dung trdn ldng ddt set ydu.
Kdt qud ghi nhdn xung ddng trong cdc tr^n ddng dat cimg nhidu nghidn ciiu bdng md hinh dd chi ra rang, trong ddng dat cac ldp ddt chju tdc dung cua tdi trgng ddng da phuang cd bidn dg bien dang va tan sd thay ddi lien tuc [1], Tir hinh 1 cho thay xung dgng ciia nen dat trong dgng dat Hyogo-ken Nanbu nam 1995 dugc ghi lgi theo hudng bdc nam va ddng tay t^i dg sdu 16m [7], Tir xung dgng nay cdc tdc gid cd the tinh todn vd lap bidu dd bidn thidn dg bidn dang theo thdi gian {hinh 2) vd quy dao cua do bidn dang trugt trdn m | t phdng ndm ngang {hinh 3). Tit hinh 3 de ddng thdy rang xung ddng ciia dat ndn trong ddng dat thd hi?n bien dang trugt ddng da phuong. Vi vay, cdc nghidn cim lidn quan den dnh hudng ciia dgng ddt len tinh chdt ddng hge ciia ddt nen phdi dugc thyc hidn theo md hinh cdt trugt dgng da phuang.
/Phuong Bac Nam .Phuong Dong Tay
j'l/PhuongBac Nam
^ Phuong Dong Tay
0 2 4 6 8 iO i : 14 16 0 ^ J.. . \ .'° ' "
Thdi gian (giay) Tlioi gian (giay)
Hinh 1. Xung d0ng ciia n^n dSl dugc ghi nh|n tore ti6p Hinh 2 Bieu do biln thien dp biln dang Iheo thdi gian Or hai phuong bic nam vd dflng tdy
^.5 ., 0 , . 0.5 j Biin dang (%)
Hinh 3. Quy dao dfl biin dang theo 2 phuong bic nam va dong tay Iren mat phang ngang
Su thay ddi tinh chdt co ly cua ddt loai set trong dieu kien tai trgng ddng chu ky da dugc nghidn Cliu trong nhidu cdng trinh vdi nhieu md hinh thiet bl thi nghiem khde nhau. Ohara vd Matsuda [11] da nghien cuu dnh hudng cua do bien dang trugt (y), sd lugng chu ky («) va he sd qud cd kdt (OCR) den su thay ddi dp luc nudc Id rdng vd tinh chdt nen lun sau cat truot bang thi nghiem cat trugt chu ky dan phuang, khdng thodt nudc theo so do didu khidn dd bidn dang (strain-controlled) Bdng thiet bi thi nghiem cat trugt don gian chu ky theo so dd didu khidn iing suat (stress-controlled), Yasuhara va Andersen [13], Yildirim vd Ersan [14] da nghien ciru dac tinh cd ket cua dat set cd ket binh thudng trong dieu kien tdc dung lien tuc ciia tai trgng chu ky khdng thoat nudc xen ke vdi thdi gian thoat nudc. Ddng thdi, cdc tdc gia cting da dk xuit phugng phap du bao sy thay ddi dp lyc nudc Id rdng trong qud trinh cat trugt va tinh todn dd liin trong giai doan tai nen ep [9, II, 13]. Ngoai ra, ndn dat dudi mdng cdng trinh xdy dyng ngoai khoi vimg Bdc cue chiu dieu kien tai trgng rit phiic tap, trong dd ngodi ung suat cat trugt hudng tam do tai trong cdng trinh, phdn tu dit dudi mdng cdng trinh cdn chiu cdc lyc cat rat Idn theo phuang ngang do tdi trgng vd su djch chuyin ciia bang. Do dd, dd nghien ciiu anh hudng cua gdc lech giiia iing suit ngang ban ddu (do tdi trgng cdng trinh) vdi iing sudt cdt trugt theo phuang ngang khac (do tdi trgng va su dich chuydn ciia bang) din quan he do bdn - dd biin dang - irng suat ciia dit loai set, DeGroot vd cdng su da su dung thiit bj thi nghidm cdt tru0 tinh don gidn true tidp Geonor (Geonor direct simple shear test apparatus) vd thidt bj cdt trugt tmh dan gidn tryc tidp da phuang (Multi-
Hinh 4, Anh chyp thiet bj thi nghiem cit trugt flOng , dan gian chu ky da phuang
directional direct simple shear test apparatus) tren ddt set Blue Boston vd ket luan rang "tinh chit khang cat khdng thoat nudc cua dat phu thudc rat ldn vao gia tri gdc lech giiia hai luc cdt trugt tren mat phdng ngang" [2]. Tuy nhien, dnh hudng cua phuang cat trugt trong tdi trgng dong chu kJ da phuang len tinh chat ddng hge ciia dat dinh (ddt loai set) vdn chua dugc nghien curu va hidu biit diy dli, mac dii dnh hudng nay ddi vdi ddt cdt dd dugc khdng dinh va nghien ciiu trong nhidu cdng trinh, cung nhu dp dung vao tinh toan thiet ke tii lau.
Ngoai ra, Matsuda vd cdng su gin day da nghidn Cliu quan hd giua do liin sau cdt trugt vdi h^ sd suy giam ling suat hiiu hieu trong ddt cdt vd dd xudt phuang phdp tinh toan sy thay ddi iing suit hihi hieu trong dat cat dudi tac dung cua tdi trgng ddng chu ky dan phuang vd da phuang thdng qua sii dung cac thdng sd dudng biin dgng [9].
