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CNG NGH X L NN V THI CNG , P PH SNG TRN NN T YU
PGS.TS L Xun Roanh
Vn phng Chng trnh bin
1. M U
Nc ta c ng b bin di trn 3200km v c rt nhiu con sng ra bin v vy cn
phi c h thng bao, chn nc. Theo thng k hin nay chng ta xy dng mt h
thng bin v ca sng tng i n nh vi chiu di khong trn 2700km. An ton
ca cc con ny li ph thuc vo cht lng kt cu ca , trong vt liu thn v
nn l rt quan trng. Trong qu trnh qun l iu cho thy hin tng thm qua
nn, hin tng mc nc ngm dng cao trong thn , hin tng ln, st, trt... v
ang xy ra mt s on . iu ny gy nhiu kh khn cho cng tc qun l an ton
. Ni dung sau y s trnh by cng ngh v phng php x l nn , p, mng
ng khi thi cng chng trn nn t yu.
1.1 Phn loi t yu theo ngun gc thnh to
Vic phn loi t yu c da theo ngun gc cu thnh, chng c hai nhm: Ngun gc
khong vt v ngun gc hu c.
Loi c ngun gc khong vt thng l st hoc st trm tch trong nc ven bin, vng
vnh, m h, ng bng tam gic chu; loi ny c th ln hu c trong qu trnh trm tch (hm
lng hu c c th ti 10 - 12 %) nn c th c mu nu en, xm en, c mi. i vi loi ny,
c xc nh l t yu nu trng thi t nhin, m ca chng gn bng hoc cao hn gii
hn chy, h s rng ln (st e 1,5 , st e 1), lc dnh C theo kt qu ct nhanh khng thot
nc t 0,15 daN/cm2 tr xung, gc ni ma st t 0 - 10
hoc lc dnh t kt qu th nghim
ct cnh hin trng Cu 0,35 daN/cm2.
Ngoi ra cc vng thung lng cn c th hnh thnh t yu di dng bn ct, bn ct mn (h
s rng e > 1,0, bo ha G > 0,8).
Loi c ngun gc hu c thng hnh thnh t m ly, ni nc tch ng thng xuyn, mc
nc ngm cao, ti y cc loi thc vt pht trin, thi ra v phn hy, to ra cc vt lng hu
c ln vi cc trm tch khong vt. Loi ny thng gi l t m ly than bn, hm lng hu
c chim ti 20 - 80%, thng c mu en hay nu sm, cu trc khng mn (v ln cc tn d
thc vt). i vi loi ny c xc nh l t yu nu h s rng v cc c trng sc chng ct
ca chng cng t cc tr s nh ni trn.
t yu m ly than bn cn c phn theo t l lng hu c c trong chng:
Lng hu c c t 20 - 30% : t nhim than bn
Lng hu c c t 30 - 60% : t than bn
Lng hu c trn 60% : Than bn
1.2 Phn loi t yu theo trng thi t nhin
Nhng nu phn theo trng thi vt ly t nhin, t yu da theo st B c phn nhm nh
sau:
BW W
W W
d
nh d
Trong : W, Wd, Wnh l m trng thi t nhin, gii hn do v gii hn nho ca t yu.
Nu B > 1 th c gi l bn st (t yu trng thi chy)
Nu 0,75 < B 1 l t yu do chy.
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2. CNG NGH THI CNG V X L THNG DNG
Hin nay c kh nhiu gii php x l nn p trn t yu, chung quy li c cc gii php
chnh sau. - Ci thin s n nh ca nn p (nh lm thoi mi p, tng chiu rng y , lm b phn
p, gim trng lng khi p, cho nn p chn su vo t yu);
- Tng kh nng chu ti ca nn bng thay i ch tiu c l (tng , C) ca t yu;
- Tng nhanh tc c kt hoc gim ln tng cng (nh lm m ct, cc ct, ct t gia c
vi, nn cc).
Ni chung cc bin php x l nn u c lin quan c vn n nh v ln. Mi trng hp c
th u c mt hoc nhiu bin php x l thch hp, vic chn bin php no cn phi phn tch
k, y .
2.2.1 X l nn bng m ct
Khi ta thay lp t yu hoc mt phn lp t yu nm di nn mng cng trnh bng m ct s
c tc dng:
- m ct ng vai tr nh mt lp chu lc, c kh nng tip thu c ti trng ca cng trnh v
truyn ti trng xung lp t chu lc pha di. Cng khn ct ca t ct ln do tng
kh nng chu ti ca nn.
- Ct c tnh p co thp do gim c ln ca cng trnh.
- Ct c tnh thm mnh nn n c tc dng tng nhanh qu trnh c kt ca nn khi chu ti trng
ngai.
- Tng kh nng n nh khi cng trnh c ti trng ngang v ct trong lp m sau khi m cht s
c lc ma st ln lm tng kh nng chng trt.
Thi cng m ct: m ct c cu to tng i n gin, nn c o vi chiu su d tng
ng vi chiu dy m ct, h s mi o ph thuc vo tnh cht t nn, chiu rng o L, sau
ct xung v m cht, vi nn bo ha nc cn tri thm mt lp vi a k thut ngn
khng cho ct chm ln vo t nn. Sau khi thi cng xong m ct tin hnh p ln trn lp
m ct. Mt ct ngang c chiu cao H, chiu rng mt b, h s mi m ( xem hnh).
Hnh 1: X l nn bng m ct
Nguyn l lm vic: Khi p trn lp m ct, m ct ng vai tr nh mt mt thot nc
nn. Di tc dng ca ti trng t p nc trong l rng nn c thot ra qua lp m ct, t
nn c nn cht nhanh hn.
m ct cn ng vai tr nh mt b phn p lm tng sc chu ti cho t nn.
Phng php m ct s dng c hiu qa nht khi lp t yu trng thi bo ha nc, chiu
dy lp t yu khng ln lm, chnh lch ct nc khng cao v gn ni xy dng c sn vt liu
ct.
K thut thi cng m ct
- Chun b mt bng thi cng tuyn . - Dng my o hoc my i o mng vi chiu su d thit k m ct.
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- Tri mt lp vi a k thut xung y h mng. Lp vi a k thut c tc dng ngn khng cho ct chm ln vo t nn trong qa trnh thi cng m ct m bo chiu dy
m ct ng thit k.
