TBM launch, collaborative U-turn and reception occur with the assistance of seal Steel sleeves. A pressure balance is built in advance within the sleeve and then the TBM passes through the portal to finish launch; before reception, the seal steel sleeves are installed in advance, followed by sand filling and water injection; the TBM maintains pressure balance within the seal steel sleeve and after sealing treatment for the tunnel ring, release the pressure, so as to finish TBM reception.

The TBM and sleeve are leveled under the action of hydraulic ram, and then translated and turned by the horizontal jack along the planned route, so as to enable the TBM and sleeve to shift from reception status to launch status.

Construction Survey before TBM Launch: 

During construction, TBM driving is controlled through the TBM automatic guide system according to the parameters such as TBM-tunnel design axis and TBM driving attitude, assisted by checking through manual re-measurement.

Design information: 

The shield tunneling segment is designed as two-sided wedge-shaped universal ring, with its outer diameter being 11,800mm, its inner diameter being 10,800mm,  its width 2m, its wall thickness 500mm and its taperness 40mm.The segments are assembled subject to 5+2+1 style, namely 5 standard rings, 2 jointing rings and 1 top sealing ring (K block). The segments are turned (to determine the position of K block) and assembled by staggered joints as per the tunnel alignment and tunneling attitude. The segments are connected with slant bolts, the circular joints are with sixteen M36 bolts and longitudinal joints are with forty six M30 bolts. The lining concrete is of C60 P12.

Preceding works of TBM launch: 

(1) The TBM working shaft end is consolidated by means of Ø1200@900 triple-tube HP chemical churning piles; the  consolidation body is 15 m long, about 42 m wide and 23.5m deep;

(2) Embedment. While constructing the main structure of working shaft, embed the shield tunnel portal steel ring in the lining wall in the end portal portal range for TBM launch and guide reference;

(3) Assembly of TBM and steel sleeve. The seal steel sleeve and the bracket are subject to integrated design; after the seal steel ring lower part and the bracket are installed, assemble and debug the TBM and mount the upper part of sleeve;

(4) Installation and debugging of slurry stations.  After installation and debugging of slurry treatment equipment, the slurry station is subject to simultaneous debugging together with TBM;

(5) Chiseling-off of portal. Chisel off partial underground continuous wall concrete within the portal steel ring.

Preceding works of TBM Reception: 

1. TBM working shaft end is consolidated simultaneously with TMB launch
2. Portal steel ring is embedded simultaneously with the TBM launch.
3. Installation of steel sleeve. The seal steel sleeve and supporting system are installed by reception gradient of -4%;
4. The portal is chiseled off simultaneously with chiseling of the launch portal.
5. After the tightness test of the steel sleeve, the sleeve is filled with sand and water.

Preceding works of TBM U-Turn: 

(1) 2cm thick plates are fully laid on the working shaft and then polished smoothly;

(2) After reception of sleeve and sealing treatment for the portal, the sand inside the sleeve is cleared off by circulating the slurry, followed by pressure relief;

(3) Disengage the hinge joint between the TBM and the trolley and tidy up the pipelines within the TBM;

(4) The TBM and the sleeve are adjusted to level with a vertically lifting jack.

(5) Plan the TBM U-turn route and fix the TBM oil cylinder pushing counteracting force point.

Preparatory works: Material Preparation: 

As per the technical specification, sufficient shield tunnel construction materials shall be procured in the whole construction process, including shield segments, bolts and waterproof material that constitute the tunnel lining, as well as auxiliary materials such as TBM grease, cable, bentonite, track, walkway of crane beam and slurry pipelines. The shield segments are prefabricated in China and then transported to the storage yard by sea. All construction materials shall go through quality inspection before arrival at Chinese port and have a certificate of quality.

