An in-tunnel jacking above tunnel protection methodology for excavating a tunnel under a tunnel in service

Tunnelling and Underground Space Technology 34 (2013) 22–37

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Tunnelling and Underground Space Technology

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An in-tunnel jacking above tunnel protection methodology for excavatinga tunnel under a tunnel in service

Xinggao Li ⇑, Chengping Zhang, Dajun YuanSchool of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

a r t i c l e i n f o

Article history:Received 26 January 2012Received in revised form 1 October 2012Accepted 3 October 2012Available online 20 December 2012

Keywords:Tunneling under existing tunnelIn-tunnel support methodology using jacksReal-time monitoringPrepared schemes for lifting above tunneland adjustments

0886-7798/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.tust.2012.10.004

⇑ Corresponding author.E-mail address: [email protected] (X. Li).

a b s t r a c t

A section of the tunnel of Dongzhimen station on Beijing metro Airport Express closely under-passing theturn-back track tunnel in service through Dongzhimen station on Line 13 was constructed by a miningmethod, termed the ‘tunnel-column method’. Due to restrictions on the surface and subsurface settle-ment of structures in the area, an in-tunnel support approach was adopted using jacks controlled by pro-grammable logic controllers (PLCs) in combination with the mining method. Jacks were arranged,grouped and slowly loaded in the new tunnel to support the overlying tunnel in service. The under-pass-ing tunnel was excavated under the protections of the lifting jacks, and the shell composed of shotcreteand lattice girders. Performance of the tunnel in service due to the under-crossing tunnel was continu-ously monitored by means of a real-time and high-precision hydrostatic leveling system together withgrating scales installed near the jacks. The measured data were timely processed, and fed back to oper-ators to readjust the procedure if required. As a result of the in-tunnel jacking, the movement of the tun-nel in service was below the allowable value, and the under-crossing project was successfully completedwithout interrupting the metro traffic.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the metro systems have been increased dramat-ically due to the large population and limited surface space in Bei-jing. So far, most of the metro station tunnels in central areas of thecity have been constructed by a mining method, termed as the‘tunnel-column method’ (Liu et al., 2000). When building tunnelsin densely populated urban areas, it is of paramount importanceto control the adverse effects of tunneling on existing structuresand utilities in the surrounding environment. In particular, tunnel-ing under an existing tunnel can induce ground movements andtunnel settlements, which, if uncontrolled, might not only producedamaging effects to the overlying structure but also pose seriousthreat to passengers in the tunnel. Therefore, protective measuresmust be considered and analyzed in the design phase to ensuresafe construction of the new tunnel as well as operation of theexisting tunnel.

There exists a great variety of literature on reducing groundmovements and neighboring structures settlements induced byshallow tunneling (e.g., Mair and Taylor, 1999). Among others,Melis et al. (1999) described the utilization of horizontal compen-sation grouting from the inside of a passage portal in the construc-tion of a metro station in Spain. Shirakawa et al. (1999) reported

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combining fore-poles and grouting in Takatoriyama road tunnelin Japan. Lunardi and Cassani (2001) related the combined applica-tion of ground reinforcement by glass-fiber components and multi-layer grouting in the excavation of two parallel tunnels crossingunderneath a station of high-speed rail lines and sidings in Italy.Kassap and Goyal (2000) demonstrated the underpinning methodfor the construction of the Boston Central Artery/Tunnel passingunder a two track metro station. Chen et al. (2006) presented theroof pipe umbrella method used in building the Chongwenmenstation tunnel underneath Line 2 of Beijing metro; Xia et al.(2006) described the ground freezing technologies employed inexcavating a metro station tunnel under the existing tunnel ofShanghai metro. Wang (2010) gave the sleeve-valve-pipe injectingnon-shrink, two-component grout method adopted in constructionof Line 10 tunnel under Line 1 tunnel of Beijing metro. It is clearthat the above commonly employed schemes and methods centeron reducing disturbances to surrounding ground of the tunnels andthen indirectly reduce settlements of overlying structures. Basedon a case study in Beijing metro construction, a direct protectionmethodology using in-tunnel jacks for excavating an under-pass-ing tunnel is presented herein. Lifting schemes with jacks arewidely used in moving and reconstruction of buildings and bridgesin China (Lan et al., 2010; Wu et al., 2011). However, it was the firsttime that an in-tunnel support methodology using lifting jacks wasemployed to control settlements of the overlying tunnel in Chinametro construction. This paper presents the design schemes,

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adjustments of schemes, the excavation method and monitoringmeasures, for the special means of installing the in-tunnel liftingsystem with jacks, used to control the tunnel settlements causedby the under-excavation.

