Ảnh hƯỞng ma sÁt Âm

I HC QUC GIA TP. H CH MINH

TRNG: I HC BCH KHOA

KHOA: K THUT XY DNG

B MN: A C NN MNG

TIU LUN MN HC

K THUT NN MNG NNG CAO

TI S 3

NH HNG MA ST M N SC CHU TI CA CC

GVHD: PGS. TS. V PHN

NHM HVTH: NHM III

LP A K THUT XY DNG K2012

1. 1. V VN BCH 10. NGUYN TIN HONG NGHA

2. THNH DANH 11. NGUYN TR NGC

3. INH HU DNG 12. V THI PHNG NGC

4. L ANH DUY 13. L PHNG

5. NGUYN HONG DUYT 14. NG HNG THNH

6. L TRNG NG 15. HN TH XUN THO

7. V MINH KHA 16. NGUYN VIT TUN

8. LIN HNG KIT 17. NG TH THANH TM

9. TRNG TH THY LOAN

TP. H CH MINH THNG 11/2012

MN HC: KT NN MNG NNG CAO GVHD: PGS. TS. V PHN

TI S 3 i

MC LC

CHNG 1: TNG QUAN V HIN TNG MA ST M ......................................... 4

1.1. nh ngha hin tng ma st m v cc thut ng lin quan ......................................... 4

1.1.2. Cc thut ng lin quan n ma st m .......................................................................... 7

1.2. Cc nguyn nhn gy ra lc ma st m ............................................................................ 8

1.2.2. Ma st m do ln di ti trng bn thn hoc p nn ................................................. 8

1.2.3. Cc ng trn nn cha kt thc c kt ........................................................................ 10

1.2.4. Mc nc ngm b h thp ........................................................................................... 11

1.3. Cc yu t nh hng n hin tng ma st m .......................................................... 13

1.4. nh hng ca ma st m n nn mng cng trnh v t liu thc t ......................... 13

CHNG 2: PHNG PHP TNH TON MA ST M ........................................... 15

2.1. Tnh SCT ca cc xt n ma st m theo tiu chun Vit Nam ................................... 15

2.1.1. Xc nh ln n nh ca nn .................................................................................. 15

2.1.2. Xc nh ln ca cc n ........................................................................................ 16

2.1.3. Xc nh chiu su vng c kh nng xut hin ma st m ......................................... 17

2.1.4. Xc nh sc chu ti ca cc c k n ma st m ...................................................... 17

2.2. Tnh SCT ca cc xt n ma st m tiu chun k thut cc cng trnh cng Nht Bn17

2.2.1. M u .......................................................................................................................... 17

2.2.2. L thuyt tnh ton ........................................................................................................ 18

2.3. Tnh ma st m theo Principles of foundation engineering Braja Das ....................... 21

2.3.1. Nguyn nhn gy ra ma st m ..................................................................................... 21

2.3.2. Phng php tnh lc ma st m .................................................................................. 21

2.4. M hnh tnh ton ma st m bng phn mm PTHH Plaxis ......................................... 24

2.4.1. Tng quan ..................................................................................................................... 24

2.4.2. M hnh tnh ton t nn ............................................................................................. 25

2.4.3. Quy trnh m hnh cc xut hin ma st m ................................................................. 26

2.4.4. ng dng phng php truyn ti trng (Load-transfer methods) xc nh ma st m

bng phn mm Plaxis 3D. ..................................................................................................... 27

CHNG 3: NG DNG TNH TON VI CNG TRNH THC T .................... 29

3.1. Xc nh nh hng ca ma st m i vi cc n. .................................................... 29

3.1.1. Cng trnh Lung cho tu bin Tr Vinh .................................................................. 29

3.1.2. Thng k a cht.......................................................................................................... 34

3.1.3. Tnh ton nh hng ma st m .................................................................................... 34


TI S 3 ii

3.1.4. Tnh ton xc nh gi tr lc ma st m theo l thuyt Braja Das. .............................. 39

3.1.5. Xc nh gi tr ma st m s dng phn mm Plaxis. ................................................. 40

3.1.6. So snh gi tr ma st m ca cc n gia cc phng php tnh. ............................. 44

3.2. Xc nh nh hng MSA i vi nhm cc bng phn mm Plaxis ........................... 45

3.2.1. M hnh tnh ton. ........................................................................................................ 45

3.2.2. Kt qu tnh ton ........................................................................................................... 45

3.3. ng dng phn mm Plaxis trong nh gi nh hng ma st m................................ 48

3.3.1. Kho st nh hng ca ma st m n cng trnh ln cn. ......................................... 48

3.3.2. nh hng ca MSA i vi mng cc ca cng trnh c khi xy dng cng trnh k

bn dng mng b ................................................................................................................... 54

3.3.3. Xem xt s dng ln v h xung ca mc nc ngm nh hng n sc chu ti ca

cc ........................................................................................................................................... 60

3.3.4. Nhn xt ........................................................................................................................ 62

CHNG 4: CC BIN PHP GIM NH HNG CA MSA ............................... 63

4.1. Khi qut ........................................................................................................................ 63

4.2. Bin php lm tng nhanh tc c kt ca nn t ..................................................... 63

4.2.1. Ni dung bin php ....................................................................................................... 63

4.3. Bin php lm gim ti trng ln t nn ...................................................................... 64

4.3.1. Ni dung bin php ....................................................................................................... 65

4.3.2. u im ........................................................................................................................ 65

4.3.3. Nhc im .................................................................................................................. 65

4.4. Bin php lm gim ma st gia t v cc trong vng ma st m ............................... 65

4.4.1. Ni dung bin php ....................................................................................................... 65

4.4.2. u im ........................................................................................................................ 66

Thi cng n gin, kinh ph thp ............................................................................................ 66

4.4.3. Nhc im .................................................................................................................. 66

4.5. Cc bin php khc ........................................................................................................ 66

4.5.1. Phng php in thm (Electro Osmosis) .................................................................. 66

4.5.2. Cch ly gia vng mi trng t nn c kh nng xy ra ma st m v b mt cc . 67

4.5.3. H thng cc bo v xung quanh nhm cc chnh ....................................................... 67

4.5.4. Phng php s dng lp bao ph bng bn Betonite v Bittum ................................ 68

CHNG 5: KT LUN, KIN NGH ............................................................................ 75

5.1. Kt lun .......................................................................................................................... 75

5.1.1. Hin tng ma st m ................................................................................................... 75


TI S 3 iii

5.1.2. Cc phng php tnh ton ma st m .......................................................................... 75

5.2. Kin ngh ........................................................................................................................ 76

TI LIU THAM KHO .................................................................................................... 78


TI S 3 4

CHNG 1: TNG QUAN V HIN TNG MA ST M

1.1. nh ngha hin tng ma st m v cc thut ng lin quan

1.1.1. nh ngha hin tng ma st m

i vi cng trnh s dng mng cc, cc c ng vo trong tng t nn c qu

trnh c kt cha hon ton, khi tc ln ca t nn di cng trnh nhanh hn tc

ln ca cc theo chiu i xung, th s ln tng i ny pht sinh ra lc ko xung ca

tng t i vi cc lm gim kh nng chu ti ca cc goi la hin tng ma st m,

lc ko xung gi l lc ma st m.

P P

AT SET MEM

hay

AT AP CO

TNH NEN LUN

AT TOT

H

(a) (b)

B

Hnh 1. Cc trong t mm v chng vo lp t tt

a. Lc ma st dng ngay v trong khi ng cc

b. Lc ma st m

Ma st m (negative skin friction) l hin tng t xung quanh cc b ln c kt ln

hn chuyn v xung di/bin dng nn ca cc; vic ny gy thm mt ti trng hng

xung ln cc. Ma st m trn cc l yu t khng th b qua khi thit k mng cc trong

khu vc mi san nn trn t yu v trong vng chu nh hng ca hin tng h mc


TI S 3 5

nc ngm. Ma st m bin ng theo thi gian, ph thuc vo tc c kt ca t v

tc ln ca cc.

Lc ma st m xy ra trn mt phn thn cc ph thuc vo tc ln t xung

quanh cc v tc ln ca cc.

Lc ma st m c chiu hng thng ng xung di, c khuynh hng ko cc i

xung, do lm tng lc tc dng ln cc. Ta c th so snh s pht sinh ma st m v

ma st dng thng qua hnh sau:

Hnh 2a. S pht sinh ma st dng.

Hnh 2b. Ma st m c lp t mi p xy ra c kt do trng lng bn thn .


TI S 3 6

Hnh 2c. Ma st m khi lp st xp c kt do thot nc hoc c thm lp

t mi p .

Qua cc hnh minh ha trn ta thy c ma st m c th xut hin trong mt phn

on ca thn cc hay ton thn cc, ph thuc vo chiu dy ca lp t yu cha c

kt. Trong trng hp ma st m tc dng trn ton thn cc th rt nguy him, sc chu

ti ca cc khng nhng khng k n sc chu ti do ma st hng ca t v cc m cn

b ma st m ko xung. Sc chu ti lc ny ch yu l sc chu ti ca mi cc, chng

ln nn t cng hay .

Trng hp cc xuyn qua lp t mm cm vo lp t cng v lp t mm ang

din tin ln do c kt bi lp gia ti trn mt hay do h mc nc ngm,, m ln

cc lp t ln hn ln ca cc th phn lp t yu c chuyn v ng nhiu hn

chuyn v ng ca thn cc bn cnh v cc lp bn trn n gy ra lc ma st m. Ngay

c trong trng hp cc ch nm trong lp t yu khng ta mi vo lp t cng, cn

gi l cc treo cng c th b tc ng bi ma st m di tc ng ca ti bn trn mt

t hoc do cc tc ng gy ra bin dng t nn nh h mc nc ngm (ln), hoc

dng mc nc ngm (n).

Lc ma st m khng ch tc ng ln mt bn thn cc m cn tc dng ln mt

bn ca i cc, hoc mt bn ca m cu hay tng chn c ta ln cc.

Khi tc ng ti ln cng trnh gy ra ln ca cc v gim dch chuyn tng i

gia t v cc (ng ngha gim ma st m), t nht phn trn v nhiu hn on


TI S 3 7

di. Trong thc t tnh ton, nhng hot ti ngn hn n c xem xt khi gy ra c

s gim ma st m.

1.1.2. Cc thut ng lin quan n ma st m

Theo Fellnius, cc thut ng trong nghin cu ma st m nh sau:

a. Hin tng ma st m (negative skin friction): l lc ma st bn c huy

ng khi t dch chuyn xung tng i so vi cc. Cc quan st lu di t cc thit b

quan trc hin trng cho thy hin tng ma st m xy ra trong hu ht tt c cc cc.

b. Lc ko xung (Dragload): l lc nn dc trc gy ra trong cc phn t ca

cc do s tch ly ma st m khi t c khuynh hng dch chuyn tng i i xung so

vi cc.

c. Mt phng trung ha (Neutral plane): l v tr dc theo cc m ti cc

lc tc dng di hn (lc ko xung cng vi ti cng trnh) cn bng vi t hp lc

(chiu dng) khng bn (bn di mt trung ha) v sc khng mi. su ny l ni

c dch chuyn tng i gia t v cc bng 0.

d. Bin dng ko xung (Downdrag): l s dch chuyn i xung ca cc do

t xung quanh cc b ln xung. ln ca bin dng ko xung bng vi ln ca

t ti mt trung ha.

e. Cng chu ti mt trc a k thut (Geotechnical axial capacity): l

t hp ca sc khng mi v ma st bn khi khng cn t trng thi cn bng tnh v s

khng tip tc dch chuyn xung. Ma st dng xy ra trn ton thn cc v lc ko

xung rt nh.

f. H s an ton cho cng chu ti a k thut (Factor of safety on

geotechnical capacity): l h s gia cng chu ti mt trc a k thut chia cho

tng ti tnh v hot ti, khng tnh n lc ko xung.

g. Cng kt cu mt trc (Structural axial strength): l cng khng

nn mt trc ca phn cc chu ti tnh v lc ko xung.

h. H s an ton kt cu ti mt trung ha (Factor of safety on structural

strength at neutral plane): l h s gia cng khng nn mt trc ca kt cu ti mt

trung ha chia cho tng tnh ti v lc ko xung, khng tnh n hot ti.