- m nn ct: Ct c chn lm vt liu lp m c ri thnh tng lp. K thut c bn nh sau :
- m th cng nng 30kg: chiu dy lp ri khong 20 cm;
- m bn rung: chiu dy lp ri khong 25 cm;
- Nu dng m bnh xch: chiu dy lp ri khong 30cm 40 cm ;
- Nu dng loi m rung c phun nc U20: chiu dy lp ri khong 100cm n 150 cm.
Khi m nn m ct bng bn rung th c th b tr mt hoc ghp hai, ba m bn rung vi
nhau, ri chia din m ra thnh nhiu khu vc nh m. m theo trnh t ng hng li, vt
m trong thi gian 15 - 20 pht trn dit m 6m2 th ct trong lp m s t n cht trung
bnh. Nu dng hn hp ct v si lm vt liu lp m th khi thi gian m 40 pht trn din
m 12m2, o cht trong lp m c th t ti D = 0,70. Khi s lt m 8 10 ln th loi ct
ht trung, c t nc c th t ti D = 0,70.
Trng hp m nn m ct bng xe bnh xch th yu cu vt xch phi st nhau. Sau khi m
mt lt ngang xong th li phi chuyn sang mt lt dc khc v c tin hnh nh vy cho n
khi t ti cht thit k. Tc di chuyn lc ban u ca xe thng vo khong 25m/pht.
i vi nn khu vc ng bng sng Cu Long, kinh nghim thc t cho hay: t nn khu
vc ny thng bo ha nc v mc nc ngm xp x mt t t nhin nn m nn m ct
bng phng php xa lc l thch hp hn c. Phng php ny dng loi xa thp di l,3 n l,4
mt v c 4 - 6 rng, mi rng ca xa di 25 - 30 cm v rng 2 - 4 cm; trng lng ton b ca xa
vo khong 4,4Kg. Khi thi cng, lp ct u tin c ri dy hn vo khong 15cm - 20cm so
vi cc lp ct tip theo. Chiu dy trung bnh ca cc lp ri (30 35)cm.
Qu trnh vn hnh ca phng php xa nh sau: u tin nng xa ln cao khong 50cm, sau
th ri t do v tin hnh lc xa ngp su dn vo trong m ct. Mi ln xa lc khong 16 ln,
c mt lt dc ri li mt lt ngang. Theo kinh nghim thi cng, mi lp ct ri ch cn xa bn
ln l t cht trung bnh.
Cc ch tiu nh gi cht lng m nn: Khi thi cng m ct, vic trc tin l xc nh cc
ch tiu m nn. nh gi cht lng m nn ngi ta thng da vo hai ch tiu quan
trng: cht v m m nn.
nh gi cht ca ct trong lp m, c th dng h s rng hoc cht tng i D.
m ct sau khi c m nn xong c th p dng mt trong hai phng php sau y kim
tra cht: phng php dng phao Kovalv, phng php xuyn tiu chun.
2.2.2 X l nn bng bc thm, ging ct
(a) Bc thm Ngi ta c th tng kh nng thot nc trong nn qua h thng bc thm thng ng. Bc thm
c ni tip vi y khi p thng qua lp ct m, chiu dy lp ct m khong 50 cm. S
phng php nh hnh biu din sau y.
Hnh 2: X l nn bng bc thm, ni tip lp ct m ngay di thn
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Hnh 3: Cm bc thm vo nn, ni thng dn vi lp m ct
Vic thi cng bc thm bng my, cn cm gim bc vo nn, mi cn nh mc, bc c gi li
trong nn. Bc thm dng lm phng tin thot nc c kt thng ng phi t c cc yu
cu sau:
Kch thc l v lc ca bc
o (xc nh theo tiu chun ASTM D4571) : 095 75 m
H s thm ca v lc (ASTM D4491) : 1.10-4 m/sec
Kh nng thot nc ca bc thm vi
o p lc 350 KN/m2 (ASTM D4716) : qw 60.10-6
m3/sec
Cng chu ko ng vi dn di di 10%
o (ASTM D4595) nhm chng t khi thi cng : 1 KN/bc
B rng ca bc thm ( ph hp vi
o thit b cm bc tiu chun ho) : 100 mm 0,05 mm
(b) Ging ct
Ging ct ch nn dng loi c ng knh khong 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 1,3 m v
khng qu 2,2 m.
Vic quyt nh chiu su ging ct hoc bc thm l mt vn kinh t - k thut i hi ngi
thit k cn phi cn nhc da vo s phn b ln ca cc lp t yu theo chiu su di tc
dng ca ti trng p i vi mi trng hp thit k c th. Khng nht thit phi b tr n ht
phm vi chu nh hng ca ti trng p (phm vi chu ln) nh ni trn m ch cn b tr
n mt su c tr s ln c kt ca cc lp t yu, t tr ln chim mt t l ln so
vi tr s ln c kt Sc d bo c sao cho nu tng nhanh tc c kt trong phm vi c b tr
ging hoc bc ny l t c tiu chun v ln c kt cho php cn li ni iu trn
trong thi hn thi cng quy nh.
Do vy ngi thit k phi a ra cc phng n b tr ging hoc bc thm khc nhau (v su
v v khong cch). Trong mi phng n b tr v chiu su u phi m bo tha mn iu
kin trn.
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Trong thi cng nn ng, 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. C th dng bt k loi t no (k c t ln hu c) p gia ti trc. Ta luy p gia
ti trc c php p dc ti 1:0,75 v cht m nn ch cn t K = 0,9 (m nn tiu
chun).
Ngoi hai phng php trn, tng kh nng thot nc trong nn, gim p lc k rng, tng c
kt cho t, ngi ta c th s dng gii php bm ht chn khng nn. Phng php tin hnh
chnh nh sau: Kim tra ch tiu c l nn, h s thm; to mn ngn cch chn khng trn mt
nn; to l ht chn khng; ht gim p trong nn bng bm chn khng.