2. Project overview

The construction site belongs to part of Dongzhimen stationproject on Airport Express of Beijing metro. The plan of the projectis shown in Fig. 1. More detailed information of the existing tunneland the new tunnel are presented in Figs. 2 and 3, and the crosssections of the existing tunnel are given in Figs. 4 and 5. The ZoneC of Dongzhimen station tunnel on Airport Express passes aboveand under the turn-back track tunnel through Dongzhimen stationon Line 13 at near zero distance and about 0.9 m respectively. Theintersection angle between the new tunnel and the existing tunnelis approximately 61.74�. The Dongzhimen station of Line 13 wasput into use in December 2002. The Dongzhimen station of AirportExpress was constructed from January 2006 to April 2008, and theunder-passing tunnel was built mainly from August 2007 to Octo-ber 2007.

In the existing tunnel there are scissors crossovers, two move-ment joints and two types of structures of cut and cover sectionand mined section. The cut and cover tunnel made of C30 concreteis a box structure with a cover of 8.8 m in depth, and is 14 m inlength, 12.3 m in width and 7.75 m in height, and the thicknessesof roof, floor and side wall of the tunnel are 1 m, 0.85 m and0.9 m respectively. The mined tunnel is 12.05 m in width and7.52 m in height, and is a double arched and composite liningstructure, consisting of primary support made of C20 concrete,waterproofing layer and secondary lining made of C30 concrete.The thicknesses of primary support and secondary lining of thetunnel are both 0.3 m. The scissors crossovers and movementjoints in the existing tunnel are especially sensitive to the under-passing tunnel excavation induced settlements, and the main chal-lenges facing the construction of Zone C of Dongzhimen station onAirport Express were to limit settlements of overlying structureswhen constructing the under-passing tunnel of Dongzhimen sta-tion on Airport Express.

It was particularly a serious concern to confine settlements ofthe existing tunnel to 15.0 mm caused by the under-excavation.This allowable movement of 15.0 mm was mainly based on theo-retical considerations and plausibility. Special measures were

Fig. 1. Plan view of c

needed to prevent the tunnel in service from excessive settlementsthat could damage the tunnel structures and influence operation ofthe existing tunnel. However, due to the restrictions by surface andsubsurface environments, it was not possible to have any workingsite above ground surface. A more direct choice was to use in-tun-nel lifting jack support methodology to control movements of theexisting tunnel. The above-passing tunnel, which was built bycut-and-cover method and caused no settlements of the existingtunnel, will not be introduced in the paper, and more attention ispaid to reducing the under-excavation induced settlements of theexisting tunnel.

As revealed by engineering geological investigation reports andshown in Fig. 3, the subsoil at the construction site consists of mis-cellaneous fill, silt fill, fine sand, medium coarse sand, silty clay andcobble soils. Because of excessive pumping of water from theunderlying cobble layer in the past, the groundwater table is belowthe bottom of the under-passing tunnel. The overburden of theabove-passing tunnel is about 8800 mm in height.

As shown in Fig. 6, the construction method adopted for the un-der-passing tunnel of Zone C of Dongzhimen station on Airport Ex-press, was termed as the ‘tunnel-column method’ (Liu et al., 2000),which was used to construct the West-Tian’anmen Station of Bei-jing metro from December 1992 to August 1999. As illustrated inFig. 6, the main steps of the construction method together withthe in-tunnel support methodology using jacks were:

(i) Two No. 1 drifts were excavated with the primary supportconsisting of sprayed concrete, lattice girder and bolts.

(ii) Reinforced concrete piles were driven into the ground fromwithin the No. 1 drifts and L-shaped support beams werecast on top of piles.

(iii) The No. 2 drift was excavated and the strip foundation wasbuilt with concrete casting, and steel props were erected.

(iv) Jacks were arranged in the No. 1 and No. 2 drifts, and the No.3 drifts were excavated.

(v) Side walls between drifts were demolished and the roof ofthe under-passing tunnel was built with concrete casting.

(vi) The remaining part was excavated and steel props were setup together with anchor cables.

(vii) The side wall and floor of the under-passing tunnel werebuilt with concrete casting.

(viii) Jacks were removed and gaps between new and old struc-tures backfilled.

onstruction site.

Fig. 2. Plan of Area EFGH in detail.

Fig. 3. Section I–I.

24 X. Li et al. / Tunnelling and Underground Space Technology 34 (2013) 22–37

Main advantages of the construction method are the excavationefficiency associated with the multi-drift operation and less distur-bance to surroundings. Hence, this method is popular in manyother projects in Beijing.

