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International Conference on Case Histories in Geotechnical Engineering
(2013) - Seventh International Conference on Case Histories in Geotechnical Engineering
02 May 2013, 4:00 pm - 6:00 pm
3D FEM Modelling of a Deep Excavation Case Considering Small-3D FEM Modelling of a Deep Excavation Case Considering Small-
Strain Stiffness of Soil and Thermal Shrinkage of Concrete Strain Stiffness of Soil and Thermal Shrinkage of Concrete
Yuepeng Dong University Of Oxford, United Kingdom
Harvey Burd University Of Oxford, United Kingdom
Guy Houlsby University Of Oxford, United Kingdom
Zhonghua Xu East China Architectural Design & Research Institute, China
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Recommended Citation Recommended Citation Dong, Yuepeng; Burd, Harvey; Houlsby, Guy; and Xu, Zhonghua, "3D FEM Modelling of a Deep Excavation Case Considering Small-Strain Stiffness of Soil and Thermal Shrinkage of Concrete" (2013). International Conference on Case Histories in Geotechnical Engineering. 46. https://scholarsmine.mst.edu/icchge/7icchge/session03/46
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wn deep excaved with a commodel considess of the soilmodel develoelastic materi
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s are frequentund space. Hoinevitably in
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oncrete should
tly used in cowever, the conduce groundmay cause darediction of grot straightforw
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Ou, Liao et a2002; Liu, Nal. 2011). Altthey provide v
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ngye bank building with a 3concrete fram
TORY COSHRINK
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nstruction prohe influence oation. The calll-strain stiffndiaphragm waabs were modation behaviooil models andl to capture th
ts et al. 2005was largely siortant details. is vital imp
mpson 1992;Ws et al. 200uts due to vaprocess is selds evident and
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; Hashash, Soimplified and Research ha
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se history modelling luded the led with a lled as an ear elastic well with inkage on behavior
ong et al. therefore s showed
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thermal e ambient red in the le (Dong,
history is and the
f concrete
s a 82.5m n building stem, with
Paper No. 3.28b 2
pile-raft foundation (Wang and Wang 2007; Xu 2007). The piles are bored piles, 0.9m in diameter, and around 60m in length. The whole project covers an area of about 7856m , and the excavation area is around 6200m . The excavation is 14.2m deep on the west side, and 12.2m deep on the east side. The project is situated in the downtown area of Shanghai, surrounded by 15 densely distributed buildings of which 8 are historical buildings with high protection standard, as well as some aged pipelines.
Fig. 1, Plan view of the deep excavation Fig. 2 shows the section view of the excavation and the supporting structures. The excavation is retained by a 1m thick diaphragm wall which is supported by horizontal beams and slabs, as shown in Fig. 3.
Fig. 2, Section view of A-A
Fig. 3, First floor underground beams and slabs
The 60m deep piles provide vertical support to the whole structure. The excavation is constructed with top-down methods, and the above buildings can be constructed to the 3rd floor at the same time with the excavation. This excavation was carefully measured during construction. Geotechnical Conditions and Soil Properties The city of Shanghai is situated at proximately 70km from the sea shore, in the large coastal plain limited by the East China Sea and the Yangtze River which is designated as the ‘Yangtze River Delta’. The subsoil of Shanghai is composed of Quaternary sediments of the Yangtze River estuary which consist of clay, loam, silt and sand, the different deposits being the final result of the variation from an estuarine to fluviatile sedimentation process (Dassargues, Biver et al. 1991). The elevation of the ground surface is typically from 2.2m to 4.8m above sea level (Xu, Shen et al. 2009). According to the site investigation report, the site is on a flat coastal plain, with ground elevation between 4.80m to 3.87m, and ground water table 0.5 to 1m below the ground surface. Based on the differences of soil characteristics, physical and mechanical properties, the soil profile can be divided into 7 sub layers, as shown in Fig. 4, with the corresponding soil properties.
60
50
40
30
20
10
016 18 20 20 40 60 0.5 1.0 1.5 0.5 1.0 0 20 40 0 10 0 10 20 30
⑧2-2
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④
③
Soil layers
Dep
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①
⑧2-1
γt (kN/m3)
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c (kPa) φ (Ο)
Note: γt=unit weight, wn=water content, wp=plastic limit, wl=liquid limit, e=void ratio, cc=compressive index, su=field vane shear strength, c=cohesive strength, φ=internal
friction angle.
Fig. 4, Geotechnical profile and soil properties Some of the soil parameters for numerical modelling are derived from Fig. 4. The unit weight generally increases with depth, but it is convenient to take its average value, roughly 18.5kN/m . For the undrained shear strength s , the data in Fig. 4 is not sufficient for numerical modelling. Therefore, a more complete S profile is collected from Dassargues, Biver et al.(1991), and Equation (1) is derived by linearizing the data. s 20 2z kPa (1) The small-stain stiffness of the soil is missing from the site investigation report, but it could be collected from publications about Shanghai clay. Stiffness at very small strain G can be measured using dynamic methods. Cai, Zhou et al.
