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Tunnelling and Underground Space Technology 26 (2011) 276–283
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Tunnelling and Underground Space Technology
journal homepage: www.elsevier .com/ locate / tust
Numerical investigation of underlying tunnel heave during a newtunnel construction
Hanlong Liu a, Ping Li a,⇑, Jinyuan Liu b
a Key Lab of Ministry of Education for Geomechanics and Embankment Engineering, Geotechnical Research Institute, Hohai University, Nanjing 210098, Chinab Department of Civil Engineering, Ryerson University, 350 Victoria Street, Toronto, ON, Canada M5B 2K3
a r t i c l e i n f o
Article history:Received 17 March 2010Received in revised form 30 July 2010Accepted 19 October 2010Available online 11 November 2010
Keywords:Foundation excavationHeave deformationConstruction schemeNumerical analysisTunnel protection
0886-7798/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.tust.2010.10.002
⇑ Corresponding author. Tel.: +86 25 8378 7772.E-mail address: [email protected] (P. Li).
a b s t r a c t
This paper presents a case study of protecting existing tunnels during the construction of a new cut-and-cover tunnel above in Nanjing, China. Various construction measures, including sequential excavation, jetgrouting, and a pile–slab retaining system were performed to control the heave of existing tunnels. Fur-thermore, a numerical analysis using a finite difference program, FLAC3D, was conducted to investigatethe influence of different construction schemes on the tunnel heave. Finally, a comparison betweennumerical results and field measurements were carried out to study the influence on the tunnel heavefrom various factors, such as the ground reinforcement depth, excavation sequence, and the skew anglebetween new tunnel and existing tunnels. The results show that when the excavation volume is small,the uplift values of existing tunnels increase nonlinearly with the increasing excavation width of eachstep. The pile–slab retaining system combining with ground treatment method can control the tunnelheave within the required limits. The optimum ground treatment scope is about 1.5 times of the excava-tion depth in this project. Compared to other construction schemes, the tunnel heave will be the smallestunder a staggered segmentation excavation method starting from the sides to the center.
� 2010 Elsevier Ltd. All rights reserved.
1. Introduction
In highly congested urban areas, more and more tunnels arebeing constructed close or above existing tunnels to meet theincreasing demands of transportation, water conveyance, etc. Un-der these conditions, it is very challenging to ensure the safety ofexisting tunnels while the excavation of the new ones isperformed.
A few case histories have been published to investigate theinteraction between new and existing tunnels. Kim (1996) carriedout a series of physical model tests on closely-spaced tunnels inkaolin clay samples and found that the interaction mechanisms de-pended on the geometry of the tunnels. Based on field observationsof the tunnel deformation caused by excavation above, Chen andZhang (2004) recommended that the heave curves of the exitingtunnels were normally distributed, and the maximum deformationpoint was at the center of the pit. Huang et al. (2006) proposed aformula to calculate the rebound of existing underlying tunnelsby using a soil unloading semi-experiential modulus derived forShanghai soft clay. Compared with beam elements, Klar and Mar-shall (2008) found that when the material stiffness is small, shellelements were more accurate for representing pipes in evaluation
ll rights reserved.
of tunneling effects on pipelines. At the same time, a jointed pipe-line bore smaller bending moments than a continuum one (Klaret al., 2008).
Numerical analyses, including both two-dimensional (2D) (Loand Ramsay, 1991; Dolezalová, 2001; Sharma et al., 2001) andthree-dimensional (3D) modeling (Liu et al., 2008, 2009) have beenapplied to investigate the interaction between new excavationsand existing tunnels. To date, however, there has been limited re-search in optimizing the construction schemes to limit the defor-mation of existing tunnels.
Based on a case history in China, this paper presents a three-dimensional (3D) finite difference (FD) analysis to investigate theinfluence on existing underlying tunnels during the constructionof a new tunnel on top. The construction process was simulatedfirst and then followed by a parametric analysis on the construc-tion optimization.