3. Protection schemes in the design phase

The protection schemes of the design phase consisted of:

3.1. Components of the lifting system with jacks

Referring to Fig. 7 and as an example, the commonly used liftingsystem with jacks comprises hydraulic jacks, displacement sensors,a SIMATIC S7-300 programmable logic controller (PLC), an in situhydraulic pump station, a computer, intake and return lines con-necting jacks and the pump station, signal lines connecting jacksand the PLCs, and other accessories such as valves and circuitblocks. With the displacement sensors (grating scales are used in

Fig. 4. Existing cut-and-cover tunnel (Section J–J).


this project), the system can easily realize the synchronous liftingof structures on and above ground surface. When it is necessaryto implement the lifting system, parameters are input into thecomputer through the human machine interface, and instructionsare transmitted to the SIMATIC S7-300 PLC. Through the calcula-tions of the central processing unit of the SIMATIC S7-300 PLC,the instructions are verified along with the measured data of thedisplacement sensors, and then the hydraulic jacks are advancedor restrained to realize the synchronous movement of structures.In case of more hydraulic jacks used in the project, two SIMATICS7-200 PLCs are added under the control of the SIMATIC S7-300PLC.

3.2. Design principles of the protection scheme

The design principles for protecting the existing tunnel whenconstructing the under-passing tunnel, in the design phase canbe highlighted as follows:

(a) Measures must be taken to prevent any collapse or excessivedeformations of the existing tunnel.

Fig. 5. Existing mined tu

(b) A special protection scheme must be devised to actively con-trol settlements of the overlying tunnel in service, and toreduce the settlements as much as possible, so that no harm-ful deformations of the tunnel can occur. Moreover thescheme should be adjustable in accordance with the moni-tored settlements of the existing tunnel based on the obser-vational method.

(c) The protection scheme must not affect operation of theexisting tunnel and interfere with quick development ofthe under-excavation, and should be efficient enough so thatthe construction could be completed in time.

In accordance with the above principles, a protection methodol-ogy of supporting the existing tunnel using jacks in the new tunnel,and corresponding schemes for different construction steps wereput forward and implemented.

3.3. Stages of lifting work with jacks

In keeping up with the development of the under-excavation,the lifting work with jacks was divided into three stages. In eachstage of the lifting work, the adjustments on movements of theexisting tunnel could be carried out. The flowchart showing thelifting schemes and associated main construction items is givenin Fig. 8.

Stage I: in step 4 of the under-passing tunnel construction, i.e.the No. 1 and No. 2 drifts were finished and the excavations ofthe No. 3 drifts started, the lifting system was installed in the drifts,and the hydraulic jacks were positioned in the No. 1 and No. 2drifts. With the excavation development of the No. 3 drift, the lift-ing of the overlying tunnel using the installed lifting system couldbe performed based on measured settlements of the existingtunnel.

Stage II: when step 5 was initiated, i.e. the side walls betweendrifts were removed piecewise and the cast roof attained its de-signed strength and vertical props set up under the roof, theadjustments and installments of hydraulic jacks were performedin time with the development of the roof construction, and the lift-ing work could be performed when necessary.

Stage III: in step 7, i.e. the under-passing tunnel was finishedand the structural concrete attained its designed strength, thework of lifting the overlying tunnel using in-tunnel jacks was per-formed based on the measured settlements of the overlying tunnel.

nnel (Section K–K).

Fig. 6. Construction steps for the under-passing tunnel (Section I–I).


Fig. 8. Flowchart of the lifting schemes and main construction items.

Fig. 7. Components of the lifting system with jacks.


3.4. Installing the lifting system and dividing the jacks into groups

3.4.1. Facilities of the lifting systemAccording to the numbering and layouts of the hydraulic jacks

in the different stages, the main facilities adopted in the lifting sys-tem were as follows:

� Two hydraulic pump stations.

� One PLC main station and two PLC substations.

� Thirty 200-ton hydraulic jacks (two ready for use).

� One grating scale for one group of hydraulic jacks.


3.4.2. Calculations of lifting force and layouts of jacksBased on the design drawings, the existing tunnel within the

zone of influence of the under-excavation was zoned for calculat-ing the weights that should be supported by jacks. It was estimatedthat the weights of the cut and cover tunnel and the mined tunnelwere about 1982 T and 1046 T respectively.

In the first stage, twenty-eight 200-ton hydraulic jacks in totalwere arranged. Six jacks were in the No. 1 drift below the cutand cover tunnel, fourteen jacks in the No. 2 drift and eight jacksin the No. 1 drift below the mined tunnel. The maximum liftingforce of the jacks was 5600 T and enough to lift the overlying tun-nel structures. Configurations of the PLCs and jacks for the firststage are presented in Fig. 9.