Tir thidt bi cat trugt dong dan gian chu k^ da phuang, chiing tdi da tiin hdnh thi nghidm trdn dit set kaolin cd kdt binh thudng, theo so do dieu khien do bidn dang tir 2 phucmg vudng gdc voi nhau. Muc dich cita cdc thi nghiem trong nghien cdu nay nhdm: (i) xac dinh anh hudng cOa phuang cat trugt (giiia don phuong vdi da phuang vd gifia da phuong cd do lech pha khac nhau), dg biin d^ng trugt (y) va sd lugng chu ky (n) len sy thay ddi dp luc nudc Id rdng trong qua trinh cdt trugt; (ii) phdt trien phuang phap mdi cho phep dy bdo sy thay ddi dp luc nudc Id rdng bing cdc thdng so dudi^
bien dang. Tinh chinh xdc cua phuang phap mdi
nay dugc kidm chiimg thdng qua so sdnh kit qud thi
nghidm vdi gid tri tinh toan cho cd trudng hgp don
phuong vd da phuong, (iii) giam thiiu ho§c tri^t
tieu dnh hudng ciia phuong cdt trugt ldn sir phdt
150
men dp lyc nudc 16 rdng trong qua trinh cdt trugt khdng thodt nudc.
2. Thf ngbi^m cat t n r g t d$ng dtm phinmg va da phtnmg
2.1. Thih bi thi nghiem
Hinh 4 la anh chup thidt hi thi nghidm cdt trugt dgng don gidn chu ky da phuong. Thdng qua hai phuang cat trugt vudng gdc vdi nhau, thidt bi ndy cho p h ^ tac dung Idn mau ddt (ddt trong hop cat) nhieu loai tai trong dgng khac nhau. Hinh 4a. b, c Id dnh chup mau dat trong hgp cdt d cdc giai doan
thi nghiem khac nhau. Hop cat trugt la hop cdt kidu Kjellman. trong dd mdu dat dugc bdo vd bang mang cao su. Ben ngodi mang cao su Id 15 din 16 vdng nhua xdp chdng len nhau. Mdi vdng nhua dudng kinh trong la 75,4mm, day 2mm. Bang each sip xdp nay, mdu ddt khdng bi biin dang ngang nhung vdn ddm bao bidn dang cdt trugt trong qua trinh thi nghiem. Be mat cua mdi vdng nhua dugc bdi bdt Silicate Magie nham giam thieu ma sdt giua cdc vdng cung nhu ddm bdo bidn dang phan bd ddng nhdt theo chidu cao ciia mdu trong qua trinh thi nghidm.
Hinh 4 (a) Mlu dat trong hgp e l l trufl'c khi co ket: (b) Mlu dat trong hop cat truoc khi thi nghi|m Ccil Inrcrt chu ky khong thoat nu'dc, (c) Miu dat sau khi kit thue thi nghiem
2.2. Mdu thi nghiem
V$t lidu su: dung trong nghidn ciru nay la dat set kaolin vdi mdt sd chi tidu co ly ca ban nhu sau; ty trgng G, = 2,83, dp am gidi hgn chay WL = 47,8%, dg dm gidl ban ddo Wp ^ 22,3% vd chi sd nen ep Cj = 0.305 De chuin bi mdu dit thi nghiem, bdt dit set kaolin khd dugc trdn vdi nudc cat ddn dd am khodng 80% (dudi dgng bim chay). Sau khi giu' cho dg dm khdng thay ddi trong 1 ngdy, huydn phii dugc hut khi trong hop chdn khdng khoang 30 phiit va sau dd dugc dd vao mang cao su da dat sdn trong hgp cit {hinh 4a) Sau do. ddt set kaolin dugc CO ket bing dp lyc thing dirng cr,,, = 49 kPa va 98 kPa trong thdi gian 40 phiit. Thdi gian cd kit nay dugc xac dinh theo kit qua quan trie dp lyc nudc lo rong ciia thi nghidm cd kdt lien hanh trong hgp cit. Sau khi cd kdt, mau thi nghiem cd dudng kinh Id 75mm. chieu cao khodng 20mm vdi he sd
rdng ban ddu e„ = 1,11-1,19 Nham ddm bdo dg bao hda cho thi nghiem trong didu kidn khdng thoat nudc, cac mau dat phai d^t hd so dp luc Id rdng (S-value) B > 0,95 trudc khi thi nghiem cit trugt.
2.3. Cdc budc thi nghiem
Sau qua trinh cd kdt, mdu ddt se chju cat trugt don gian chu ky dan phuong va da phuong, khdng thoai nudc theo thdng sd dau vao gdm sd lugng chu ky. dd bidn dang truot va dd lech pha da dugc Igp trinh (hmh 5) Sau khi ket thiic cdt trugt, van thodt nudc duac md va dp lyc nudc Id rdng {U,i^„) tich luy trong qua trinh cat se duoc thoat, Dd liin vd dp luc nudc Id rdng trong mau ddt duoc theo doi theo ihdi gian la 60 phiit.
Tdt ca cac mau ddt dugc thi nghiem cat trugt don phuong vd da phuong (6 ^ 20°, 45^, 70° va
90°) trong diiu kidn khdng thodt nudc. Bien do dg biin dang thay ddi tir ; ' = 0,05% d i n / = 3,0% va sd lugng chu ky Id « = 10, 20, 50, 100 va 200. Biin dgng c i t trugt tdc dung len mdu d i t cd dang hinh sin vdi t i n s d / = 0,5 Hz. Hinh 6 thd hidn md hinh bidn dgng dat trung cua mdu trong diiu kien cat trugt dan phuong {hinh 6a) va da phuang vdi d = 90° {hinh 6b). Do biin dang trugt dugc xac djnh bdng ty sd giUa bien do bidn dang ngang ldn nhdt ^ v d i chidu cao ban ddu ciia mau.