2. Engineering background
The project was located in the northeast of Nanjing, China. Anew cut-and-cover tunnel was 572.08 m long, which was dividedinto a middle tunnel of 260 m long and a west–east approach of312.08 m long. The maximum longitudinal slope was 4.75%, the in-ner width was 10 m, and the net height was 4.73 m. Moreover, the
H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283 277
new tunnel had a 650 mm thick roof, an 800 mm thick floor slab,and two 600 mm thick sidewalls.
0.6m 0.6m
Existing shallow tunnel
0.8m
0.65m
9.0m 11.2m
0.6m 0.6m
New tunnel
0.8m
0.65m
6.18
m
11.2m
(a) Shallow tunnels
t=0.3m
R3.1m
6.8m
R3.1mt=0.3m
12.8m 6.8m
Left Right
(b) Shield tunnels
Fig. 1. Characteristic dimensions of tunnels.
New tunnel
Existing double-track shield tunnels
70 degrees
A
AExisting shallow tunnel
P20 P9
P3
Settlement plates for shield tunnels
CC
P4
P5
P6
P7
P8
P16
P1
P10P11
P12
P13
P14
P15
P2
Left Right
Anti-uplift pile
Strip slab
Axial force gauges for strutsInclinometers for supporting piles
I3
F3
I4I1I2
F1F2
I6I5 I7 I8
P17
P18 P19
(a) Plan layout
Ground level
Existing double-track shield tunnels
Existing shallow tunnel
New tunnel1.95m
6.13m
2.15m
6.80m
Anti-uplift pile
(b) Sectional view of A-A
Fig. 2. Layout of existing tunnels and new tunnel.
Table 1Soil profile at the site.
Depth (m) Void ratio Liquid limit Plastic limit SPT Descrip
0.0–4.5 0.99 35.5 21.3 4 Yellow4.5–7.8 0.76 31.7 25.7 11 Cyan an7.8–17.5 1.03 34.4 21.3 3 Grey m17.5–35.0 0.70 37.6 21.5 20 Grey sil35.0– 47 Greyish
A total of four tunnels were in this section, including two shal-low side-by-side parallel tunnels (one was an existing tunnel andthe other was the new one) with their dimensions shown inFig. 1a and the other existing double-track shield tunnels of MetroLine 1 below these shallow tunnels. The characteristic dimensionsof shield tunnels, i.e. the thickness of tunnel linings (t = 0.3 m), theradius (R = 3.1 m) are shown in Fig. 1b. The skew angle betweentwo double-track shield tunnels and the new tunnel was 70�. Thetop slab of the shield tunnels was 10.23 m below the ground sur-face. The minimum clearance between the pit bottom and the tun-nel crown was 2.15 m (Yang and Wang, 2003). Both the plan andsectional views of these tunnels are shown in Fig. 2.
The soil profile on the site consists of silty clay with a depth of30.5 m overlain by a miscellaneous fill with an approximate 4.5 mthickness. Immediately beneath the silty clay, moderately weath-ered diorite porphyrite can be found. The groundwater table fluc-tuates seasonally between 1.0 and 2.0 m below the groundsurface (the watertable was assumed to be 1.5 m deep in the anal-yses). The ground profile is briefly described in Table 1.
3. Tunnel structural design
3.1. Retaining pile of the pit
The new tunnel pit was 12.8 m wide, 7.8 m deep, and 420 mlong. A total of fourteen secant piles and fourteen bored piles wereused in the retaining wall above the existing shield tunnels. The0.8 m diameter secant piles were 16.50 m long and the 1.2 m diam-eter bored piles varied in length from 8.20 m to 16.50 m. The jetgrouting columns with a diameter of 0.8 m and a lap length of0.3 m were also used behind the retaining wall as a waterproofcurtain.
In order for the existing tunnel linings to have a contact withthe supporting piles, the soil around the existing tunnels was trea-ted by sodium silicate chemical grouting. In addition, two rows of609 mm diameter steel pipes with a wall thickness of 12 mm wereinstalled as struts with a horizontal spacing of 4.8 m and a verticalspacing of 3.8 m.