In the second and third stages, eight 200-ton jacks were ar-ranged below the mined tunnel and twenty 200-ton jacks arrangedbelow the cut and cover tunnel. Underneath the mined tunnel thejacks were positioned along the top of the L-shaped beam and un-der the bottoms of the middle wall and side walls of the minedtunnel. Underneath the cut and cover tunnel, the jacks were posi-tioned along and under the bottom of the side walls and the floorcenterline of the cut and cover tunnel. The configurations of thePLCs and jacks for the second and third stages are given inFig. 10. It is observed from Figs. 9 and 10 that in the first stagethe jacks were divided into six groups and in the second and thirdstages the jacks were divided into eight groups.

4. Monitoring movements of the overlying tunnel andinformation management

Obviously, both the lifting scheme and the settlement control ofthe existing tunnel were heavily dependent on accurate and timelyinformation, as well as the ability to use the information effectivelyin the decision-making process. Meanwhile, there was a clear de-mand for real-time monitoring and availability of results to effec-tively manage any unforeseen events during the construction ofthe tunnel below the existing tunnel. So an automatic and high-

Fig. 9. Configurations of the jacks

precision hydrostatic leveling system was employed for continu-ous monitoring of movements of the existing tunnel.

As shown in Figs. 11–13, the system is mainly composed of thehydrostatic leveling monitor fixed to the mounting bracket on thetunnel sidewall and floor, the data collection unit, and a computer.As shown in Fig. 14, attention was focused on the behavior of themovement joints, and sixteen measuring points (MPs) were ar-ranged to monitor vertical movements of the existing tunnel. Themeasured movements were important sources of constructionfeedback and decision-making when executing the lifting work.

The transfer and flow of measured data among the parties in-volved is of vital importance to the in-tunnel lifting work, and acollaborative-work environment provided facilities for sharingdata, tracing decisions, and communicating the information byefficient means. A joint team composed of the designer, the builder,the inspection engineer, the owner and the administrator of theoperating tunnel was established, constituting an indispensabletool for the efficient and effective management of the settlementcontrol of the existing tunnel. During construction of the under-passing tunnel, the measured data were well recorded and sharedamong members of the team. Discussion and analysis were readyto be conducted in the light of movements of the existing tunnel,and parameters of the lifting system, to decide whether, how andwhen to perform the lifting work.

In the paper, positive movements represent uplifts and negativemovements represent settlements.

5. Test of lifting the overlying tunnel using in-tunnel jacks andadjustments of the lifting schemes

5.1. Test of lifting the overlying tunnel using jacks

As at August 9, 2007, the maximum differential settlement andsettlement of the existing tunnel had reached 4.8 mm and morethan 10.0 mm respectively. In order to restrain the trend of settle-ment of the existing tunnel, and ensure its safe operation, it was

and PLCs for the first stage.

Fig. 10. Configuration of the jacks and PLCs for the second and third stages.

Fig. 11. Makeup of the hydrostatic leveling system.

Fig. 12. Hydrostatic leveling monitors on sidewall.Fig. 13. Hydrostatic leveling monitor on floor.


decided that the lifting methodology using in-tunnel jacks shouldbe employed. Before executing the lifting scheme, a test of liftingthe overlying tunnel using the in-tunnel jacks was performed onAugust 11, 2007. Objectives of the lifting test are summarized asfollows:

(i) To verify the contact between jacks and above shotcretelining.

(ii) To preload the foundation underneath the jacks.(iii) To ensure the lifting force of installed jacks was able to lift

the overlying structures and determine the needed liftingforce to raise the overlying tunnel.

(iv) To examine the actual effect of the lifting methodology andpreview the performance of the existing tunnel and makeimprovements.

Fig. 14. Measuring points (MPs) layout.

Fig. 16. Loading processes of jacks on August 11.


The measured movements by four grating scales below fourcorners (see Fig. 10) of the cut and cover tunnel, the loading pro-cesses of jacks and changes of vertical movements at the MPs overthe period of lifting tests are presented in Figs. 15–17 respectively.

As observed from Fig. 15, the synchronous lifting of the overly-ing tunnel was more or less difficult to realize due to the unevenrigidities of initial supports of the tunnels and the imperfect layoutof jacks in the limited underground space. There exist differencesamong vertical movements at action points of lifting jacks, whichwill induce the additional internal forces in the overlying tunnelstructures. Therefore, the loadings of lifting jacks should be per-formed slowly. Once fractures occurred in the existing structures,the lifting work should be terminated. The loading process of jackspresented in Fig. 16 remains roughly the same, but is not in fullagreement mainly on account of imperfect layout and groupingof jacks. The measured movements of the overlying tunnel inFigs. 15 and 17 showed that the installed jacks were able to raisethe above tunnel. It can be argued that in Fig. 17 the vertical move-ments are a reflection of differential settlement from the jacksfoundation settlements and total vertical movement. It may beused to show settlements of the jacks.