Hinh 6 thi hidn kdt qud ghi lai sdng ciialuc cdt trugt chu ky vd hinh 7 Id quy dao ciia do bidn dang trugt trdn mat phang ndm ngang cho cac thi nghiem cdt truot chu ky dan phuong vd da phuang cd dd bidn dang y^ 1,0%. Trong mdi thi nghiem, didu kien cdt trugt dan phuang va da phuang cd do lech pha khde nhau dugc mdy tinh dieu khidn tir phuang X va F vudng gdc vdi nhau. Trong thi nghidm cdt trugt ddng chu ky dan phuang, bidn dgng trum tac dung ldn mdu ddt chi tu mdt phuang (ddi vdi nghien ciiu nay la phuang X, hinh 6a) ndn
quy dgo ciia do bien dang cat trugt tao ndn dudng t h i n g {hinh 7). Trong cdc thi nghiem c i t tnrgt dgnjl chu ky da phuang, bidn dang trugt tdc dung dong thdi len rodu d i t tii phuong A'(;^) vd phuong Y{yii vudng gdc vdi nhau. B i i n dang trugt ciia hai phuang nay cd bien dd bien dang bang nhau {y^ 1,0%) nhung cd dp lech pha khac nhau {hinh 6b), do do hinh dgng ciia dg bien dang thay doi tii dudng elip (0 = 20°) ddn dudng tron {6 = 90°) {hinh 7). Anh hudng cua phuong cdt trugt va dg lech pha len hinh dang cua dudng bien dang the hidn ro trong hinh 7 ndn nd cd vai trd quan trgng, dnh hudng d i n tinh chat ddng hpc ciia dat.
£>x
'T^'
Sonphuong Daphirong(ff=!l(y) Hinh 5. Mo hinh mlu ddt bi cit tnjgt
chu ky dan phuong (a) va da phi/ong vdi 0= 90° (b)
DD
3-0
Thdi gian (giay) (c) D a p h u o n g ((9=45°) Kaolin; -
AAi
= I 0% DotiphiTdEig
A A . A A A A
.\]\j\l\l\i\J\l\]\J\o
— H i t f o t i g X Thdd gjan (giay) (a) D o n p h u o n g Kaolin:' = 1.0%
[ \li \l W-* M
— Phiftfagjr
9 - 4 5 "
W> U vl- W \li \ 0
phtfcftigy
Kaolin; 7 = 1.0%; 9 =20^^
Phtfgng.;^: P h t f d h g r Thoi gian ( g i a y ) (b) D a phtSrtiig ( 5 ^ 2 0 ° )
— P h i K m g X P h u D t a g r T h d i gian (giay) ( d ) D a p h i i o n g ( ^ = 7 0 ' ' )
-VhvicfngX Thuftfngy Thdi gjan ( ^ i y ) ( e ) D a p h u c f t i g ( 5 = 9 0 0 )
6. Kdt qua ghi lai song cit tni'gt chu ky trong Ihi nghidm d t trugt dgng chu ky don phuang va da phuong voi dfl bien dang ;-= 1,0%
3. Ket q u i vd thao lu^n
3.1. Ap Iffc nu&c Id rdng Id hdm sd cda iK biin dang (yi vd sS lu^g chu ky (n) - phuong phdp tmh todn truyen thong
3.1.1. Su thay ddi cua dp luc nu&c Id rdng trong qud trinh cdt trugi ddng chu ky khong thodt nuac
Dudi tac dgng ciia tdi trgng ddng chu ky trong didu ki?n khdng thoat nudc, dp luc nudc Id rdng (Udyn) trong dat se tang len ciing vdi qud trinh gia tdi (hay tang ldn theo sd lugng chu ky), Ket qua ghi Igi su thay ddi cua dp lyc nudc Id rdng trong cdc thi nghiem cat trugt ddng chu ky don phuang va da phucmg khdng thoat nudc cho do bien dang y= 0 , 1 % , 0,4% vd 2,0% dugc ve trong hinh 8. Kdt qud thi nghiem cho thay ap lyc nudc Id rdng trong mau ddt tdng theo sd lugng chu ky va tai ciing sd lugng chu ky, do bidn dgng Idn hgn sd cho kdt qud
dp lyc nudc Id rdng cao hon. Ngoai ra, tai cimg mgt gid trj do bien dang, dp lyc nudc Id rdng trong didu kidn tdi trgng ddng da phuang ( ^ = 2 0 ° , 45°, 70° and 90°) cao hon rat nhieu so vdi gia tri trong thi n ^ i e m cat trugt ddng don phuang; vd ddi vdi cdc thi nghiem dgng da phuong, ap lyc nudc Id rdng tang theo gia in cua do lech pha. Ket qud tuong ty cd the thdy trong hinh 9 va 10; trong hai hinh nay he sd dp luc nudc Id rdng, dmh nghTa bang UdyJa\.Q vdi cr'^j la irng suat hiiu hidu, dugc ve vdi sd lugng chu ky (hinh 9) vd do lech pha (hinh 10).