Theo Fellnius, v tr mt trung ha l hm s ca s cn bng ca cc lc ct dc

thn khi chng c huy ng hon ton. Cc lc v sc khng l kt qu qu trnh

ln ca t l do s khc bit v cng ca t v cc. Yu cu tuyt i tha mn


TI S 3 8

phng trinh cn bng l lc ct pht trin dc phn chnh trn thn cc c du m v

phn di cc c du dng. Vng chuyn tip t m sang dng c gi l mt trung

ha. Mt s t trng hp v tr mt trung ha nm trong lp t ang ln, hay trong lp

t tt hn hoc trong lp t t ln.

Khi thay i lc tc dng ln u cc th v tr mt trung ha s thay i do kt qu

ca s cn bng lc mi.

Mt trung ha l ni cc v t dch chuyn nh nhau hay ni cch khc l ni

khng c s dch chuyn tng i gia cc v t. iu ny c ngha l khi gii bi ton

ln ca nhm cc l cng vic tm ra ln ca mt trung ha.

ln ma st m ph thuc vo cc yu t sau (Brejum,1973):

Vt liu cc.

Phng php thi cng cc.

iu kin t nhin ca t nn.

Vn tc dch chuyn tng i gia t v cc.

1.2. Cc nguyn nhn gy ra lc ma st m

Mt iu d dng thy rng, mt d c ln ca lp t xung quanh cc, lc ko

xung (ma st m) s khng xut hin nu s chuyn dch xung pha di ca cc di

tc dng ca tnh ti v hoc hot ti ln hn ln ca t nn. V vy mi quan h gia

bin dng ln ca nn v bin dng ln ca cc l nn tng c bn lc ma st m xut

hin.

Qu trnh xut hin ma st m c c trng bi ln ca t gn cc v tc

ln tng ng ca t ln hn ln v tc ln ca cc xy ra do tc ng ca ti

trng. Trong trng hp ny t gn cc nh bung khi cc, cn ti trng thm s cng

vo ti trng ngoi tc dng ln cc. Thng thng hin tng ny xy ra khi cc xuyn

qua lp t c tnh c kt v dy ln, khi c ph ti tc dng ln mt t quanh cc.

1.2.2. Ma st m do ln di ti trng bn thn hoc p nn

Khi nn cng trnh c tn cao, gy ra ti trng ph tc dng xung lp t bn

di lm xy ra hin tng c kt cho lp nn bn di, hoc chnh do ti trng bn thn

lm cho lp t nn p xy ra qu trnh t c kt. Ta xt cc trng hp c th sau:


TI S 3 9

a. Trng hp 1:

Khi c mt lp t st p trn mt tng t dng ht m cc s xuyn qua n, tng

t s c kt dn dn. Qu trnh c kt ny s sinh ra mt lc ma st m tc dng vo cc

trong sut qu trnh c kt.

b. Trng hp 2:

Khi c mt tng t dng ht, p pha trn mt tng st yu, n s gy ra qu trnh

c kt trong tng st yu v to ra mt lc ma st m tc dng vo cc.

c. Trng hp 3:

Khi c mt tng t dnh p pha trn mt tng st yu, n s gy ra qu trnh c

kt trong c hai tng t p v tng t st v to ra lc ma st m tc dng vo cc.

St

Set ap

L

Z

Hf

L

Hf

Z

St

Cat ap

L

Z

Hf

Ct

Set ap

(a) (b) (c) Hnh 3. Cc trng hp xut hin ma st m do tn nn .

Trng hp cc cc c ta trn tng t cng v c tn ti ti trng b mt, xy

ra trong cc trng hp sau y:

d. Trng hp 4:

Vi tng ct lng s c bin dng ln tc thi, c bit khi t nn chu s rung

ng hoc s giao ng ca mc nc ngm; s tc ng ca ti trng b mt s to ra

s bin dng ln.

e. Trng hp 5:

i vi nn st yu, xu hng xy ra bin dng ln c th rt nh nu nh khng

chu tc ng ca ti trng b mt. Nhng d sao khi khoang to l s gy ra s cu trc

li ca nn st v vy bin dng ln (nh) ca nn st s xy ra di tc dng ca ti

trng bn thn ca nn st.

f. Trng hp 6:


TI S 3 10

iu hin nhin l gn nh bt k s p no s to ra bin dng ln theo thi gian

di tc dng ca trng lc.

Vic xc nh mi quan h ln ca t nn pha trn v ca cc l cn thit

ra gii php x l ph hp i vi vn . Trong cc trng hp ni m t nn

phn trn ln xung pha di ln hn ln ca cc, mt gii php an ton c th c

c khi gi thit ti trng truyn hon ton ti nh ca lp t nn pha di.

1.2.3. Cc ng trn nn cha kt thc c kt

Trong thc t mt tnh hung thng xuyn gp phi trong thit k cu ng ni

m lc ma st m c th xy ra. Cc cc c thi cng xong trong khi nn t cha

kt thc c kt, m cu c xy dng v t nn c p. ln ca nn t dc

theo thn cc c th rt kh khn loi b, v vy lc ma st m thng xy ra vi dng

kt cu nh hnh 5.3, thm ch cn c khuynh hng to ra chuyn dch ngang ca m

cu, nhng s dch chuyn ny c th gim th nu ta s dng mt s gii php thit k

nn mng hp l.

Ma st m ch xy ra mt bn cc do phn ng vo cu c lp t p cao lm

cho lp t bn di b ln do phi chu ti trng ca lp t ny, cn phn bn kia m

(b sng) khng chu ti trng p nn lp t khng b ln do ti trng ngoi, do cc

khng nh hng ma st m. V vy, mt bn cc chu ma st m cn mt bn chu ma

st dng.


TI S 3 11

Hnh 6: Hin tng ma st m do vic ng cc m cu vo nn t yu cha kt

thc c kt hoc cn trng thi t nhin

1.2.4. Mc nc ngm b h thp

Vic h mc nc ngm lm tng ng sut thng ng c hiu ti mi im ca t

nn. V vy, lm tng ln c kt ca nn t. Lc , tc ln t xung quanh cc

vt qu tc ln ca cc dn n xy ra hin tng ko cc i xung ca lp t xung

quanh cc.

Hin tng ny c gii thch nh sau: khi h mc nc ngm th:

Phn p lc nc l rng u s gim.

Phn p lc c hiu thng ng h ln cc ht rn ca t tng.

Xem biu tng quan gia u v h trong trng hp bi ton nn mt chiu v

ti trng q phn b u khp.

Nhp Cu

Tng t p

Tng t yu

Tng chu lc

M cu


TI S 3 12

Hnh 7: Biu tng quan gia p lc nc l rng u v p lc c hiu thng

ng ln ht rn ca t h trong trng hp bi ton 1 chiu v ti trng ngoi phn

b u khp.

Trong :

h = p = const: ng sut ton phn.

Ha : vng hot ng ca ng sut phn b trong t.

t bnh thng: Ha tng ng vi chiu su m ti bth 2.0 .

t yu: Ha tng ng vi chiu su m ti bth 1.0 .

bt : ng sut do trng lng bn thn ca lp t c chiu dy Ha.

Theo TCVN 205 1998: Hin tng ma st m nn c xt trong cc trng

hp sau:

S c kt cha kt thc ca trm tch hin i v trm tch kin to.

S tng cht ca t ri di tc dng ca ng lc.

S ln t ca t khi b ngp nc.

Mc nc ngm h thp lm cho ng sut c hiu t tng ln, dn n tng

nhanh tc c kt ca nn.

Nn cng trnh c tn cao ln hn 1m trn nn t yu.

Ph ti trn nn vi ti trng t 2/2 mT tr ln.

S gim th tch trong t do cht hu c b phn hy

P

h

H

at yeu

Lp at tot

q

q

u = 0min

u

h

a

u max

z= q = const

z


TI S 3 13

1.3. Cc yu t nh hng n hin tng ma st m

Ma st m l mt hin tng phc tp v n ph thuc vo nhiu yu t nh:

Loi cc, chiu di cc, phng php h cc, mt ct ngang ca cc, b mt

tip xc gia cc v t nn, s co nn n hi ca cc.

t tnh c l ca t, chiu dy ca lp t yu, tnh trng n ca t.

Ti trng tnh ton (chiu cao p nn, ph ti).

Thi gian cht ti n khi xy dng cng trnh.

ln ca nn sau khi ng cc, ln ca mng cc.

Quy lut phn b ma st m trn cc.

Tr s lc ma st m c lin quan ti s c kt ca t, ph thuc trc tip vo ng

sut hu hiu ca t xung quanh cc. Nh vy lc ma st m pht trin theo thi gian v

c tr s ln nht khi kt thc qu trnh c kt ca t.

Bt k mt s dch chuyn no xung pha di ca nn t i vi cc u sinh ra

lc ma st m. Ti trng ny c th truyn hon ton t t nn cho cc khi mi tng

quan v chyn v khong t 3mm n 15mm hoc 1% ng knh cc. Khi chuyn v

tng i ca t ti 15mm th lc ma st m c pht huy y . Mt iu thng

c gi thit trong thit k khi cho rng ton b lc ma st m s xy ra khi m c mt

s chuyn dch tng i ca t nn c d on trc.

1.4. nh hng ca ma st m n nn mng cng trnh v t liu thc t

Khi cc trong t, th sc chu ti ca cc c th hin qua thnh phn ma st

(dng) xung quanh cc v sc khng mi cc. Khi cc b nh hng lc ma st m th

sc chu ti ca cc s gim v lc ny ngoi chuyn v ca cc, cc cn phi gnh thm

mt lc ko xung m ta gi l lc ma st m. Ngoi ra qa trnh c kt ca lp t,

gy nn khe h gia i cc v lp t di i, gia cc v t xung quanh cc, t

gy tng thm ng lc ph tc dng ln mng cc. i vi t trng n, ma st m c

th gy nn ti trng ph rt ln tc dng ln mng cc.

Trong mt s trng hp lc ma st m kh ln, c th vt qua ti trng tc dng

ln u cc nht l cc c chiu di ln. Chng hn (1972) Fellenius o qu trnh pht

trin ca lc ma st m ca hai cc b tng ct thp c ng qua lp t mm do dy

40m v lp ct dy 15m cho thy: s c kt li ca lp st mm b xo trn do ng cc

to ra lc ko xung 300kN trong thi gian 5 thng v 16 thng sau khi ng cc th

mi cc chu lc ko xung 440kN.


TI S 3 14

Johanessen v Bjerrum theo di s pht trin hin tng ma st m ca cc thp

xuyn qua lp t st dy 53m v mi cc ta trn nn . Lp t p bng ct dy 10m,

qu trnh c kt ca lp t st gy ra ln 1.2m v lc ko xung khong 1500kN

mi cc. ng sut mi cc t tnh t ti 190kN/m2 v c kh nng xuyn thng lp

.

i vi vic s dng ging ct: ma st m lm hn ch qu trnh c kt ca nn t

yu c dng ging ct. Hin tng ma st m gy ra hiu ng treo ca t xung quanh

ging ct, lp t xung quanh ging ct bm vo ging ct lm cn tr ln v cn tr

qu trnh tng kh nng chu ti ca t nn xung quanh git ct.

Qua phn tch trn cho thy tc dng chnh ca lc ma st m lm gia tng lc nn

dc trc cc, lm tng ln ca cc, ngoi ra o lp t p b ln to ra khe h gia

i cc v lp t bn di i c th thay i mmen ung tc dng ln i cc. Lc ma

st m lm hn ch qu trnh c kt thot nc ca nn t yu khi c gia ti trc v c

dng ging ct, cn tr qu trnh tng kh nng chu ti ca t nn xung quanh ging ct.

Ngoi ra lc ma st m lm tng ti trng ngang tc dng ln cc.