Through the lifting test the lifting of the overlying tunnel waspreviewed and the minimum hydraulic pressure of jacks was mas-

Fig. 15. Recordings by grating scales on August 11.

tered, and from Fig. 16 it was found to be about 10 MPa averageload on the eight jacks. Of course, this pressure should be increasedwith the construction development of the overlying tunnel andsurface backfilling.

5.2. Adjustments of the lifting schemes

Adjustments of the lifting schemes were made based on the lift-ing test, and are given as follows:

(i) The forces of jacks directly acted on the primary support ofthe under-passing tunnel instead of the floor of the existing tunnel.The following are the reasons. When excavating the under layerand primary support of the existing tunnel, it was found that theexposed structure face was rough and uneven. There was no en-ough time to chisel and level off the face. So the initial supportof the under-passing tunnel consisting of welded wire mesh, latticegirders and sprayed concrete was employed to directly bear thelifting forces.

As shown in Fig. 18, a jack directly supported a steel plate thatcontacted the initial support of the under-passing tunnel. Both thequality and properties of the shotcrete lining initial support couldbe equal to those of cast in place C20 concrete with proper care and

Fig. 18. Schematic diagram of lifting jack installation.

Fig. 17. Measured vertical movements of the existing tunnel on August 11.


control of the total placement procedure, and could meet the de-mand of lifting the above tunnel.

As shown in Fig. 19, each group of lifting jacks was equippedwith one grating scale installed near the lifting jacks. The gratingscale was used to measure vertical movements of the sandwichbetween the steel plate and the overlying tunnel. The sandwichcomprised the initial support of the under-passing tunnel and

Fig. 19. Installed grating scale near lifting jacks.

the under layer of the existing tunnel, and its movements wereof great importance to regulating the jacks.

(ii) Considering the existence of the moving joints, the forces ofthe jacks were changed mainly in accordance with measuredmovements by four grating scales below the four corners.

(iii) On account of the uneven rigidities of the under layer of theexisting tunnel and the initial support of the under-passing tunnel,it was impossible in practice to realize the synchronous lifting onlyin the light of measured movements by grating scales. Measuredmovements of the existing tunnel by the hydrostatic leveling sys-tem were indispensable to realizing the synchronous movement ofthe existing tunnel.

(iv) Based on the construction period, the lifting jacks playedthe role of temporary support during construction. As at October24, 2007, the secondary lining of the under-passing tunnel wascompleted to provide firm support. At the same time, fracture oc-curred in the primary support of the overlying mined tunnel, and itwas time to cease further lifting of the existing tunnel.

6. Implementation of lifting schemes

The cast floor and roof of the above-passing tunnel wereperformed on September 24 and October 15, 2007 respectively.Considering loading of concrete casting would further increasethe settlements of the existing tunnel, the lifting schemes for thesecond and third stages were carried out on September 28,

Fig. 20. Revised configuration of hydraulic jacks.

Fig. 21. Recordings by grating scales on September 28.

Fig. 22. Loading processes of jacks on September 28.


September 29, October 18 and October 21, 2007 respectively to re-sist the rapid settlements of the existing tunnel.

During the process of the lifting exercise, the synchronous up-ward movement of the existing tunnel was achieved, based onthe measured movements by grating scales and hydrostatic level-ing monitors, and through slow loading of the jacks.

As at October 15, 2007, the hydraulic pressure of the liftingjacks had reached 27 MPa and the total lifting force was 3500 T.According to the performances of the existing tunnel, the configu-ration of the hydraulic jacks was adjusted on October 21, 2007. It isnecessary to point out that steel props were erected to support theoverlying structure before demolishing the stressed jacks. Asshown in Fig. 20, four 200-ton hydraulic jacks were added andthere were 24 jacks below the cut and cover tunnel, and the totaljack force rose to 3810 T, which is about 3.04 times the weightof the overlying tunnel within the zone of influence. Because a partof the initial support of the above mined tunnel fractured, the lift-ing work was terminated on October 25, 2007.

6.1. Recordings and measurements on September 28

Presented in Figs. 21–23 are readings from the grating scalesand the computer connected to the data collection units of the lift-ing system, and measured movements by hydrostatic levelingmonitors on September 28, 2007. The development of hydraulicpressures in lifting jacks demonstrate similar trend of slow in-crease in pressure except the decrease in hydraulic pressure ofjacks of Group 4 at the later stage. The final hydraulic pressure isin the range of 14–17 MPa. Similarly, the upward vertical move-ments of the sandwich between the steel plates and the overlyingtunnel show a trend of slow increase, and the maximum uplift isabout 2.2 mm at Corner 4, and the minimum uplift is 0.9 mm atCorner 1. Under the actions of lifting jacks, the measured resultsof DJJ2–DJJ8 show the upward movement of 0.2–0.8 mm of theexisting tunnel. The maximum uplift occurs at DJJ7 under whichthere are two groups of hydraulic jacks. The recordings of DJJ1 re-main almost unchanged perhaps due to less lifting forceunderneath.