Tir cdc ket qua thi nghiem vua ndu cd thd thay rang phuang cat trugt, bien do do bien dang vd sd lugng chu ky la nhflng thong sd quan trgng dnh hudng ddn su hinh thanh va phdt tridn ciia dp lyc nudc Id rdng trong ddt dinh chiu tai trpng ddng chu ky dan phuong va da phuang trong dieu kien khdng thodt nudc.
Kaolin
•!-10'i
r
yjrphKcmg
••JUJU
ir'^'&fc ._, a-f-
"--'"'
!±™,i
h
-.•^^:^[r-if'
CJ C'S
7 06
^ ^ 1
-
« 02
"
..,W-
\jl fh- •/^M/
wm
^ r i-n
^
T r o i ' i J _ Dm
1'..J5
W"'
Bi^ndangphucmgjr /;i-C'o)
Hinh 7 QQy dgo cua aufrng bi^n dgng trLrpI tr§n mat phang n i m
ngang trong thi nghiem c i t trugt dpng chu ky don phuwng va da ptiiKyng vdi do bi^n dgng / = 1.0%
Hinh 8 Ket qua the hi^n SU" thay doi cua tip lu'C n u o c 10 rflng (L/K,,,) du'oc ghi lai trong thi nghiem c i t t r u o l dpng chu ky d o n phu-ong va da phuong voi y = 0 , 1 % , 0,4% va 2.0%
Hinh 9, Quan he giua he s6 Sp lyc nu'O'c 16 rong {Ua~,Jo\a\ voi s6 iLj-gng
chu ky (n) trong cac thi nghidm c i t tnyat dpng chu ky d o n phipang vS fla phifong vol •/= 0 , 1 % , 0,4% va 2,0%>
fc^^^^^^^fv" r " \ ^ n i J
[ / ''"""
j A /= ] o°.b
i—--!—
1 Kaolin I f f , , , - 49kPa r n= 10 ,
V= i w i
— . ^ - ^ 1
• - 0 4 o ,/=0 05° =
. / - 2 0%
. .' = 005°^
~ ^ ^ g b n , b
cm piiuoiig -Q^ i^j-ij pij3 Q
( a ) n = 10 (b)fj = 200
Hinh 10, S y thay doi cua h? s6 Sp lyc nufl'c Ifl rong vfl'i dp i f c h pha (0) cho cac gia tn dp bien dang va so l u g n g chu ky khac nhau
3.1.2. Cdng thitc tinh todn
Nam 1984, Ohara va cdng su xac dinh r&ig khi ddt set kaolin cd kdt binh thudng (OCR = 1) chiu cdt trugt dpng chu ky trong dieu kien khdng thoat nudc thi dp lyc nudc Id rdng se tang theo sd lugng chu ky (M). Cdc tac gid sau dd dd xdy dyng cdng thiic tinh todn thd hidn mdi quan he giira he sd dp lyc nudc Id rdng vd sd lupng chu ky bang hdm hypecbon nhu sau:
dyn
a-^-jinTrong dd a va p la tham sd thi nghidm phu thudc vdo dd bidn dang (y) va duac bieu dien:
a=A{yr (2)
^Cy (3)
Hdng sd thi nghidm A, B, C va m trong cdng thirc (2) va (3) cd thd xac djnh thyc nghiem bang phuong phdp "curve-fitting". Chi tidt ap dung phuang phap ndy dd xdc dmh gia tri cho A, B, C va m cd thd tim thay trong cdc cdng trinh nghien ciiu trudc day [10, 11].
Nam 1988, Ohara va Matsuda [11] phat tridn cdng thirc (I) dd dp dung tren dat set kaolin qud cd kdt (OCR = 2, 4 vd 6) chiu tdc dung tai trpng ddng chu ky don phuang trong didu kien khdng thodt nitdc. Ban phdt trien mdi ciia cdng thiic nay cd phd dp dung rat rdng ddi vdi do bidn dang (y= 0,1%
ddn 3,0%) cung nhu dugc sii dung trong tinh todn do lim cho cac Idp dat dinh do tai trgng ddng chu ky don phuong gdy ra
3 1.3. Tinh todn he so dp lire nuac 16 rong Trong hinh 11, he so dp lyc nudc Id rdng dugc ve theo sd lugng chu ky cho dat set kaolin cd ket binh thudng chiu tai trgng ddng chu ky don phuang va da phuong vdi do bien dgng y^ 0,1%, 0,4% va 2,0% Ky hidu trong hinh nay Id kdt qua thi nghiem va dudng cong net dut va net lidn thd hidn kdt qua tinh todn bdng cdng thirc (1) tuong ling cho trudng hgp don phuang vd da phuang, Dd dang thay rang ket qua tinh todn phii hgp vdi kdt qua thi nghiem
KaoliiKo*ri, = 49kPa
S6 l i n ^ g chu If^ n Hinh 1 1 . He so ap lyc n u o c lo rflng ve theo s6 lugng chu ky cho cac thi nghiem cat tni-gt chu ky d o n phuang va da phifong {&= 20°, 45°, 70° and 90°) vo'i ?-= 0.1%,
0,4% va 2,0%
Quan he gifla he sd dp luc nudc Id rdng vd logarit do bidn dang dugc thd hidn trong hinh 12 vd 13 cho cdc didu kidn phuong cat trugt va sd lugng chu ky khac nhau. Ki hieu trong hai hinh nay thd hien kdt qud thi nghiem trong khi gia tri tinh todn theo cdng thiic (1) dugc thd hidn bang dudng cong net lidn (da phuong) vd net diit (don phuang). Tir kdt qua trong 2 hinh tren cho thay dp bidn dang vd sd lugng chu ky cang ldn thi he sd dp luc nudc Id rdng cang cao va tai cdng do bidn dgng vd sd lugng chu ky, he sd dp lyc nudc Id rdng trong thi nghidm da phuang Idn ban gia trj trong dieu kidn don phuang. Ddi vdi cac thi nghiem da phuang thi he sd dp lyc nudc Id rdng tang theo do lech pha.
