Danh gia on djnh cua tru dat tron xi mang gia co nen dat du'O'i nen du'O'ng c6 xet den pha hoai tru do bj uon Stability evaluation of embankment on soft soil foundation improved by dcm columns in v\/hich the bending failure mode of dcm column is concerned with
Ngay nhan bai: 15/9/2014 Ngay siTa bai: 5/11/2014 Ngay chap nhan dang: 10/12/2014
T 6 M TAT:
Qui trinh tinh toan danh gia on dinh cua tru d3t trdn xi mang dildi nen dUdng hien nay chi chu yeu m6i xem xet den sii pha hoai ciia tru do cdt gay ra thong qua he so on dinh trif^ tru tron tinh theo phuong phap can bang gidi han. Tuy nhien ket qua thi nghiem ly tam cua Kltazume va Inagaki chi ra ring pha hoai cua tru dat xi mang co the do uon gay ra. Vi the, van de danh gia on dinh cua nen dUcfng tren nen dat dUdc gia cUcJng bang tru dat xi mang co xet den tinh pha hoai do uon cua tru duoc dat ra mang tinh cap thiSt. Ngi dung chinh ciia b i i bao nay la phan tich ly thuydt k^t h^p vdi viec danh gia on djnh n^n dUcfng bang phiidng phap can bang gidi han nham xac dinh dang pha hoai cua tru dat xi mang (co the do uon hay do cat). Tif do viec danh gia chinh xac he so an toan on dinh co xet den cac dang pha hoai khac nhau se du^c xac dinh day du.
TH khoa: tru dat Iron xi mang, nen diidng, pha hoai do uon, dat yeu
ABSTRACT
Curently, calculation process to evaluate the stability of embankment on soft soil foundation improved by deep soil cement columns (DCM) only essentially determined by limit equilibrium method in which the DCM columns will be failed du to shear mode. However, centrifuge tests performed by Kitazume and Inagaki shows that the DCM columns under embankment is likely to be failed due to bending or tilting. Therefore, stability evaluation of embankment on soft soil foundation improved by DCM columns in which the bending failure mode of DCM coliunn is concerned with is very urgent. The main content of this paper is to analyze the theory combined with limit equilibrium method in order to determine failure mode of DCM column (maybe due to ending or shear failure). And then, exact evaluation of safety factor will be estimated in accurate manner.
Keywords: DCM column, embankment, bending failure mode, soft soil
TS. Nguyen Minh Tam
Giang vien, Khoa Ky Thuat Xay Diing, Tnidng Dai Hoc Bach Khoa - Dai Hpc Qudc Gia Tp.HCM
Email: [email protected] Dien thoai: 0903863348
Nguyen Minh Tam
d o chuyen w 6 t o Idcfi tdi 1. Gidl t h i e u
(5n d m h cCia nen diicmg tren dit 6uac gia ciidng bang t r u dat tr6n xi mang c6 the duoc phan tich bSng phuong phap tinh toan sfl nhu phuong phap phan td hCTu han (FEM) hoac bang phuong ph^p c3n bhng gidl han. Phuong phap phan tich sfi doi hoi nhu'ng t h d n g so vat lieu h < ^ 1^ n h u duoc xac dinh bang nhdng tht nghi&m d hien trUdng vk thi nghl€m trong phong hoac la tren cac t r u dUoc khai quat d hien trudng vh tren dat khong dn ^ n h . M o t p h u o n g phap dUoc xem la d o n gian hon 1^
phuong phap can bang gidi han. Trong phan tich theo phuong ph^p nky thi fln d m h cUa nen dudng duoc phan tich bSng gia thuyet rang phci hoai cua tru va cOa dat giUa t r u xSy ra doc theo mat trUcft cung tron. Trong viec phan tich cung truot tron, cudng d d c3t hdn hop can c d vao cudng dp cSt cua dat, cUdng d o
c3t cCia tru va t^ so dien ^ tich thay the dUoc sU I
dung cho tinh toan.