TI S 3 15

CHNG 2: PHNG PHP TNH TON MA ST M

2.1. Tnh SCT ca cc xt n ma st m theo tiu chun Vit Nam

2.1.1. Xc nh ln n nh ca nn

Xc nh ln n nh ca nn S theo kt qu th nghim nn c kt da vo

ng cong e-p hoc theo e-log(p)

Tnh ln ca nn theo phng php tng phn t:

i

n

i i

iin

i

i he

eesS

1 1

21

1 1

Phng php tng phn t c hai dng biu thc tnh ln nh sau:

i

n

i

i

n

i

i

i

n

i

ivi

n

i

i

hpE

sS

hpmsS

11

11

Tnh ln nn theo h s c kt:

Trng hp 1: i vi t c kt thng ta c:

0

00

00

log1

1p

ppH

e

CS

p

pOCR cc

Trng hp 2: i vi t c kt nh trc:

c

ccsc

p

ppH

e

C

p

pH

e

CSpppp

00

00

0

0

00 log1

log1

Trng hp 3: i vi t c kt nng:

0

00

0

0 log1 p

ppH

e

CSppp sc

Trong :

P0: ng sut do trng lng bn thn )/(2mkN .

p :p lc tng thm do ti trng cng trnh gy ra ti gia lp t cn tnh

ton )/( 2mkN .

cp :p lc tin c kt )/(2mkN .

e0:h s rng ban u.

Cc:ch s nn .


TI S 3 16

Cs:ch s n.

H0: chiu cao lp t tnh ton (m)

2.1.2. Xc nh ln ca cc n

ln ca cc n c xc nh gm ba thnh phn:

bm SSLS

Trong :

L : Bin dng n hi ca cc.

Sm: ln ti mi cc do ti cng trnh truyn xung mi cc.

Sb: ln ti mi cc do ti truyn dc theo thn cc.

Bin dng n hi thn cc:

cp

tb

EA

LQL

Trong :

Ap: Din tch tit din ngang ca cc.

L: Chiu di tnh ton ca cc.

Ec: Mul n hi ca cc.

Qtb: Lc nn trung bnh tc dng ln thn cc.

ln ti mi cc do ti cng trnh truyn xung mi cc.

0

2 )1(

E

BqS

pthuc

m

Trong :

qpthuc: Sc khng mi n v ti trng lm vic.

B: Cnh cc.

E0: Mdul n hi t di mi cc.

: H s Poisson (st 43.0 , st pha 43.03.0 , ct pha 3.025.0 ,

ct 25.0 .

: H s ph thuc vo hnh dng cc : cc vung 88.0 , cc trn

79.0 .

ln ti mi cc do ti truyn dc theo thn cc.

0

2 )1(

E

BfS bsthucb


TI S 3 17

b : H s ph thuc vo mnh ca tng on cc.

i

ib

B

L35.02

sthucf : Sc khng bn n v ti ti trng lm vic tnh trung bnh cho tng

on cc.

2.1.3. Xc nh chiu su vng c kh nng xut hin ma st m

Vng ma st m xut hin khi cc qua lp t yu cha c kt v c ln ln hn

tc ln ca cc. Ma st m tc dng ln cc v to lc cng vi cc chuyn v ln

nhanh hn.

Mt cng thc xc nh vng nh hng ma st m nh sau:

HzS

Sd 1

Trong :

Sd: ln ca cc n

S: ln n nh t nn

H: Chiu dy lp t yu

2.1.4. Xc nh sc chu ti ca cc c k n ma st m

u f si i p pQ u m f z A q

Trong :

u: chu vi cc

mf: h s lm vic ca t mt bn cc

fs: sc khng bn n v, tnh trung bnh cho ton on cc

Zi: b dy ca lp t tnh ton

Ap: din tch tit din cc

qp: sc khng mi n v

2.2. Tnh SCT ca cc xt n ma st m tiu chun Nht Bn

2.2.1. M u

Nu cc xuyn qua lp t m chu nh hng c kt th ma st m s c tnh n

khi tnh ton sc chu ti cho php dc trc ca cc

Ch gii : Khi cc xuyn qua lp st mm ti lp a tng chu lc, lc ma st t

lp mm s tc ng hng ln pha trn v chu mt phn ti trng tc ng ln u cc.


TI S 3 18

Khi lp st c kt th bn thn cc c bi sc chu ti ca tng chu lc v hu nh

khng ln, hng lc ma st theo hng ngc li.

Lc ma st trn ton b chu vi xung quanh cc by gi dng chng li ti trng tc

dng ln u cc. Lc ma st hng xung pha di v tc dng ln ti trng chn

cc. Lc ma st hng xung pha di trn ton b chu vi xung quanh cc c gi l

ma st tip xc m hay ma st m.

2.2.2. L thuyt tnh ton

Trng hp 1:

Gi tr ma st m vn cn mang tnh l thuyt v cha c c th, tuy nhin gi tr

ln nht c th c xc nh t phng trnh:

snf fLR .. 2max, (1)

Trong :

Rnf,max: Lc ma st tip xc ma st m ln nht (cc n) KN.

: chu vi xung quanh cc (chu vi ca din tch kn trong trng hp cc

thp c tit din ch H) (m).

L2: Chiu di ca cc trong lp t c kt (kN/m2).

sf : Cng lc ma st tip xc trong lp t c kt.

Trng hp 2

Trong trng hp trn , sf : trong nn t st i khi c ly bng qu/2. Nu lp ct

kp gia lp st c kt , hoc nu lp ct nm trn lp st c kt , th chiu dy lp ct s

nm trong L2 . Ma st thnh bn trong lp ct i khi c xt n tnh ton cho sf .

Gi tr ln nht ca ma st m trong trng hp ny c th hin trong Phuong trnh:

)2

2( 2max,cu

snf

LqLNR

Trong :

Ls2: Chiu dy lp ct nm trong L2.

Lc: Chiu dy lp st nm trong L2.

L2 = Lc + L s2.

2sN : Gi tr trung bnh ca lp ct c chiu dy Ls2.

qu: Cng trung bnh nn n hng ca lp st c chiu dy Lc.


TI S 3 19

Trng hp 3:

Trong trng hp nhm cc, ma st tip xc m c th c tnh ton bng cch coi

s lm vic ca nhm cc nh mng n su. Lc ma st tip xc m cho mt cc c

tnh ton bng cch chia cho s cc.

n

LAUHsR

g

nf

2

max,

(2)

Trong :

Rnf,max: Lc ma st tip xc ma st m ln nht (cc n) KN.

U: Chu vi ca nhm cc.

s : Cng ct trung bnh ca t trong phm vi chiu di L2.

Ag: Din tch y nhm cc.

: dung trng trung bnh ca t trong phm vi chiu di L2.

n: s lng cc trong nhm.

Phng trnh (1) v (2) a ra gi tr ln nht ca ma st tip xc m. Gi tr thc t

s ph thuc vo tr s ln c kt v tc ln, tnh cht t bin ca t st c tnh cht

bin dng ca tng chu lc.

Trng hp 4:

Khi tnh ton sc chu ti cc dc trc cho php ca cc, c mt vi trng hp thay

i trong vic nh gi nh hng ca lc ma st m c xc nh bng cch kim tra

bt k mt lc truyn ln mi cc vt qu c hai gi tr ti trng chu un ca nn ti

mi cc v cng nn khi un ca tit din ngang ca cc. iu c ngha l sc

chu ti cho php dc trc trong iu kin bnh thng Ra s tha phng trnh:


TI S 3 20

max,

max,2.1

1

nfefa

nfpa

RAR

RRR

Trong :

Ra: sc chu ti dc trc cho php (trong iu kin bnh thng).

Rp: Sc chu ti cui cng ca cc (gi tr ti hn).

Rnf,max: lc ma st m thng tip xc max nht (gi tr thng nh hn cc

n hoc nhm cc).

:f cng nn khi un ca cc.

Ae: tit din ngang c hiu ca cc.

Gi tr sc chu ti ti hn ca cc Rp c th ly ti 300NAp trong phng trnh (1).

Nu cc xuyn vo a tng chu lc, lc ma st tip xc trong a tng chu lc c th

bao gm c trong sc chu ti ti hn.

112300 sspp LNNAR

Trong :

Rp: Sc chu ti mi cc (gi tr ti hn).

N: Gi tr N ca nn ti mi cc.

Ap: din tch mi cc.

Ls1 = L1: chiu di cc nm trong tng chu lc (nn ct).

1sN : Gi tr N trung bnh trong min Ls1.

: Chu vi cc.


TI S 3 21

2.3. Tnh ma st m theo Principles of foundation engineering Braja Das

2.3.1. Nguyn nhn gy ra ma st m

Ma st m l lc ko xung do t xung quanh tc dng ln cc. Lc ny xut hin

di nhng iu kin sau:

Cc xuyn qua lp st p nm trn lp t ri: qu trnh c kt ca lp st p

theo thi gian s gy ra lc ko xung tc dng ln cc (hnh a).

Cc xuyn qua lp ct p nm trn lp st mm, qu trnh c kt ca lp st do

ti trng t p s gy ra lc ko xung tc dng ln cc (hnh b).

H mc nc ngm lm cho ng sut hu hiu trong t tng ln v do lm

tng tc c kt ca t st gy ra lc ko xung tc dng ln cc.

Trong mt s trng hp, lc ko xung c th ln n mc gy ra ph hoi cho

mng.

2.3.2. Phng php tnh lc ma st m

Trong phn ny tc gi gii thiu hai phng php tnh lc ma st m.

a. Trng hp 1: Lp st p nm trn lp t ri

Theo phng php , lc ma st m (ko xung) n v tc dng ln cc l:

fn = Kotan

Trong :

K = h s p lc t: K 1 sin '

o = ng sut hu hiu theo phng ng ti su z = fz

f = trng lng ring hu hiu ca lp st p


TI S 3 22

= gc ma st gia cc v t 0.5-0.7 '

Tng lc ko xung tc dng ln cc l:

2

'tan')'tan'(

2'

0

'

ffH

fn

HpKzdzpKQ

f

vi Hf = chiu cao lp st p.

Nu lp st p nm trn mc nc ngm, trng lng ring hu hiu f, nn c

thay th bng trng lng ring t nhin.

b. Trng hp 2: Lp ct p nm trn lp st

Trong trng hp ny, ma st m xut hin trong on t z = 0 n z = L1, theo

su mt trung ha (Vesic, 1977). su mt trung ha c xc nh nh sau (Bowles,

1982)

'

2

'2

)( ''

1

1

ffffff HHHL

L

HLL

Vi f, = trng lng ring hu hiu ca lp ct p v lp st bn di.

i vi cc chng, su trung ha c th xem l v tr mi cc

(L1 = L - Hf)

Vi mi gi tr L1 lc ko xung c xc nh nh sau:

Lc ma st m n v ti su trong khong z = 0 n z = L1 l:

fn = Kotan, vi

K = K 1 sin '

o = fz + z

= 0.5-0.7 '

2

'tan'''tan''tan''

2

11

''

00

11

pKLLHpKdzzHpKdzpfQ ffff

LL

nn Nu

lp st v lp ct p nm trn mc nc ngm, trng lng ring hu hiu nn thay

bng trng lng ring t nhin. Trong mt vi trng hp, cc cc c th c ph bn

ngoi bng lp bitumen trong vng ko xung trnh hin tng ny.

Bjerrum et al. (1969) cng b kt qu kim tra lc ko xung khi th cc ti cng

Oslo, Norway (k hiu l cc G trong trong bi bo gc). Nghin cu ca Bjerrum et al.

(1969) cn c phn tch bi Wong v Teh (1995) trong gii hn ca cc khoan n

gc su 40 m. Hnh c m t s lc t v cc. Wong v Ted tnh ton theo cc s

liu sau:


TI S 3 23

Lp t p c khi lng ring t nhin f = 16 kN/m3, trng lng ring

bo ha sat = 18.5 kN/m3, do f = 18.5 - 9.81 = 8.69 kN/m

3 v Hf = 13 m

Lp t st: Ktan 0.22, trng lng ring bo ha hu hiu = 19.9.81

= 9.19 kN/m3

Chiu di cc L = 40 m, ng knh D = 500 mm.

