NH GI C KT CA NN T SAU KHI X LHong Trung Hu K s a k thut Japan
Port Consultant -Nippon Koei Joint venture
1. T VN : Nn t yu thng c x l bng nhiu phng php khc nhau, trong
phng php cm PVD kt hp gia ti l mang li nhiu hiu qu v l phng php kh
ph bin hin nay cc d n ti Vit Nam. Tuy nhin, vic nh gi d ti cng nh
nh gi c kt ca nn t sau khi x l cn thiu nhiu ti liu tng hp. Trong bi
ny, trnh by cc phng php nh gi c kt nn t yu bng cc phng php nh sau:
Phng php da trn s liu quan trc ln v mc nc. Phng php nh gi qua cc s
liu th nghim hin trng. Phng php nh gi qua s liu th nghim trong phng
da trn mu nguyn dng t cc h khoan. Trong phn ny s i nh gi chi tit
tng phng php. 2. PHNG PHP DA TRN S LIU QUAN TRC LN V QUAN TRC MC
NC: 2.1. Tng quan: Phng php ny da trn s liu quan trc ln nh gi c kt.
Trong , kt hp quan trc mc nc c nh gi chnh xc hn. 2.2. u im: L phng
php chnh trong vic nh gi c kt nn ng. Trc quan v r rng. nh gi trc
tip. 2.3. Nhc im: Vic khng quan tm n mc nc trong qu trnh thi cng
cng nh mc nc tng lai c th s dn n d bo c kt khng ng.
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o ng
Hnh 1. S ha ln c kt trong thi gian xy dng v khai thc. Trong :
Spf(1): ln cui cng s cp d kin nu khng d ti. Spf(2): ln cui cng s cp
d kin nu d ti. ST: ln cui cng th cp d kin. 2.4. Trnh t nh gi nh
sau: Bc 1: V cc ng quan trc ln, cao p v cao mc nc nh hnh th
hin.
Bc 2:
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Da trn s liu quan trc tin hnh d bo ln cui cng theo phng php
Asaoka. V bn cht phng php ny nh sau: Asaoka chng minh rng nu chia
ng cong quan trc ln thnh nhiu im Si c cc khong thi gian bng nhau th
khi v trc th vi trc honh l Si v trc tung l Si+1 th cc im l ng thng.
iu ny ch ng khi ti trng tc dng l hng s, thng ng vi giai on p ti ln
nht (nh hng mc nc cng s lm cho cc im lch nhau). C th s ha nh
sau:
ln cui cng l im giao nhau gia ng ni cc im v ng k t gc ta gc 45
.
Bc 3: Tin hnh bi ton phn tch ngc: dng m hnh tnh ton li ln ti tng
thi im khc nhau vi cc chiu cao khc nhau v mc nc quan trc khc nhau.
iu chnh cc thng s ban u
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nh Cc v Cs ng tnh ton ph hp vi ng quan trc thc t. Trong hnh bn
di: ng mu xanh l ng tnh ton da trn m hnh iu chnh cc thng s Cc v Cs.
So snh kt qu tnh ton ny vi kt qu tnh ton bc 2 c ln cui cng tng ng
nhau. Da trn ng cong ln ny, c th d bo thi gian c kt t c 90% (hoc
khc) c th d bo ngy d ti hp l.
Ghi ch: c th thy c rng, trong qu trnh p t. Khi p cng cao th mc
nc cng cao. V vy, cn phi bm nc thot ra nn ng. Khi nc h xung th ln s
tng ln. S l nguy him nu khng quan tm n mc nc. V sau mt thi gian mc
nc s gim lc ln s tng ln (nu mc nc vn vy th ln vn tip din nh quan
trc d bo). V vy, cn c thm bc d bo mc nc tng lai. Bc 4: Tnh ton ln
th cp Thng thng cc d n Vit Nam t khi tnh ton ln th cp. Hu ht u ly
tiu chun 90% c kt ca ln s cp tin hnh d ti, mt s d n kin ngh ly 80%
c kt ca ln th cp d ti. Vic tnh ton ln th cp phn nhiu d bo cao ln
sau mt thi gian khai thc, m bo cao ny nm trn mc nc cho php (thng cc
d n Cng).
