Arezki Touat, Senior Engineer-Systra Paris, Serge Montens, Director of Bridge Department-Systra Paris and Jean Charles Vollery, Civil Works Director-Systra Asia Pacific and Chief Executive of Systra-Mva Consulting (India Pvt. Ltd.)

Introduction

For the bypass of Kota (RJ), as part of the NHDP Phase II project; NHAI has planned to make the NH-76 cross the Chambal River. Since this crossing was falling in the “Chambal gavial wild life sanctuary,” it was decided to cross the river width of about 300m with a single span, and hence reduce crocodile wild life disturbance by avoiding any pier in the river bed.

The solution proposed by the construction JV Hyundai-Gammon to achieve this goal, was a concrete cable stayed bridge with a main span of 350m as projected by Systra.

When completed, this bridge will be the first axial suspension cable stayed bridge, ever built in India.

Bridge Description

The selected deck consisted in a single cell box girder of 30,2m width, stiffened at every 3,5m by transverse ribs. This very large width was required to position the 6 lanes carriageway with the 3m central median. Two footpaths of 1,5m width are also located on the deck sides.

The deck is supported by sliding spherical or pot bearings on all piers, except on shortest pylon pier where it is rigidly connected. Location of this pylon near the cliff, has required special geotechnical and geophysical investigations, to ensure cliff stability.

The stays are in a single plane with a semi harp arrangement, and are anchored every 7 m in the deck. The 80m high pylons, receive the passive anchorages of the stay cables.

Thanks to the good rock quality below, all lateral piers of the cable stayed bridge are founded on, spread footing, and pylons foundations are composed of two 4,5 m diameter shafts.

The structure will be cast in situ: on scaffoldings for the lateral spans and using the cantilever method for main span

Site Investigations

Topographic investigations

The surveys were related to ground and vertical face of the cliff.

The ground survey has been performed to localise bridge axis, determinate pier positions, and ground levels at foundations location. It has also confirmed that there was no major topographic anomaly on site.

The survey of the cliff was required to create in 3d the shape of the cliff, in order to evaluate with accuracy the cliff stability; since one of the pylons (p5) is located at only 30 m from cliff edge.

Geotechnical investigation

Geotechnical investigations were divided into three main jobs: site investigations, geophysical survey and satellite imagery.

A) site soil investigations consisted in boreholes drilling for determination of rock parameters, by in-situ and laboratory tests as detailed hereafter:

In-situ Tests:
  • permeability
  • CaCO3 content
  • visual identification (tcr, scr & rqd) for each run

Laboratory Tests:

  • Dry density
  • water absorption, porosity
  • specific gravity
  • point load index (pli)
  • resistance tests (ucs)
  • CaCO3 content
  • elastic modulus and poisson ratio
In addition to the above tests, a petrographic analysis of the rock, and a water chemical analysis were also performed.

Number and depth of boreholes was governed by foundation type (pile shaft or footing): one borehole was required for common pier foundations, and three boreholes at pylon foundations location up to 40m depth. The number of boreholes at the pile shafts location was required due to the type of tests to be performed (cross hole tests).

The results have shown that the site geology is mainly composed of hard siliceous sandstone. Due to temperature effects and climatic variation, metamorphism takes place on the 10 first meters which increases the percentage of silica: sandstone transforms into quartzite. This part generally creamish to brownish colored is more fractured. At a lower depth, the sandstone becomes more ferruginous.

CaCO3 tests have indicated that there was no trace of calcium carbonate in the rock samples. Ucs test have shown that the rock has an important resistance: between 100 and 300 mpa. Youngs modulus is equal to 17 500 mpa for intact rock, and poisson’s ratio is equal to 0.21.

Chemical analysis of water has shown that there was no presence of aggressive elements, which could affect structure durability.

At pylon located near the cliff (p5), there are different sets of joints which show no connectivity, except for joints with 45° angles. In addition, a thin layer of soil has been found at 10m depth, which has led us to propose a geophysical survey to verify that this layer do not extend to the cliff, and therefore do not affect cliff stability.

A slope stability calculation was afterwards conduced considering network of fractures, block stability and low cohesion on sliding surfaces.

B) Geophysics survey includes cross-hole tests at pylons location and surface survey using the electrical method.

Cross hole tests have allowed to measure velocity into the rock along the bore length. The tests have confirmed the presence of weak rock at 13m depth. On the remaining height values were consistent and no drastic variation is observed. This test has also confirmed the absence of cavities or channels.

Surface geophysics survey has allowed to develop a complete 3D model of rock resistivity as shown in the next scheme.

The geophysical investigation has concluded that the discovered weak zone (with low resistivity), do not extend up to the edge of the cliff and consequently do not affect the stability of the cliff.

C) Satellite imagery has allowed to confirm the geology of the area, and to determine river bed profile since no access was possible from the river for environmental reasons. This investigation has also confirmed that there were no cavity below water level, which could extend below the pylon P5 foundations.

General Design Features

Foundations

All lateral piers of the bridge are supported on rectangular footings, resting on safe rock. Pylon piers foundations are composed of two vertical shafts of 4,5m diameter, with a maximum length of 15m. For shafts design, friction and reduced end bearing were considered in order to reduce the settlement. The rock mass ratio (rmr) classification was used to determine the bearing parameters following the AASHTO LRFD code.

The pilecaps above the shafts have a variable depth; from 2m at the extremities to 5m at pier axis, this shape has been chosen in order to optimize quantities by placing the concrete only where it is required. Concrete grade for all foundations is m47.5.