6.2. Recordings and measurements on September 29

Relevant recordings of the lifting work on September 29, 2007are given in Figs. 24–26. The loading processes of the jacks

Fig. 23. Measured vertical movements of the existing tunnel on September 28.

Fig. 25. Loading processes of jacks on September 29.

Fig. 26. Measured vertical movements of the existing tunnel on September 29.

Fig. 27. Recordings by grating scales on October 18.


resemble each other and overall display the trend of slow increasewith time, and the ultimate hydraulic pressure is within the rangeof 17–21 MPa. Likewise, the vertical movements of the sandwichincrease with time, and the maximum uplift is about 1.9 mm atCorner 4, and the minimum uplift is 0.6 mm at Corner 1. The at-tained results of DJJ2–DJJ8 show the upward movements of 0.2–0.9 mm of the existing tunnel, and the maximum uplift occurs atDJJ7. The readings of DJJ1 almost remain unchanged.

6.3. Recordings and measurements on October 18

Readings on October 18, 2007 from the grating scales, the liftingsystem and the hydrostatic leveling system are displayed inFigs. 27–29. The hydraulic pressure of lifting jacks increases slowlywith time at the earlier and middle stages, but some readings de-crease slowly with time at the later stage, and the final hydraulicpressure is about 18.5–30.5 MPa, except that the jacks of Group 2break down. The upward movement of the sandwich increaseswith time on the whole, and the final maximum movement isabout 3.1 mm at Corner 4, and the minimum movement of0.9 mm occurred at Corner 1. The attained results of DJJ1–DJJ8show the upward movements of 0.2–1.4 mm of the existing tunnelin the end. Overall, it seems that the vertical movements of theexisting tunnel increase with first minutes and remain unchangedfor the rest of the time. The maximum uplift of the existing tunneloccurred at DJJ7, and the minimum uplift at DJJ1.


It is necessary to point out that on account of the positionadjustments of jacks on October 21, 2007, only the final move-ments of the existing tunnel were obtained.

Fig. 24. Recordings by grating scales on September 29. Fig. 28. Loading processes of jacks on October 18.

Fig. 29. Measured vertical movements of the existing tunnel on October 18.Fig. 31. Loading processes of jacks on October 21.


Relevant results of the lifting work on October 21, 2007 aredrawn in Figs. 30–32. Obviously, the grating scale at Corner 1(see Fig. 10) has a breakdown. On account of position changes ofthe jacks, their hydraulic pressures and the grating scale measuredmovements are decreased at the former half stage and increased atthe latter half stage. The ultimate hydraulic pressure is within theranges of 17–22 MPa and the final reading of grating scales is about0.1–0.9 mm. The obtained results from the hydrostatic levelingmonitors show the settlements of 0.1–1.2 mm of the existing tun-nel at DJJ1–DJJ6, and the little uplift of about 0.2 mm at DJJ7–DJJ8.


Recordings of the lifting work on October 25, 2007 are pre-sented in Figs. 33–35. The hydraulic pressure of the jacks has a lit-tle increase at the initial stage, and then demonstrates the trend ofdecrease with time, and finally approached zero. Obviously, thegrating scale at Corner 1 broke down at the middle and later stages.Readings from other grating scales show that the deformation ofthe sandwich exhibits the same development process as that ofthe hydraulic pressure of the jacks, and reached about 2 mm inthe end. The measured results of DJJ1–DJJ8 show the overlying tun-nel increased in uplift at the initial stage, and then settled, and fi-nally remained almost stationary at the latter half stage. The finalmaximum settlement of the overlying tunnel is around 1.4 mmand at DJJ2.

6.6. Final deformations of the existing tunnel

When the under-passing tunnel was finished, the measuredmaximum settlement of the existing tunnel was about 11.2 mmand less than the allowed settlement of 15.0 mm. Attained maxi-


mum differential settlements at the moving joints of the existingtunnel are presented in Table 1. Though the lifting work was per-formed five times, the maximum settlement of the existing tunnel,and the maximum differential settlement of the moving joints in-creased about 12% and 19% respectively. On the other hand, with-out performing the lifting work the existing tunnel would have alarge increase in settlements and the final settlements of the tun-nel would have certainly exceeded the predefined allowable move-ment. The lifting work is an active defense methodology, and canreduce adjacent tunneling induced settlements of the existingtunnel.

