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MT S TN TI TRONG D TNH SC KHNG DC TRC CC KHOAN NHI THEO CC TIU
CHUN 22TCN 272-05 V AASHTO LRFD 2007
ThS. Ng Chu Phng, B mn Cu Hm CS2
PGS.TS. Trn c Nhim B mn Cu Hm
Trng i hc Giao thng Vn ti TM TT
Bi bo tm tt mt s tri nghim thc t trong vic d tnh sc khng dc trc
ca cc khoan nhi m tr cu theo cc tiu chun hin hnh trong nc v quc t,
qua nu ln nhng tn ti khi p dng 22TCN 272-05 v AASHTO LRFD 2007, t c
nhng kin ngh b sung, cp nht theo hng hp l hn trong tnh ton thit k
mng cc khoan nhi m tr cu, nhm gip cc k s thit k c th tip cn d dng v
lm ch c nhng kt qu tnh ton ca mnh trn c s hi nhp.
1. T VN
Vic phn tch nh gi cc tn ti thuc cc lnh vc ca tiu chun hin hnh
cng nh nghin cu pht trin chng tr thnh vic thng nin vi ngun knh ph
rt cao cc nc pht trin. in hnh trong lnh vc ng dng cc khoan nhi, Tng
Cc ng b Lin bang M (FHWA) ch tr nghin cu pht trin v xut bn tng cng
4 ln tuyn tp v Thit k v thi cng cc khoan nhi (Nm 1977, 1988, 1999 v
2010). Theo , cc ln ti bn ca tiu chun AASHTO LRFD 1994, 1998, 2004,
2007 v 2010 cng c b sung, cp nht v chnh l trn cc kt qu nghin cu
[3].
Vit nam, vi hn mi nm p dng tiu chun 22TCN272-05 (AASHTO LRFD
1998), n nay vic nghin cu pht trin b sung cp nht cn rt hn ch. Tuy
nhin, cng pht hin c nhng tn ti, nht l trong lnh vc d tnh sc khng ca
cc khoan nhi, nh c qu nhiu phng php tnh sc khng dc trc cc khoan nhi
ca cc tc gi t nhng nm trc 1990, khng c h s sc khng cho trng hp a
cht l t ri. T dn n kt qu tnh ton thit k thiu chnh xc ph thuc kh
nhiu vo nhn nh ch quan ca ngi thit k, c th lm tng chi ph u t hoc km
an ton.
Vi c im tiu chun 22TCN272-05 da trn nn tng tiu chun AASHTO LRFD,
vn l mt tiu chun m, nn rt cn c nghin cu pht trin v cp nht thng xuyn
trn c s hi nhp cc tiu chun quc t. Do vy, thng nht trong tnh ton, ng
thi to ra c hi trao i rng ri ca cc ng nghip cng nh vic t vn cho Ch
u t, bi vit ny i su vo vic phn tch nh gi mt s tn ti trong tnh ton
sc khng dc trc cc khoan nhi khi p dng tiu chun 22TCN 272-05 v
AASHTO LRFD 2007. Trong bi vit ny ch cp n sc khng dc trc ca cc
khoan nhi theo iu kin t nn l t ri v dnh.
2. NHNG TN TI KHI P DNG TIU CHUN 22TCN 272-05 V AASHTO LRFD 2007
D TNH SC KHNG DC TRC CC KHOAN NHI:
Sc khng danh nh hay sc khng tnh ton (Qu hay QR) ca cc khoan nhi
bao gm sc khng mi cc (Qb hay Qp) v sc khng hng (Qs). Cc sc khng, Qn
hay QR, ca cc khoan nhi c tnh theo cc cng thc sau y
[1],[6],[7],[8],[9]:
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u p s p p si si1
Q Q Q q .A q An
i== + = + (1)
Q Q Q Qu qs s qp pR = = + (2)
Trong : Qu: Sc khng danh nh ca cc n (N); QR: Sc khng tnh ton
(N); Qs: Sc khng thn cc (N); Qp: Sc khng mi cc (N); Ap: Din tch mi
cc (mm2), qp: Sc khng n v mi cc (MPa) (xem Bng 2.&4.); Asi: Din
tch b mt thn cc lp th i (mm2), qs: Sc khng n v thn cc ca lp t th i
(MPa) (xem Bng 3.&4.)v n: S lp t dc theo cc khoan nhi; : H s sc
khng, qs: H s sc khng mi cc, qp: H s sc khng bn thn cc (xem Bng
1).