3.2. Supporting of resistant heave for shield tunnels
The following criteria were set in this project in order to ensurethe safe operation of existing tunnels during and after the new tun-nel construction:
(1) The maximum allowable settlement 615 mm.(2) The maximum uplift value <10 mm.(3) The curvature radius of deformation P15,000 m.
A series of reinforcement measures were used to control thetunnel heave in this project, including the jet grouting groundtreatment method and a pile–slab retaining system. The high pres-sure jet grouting was used to treat the pit soil within the depthfrom 4.5 m to 17.5 m below the ground surface. The reinforcementscope was 500 mm away from the shield tunnel linings. The pile–
tions General properties
and grey miscellaneous fill Heterogeneity and low strengthd grey silty sand with silty clay Medium compressibility and low strength
ucky silty clay High compressibility and low strengthty clay Medium compressibility and low strengthyellow diorite porphyrite High compressibility and high strength
Ground level
Existing double-track shield tunnels
Anti-uplift pile
New tunnel
Strip slab
Sodium silicate chemical grouting
1.95m
6.13m
24.0m
Fig. 3. Section view of the pile–slab retaining system (C–C section).
12.0m
9.0m
6.0m
0.0m
-3.0m
3.0m
Elevation at the start of excavation
After 5 days
After 7 days
After 9 daysAfter 17 days
After 18 days
After 19 days
After 21days
After 23 days
After 25 days
After 14 days
Elevation at the end of excavation
Ele
vatio
n
Excavation sequence:
Exiting double-track shield tunnels
-6.0m
-9.0m
After 3 days
After 13 days
N
Fig. 5. Excavation sequence of new tunnel pit.
278 H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283
slab retaining system consisted of anti-uplift piles and P-shapedstrip slabs, as shown in Fig. 3. The 0.8 m diameter and 24 m longanti-uplift piles were spaced 1.4 m, 1.8 m and 1.9 m between eachother. There were totally seven concrete slab strips cast with C50concrete mixed with early strength agents. These slab strips werenumbered r–x, where the dimensions of number r strip were2000 mm (width) � 600 mm (thickness), and those of the otherswere 1800 mm (width) � 400 mm (thickness). A rigid connectionwas used at the joints between the anti-uplift piles and the stripslabs. The details of the supporting system for the new tunnelare shown in Fig. 4.
3.3. Field instrumentation and monitoring
The heave of the existing double-track shield tunnels was thecritical issue in this project. Therefore, a field monitoring programwas commissioned prior to excavation. The instrumentation con-sisted of inclinometers, axial force gauges and settlement plates.A total of 16 settlement plates (P1–P16 as shown in Fig. 2a) wereinstalled in the soil close to the crown of double-track shield tun-nels, and four (P17–P20) were arranged at the two sides of thetunnels.
4. Construction sequence
The elevation of the ground surface was 10.5 m and that of thebottom of the excavation was 2.7 m. The field construction wascarried out in the following sequence:
1:2
Waterproofcurtain
1:21st strut
Retaining wall
Ring beem 1.1 0.8m
Jet grouting
3 1.8m 3 1.8m2.0m
2nd strut
6.2m
0.4m
0.6m
Exiting double-track shield tunnels2 1.8m 2 1.9m 2 1.8m 1.4m1.4mAnti-uplift pile
1.00m
3.50m
2.15m
6.80m
3.30m
NS
Fig. 4. Retaining system for new tunnel pit.
(1) Work preparation before excavation. This work consisted ofinstallation of anti-uplift piles, installation of secant pilewalls, and ground treatment using high pressure jetgrouting.
(2) Excavation and installation of the first level struts. The pre-stressed force of 360 kN was applied on the struts.
(3) Full excavation to a depth of 4.5 m. The soil was divided intothree layers with an approximate thickness of 1.5 m each.
(4) Installation of the second level struts. The positions werestaggered in the horizontal direction with respect to the firstlevel struts.
(5) Sequential excavation of the bottom soil. The soil wasdivided into seven strip segmentations, as shown in Fig. 4.The sequence used in the site was to excavate from the cen-ter to the two sides. A concrete slab was cast immediatelyafter each segment excavation.