Ma st m trn mt cc cng Oslo, Norway (theo Bjerrum et al. (1969), Wong v

Ted (1995))

Cc trong trng hp ny l cc chng, chiu di L1 = 27 m, lc ko xung ln nht

tc dng ln cc c tnh theo cng thc:

2

'tan''2132'tan'

2

11

, KpL

LpKQ ffn hay

23482

22.019.95.027271169.821622.05.0

2

nQ kN

Gi tr ln nht o c Qn khong 2500 kN (hnh d), kh gn so vi gi tr tnh

ton.


TI S 3 24

2.4. M hnh tnh ton ma st m bng phn mm PTHH Plaxis

2.4.1. Tng quan

Phng php phn t hu hn l mt phng php ph bin nht trong khoa hc k

thut. N c ng dng rt thnh cng vo gii quyt nhiu vn trong nhiu lnh

vc khc nhau. S pht trin ca phng php PTHH trong lnh vc xy dng to ra

nhiu bc tin ng k: gii quyt nhng bi ton gii tch m nhng phng php gii

tch thng thng ph tn rt nhiu thi gian.

Plaxis l mt phn mm phn t hu hn tnh ton a k thut vi nhiu tnh nng

vt tri nh:

M phng s lm vic chung ca t v kt cu mng qua h thng li phn t

hu hn v cc phn t tip xc gip cho vic m phng tng gic gia t v cc gn

vi lm vic thc t hn. Xt n quan h gia lc v chuyn v trong bi ton chuyn v

nt t tnh ton cc thng s ni lc cc phn t thnh phn.

M phng bi ton theo qu trnh thi cng: mi giai on thi cng ng x t v cc

c nhiu im khc bit so vi thit k ban u nn c th xy ra nhng tc ng n

cng trnh dang thi cng cng nh cng trnh ln cn tnh nng ny gip ngi thit k

nh gi c tnh hp l phng php thi cng v hon thin hn bc thit k nn

mng.

Tnh ton bi ton c kt thm theo thi gian da trn cc l thuyt c kt thm

phn mm gii bi ton c kt thm v ln c kt vi cc thng s t nn do ngi dng

nhp vo v cc thng s ng sut bin dng t cc bc tnh ton phn t hu hn cc

bc trn. Nh ta xt c nhng nh hng ca qu trnh c kt tc ng n cng

trnh.

Xt n nhng nh hng ca vic tng gim mc nc ngm.

Nhn xt: vi cc tnh nng trn Plaxis l mt phn mm rt ph hp gii quyt bi

ton a k thut ni chung v bi ton thit k mng cc ni ring. Qua cc bc tnh

ton theo tin thi cng cng nh tnh ton c kt thm ta xt c tt c cc nh

hng tc ng ln cc, chuyn v tng i gia cc v t, cc ng sut pht sinh trong

cc theo thi gian t a ra phng n thit k kh thi nht.


TI S 3 25

2.4.2. M hnh tnh ton t nn

t nn c m hnh ng x nh mt vt liu n hi tuyn tnh v tuyt i do

(linear elastic perfect plastic material) v hm do ca n c nh ngha theo tiu

chun ca Mohr Coulomb.

y l mt m hnh tnh ton t nn c ng dng rng ri trong thc t vi b

thng s c xc nh c xc nh t cc th nghim a cht.

ng x thot nc (long-term) v khng thot nc ca t (short-term)

Xem xt s pht trin ma st m trong cc thng qua qu trnh c kt ca t do

vic tnh ton ng x ca t phi l mt qu trnh di hn (ng x thot nc).

Trong phn mm plaxis hai ng x trn c th c m phng theo 2 trng thi tnh

ton Drained (Single analysis) v Undrained (Double analysis). Ta s dng ng x

Undrained cho tnh ton cc lp t st, i vi cc lp ct s dng ng x Drained v ct

thot nc nhanh sau khi cht ti.

Cc thng s ca m hnh Mohr Coulomb: c xc nh da trn phng php

phn tch ng sut hu hiu (thng s ca chnh bn thn khung ht khi nc thot

hon ton do c kt).

a. Thng s sc khng ct ca t hu hiu: lc dnh c, gc ma st c trng

cho sc chng ct v gc ma st ca chnh khung ht t. Hai thng s ny xc nh t

th nghim nn ba trc (CU, CD) tuy nhin trong gii hn ti khng c y cc

thng s t nhng th nghim trn nn ch dng cc s liu t th nghim ct trc tip.

b. Thng s v cng E xc nh t th nghim nn 3 trc CD tuy nhin th

nghim hu nh khng c thc hin v phc tp v tn thi gian do gi tr cng

E xc nh thng qua th nghim UU hay thi nghim nn c kt.

Xc nh E t th nghim UU

Hnh II.3: Biu quan h ng sut bin dng


TI S 3 26

T th xc nh c Eu50. Suy ra E theo cng thc

5.0,)'1(2

'

)1(2

u

u

u EEG

5.1

)'1('

u

EE

c. h s Poisson do khng c th nghim xc nh h s trn nn ta ly theo

cng thc kinh nghim:

i vi st: =0.25+0.00225PI , PI l ch s do.

i vi ct: i vi t ct: o

o

t

2545

253.01.0

0

, t l gc ma st ca t.

Xc nh modun n hi Eoed t th nghim nn c kt: cng da trn th nh

hnh trn xc nh Eoed.

d. Cng thc tng quan gia Eoed v E:

e. Xc nh h s thm kx, ky t th nghim nn c kt thng qua biu

chuyn v ca mu theo thi gian.

T th xc nh h s c kt cv

T suy ra h s thm k theo cng thc: 12

12);1

(

eea

e

aCk v

o

v

nv

2.4.3. Quy trnh m hnh cc xut hin ma st m

Nguyn nhn chnh xc nh gi tr ma st m l s chuyn v tng i ca t nn

ln hn chuyn v ca cc. Do m xc nh c ma st m ta cn 2 gia on c

bn.

Tc dng lc ln u cc:

Hnh II.4: Biu pht trin ln c kt theo thi gian -

lg(t)


TI S 3 27

Ti trng tc dng phi nh hn gi tr sc chu ti thit k Qa ca cc. V nh

vy gi tr ln ca cc l hp l v ph hp vi thc t.

Nu gi tr Po nhp vo qu ln th ln ca cc ln nhng ma st m nu c

cng nh hng khng ng k.

Do cn phi c bc tnh ton xc nh gi tr SCT ca cc trc khi

nhp lc vo phn mm.

To chuyn v tng i ca cc v t nn: chuyn v tng i ny c th do:

C kt ca nn khi c ti trng ngoi nh : t p hoc cc ti trng thi cng

(ca chnh cng trnh hoc cng trnh ln cn)

S gim mc nc ngm.

2.4.4. ng dng phng php truyn ti trng (Load-transfer methods) xc nh ma st

m bng phn mm Plaxis 3D.

Phng php truyn ti trng s dng mt b ng cng n gin phn tch ng

x ca ti trng dc trc trong cc n cng nh nhm cc nh sau:

H 2.1. Minh ha ma st m trong cc.

a. Hin tng ma st m; b. S truyn ti trng dc trc trong cc;

c. Bin dng ln ca cc v t.


TI S 3 28

ng dng cc cng c xut kt qu ca phn mm Plaxis 3D ta d dng xem xt

c truyn ti trng dc trc ca cc t xc nh gi tr lc ma st m.

H 2.2. Biu truyn truyn ti trng dc trc trong cc.

Gi tr ti trng tc dng: Po=400kN.

Gi tr lc tng lc dc ln nht Ps=717kN.

Gi tr lc ma st m: Pn=Ps-Po=717-400=317kN .


TI S 3 29

CHNG 3: NG DNG TNH TON VI CNG TRNH THC T

3.1. Xc nh nh hng ca ma st m i vi cc n

3.1.1. Cng trnh Lung cho tu bin trng ti ln vo sng Hu Tr Vinh

D n (LSH) do Cc Hng Hi Vit Nam (CHHVN) lm ch u t (i din l

BQLDA Hng Hi III). D n c xy dng vi mc tiu m lung tu n nh, lu

di cho tu bin trng ti 10,000DWT (y ti) n 20,000DWT (gim ti) ra vo,

m bo thng qua lng hng ha ca khu vc BSCL.

V tr on ngoi bin


TI S 3 30

Ton b tuyn lung di khong 107km, c chia ra lm 04 on bao gm:

- on lung sng Hu (t i An n Cng Cn Th) di khong 73km;

- on knh Quan Chnh B di khong 19km;

- on Knh Tt di khong 9 Km, on ny o mi hon ton;

- on lung Bin di 6km.

Ton b tuyn lung nm trong khu vc ng bng sng Cu Long. a hnh khu

vc d n rt bng phng khng c ni i hay chng ngi vt qu cao hay qu ln che

chn.

Nhm tng cng an ton giao thng hng hi thng qua gim st, thng tin nhanh

chng kp thi v iu kin ca lung, cng v cung p nhng thng tin nh hng cng

nh nhng cnh bo ng tin cy. Ti nhng v tr trng yu, cc h thng hng hi in

t AIS c pht trin v lp t phi c kh nng cung cp nhng thng tin chnh

xc cho c quan c thm quyn


TI S 3 31

Thp tr

Tt c cc tr c cung cp & lp t phi p ng cc yu cu sau:

Tr bng li thp cao 40 mt, vi thit k v xy dng theo module lp t

nhanh chng v an ton.

Cc c im chnh ca ta thp s l:

Mt ct ngang hnh vung, theo mu thp Eiffel.

Thit k chu c vn tc gi hn 200 km/h.


TI S 3 32

Thang nghing i ln trong tr v c chiu ngh mi 5 mt vi tay vn an

ton.

Tng trn nh rng ri v n nh, kch thc 4x4 mt. y l khu vc t thit

b v l khu vc lm vic vi cc lan can bo v cho php lp t h thng hon

chnh.

Cu trc thp hon chnh c lm t thp m km nhng nng loi S275JR

hoc cao hn, kh nng chng n mn cao.

H thng chng st bao gm b thu st a inh v dy dn, tm ni t c gi

c nh.

Ti mi v tr chn thp thp d kin t h mng cc vi cc kch thc chnh

nh sau:

Cc BTCT cnh 30x30cm, di 30m, gm 3 on cc ni li, cc c ng vo

tng t c cc thng s a cht nh sau:


TI S 3 33

Mc nc ngm nm ngay mt t t nhin.


TI S 3 34

3.1.2. Thng k a cht.

Thng s Lp 3a Lp 3b Lp 4b Lp 5

Chiu dy (m) 17.9 5.6 1.5 18

sat (kN/m3) 16 20 192 19.6

C (kN/m2) (UU) 9

C (kN/m

2) (CD) 7.7 24.5 6 9.1

(0) (UU) 6

(0) (CD) 23.6 27.1 27 27.2

0.25 0.3 0.3 0.3

E (kN/m2) 2000 7000 15000 16000

0.25 0.3 0.3 0.3

kx (m/day) 7.50E-04 5.10E-05 1.50E+00 7.50E-06

ky (m/day) 5.00E-04 3.40E-05 1.00E+00 5.00E-06

3.1.3. Tnh ton nh hng ma st m

M un n hi ca cc E = 2.65.106 T/m2

Lc tc dng ln u cc P = 40T, = 0.8.

a. Xc nh gia tng p lc bn trn

Nhm tn to bi, nng cao trnh nn, ngi ta p mt khi t c chiu cao 2m,

sat = 19.0 kN/m2.

p lc gy ln:

p 19.0x2 38.0 (kN/m2)

b. Tnh ton ln cho lp t yu v ln ca cc

Tnh ln ca lp st yu:

n

ii i

i 1 i

0.8S p h x38x17.9 0.272

E 2000

(m)

Tnh ln ca cc

Bin dng n hi ca bn thn cc

36

p c

PxL 40x30l 5.031.10

A xE 0.3x0.3x2.65x10

(m) = 0.5 (cm)

ln ca t mi cc (khng xt t p)


TI S 3 35

2

p

m

i

q B (1 )S

E

ng sut hu hiu do nn gy ra ti mi cc

' '

vp i ixH 6.0x17.9 10x5.6 9.2x1.5 9.6x5 225.2 (kN/m2)