Ngoai ra, mdt sd ket qua thi nghiem cat trugt ddng chu ky da phuang dudi ap lyc thang dirng 98kPa cung dugc thd hien bang ki hieu trong hinh 12a, b.
Cd the thdy rang sy chenh lech he sd ap lyc nudc Id rdng giua hai gid tri dp luc thang diing (49kPa va 98kPa) la khdng dang ke, Ket qua nay phii hop vdi ket qua thu dugc trudc day cho trudng hgp tdi trpng ddng chu ky don phuang [8, 11]. Do do cd thd ket luan rang, trong gidi ban tai trgng thdng dirng tir 49kPa den 98kPa thi anh hudng ciia dp luc thang dung len sy thay doi ciia dp lyc nudc Id rdng Id khdng dang ke va do do, kdt qud tinh toan trong hinh 12 vd 13 va cdng thirc (1) cd the dp dung vao cdc trudng hop tai trpng thdng diing khac nhau, it nhdt trong pham vi tir 49kPa den 98kPa
Tuy nhien, so sdnh chi tidt han ket qua thi
nghiem va ket qua tinh toan trong hinh I2a, b cho
thdy khi n < 50 thi ket qua thi nghiem va tinh todn
phii hgp vdi nhau cho ca tnrdng hgp dan phucmg vd da phucmg. Nhung khi n > 50, kdt qua tinh toan cho trudng hgip da phucmg (hinh 13b) khdng chinh xdc bdng kSt qua trong didu ki^n don phuong {hinh 13a). Vi vgy, khi tinh toan dp luc nudc Id rdng
• ^ 0.8
trong ddt dinb chiu tai trgng dgng chu ki da phuong thi cdng tbuc (1) chi cd thd ap dung cho trudng hgp n < 50. Khi n > 50, cdn thidt phdi phdt tridn cdng tbuc (1) hoac de xuat phucmg phap tinh toan mdi phii hgp hon.
1>
• e = 9tf'((T',fl=49kPa) oe=70"
oe=45o
• e=2o«
cDonphuong "
H e = 90"(G'rti=98kPa)
—Tmh loan ^pfauong) '- -Tinh loin ^im phuong)
v*,®° / /
tw ''"
11
A.'i .t_J^
Kaolin n ^ l O
Kaolin
n = 200 , i
7
ff J k
1 j
V
6• e = 90"(o'rf,=49kPa)
•.6=70"
'>e=45«
oE)oii phucmg "
H6 = 900(tj'rt=98kPa)
—Tinh lo an ^ phinmg) - -Tmh loan (Bun phuimg)
0.1 1 10 0.01 0.1
Dpbien dsngtnrpt, ;•(%) ^h\ea dangtnrgL y(%)
(a)n=10 (b)n=200 Cra h$ s6 dp lyc nuoc (fl rflng va dfl bien dang cho cac thi nghifm cat t r u ^ chu ky don phuang vS
da phu'ong v&i s6 lugng chu kJ kh^c nhau
0
0 n = 200
* n = 100 a n = 50
° n = 20
• n = 1 0 Tinh 1 0 ^
— ( D o n phirong)
.i
9'' §;•
' « ' , « Kaolin
•I'X c i ' „ - 4 9 k P . , « Ben phuimg
0 n - 2 0 0
»: n = 100
•0 n = 5 0
• 11 = 20
• n = 1 0
. _ J
™gl OQ
e M ^
B/ZM//
Iff
i/ji
gp#
Kaolin CT'^o=49kPa 9 ^ 9 0 "
Dp bien dang trugrt,;-(%) (a) Don phuang
Do bien d?ng tru(;t. /(%) (b) Da phuong (C= 90°) Hinh 13, Quan h0 giua h# s6 ap lyc nufl'c 16 rflng vol dg bien dang cho thi nghiem cat tnrat chu ky don phuong vd
da phuong (9 = 90°) vfl'i so lugng chu ky khac nhau 3.2. Ap IfTc nu&c Id rdng theo hdm so cua dp bien
dgng tich lHy (G*J -phirongphdp linh todn mdi 3.2.1. Dinh nghTa edc thdng sd dudng biin dgng
Fukutake vd Matsuoka xdy dung mdt md hinh ggi Id Md hinh Bowl dd ludn giai sy di chuydn hat ddt ciia ddt lo^i cat trong didu kidn cdt trum ddng chu ky da phucmg thodt nudc. Trong md hinh nay, dudng bien d^ng trugt tren m§t phdng ngang cd thd md td bdng hai thdng sd la dp bidn d^ng tdng hop - r vd dg bidn dang tich luy - G* [3]. Trong do, dd
bidn dang tdng hgp la khodng cdch hien tai cua hat ddt so vdi vi tri ban ddu, thdng sd nay lien quan ddn su dich chuydn cua hat ddt trong qud trinh cat trugt vd do bien dang tich liiy Id chidu ddi ciia dudng bien dang trugt trong qua trinh cat trugt, nen thdng sd nay lien quan ddn miic do phd hiiy cdu tnic ciia mau ddt {hinh 14). Hai thdng sd tren dugc md td bdng cdng thiic (4) va (5) nhu sau:
=ZAG =ZjAr^+Ar:,
(4)r=f>^^ (5)
Vdi Aft- vd A^f ddt trung cho phdn td tdng len ciia dd bien dang trugt lan l u 0 theo phucmg X
yy
Gy^ p^\
0 . - • •
r.