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Hnh 3. ng d bo ln da trn s liu quan trc. Ghi ch: k hiu ng nt t
mu l ng ng sut tng v hiu qu do ti trng t p gy ra. 3. PHNG PHP NH GI
QUA S LIU TH NGHIM HIN TRNG: 3.1. Tng quan: Phng php ny da trn s
liu th nghim hin trng nh gi c kt. Phng php ny thng c dng nh gi b
sung cho phng php nh gi da trn s liu quan trc ln. 3.2. u im: Trc
quan v r rng. nh gi trc tip. 3.3. Nhc im: Thit b th nghim vn cha cn
nhiu. Chi ph thc hin th nghim t. Kt qu s kh quan hn khi c s liu th
nghim ca cng loi thit b, cng mt v tr trc v sau khi gia ti. Hai hnh
thc c p dng ph bin: Hnh thc 1: th nghim xc nh cng khng ct khng thot
nc xc nh t c kt dng th nghim xuyn cn tnh CPTu v th nghim ct cnh hin
trng FVST. Hnh thc 2: th nghim tiu tn p lc nc l rng dng th nghim
xuyn cn tnh CPTu. 3.4. Sau y s gii thiu 2 hnh thc trn. a. Hnh thc
1: a.1. Th nghim CPTu: Nguyn l: Dng mi cn xuyn tiu chun c b phn u d
ghi li sc khng mi, thn v p lc nc l rng lin tc n chiu su xuyn.
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Hnh. Mi cn
Hnh. Cu to mi cn
Hnh. Lp t dn nn xuyn cn
Hnh. Lp t xuyn cn
Hnh. Thit b o xuyn Kt qu xuyn:
Hnh. Biu xuyn
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Ghi ch:qc [MPa]0 0 1.0 2.0 3.0
fs [kPa]4.00 13 25 38
u2 [MPa]500
Rf [%]1.0 2.0 3.0 4.0
0.50 1.00 1.50 2.00 0
Depth [m]
1 2 3 4 5 6 7 8
-
Line mu xanh: sc khng mi qc. Line mu : sc khng bn. Line mu nu: p
lc nc l rng u. Line mu en: t s ma st gia sc khng mi v thnh bn.
2.5
Pre-dril ling
-
Clay (3)
9 10Si lty clay to clay (4)
11 12 13 14 15 16 17 18 19 20 21 22 23
-
-
Clay (3)
24 25 26 27 28 29 30 31 32 33 34 35 36 37 380.057 >
Si lty clay to clay (4)
39 40 41 42
Sand (9)
43 44 45 46 47
10.105 > 10.280 >
0 0.25 0.50 0.75 1.00 Uo [MPa]Location:U2
Position: Client:
Ground level:
Test no:
Tan Thanh - BRVTProject ID: Date:
7 Toa - Toyo Joint Venture 20100518Page:
CPTU-LOT12Scale:
1 : 200Fig:
Cone No: 40621 Tip area [cm2]: 10 Sleeve area [cm2]: 150
Project:
Cai Mep International Container TerminalFile:
1/1 CPTu-LOT12.cpd
a.2. Th nghim ct cnh hin trng (Field Vane Shear test-FVST):
Nguyn l: Dng cnh hnh ch thp tin hnh khong 2m ct mt im xc nh Su trc
tip.
Hnh. Cu to cnh
Hnh. Cu to ng bc (nu ct trong l khoan th khng cn)
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Hnh. Bn quay ct cnh v ng h c. a.3. Tnh ton:
Hnh. c v ghi s liu
Cng khng ct khng thot nc xc inh theo: Su = (qT svo) / Nkt qT =
qc + (1-a) . u Da vo biu xuyn trn ta c th xc nh c qT (qc v u bit,
gi tr a l h s c ly t calibaration (chng ch hiu chun) ca nh sn xut.