Cc c ngm vo lp 5: = 27.2 0; C = 9.1 (kN/m2)

Tra bng:

q

c

N 16.26

N 29.69

N 13.15

Cng t nn di mi cc:

p c vp qq 1.3CxN N 0.4 dN

pq 1.3x9.1x29.26 225.2x16.26 0.4x9.6x0.3x13.15

qp = 4.028x103 (kN/m

2)

Vy: 3 2

m

4.028x10 x0.3x0.88x(1 0.3 )S 0.057

17000

(m)

ln ca t thn cc

2n

si ib

i 0i

f (1 )S B

E

Lp t 3a: = 6 0; C = 9 (kN/m2); sat = 16 (kN/m3)

'v3a17.9

6x 53.72

(kN/m2)

fs3a= (1-sin6)x53.7xtg6+9=14.05(kN/m3)

3a3aH 17.9

2 0.35x 2 0.35x 4.704B 0.3

3a

2 0.35

H3a

d

1

2

4.704

Sb3a

fs3a

d 3a

1 3a

2

E3a

9.296 103

(m)


TI S 3 36

Lp t 3b: = 27.1 0; C = 24.5 (kN/m2); sat = 20 (kN/m3)

'v3b5.6

6x17.9 10x 135.42

(kN/m2)

s3bf (1 sin 27.1)x135.4xtg27.1 24.5 62.224 (kN/m3)

3b3bH 5.6

2 0.35x 2 0.35x 3.512B 0.3

2

b3b

62.22x0.3x3.51x(1 0.3 )S 0.0085

7000

(m)

Lp t 4b: = 27 0; C = 6 (kN/m2); sat = 19.2 (kN/m3)

'v4b1.5

6x17.9 10x5.6 9.2x 170.32

(kN/m2)

s4bf (1 sin 27)x170.3xtg27 6 53.38 (kN/m3)

4b4bH 1.5

2 0.35x 2 0.35x 2.783B 0.3

2

b4b

53.38x0.3x2.78x(1 0.3 )S 0.0027

15000

(m)

Lp t 5: = 27.2 0; C = 9.1 (kN/m2); sat = 19.6 (kN/m3)

'v55

6x17.9 10x5.6 9.2x1.5 9.6 201.22

(kN/m2)

s5f (1 sin 27.2)x201.2xtg27.2 9.1 65.24 (kN/m3)

55H 5

2 0.35x 2 0.35x 3.429B 0.3

2

b5

65.24x0.3x3.43x(1 0.3 )S 0.0038

16000

(m)

b b3a b3b b4b b5S S S S S 0.009 0.0085 0.0027 0.0038 0.024 (m)

ln ca cc

d m bS l S S 0.005 0.06 0.024 0.09 (m)

c. Xc nh chiu di cc chu MSA

Chiu su nh hng n ma st m

d 3aS 0.009

z (1 )H (1 )x17.9 12S 0.272

(m)

d. Xc nh SCT ca cc khi khng c MSA


TI S 3 37

Sc chu ti cc hn ban u ca cc khi cha c khi p

p p pQ q xA 4028x0.3x0.3 362.52 (kN)

s s iQ u f xL

sQ 4x0.3x(8.55x17.9 62.22x5.6 53.38x1.5 65.24x5) 1208 (kN)

pS

a

QQ 1208 362.53Q 724.61

2 3 2 3 (kN)

e. Xc nh SCT ca cc khi c MSA

TH1: Tnh SCT cc hn (tc thi) ca cc khi st n ma st m

Lp 3a: on ma st m di 12m

'v3a12

6x 362

(kN/m2)

s3af (1 sin 6)x36xtg6 9 33.2 (kN/m3)

on ma st dng di 5.9m

'v3a5.9

6x12 6x 89.72

(kN/m2)

s3af (1 sin6)x89.7xtg6 9 33.2 (kN/m3)

p p pQ q xA 4028x0.3x0.3 362.52

snew s iQ u f xL

sQ 4x0.3x( 27.4x12 33.2x5.9 62.22x5.6 53.38x1.5 65.24x5) 747 (kN)

pS

atk

QQ 747 362.53Q 494

2 3 2 3 kN

gim SCT khi st ma st m:

a atkQ Q Q 724 494 230 (kN)

TH2: Tnh SCT cc hn (cui giai on ln c kt) ca cc khi st n MSA

Thi gian lp t st t c kt U=85%

Cc thng s t phng th nghim: 316 /kN m , K = 5e-4 m/ngy,

H = 18m.

Quan h ng sut v h s rng:

P(kN/m2) 0 12.5 25 50 100 200 400

e 1.58 1.57 1.551 1.518 1.396 1.221 1.027


TI S 3 38

H s nn a : e

aP

= 0.0018

H s c kt Cv : w

(1 )v

K eC

a

= 0.066 m

2/ngy

Nhn t thi gian: Tv: 2v

v

C tT

H

=2.03x10

-4xt

Khi mc c kt Uv = 85% th

Tv = 1.718 - 0.933logx(100 - Uv ) :

1.718-0.933xlog(100-90 )= 2.03x10-4xt t = 1986 ngy = 7 nm.

T th nghim c kt thot - nc (CD). Cc thng s c (kN/m2); (0) ca

Lp 3a nh sau:

c =7.7 (kN/m2); = 23.6 0

Tnh ton tng t nh trn ta c cc kt qu nh sau:

Sc chu ti khi cha c t p.

pS

atk

QQQ 807

2 3

Lp 3a: on ma st m di 11.65m

'v3a11.65

6x 34.962

(kN/m2)

s3af 27.26 (kN/m3)

on ma st dng di 6.25 m

'v3ad 88.66 (kN/m2)

s3af 33.06 (kN/m3)

p p pQ q xA 4028x0.3x0.3 362.52

snew s iQ u f xL 772.2 (kN)

pS

atk

QQQ 507

2 3

gim SCT khi st ma st m:

a atkQ Q Q 807 500.7 300 (kN)

kN

kN


TI S 3 39

Kt lun:

Khi c k n ma st m SCT ca cc gim 23 Tn (khong 30%) trng

hp nn t yu ln tc thi. Trng hp nn t yu ln c kt th SCT ca cc gim

30Tn (khong 37.5%).

Dn chng thm t ti liu phn tch v tnh ton mng cc ca Thy V

Phn c on: Nm 1972 Fellenius o qu trnh pht trin lc ma st m ca hai

cc btng ct thp c ng qua lp t st mm do dy 40m v lp ct dy 15m

cho thy: s c kt li ca lp t st mm b xo trn do ng cc to ra lc ko

xung 300KN trong thi gian 5 thng v 16 thng sau khi ng cc th mi cc chu

lc ko xung l 440KN

3.1.4. Tnh ton xc nh gi tr lc ma st m theo l thuyt Braja Das

Trng hp tnh ton: lp t p l t ri (ct) nm trn lp t st

Chiu su ti v tr trc trung ha:

' '

1 ' '1

2

2

f ff f f fH HL LH HL

L

Trong :

L1 chiu su ti v tr trc trung ha

L chiu di cc 30m

Hf chiu cao lp t p 2m

'f trng lng ring hu hiu ca lp ct p 19 KN/m3

' trng lng ring hu hiu ca lp st bn di 16-10= 6 KN/m 3

6

2192

6

219

2

230)230(

1

1

LL

1L = 18.35 m

Tng lc ko xung trong cc l:

Q2

''')'(

2

11

'' tgKpL

LtgHKp ffn

Trong :

p chu vi cc 0.3x4= 1.2 m 2

K ' = 1- sin ' = 1- sin23.5 0 =0.6

'f trng lng ring hu hiu ca lp ct p 19 KN/m3


TI S 3 40

gc ma st gia cc v t = 0.5- 0.7 ' = 0.5- 0.7x0.41= 0.213

L1 chiu su ti v tr trc trung ha 18.35 m

' trng lng ring hu hiu ca lp st bn di 16-10= 6 KN/m 3

Hf chiu cao lp t p 2m

Q n 2

213.066.02.11818213.02196.02.1

2 tgtg

= 260kN

3.1.5. Xc nh gi tr ma st m s dng phn mm Plaxis.

M hnh cc n.

H 3.1 Mt bng cc v cc mt phng lm vic.

Cc c di 30.5m t su -1.5 n -32m.

Hnh tr h khoan khoan.


TI S 3 41

H3.2. Hnh tr h khoan cc lp t vi a cht tng t nh mc 3.1.2.

Li phn t hu hn

H.3.3. Li PTHH cng gn cc c chia cng mn.


TI S 3 42

Cc phase tnh ton.

Kt qu tnh ton.


TI S 3 43

Nhn xt:

Gi tr ma st m v v tr mt phng trung ha tng dn theo thi gian c

kt.

Kt thc qu trnh c kt ma st m t cc i, tuy nhin qu trnh c kt

din ra rt lu (trong bi ton ny c kt t 99.999% l 43 nm) do lp

t st yu dy. V s thay i gi tr ca ma st m khng ln t 3 nm n

c kt hon ton ma st m tng: khong 5%

-40.0

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

-800 -700 -600 -500 -400 -300 -200 -100 0

Loading

1-Year-Consolidation

3-Year-Consolidation

Minimum-Pore-Pressure


TI S 3 44

Do ta khng cn thit tnh ton ma st m n ht qu trnh c kt ca

t gim thi gian c k ta ch cn tnh ton gi tr ma st m khi mc

c kt ca t khong 85% (t 3-7 nm ty theo loi t)

3.1.6. So snh gi tr ma st m ca cc n gia cc phng php tnh.

Bng tng kt kt qu:

Tiu chun Giai on Gi tr lc MSA

(kN)

V tr mt trung ha (m)

TCVN Tc thi 230 12

C kt hon ton 300 11.65

DAS C kt hon ton 280 18.35

PLAXIS Tc thi 260 12.1

C kt hon ton 317 17.4

Nhn xt:

Cc phng php u cho gi tr ma st m gn nh tng ng nhau. Tuy nhin

gi tr mt phng trung ha ca cc phng php l khc nhau.

Theo tiu chun VN nhm xut xt ma st m trong 2 trng hp thot nc v

khng thot nc k n s pht trin ca sc chng ct theo thi gian c kt tuy nhin

cha k n s pht trin ca modun n hi do kt qu gi tr mt phng trung ha

gn nh khng i.

S dng phn mm Plaixs cho thy nhng thay i ny rt nhiu v ph hp vi

kt qu nghin cu ca cc nh khoa hc trc. Tuy nhin vic tnh ton i hi cc

thng s u vo phi chun xc v da trn s lm vic thot nc (CD) ca t nn.

Tuy nhin xt phng php ny khng a ra c sc chu ti ca cc l bao nhiu nn

vn phi tnh ton SCT theo tiu chun kim tra cc c m bo kh nng chu lc hay

khng.


TI S 3 45

3.2. Xc nh nh hng MSA i vi nhm cc bng phn mm Plaxis

3.2.1. M hnh tnh ton.

Vi cc thng s v cc v t nn tng t nh mc 3.1 ta m hnh xc nh gi

tr lc ma st m trong nhm cc 9 cc nh sau:

B tr cc: khong cch 3 cc l 3D v cc cch mp i l D. Cc cc c

nh s kho st nh hnh di.

Ti trng tc dng ln i: F=3500kN.

M hnh tng t vi thi gian c kt l 400 ngy. Ta c cc kt qu nh

sau:

3.2.2. Kt qu tnh ton

1 2

3


TI S 3 46

Chuyn v tng ca nhm cc v t nn

Chuyn v tng i ca nhm cc v t nn

S truyn ti trng trong cc.


TI S 3 47

Nhn xt:

T kt qu trn ta thy rng ma st m nh hng ln n cc cc bn bin

v gim dn i vi cc cc bn trong.

Gi tr ma st m thay i dn nh sau.

Cc n 1 2 3

Gi tr (kN) 717 711 619 443

% 100 99 86 62

Cc trng tm mng chu nh hng ca ma st m nh nht ch bng 62% gi

tr tnh ton ma st m cho cc n. Do nu qu trnh tnh ton thit k mng nu c

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

-800.0 -700.0 -600.0 -500.0 -400.0 -300.0 -200.0 -100.0 0.0

3

1

2

Single


TI S 3 48

k n ma st m m khng xt n s nh hng ca h s nhm th chi ph thit k rt

cao.

