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Construction of Road Under Bridge by PrecastRCC Box 2 x 5.00x 5.00 m by Box Pushing

Technique in Hubli-Bangalore Section

Dr. R. S. Dubey *Prem Narayan **

Synopsis : Due to growing traffic on road as well as on rail, necessity ofcrossing the rail and road is on the steep rise. To construct such opening,with minimum disruption to train services and road traffic, is a challenge tothe Railway Engineers. Box pushing technique is one of the suitable methodsfor such situations. This paper deals a real case of Box Pushing carried outunder train running traffic condition by providing Road under bridge for LevelCrossing No. 261 between Hubli - Bangalore line. This paper mainly dealsexecution part of the Box pushing and precautionary measures for successfulcompletion of work.

1.0 IntroductionGovernment of Karnataka (GOK) has proposed two lane sub waydue to up gradation of Karwar- Ilkal State Highway 6. As the TVU isless than 1 lakh, hence the cost is fully borne by GOK. The scope ofwork was construction of subway including approaches. This subwaywas of 2x5. 0x5.0 m road under bridge (RUB) to be provided forelimination of level crossing no. 261 at km 423/1-2 between Gudgeriand Yelvigi stations in Hubli-Davangere section. This bridge involved2200 Cum of Reinforced concrete of M30 grade with 330 tons ofsteel reinforcement. The construction was done by box pushingtechnique involving 180 tonne capacity hydraulic jacks. The RCCbox was cast in segments of 8.5 to 11.00m length. A drag sheet wasused to avoid the friction between the formation earth and the boxduring pushing operation.This work of RUB will also eliminate the Manned Level Crossing No.261 which will ensure safety of running trains as well as saving ofman power as 3 men are deputed for the gate.* Chief Engineer (Construction), South Western Railway, Hubli** Dy. Chief Engineer (Construction), South Western Railway, Hubli

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2.0 Site PlanThe state high way is crossing the Hubli-Banglore Railway line atKM 423/1-2 at a skew angle of 65 degrees and track is on 2.8 degreescurve. Either side of track is on bank, hence subway was foundfeasible instead of ROB. Considering 5.00 m vertical clearance forroad traffic, top of bottom slab of RCC box is decided duly takingconsideration of depth of track and 0.75 meter earth cushion (1.00earth cushion is preferred for box pushing). Alignment of road ismade straight duly eliminating road curves. To cater for traffic, twoRCC boxes of size 5.00 x 5.00 meter are pushed independentlykeeping 1.00 meter gap between boxes.

3.0 Salient Features1) Box size - 2x 5.00x5.00 meters2) Number of RCC segments - 4 numbers on Hubli end and 5

numbers on Bangalore end.3) Box length - 43.00 m Hubli end and 53.50 m Bangalore end.4) Top and bottom slab thickness - 500 mm5) Side walls thickness - 500 mm6) Quantity of RCC in boxes - 1100 cum7) Quantity of RCC in road works - 800 cum.8) Quantity of RCC in retaining walls - 300 cum.9) Quantity of total steel involved - 330 Tonnes.

Fig. 1 : Location Map

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10) Hubli end box pushing was commenced on 26.08.11 andconcluded on 29.09.11 (33 days). A total length of 43 meterwas pushed achieving an average progress of 1.3 meter perday.

11) Bangalore end box pushing was commenced on 02.10.11 andconcluded on 02.11.11 (30 days). A total length of 53.50 meteris pushed achieving an average progress of 1.78 meter per day.Removing the earth from inside the box was done manuallywhere the strata was loose. Hard strata were removed byengaging Poclain -200.

12) Total 18 hydraulic jacks of 180 tonne capacity each and 3 powerpacks were used during pushing operation of the box segments.Minimum force of 120 tonne and maximum force of 600 toneswas applied during pushing of segments. Minimum 2 andmaximum 6 jacks were engaged for segments.

Fig. 2

Fig. 3 Typical box reinforcement details

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13) Drag sheets 1.00 mm thick 0.90 m wide, 3 numbers in 4 rows ineach box were used.

4.0 Details of Concrete usedMix design for RCC was got done from "B. V. Bhoomaraddi Collegeof Engineering and Technology , Hubli. A compressive strength of24.7 N/mm at 7 days and 40.57 N/mm at 28 days was achieved inthe design mix. Concrete cubes were tested at site duly establishingthe concrete testing machine. During the progress of casting of theboxes, 156 cubes were tested for 7 days strength and 156 cubes for28 days strength. Average strength of 35 N/mm for 7 days and 44N/mm for 28 days respectively was achieved.4.1 Design MixThe mix design for M30 grade concrete is adopted for the RCCwork. The ratio of 1: 1.35: 2.58 was adopted as per mix design.Water cement ratio was 0.42. In addition super plasticizer (SP430)at 0.3 % (minimum) by weight of cement was used.

5.0 Procedure of Casting of RCC Box & Pushing Operationz Install necessary temporary engineering indicators as per rules

in force.z Impose speed restriction of nonstop 20 kmph over proposed

bridge site.5.1 Thrust BedThrust bed is one of the important items of the box pushing technique.The location and level should be carefully decided in such a waythat entire length of the box is pushed to the desired location. Toachieve the desired location of the box, the gap between the end ofthe box and beginning of the thrust bed shall be kept 2 to 3m.The thrust bed, thrust beam and keys are designed with RCC toresist the required thrust exerted by jacking force and to transfer itto soil at bottom and sides. In cohesive soils even shallow piles arerequired to transfer the load. Provision for jacking supports is madeby providing suitable pockets in the bed to accommodate pinsupports. After completion of jacking the thrust bed is utilized asfloor bed and is left in place.