ng sut svo l gi tr ng sut bn thn tng, c tnh t trn mt t n ti im tnh
ton, vi svo = gi.hi gw.hj (vi gi dung trng ca t, hi: chiu dy lp t
th i, hj l chiu cao ct nc tnh ton c tnh t im ng sut n mc nc tnh
ton) Gi tr Nkt l gi tr tng quan. Gi tr ny thng nm trong khong
15-22. Vic xc nh Nkt s xc nh c gi tr Su. Xc nh Nkt da trn biu tng
quan Su t FVST v (qT svo) t CPTu.1200 1100 1000 qt-SIGvo 900 800
700 600 500 20.00 y = 16.243x + 183.7 R2 = 0.8574
25.00
30.00
35.00
40.00 Su
45.00
50.00
55.00
60.00
a.4. nh gi: Nh vy, ta c ng Su lin tc. c th bit c tha mn c kt cha
th cn c mt ng Su lin tc khc, ng l ng c d bo t c c kt yu cu. Xc nh
nh sau: Su(mc tiu) = a.sv H s tng quan a gia Su v sv (ng sut hiu
qu) c xc nh thng qua cc l khoan v tin hnh ct cnh. Vng Tu, gi tr ny
tm khong (0.22~0.25). Hnh sau minh ha tng quan: Page 8/16
Nh vy, khi a ln cng th nu ng khoan kho st nm bn phi ca ng Su(mc
tiu) th xem nh t.su(kPa) 0 10Su(ban u) Su(m c tiu)
20
40
60
80
100
120
140
160
5
Su(FVST) Su(kim tra CPTu)
0
-5
b. Hnh thc 2: th nghim tiu tn p lc nc l rng dng th nghim xuyn cn
tnh CPTu. Nguyn l: dng th nghim xuyn cn CPTu, xuyn n mt chiu su nht
nh (mi l ty theo yu cu c th lm n 4-5 im). Tin hnh ngng xuyn v lm
tiu tn p lc nc l rng. Th nghim tin hnh trong 2000s. Kt qu lm tiu tn
ti mt im nh sau:
Elevation (m)
-10
-15
-20
-25
-30
-35
-40
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X l s liu: Bc 1: Tin hnh chuyn d liu o c t Umax v zero.
Bc 2: S dng phng php Hyperbolic ngoi suy tm gi tr p lc nc l rng
cui cng Uf (y l p lc nc l rng tnh, n nh, nhng gi tr o c l p lc nc l
rng ng). V c bn phng php d bo theo Hyperbolic nh sau:
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Trn th sau chuyn i, chn mt khong cc s liu v ly im u lm im gc. Nh
th hin hnh sau:
Tin hnh v th nh hnh trn vi trc honh l (T To) v trc tung l (T To)
/ (U Uo). Da trn cc im v, s c mt on cc im gn nh nm trn ng thng, k
ng thng qua cc im , xc nh gc dc a. Gi tr p lc nc l rng tnh (hay p
lc nc l rng cui cng) l: Uf = Uo + 1 / a
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Sau khi lm tng t cho tt c cc im, tin hnh v trn cng mt th. Trn
th, v ng p lc thy tnh P = gw . h. Sau khi gia ti, p lc nc s tng ln
mt gi tr tng ng vi p lc gia ti (c xc nh nh bn di). Gi tr ny sau s
tiu tn v gim dn. c kt t 100% khi gi tr tiu tn ny nm trn ng p lc nc
thy tnh.
Tnh ton c kt:
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Trong : U: p lc nc l rng c tnh t th nghim tiu tn nc l rng. HSP:
p lc thy tnh (Hydro static pressure). EPWP: p lc nc l rng thng d
(Excess pore water pressure) = (U HSP) Max Consolidation Stress: p
lc c kt ln nht (tnh ton nh bn di). c kt (deg. Of Cons.) % xc nh nh
sau: (Max Consolidation stress EPWP) / Max Consolidation stress *
100% * Tnh ton p lc c kt ln nht:
4. PHNG PHP NH GI QUA S LIU TH NGHIM TRONG PHNG DA TRN CC MU
NGUYN DNG T CC H KHOAN: 4.1. Tng quan: Phng php ny da trn s liu th
nghim trong phng qua cc mu nguyn dng c khoan t cc l khoan. 4.2. u
im: n gin, thit b th nghim ph bin. Chi ph r. 4.3. Nhc im: - Kh kim
sot kt qu cng nh mu trong qu trnh vn chuyn v bo qun. - Thi gian c
kt qu rt chm. - Khng th hin kt qu y v lin tc. 4.4. nh gi mc c kt