Tuy nhien, phuOng phdp nciy chUa xem xet den pha hoal cua tru d o momen ma theo ket q u ^ t h i nghidm ly tam cOa Kltazume et al (2000) va Inagaki et al (2002) da chi ra rang p h ^ hoai cCi cac tru dat trdn XI mSng chu yeu la do uon. Hinh 1 la so do t h i nghiem ly tSm cila Kitazumi (Kitazumi ' et al, 2000) sd dung tai trong nghieng dat tr^n nen dat set y^u duoc gia cudng vdl tru dat tron xl mang va hlnh 2 md ti kit quci cua t h i nghidm ly tam, da chi ra phd hoai d o uon va L do XD I f c h cOa tru.
CDIT (2002) cung da dUa ra canh bao rSng pha hoal cua tru DCM cd the xay ra hien tuong n d t gay va sup I do udn vi xo lech va fln dinh bin trong cilia tru DCM cung nSn dUOc
Tdp cdT i n o d t nuac ~
don VI ghi bdn vS cm ghiemlyt3m(Kilazuine. .2000)
H'mh2 Pha hoaixB lech tru vau6n tru (Kitai
iiOBUfEHBlV 4.2015
xem xet t r o n g thiet k^. Bdi viy vide dinh gia pha hoal cCia t r u DCM ben dUdi nen dUdng dUdi nhieu di^u kiin khic nhau, nhU \i, do ufln, do cSt, nen... Ia mot viec lam can t h i & trong thiet k l nen dUdng thUc te.
2. T h l ^ lap cAc c6ng thfifc ddnh gia do bin khdng c^t cua tru dat trdn ximang cd x ^ din phA hoai cua tru do bj u6n
Ong x d v^ sdc khang c i t cOa t r u xl mang mi duoc dat vSo trong vUng chu dfing vi trang v£ing cSt ciia mot mat trUOt gia dinh tal gia tri phd hoai ct!ia nen dudng duqc phan tich hin dudl. GiA thuyet rSng phd hoai cua dat khong 6n dinh giUa tru x3y ra doc theo be mat cCia mat trUot trong vCing chU ddng ma no nghidng mdt gfic a so vdi phuong ngang. SUc khdng cSt Idn nhSt cila tru doc theo mfit mat trUOt gia dinh bi chi phfii bdi dng xiS va kieu phd hoal cd the cilia tru. Kieu pha hoai cila t r u cfi thi theo mdt mat trU^t khdc khi nen dudng pha hoai. Nhdng kieu phd hoai khac nhau phu thuoc vao nhdng yeu to nhu Id tdi trong doc true t r o n g t r u va trong dat khfing on dinh, dudng kfnh cila tru, c h i l u ddl va khoang each cua t r u . Hinh 3 chi ra 3 dang phd hoai tru cd the xay ra.
l\:r 1 r ~ T,"
n.
u U
Dgng a Dqng b Dang c Hinh 3. Khung dang pha hoai cd the cua tin 2.1 Dgng phd hogi a
Khi dang pha hoal ndy xdy ra (hinh 4), mfit khdp ddn hoi phdt tnen ben dudl mat t r u q t d do momen uon Idn nhat vUdt qud Ichd nang chiu momen cCia t r u . Phdn cao hOn cua t r u se xoay quanh khfip ddn hfii khi n i n d u d n g pha hoai Kieu phd hoai nay xSy ra khi c h i l u sau cCia khfii dat truqt d tren mat t r u q t thi nhfi so vdl chilu ddl ciia tru. Gid thuyet t r o n g trUdng hqp nay rSng tru quay n h U m f i t xuong cdng quanh khdp ddn hdi va r i n g sU quay cila t r u la tUong dfii ldn.
Ifinh 4. Pha hoai dang a
Luc cdt Ta cd the t i m dUOc tU can bdng ciia nhCing phdn khdc nhau cUa t n j , xem hinh S. Cdng thdc t i n h todn c6 the t i m duoc tU cdn b3ng dif dinh theo hai phuong ngang vd t d can tiSng mornen vd tU dieu kien bien cho luc cSt vd momen Gid thuyet rSng khdp ddn hdi phdt t r i l n tai c h i l u sau (H,+a) b#n dudl dinh ciJa tru d d d luc cdt bdng.