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5.2 Casting of Precast RCC Box UnitsThe precast box is cast in segments of convenient lengths. The boxsection is designed as per IRS codes of practice for 25 T loading2008.First RCC box segment (leading segment) is cast over the well setand leveled thrust bed. The front face of the box was cast in slopedmanner to match the shape of cutting edge and was integrated tothe concrete. The cutting edge was provided all around the box as itacts as shield preventing falling of soil from top and sides. A steelrear shield is also provided which houses and guides the subsequentsegments.

Reinforcement detail of main & auxiliary thrust bed.

View of thrust bed after casting

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5.3 Function of Front Cutting ShieldThe first segment of the precast box was provided with a speciallydesigned structure named as cutting shield which formed the cuttingface with a cutting edge fabricated from 16 to 20 mm MS plates andhoused on RCC box section with suitable anchor bolts. The cuttingedge was provided with stiffeners at regular interval through theface of box. On completion of jacking the outer shell of the shiedwas cut and removed. The function of the front shield is to supportthe soil on top and sides during jacking.

Fig. 4 : Side view of cutting edge

Fig. 5 : Front view of cutting edge

0.800.31

5.15

0.50

0.310.302.30

6.00

5.00

5.156.15

0.50

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5.4 The function of Rear Jacking ShieldThe rear jacking shield was provided by anchoring steel plate onthe face of the bottom slab of RCC box suitably designed to distributethe jacking load uniformly on concrete area. The function of rearshield is to house and guide the following segments. It also functionsas a hood for supporting the soil on top and sides between the twounits.

5.5 Intermediate Jacking StationAs total length of box is cast in segments, each segment is pushedturn by turn with necessary jacking force. Necessary intermediatejacking stations are provided with jacking pockets in bed & walls.

5.6 Pushing operation of the boxTo reduce the frictional resistance, a thin film of grease and thickgauge plastic sheets are provided between the top of the thrust bed& bottom of the box. This is done before casting of the bottom slabof the box. A glossy epoxy layer is also provided to reduce frictionon the top and prevent the distribution to the earth and ballast massover box during pushing operation.

Front Cutting Shiield

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Provision of thick gauge plastic sheet over a thin film of greaseWhere mass of embankment is less or the soil is of poor qualitydrag sheet system is also considered for least resistance of frictionand disturbance to the mass above box.

Fig. 6 : Drag Sheet Arrangmets

DRAG SHEET CONSISTS OF GALVANISEDALUMINUIM SHEETIN ROLL, 1 MM THICK AND 0.90M WIDTH

STEEL PORTAL FORANCHORING DRAG SHEET

DRAG SHEET 3LAYERS, 4 ROWS

G. I PIPES SUPPORTINGDRAG SHEETS

20 MM SLOT 1.00 LONGTO GUIDE DRAG SHEET(SLOTS 4 NO.)

STIFFENERS20 MM PLATE

CUTTING EDGEWITH 20 MMSTEEL PLATES

PILOTSEGMENT

BOXSEGMENT

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Drag sheet arrangement over box

Drag sheet arrangement over box with anchoring portal

Auxiliary beds are also provided for casting of the other segmentswith thin film of grease and plastic. Segments are then brought inalignment of pushing as and when required as pushing progresses.With the progress of jacking, the front unit with shield penetratesinto the embankment. There after excavation within the shield isdone either manually or mechanically and the excavated stuff istransported outside the working area.The progress of pushing is kept continuous and the system of shiftingof remaining box segments from auxiliary bed and bringing theminto alignment is adopted till total length is pushed.

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5.7 Hydraulic EquipmentSufficient numbers of Jack units are provided in series for distributionof pushing load evenly on the face of the concrete, and all jacks areoperated simultaneously with a common power pack, which suppliesuniform flow of pressure through network of hydraulic pipescommencing from front unit to rear unit. Jacking force is applied insequence. The pushing cycles are repeated till total pushing iscompleted.

Fig. 7 : Position of jacks between segments

Fig. 8 : Position of jacks on thrust bed

Box segment

Movement of box

Jack 180 ton capacity

Box segment

20 mm plate embededin concrete

6.15

Spacers(steel)

Steel pinPin Pockets0.6 x 0.6 x 0.6 size

Thurst bed

End box segment

Jacks 180 Ton capacity

20mm plate embedded in concrete

2.50 2.50 2.50

6.15

1.00

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Jacking of end segment of box from thrust bed

Jacking of intermediate box segment taking supportfrom adjoining box segment

6.0 Precautionary Measures for Successful Operation of BoxPushing

6.1 Cutting edge of front shield is fabricated with adequate thicknessof steel plate and the front edges of the steel plate are sharpenedto facilitate penetration into the soil.

6.2 Cutting edge shall be projected more at the top with respect tobottom slab to prevent falling of earth from top during excavation.

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6.3 Leading segment shall be pushed at least 10cm less than thelength of the rear shield in one operation.

6.4 To prevent caving of earth during excavation quantity of earthshall be removed to barest minimum duly following the slop ofcutting edge.

6.5 Guide channels to be provided in the thrust bed to guide thesegments to ensure straight alignment.

7.0 ConclusionThe box pushing method adopted for elimination of this LevelCrossing is found suitable and convenient as per the site conditions.Numbers of construction joints in this method are reduced compareto cut and cover method. Requirement of Mega Line Block couldalso be avoided by adopting this technique. Though the cost ofconstruction in this method is more, it is adopted mainly due to non-availability of Mega Line Block and due to non-availability of longertemporary girders (R. H. Girders). The box pushing work has beensuccessfully completed without disturbance to track as Drag Sheethas facilitated for smooth insertion of Box through formation.

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