2.2.3 S dng vi a k thut gia c
i vi nhng on tng i cao, cn thi cng trong mt ma qua vng t yu c th dng
vi a k thut gia c nn v thn . t cc lp vi a k thut ln b mt phn cch gia
thn v nn , ng thi t cc lp vi a k thut cc cao trnh khc nhau trong thn
nm song song vi mt nn. Lp vi a k thut t mt nn c tc dng phn cch nn v
thn , lm cho khi t p khng b ln chm vo nn, p lc t p phn b tng i
ng u vo mt nn to iu kin cho nn c kt t t. Lp vi t nm ngang trong thn c
tc dng phn b p lc u theo tng cao trnh mt ct ngang , tng bn chng trt ca
khi t p v gim mt ct ngang .
Hnh 4: X l bng vi a k thut tng n nh bn
(a) Kim tra n nh bn ca vi
S dng gii php ny, khi tnh ton thit k phi bo m iu kin sau:
F Fcp
Trong :
F l lc ko m vi phi chu (T/m)
Fcp l lc ko cho php ca vi rng 1 m (T/m)
Lc ko cho php ca vi Fcp c xc nh theo cc iu kin sau:
iu kin bn ca vi:
FF
kcpmax
Trong :
Fmax l cng chu t ca vi kh 1 m (T/m)
k l h s an ton; ly k = 2 khi vi lm bng plieste v k = 5 nu vi lm bng pliprpilen hoc
plithilen
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iu kin v lc ma st cho php i vi lp vi ri trc tip trn t yu :
F h fcp d i0
1
F h fcp d i 0
2
Trong :
1 v 2 l chiu di vi trong phm vi vng hot ng v vng b ng. d l dung trng ca t p;
f ' l h s ma st gia t p v vi cho php dng tnh ton.
hi l chiu cao p trn vi (thay i trong phm vi 1 v 2 , t hi = h n hi =0, biu thc trn l tng lc ma st trn vi trong phm vi vng hot ng v vng b ng:
f k2
3tg
Trong :
l gc ma st trong ca t p xc nh tng ng vi cht thc t ca nn p hoc ca
tng m ct nu c ( );
k' l h s d tr v ma st, ly bng 0,66.
Vi a k thut dng tng cng n nh cho nn p trn t yu c th c b tr mt hoc
nhiu lp (1 - 4 lp), mi lp vi xen k ct p dy 15 - 30 cm ty theo kh nng lu ln. Tng
cng chu ko t ca cc lp vi phi chn bng tr s Fmax c xc nh nh ni trn.
Lu : Cc lp vi pha trn nm trong ct p (mt trn v mt di u tip xc vi ct) th tr
s Fcp tnh theo iu kin trn c nhn 2, t tnh ra tng lc ma st cho php ca cc lp vi.
(b) K thut thi cng vi a k thut
Chun b nn:
- Trc khi tri vi a k thut, mt nn phi c san hoc lp t cao thit k v m n cht yu cu. B mt tip xc vi vi phi tng i phng, m bo cho
vi tip xc tt vi nn. Nhng vt cng sc nhn phi c dn sch khng lm
hng vi.
- Sau khi chun b nn xong, tri vi trc tip ln mt t c chun b theo yu cu t vi trn. Cng cc thm vi lm cng lc vi vic san gt, lin kt cc bng vi k
thut vi nhau bng khu li vi nhau hoc tng chiu rng phn vi ph chng ln
nhau, tuy theo cc c trng ca t nn, cao trnh mt ct ngang m bng ny ph
chng ln bng kia t 0,3 m n lm
- Thi cng vt liu p u tin, th iu quan trng l n nh lp p u tin trn nn t yu cho php cc thit b xy dng i vo thi cng, lp u tin c p x
dng xe t loi nh v cch git li trnh s tip xc ca bnh xe ln vi. p
dng phng php theo di hp i xng t ng trung tm gi cho qa trnh
thi cng lun lun c dng ch U, vic thi cng nh vy s hn ch c s dch
chuyn ngang ca lp t p. Vic thi cng mi dc dng khun c gc ph hp vi
mi dc thit k.
- Sau khi thi cng lp u tin li tri vi lm nh trn sau thi cng tip, c nh th thi cng n cao trnh thit k. Lu trong qa trnh thi cng, ngi thi cng phi chu
trch nhim m bo vi khng b ph hoi khi t vi m; trong nhng trng hp
cc thit hi nhn thy trn vi, nh thu phi bo ngay cho cc k thit k c bin
php gia c kp thi v cc lp tip theo.
2.2.4 X l nn bng b cy
Trong nhng trng hp c th, do tng t yu di qu su, khi x l cc gi php khc k
trn cha thnh cng th ngi ta c th dng gii php b mng gia c y khi p rt hu
hiu. p t trn b lm bng g, trm, tu l da, b cnh cy l mt trong nhng phng php
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s dng lu i, tng c x dng thnh cng trong xy dng . Khi s dng b cy c
nhng tc dng chnh sau:
- M rng din tch truyn ti trng, lm cho nn thin nhin chu mt ti trng phn b u. - C th ngn khng cho mt trt su xuyn qua nn . - Ngn khng cho ct, t chm su vo nn t yu v nc cun tri t p.
Cc loi t mm yu thng c tnh nn ln ln v mc nc ngm cao do sau mt thi gian
ngn nn ln c kt b c th chm xung di mc nc ngm s kh mc nt nn thi gian s
dng c ko di n khi nn c kt xong.
Da trn vt liu s dng c th chia b thnh 2 loi: B mm v b cng.
B mm c lm bng cc b cnh cy hoc cy con nh: trm, tre, tu l da, s vt c ng
knh (2-5) cm thng c dng p ln bin v quai m ly. Ngoi ra b mm cn
c dng lm lp lt trn nn t yu trc khi lm lp m ct thay cho lp vi a k thut.
Hnh 5: X l nn bng b cy b mng
B cng thng c lm bng tre hoc g c ng knh ln ghp li.
Phng php p trn b c u im l thi cng n gin, trng lng nh do nhng ni
c sn vt liu lm b thi y cng l mt phng n kh thi. Tuy nhin vic tnh ton c th cu
to ca b, c bit kh nng dng nhng ni mc nc ngm khng n nh cha c nghin
cu su m thng l b tr cu to theo kinh nghim.