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p excavation. Tig. 6.
Fig. 6, Layout observed item
he dynamic cugh several typG is citied and
20 2z
tiffness propehown in Equat
11 9.2
.
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that it has adv
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environmentalve field measuprocess to mThe detailed i
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characters of pes of laboratod generalised a
MPa
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and
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FEM MODE Model Descri The FEM structures anshows the exc
100
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Ux=Uy
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L AND INPU
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400 m
y=0
Ux=Uy=0
g. 7, Soil mesh
the diaphragmFig. 9.
g. 8, Diaphrag
acement, 1~rizontal displa
1~ 32; splacement ou
el, 1~ 5; nt outside the 1~ 12, 13~pelines, inclu
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UT PARAMET
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undary conditio
400 m
Ux
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Ux=
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upport system
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3
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x=Uy=0
x=Uy=0
is shown
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soil and the ahedral elemeular to model rnal force andven that it tenund settlemenment caused but the wall cenEM (Dong, Belled with 2-elled with 4el has totalldratic elementhe linear ele
excavation solidation andulations are co
ut Parameters
soil is modell9) developed AQUS througsider the smallace model, fo
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upporting syste
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10, Nested yi
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beneficial benerface of the kness in geomes and beamsts. The slabsll elements. d 116,756 nos are almost snger time to ru
ained conditsidered here. percomputer.
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odes. same un.
ions. The
ulsby d into ), to yield
work-inear ffects ecent ieved pe as
The stiffnessstrength in(1) and Equaparameters foFig. 5. To invsoil models model, are al The diaphragproperties to et al. 2005).
/ 0this case hist30 , In most analyare modelledthe design stiand workmanPotts et al. 19In the analymaterial but idue to tempeambient tempBoone and CThe Coefficieassociated walso based on ANALYSIS The strategy which is beliproblem and other analyseanalysis and excavation be The central aof the case hAnd in other time. The ruTable 2. Theas well as the The focus of soil models excavation binto two grou RESULTS IN There is a modelling anpresent all of
s at very smncrease linearlation (2) respor the nested yvestigate the inin ABAQUSso used.
gm wall is consider the j. The out-of-
0.1 is adoptedtory. The ela0.2.
yses from exid as linear elaiffness to consnship of the co993). Althoughyses here, thincluding the trature changeperature varia
Crawford 2000ent of Therma
with temperatun back analysi
STRATEGIE
of the analyseieved to consthe result agr
es are groupefield data to
ehaviour.
analysis is conistory to obtacalculations,
un ID and dese numerical ree field data.
f the calculatioand therma
ehaviour. Theups and the res
NTEPRETAT
large amounnd field measuf them here.
mall strain ly with depth,pectively. Theyield surface mnfluence of so, linear elasti
modelled wijoints in the w
f-plane and ind based on soastic propertie
isting publicatstic material wsider open acconstruction (Sh simple, it ha
hey are modethermal contra
e during concration(Whittle,0; Hashash, Mal Expansion (ure change ∆s.
ES AND CALC
es is to focus sider every imrees best withed and comp
o investigate
nducted basedain a group of
only one parascription are esults are show
ons is to invesal shrinkage erefore, the casults are comp
TION
nt of data urements, but Therefore, on
and the u, as shown in e small-strainmodel are derioil models, twic and the Tr
ith anisotropiwall (Zdravkon-plane stiffnome back an
es of concrete
tions, beams with 20% redcess, shrinkagSimpson 1992as no physicalelled as lineaaction which irete curing pro Hashash et
Marulanda et a(CTE) is10∆ 35
CULATIONS
on one centramportant aspeh the field datpared with ththeir influenc
d on the backf optimised paameter is chanshown in Tab
wn in the next
stigate the infof concrete
alculations arpared in each g
from both nit is not appr
nly some sele
4
undrained Equation
n stiffness ived from
wo built-in resca soil
ic elastic ovic, Potts ness ratio nalyses of e are
and slabs duction of ge, cracks, ; St. John, l meaning. ar elastic is evident ocess and al. 1993; al. 2003). 10 / , which is
S
al analysis ect of the a. All the
he central ce on the
k analysis arameters. nged each ble 1 and t sections,
fluence of e on the re divided group.
numerical opriate to
ected data
Paper No. 3.28b 5
from the field measurements is compared with numerical results, as shown in Fig. 11.