Hinh 14. Khai niem dfl bien dang tich luy (G*) va do bien dang t6ng hop (F) trong cdt tn^gt chu ky
da phuong (theo Fukutake va Matsuoka [14]) Quan hd giila dp bidn dang tich liiy (G*) vdi do
bidn dang trugt {f) cho sd lugng chu k)- khac nhau (n = 10, 20, 5 0 , 1 0 0 vd 200) ldn lugt dugc trinh bdy tren hinh 15a, b cho trudng hop dom phuang vd da phuang. Ki hieu trong hinh la kdt qua thi nghidm va cdc dudng net lidn la gia tri tinh todn theo cdc cdng thirc sau:
- c a t trugt chu ky d a n phuang:
G * = « ( 3 . 9 5 0 / + 0 . 0 5 2 3 ) (6) - Cdt trugt chu ky da phuong:
G * - « ( 5 . 9 9 5 ; ' + 0 . 3 5 1 0 ) (7) Gia tri tinh toan theo cdng tbuc (6) va (7) hodn toan phii hgp vdi kdt qua thi nghiem. Tii hinh 15 cd thi k i t ludn rdng dp bidn dang tich luy Id ham sd ciia dd biln dang (y) vd sd lugng chu ky (n). Khi sd lugng chu ky biing nhau, G* tang theo ty le vdi y, vd khi so sdnh hinh 15a. b tai cung dp bien dang, G* trong trudng hgp da phucmg Idn hon trong trudng hgp don phuang va dp chdnh lech giiia chiing tang ldn theo sd lugng chu k>'.
3000 g.2500
3 J500
= 1000
— G* = n (3-950'/ + 0,0523) . Kaolin
c',o=49kPa IXra phinmg
^ n = 200
^ 1 1 = 1 0 0
^ n = 5 0
^ " = ^ ° ^ n = 1 0 2500 2000 1500 1000 500
— G ' = n (5.995 Kaolin ff,fl = 49kPa Da phucmg . 9 = 901)
, = 9 = 70" J ' 6 = 45" j r , ' 6 = 20" 9^
+ 0.3510)
^ n
/ "
, n
-^°
- 2 0 0
100
= 50 - 2 0 10
Dp bien dang truot. y {"'") (a) cdt trugt chu ky dan phuong
Dobiendangtnrot. >"(%) (b) cat t n j ^ chu ky da phuong Hinh 15, Quan he giCra dfl bign d ig trugt tich luy (G') vfl'i do bien dang (y) va so l u g n g chu ky (ri) trong cat fruot
chu ky don phL^ong (a) va da p h u a n g (b) 3.2.2. Quan he giira he so dp hec me&c 16 rong
(Udyjiy'vo) v&i do bien dgng tich ISy (G*) Nham thd hidn tinh uu vidt cua dp bien dang tich lily (G*) trong md td sy thay ddi dp lyc nudc Id rdng, quan he giua he sd dp iyc nudc Id rdng vd G* dugc trinh bay tren hinh 16 cho sd lugng chu ky w = 50 (sd lieu tuong ty trong hinh 9). Hai dudng net diit vd net lidn lan lugt la gia tn ctia Udyjar'vi) sau 50 chu ky cho trudng hgp dan phuang va da phuong. Co thi thdy rdng, tai ciing gid trj dp bidn dang vd sd lugng chu ky, dp bidn
dang tich luy trong thi nghidm da phuong ldn hon trong don phuang vd ddi vdi cat trugt chu ky da phuong cd dg lech pha khac nhau thi thdng sd nay tang theo dp lech pha, va G* cang Idn thi h? sd dp lyc nudc Id rdng cang cao. Nhu da de cap trudc day, dp lyc nudc Id rdng phu thugc rdt ldn vao phuong cat trugt (giira dan phuang va da phuang) nen su khde nhau ve he sd dp lyc nudc Id rdng giiia thi nghidm da phuong va don phuang la rdt rd rang (hinh 12). Tuy nhien, khi so sdnh hai dudng nit lidn vd net diit trong hinh 16 thay chiing gdn nhu trung nhau va didu nay cho thay sy khac nhau ve 156
h? s6 ap Iyc nudc l6 rong giiia thi nghiem dem phuong va_,da phucmg (hay dnh hudng ciia phucmg
DO bien dang tich lily, G* (%) Hinh 16. Quan h0 giO'a h$ s6 Sp luc nudc 15 rflng va do bien
dgng tich lOy trong thi nghidm cat trugl chu ky iJon phu'O'ng vS da phuong thyc hi#n cho r= 0,1%, 0,4% vS 2,0%
Cdt trugt len kp luc nudc 16 rdng) cd thd dugc tridt tieu khi su dung do bidn dang tich liiy G*.
Kaolin 8=900 a',o=4<»kPa e = 7 f f = \ . n = 5 0 \ ,
9=45" \ j j 9=20" V/fT
—Dudng noi ^ a phuoiig)\^Cn
—©irong noi (Don phatm^^fa j^tg[l=23X'
^ 6 - 7 C f t j P - 6 = 4 5 "
- D o n phuong /e=90<' / p a = 7 0 "
Jr9-45'' ,
— e=2o° }7=ai'/.
on phuong
Tioliioj, TklltOSI
• e=90"
D e - 7 0 "
• ' 6 = 45"
nfftt phuong)
^mplimTl^ _ / i
• 6 = 2 0 "
^ Don phuong
, -^snr.'.