Bng 1. H s sc khng ca cc trng thi gii hn cng a k thut cc khoan
nhi n chu ti trng dc trc theo tiu chun 2TCN 272-05 v AASHTO LRFD
2007
22TCN 272-05 [1] AASHTO LRFD 2007 [8] t/iu kin Phng php H s sc
khng Phng php
H s sckhng
Sc khng bn trong t dnh
Phng php (Reese & O'Neill 1988) 0.65
Phng php (Reese & O'Neill, 1999) 0.45
Sc khng ti mi cc t dnh
Tng ng sut (Reese & O'Neill 1988) 0.55
Tng ng sut (Reese & O'Neill 1999) 0.4
Sc khng bn trong t ri
Touma & Reese (1974) Meyerhof (1976)
Quiros & Reese (1977) Reese & Wright (1997) (Reese &
O'Neil 1988)
Xem cptrong iu 10.8.3.4
(khng c)
Phng php (Reese & O'Neill, 1999) 0.55
Sc khng ti mi cc trong t ri
Touma & Reese (1974) Meyerhof (1976)
Quiros & Reese (1977) Reese & Wright (1997) (Reese &
O'Neill 1988)
Xem cptrong iu 10.8.3.4
(khng c)
Reese & O'Neill (1999) 0.50
Nhn xt: Trong tiu chun [1], khng c h s sc khng khi d tnh sc khng
dc trc cc khoan nhi trong t ri; H s sc khng khi d tnh sc khng trong
t dnh trong tiu chun [1] ln hn tiu chun [8] t 37,5% n 44,4%;
Bng 2. Tm tt cng thc tnh sc khng n v ca cc khoan nhi trong t dnh
theo tiu chun 22TCN 272-05 v AASHTO LRFD 2007
22TCN 272-05 (Reese v ONeill,1988) [1] AASHTO LRFD 2007 (Reese v
ONeill,1999) [8] Sc khng bn, qs (MPa)
(Phng php ) Sc khng mi, qp (MPa)
(Phng php tng ng sut) Sc khng bn, qs (MPa)
(Phng php ) Sc khng mi, qp (MPa)
(Phng php tng ng sut) s uq S=
Su(MPa) 0.9 -
4p c uq N S= , y 6[1 0.2( / )] 9cN Z D= +
Nc =0.33 vi Su
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Nhn xt: Khc bit trong phng php , theo [1], l h s t 0.31-0.55 c
tra bng ph thuc vo ch s Su (t 0.2-0.9), cn Phng php , theo [8], th
h s t 0.45-0.55 c tnh t cng thc ph thuc vo ch s Su/Pa; Phng php tng
ng sut tnh sc khng mi theo tiu chun [1] v [8] v c bn l khng thay i,
ch hiu chnh li gi tr cn di ca h s Nc0.67.
Bng 3. Tm tt cng thc tnh sc khng bn n v, qs(MPa) ca cc khoan nhi
trong t ri theo tiu chun 22TCN 272-05 v AASHTO LRFD 2007
Tiu chun 22TCN 272-05 [1] AASHTO LRFD 2007 [8] Touma v Reese
(1974): ' tan 0.24s v fq K MPa = < y: K=0.7 i vi 7500bD mm
K=0.6 i vi 7500 12000bmm D mm< K=0.5 i vi 12000bD mm>
Khng c
Meyerhof (1976): 0.00096sq N= Khng c Quiros v Reese (1977):
0.0025 0.19sq N MPa= < Khng c Reese v Wright (1977): Vi 53,N
0.0028sq N= Vi 53 100N< , 0.00021( 53) 0.15sq N= + Khng c
Reese v ONeill (1988): ' 0.19vs
q MPa= vi 0.25 1.2 y 31.5 7.7 10 z =
(Reese v ONeill,1999): ' 0.19s vq MPa= vi 0.25 1.2 y 31.5 7.7 10
z = , vi 60 15N
360 (1.5 7.7 10 )15N z = , vi 60 15N <
Ghi ch: v: ng sut thng ng hu hiu do trng lng bn thn ca t gy ra
ti on cc ang xt (MPa); f: gc ma st ca t ri (); Db: Chiu su chn ca
cc khoan trong lp chu lc l ct (mm); N: S ba m c ng SPT xuyn t 300mm
(ba/300mm); N60: S ba m c ng SPT xuyn t 300mm hiu chnh v 60% nng
lng hu ch; z: xuyn t ca cc khoan nhi(mm); Pa: p sut kh quyn
(MPa).