The excavation sequence and corresponding times for new tun-nel pit are shown in Fig. 5. More details can be found in Yang et al.(2005).
5. Numerical analyses of tunnel interaction
5.1. FD analysis model and boundary conditions
The numerical analyses were performed using the finite differ-ence (FD) analysis software, FLAC3D (Itasca, 2005). Because of theasymmetric nature of the excavation, it was necessary to model allexisting tunnels in three-dimensional (3D) condition, as shown inFig. 6a. In order to balance the boundary effect and the computa-tional efficiency, the model dimensions were selected to be180 m (length) � 60 m (width) � 35 m (depth). The 35 m verticaldepth was approximately five times the excavation depth, as sug-gested by Li et al. (2008). The tunnel linings were constrained tomove with surrounding soil and the effect from the installationprocedure was ignored. As for boundary conditions, the displace-ments were set to be zero in the three directions with no horizon-tal and vertical movements allowed at the bottom of the mesh. Themovements in two horizontal directions were restrained, and onlyvertical movement was allowed on the four side boundaries.
All analyses modeled in terms of drained conditions. The tunnellinings were modeled as a continuum shell without joint consider-ation. A total of 15,696 eight-noded hexahedral elements were
(a) Geometric model
(b) Tunnel linings
(c) Anti-heave supports
Anti-uplift pileRetaining wall
Internal strut Existing double-track shield tunnels
N
3.0m
6.0m
0.0m
-3.0m
-6.0m
XYZ
Existing double-track shield tunnels
Existing shallow tunnel
New tunnel pit N
Existing double-track shield tunnels
70 degrees
New tunnel Existing shallow tunnel
Fig. 6. 3D finite difference mesh used in the study.
Table 2Physical and mechanical parameters of soil.
Depth (m) x (%) H (m) c (kN/m3) c (kPa) u (�) Er (MPa) K0
0.0–4.5 35.1 4.5 18.0 20.0 15.0 28.8 0.454.5–7.8 27.1 3.3 18.8 6 29.7 78.1 0.407.8–17.5 36.5 9.7 17.6 12.0 12.0 23.7 0.5317.5–35.0 25.4 17.5 19.6 16.0 35.0 42.7 0.43
Notes: c and u are the cohesion and the friction angle of the soil, respectively, x isthe water content, c is the unit weight, K0 is the lateral pressure coefficient, Er is therebound Young’s modulus (under condition of small strain), all of which wereobtained from the laboratory tests. H is the soil sampling depth during geotechnicalinvestigation.
Table 3Properties of structural members used in the analyses.
Name c (kN/m3) E (MPa) m A (m2) Ix (m4)
Reinforced soil 20.0 1.37 � 103
Steel strut 78.0 2.00 � 105 0.3 0.023 1.00 � 10�3
Anti-uplift pile 25.0 3.00 � 104 0.2 0.500 2.01 � 10�2
Notes: E is the Young’s modulus, A is the sectional area, and Ix is the inertia moment.
H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283 279
used for the soil mass, and 9152 three-noded shell structural ele-ments were used for tunnel linings, as shown in Fig. 6b. Further-more, the piles and the struts were modeled by 820 two-nodedpile structural elements and 74 two-noded beam structural ele-ments respectively, as shown in Fig. 6c.
5.2. Material models and parameters
The concrete structure units, including retaining walls, tunnellinings, and strip slabs were modeled as an isotropic linear elasticmaterial with a Young’s modulus of 30 GPa and a Poisson’s ratio of0.2. The soil was simulated by using an elasto-plastic constitutive
relationship based on the Mohr–Coulomb criterion with the re-lated parameters listed in Table 2.
The rebound bulk modulus Kr and rebound shear modulus Gr
were determined by using the following equation:
Kr ¼Er
3� ð1� 2mÞ ð1Þ
Gr ¼Er
2� ð1þ mÞ ð2Þ
where m is the Poisson’s ratio and Er is the rebound modulus (undercondition of small strain) obtained from conventional triaxial tests.