7. Discussion

7.1. Effects of the lifting work on the existing tunnel

It is observed from Figs. 21, 24, 27, 30 and 33 that the maximumdifference between measured movements by any two gratingscales is about 1.3 mm on September 28 and 29, 2.2 mm on Octo-ber 18, and 0.8 mm on October 21 and 25. Similarly, the hydro-static leveling system measured results in Figs. 23, 26, 29, 32 and35, show that the maximum differential vertical movement ofthe existing tunnel is about 0.8 mm on September 28, 0.9 mm onSeptember 29, 1.4 mm on October 18, 1.2 mm on October 21,and 1.4 mm on October 25. Hence, the synchronous movement ofthe existing tunnel using lifting jacks was basically realized dueto the improvements made after the lifting test. The smooth com-pletion of the under-passing tunnel can be mainly attributed to theresumed lifting work, the real-time hydrostatic leveling system inthe existing tunnel, and the construction approach used.

Similar phenomena for vertical movements are observed fromFigs. 21, 23, 24, 26, 27 and 29 that uplifts increase initially and thenremain almost unchanged later on. This is mainly because of theincrease in the influence zone of the existing tunnel with the in-creased jacking force, and the construction of the above-passingtunnel may also have something to do with the phenomena. Thatthe uplifts in Figs. 33 and 35 increase initially and then decreaseslater on, is closely related with the loading/unloading of the jacks.

7.2. Lessons learned in the lifting work

It is obvious that the amount, rational distribution and groupingof jacks are of paramount importance to achieving the synchronousupward movement of the existing tunnel. At the initial stage ofconstruction, only the No. 1 and No. 2 drifts were excavated, andthe jacks arranged and grouped in these drifts. With the accom-plishment of upper face excavation of the under-passing tunnel,more space was provided, and adjustments of layout and groupingof the installed jacks were possible. As a result of the adjustment,configurations and grouping of the jacks for the second and third

Fig. 32. Measured vertical movements of the existing tunnel on October 21.


Fig. 34. Loading processes of jacks on October 25.

Fig. 35. Measured vertical movements of the existing tunnel on October 25.


stages were formulated. There are still many unknowns in con-struction of the under-passing tunnel. More improvement of thelifting scheme was still required in combination with measuredmovements of the existing tunnel, which led to the increase inthe amount of lifting jacks on October 21, 2007. But from Figs. 30–32, we can see that the process of first unloading and then reload-ing the jacks on October 21, 2007 was unreasonable because eventhe amount of hydraulic pressure of the lifting jacks had exceededtheir initial values, and the lifting work resulted in more than1.0 mm settlement of the existing tunnel in the end.

7.3. Utilizations of monitoring and measuring system

From Figs. 21, 23, 24, 26, 27, 29, 33 and 35, it is observed thatthe final readings on September 28, 29 and October 18, 25 by grat-ing scales are largely different from those by hydrostatic levelingmonitors. The main reason was that there existed the soft sand-wich made up of the initial support of the under-passing tunneland the under layer of the existing tunnel between the jacks andthe overlying tunnel. To effectively manage any unfavorable eventsduring construction of the tunnel under the existing tunnel, it wasindispensable that a real-time and high-precision monitoring sys-tem (hydrostatic leveling system in this project) be installed inthe existing tunnel to help realize the synchronous lifting of theexisting tunnel when executing the lifting schemes.

7.4. Loading/unloading speed of the lifting jacks

The damage and fracture of a reinforced structure are greatlyinfluenced by the loading/unloading rate. Experimental results byQian et al. (1995) indicated that under high rate loading the max-imum crack growth increases with the loading rate. At the sametime, very low rate of loading/unloading means more time andmore risk to the construction of the under-passing tunnel. Hence,it is of great importance to use reasonable loading and unloadingrates of hydraulic pressure jacks. The average loading/unloadingrate can be determined by linear regression method. Taking thedata of Group 3 jacks on September 28 for example, the methodis illustrated in Fig. 36. The fitted regression equation isy = 0.50x + 7.78, where y is the dependent (explained) variableand represents hydraulic pressure, x is the independent variable(explanatory) and represents time, and r2 is the calculated coeffi-cient of determination. The slope of 0.50 MPa/min is the deter-mined average loading rate.

Considering the unloading and loading process experienced onOctober 21, two linear regressions were performed to fit the datarecorded in the former half and latter half stages respectively.The results are presented in Fig. 37 using the recordings of jacks

Table 1Maximum differential settlements of the movement joints.