Bng 4. Tm tt cng thc tnh sc khng mi cc n v, qp(MPa) ca cc khoan
nhi trong t ri theo tiu chun 22TCN 272-05 v AASHTO LRFD 2007
Tiu chun 22TCN 272-05 [1] AASHTO LRFD 2007 [8] Touma v Reese
(1974):
Ri: qp(MPa)=0.0 Cht va: qp(MPa)=1.5/K Rt cht : qp(MPa)=3.8/K
y: K=1 i vi Dp500mm; K=0.6Dp i vi 500mm Dp, ch dng khi
Db>10D
Khng c
Meyerhof (1976): ( ) 0.013 0.13p corr b p corrq MPa N D D N=
< ,vi ct
60
Khng c
Reese v ONeil (1988): ( ) 0.057pq MPa N= , vi 75N ( ) 4.3pq MPa
= , vi 75N >
(Reese v ONeill,1999): qp(MPa)=1.2N60, vi 0.057N60 50 (Trong cng
thc ny, h s 1.2 khng ng cho h n v SI, ng ra phi l 0.057)
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Tiu chun 22TCN 272-05 [1] AASHTO LRFD 2007 [8] 0.8' '
600.59 *p a v vq N p = , vi N60 >50, Pa=0.101MPa
Ghi ch: Dp: ng knh chn cc khoan (mm); Db: Chiu su chn ca cc
khoan trong lp chu lc l ct (mm); D: ng knh cc(mm); v: ng sut thng
ng hu hiu do trng lng bn thn ca t gy ra ti on cc ang xt (MPa); N: S
ba m c ng SPT xuyn t 300mm (ba/300mm); N60: S ba m c ng SPT xuyn t
300mm hiu chnh v 60% nng lng hu ch; Pa: p sut kh quyn (MPa).
Nhn xt: Tiu chun [1] s dng 5 phng php ca 5 tc gi khc nhau d tnh
sc khng ti hng v mi cc, cn tiu chun [8] ch dng phng php (Reese
& O'Neill (1999)) cho d tnh sc khng hng v phng php Reese &
O'Neill (1999) cho sc khng mi; Phng php [1], h s c tnh theo mt cng
thc, ph thuc vo su; cn Phng php [8] th h s c tnh theo mt trong hai
cng thc, ph thuc su z v ch s N60; l thuyt ca phng php tnh sc khng
mi theo [1] ch ph thuc vo gi tr N, trong khi theo tiu chun [8] th
ph thuc vo c gi tr N60 v t s Pa/v.
3. KT QU NGHIN CU V BNH LUN:
lm sng t cc nhn xt trn, nhm tc gi vn dng tiu chun 22TCN 272-05 v
AASHTO LRFD 2007 [8] d tnh sc khng dc trc cc khoan nhi trong mt on
cu cn thuc d n ng cao tc TP. H Ch Minh-Long Thnh-Du Giy:
c im cng trnh: on cu cn dm Super T thuc gi thu 1a (Km4+000.00 n
Km7+500.00); mi mng m, tr c bng 6 cc khoan nhi, ng knh cc 1,2m,
chiu di cc 65,5m-74m. Cc xuyn qua 3-4 lp t trong 5 lp t sau: Lp 1:
Bi do, chiu dy thay i t 2.0m (l khoan T50L) n 22.8m (T78L), gi tr
SPT t 0-4; Lp 2: St pha ct, cng va, chiu dy t 0.48m(T26L) n
24.0m(T5R), gi tr SPT t 4-27; Lp 3: Ct st, cht va n cht, chiu dy ca
lp cha xc nh chnh xc, chiu dy ln nht khoan c l 77.35m (T29R), gi tr
SPT t 10-49; Lp 4: St pha ct, rt cng n rn, chiu dy thay i t 1.4m
(T60L) n 11.10m (T21R), gi tr SPT t 31-84; Lp 5: Ct st, ct bi, cht
n rt cht, chiu dy ca lp cha xc nh chnh xc, chiu dy ln nht khoan c
16.00m (T18L), gi tr SPT thay i t 39 n 70. Kt qu th tnh thc hin cho
8 cc th, trong c 1 cc th n 300%Pmax v 7 cc th n 150%Pmax, vi Pmax l
sc chu ti thit k tnh ton tc dng ln u cc c gi tr t 5.018KN-5.207KN,
[2].