2.2.5 X l nn bng m cc ct
Phng php c kt thot nc phng thng ng p dng khi t yu tng i dy, thi gian
c kt thot nc ca t nn di. rt ngn thi gian c kt, cn rt ngn khong cch thot
nc bng cch b tr cc hnh lang thot nc theo phng thng ng, ng thi trn b mt t
nn li ph lp ct thot nc v lp gia ti nhm y nhanh c kt. Hnh lang thot nc phng
thng ng c th l cc ct hoc bc thm (nh trnh by trn).
(a) Kch thc s b
Cc ct c to thnh nh ng cc ng thp vo t bng my ng cc, nhi ct vo cc ng
v ri rt vch ng thp ln. ng knh cc ct thng thng khong 2030cm, nu di nc
th khong 3040cm. Khong cch gia cc cc ct thng trong khong 8 n 10 ln ng knh,
chiu di khng qu 20m. dy lp ct thot nc trn nh cc cc ct thng ly 0,30,5m
trn kh, 1m di nc.
Khong cch gia cc cc ct:
L = 1,904dc (1 +eo)/ (eo enc)
Trong :
dc - ng knh cc ct
eo H s rng ca t thin nhin trc khi nn ct
enc H s rng ca cc ct
Chiu su chn cc ct :
Chiu su ti ng sut z 0,5 n
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n = tn .Z
Z - Chiu su tnh ng sut
(b) K thut thi cng cc ct
- Chun b mt bng thi cng tuyn ;
- Dng cc tm chng ly v ray vn chuyn my khi ng cc;
- Dng ba ng cc v hai ng thp ng knh 40cm, di 4,5m nng 450kg, mi nhn ca ng
thp c 4 cnh lp bn l. nn cht ct trong cc, d dng 2 chy m bng st di 4m, ng
knh 35cm, hai kch 50T phng khi rt ng khng ln trong qa trnh thi cng.
Hnh 6: X l nn bng m cc ct
Trnh t thi cng nh sau:
- Trc tin di chuyn my ng cc n v tr thit k, k m cho my cn bng v vng chc,
iu chnh cho tim ba trng vi tim cc, tip theo dng ti ca ba dng ng ln mi nhn
ng thp ng vi tim cc.
- H ba chn trn u ng, iu chnh cho ng thp thng ng ri ri bt u h ba ng cc
ti cao trnh thit k; ko cc ln 1m 4 cnh mi cc m ra, ct xung, dng ti ca ba ko
chy m ln cho vo ng thp v h ba ng 3 ln ln cht ct, sau buc chy m vo ba
ko ba ln, ti th dng ko ng thp ln. Tip tc ko ng thp ln lm ct vo ng thp;
h chy m v ba ng 3 ln nn cht ct. Tip tc ko ng ln lm na, ct; h ba ng
nh trc, c nh th ko ng ln, nhi y ct v dng chy m cht cc ct.
- Sau khi thc hin xong cc ct, cn tin hnh kim tra xc nh trng lng th tch, h s rng
ca t, cng nh cc ch tiu c l cn thit khc khong cch gia cc cc ct. Nhng tr s
ny yu cu phi ph hp vi cc s liu tnh ton trong thit k.
( c) Tnh ton sc chu ti ca nn sau khi x l y bng cc ct
Phng trnh c bn cn bng lc [8]
Po. A = Pcol. A col + P sol.A sol Hay
= (Po/Psol) = 1 + (n-1)a
Trong :
Po l ng sut chu ti ca nn sau khi x l (kPa)
A - din tch ton b nn (m2)
Pcol - ng sut chu ti ca cc ct (kPa)
A col Din tch cc ct (m2)
P sol - ng sut chu ti ca cc t nn (kPa)
A sol Din tch cn li ca nn (tr phn cc chim ch), (m2)
n = Pcol/ Psol
a = Acol/A.
Cc thng s trn bit c th tnh c kh nng chu ti ca nn sau khi nm cc ct.
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Khi tnh cc ch tiu c l ca nn c th hin qua cc cng thc sau:
c = m.ccol + ( 1-m) csol
tan = m.tancol + (1-m) tansol c - lc dnh ca nn sau khi x l y (kPa)
csol lc dnh ca t nn
ccol- Lc dnh ca vt liu cc ct
tan gc ma st trong ca t nn sau x l
tancol - gc ma st trong ca cc ct
tansol - gc ma st trong ca nn sau x l .
Gi tr m nm trong khong a m a. (Pcol/Po) .
Hnh 7: M hnh tnh ton sc chu ti ca nn sau khi x l
2.2.6 Gia tng cht nn bng m nn
Khi cn gia cng vng t yu ng bng, ven bin, a hnh bng phng v rng ri, chiu su
lp t yu khng ln hn 8m ngi ta c th dng cc bin php m nn.
a) m nn b mt
Hnh 8: Gia cng nn t yu bng m ln p c kt hp xung lc
Bin php ny ch dng khi nn t m v chiu dy di 2,5m.
b) m ba nng
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Hnh 9: Gia cng nn t yu bng cch m nn su
( c) m chn ng su
Trong trng hp my i li c trn nn p th ngi ta c th s dng my m su, chn
rung cng bc ht ct dich chuyn. Nguyn l m ging nh m su trong thi cng b tng.
Hnh 10: m chn ng su gia c t yu
2.3 p c kt
Trong trng hp nn p khng sc chu ti trn truyn xung, trong trng hp ny nu
khng gia c nn th c th dng gii php p gia ti, tng c kt, ch n khi sc chu ti ca
nn tng ln, sau s p tip. Gii php ny chng c tn l p giai on, tng c kt.
2.3.1 Chiu cao gii hn khi p
i vi nhng tuyn khng cao v cho php ko di thi gian thi cng, th bin php hiu qu
l chia chiu cao thnh hai hoc ba lp v p nng cao dn trong nhiu nm to iu kin
cho t nn c kt tng kh nng chu ti.
Chiu cao gii hn cho php [Hgh] ca khi t p theo kh nng chu ti ca nn t yu [Hgh]
c tnh theo cng thc sau:
[Hgh]=
u
K
C.14,5
Hay [Hgh]=)tg.14,51(xK
xC14,5
w -
Trong :
Cu- Sc khng ct ca t nn thc hin trn th nghim nn ba trc theo s
khng c kt, ct nhanh khng thot nc;
C, - lc dnh v gc ma st trong ca t xc nh trn my ct phng, th nghim
trong iu kin t khng c kt, ct nhanh khng thot nc;
- khi lng ring ca t p
11
K- h s an ton, chn K=1,25.