The jet grouting reinforced soil was treated as an elastic mate-rial, because the mechanical properties of the soil were signifi-cantly improved due to jet grouting. The related parameters ofreinforced soil, steel struts and anti-uplift piles used in the analy-ses are summarized in Table 3. The reinforced soil values of E wereobtained from triaxial tests, as recommended by Cao (2006), andthe others were taken from the lookup table and calculated in SIunits.
5.3. Excavation simulation
In this study, null models were used to simulate the excavation.The deformation caused by excavating existing tunnels was resetto zero before the new tunnel construction. Therefore, the calcu-lated uplift values were actually the incremental deformationdue to new tunnel excavation. In addition, the influence of strutinstallation was ignored to simplify the problem. The monitoringpoints were set up in the mesh in order to compare the results withmeasurements in the field during excavation.
6. Results and comparisons
6.1. Comparisons between computed results and measured data
The real excavation process was simulated first in the analyses.Fig. 7 shows the comparison between the measured tunnel heavesand computed ones. The FD results are generally larger than themeasured ones. However, the changes with time are similar toeach other. The larger deformation in the FD analysis may becaused by treating the pit soil as an isotropic soil which couldnot reflect the anisotropic characteristics of the in situ soil. The ra-
0 3 6 9 12 15 18 21 24 270.0
1.5
3.0
4.5
6.0
7.5
Tun
nel h
eave
(m
m)
Time (day)
P2 (computed) P2 (measured) P1 (computed) P1 (measured)
(a) The change of tunnel heave with time at the strip slab
-15 -10 -5 0 5 10 150
2
4
6
8
10
Tun
nel h
eave
(m
m)
Coordinates along the axial direction (m)
Right tunnel (measured) Right tunnel (computed) Left tunnel (computed) Left tunnel (measured)
(b) The change of tunnel heave along the axial
mid-point of No.
direction of shield tunnels
Fig. 7. Comparison of measured and computed heaves of existing tunnels.
0 5 10 15 20 25-6
-4
-2
0
2
4
6
Hor
izon
tal d
ispl
acem
ent (
mm
)
Time (day)
measuredcomputed
Fig. 8. Comparison of horizontal displacements on the top of retaining walls.
280 H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283
tio of the final measured tunnel heave, dv, to the excavation depth,H, is approximately 0.071%, which is similar to that reported previ-ously by Chen and Hsiung (2009). In addition, both the computedand measured results of the left shield tunnel (monitored by P1)are less than that of the right shield tunnel (monitored by P2). Infield operation, the excavation sequence of the pit bottom soil onleft tunnel has been changed into r ? s ? u ? t ? w ?v ? x (see Fig. 5); this might be the main reason for the asym-metric deformation of the twin tunnels. It also can be seen thatthe maximum heave occurs at the center of excavation in Fig. 7b,which is the same as the finding by Chen and Zhang (2004). Finally,it is worth noting that all the uplift values were less than 10 mm,which satisfied the deformation criteria of existing shield tunnelsin this project.
The comparison between calculated and measured data of thehorizontal movement at the top of retaining wall is given inFig. 8, where the negative value means the movement towardthe inside of the pit. The final computed horizontal displacementwas �1.22 mm compared to a field measurement of �5.90 mm.The small horizontal displacement of retaining walls demonstratesthat the retaining system was very stable. Moreover, the curve ofmeasured data shows that the horizontal displacement was signif-icantly influenced by construction activities, for example excava-tion, heaped load and bracing placement.