Tc gi s dng 2 phng php: phng php (b), dng cng thc ca tc gi Reese
and O'Neill (1988) theo 22TCN272-05 v phng php (c) ca tc gi Reese
and O'Neill (1999) theo AASHTO LRFD 2007. Trn c s so snh vi t hp 3
phng php: Reese and Wright (1977)-Meyerhof (1976)-Reese and O'Neill
(1988), 22TCN272-05 m n v T vn thit k p dng v i chiu vi kt qu th ti
tnh. Kt qu tnh ton, phn tch nh gi c thng k bng 5.&6., [2],
[4].
Bng 5. Kt qu d tnh sc khng danh nh ca cc khoan nhi n (Qu) theo
tiu chun 22TCN 72-05 v AASHTO LRFD 2007 so vi kt qu th ti tnh
Phng php (a), (KN) [1]
Phng php (b), (KN) [1]
Phng php (c), (KN) [8] ST
T V tr th nghim Qu Sai lch Qu Sai lch Qu Sai lch
Ti trng th,Qmax
(KN) Ghi ch
(1) (2) (3) (4)=(3)-(9)/(3 hoc 1,5) (5) (6)=(5) -(9)/(3
hoc 1,5) (7) (8)=(7)-(9)/(3
hoc 1,5) (9)
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1 T96 18.231 13.024 19.874 14.667 19.889 14.682 15.621
=300%Pmax2 T17L6 17.253 12.229 18.134 13.110 17.762 12.738 7.536
=150%Pmax3 T20L6 20.294 15.270 21.161 16.137 19.783 14.759 7.536 Nt
4 T13L6 14.202 9.178 16.940 11.916 16.169 11.145 7.536 Nt 5 T15L6
17.001 11.977 17.745 12.721 17.717 12.693 7.536 Nt 6 T9L6 24.156
19.132 27.763 22.739 20.575 15.551 7.536 Nt 7 T1L4 16.317 11.179
16.948 11.811 16.948 11.811 7.706 Nt 8 T7L2 19.693 14.674 19.748
14.729 19.615 14.596 7.529 Nt
(KN) 18.393 13.333 19.789 14.729 18.557 13.497 7.559 (KN) 3.016
3.029 3.557 3.570 1.608 1.604 2.851 =/ 4,40 4,13 8,41
tin cy 99,999% 99,998% 100,000%
Ghi ch: (a)=Reese and Wright (1977)-Meyerhof (1976)-Reese and
O'Neill (1988), 22TCN272-05;(b)=Reese and O'Neill (1988),
22TCN272-05; (c)=Reese and O'Neill (1999), AASHTO LRFD 2007;=Gi tr
trung bnh; = lch chun; =Ch s tin cy [5]. Bng 6. Kt qu d tnh sc khng
tnh ton ca cc khoan nhi (QR) theo tiu chun
22TCN 72-05 v AASHTO LRFD 2007 so vi kt qu th ti tnh Phng php
(a),
(KN) [1] Phng php (b),
(KN) [1] Phng php (c),
(KN) [8]
STT V tr th
nghim QR Sai lch QR Sai lch QR Sai lch
Ti trng th,Qmax
(KN) Ghi ch
(1) (2) (3) (4)=(3)-(9)/(3 hoc 1,5) (5) (6)=(5) -(9)/(3
hoc 1,5) (7) (8)=(7)-(9)/(3
hoc 1,5) (9) 1 T96 6.993 1.786 8.507 3.300 7.530 2.323 15.621 =
3 0 0 % P m a x2 T17L6 6.986 1.962 7.332 2.308 7.096 2.072 7.536
=150%Pmax3 T20L6 8.268 3.244 8.549 3.525 7.772 2.748 7.536 Nt 4
T13L6 5.980 956 7.013 1.989 6.364 1.340 7.536 Nt 5 T15L6 6.978
1.954 7.268 2.244 7.059 2.035 7.536 Nt 6 T9L6 9.740 4.716 8.794
3.770 8.079 3.055 7.536 Nt 7 T1L4 6.641 1.504 6.891 1.753 6.812
1.675 7.706 Nt 8 T7L2 8.043 3.023 8.068 3.048 7.697 2.677 7.529
Nt
(KN) 7.453 2.393 7.803 2.742 7.301 2.241 (KN) 1.179 1.202 763
761 567 576 =/ 1,99 3,60 3,89 R,
tin cy 0,97673 0,99984 0,99995
Ghi ch: Nhng gi tr h s sc khng khng c trong tiu chun 22TCN272-05
c s dng tm theo tiu chun AASHTO LRFD 2007; h s nhm cc l 0,7.