Nu chiu cao yu cu ca (h) thp hn chiu cao gii hn cho php, h Hgh, ta c
th p trc tip trn nn thin nhin.
Nu h Hgh tc l t nn khng kh nng chu ti trng ca th phi tm gii php
nng cao sc chu ti ca nn t yu trc khi p ng chiu cao yu cu.
2.3.2 Xc nh cc thng s p
- Xc nh chiu cao an ton Hat ca khi t p vi nn t yu t nhin:
Chiu cao an ton Hat ca khi t p c dung trng s l:
Hat=
uxC14,3
Cu - l sc chng ct ca t nn. Nu khng c s liu th nghim xc nh Cu trn my ba trc,
c th s dng s liu Cu, C ca t nn xc nh trn my ct phng v s dng biu thc:
Hat=)tg.14,51(x
xC14,3
w -
- Chn chiu dy lp t p t u l h1:
m bo n nh nn t yu di , chiu dy lp t p th nht (h1) khng nn vt qu
chiu cao an ton (Hat) i vi t nn, tc l:
h1 Hat
Trong trng hp cn thit c th chn Hath1[Hgh].
- Xc nh thi on cn ngng thi cng T1:
Sau khi p xong lp h1, cn ch mt khong thi gian T1 t nn t c c kt yu cu
Ut di p lc lp t th nht P1=h1;
T1= t.n1 )
h
H(
Trong : n - ch s c kt; ch s n ph thuc vo ch s do (Ip) v ch s st IL ca
t; i vi t bn, t dnh trng thi nho, do nho chn n=2;
t - thi gian nn mu t c chiu cao h (h=2cm) di p lc P1 = h1 t n c kt
yu cu U1 trong phng th nghim.
H1 - chiu dy lp t nn chu nn bi p lc P1 = h1 ca lp t p th nht.
Trong trng hp khng c s liu th nghim sc chng ct theo thi gian v theo mc
c kt c th tnh ton thi gian c kt cho mi lp p theo cng thc:
t = 2
v
v
T h
C(Tv c xc nh t quan h gia c kt U v Tv)
- Tnh ton kim tra s n nh ca t nn sau thi on T1 xc nh chiu dy p t
lp th hai (h2):
Sau thi gian T1 do tc dng nn c kt c trng chng ct ca t nn s t cu, Ccu. So
vi gi tr u, Cu trng thi t nhin ban u sc chng ct ca t nn s tng:
cu cu u
cu cu cu
C C C
Tnh ton kim tra chiu cao an ton, chiu cao gii hn cho php ca sau thi on T1
vi cc gi tr sc chng ct tnh ton theo mc gia tng trung bnh:
tt cu
cu u
tt cu
cu u
CC =C +
2
2
-
Chiu cao an ton ca khi t p sau thi on T1 l:
12
Hat=)tg.14,51(x
xC14,3tt
cu
tt
cu
- -
Chiu cao gii hn cho php ca khi t p sau thi on T1 l:
[Hgh]=)tg.14,51(xK
xC14,5tt
cu
tt
cu
-
So snh nu H [Hgh], ch cn p thm lp th hai c chiu dy h2 l:
h2 = H - h1
Nu H > [Hgh], chiu dy lp t th hai ch c chn:
h2 = [Hgh] - h1
p xong lp t th hai phi ch sau mt thi on T2 mi p tip.
2.3.3 Thc hin vic kim tra trng thi c kt ca t yu di nn p bng cc bin php
sau - o p lc nc l rng;
- o ln ca lp t yu;
- Xc nh tng ca lc dnh Cu bng th nghim ct cnh;
S dng cc phng php tnh ton kim tra, m bo th vic thi cng c thc hin,
cn nu khng phi ko di thi gian ch c kt hoc tm kim gii php khc ph hp nht.
2.2.4 D tnh ln c kt theo thi gian trong trng hp thot nc 2 chiu
Trong trng hp ny c kt U t c sau thi gian t k t lc p xong c xc nh theo
cng thc sau:
U = 1 (1Uv) (1Uh)
Trong :
Uv l c kt theo phng thng ng vn c xc nh nh ni trn,
Uh l c kt theo phng ngang do tc dng ca ging ct hoc bc thm.
c kt theo phng ngang Uh c xc nh nh sau:
U 1 exp 8T
F n F Fhh
s r
Trong :
Th l nhn t thi gian theo phng ngang:
T C
thh
2
Vi l khong cch tnh ton gia cc ging ct hoc bc thm:
- Nu b tr ging hoc bc thm theo kiu vung
= 1,13 D
- Nu b tr theo kiu tam gic
= 1,05 D
D l khong cch gia cc tim ging hoc bc.
H s c kt theo phng ngang Ch (cm2/sec) cng c th c xc nh thng qua th nghim nn
ln khng n hng i vi cc mu nguyn dng ly theo phng nm ngang theo TCVN 4200-
86. Nu vng t yu c kt gm nhiu lp t c Ch khc nhau th tr s dng tnh ton l tr s
Ch trung bnh gia quyn theo b dy cc lp khc nhau .
13
giai on lp d n kh thi, cho php tm dng quan h sau xc nh tr s Ch a vo tnh
ton:
Ch = (25)Cvtb
Vi Cvtb
c xc nh nh ni trn.
F (n) l nhn t xt n nh hng ca khong cch b tr ging ct hoc bc thm, c xc
nh ty thuc vo n = dw/d (vi d l ng knh ca ging ct hoc ng knh tng ng ca
mt bc thm) theo cng thc:
F(n) n
n 1ln(n)
3n 1
4n
2
2
2
2
;
Fs l nhn t xt n nh hng ca vng t b xo ng xung quanh bc thm (lm h s thm
trong vng b gim i).
Fr l nhn t xt n nh hng v sc cn ca bc thm.
Khi dng ging ct th khng xt n 2 nhn t ny (tc l xem Fs = 0 v Fr=0) cn khi p dng
bc thm th chng c xc nh nh ni trn.