Hinh5 SUphdnboaplucdatxungquanhtruDCMchodang pha hoai loai 3
Gia thuyet rdng dp luc dat xung quanh thi vudng gdc vdi try va gid thuyet he sfi kha ndng chiu tdi la k va cudng do cdt khdng thodt nudc c^ cua dat ben dUdi mdt trUcrt.
LUc cdt Ta cd t h i duoc danh gid t d mfit su can bdng d u d i n h cho phan cua t r u md duoc dat d dudl mat truqt gia dinh,
T^ = k c ^ d a ' (1) Trong do
k = he so khd nang chiu tdi;
c^ = cudng do cdt khdng thoat nUdc cua dat khfing on dinh;
d = d u d n g kinh t r u ;
a = khodng each giCfa mat phd hoai va khdp dan hoi.
Can bdng dU ^ n h cho phdn cila tru dUoc dat tren mat trUot
T , = k c , d b - k c ^ d ( H , - b ) (2) Trong do
H, = c h i l u dai cua t r u tren mat truqt;
b = c h i l u ddi, xem hlnh 5
Chieu dai b CO t h i duqc xac dinh t d cfing t h d c ( l ) v a ( 2 ) .
b = l ( a ^ H , ) (3, Cdn bdng m o men cho phdn cda try dUOc
dat ttin khdp dan hdi tai c h i l u sdu (H,+a) dUcJc bleu dien theo cdng thdc:
M „ + ( H , - b ) k c „ d [ b + ^ ^ | =
N l u b t u c6ng thUc (3) duac the vao cfing thdc (4), khd nang chju mfi men M^ cila tru cd the duoc trinh bdy theo cong thUc sau:
thU hai phdt t n l n nhU phd hoai dang b trong d d md men uon Idn nhat dat den kha ndng chju md men cila t r u (hlnh 6).
Cf dang pha hoai b, Iyc catT^ c d t h e t i m dUac tis can bang cUa nhUng phan khdc nhau cua tru vd tiy dieu kien bien vd'i luu y lUc cdt va mo men n h u chi ra d hinh 7. Cdng thdc tinh todn c6 the t i m duoc tU mdt cin bdng d y ^ n h theo phUOng ngang vd m o t can bdng momen.
Can bang dU dinh cho phdn cOa tru dUoc ddt tren mat trUcrt rut ra cfing thdc:
T^_ = kc^da (7)
u J i " ' H'inh 6 Pha hoai dang b
k c , d f 3 , ^ H p M„=—**— -a^+H,a—!-
" 2 U • 2J (5) LUc cdt cd quan he vdi khd nang chiu mo men M^ bdi cdng thUc (1) va (5). B i n g each thay t h i c h i l u ddl trong cdng thdc (1) vdo cong thUc (5), t a c d cdng thdc:
J U f H , ? 4 M„ i f H , (6) 2.2 Dgng phd hogi b
Khi c h i l u sdu cila khfii dat trUcrt gia tang thi mfi men ufln Icm nhat trong tru tren mat phd hoai cung gia tdng Khi m d men ndy dat den khd ndng chju m o men cila try mot khdp dan hfii
S! J i
T=0 Mu Hinh 7. Slf phin boap lUc dat xung quanh tiu DCM cho dang ph^ hoai loai b
Bi phdn tich dOn gidn, gid t h u y i t rdng M^
khfing doi doc theo cac phan da xem x^t cua tru V\ic trinh bay ndy cfi the nhan duqc tU khd nang chiu m d men M^^ tai can bang cdc phan cua tru ma dUoc ddt gida 2 khdp dan hoi.