6.2. Influence of different reinforcement measures
In order to fully understand the heave of existing tunnels, theinfluence of different reinforcement schemes were investigatedin this study: including (1) without any reinforcement, (2) onlyjet grouting ground treatment, (3) only the pile–slab retaining sys-tem, and (4) the combination of jet grouting ground treatmentwith pile–slab retaining system. The results show that the maxi-mum heave values at the crown of shield tunnels happened inthe first case, but the final value could not be reached due to thefailure of retaining walls and the numerical convergence issue.The final tunnel heave was 11.1 mm for the second case comparedto 15.3 mm for the third case and 6.9 mm for the fourth case. Thecorresponding ratios of the final tunnel heave, dv, to the excavationdepth, H, are 0.14%, 0.20%, and 0.09%. It indicates that jet groutingground treatment is more efficient than the pile–slab retaining sys-tem in limiting the tunnel heave. However, the tunnel heave inboth cases exceeded the deformation criteria, 10 mm, in this pro-ject. It is also important to point out that the combination of bothreinforcement schemes provided a satisfactory heave value.
The change of the tunnel heave with increasing jet groutingtreatment depth is shown in Fig. 9a, where the pile–slab retainingsystem was used in all the cases. As expected, the final heave val-ues were reduced with increasing treatment depth; however, theefficiency of ground treatment was not linearly increasing withthe increasing treatment depth. For a treatment depth lower than13 m, the final uplift values were mainly influenced by the excava-tion depth of 7.8 m in the project.
Furthermore, the deeper the excavation, the more significant ef-fect of the reinforcement, as shown in Fig. 9b. The most efficienttreatment depth was found to be 1.5 times the excavation depth.
6.3. Influence of different excavation sequences and sizes
The sequential excavation considering excavation time and vol-ume, namely the time–space effect, has been proved as an efficientmethod for controlling the heave of existing tunnels (Chen andZhang, 2004; Yang et al., 2005; Huang et al., 2006). In order to eval-uate the influence of excavation sequences, three sequence condi-tions were analyzed in this study: The first was from the center tothe two sides (r?st?uv?wx, as shown in Fig. 5); the sec-ond from the two sides to the center (wx ? uv ? st ? r);and the third, a staggered one, from the two sides to the center(wx ? st ? uv ? r). The uplift values caused by the threedifferent excavation sequences at the crown of shield tunnels areshown in Fig. 10.
17
16
15
14
13
12
114 5 6 7 8
Pit s
oil r
einf
orce
men
t dep
th (
m)
Tunnel heave (mm)
P2 (computed) P1 (computed)
(a) Final tunnel heave
10
8
6
4
2
00 2 4 6 8
Exc
avat
ion
dept
h (m
)
Tunnel heave (mm)
Reinforcement depth (P2 computed)
16m 14.5m13m 11.35m
(b) Incremental tunnel heave
Fig. 9. Change of tunnel heave with different pit soil reinforcement depths.
H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283 281
It can be seen that the tunnel heave in the first case was small-est at the beginning and then developed rapidly to the final largest
4.5
5.0
5.5
6.0
6.5
7.0
7.51 2 3 4 5
Excavation sequence
Tun
nel h
eave
(m
m)
Segmentation excavation steps
Fig. 10. Change of tunnel heave with different excavation sequences.
value of 7.1 mm. The uplift value in the second case changed rever-sely as the sequence was reversed with a final heave value of6.7 mm. The tunnel heave changed between the first and the sec-ond cases in the third sequence and finally reached 6.5 mm afterconstruction. The tunnel heave for the third case of 6.5 mm wasthe smallest, where the excavation sequence was a staggered onestarting from the two sides to the center.
When the soil strips at the staggered two sides were removed,the additional stress generated by the reserved soil strips (Nos.r, u, and v) was the largest in the soil below pit basement. Thismethod of staggered segmentation excavation made full use of thetime–space effect. At the same time, the pile–slab retaining systemwas constructed immediately after excavation at the two sides,which would help limiting the heave.
The change of frictional force around anti-uplift piles with thepile length is shown in Fig. 11. It is obvious that the force valuescaused by the staggered excavation sequence starting from thetwo sides to the center are generally smaller than others, whichis consistent with the tunnel heave. Meanwhile, the frictional forceexperiences several decreasing and increasing cycles due to thefact that mechanical properties of the layered soil are quitedifferent.
The influence on the tunnel heave from the excavation volumein each step is shown in Fig. 12, where no anti-uplift piles wereused. It is found that when the excavation volume is small, the tun-nel heave increases nonlinearly with increasing excavation widthwith all excavation lengths being kept constant at 13.5 m. How-ever, the influence of strip width gradually reduces when the exca-vation width increases to a certain level, 6.4 m in this study.