2.3 X l nn bng cc ximng t
Cng ngh pht p lc cao Jet grouting
Cng ngh Jetgrouting c pht minh Nht Bn nm 1970, nm 1974 mt cng
ty mua li pht minh trn v ra i cng ty Technicwell. Hin nay cng ngh ny
pht trin ra nhiu nc v c ng dng kh ph bin khi gia c nn t yu, tng
cng kh nng chng thm ca nn.
c im phng php
Jet-grouting to ra ct t gia c t va pht v t nn. Ni lin cc ct gia c ny s to
thnh mng chng thm hay tng chu lc cho kt cu pha trn. Nh tia nc v va
phun ra vi p sut cao (200 - 700 atm), vn tc ln ( 100m/s), di tc dng ca p lc
dng tia rt cao cc phn t t xung quanh l khoan b xi ti ra v ho trn vi va pht,
sau khi ng cng to thnh mt khi ng nht gi l Soilcrete (tm dch l btng t).
Soilcrete trong t ng vai tr n nh nn v chng thm.
- Cng chu nn ca Soilcrete t 20 250 kg/cm2, ph thuc vo: + Loi va, nu l va xi mng th ph thuc hm lng xi mng v t l t cn li
trong khi Soilcrete .
+ Loi t nn, nu nn bn c th t 20 50 kg/cm2, nu nn cui si c th t
150 250 kg/cm2.
- Hiu qu chng thm ca Soilcrete t c bng cch la chn loi va thch hp, trong trng hp cn thit phi cho thm Bentonite.
Phm vi ng dng cc hnh thc khoan pht ph thuc vo tng loi t v cng ngh
thi cng. Vi loi t no c th s dng phng php ph hp.
Cng ngh khoan pht - Jet-grouting
14
Hin nay trn th gii pht trin ba cng ngh Jet-grouting, u tin l cng ngh S, tip
theo l cng ngh T, v gn y l cng ngh D. c trng k thut ca mi loi cng
ngh ny xin tm tt nh sau.
(a) Cng ngh n pha-Soilcrete S
Va pht phun ra vi vn tc 100m/s, va ct t va trn va vi t mt cch ng thi,
to ra ct Soilcrete ng u vi cng cao v hn ch t tro ngc ln. Cng ngh
n pha dng cho cc ct Soilcrete c ng knh va v nh 0,4 ~1,2m.
( b) Cng ngh hai pha Soilcrete D
y l h thng pht va kt hp va vi khng kh. Hn hp va t-ximng c bm
p sut cao, tc 100m/s v c bao bc bi mt tia kh nn. Dng kh nn s lm gim
ma st v cho php va xm nhp su vo trong t, do vy to ra ct Soilcrete c ng
knh ln. Tuy nhin, dng kh li lm gim cng ca Soilcrete so vi phng php pht
n pha v t b tro ngc nhiu hn.
Cng ngh ny ch yu dng thi cng cc tng chn, cc v ho chng thm.
Hnh 11: Nguyn l Jet grouting cng mngh 1 pha v 2 pha
(c) Cng ngh ba pha-Soilcrete T
Qu trnh pht c c va, khng kh v nc. Khng ging pht n pha v pht hai
pha, ban u nc c bm vi p sut cao kt hp vi dng kh nn bao bc xung quanh
dng nc y kh ra khi t. Sau va c bm qua mt vi ring bit nm di
vi kh-nc lp y khong trng ca kh. Pht ba pha l phng php thay th t m
khng xo trn t.
Cng ngh Soicrete T s dng lm cc cc, cc tng ngn chng thm, c th to ra
ct Soilcrete ng knh n 2m.
Hnh 12: Nguyn l Jet grouting cng ngh 3 pha
15
Ngoi cng ngh trn t trn, hin nay ngi ta cn s dng cng ngh trn kh. Nguyn
l c bn l dng li dao ct ph ri t nn, pht bt gn kt vo t xi ti, i cho
cht gn kt pht trin cng , to khi mi chnh th. Cng ngh ny c minh ha
qua hnh sau.
Hnh 13 : M hnh cng ngh trn su ( trn kh)
16
Hnh 14 : My khoan song ng trn su
Tnh ton thit k cc thng s khoan pht
Cc yu t nh hng n kt qu gia c t bng Jet grouting l: loi t; sc chu
ti; dung trng; cp phi ht; hm lng nc; gii hn Atterbug.
Cc thng s cn xc nh trong Jet grouting l ng knh ct Soilcrete; tc thi
cng; tnh cht c l ca ct Soilcrete mi to ra v hiu qu kinh t. Mi thng s
ng vi mt loi t mt v tr nht nh c xc nh qua tnh ton. Tuy nhin,
cn phi tin hnh cc th nghim hin trng nhm tm c cc thng s thch
hp. C th tham kho bng di y.
Bng 1: Cc thng s cho Jet grouting
Kiu khoan pht 1 pha 2 pha 3 pha
Thng s n v Min Max Min Max Min Max
p sut tia va Mpa 20 60 30 60 3 7
Lu lng va l/pht 40 120 70 150 70 150
p sut tia kh Mpa 0.6 1.2 0.6 1.2
Lu lng kh l/pht 2000 6000 2000 6000
p sut tia nc Mpa 20 50
Lu lng nc l/pht 70 150
ng knh l pht
va
mm 1.5 3 1.5 3 4 8
ng knh l pht
nc
mm 1.5 3
17
ng knh l pht kh
(ng trc) mm 1 2 1 2
Tc vng quay cn
khoan
v/pht 10 25 5 10 5 10
Tc rt cn cn
khoan
cm/pht 10 50 7 30 5 30
Tnh ton sc chu ti ca cc Xi mng-t
Hin nay, vn tnh sc chu ti v bin dng ca nn t gia c bng ct ximng-t vn cn l
vn cn phi tip tc nghin cu pht trin. C 2 quan im chnh nh sau:
- Quan im tnh ton nh mng cc. Theo quan im ny i hi tr phi c cng tng
i ln v cc u tr ny c a vo tng t chu ti. Khi lc truyn vo mng s ch yu
i vo cc ct ximng-t (b qua s lm vic ca t nn di y mng). Trong trng hp tr
khng a c xung tng t chu lc th c th dng phng php tnh ton nh tnh ton vi
cc ma st. Quan im ny s dng khi tnh ton b tr cc di y mng cng trnh.
