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This document briefly explains Box Pushing Methodology for construction of RUB under railway tracks.
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CONTENTS
1.0 INTRODUCTION...................................................................................................2
2.0 STRUCTURAL COMPONENT.................................................................................3
2.1 Thrust bed..................................................................................................3
2.2 Precast R.C.C. Box......................................................................................4
2.3 Front Cutting Shield & Rear Jacking Shield.................................................5
3.0 PUSHING OPERATION OF THE BOX.....................................................................6
3.1 Hydraulic Equipment..................................................................................7
3.2 Intermediate Jacking Station......................................................................8
4.0 DESIGN EXAMPLE.............................................................................................10
4.1 Introduction..............................................................................................10
4.2 Box pushing methodology........................................................................10
4.3 Analysis and Design methodology............................................................10
4.4 Material properties...................................................................................11
4.5 Primary load cases...................................................................................11
4.6 Detailed design calculations.....................................................................11
5.0 CONCLUSION....................................................................................................11
APPENDIX A
APPENDIX B
1.0 INTRODUCTION
This Present day Intensity of Traffic, both Rail & Road due to the fast
development of Industries and other Infrastructures, is very heavy and so it cannot
the disturbed, for construction of under bridges or Canal Crossings, drainage etc
by conventional construction methods. So to construct such structures, Box
Pushing Technique is developed where in R.C.C. Boxes in segments are cast
outside and pushed through the heavy embankments of Rail or Road by Jacking.
The required thrust is generated through thrust bed, as well as line and level of
precast boxes are also controlled
2.0 STRUCTURAL COMPONENT
2.1 Thrust bed
The thrust bed, thrust Beam and keys are designed in R.C.C. to resist the
required Thrust extorted by Jacking force and transfer it to soil at bottom and
sides. In cohesive soils even shallow piles are required to transfer the load.
Provision for jacking supports is made by providing suitable pockets in the thrust
bed to accommodate pin supports. During the construction of precast box
segments on the thrust bed, the thrust bed are filled with sand or any other filling
material, and the same is covered with screed layer of cement mortar of 50mm
thickness.
2.1 Casting bed showing jacking pin pockets in position
2.2 Construction of casting bed
2.2 Precast R.C.C. Box
The Precast Box is cast in segments of convenient lengths of Total pushing
length. The Box section is designed as per IRS / IRC codes of practice for loading.
Concrete grade normally kept as M-35. Front face of the precast box segment
which pierce into the embankment is provided with inclined edge as shown in the
following figure.
2.3 Front face of concrete precast segment
2.3 Front Cutting Shield & Rear Jacking Shield
The First Segment of the precast box is provided with a specially designed
structure named as cutting shield which forms the front working face with a cutting
edge fabricated from 16 to 20 mm thick M.S. Plates and housed on RCC Box
section with suitable anchor bolts. Necessary strengthening is provided by
stiffeners and face plates of Box in front on completion of jacking the outer shell of
the shield is cut and removed
The Rear Jacking shield is provided by anchoring steel plate on the face of
Bottom slab of RCC Box suitably designed to distribute the jacking load uniformly
on concrete area.
3.0 PUSHING OPERATION OF THE BOX
The purpose of pushing of Box is to form a horizontal opening below
ground or through embankments, can say a tunnel, by providing precast box units
underground or an embankment, without disturbing the overhead amenities, like
traffic & structures.
To reduce the frictional resistance, a thin film of Grease and thick gauge plastic
sheet is provided between the top of the thrust bed & Bottom of the Box. This is
done before casting of the Bottom slab of the box. A Glossy Epoxy layer is also
provided to reduce friction on the top and prevent the disturbance of the mass
over box to least during progress of pushing
Where mass of embankment is less or the soil is of poor quality, drag sheet
system is also considered for least resistance of friction and disturbance of mass
above box. This avoids disturbance of track structure due to the pushing operation
where minimum depth of earth cushion is provided over the top of box. Typical
drag sheet arrangement is shown in the following figure.