• ^ " ^ ^ ^ _ (8) Khodng cdch giOa khdp dan hoi vd mat trUOt CO t h i duoc xdc dinh theo cdng thdc sau
a =
l ^ (9)
\ ! k c „ d
LUc cdtT^ dua theo mat pha hoai cd mfli lign quan vdi kha ndng chju momen M^ cCia t r u bdng each t h i cfing thUc (9) vao trong cfing thdc (7), LUc cdt Tj^ cd t h i duqc danh gia theo cfing thdc:
T . = V 2 M . k c . d „ „ , Co mfit vdi sU khdng c h i c chdn v l dfi Ifln
cua h^ sfl khd ndng chiu tdi k vd cila sU phdn bd cUa dp luc dat xung quanh dqc theo tru. Do dd, v l ^ t h i nghiem la can t h i l t dexac dinh gid trj h ^ sfi khd ndng chiu tdi k
2.3 Dgngphdhogic
Hlnh 8 bieu d i l n dang phd hoal c d dfi dang pha hoai ndy duqc ddc trUng bdng bien dang ciia mdt khop dan hfii d ben trfen mdt mdt trUot
6 do momen uon Idn nhat vUOt qud kha nang chju momen cila tru. Tai gia tri phd hoai cua nen dudng, phan thap hon cua try quay quanh k h i ^ dan hoL K i l u pha hoai cfi the khi c h i l u sdu cua mat t r u o t gia djnh la Idn so vdi c h i l u ddi cua tru.
Su phan bo cua ap l U c d a t x u n g quanh, cila lUc cat, va cUa mfi men uon doc try.
Cfing thdc chi phfii cho kieu pha hoai nay cd the duqc rut ra t d can bangcua nhUng phan khac nhau cCia tru trong cung mfit cdch nhU pha hoai dang a Luc cat T^ doc theo mat trUqt gia dmh cd the dUoc danh gia t d h a l sU can bang dU djnh va t d mot sU cdn bdng mfi men va tU d i l u kien hiin vdt luu •} den cat va uon (hinh 9).
Sugidi thich dUdl day cho lUc catT_. dUoc rut ra t u can bang dU dinh cho phan cao hon cila tru d t r e n mat trUqt
T = k c d a (11)
bieu d i l n nhU m f i t ham sd cOa kha nang chiu mo men M^ cua tru. Bang each thay the a trong cfing thUc (11) vao cdng thdc (15) ta se cd tai chieu sdu H, = L-H,.
Hinh S. Phd hoai dang c
Hlnh 9.5if phan boap lUcdatxung quanh tru DCM dio dang phi hoai loai c
Can bang dU dmh cho phan thap han cua tru:
(12) chieu dai ciia try hin dUdi r, - k c „ d b - k c ^ d ( H 3 - b ) Trong dfl Hj mat truot;
Chieu dai b co the dUoc xac dinh theo cdng thUc sau
Mfii quan he sau duqc nit ra tU sU can bang momen cho phdn ciia tru ma dUoc dat ben dufli khdp dan hfii.
M.+(H,-b)kc.dCb+-yi^l.
Neu lay b trong cfing thUc (13) t h i vao cfing thdc (14), khi do kha nang chiu momen M^ cua try cfi the dUOc b i l u dien bdi cong thdc:
LUc cSt T^ cho k i l u phd hoal nay cd the duqc
- k c „ d '
UrL-H^Y 4 M, 9I d J '^3kc„d'
i f L -
3. So do khdi tinh toan xac dinh cac dang pha hoai cua try dait tron xi mang
Viec t i n h todn ddnh gid phd hoai ciia try dat trdn xi mang cho cfing trinh thuc te cfl the duqc t h y c hien theo s o d d n h u the hien d hinh 10.