- Quan im tnh theo nn tng ng, c ngha l nn t sau khi x l c ci thin, tnh
cht c l nh mt nn tng ng (td ; td ; Etd );
Di y gii thiu phng php tnh ton thit k cc ximng t c trnh by trong Quy
phm Thng Hi (Trung Quc) v TCXD 385-2006 (B Xy Dng)
* Tnh theo quan im mng cc
a) Lc chu ti cho php ca ct n xi mng t nn xc nh thng qua th nghim ti trng ct
n, cng c th c tnh theo cng thc sau:
Pa = . cu. AP
Hoc Pa = UP . qsi i +. AP. qP Trong cng thc trn:
Pa - lc chu ti cho php ct n (kN);
fcu - tr s bnh qun cng khng nn (kPa) ca mu th xi mng t trong phng
(khi lp phng vi chiu di cnh l 70,7mm) c cng thc phi trn xi mng t nh ca thn
ct, 90 ngy tui v trong iu kin bo dng tiu chun;
AP - din tch mt ct ca ct (m2);
- h s trit gim cng thn ct, c th ly 0,3 ~ 0,4;
UP - chu vi ca ct (m);
qsi - lc ma st cho php ca lp t th i xung quanh ct. i vi t bn c th ly 5~8
kPa; i vi t ln bn c th ly 8~12 kPa; i vi t st c th ly 12~15 kPa;
i - chiu dy ca lp t th i xung quanh ct (m); qP - lc chu ti (kPa) ca t mng thin nin mi ct;
- h s trit gim lc chu ti ca t mng thin nhin mi ct, c th ly 0,4 ~ 0,6.
Lc chu ti t mng hn hp ct xi mng t chu lc nn thng qua th nghim ti trng
mng t hp xc nh, cng c th c c tnh theo cng thc:
asp s
p
m.Pf (1 m)f
Trong cng thc trn:
sp - lc chu ti cho php ca mng t hp (kPa);
s - lc chu ti cho php ca t mng thin nhin gia cc ct (kPa);
m - t l phn b din tch ct v t;
- h s trit gim lc chu ti ca t gia ct. Khi t mi ct l t yu, c th ly 0,5 ~
1,0; khi t mi ct l t cng, c th ly 0,1 ~0,4. Cng c th cn c yu cu cng trnh t ti
lc chu ti cho php ca mng t hp, tm t l phn b din tch ct v t theo cng thc:
sp s
a P s
f .fm
P A .f
18
Khi b tr mt bng ct xi mng t chu lc c th cn c yu cu v lc chu ti v bin
dng ca nn mng i vi kin trc phn trn cng nh c im kt cu phn trn, s dng cc
hnh thc gia c nh hnh tr, kiu tng, hnh vy quanh hoc hnh khi, ct c th ch b tr
trong phm vi mt bng nn mng. Chiu di ct phi cn c cc yu t nh cu bin dng ca
khi kin (cu) trc v kt cu mng xt.
X l nn mng ct c th b tr ct theo hnh vung hoc tam gic u, tng s ct cn
dng tnh theo cng thc:
p
m.An
A
Trong cng thc trn:
n - tng s ct;
A - din tch y nn mng (m2).
Khi ct xi mng t chu lc c t l phn b t v ct tng i ln (m>20%), ng thi
li khng b tr theo hng n, phi coi chm ct xi mng t vi t gia ct l mt mng nng
ton khi quy c. kim tra cng lp t mm yu di y mng nng ton khi quy
c, p dng cng thc:
sp s s s 1
1
f .A G A .q f .(A A )f
A
Trong cng thc trn:
fsp - lc nn mt y mng nng ton khi quy c (kPa);
G - trng lng mng nng ton khi quy c (kN);
As - din tch b mt bn mng nng ton khi quy c (m2);
qs - lc ma st bnh qun b mt bn mng nng ton khi qui c (kPa);
fs - lc chu ti cho php ca t cnh mp mng nng ton khi qui c (kPa);
A1 - din tch mt y ca mng nng ton khi quy c (m2);
f - lc chu ti cho php ca y mng sau khi chnh sa mt y mng nng ton khi
quy c (kPa).
b) Tnh ton bin dng: Tnh ton bin dng ca t mng hn hp ct xi mng t chu lc phi
bao gm tng ca bin dng co nn ca cm ct xi mng t v co nn bin dng ca lp t
cha gia c di mi ct. Trong tr s bin dng co nn ca cm ct c th cn c kt cu
phn trn, chiu di ct, cng thn ct v.v... ly 20 - 40 mm theo kinh nghim. Tr s bin
dng co nn ca lp t cha gia c di u ct XM tnh ton nh t nn thin nhin cha
gia c.
* Tnh theo quan im nn tng ng
Quan im ny thch hp khi s dng cc ximng t gia c nn khi p. Khi mt
cc (m) nn nm trong khong 12 ~ 20%, ng thi hm lng ximng khng nn vt qu
300 kg/m3 cc. iu ny nhm hn ch s khc bit nhau qu ln gia phn mng v phn khi
p bn trn.
Nn sau khi gia c c xem nh nn ng nht vi cc s liu cng t, Ct, Et c
nng cao so vi , C, E ca t nn khi cha gia c. Cng thc quy i tng ng t, Ct, Et
da trn cng ca cc XM, t v din tch t c thay th bi cc XM.
Gi m l t l gia din tch cc xi mng-t thay th trn din tch t nn.
m =s
p
A
A
t = mct+(1-m)nn
Ct = mCct+(1-m)Cnn
Et = mEct + (1-m)Enn
Trong : Ap - Din tch t nn thay th bng ct XM (tc l din tch cc XM chim ch);
As- Din tch t nn cn gia c.
Theo phng php tnh ton ny, bi ton gia c t c 2 tiu chun cn kim tra:
19
- Tiu chun v cng : t , Ct ca nn c gia c phi tha mn iu kin sc chu ti
di tc dng ca ti trng cng trnh.
- Tiu chun bin dng: Mun bin dng ca nn c gia c Et phi tha mn iu kin
ln ca cng trnh.