Figure 3.4 Drag sheet arrangement over box
Figure 3.5 Drag sheet arrangement over box with truss
Auxiliary beds are also provided for casting of the other segments with thin
film of grease and plastic and segments are then brought in alignment of pushing
as and when required as pushing progresses.
With the progress of jacking, the front unit with shield penetrates into the
embankment and there after excavation within the shield is done either manually
or mechanically and the excavated stuff is transported outside the working area.
After the first segment advances to the end of the thrust bed, the second
segment is brought in alignment of the front Box & intermediate jacking space is
lift between the two. The progress of pushing is kept continuous and the system of
side shifting of remaining box segments from Auxiliary bed and bring in pushing
alignment is adopted till total length is pushed.
3.1 Hydraulic Equipment
The entire Box Pushing system is matter of proper selection of Hydraulic
equipments, as tremendous driving force is required for pushing of medium to
large size of Boxes. So for this, Number of Jack units are provided in series for
distribution of pushing load evenly on the face of the concrete, and all Jacks are
operated simultaneously with a common power pack, which supplies uniform flow
of pressure through network of Hydraulic pipes of required pressure capacity,
commencing from front unit to rear unit. Jacking force is applied in sequence. This
way pushing cycles are repeated till total pushing is completed.
Figure 3.6 Jacking of end segment of box from thrust bed
3.2 Intermediate Jacking Station
As total length of Box is casted in Segments, each segment is pushed turn
by turn with necessary jacking force and so for this necessary intermediate jacking
stations are provided, with jacking pockets in bed & walls.
Figure 3.7 Jacking of intermediate segment
4.0 DESIGN EXAMPLE
4.1 Introduction
Design of RUB which is proposed for a rail connectivity taking off from
railway station is covered in this document. The RUB is a RCC box of size 1 x 7.5
x 5.888m which has to be constructed below the railway track. It is proposed to
use Box pushing methodology for making the underpass without interrupting the
rail traffic above it. Barrel length of the box is 22.0m.
4.2 Box pushing methodology
Box pushing Methodology is a well suited construction method for making
underpass below the rail tracks without disturbing the rail traffic. Number of RCC
box segments to be pushed is depending upon the barrel length of the RUB. In
this project, two box segments of 11.0m each is used. The entire operation takes
place by pushing the box against thrust bed, and removing the soil at the front end
of the first box segment. Thrust bed consists mainly of thrust wall, pin pockets and
keys for additional resistance. Thrust bed is designed for box pushing force which
depends on the frictional resistance of the soil.
4.3 Analysis and Design methodology
Analysis of the box has been done by moment distribution method
by considering all the loads on it and with suitable load combinations. IRC-6 : 2010
has been followed for arriving the effect of road traffic on the base slab. In addition
to these loads pushing force on the box and frictional resisting force due to soil
also considered for the design. Design bending moments and design shear forces
have been taken from the moment distribution method. The design of structural
elements has been carried out using IRS codes - Limit state method as per codes:
IRS Substructure Code, Concrete Bridge Code and Bridge rules.
4.4 Material properties
Table 4.1 Material Properties
Material Grade
Grade of Concrete for box M35
Grade of Steel reinforcement Fe 500
Grade of Steel reinforcement for Shear Fe 415
4.5 Primary load cases
i. Self weight, and earth fill over the box
ii. Super imposed dead load
iii. Rail Live load and traction forces (Bridge rules, IRS)
iv. Box pushing forces and resisting forces from the soil
v. Earth Pressure
vi. Surcharge Pressure(Due to DL & LL)
vii. Road traffic loads
4.6 Detailed design calculations
Detailed load calculation, analysis, Calculation of pushing force, Design of
RUB and thrust bed are provided in Appendix A
5.0 CONCLUSION
The entire system is purely technical, dependent on Hydraulic system, &
safest method of crossing underground / Embankment, without disturbing
overhead traffic / structures for R.U.B., Canal siphon and other drainage
crossings.
Appendix A
Design Calculations
Appendix B
Drawings