4 . Ap dung tinh toan cdng trinh cu the 4.1 Md td cdng trinh
Mat cat ngang dai dien cCia nen dUdng tren nen dd't duoc x d ly bfli cdc try DCM dUqc the hien fl hlnh 11. Nen dUdng duqc dap bang cat cd do chat k=0.98, b l day Idp dap 6,0m. Le dudng ddp bdng dat cfi do chat k=0.9S, mdi ddc ta\uyVi Tain bo neiy dUdng dUflc dat trfen dat t u nhien c6 cdc ldp dia chat nhUsau: tren mat la Iflp bun set lan vat chat hdu co, mau xdm den trang thdi chay, c h i l u ddy trung binh cCia Idp Id 10,5m;
ldp 2 Id Idp set pha cdt mdu ndu vdng xdm trang, trang thdi d^o mem, be ddy trung binh cua Idp la 4.0m, Iflp 3 la Iflp set mau nau do, trang thdi deo cdng. Cdc chi tieu co ly nhU sau Iflp 1 cfl V=l 5.1 kN/m^ lUc dinh c=7 kPa, gfic noi ma sat <p - 3''57', mo dun tfing b i l n dang E^,=420 kPa; Idp 2 CO Y=l 9.6kN/m^ lUc dinh c = l 7kPa, gdc noi ma sdt tp=l 8''25', mfl dun tong bien dang E^^2370 kPa; ldp 3 co y=19.9 kN/m^ lUc dmh c=36 kPs, gdc nfii ma st (p=15''04', mo dun tfing bien dang E^j=3750 kPa; Idp cat d i p cfl y =17.0 kN/m^ lUc dinh c^l.O kPa. gfic noi ma sat (p=38°, mfi dun dan hoi E =35000 kPa.
1 ^ ' ! 1 U I | . H " iLiJ IMJ U I 11' L I
i;c
d m h cila nen d u d n g ndy theo pf J f n g phdp can bang gidi han, cac try dat trfin xi mdng dUOc xem n h u b i pha hoai do cat. SU phd hoal t r u co kha nang do bi ufin thi v i n chUa duqc de cap trcjng tinh todn ndy
— 1 fll)^™'' ^'° ^" ^"^ "^" ^''*'9 9'" "
I bgrig tru dot trpn xi mdng cd xet din phd hogi eda tru do uon.
LUc tac d y n g theo phUOng ddng len try tai vi t r i mat t r u q t gidi han la Q=60.462 kN va vl t h i dng suat phdp t u y i n tren t r u tai vi t r i mat trUqt gidi han la 120.72 kPa. CUdng d d nen mfit tryc khfing thodt nudc cila vat lieu tru la 150 kPa vd cUdng dfi cat khdng thoat nudc ciia ddt xung quanh t r u la 7kPa
Sdc khdng mfimen CLia try M^ ^ = 6.4000 kN.m
LUc cdt gidl han (tinh theo sdc khdng momen cua tru) tai vi tri mat t r u q t theo cac dang pha hoai
T^ = 68.7906 kN
\ = 25.3992 kN T - 1 1 0 . 1 8 2 4 kN
iJng sudt cat gifli han (theo sdc khang momen cua try) tal vi trf mat trUot theo cdc dang pha hoai:
T^ = l 36.8545 kPa T^ = 505301 kPa T^ = 219.2009 kPa
Va do do tru bi phd hoai do momen ufin 6 loai dang b
Neu chon he sd an toan khi phdn tich on Hinh 10 So3okh8ilinhtoanKacdinhcacdarg pha hoaicSa tru dattr5nximang
Hinh 11. Mat cat ngang cfla difiing 4.2. Bdnh gid on dinh triidt theo phUdng phdp cdn bdng gidi hgn.
N i n ddt dUOc gia co bang try dat trfin xl mang den h i t c h i l u day cua Idp ddt yeu t h d nhat, chieu ddi tru dat trfin xi mang la 10.5m, be rfing gia cfi gfim 6 hang try don. Cdc t h f i n g so vat lieu ciia tru dat trfin xi mang n h u sau; y^ = 15.5 kN/m', luc dfnh c^=75kPa, gflc noi ma sat q)^_=0°, m f i dun dan hoi Ejj,= 100 q^ = 15000 kPa.