Preparatory works: Equipment Preparation: 

The following equipment shall be prepared:

TBM, shield accessories, generator, slurry separating machine, mortar mixing machine, capacitor carrier, crane, injection machine, gantry crane, air compressor, electricity cable, electricity distribution box, illumination, ventilation equipment, refrigeration equipment ,Seal steel sleeve, jack, hydraulic pump, etc

Aided design of TBM launch, reception: 

1) Principle of sleeve auxiliary construction

The project is located in Chittagong, Bangladesh. At the working shaft end, the geological conditions are complex with a long rainy season and the underground water communicates with the riverway and sea water. At the time of TBM launch or reception, the water saturated sand layer is very prone to water and sand burst, resulting in seal failure. So,we will change the tunneling assistance “sliding Ring steel + freeze” to steel sleeve assistance. 

2) Principle of steel sleeve launch assistance

The steel sleeve changes the original sliding seal (portal heavy curtain rubber + wire brush seal) to welding seal; the TBM is located in the steel sleeve to form an annular seal space; before passing the portal, it establishes slurry balance to avoid stratum leakage and loss, so that the working occurs in safe and stable condition.

3) Principle of TBM reception with assistance of steel sleeve:

The steel sleeve is installed before TBM reception. Similar to the optimized sealing method during launch, the sleeve is filled with sand and injected with water to simulate the stratum condition and pressure and extend the driving length. Normal driving shall occur within the sleeve until the pressure balance disappears, so as to avoid stratum water and soil loss at the working shaft end, keeping safe and stable operation. 

4) Turning with assistance of steel sleeve

Lower part of steel sleeve is provided with a vertical jacking system; after TBM reception, disengage TBM with the rear associated equipment and level the TBM and sleeve in vertical direction at the same time; and translate and U-turn by using a jack and a hydraulic power pack

5) Design of seal steel sleeve

The assistive steel sleeve for TBM launch, reception and U-turn is integrated with the pedestal; the assistive structure of steel sleeve comprises extended steel ring (welded with the portal steel ring), steel sleeve body, integrated bracket, end cap, discharge outlet, air inlet, pressure relief opening and pedestrian door. 

Composition of assistive seal steel sleeve is shown in Figure 

Steel sleeve material and structure

A. Outer diameter of steel sleeve is 12,924mm and inner diameter thereof is 12,484mm, the clearance between the sleeve and TBM body is 182mm, and the four-bearing rail is 150mm×150mm square steel;

B. The steel sleeve is composed of 4 sections, each consisting of 90° bottom and support, and nine 30° fan-shaped blocks;

C. The sections are located with a positioning device and the circular joints and longitudinal joints are subject to welding;

D. The sleeve is 20mm thick steel plate on which is provided with a rib plate for maintaining its stiffness;

E. Each section of sleeve is connected via flange; the flange is 100mm thick steel plate that is connected with M30 bolts; and in the middle of the flange, there are two O rubber rings;

F. End ring: a force transfer device between the negative segment and counter-force bracket, and there is a jack in the end ring.

Design of Reaction Rack: 

(1) Design Principle of Reaction Rack

The counterforce bracket provides counter-force for TBM during TBM launch and reception and transfers the counter-force to the working shaft structure via the rear support of the counterforce bracket. Its function should meet: 

a) The counterforce bracket is matched with the sleeve, meeting the support counter-force demand of TBM launch and reception;

b) Consideration shall be given to the horizontal transport boundary of segment for the structural support at the open cut concealed burying section where the counterforce bracket is adjacent to the working shaft;

c) It is easy to install, remove and transport the counter-force bracket;

d) The counterforce bracket is vertical to the TBM launch or reception axis.

e) Reactive force is transmitted safely and effectively to the structure.

2) Counter-force bracket design

The counterforce bracket is designed with Q235B steel structure and easy for installation and removal for repeated use. The counterforce bracket is composed of stand columns at both sides, horizontal steel beam and four inclined steel beams. The steel stand columns are welded box beams, being 900mm in transverse width (parallel to the tunnel axis) and 1,000mm in longitudinal width; the inclined cross-bar is welded on the stand column, between them is provided a vertical steel beam for reinforcement, and high-strength bolts are employed to enhance the stand column stiffness.

(3) Stress calculation and analysis of counter-force bracket

On the basis of structural and supporting form design of counter-force brackets, a 3D model is created in Creo software and then imported into Abaqus for finite element analysis.  