6.4. Influence of skew angles between existing tunnels and new tunnel
The skew angle between existing double-track shield tunnelsand the new cut-and-cover tunnel was 70� in this project, as shownin Fig. 6b. The skew angle will have a significant influence on exist-ing tunnels. In order to evaluate this influence, a series of analyseswere performed with different skew angles, such as 70� and 90�.The different skew angles were obtained by changing the relayedmesh in the model.
The change of tunnel heave with the skew angle is shown inFig. 13 along with the field measurements for the skew angle of70� case. It is noted that the shield tunnels heave similarly withthe construction sequence with a larger deformation occurred inthe skew angle of 70�. As far as shield tunnels are concerned, the
25
20
15
10
5
030 1 2 4 5 6
Frictional force 105 N
Ant
i-pi
le le
ngth
(m
)
Excavation sequence
Fig. 11. Change of frictional force along the anti-uplift pile length.
0 2 4 6 8 10 12 148.5
9.0
9.5
10.0
10.5
11.0
11.5
Tun
nel h
eave
(m
m)
Excavation width in total (m)
Excavation width in each step 1.2m 1.7m 2.4m 6.4m
Fig. 12. Tunnel heave under different excavation widths in each step.
2 3 4 50
2
4
6
8
70 degrees (measured)70 degrees (computed)90 degrees (computed)
Tun
nel h
eave
(m
m)
Construction sequence (discussed in 4)
Fig. 13. Influence of skew angle on existing tunnels.
282 H. Liu et al. / Tunnelling and Underground Space Technology 26 (2011) 276–283
smaller the skew angle, the more the unloading. This finding isconsistent with the research by Qing (2007).
6.5. Influence on existing shallow tunnel
Based on the analyses, there were 5.2 mm heave and 1.3 mmhorizontal displacement at the top of the existing parallel shallowtunnel. It indicates that the influence on the existing shallow tun-nel from the new tunnel construction is smaller than the shieldtunnels below.
7. Conclusions and discussions
This paper presents a numerical analysis on the heave of exist-ing underlying tunnels below a new cut-and-cover tunnel con-struction based on a case history in Nanjing, China. The analyseswere performed using FLAC3D and compared to the field measure-ments. In addition, a parametric analysis was conducted to inves-tigate the influence of various construction parameters on tunnelheave. The following conclusions can be reached from this study:
(1) The combined jet grouting with the pile–slab retaining sys-tem could control the heave of existing tunnels efficientlyduring the new tunnel excavation.
(2) The jet grouting treatment in pit soil alone is more efficientthan the pile–slab retaining system in controlling the heaveand its optimum reinforcement depth is about 1.5 times theexcavation depth for this project.
(3) The sequential excavation considering the time–space effectis an effective control method for existing tunnel heave. Inorder to reduce the heave values, the sequence of staggeredsegmentation excavation from the two sides to the center issuggested.
(4) Under conditions of constant excavation lengths and smallstrip width, the tunnel heave increases nonlinearly withthe increasing excavation width in total, and the influenceof strip width gradually reduces when the excavation widthincreases to a certain level.
(5) The influence of the new tunnel is smaller on the existingparallel tunnel than that on the two existing underlying dou-ble-track shield tunnels.
In fact, the response of existing tunnels due to new constructionis very complex. It must be pointed out that the influence factorssuch as water buoyancy, the lining stiffness reduction due to liningjoints, squeezing effect of retaining walls and rheological behaviorof soft soil were not considered in this study. These factors willhave some influence and need further study.
Acknowledgments
The authors would like to acknowledge the Nanjing MunicipalDesign and Research Institute Co., Ltd. for permission to use thefield data published in this paper and the Fundamental ResearchFunds for the Central Universities No. 2010B14914 provided bythe Ministry of Education, China. The editorial help from Ms. CindyCostain of Ryerson University is also greatly appreciated.
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