PhUflng phap can bdng gidi hcin sd dung phdn mem tinh on d[nh mai dflc Geo Slope da duqc thUc hien va k i t qud h& sfi an toan duocxdc dinh la 1.377 n h U c h l r a d h i n h 1 2 . T r o n g d d n h g i d o n
1 1 2 HSfElSIl 4.2015
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CWNH GIA ON Dlf*4 TRUOT CU=, HE
I . - . . v , '^
'-_
Ci_
Jl
\
r
111 t • '>
:
17
1/2 Be rong (m) ih 12.flanh gia on dinh tuct nen dirong theo phinmg phap tan bang giOi han
79.218 56-746
• ^ - 2 7 264
60.462 Hlnh 13. Cat l^rc tac dung l§n 1 tiu flit xi mSng Sfth t r u q t F = 1, kl>i d d cudng d d khdng cdt k M n g thodt nUdc gidl han theo tSeu k i f n t i t f ^ cOa vdt lieu t n j c „ ^ = 7SkPa. Tuy nhi^n.
khang cat khdng thoat nudc gidi han theo d i l u kien trUqt ciia vdt lieu tru c^ ^ = 54.5kPa vd tuy nhifen cUdng d d khang cat gidi han (theo sdc khang momen) dang b Id 50 53kPa. Tru cung se bj phd hoai do momen truflc khi xdy ra phd hoai truqt.
5. K ^ i u a n
Qua nghien cdu ndy, mfit vdi cong thdc tinh todn sdc khdng cat ciia try dat xi mdng theo so dd try bl phd hoal do uon gdy ra da duqc thiet lap vd vl the viec danh gid fin dinh cua try dat xi mang se mang tfn toan dien han trUcic day thay vi chi danh gid on dinh theo mat truot tru trdn hi tru bj phd hoal do cdt.
Cd mdl lifen he giUa pha hoal do momen ufin vd pha hoai truot. Pha hoai do momen ufln phu thufie vao chieu sdu ciia mat trUot va vj tri hinh thanh khdp uon so vfli mat trUqt; neng ddl vdi phd hoai dang b thi chi phu thuoc vao vi tri hinh thanh khdp uon so vdl mdt truoL Do do khi danh gid fin dinh trUot can t i l n hdnh song song vdi vifec ddnh gid dn (Knh do mdmen ufln, qua d d cd t h i xdc dmh duoc cdc dang pha hoai vd hfe so an todn o n djnh tong the.
Td nhUng k i t qud kiem toan cho cfing trinh cu the thay rdng pha hoai do momen ufin cila tru ddt xi mdng thudng xdy ra trUflc khi xdy ra pha hoai do cat. Trong cac dang pha hoai do mfimen ufin, phd hoai dang b thudng xdy ra nhat vi cd lUC cat huy dfing gidi han nhd nhat.
So dfi pha hoai try cung phu thufie vdo vi tri cua tru dUOc chen vdo ben dUdi nen dUdng NhQng tru duoc dat trong viing chu dfing va viing bi ddng sfe xdy ra phd hoai do momen udn trudc khi phd hoai do cdt.
momen) dang b Id S0.53kPa. Try sfe bj phd hoai do momen trudc khi xdy ra pha hoai truqt.
N l u chgn he so an todn khi phan tich on cudng do khdng cdt gidi han (theo sdc khdng ^ n h truot F^ = 1.377 (hlnh 12), khi dd cUdng dfi ^^ ^^
TAl UEU THAM KHAO
1. CDIT (Coastal Development Institute of Technology).
(2002). The Deep Minng Method. Pnnaple, Design and Constmction. A.A, Balkema The Netherlands.
2. Inagaki, M., Abe, l , Yamamoto, M.. Nozu, M., Yanagawa, V, and Ll, L (2002), Behavior of Cement Deep Miiring Columns under Road Embankment. Physical Modeliing in Geotechnlcs, 967-972.
3. Kltazume, M, Okano, K., and Miyajima, S. (2000),
"Centrifuge model tests on failure envelope of column type deep mixing method improved ground* Soils and Foundations, 40(4),