Cn c thm nhiu nghin cu tng th v vn ny, trn th gii cng c

nhiu ti nghin cu v nh hng ca ma st m cho nhm cc tuy nhin nhng

nghin cu ny c thc hin gn y nn cha ph bin kt qu. nc ta vi a cht

cc vng nh qun 7, 8, Nh B, ng Bng Sng Cu Long rt thun li cho s

pht trin ca ma st m. Do cn c thm nhiu nghin cu mi v nh hng ca ma

st m i vi nhm cc thit k cng trnh hiu qu v kinh t nht.

Vn dng kt qu trn ta ra mt phng php lm gim nh hng ma st m l

to nhm cc trong cc cc bin s c thit k vi cng di cng nh kh nng chu

lc chu lc ko xung v cc tm mng ch yu chu ti trng cng trnh.

3.3. ng dng phn mm Plaxis trong nh gi nh hng ma st m.

Trong phn l thuyt v ma st m ta bit nhng trng hp ma st m gy

nh hng ln cng trnh, nhng nh hng c th nh th no v c tnh ton c

hay khng?

i vi phng php gii tch ta kh c th nh gi c ht cc nh hng

ny. ng dng cc cng c m hnh v phn tch mnh m ca Plaxis nhm tiu lun xin

xut phng php nh gi nh hng ma st m cho mt s bi ton thc t nh sau:

3.3.1. Kho st nh hng ca ma st m n cng trnh ln cn.

D kin bi ton:

Cn xy dng mt cng trnh mi c 3 tng hm st bn cng trnh hin hu.

Cng trnh hin hu s dng mng cc p BTCT tit din 30x30, cc c sc chu ti l

40T.

Cng trnh hin hu s dng mng cc, cc gn nht nm st mp cng trnh

chun b xy dng. trnh nh hng n cng trnh k bn, tng tng hm ca

cng trnh mi s cch mp cng trnh c 3m.

Thi cng theo phng php h, mc nc ngm ti mt t t nhin. Trong sut

qu trnh thi cng phi tin hnh h thp mc nc ngm m bo h mng c

kh ro.


TI S 3 49

M hnh bi ton

Quy trnh h mc nc ngm theo tng giai on o t:

Trng thi ban u: MNN cao 0.000


TI S 3 50

H MNN ln 1: cao MNN trong h o l -4.000




TI S 3 51


Bng thng s u vo ca cc BTCT 30x30cm


TI S 3 52

Bng thng s u vo ca tng barette:

Bng thng s u vo ca thanh chng ( thp H):

Kt qu cc Phase t 1 n 18:


TI S 3 53

Kt qu cc phase t 1 n 18

Ni lc pht sinh trong cc:


TI S 3 54

Ni lc thn tng:

Kt lun: Khi h MNN trong nn cha c kt gy ra ma st m.

3.3.2. nh hng ca MSA i vi mng cc ca cng trnh c khi xy dng cng trnh

k bn dng mng b

Cng trnh mi s dng mng b trn nn t t nhin. Cao y mng

l -2.000m.

Cng trnh c s dng mng cc, cc 30x30, ti thit k tc dng ln cc l 40T.

Cao y i mng l -2.000m.

Xem xt nh hng ca mng b n SCT ca cc ca cng trnh k bn.

Cng trnh c c m phng nh sau:


TI S 3 55

Mt s kt qu:

-35

-30

-25

-20

-15

-10

-5

0

0 20 40 60

Series1


TI S 3 56

Ni lc ging mng:

M hnh khi c cng trnh mi:

Thng s cc BTCT 30x30cm:


TI S 3 57

Thng s ging mng D 30x60cm:

Thng s i mng dy 80cm:


TI S 3 58

Thng s mng b dy 30cm:

Kt qu cc Phase :


TI S 3 59

Kt qu ni lc pht sinh trong cc:

Ni lc tng thm trong cc khong 32.5T

Ni lc ging mng:

-35

-30

-25

-20

-15

-10

-5

0

0 50 100

Series1


TI S 3 60

3.3.3. Xem xt s dng ln v h xung ca mc nc ngm nh hng n sc chu ti

ca cc

D kin bi ton:

Cng trnh ven sng, mc nc ngm thay i theo ma, qu trnh dng ln v h

xung ca nc ngm s lm cho t c kt hn, gy nn ma st m.

M phng 1 cc nhi D = 600mm, ti trng tc dng ln cc N = 500T.

Mc nc ngm h thp nht vo ma kh, cch mt t t nhin 20m.

Tin hnh tng, gim mc nc ngm 4 ln quan st.

M hnh bi ton 2D:

Kt qu cc phase:


TI S 3 61

Cc kt qu thu c:

H MNN ln 1 Tng MNN ln 1

Lc dc trong cc sau khi MNN sau 4 ln h, tng mc nc ngm:


TI S 3 62

Nhn xt: Lc dc trong cc tng ln do qu trnh h, tng mc nc ngm.

Trong tnh ton thit k phi tm hiu k v a cht thy vn c kt qu hp l nht.

3.3.4. Nhn xt

C th thy rng vi phn mm Plaxis ta c th x l c rt nhiu vn

ca bi ton a k thut.

Nhng v d trn nh mt gi v cc tng thc t sau ny. Lm sao m

phng c ng x thc t ca t nn l vn ht sc quan trong. C th ta m

phng c qu trnh lm vic ca nn theo cc Phase ging nh thc t nhng nhng

thng s ng ca t nn tng ng vi qu trnh l bao nhiu? Gi tr c thay

i theo thi gian khng? M hnh th no k n nh hng ca cc gi tr ?..

l mt s cu hi cn t ra trc khi s dng phng php ny v phn

mm tuy mnh nhng cc thng s u vo khng chun th kt qu s khng ng

vi thc t.


TI S 3 63

CHNG 4: CC BIN PHP GIM NH HNG CA MSA

4.1. Khi qut

T thc nghim hin trng ch ra rng lc ma st m nh trn cc cc ngn

c chiu di khng vt qu 8m, nn n c th c b qua (M.G.Khare v

S.R.Gandhi).

i vi cc cc c chiu di trung bnh, ma st m c xt n v gii php

khc phc c th c chn l tng kh nng chu ti ca cc, bng cch tng chiu

di hoc gim khong cch cc cc.

Tuy nhin, khi m ma st m qu ln, gii php tng kh nng chu ti ca cc

khng cn kinh t hay t hiu qu, th cn phi s dng mt trong cc bin php gim

ma st m. Xut pht t ngun gc v s hnh thnh ma st m, bin php khc phc

ma st m c th chia lm 3 nhm chnh:

Bin php lm tng nhanh tc c kt ca nn t.

Bin php lm gim ti trng ln t nn.

Bin php lm gim ma st gia t v cc trong vng ma st m.

4.2. Bin php lm tng nhanh tc c kt ca nn t

Gim ti a ln cng nh tc ln cn li ca nn t trc khi thi cng cc

bng cc bin php x l nn nh gia ti trc bng t p hoc chn khng, kt hp

vi cc gii php tng nhanh qu trnh c kt, ngha l qu trnh tiu tn p lc l rng

thng d trong nn bng cc ct hay bc thm,v.v...

y cng cn phi ch rng, qu trnh bm nc ngm ca cc cng trnh ln

cn cng pht sinh ra qu trnh c kt trong nn, t cng c th pht sinh ma st m

tc dng ln cc. Do , cn ht sc trnh hin tng bm ht nc ngm xung quanh

cng trnh mng cc m khng kim sot phm vi cng nh mc nh hng ca n

i vi cng trnh mng.

4.2.1. Ni dung bin php

i vi cng trnh c thi gian thi cng khng gp, cng trnh c h mng cc

trong t yu cha c kt. gim ma st m, c th b tr cc phng tin thot nc

theo phng thng ng (ging ct hoc bc thm) nn nc c kt cc lp su trong

t yu di tc dng ti trng p s c iu kin thot nhanh (thot theo phng

nm ngang ra ging ct hoc bc thm ri theo chng thot ln mt t t nhin). Tuy

nhin, m bo pht huy c hiu qu thot nc ny th chiu cao nn p ti


TI S 3 64

thiu nn l 4m, do nu nn p khng ln th ta kt hp vi gia ti trc

pht huy hiu qu ca cc ng thm thng ng.

Khi s dng cc gii php thot nc c kt thng ng nht thit phi b tr tng

ct m.

Ging ct ch nn dng loi c ng knh t 35-45 cm, b tr kiu hoa mai vi

khong cch gia cc ging bng 8-10 ln ng knh ging.

Nu dng bc thm th cng nn b tr so le kiu hoa mai vi c ly khng nn

di 1,3m v khng qu 2,2m.

Khi s dng cc gii php thot nc c kt thng ng nn kt hp vi bin

php gia ti trc v trong mi trng hp thi gian duy tr ti trng p khng nn

di 6 thng.

Hnh 4.1. S b tr gia ti trc kt hp ging ct lm tng nhanh qu trnh c kt ca t

u im: c th p dng cho c cc ng v cc khoan nhi.

Nhc im: cn thi gian thi cng lu v mt bng ln (nu c p gia ti).

4.3. Bin php lm gim ti trng ln t nn

Kim sot v hn ch n mc c th ng sut phn b trong nn t yu do ti trng

cht thm trong khi thi cng cng trnh cng nh sau khi cng trnh c a vo s

D L

Khoi at ap (thng la cat)

kh

kvkv

kh

kv

kh

kv

kh

kv

kh

kv

kh

Tru thoat nc

em cat

H

co ket trc lp at yeu nham lam giam ma sat am

Hnh 3-1: Gia tai trc ket hp dung gieng cat thoat nc lam


TI S 3 65

dng. Gii php c th c a ra l s dng sn gim ti trn h cc hoc l nn t

p trn h cc vt liu trn (t trn xi mng, t trn vi,...) c lt vi a k thut.


i vi cc cng trnh c ph ti l hng ha, vt liu, container, . ti trng ph

c gi tr ln th dng cc sn b tng c x l cc t ph ti.

Trong cng trnh giao thng, sn gim ti (b tr cho nn ng p cao sau m

cu), ngy cng c s dng rng ri, t p nn c p ln sn gim ti ch khng

tc dng trc tip ln nn t yu bn di. Cc d n ln khu vc ng bng sng

Cu Long s dng gii php sn gim ti nh: cc cu Hng Li, M Thanh, Rch

Mp thuc d n xy dng tuyn ng Nam Sng Hu.

Trong trng hp ny, lc ma st m gim ng k do ph ti c truyn xung

tng t tt c kh nng chu lc. Nh vy ti trng ph s t nh hng n lp t c

tnh nn ln cao t lm gim ln ca t nn, dn n gim lc ko xung ca t

xung quanh cc.

4.3.2. u im

Bin php ny d thi cng, lm gim ng k lc ko xung ca cc, an ton v k

thut. Bin php ny c bit thch hp vi cc cng trnh c xy dng tn nn cao

trn nn t yu ln nh hin nay.

4.3.3. Nhc im

Xt v mt kinh t th cha t hiu qu cao.

Trong nhm ny cn phi k n phng php x l nn bng phng php c kt

chn khng. Cc phng php ny mi c p dng ti Vit Nam c hiu qu rt ln

tuy nhin gi thnh cn cao.

4.4. Bin php lm gim ma st gia t v cc trong vng ma st m

Gim ma st, s dnh bm gia b mt t v cc trong phn nn c xut hin ma st

m. Trong nhm gii php ny bao gm nhiu phng n c nghin cu, chng

minh v bo co trong cc bi bo ca nhiu tc gi.


To lp ph mt ngoi ngn nga tip xc trc tip gia cc v t xung quanh

lm gim ma st thnh bn gia cc v lp t nn xung quanh cc.