Nhn xt: Sc khng danh nh v sc khng tnh ton cc khoan nhi n tnh
theo phng php (c)-[8] c tin cy R cao nht, tng ng l 1,00 v 0,99995;
k n l phng php (b)-[1], tng ng l 0,99998 v 0,99984 v thp nht l phng
php (a)-[1], 0,99999 v 0,97673 (xem cc Bng 5 v 6).
4. KT LUN V KIN NGH
Qua cc ni dung phn tch nghin cu trn, c mt vi kt lun v kin ngh v
d tnh sc khng dc trc cc khoan nhi theo t nn nh sau:
Tiu chun 22TCN272-05 ( v c AASHTO LRFD 1998, 2004) cn nhiu tn ti
nh nu qu nhiu phng php tnh sc khng dc trc cc khoan nhi ca cc tc gi
t nhng nm trc 1980; h s sc khng ln hn [8] t 37,5% n 44,4% v khng c
cho
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trng hp t ri. Kt qu tnh ton so vi kim nh thc t cho thy bin sai
lch ln v tin cy cc phng php l khng cao, t 97,673% n 99,984%;.
Tiu chun AASHTO LRFD 2007 (SI), AASHTO LRFD 2010 (US) c bn gii
quyt nhng tn ti ca cc phin bn ban hnh t nm 2004 tr v trc. Tuy nhin,
khi chuyn i h n v gia US sang SI c nhm ln h s trong cng thc
(10.8.3.5.2c-1) [8]. Kt qu tnh ton so vi thc t cho thy bin sai lch
nh v tin cy cc phng php tnh kh cao, ti 99,995%.
Kin ngh, trc mt p ng vic tnh ton sc khng dc trc cho cc khoan nhi
m bo c tin cy cn thit trong thc tin thit k hin nay, cn c ngay hng
dn b sung cp nht cho tiu chun 22TCN272-05 trn c s tiu chun AASHTO
LRFD 2007 (SI) t cc c quan qun l ngnh. V lu di, cn phi c nhng cng
trnh nghin cu nh gi chuyn su v vic s dng cc khoan nhi ph hp vi iu
kin Vit Nam da trn vic s dng l thuyt tin cy kt hp vi cc s liu thc
nghim ca cc khoan nhi ngoi hin trng a cc iu kin, cc thng s ph hp vi
iu kin a cht, phng php thi cng cc khoan nhi. T kin ngh s dng phng
php (m hnh) tnh ton cng nh xut cc h s sc khng ph hp vi iu kin Vit
Nam a ra vic d tnh sc khng cc khoan nhi gn st vi sc chu ti thc
t.
TI LIU THAM KHO [1]. B Giao thng vn ti (2005), Tiu chun thit k
cu 22TCN272-05. Nxb Giao thng
vn ti, H Ni, 2005. [2]. H s thit k, Bo co kho st a cht, thuyt
minh tnh ton(2008) v bo co th
ti tnh(2010) gi thu s 1A, D n ng cao tc Thnh ph H Ch Minh-Long
Thnh-Du Giy.
[3]. V Cng Ng, Nguyn Thi (2006), Mng cc, phn tch v thit k, NXB
Khoa hc v K thut, H Ni, 2006.
[4]. Ng Chu Phng v cng s (2011), Phn tch, nh gi v d tnh sc khng
dc trc cc khoan nhi mng m tr cu trn nn t yu theo tiu chun thit k cu
22TCN 272-05 v AASHTO LRFD 2007, ti NCKH cp trng, i hc GTVT.
[5]. Trn c Nhim (1996), Cc phng php xc sut v l thuyt tin cy
trong tnh ton cng trnh,Nxb i hc Giao thng Vn ti, H ni, 1996.
[6]. AASHTO(1998), LRFD Bridge Design Specifications 2th
Edition, American Association of State Highway and Transportation
Officials, Washington D. C.
[7]. AASHTO(2004), LRFD Bridge Design Specifications, 3th
Edition. American Association of State Highway and Transportation
Officials, Washington, D.C.
[8]. AASHTO(2007), LRFD Bridge Design Specifications (SI), 4th
Edition. American Association of State Highway and Transportation
Officials, Washington, D.C.
[9]. AASHTO(2010), LRFD Bridge Design Specifications (US), 5th
Edition. American Association of State Highway and Transportation
Officials, Washington, D.C.