qua 2 phng php: - Phng php c tnh bng th nghim nn khng n hng
(Unconfined Compression test) - Phng php c tnh bng th nghim c kt (
Consolidation Test). a. Phng php 1:Phng php c tnh bng th nghim nn
khng n hng (Unconfined Compression test)
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Cng khng nn khng n hng gia tng nh qu trnh c kt. Cng khng nn khng
n hng sau khi c kt (quf) c th c tnh bng cng thc nh sau: quf = quo +
2 . P . Cu/P (xut pht t cng thc quf = 2 Cu = 2(Cuo + P.U%.tanj) =
quo + 2 . P . (U%.tanj) ) Trong : quo : cng khng nn khng n hng ban
u. P : tng ng sut hiu qu so vi ban u. Cu/P: h s gia tng cng
(U%.tanj), trong trng hp ly ln nht th Cu/P = tanj. (vi j l gc ni ma
st hu hiu ca t) Theo quan h trn, c kt c c tnh nh hnh sau:
V vy, c kt do ng sut hiu qu: Up = (Din tch t gch) / (Tng din tch
abcd) c kt do bin dng: Ue = log(1 + Up.(A 1)) / logA Vi A = (quo +
qu) / quo b. Phng php 2: Phng php c tnh bng th nghim c kt (
Consolidation Test). p lc tin c kt Pc gia tng theo qu trnh c kt, c
tnh nh sau: Pcf = Pco + P Pco: p lc tin c kt ban u. P: tng ca ng
sut hiu qu. Theo quan h trn, c kt c c tnh nh hnh sau:
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V vy, c kt do ng sut hiu qu: Up = (Din tch t gch) / (Tng din tch
abcd) c kt do bin dng: Ue = log(1 + Up.(A 1)) / logA Vi A = (Pco +
P) / Pco 5. KT LUN: Trong 3 phng php ph bin trn, th phng php s dng
s liu quan trc l c s dng nhiu nht v cho kt qu trc quan nht. Hai
phng php th nghim trn nhm b sung thm cho vic nh gi chnh xc. Trong ,
phng php th nghim hin trng cho kt qu tt hn vi thit b ti u hn. Phng
php khoan ly mu s c tt hn nu nh vic ly mu c thc hin bng ng ly mu
pittong thay cho loi ng ly mu thnh mng thng thng nh a s cc cng ty
hin nay. Cng cn ni thm rng, mc d phng php da trn s liu quan trc l
trc quan. Tuy nhin, vn s dng phng php phn tch v s liu thc t to nn
mt s trng khp tng i l vn kh nu khng s dng mt chng trnh tnh ton. Bi
l, ti mi thi im s c mc nc khc nhau, trong d bo mc nc tng lai s gim
ng k so vi lc gia ti cng s l vn ng ni. V nu ly mc nc trung bnh tnh
th cn xc nh c rng: khng phi nn ln bao nhiu th mc nc h xung by nhiu,
c ngha rng: ng vi mt ti trng ngoi, th khi mc nc khng i (nm trong
phm vi nn) v xy ra ln th ti trng s nh i ch khng phi l hng s, bi vic
tnh ton y l theo ng sut hu hiu, chnh iu ny s lm ln nh i (ci ny xy
ra kh nhiu cc bng tnh ca VN). Tuy nhin, y l mt vic lm kh gii quyt v
c tm chp nhn VN. Trong phn ti, s tin hnh cc bc thi cng nn t yu bng
PVD kt hp gia ti vi cc hnh nh v phn tch, c th: Thi cng lp to mt
bng. ng bc thm. Ri bc thm ngang. Page 15/16
Thi cng cc rnh thu nc, ging bm nc. Ri vi a. Lp t bn o ln, bn o
chiu dy ct. Trnh t v nguyn tc lp t cc thit b quan trc: thit b o ln
su (extensometer), o p lc nc l rng (piezometer), quan trc mc nc tnh
(ng o standpile), bn o ng sut tng (Earth pressure Cell), thit b
quan trc chuyn v ngang (inclinometer).
p t ch ln. Quan trc ln. Cc th nghim trc khi d ti. Kt thc qu trnh
d ti. Th nghim ln mt ng bng quan trc gia ti tng ng vi ti mt ng.
(Trong phn ny s nu tng quan v phng php bm ht chn khng v cc hnh
nh thi cng thc t t lc bt u n khi tin hnh bm ht).
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