•jj
Jp ^
fc * 2
f
^ n = 200
ff^n=50
^ n = 1 0 Kaolin o',^,= 49kPa y = a 0 5 % - 3 . 0 % 1 10 100 1000
Do bien dang tich luy, G* ("/o) Hinh 17, Quan he gilja tie sfl ap luc nucic Ifl rflng va logant
cua dg bi^n dang tich lOy cho cac di^u ki?n phuong cat truot, dfl bi^n dang va s6 tugng chu ky khac nhau 3.2.3. H4 so dp luc nudc 16 rong Id hdm so ciia do
biin dgng tich liiy G*
Nam 2011, Matsuda ciing cpng sy da de xuat phuang phap mdi tinh toan su thay ddi ciia irng suat h&u hieu trong ddt loai cat dudi tac ddng ciia tai trpng chu ky da phuong thdng qua sii dung do bien d^ng tich luy (G*) nhu sau:
Acr„
a + pG*
(8)trong do ACT\. la dd suy gidm ciia iing sudt hfru hi^u
Ddi vdi didu kidn khdng thoat nudc thi do suy giam ciia irng suat httu hieu can bang vdi gia tn tdng Idn ciia dp iyc nudc Id rdng, nghia la Ua^„ = I ^cr'^l. Do dd, khi iing dung cho thi nghidm cat trugt chu ky khdng thodt nudc thi cdng thiic (8) trd thanh:
v.. G*
H4 sd a vd /? trong cdng thiic (8) va (9) tuong ty nhu trong cdng thiic {1).
Quan h? giiia he so dp lyc nudc id rdng va logarit dg bidn dang tich luy dugc the hidn trong hinh 17 cho cac didu kien phuang cat trugt, dd bien d^ng va sd lugng chu ky khac nhau. Ky hidu
trong hinh Id ket qud thi nghiem vd dudng net lien vd net dirt tuong img vdi gid tri tinh todn b3ng cdng thue (9) cho trudng hgp don phuong va da phuong, Nhin chung, gid tri tinh todn phii hgp vdi kdt qua thi nghidm cho tdt ca cac dieu kidn thi nghiem khac nhau, Ngoai ra cd the thay trong hinh 17, dp bien dang tich lijy C* tang theo dg bien dang trupt va sd lugng chu ky, va dp bien dang tich luy cang Idn thi he sd dp luc nudc Id rdng cang cao.
Trong hinh 12a. b, tuong iing vdi mdi sd lugng chu ky, he sd dp lyc nudc Id rdng giiia thi nghiem da phuang va don phuong chdnh lech nhau rdt Idn trong gidi ban dp bien dang tir 0,05"/o den 3,0%.
Tuy nhien, khi sir dung dd bien dang tich luy G*, thi sy chdnh lech nay bi bidn mdt, ciing nhu thdng qua ket qua thi nghidm va gia tri tinh todn trong hinh 17. Do do. sy chdnh lech he sd dp lyc nudc lo rdng giiia cat trugt chu ky don phuong vd da phuang hay anh hudng ciia phuang cat trugt len sy thay ddi dp lyc nudc Id rdng cd the bj triet tieu khi sii dung hdm sd ciia do bidn dang tich luy.
4. Ket Luan
Ket qua ghi tai tir may tinh ve sy thay ddi ciia ap lyc nudc Id rdng trong qud trinh cdt trugt khdng thodt nudc va quan he giiJa he sd dp luc nudc Id rdng vdi do lech pha va dp bien dang cho thay rdng phuang cat rrugt, bidn dd bidn dang trugt vd sd luong chu ky cd dnh hudng rat ldn den sy phdt
trien ciia dp luc nudc Id rdng trong qua trinh cat trugt ddng chu ky khdng thodt nudc. Do bien dang vd sd lugng chu ky cdng ldn thi he sd dp luc nudc Id rdng cang cao. Tai cung gia tri dp bidn dang vd sd lugng chu ky, dp Iuc nudc Id rdng do cat tnrgt da phuong gdy ra Idn ban rat nhieu trong thi nghidm don phuong. Ddi vdi cdc thi nghidm da phuang thi dp luc nudc Id rdng tang theo dp lech pha.