C th dng cc cng thc gii tch v cc phn mm a k thut hin c gii quyt bi
ton ny.
2.4 Phng php gia c ho hc Dng gia cng mng v tng chn, tng sc chu ti ca cc, bo v mng chng cc tc
nhn n mn, gia c mi h o v cng trnh t.
Vt liu c bn gia c bng silicat l thu tinh lng - dung dch keo ca silicat natri
(Na2O. nSiO2 + mH2O). Tu theo loi, thnh phn v trng thi ca t cn gia c m dng
mt hay hai dung dch silicat ho.
Loi mt dung dch c da trn dung dch to keo bm vo trong t gm 2 hoc 3 cu
t. Ph bin nht l xit phosphosilicat, oxit lu hunh-nhm-silicat, xit lu hunh-fluo-
silicat, hydro-fluo-silicat v..v.. Phng php mt dung dch thch hp cho t ct c h s
thm 0,5-5m/ngy m.
Phng php 2 dung dch dng gia c t ct c h s thm n 0,5m/ngy m v gm
2 ln bm ln lt vo t 2 dung dch silicat Na v clorua Ca. Kt qu ca phn ng ho
hc l to ra xit keo silic lm cho t tng bn (n 2-6Mpa) v khng thm nc.
Phng php in ho silicat l da trn s tc ng t hp ln t ca hai phng php:
silicat ho v dng in 1 chiu nhm gia c ct ht mn qu m v ct c h s thm u
0,2 m/ngy m.
Phng php amniac ho l da trn vic bm vo trong t hong th ( loi tr tnh
ln sp) kh amniac di p lc khng ln lm.
Silicat ho bng kh gas dng lm cng silicat Na. Phng php ny dng gia c t
ct (k c t cacbonat) c h s thm 0,1-0,2 m/ngy m cng nh t c hm lng hu
c cao (n 0,2). bn ca t gia c c th n 0,5-2MPa trong thi gian ngn.
Phng php thm nhp nha dng gia c t ct c h s thm 0,5-5m/ngy m
bng cch bm vo trong t dung dch nha tng hp (cacbonic, phenol, epoxy..). Tc
dng ca nha ho s tng ln khi b sung vo dung dch mt t axit clohydric (i vi t
ct). Thi gian keo t rt d iu chnh bng lng cht ng cng. t c gia c bng
nha ho s khng thm nc v cng chu nn 1-5Mpa. Ngoi vic gia c nn,
phng php ny cn dng gia c vng s o xuyn ca cng trnh ngm. Tu theo
cch t ng bm, c th gia c t cc v tr khc nhau: thng ng, nghing, nm
ngang v kt hp cn s trn mt bng c th theo dng bng di, di ton b mng,
gia c cc b khng ni kt hoc theo chu vi vnh mng. Vic chn phng php v s
gia c ph thuc ch yu vo tnh cht ca nn, hnh dng v kch thc ca mng cng
nh ti trng tc dng ln mng.
3. X L NN P PH SNG TRN THM BIN
p ph sng t trn bi, thm bin ni c ct nc tng i su, a cht phc tp, s
gy ra rt nhiu kh khn trong x l v thi cng. Mt v d in hnh l trong xy dng
cng Dung Qut, khi kho st nn bin khng pht hin c ti bn, ch khi thi cng mi
pht hin. Khi lng ti bn ln n triu mt khi. Cng ngh x l loi ly c th tm
tt nh sau.
3.1 Cng ngh nhi cc ct do T vn Nht Bn xut
S dng x lan mang my nn ct ct. Nn ng bao xung nn ti cao trnh y thit k,
ct c ch trong x lan, ct vo phu np, nhi ct co trong ng thp rng, m nn
ct dn ngang to nn ct ct. Rt ng ln lm on sau.
20
Hnh 15: My thi cng cc ct bng my nn p lc, xoay to l
Hnh 16: S cu trc my ng cc ct trn bin
Working Vessels
21
Hnh 18: Quy trnh thi cng nn cc ct trn nn bin
Hnh 19: Quan h nng ng v th tich nhi ct
Hnh 20: ng bao ng knh ct s dng cho cc nhi.
3.2 Phng n x l do t vn H Lan thi cng
22
T vn Haskoning ca H Lan xut gii php ht ton b ti bn bng tu ht cng
sut ln, chuyn bn ht bng x lan hoc bm y ra xa ( vo b to bi), sau s dng
s lan ht ct cch cng trnh 10KM, vn chuyn v chn cng trnh, nh v m cht ct
trong nc bng m chn ng, trc cng. Sau ri lp th ln trn trc khi thi
cng thn p ph sng.
4. KT LUN
Trong xy dng cng trnh , p ph sng, ng giao thng... trn nn t yu cn phi
x l nn trc khi t cng trnh ln. Gii php p dng c th l: cc ct, bc thm, gia
cng vi a k thut, gin b mng trn mt nn, hoc dng gii php khoan pht trn
su. Tuy nhin cha phi hon ton l th, ngi ta c th p c kt giai on ch tng
ch tiu c l , sau p tip. Mi gii php u c c s la chn m bo k thut v
kinh t nht. Bi vit ny c son tho trn c s ca cc ti liu lin quan v x l nn
cng trnh ven bin , bi ging chuyn v x l nn ht ri. Bn c c th tm hiu
thm t cc ti liu tham kho trch dn.
TI LIU THAM KHO
1) 14 TCN 1-2004: Quy trnh k thut pht va gia c . 2) Quy phm k thut thi cng khoan pht va xi mng cng trnh thy cng SL- 62-
94 ( Tiu chun ca Trung Quc).
3) Cng ngh khoan pht p lc cao trong x l nn t yu. PGS,TS Nguyn Quc Dng, NXB Nng nghip 2005.
4) Tuyn tp cc bo co hi tho khoa hc chuyn khoan pht x l nn, Hi p ln VN, 2007.
5) L nh Chung&nnk (2009); Nghin cu xy dng s cng ngh thi cng cc cng trnh thy li, thy in.
6) Hng dn thit k bin (2010), Chng trnh khoa hc cng ngh xy dng bin v cng trnh thy li vng ca sng ven bin, B NN&PTNT.
7) CUR Report 162 (1996), Building on soft soils. A.A Balkema, Rosterdam.
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