Bitumen thng c s dng ph xung quanh cc bi v c tnh do nht ca

n, ng x nh vt liu rn n hi di tc ng ti tc thi (ng cc) v nh cht lng


TI S 3 66

nht vi sc chng ct nh khi tc di chuyn thp. Nhng thnh cng s dng

bitumen lm gim lc ko xung ph thuc nhiu vo cc yu t nh lai v tnh cht

ca bitumen, mc thm nhp ca ht t vo bitumen, s ph hng ca bitumen khi

ng cc, v nhit mi trng.

Theo kt qu nghin cu nh hng ca lp ph bitumen lm gim ma st m trong

cc ca Brons (1969), kt qu nghin cu cho thy lc ma st m gim khong 90% so

vi trng hp khng dng lp ph mt ngoi.

Theo kt qu nghin cu ca Bjerrum (1969), i vi cc dng lp ph bitumen v

dng bn bentonite bo v khi h cc th lc ko xung gim 92%. Trong trng cc

dng bn bitonite gi n nh th lc ko xung gim 15%.

V vy c th kt lun: lp ph bitumen c tc dng lm gim lc ko xung khong

75%. Tuy nhin, nu khng c bn bentonite khi h cc th tc dng ca bitumen ch cn

khong 30% m thi, do lp ph bitumen b ph hng trong qu trnh h cc. Do

chiu dy lp ph bitumen nn vo khong 4-5mm nga cho trng hp b xc khi

h cc.

4.4.2. u im

Thi cng n gin, kinh ph thp

4.4.3. Nhc im

Ch c th p dng cho cc ng, khng p dng c cho cc khoan nhi.

Ngoi ra, ngi ta c th khoan to l c kch thc ln hn kch thc cc trong

vng chu ma st m, sau thi cng cc m vn gi nguyn khong trng xung quanh

v c lp y bng bentonite.

4.5. Cc bin php khc

4.5.1. Phng php in thm (Electro Osmosis)

Ni dung bin php

Phng php ny c s dng nhm lm gim tm thi lc dnh gia t (st) v b

mt cc thp. Bjerrum et al.(1969) trnh by k thut in thm vi cc cc thp l cc

u cc m, nhm lm gim ma st m (Electro-osomosis gy ra s di chuyn cc ht

proton tch in dng xuyn qua mt mng trao i proton (proton exchange membrane

- PEM), t dn n hin tng cc phn t nc b ko t cc dng sang cc m)


TI S 3 67

Trong cc th nghim hin trng, khi dng in trong cc cc thp ng vai tr l

cc u cc m c tng t 4A n 80A, th ma st m gim n gi tr nh c th c

b qua.

u im

u im vt tri ca phng php l c kh nng hi phc lc dnh hu ch gia

t v cc sau khi qu trnh pht sinh ma st m kt thc.

Nhc im

Phng php ny ch c tc dng tc thi trong khi tc dng dng in. Do , dng

in cn phi duy tr cho n khi nn t xung quanh cc t ln n nh, tc ln

khng cn ln hn so vi cc.

Nhn chung, phng php ny gi thnh cao hn so vi cc phng php khc do

him khi c s dng.

4.5.2. Cch ly gia vng mi trng t nn c kh nng xy ra ma st m v b mt cc

Ma st m trn b mt cc c th b loi tr bng cch s dng mt ng bao bn

ngoi cc trong phm vi c kh nng xut hin ma st m.

Tuy nhin phng php ny khng nn s dng trong trng hp kh nng chu ti

ca cc cn n thnh phn ma st bn (ma st dng) sau khi kt thc qu trnh pht

sinh ma st m. V nu nh chiu di pht sinh ma st m l tng i ln th tnh kinh t

khng cao.

S cch ly gia t v cc cng c th t c vi cc cc c vut thon. Cc th

nghim m hnh ca Sawaguchi (1982) cho thy rng cc c vut thon gim lc

ko n 90% so vi cc cc thnh thng.

4.5.3. H thng cc bo v xung quanh nhm cc chnh

Phng php ny s dng mt h thng cc cc ng chu ti b tr gn nhau, xung

quanh c bao bc bi cc cc gi l cc bo v. Chnh cc cc bo v ny s chu ma

st m.

Okabe (1977) bo co mt ng dng thnh cng theo phng php h thng cc

bo v, ca cng trnh trm chuyn (ng st) Nht. Cc cc bo v bn ngoi chu

mt lc ma st m l 3500KN trong khi cc cc bn trong (cc chu ti trng chnh

ca mng cng trnh) li gn nh khng c xut hin ma st m.


TI S 3 68

Tuy nhin, y cng thy rng, phng php ny i hi cc cc cu ti phi b tr

gn nhau, iu ny lm gim h s nhm cc. V gi thnh c th i ln cao. Do cn

phi cn nhc v kh nng chu ti ca nhm cc cng nh tnh kinh t ca phng php.

4.5.4. Phng php s dng lp bao ph bng bn Betonite v Bittum

4.5.4.1. Phng php bao ph bng bn Betonite

Cc kt qu th nghim ca Brons et al. (1969) v Bjerrum et al. (1969) ch ra

rng, lp bn bentonite c kh nng lm gim lc dnh gia t v cc.

Edwards v Visser (1969) cng trnh by nghin cu ca mnh v vn ny

(1969). Trong nghin cu ny, cc c ph mt lp bentonite dy t 30mm 40 mm.

Lc ma st m trn cc cc c bc lp Bentonite khong 120KN, trong khi trn cc

cc khng ph Bentonite l 700 800KN. Lp ph bentonite thng s dng cho cc

c ti ch (cc khoan nhi).

4.5.4.2. Phng php bao ph bng sn Bittum

Vic sn ph b mt cc bng cc loi vt liu c tnh nht gim ma st l gii

php kh thi v kinh t nht trn cc cc c sn (Baligh et al. 1978).

Cc vn t ra l:

Xc nh loi vt liu ph hp s dng.

Xc nh chiu dy cn thit.

Quy trnh thi cng.

4.5.4.3. Xc nh loi vt liu

a. Ni dung

M.G.Khare v S.R.Gandhi hc vin cng ngh Madras, Chennai, n tin

hnh nghin cu trn m hnh b mt ma st gia cc v t th, s dng b thit b ct

trc tip. B mt cc c m hnh bng khi thp mm kch thc 8,5mm x 8,5 mm x

2,8mm. t s dng trong nghin cu ny cho nh trong bng.


TI S 3 69

B thit b ct trc tip c ci tin:

Hnh 4.2. S th nghim ct trc tip trn lp thp c sn ph v ct

Nghin cu c tin hnh i vi 2 loi vt liu sn ph l Shalikote

(T-25) TM v bittum mc 30 - 40. Shalikote (T-25) cng l mt loi sn phm t bittum,

nhng c nht thp hn.

Ct c cha trong hp ny c lm cht n cht tng i l 70%. Tc

th nghim ct l 0,25mm/min.

Cc mu c ph lp Shalikote (T-25) v bittum vi chiu dy ln lt l 2mm,

3mm, v 5mm.

Tt c cc th nghim c tin hnh nhit phng l 31oC.


TI S 3 70

Hnh 4.3. ng sut ct

b. Kt qu

S gim ng sut ct trong cc th nghim ny c xem nh l nh hng ca lp

sn ph i vi s gim ca ma st m. V kt qu th nghim sc khng ct d c

a ra nh gi mc nh hng ca lp ph bng Shalikote (T-25) TM v ca

bittum.

Cc th nghim trn Shalikote (T-25) TM cho thy mt s gia tng ban u ca ma

st b mt sau gim ng k khi mu b ct.


TI S 3 71

i vi bittum, ma st b mt gia tng khi cc ht ct xuyn vo trong lp ph, sau

th hu nh khng i. Th nghim cho thy rng, ma st b mt c huy ng y

tng ng vi dch chuyn tng i ch khong vi milimet.

Cc kt qu th nghim cho thy rng, lp ph bittum t c gim ng sut ct

d ln nht vi mi gi tr ng sut php v chiu dy so vi Shalikote (T-25)TM.

hnh 4.3 ti ng sut php bng 25 KN, ng sut ct trn cc mu c ph 1 lp

Shalikote (T-25) TM dy 1mm v 1.36mm cao hn so vi cc mu khng c ph.

iu ny c th l do thnh phn lc dnh ca Shalikote (T-25). Tuy nhin trong cc mu

c qut bittum, iu ny hon ton khng xy ra.

cc mu ph Shalikote (T-25) TM cho thy mt s co ngt chiu dy lp sn ph.

Chiu dy ban u sn ph l 2mm, 3mm v 5mm, nhng sau khi bo dng th ch cn

li tng ng l 1mm, 1.36mm, v 2.16mm. S co ngt ny c th gy nh hng khng

tt do c th pht sinh hin tng nt n lp ph. Shalikote (T-25)TM c kh nng lm

gim ng sut ct khong t 23% n 60%.

i vi bittum cho thy kh nng gim ng sut ct mt cch ng k. Trn cc

mu ph bittum, ng sut ct gim t 85% - 97% khi so snh vi cc mu khng ph.

Khi chiu dy lp ph t 2mm 5mm th ng sut ct gim ng k. Khi chiu dy l

3mm, ng sut ct gim n bng hoc ln 90%, v thch hp vi cc trng hp thc

t. Do , chiu dy lp ph bng bittum ch cn dy khong 3mm l c th kh nng

gim lc ma st m.

c. Kt lun

Trong nghin cu ny, cc nghin cu thc nghim c tin hnh so snh

gia Shalikote (T-25) TM v bittum mac 30-40 vi cc chiu dy 2mm, 3mm v 5mm.

Shalikote (T-25) TM c th gim ng sut ct t 30% - 50%. Tuy nhin, nhc

im ln nht ca n l c th pht sinh cc vt nt do co ngt khi sn ph ln b mt

cc.

Bittum c th nng gim ng sut ct ln nht. ng sut ct c th gim t 80%

n 97%, ph thuc vo ng sut php v chiu dy lp sn ph. Chiu dy lp ph

c khuyn nn s dng l khong 3mm.

nh hng ca lp sn ph i vi vic gim ma st m ph thuc vo cc c tnh

ca cc, t v vt liu sn ph. Nh vy, qua th nghim ny, v cng ging nh theo


TI S 3 72

kin ca cc tc gi, vt liu bao ph c nht, tnh mm cng ln th kh nng gim

ma st m cng cao (in hnh l so snh gia 2 vt liu l Shalikote (T-25) v bittum).

Nu bittum c s dng th bittum c kim ln cng cao th kh nng gim ma

st cng ln. Vi cc th nghim thc t hin trng v cc th nghim trong phng

(Johannesen et al. 1965; v 1969; Walker v Darwall. 1973, Clemente 1979 v 1981, v

Fellenius 1975 v 1979) cng a ra kt lun rng, vi chiu dy khng ln hn 1/16 in

(bng 1 2mm), vi loi bittum c kim ln bng hoc ln hn cp 80 / 100, s c th

gim ng k lc ct gia b mt cc v t ti cc tc chuyn dch tng i thc t

gia t v cc.

Ngoi nguyn nhn gim ma st gia t v cc, tnh nht cng cao th kh nng

bm dnh ca vt liu bao ph ln b mt cc cng ln, kh nng b co ngt, nt n do

nh hng thi tit cng nh.

Do , theo kin ca nhiu tc gi khuyn nn s dng loi bittum c kim ln

t 80/100. y l loi vt liu c cc c tnh ph hp, p ng c yu cu v c sn

trn th trng.

4.5.4.4. Chiu dy lp bao ph

Theo kt qu th nghim trn, ta cng c th thy rng, chiu dy lp bao ph cng

ln th kh nng gim ma st m cng cao. Tuy nhin, v cc l do kinh t v iu kin

thi cng, cng nh trong qu trnh lp ph lm vic, nn cn phi xc nh chiu dy

gim ma st m n mt gi tr mong mun. Khng phi lp bittum c chiu dy cng

ln th cng tt.

Theo kin kin ca M.G.Khare v S.R.Gandhi, th chiu dy lp ph yu cu l

khong 3mm.

Cc kt qu o c ca Brons et al. (1969) v Bjerrum et al. (1969) cho thy rng

ch cn mt lp bittum sn ph mng cng lm gim ma st m. Cc kt qu th

nghim ca Bjerrum et al. (1969) cho thy mt lp bittum sn ph dy 1mm c th lm

gim 90% ln ca lc ko (ma st m) so vi cc cc khng sn ph.