Phuang phap tinh toan ap lyc nudc Id rdng truydn thdng do Ohara va Matsuda (1984) de xuat (cho didu kien cat trugt don phuong) cd the dp dung cho dieu kidn cat trugt don phucmg va da phucmg khi « < 50. Khi n > 50 gid tri tinh toan trong dieu kidn da phuong khdng chinh xac nen can thiet phdi phat trien phuong phdp cii hoac de xudt phuong phap mdi cho phep tinh todn he sd dp lyc nudc Id rdng trong dieu kien cat trugt chu ky da phuong vdi do bidn dang va sd lugng chu ky khac nhau,
Nhdm kiem tra anh hudng cua tdi trgng thang diing Idn tinh chat dpng hge cua dat, mdt sd thi nghiem cat truot ddng chu ky da phuong {$ = 90°) duoc tidn hdnh trong didu kidn tai trgng thdng diing CTyo = 98 kPa, Ket qua thi nghiem cho thdy su khac nhau ciia tai trpng thang ddng, tdi thilu giiia 49kPa va 98kPa, khdng anh hudng nhidu ddn su phdt tridn ciia dp luc nudc 16 rdng trong didu kien cdt trugt chu ky da phuang. Ket qua nay phu hgp vdi kdt ludn trudc day cho dieu kien cdt truot chu ky don phuong
Thdng qua sii dung thdng sd bidn dang mdi la do bien dang tich liiy G*, chiing tdi da phat triln vd dp dung thanh cdng phuang phap mdi cho phep tinh todn he sd dp lyc nudc Id rdng trong cdc didu kien thi nghiem khac nhau: don phuong, da phuong cd dp lech pha khac nhau, bien dp biln d^ng vd sd lugng chu ky khac nhau. Ddng thdi, bang viec su dung thdng sd mdi nay, chiing ta cd the tridt tieu dnh hudng cua phuang cdt trugt len sy thay ddi dp lyc nudc Id rdng trong qua trinh cdt trugt khdng thoat nudc.
TAI LIEU DAN
[1] Ansal. A.. lyisan, R. and Yildirim. H., 2001; "The cyclic behavior of soils and effects of geotechnical factors in microzonation", Soil Dynamics and Earthquake Engineering, Vol 21 No. 5, pp. 445-452.
[2] DeGroot, D. J., Ladd, C. C. and Germaine, J. T., 1996: Undrained multiduectional direct simple shear behavior of cohesive soil, J. Geotechnical Eng., ASCE, Vol. 122, No. 2, pp.91-98.
[3] Fukutake. K. and Matsuoka, H. A.. 1989:
"Unified law for dilatancy under multi-directional simple shearing". Journal of JSCE Division C.
J S C E , V o l . (412/III-l),pp. 143-151 (tidng Nhat) [4] Hyodo, M., Yamamoto. Y. and Sugiyama, M., 1994: "Undrained cyclic shear behavioiBi of normally consolidated clay subjected to initial static shear stress". Soils and Foundations, Vol. 34, No. 4, pp. 1-11.
[5] Matsuda, K, 1997: "Estimation of post- earthquake settlement-time relations of clay layers". Journal of JSCE Division C, JSCE, Vol.
568(111-39), pp. 41-48 (tidng Nhdt).
[6] Matsuda, H. and Ohara. S., 1989:
"Threshold strain of clay for pore pressure buildup", Proc. of 12" Worid Conf on Soil Mechanics and Foundation Engineering, Rio De Janeiro, pp. 127-130.
[7] Matsuda. H and Nagira. H. 2000:
"Decrease in effective stress and reconsolidation of saturated clay induced by cyclic shear", Joumal of JSCE Division C, JSCE, Vol. 659(111-52), pp, 63- 7 5 ( t i l n g N h a t ) .
[8] Matsuda. H., Shinozaki, H. Okada. N.
Takamiya, K. and Shinyama, K, 2004: "Effects of multi-directional cyclic shear on the post- earthquake settlement of ground", Proc. of 13'"
World Conf on Earthquake Engineering, Vancouver, B,C., Canada, Paper No. 2890.
[9] Matsuda. H.. Andre. P. H, Ishikura, R.
and Kawahara, S., 2011: "Effective stress change and post-earthquake settlement properties of granular materials subjected to multi-directional cyclic simple shear". Soils and Foundations, Vol.
5 1 , N o . 5, pp. 873-884.
[\Qi\ Ohara. S. Matsuda. H. and Kondo. Y..
1984: "Cyclic simple shear tests on saturated clay with drainage", Joumal of JSCE Division C, JSCE, Vol. (352/II1-2), pp. 149-158 (tidng Nhat).
[\\]Ohara, S. and Matsuda. H, 1988: "Study on the settlement of saturated clay layer induced by cyclic shear". Soils and Foundations, Vol. 28, No.
3,pp, 103-113.
[\2]0hmachi, t'., Kawamura. M. Yasuda, S..
Mimura, C andNakamura, Y., 1988: "Damage due after cyclic loading", Soils and Foundations, Vol.
to the 1985 Mexico Earthquake and the ground 31, No. 1, pp. 83-94.
conditions". Soils and Foundations, Vol. 28, No. 3, r,.-, J^-,J- • rr J r- r, onm , - Q , ^ g [l4]Yildinm, H. and Ersan, H., 2007:
"Settlements under consecutive series of cyclic [13] Yasuhara, K. and Andersen, K. H. 1991: loading". Soil Dynamics and Earthquake
"Recompression of normally consolidated clay Engineering, Vol. 27, No. 6, pp. 577-585.
Study on excess pore water pressure changes of saturated soft clay subjected to undrained uni-directional and multi-directional cyclic simple shears
In this paper, normally consolidated specimens of kaolinite clay were tested under undrained uni-directional and multi-directional cyclic simple shears. It is s h o w i from the test results that shear strain amplitude ( ^ , cyclic shear direction (phase difference (0)) and the number of cycles (n) have significant effects on the changes of excess pore water pressure during cyclic shearing. However, the effect of cyclic shear direction on the excess pore water pressure can be eliminated by using a new parameter - cumulative shear strain (G*). Conventional estimation method of excess pore water pressure can not be used for the case of multi-directional cyclic shear with the number of cycles larger than 50 (n > 50). Consequently, a new estimabon method, which is a function of cumulative shear strain was proposed This method can be applied to various experimental conditions including uni-directional and multi-directional cyclic simples shear with different shear strain amplitudes and number of cycles.