P9
P8
P3 AA9
L12
Wall 1 Wall 2
Wall 4
Wall 6
Wall 8
L07
AA12
Line 3
Line 1
Fig. 11, Field instrumentations The comparison is focused on one wall deflection at wall centre (P9) and one at wall corner (P8), and the ground settlement along Line 1, Line 2 and Line 3. And it is believed that the data can reflect a complete picture of the excavation behaviour. In this section, results from two sets of calculations are presented, in order to demonstrate the influence of soil models and thermal shrinkage of concrete on the excavation behaviour. Influence of soil models To investigate the effect of different soil models, besides the central analysis which considers the small-strain stiffness of the soil using the nested yield surface model, three other runs are conducted with simpler soil models, as shown in Table 1. The results are compared with the filed data.
Table 1 FEM Runs and Description Run ID Description Central analysis
Soil Model: Nested-yield surface model, stiffness and strength increase linearly with depth; Wall Model: anisotropic elastic, E E⁄ 0.1, Beams and Slabs: elastic, α 10 10 / , ∆T 35
SME Same as central analysis except that the soil model is linear elastic with constant soil parameters
9 , 0.49 SMTC Same as central analysis except that the soil model
is Tresca with constant soil parameters; 9 , 0.49, 50
SMTV Same as central analysis except that the soil model is Tresca and soil stiffness and strength increases linearly with depth;
180 , 0.49, 20 2 To make these analyses comparable, some assumptions of the soil parameters are adopted. For linear elastic analysis and Tresca soil model with constant soil properties, the stiffness G is adopted as G (stiffness at 50% of the shear strength) from
Fig. 5, and the strength S is taken at the depth of 15m , roughly half of the wall depth. For Tresca soil model with variable soil parameters, G 0.180G , and G S⁄ 1000 is used here, so G 180S . The results of calculations in Table 1 are shown in Fig. 12 - Fig. 16, together with the field data.
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100-32-30-28-26-24-22-20-18-16-14-12-10
-8-6-4-20
Wal
l dep
th /m
Wall deflection /mm
Field data Central analysis SME SMTC SMTV
P9
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100-32-30-28-26-24-22-20-18-16-14-12-10
-8-6-4-20
Wal
l dep
th /m
Wall deflection /mm
Field data Central analysis SME SMTC SMTV
P8
Fig. 12, Wall deflection at P9 and P8 From Fig. 12, it is shown that the central analysis agrees well with the field data. Tresca soil model with variable stiffness and strength soil parameters could also capture the pattern of wall deflection very well. But the linear elastic model and Tresca soil models with constant soil parameters perform rather poorly, and therefore they are not suitable to use for prediction purposes. The ground settlement along Line 1 is shown in Fig. 13. Again, the results indicate that the central analysis with the nested yield surface model captures the ground movement very well because it considers the small-strain stiffness of the soil. But the other three, even the Tresca soil model with variable soil properties which could predict the wall deflection well, fail to produce both the pattern and magnitude of the ground movement. For the linear elastic and Tresca soil model, the ground movement around the excavation is upward which contradicts with the field data. If they were used for the prediction of the adjacent infrastructures around the excavation, the result would be misleading. Therefore, in order to get reasonable results for ground movement, the small-
Paper No. 3.28b 6
strain stiffness must be considered in the analysis.
0 5 10 15 20 25 30 35 40-30
-20
-10
0
10
20
30
40
50
60
Gro
und
settl
emen
t /m
m
Horizontal distance from wall /m
Field data Central analysis SME SMTC SMTV
Fig. 13, Ground settlements along Line 1 The ground settlement along Line 2 and Line 3 is shown in Fig. 14. Again, it demonstrates that when the small-strain stiffness of soil is considered the numerical result can capture the ground settlement. Otherwise, the results are disappointing when compared with the field data.
0 10 20 30 40 50 60 70-30
-20
-10
0
10
20
30
40
50
60
70
80
Gro
und
settl
emen
t /m
m
Horizontal distance /m
Field data Central analysis SME SMTC SMTV
Line 2
0 10 20 30 40 50 60 70 80-30
-20
-10
0
10
20
30
40
50
60
70
80
Gro
und
settl
emen
t /m
m
Horizontal distance /m
Field data Cental analysis SME SMTC SMTV
Line 3
Fig. 14, Ground settlements along Line 2 and Line 3 Influence of concrete thermal shrinkage To investigate the influence of thermal contraction of concrete and different temperature change on the excavation behaviour, results from another two runs, as shown in Table 2, are
compared with the central analysis as well as the field data. The results are presented as below.