Portal steel ring embedment 

After the working shaft structure is constructed, the portal steel ring is embedded in advance. The portal steel ring is fabricated by pieces; after assembling on the ground, the pieces are to be welded together and located. The portal steel ring is provided with Ø32×4 injection pipes, installing one every 18°, in total 20 ones. 

Slurry preparation and circulation

The slurry used for the Air Pressure Slurry Balance Tunnel Boring Machine shall be of bentonite processed in the special slurry pit. The slurry pit is located in the Slurry Separating Station. 

Raw materials of slurry are bentonite and water, and the proportion of bentonite and water (mix ratio) is 1/0.135. The raw materials are mixed as slurry with a high-speed rotary mixer after being put into the preparation barrel according to a certain mix proportion. The weight ratio of water and bentonite is 1:0.135(mix proportion). After mixing, the slurry is stored in a slurry storage pool for standby and will be pumped into the slurry circulating pool after 24h for use. Indexes of the newly-prepared slurry and the slurry put into circulation for use are shown below: 

Twisting measure during TBM launch

(1) Sand is filled in the sleeve to provide force of friction and resist against twisting;

(2) TBM dead weight provides torsional force;

(3) The water pressure of shield cutting provides torsional force;

(4) An anti torque rib is welded on the TBM to resist against torque;

(5) Possibility to optimize TBM cutter head is discussed to reduce sliding friction (in discussion).

TBM Reception 

Breakthrough survey control

• Densify breakthrough survey to ensure TBM can smoothly enter the portal ring;
• TBM attitude and elevation are heightened appropriately.

Grouting management

• Synchronous grouting management;
• Secondary compensation grouting management.

Segment tensioning

While the TBM arrives at the last 10 rings, the segment is tensioned to avoid segment instability due to reduction in oil cylinder thrust. 

TBM entering steel sleeve and advancement

• Control the tunneling parameters, reduce thrush, control torque, and reasonably set the center cutting water pressure;
• Slow down the tunneling speed and strengthen synchronous grouting and secondary compensation grouting;
• TBM enters the sleeve and drives and assembles 2 ring segments within the sleeve;

Pressure relief and portal seal

• Perform secondary compensation grouting for 5 ring segment at the portal and check the grouting quality through inspection hole;
• Perform grouting with injection pipe for the portal embedded steel ring to observe the grouting and blocking quality;
• Start slurry circulating flow to clean the slag in the sleeve;
• Dismantle the extended steel ring, and block the portal by vertical position welding between the portal embedded steel ring and the embedded steel plate behind the segment wall.

Treatment of TBM

• Dismantle the pipelines and cables between the TBM and lorry equipment;
• Consolidate the segment erector;
• Disengage the constraint between the TBM and the trolley;

TBM translation and U-turn

Work principle: After leveling of TBM, the working shaft is made available for paving steel plate and welding counter-force devices, installing jack groups, and connecting hydraulic power pack for horizontal U-turn. Counter-force device is dynamically welded with movement of TBM, so as to constantly provide jacking counter-force.

Work process of translation and U-turn: 

STEP 1: Jack up the TBM to leave the portal

STEP 2: translate TBM

STEP 3: Lay temporary track and hoist out the TBM lorry

STEP 4: Constantly adjust the counter-force points to U-turn the TBM

STEP 5: Install vertical jack, jack up TBM sleeve to install gradient adjusting pier, install the TBM and sleeve in place. 

Quality Assurance 

(1) Strictly control the TBM driving parameters and keep the records;

(2) The segments shall be used after passing inspection and assembling can be performed only after the waterproof material is properly attached;

(3) Synchronous grouting is subject to index control with grouting pressure and amount;

(4) Control the TBM attitude and keep its tail clearance;

(5) The shield tunneling data in the operating room is given feedback to the ground monitoring room timely for management personnel to make analysis decisions. 

(6) The instruments used for quality inspection of shield tunnels are all calibrated by the relevant Chinese organization and are within the validity period.

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