Mt vn na c lin quan n chiu dy lp ph l lm sao bo m cht

lng lp ph trong qu trnh lu tr, v trnh b bc trc trong qu trnh h cc, c bit

l khi h cc xuyn qua cc lp t ht th. Theo kinh nghim ca Fellenius, th chiu

dy lp t ht th ch cn vi mt l gy ra nguy c co rch lp bittum rt ln.

tng c cc kin cho rng, lp ph cng mng th cng d b try rch trong qu trnh


TI S 3 73

h cc. Trong mt bi ton c th c a ra ca cc tc gi Alphonus, I.M.Claessen, v

Endre Horvat (1974), chiu dy lp ph c ngh ln ti 10mm (1cm). Tuy nhin,

theo quan im ca Fellenius (1975), chiu dy 10 cm l khng thc t, n qu dy,

khng ch l qu mc, m cn l khng th thc hin c nu khng s dng cc

bin php c bit trong qu trnh thi cng sn ph, cng nh trong qu trnh bo dng,

lu tr trnh b bin dng do hin tng ho mm. Hn na, Fellenius cng khng

ng vi kin ca cc tc gi trn, v theo ng, kh nng b co rch lp ph cng ln

nn nh lp ny cng dy. Cc bi bo ca Bjerrum et al, cng a ra kin rng cc cc

c h xuyn qua mt lp t ht th cng c th b co rch lp bittum d cho lp ny

dy hn.

Nhiu kinh nghim c c kt t th nghim trong phng v hin trng cho

thy rng, ngoi tr m bo khi lng bittum cn ti thiu, mt lp sn ph mng

khong 1mm n 2mm vi cc loi bittum mm, Mc t 60/70 tr ln l ph hp bi n

hn ch s chy mm trong qu trnh lu tr v s bc tch trong qu trnh h cc.

Hn na, theo nhn nhn thc t ca Fellenius, khng cn phi i hi ma st m

phi b loi tr khong 90%, hay ngha l ch cn t khong 80%. Do , ch cn cc

loi bittum thng thng c bn trn th trng cng thch hp. Fellenius ngh nn

s dng bittum c kim ln 85/100, theo h thng phn loi ca American Society for

Testing and Material (ASTM) D-946, vi chiu dy sn ph t 1mm n 2mm.

ng thi, theo kin ca Fellenius, thi tc ln, hay bin dng ct thc t ngoi

hin trng rt nh so vi tc th nghim trong phng. Theo kt qu th nghim ct m

Fellenius tng thc hin vi mu t st trn b mt btng c ph lp bittum mac 120 ,

ng sut ct li gia tng khi tc bin dng tng. Do , c th nhn nhn rng, ng sut

ct ca b mt tip xc gia cc v t thc t trong nn s nh hn rt nhiu so vi cc

kt qu th nghim trong phng. Nh vy, chiu dy i hi thc t ca lp ph bittum

cng s nh hn so vi cc kt qu th nghim.

4.5.4.5. Thi cng lp ph bittum

Theo nh khuyn ngh ca Fellenius cng nhiu tc gi khc, loi bittum thch hp

s dng c mc t 85/100 theo ASTM D-946.

Loi bittum ny c th c sn hoc trt ln b mt cc (c th thi cng cng

trng hoc ti nh my) sau khi c un nng n trng thi ho lng, khong

175oC.


TI S 3 74

Trong trng hp cc cc c sn, cn thit phi m bo dnh ca lp bittum

vi b mt cc. Phng php r nht l ho tan bittum vi cc loi dung mi thng

thng (nh du ho, hoc xng) n khi ho lng ri sau sn ph ln b mt cc.

c bit khi trong iu kin kh hu lnh, vic thi cng v s dng bittum nng l

rt kh khn v c . Do , trong cc trng hp ny, cn phi pha bittum vi mt

loi dung mi nhm lm mm bittum sao cho nhit un ch cn t 75oC l . Tuy

nhin, bittum lng phi c kh nng ng cng nhanh chng nh c tnh gc ca n,

nhm m bo lp ph n nh trn b mt cc trong qu trnh lu tr v khi h cc vo

trong t. Loi bittum nh vy trn th trng gi l RC, cut back bittum (hay cn

gi l nh tng phn tch nhanh).

4.5.4.6. Nhc im ca lp sn ph bittum

Nhc im ln nht ca phng php ny l ma st dng khng th ph hi

trong vng c sn ph lp bittum khi xt v lu di (sau khi chm dt qu trnh pht

sinh ma st m).

Theo kinh nghim ca cc tc gi Fellenius, Briaud, v nhiu tc gi khc, th gi

thnh cc cc c sn ph bittum ln hn cc cc khng sn ph dao ng trong

khong t 10% - 20%. Khong chnh lch gi thnh ny khng phi l nh. Do ,

trong mi trng hp thit k, cn thit phi tin hnh nghin cu cc bin php gim

ma st m n mt mc cho php, bao gm c s so snh gi thnh ca cc phng

n khc nhau.


TI S 3 75

CHNG 5: KT LUN, KIN NGH

5.1. Kt lun

5.1.1. Hin tng ma st m

Ma st m hay sc khng bn m l mt thnh phn lc sinh ra trong cc trng hp

ln ca lp t xung quanh cc ln hn ln ca cc.

C rt nhiu yu t nh hng n ma st m (ch tiu c l ca t, mc nc

ngm, loi cc, cc kch thc ca cc, c kt thi gian c kt ca t) cho nn vic

xy dng mt m hnh tnh ton tng qut cho bi ton ny rt phc tp.

Ma st m pht trin theo thi gian v t gi tr ln nht khi kt thc c kt.

Lc ma st m t l vi p lc ngang ca t tc ng ln cc v tc ln c kt

ca t, hin tng ma st m s kt thc khi ln c kt chm dt, lc by gi ma st

ca t v cc s tr thnh ma st dng.

Lc ma st m khng ch tc ng ln mt bn ca cc m cn tc dng ln mt bn

ca i cc, hoc mt bn ca m cu hay mt tng chn c ta ln cc.

Khi tng chiu cao t p (hoc ph ti) th ma st m tng rt nhanh giai u v

chm li giai on sau, v khi chiu cao p tng n mt gii hn no th ma st

m s tng khng ng k (c th l const). Kt qu tng t cng xy ra i vi chiu

su vng chu nh hng z.

Tu theo chiu cao ca lp t p (hoc ln ca ph ti) v chiu dy ca tng

t yu m chiu su vng chu nh hng ca ma st m c th khng ch trong vng

t yu m c th nh hng sang c lp t tt bn di (khi ln ca lp t tt ln

hn ln ca cc).

5.1.2. Cc phng php tnh ton ma st m

Nhm tiu lun tin hnh 3 phng php tnh ton ma st m v nhn thy rng

kt qu 3 phng php ny l tng ng nhau chng t cc phng php tnh l chun

xc.

Trong phng php thit k theo TCVN l phng php c xut s dng

trong thit k mng cc nc ta. Phng php ny n gin ph hp vi cc quy trnh

quy phm v iu kin a cht Vit Nam.

Phng php theo nh khoa hc DAS cho php xc nh nhanh chng gi tr ma st

m v c th dng d bo nh hng ca ma st m trong thit k s b.


TI S 3 76

S dng phn mm Plaxis l mt phng php mi m nhm n lc nghin cu

v cho ra cc kt qu rt ph hp vi nhng nghin cu trc. Vi phng php ny ta

c th nghin cu su thm cc vn v nh hng ca ma st m m cc phng php

gii tch trc y rt kh khn thc hin.

5.2. Kin ngh

Trong thit k nu c xt n nh hng ca ma st m cn tnh ton c hai ng

x thot nc v khng thot nc ca t v phn tch nh hng ca ma st m l qu

trnh phn tch ng x di hn (ng x thot nc) ca t nn i hi phi c y cc

thng s ng x thot nc t cc th nghim a cht. Nu ch s dng cc thng s t

cc th nghim n gin nh ct trc tip, th nghim nn nhanh th nhng kt qu thu

c s khng chnh xc em li ci nhn khng ng v ma st m tng chi ph thit k

nhng khng mang li hiu qu.

Cn xem xt nh hng ca ma st m trong cc trng hp sau:

Cc xuyn qua cc lp t yu vi c kt cn b nh st yu, bn, than bn

nhng ni nn cng trnh c nng cao vi vi chiu dy ln hn 1m trn lp

t yu hay ph ti nn vi ti trng ln t 20kPa tr ln.

Mc nc ngm b h thp ng k do qu trnh thi cng cng trnh mi hay do

khai thc nc ngm trong thnh ph.

Qu trnh thi cng cc p ca mt s cng trnh xy chen c th gy ra ma st m

i cc cc bin ca cng trnh c.

Thit k cc ni trn nn ang c kt.

S dng cc gii php khc phc cn nm r cc iu kin ca cng trnh v

nhng u nhc im ca phng php c la chn ph hp. Mi phng php u

c u v nhc im ring v ph hp vi cc cng trnh khc nhau nn cn phn tch

la chn tht k qu trnh thit k l hiu qu v kinh t nht.

nh hng ca ma st m cho nhm cc l mt vn cn nghin cu su thm

nc ta. V i vi a cht cc vng nh qun 7, 8, Nh B, ng Bng Sng Cu

Long lp t yu trn mt dy, rt d xy ra hin tng ma st m tc dng ln cc khi

san lp mt bng. Nu khng c mt nhn nh chnh xc v mc nh hng ca ma

st m n cc trong nhng khu vc ny d dn n cc s c khi s dng cng trnh.

Nhng tnh n ma st m ma khng k n nh hng ca nhm cc th chi ph cng


TI S 3 77

trnh s tng. Nhng gi tr tng cng nh tit kim l bao nhiu th cn c nhng nghin

cu c th xc nh.

MN HC: K THUT NN MNG NNG CAO GVGD: TS. V PHN

TI S 3 78

TI LIU THAM KHO

[1]. PGS. TS . V Phn, Hong Th Thao, Phn tch thit k Mng Cc, NXB H

Quc Gia H Ch Minh, 2010.

[2]. Joseph E.Bolwes, Foundation analysis and design fifth edition, Page 1053,

McGraw-Hill Book Co Singapore, 1997.

[3]. TS. u Vn Ng, Nghin cu nh hng ca ma st m n cng trnh v cc

bin php lm gim thiu ma st m, Science & Technology Development, Vol

12, No.06 2009.

[4]. Nhm 3 KTXD2008, Bo co tiu lun mn hc Mng cc Ma st m.

Thng 6/2009.

[5]. Shen Ruifu, A Thesis Submitted For The Degree of Doctor of Philosophy

Departmentoof Civil Engineering National University of Singapore, 2008.

[6]. Dr Bengt H. Fellenius, 1984, Negative skin friction and Settlement on pile.

[7]. TS. Nguyn Minh Tm, Bi ging ng dng Plaxis trong tnh ton a k thut.

[8]. Trn Vn Vit, Cm nang dng cho k s a k thut, Nh xut bn xy dng.

[9]. V Cng Ng v Nguyn Thi, Mng cc phn tch v thit k, NXB khoa hc v

k thut.

[10]. Tiu chun thit k mng cc - TCXDVN 205-1998.

[11]. Tiu chun thit k cu - 22 TCN 272.

[12]. Chu Ngc n, C Hc t, NXB i hc Quc gia TP. H Ch Minh

[13]. Richard P. Long & Kent A. Healy, thng 3/1974, Final report Negative skin

friction on pile.

[14]. Ali Sharif, Negative skin frition on single pile in clay to direct and indirect

loading, 1998.

[15]. E.E.Alonso, A.Josa and A.Ledesma, Negative skin friction on piles: A simplified

analysis and prediction procedure, 1984, Geo1technique 34, No.3, 341-357.

[16]. Manuals Plaxis 2D Foudation, 2001, A.Abalkema Publishers.

[17]. Cc hnh nh biu trong Phn 2 mc III l cc biu kt qu tnh ton ca

phn mm Plaxis 2D Foundation.

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