Table 2 FEM runs and description
Run ID Description Central analysis
Soil Model: Nested-yield surface model, stiffness and strength increase linearly with depth; Wall Model: anisotropic elastic, E E⁄ 0.1, Beams and Slabs: elastic, α 10 10 / , ∆T 35
ANE1T30 Same as central analysis except ∆T 30 ANE1T40 Same as central analysis except ∆T 40 The results of the analyses listed in Table 2 are shown in Fig. 15~Fig. 16, together with the field data.
0 5 10 15 20 25 30 35 40 45 50-32-30-28-26-24-22-20-18-16-14-12-10-8-6-4-20
Wal
l dep
th /m
Wall deflection /mm
Field data Central analysis ANT30E1 ANT40E1
P9
0 5 10 15 20 25 30 35 40 45 50-32-30-28-26-24-22-20-18-16-14-12-10-8-6-4-20
Wal
l dep
th /m
Wall deflection /mm
Field data Central analysis ANT30E1 ANT40E1
P8
Fig. 15 Wall deflection at P9 and P8 The results show that the wall deflection is sensitive to temperature change inside the concrete during curing process. When the concrete cools down by 5 , the beams and slabs shrink and the wall deflections increase around 3mm. But the wall deflection increment at P8 is slightly smaller than that at P9 due to the corner effect. Therefore, this effect should not be neglected in the analyses. The ground settlement behind the wall along Line 1 is plotted in Fig.16, together with the field data.
Pape
The arouwhic The 3 are
Fig. and sensichanwall
er No. 3.28b
0 5-30-28-26-24-22-20-18-16-14-12-10
-8-6-4-2024
Gro
und
settl
emen
t /m
m
Fig. 16
results showund 3mm whech is accompa
ground settleme shown in Fig
0 10-30-28-26-24-22-20-18-16-14-12-10
-8-6-4-20
Gro
und
settl
emen
t /m
m
0 10-30-28-26-24-22
-20-18-16-14-12-10-8-6-4-2
0
Gro
und
settl
emen
t /m
m
Fig. 17, Grou
17 is consisteindicates th
itive to thermnge behind the
centre.
10 15Distance
6, Ground settl
ws that the gren the temperanied by the in
ments behind g. 17, together
0 20 30Horizonta
20 30Horizont
Fiel Cen ANT ANT
Li
und settlemen
ent with the gat ground seal contractione wall corner
20 25 3e from wall /mm
lements along
round settlemrature of concncrease of the
the wall alonr with the field
40 50al distance /m
Field data Central analysis ANT30E1 ANT40E1
Line 2
40 50 60tal distance /m
d datantral analysisT30E1T40E1
ne 3
nts along Line
ground settlemettlement beh
n of concrete. is smaller tha
30 35 40
Field data Central analysis ANT30E1 ANT40E1
Line 1
g Line 1
ment increasecrete reduceswall deflectio
ng Line 2 and d data.
60 70
0 70 80
2 and Line 3
ment along Lihind the waBut the settleman that behind
s by 5 ,
on.
Line
ine 1 all is ment d the
CONTOUR D The displaceshown belowsoils and stru The ground v18.
Fig
The largest gcentre aroundaround the cowall, the largis also evidensoil displacemincreases, andzero.
As shown in at the wall cdeflection at shape of the
DISPLAY
ement contouw, from whicuctures is clear
vertical displa
g. 18, Ground
ground settlemd the excavatorner due to thger is the grount inside the ment decreasd the displace
Fig. 19, W
Fig.19, the lacentre area clothe corner is sdiaphragm w
urs from thech the displacrly seen.
acement conto
d vertical displ
ment is concention, while thehe soil archingund settlementexcavation dues as the dist
ement around
Wall deflectio
argest wall defose the excavsmaller due to
wall also affec
e central anacement distrib
our is displaye
lacement (m)
ntrated behinde settlement ig effect. The lt area. The baue to stress retance from exthe boundary
n (m)
flection is convation depth, o the corner efcts the wall d
7
alysis are bution of
ed in Fig.
d the wall is smaller longer the asal heave elief. The xcavation
y is nearly
ncentrated while the ffect. The
deflection.
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The fieldat thcond
Fig Fig. usefuundeand bene CON The probnon-procof diprovcons In ordetaimod
ACK
er No. 3.28b
displacement d measuremenhe toe. This
ditions and wa
g. 20, Displac
20 shows theul to comparerstand the perbending mom
eficial for desi
NCLUSIONS
3D numericalblems, which c-linear materedures. From isplacements a
vide rather usstruction.
rder to captureiled structure el should cons
1. The smaconsideremovemenand strenwall defmovemenconstant suitable t
2. Concretechange temperatuproblems
KNOWLEDG
at the wall bont it is usuallys assumptionall depth.
cement distribu
e deformationre with the frformance of
ment can also ign purposes.
l analysis is acould considerrial propertithe numerica
and stresses isseful informa
e the main exand construc
sider the follo
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ature bient ation
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