Date September 28 (mm) September 29 (mm) October 18 (mm) October 21 (mm) October 25 (mm)

Differential settlement 5.8 5.5 6.4 6.6 6.9

Fig. 36. Determining the average loading speed using the linear regression method.

Fig. 37. Determining the average loading and unloading speeds using two linearregression equations.


of Group 3. Likewise, two slopes of �3.53 and 9.02 MPa/min of thefitted regression equations are the calculated average unloadingand loading speeds in the former half and latter half stagesrespectively.

Using above methods, the calculated slopes and coefficients ofdetermination are listed in Table 2. In each cell of Table 2, the for-mer figure is the average loading or unloading speed, and the latterin brackets is the corresponding coefficient of determination. Thepositive number means loading rates and negative numbers repre-sent unloading rates.

Table 2Average loading/unloading rate of hydraulic jacks.

Group of jacks Group 1 Group 2 Group 3 Group

August 11 1.65(0.84) 1.51(0.88) 1.19(0.97) 1.86(September 28 0.51(0.85) 0.47(0.89) 0.54(0.95) 0.75(September 29 0.37(0.66) 0.37(0.81) 0.60(0.97) 0.50(0October 18 2.87(0.98) – 1.77(0.90) 1.40(0October 21 �3.84(0.96) �3.53(0.99) �3.54(0.96) �3.84(

8.83(0.95) 8.26(0.97) 9.02(0.98) 8.56(October 25 �2.82(0.86) �2.64(0.84) �2.87(0.92) �2.73(

The Italic Value Shows coefficients of determination of the linear regressions.

Apply the strikethrough format to slopes whose coefficient ofdetermination is less than 0.8 in Table 1 and discard these slopesin the following analysis. It is observed that the jack loading rateon October 21 of 7.68–9.02 MPa/min is too high and unreasonable,which is the main reason for cracking of initial support of the over-lying mined tunnel. The rest of the slopes of about 0.44–2.87 MPa/min are reasonable loading rates. For judging the rationality of theunloading arte on October 21 of 3.53–4.01 MPa/min lacks enoughevidence, no discussion on this unloading rate is given here. Con-sidering the fact that a stable state occurred at the later stage inFig. 35, it is concluded that the unloading rate of 2.54–2.87 MPa/min on October 25 was reasonable.

8. Conclusions

Experience indicates that, such an in-tunnel protection method-ology using jacks, when combined with cautious tunneling proce-dures, was able to effectively control the extent and degree ofinfluence of the under-excavation on the overlying tunnel. Basedon the above statements and discussions, the following conclu-sions are drawn:

(i) The completing of the settlement control of the existing tun-nel in construction of the under-passing tunnel using the in-tunnel jacking above tunnel methodology lies in the pre-pared schemes, the lifting test, adjustments of the schemes,the construction method of the under-passing tunnel, andmost important of all, the application of the high precisionand automatic hydrostatic leveling system. Monitoringplayed an important role in this project, modern instru-ments with advanced accessories, timely processing of mea-sured data, and feed-back to operators resulted in aninformation-oriented lifting work. This ensured the settle-ment control of the existing tunnel without interruption ofmetro traffic.

(ii) Once lifting work is started it is unreasonable to unload thesupporting jacks and move them, and the suggestedapproach is to add jacks where needed.

(iii) Based on the lifting work presented, the loading rate of 0.44–2.87 MPa/min and the unloading rate of 2.54–2.87 MPa/minare reasonable.

(iv) The lifting work using in-tunnel jacks is an active defensemethodology and can be used effectively to reduce settle-ments of existing tunnels caused by under-excavation.

4 Group 5 Group 6 Group 7 Group 8

0.90) 1.57(0.96) 1.57(0.94) 1.26(0.85) 1.27(0.94)0.74) 0.47(0.91) 0.45(0.91) 0.44(0.90) 0.46(0.90).77) 0.44(0.66) 0.22(0.63) 0.31(0.51) 0.34(0.55).75) 1.56(0.92) 1.27(0.92) 0.82(0.49) 0.89(0.56)

0.97) �3.54(0.96) �3.84(0.97) �4.01(0.96) �3.74(0.98)0.95) 9.02(0.98) 8.56(0.95) 7.68(0.87) 7.93(0.91)0.88) �2.87(0.92) �2.73(0.88) �2.66(0.85) �2.54(0.89)


Acknowledgments

The support from the Chinese National Natural Science Founda-tion (No. 50978017 51008015) and the Fundamental ResearchFunds for the Central Universities of China (2011JBM076) areacknowledged. The authors are grateful to two reviewers for theirconstructive comments.

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An in-tunnel jacking above tunnel protection methodology for excavating a tunnel under a tunnel in service