Empty ducts will be provided on p5 pile cap for future active anchors, in case it becomes required.

Substructures

The piers have a rectangular shape for lateral piers and a cross shape for pylon piers. Although this cross shape is unusual, it has been chosen for structural reason. The main loads come from the pylon, so it is logical to put some material just below the rectangular axial pylon. The other part of the load comes from the deck webs and its diaphragm, so it is logical to put some material on a rectangle located just below the diaphragm. This leads to a cross shape.

The pier P5 is rigidly connected to the deck, whereas the second pylon pier (P4), has a pier cap supporting four spherical bearings of 7000t capacity each.

Seismic Design

The city of Kota is located in seismic zone II, with a peak ground acceleration of 0,1 g. The design response spectrum was defined as per IRC code.

To estimate earthquake forces on the structure, a 3D model of the bridge has been realised, including part of the access bridges in order to have the correct effects on transition piers.

Earthquake effects were evaluated for service stage and for the most unfavorable construction stage. The effects of all modes were combined together using complete quadratic combination CQC. Response modification factors were considered as per AASHTO LRFD, with distinction between wall type, and column type of piers for each horizontal direction.

Bearings

Lateral spans are supported on pot bearings with a maximum vertical capacity of 1200t in service stage. These bearings shall also resist uplift forces evaluated to a maximum of -380t at strength limit state.

The four spherical bearings located on p4, have a maximum vertical capacity of 7 000 t.

Wind stability

Deck stability under wind loads was tested in wind tunnel, and has required the change of the shape of the sound barriers (required to reduce environment disturbance), in order to satisfy vortex shedding equirements. Flutter stability was satisfied in service and construction stage for different wind incidences.

After validation of aerodynamic behavior of the deck, the bridge sectional model has been used to determine the different aerodynamic coefficients (drag, lift and torsion), required for the numerical calculation of wind forces (buffeting analysis). This calculation has been performed after also, the numerical definition of the turbulent wind model.

Obtained loads were used for the design of all elements of the structure.

Pylons

The pylons are 80m high above top of deck with a constant width of 3m, and a variable length: from 7 m to 4 m. The concrete grade used is m60.

The pylons contain a steel frame where are located the passive anchorages of stay cables. This steel frame is composed by 20 steel boxes (one for each pair of stay cable), that take the horizontal component of the stay cables. The vertical component is transmitted to the pylon through shear studs, located on the laterals sides of the steel boxes.

A manhole with a minimum dimension of 800x1500 mm is provided inside the pylon.

The pylon structural resistance has been verified under dynamic wind loads, and a through detailed buckling analysis (2nd order analysis), taking into account geometrical and material non linearities.

Stay Cables

The stay cables are composed of individually sheathed strands having a triple protection: galvanisation, wax filling and individual polyethylene sheath. The external cable duct has helicoids in order to eliminate rain and wind induced vibrations.

The strands have seven wires of class 1860 mpa and stay cables unities vary from 58 to 91 strands.

Anti vibration devices will be provided for the longest stay cables.

Deck

The concrete deck (M60 grade), is prestressed longitudinally using internal and external tendons, and transversally using only internal tendons.

The external tendons are located mainly in lateral spans that are longitudinally prestressed using only external tendons. Some external tendons are also provided for the continuity prestressing of main span. Provisions for future external prestressing are also provided.

Internal tendons are used for cantilever tendons, cyclic tendons and some of the continuity tendons of main span.

Transverse prestressing is composed of 13T15 tendons in the ribs and 4T15 tendons in the slab. At stay cables anchorage location, a diaphragm wall prestressed diagonally, permits the transmission of forces from the lateral webs, to the stay cable anchorage located at the center.

For the transverse analysis of the bridge, one third of the bridge has been modelized using finite elements.

To reduce uplift forces on lateral piers, the box girder will be filled with concrete at these locations.

Pier segment below pylon P4 has been studied using volumic finite elements, as the transfer of pylon vertical load to the four spherical bearings was not easy to apprehend. This study has confirmed that a transverse prestressing at the bottom of the pier segment was required.

The longitudinal analysis has been performed considering second order effects, construction stages and time dependent effects.

Special load cases particular to the design of cable stayed bridges and cantilever construction were considered such as cable braking or replacement, differential temperature in deck and stay cables, accidental falling of travelling formwor.

A complete AutoCad 3D model of the bridge was created in order to generate the prestressing layout and ensure its feasibility. This model has also been used for the preparation of reinforcement shop drawings.

Service life of the Structure

The identified risk for structure durability is the concrete carbonation. Based on a present CO2 concentration in the air estimated to 350 ppm, and an expected increase due to road traffic to 450 ppm, the service life of the structure has been estimated using the Papadakis & al. model.

Estimation of structure design life is defined as the time required for carbonation to reach the first layer of reinforcement.Based on an external concrete cover for pylons and deck box-girder of 40 mm, the required design life of 100 years has been reached for a defined minimum concrete carbonation resistance.

It should be also noted that with these methods, we are only evaluating the time of initiation of the reinforcement corrosion, corresponding to the time required for carbonation to reach the first ayer of reinforcement. But in reality, the life time of the structure is largely higher as shown in the next scheme:

Construction

The construction of the bridge has started in December 2007, and is scheduled to be completed in 2011. The lateral spans of the bridge will be cast in situ on scaffoldings, starting by the spans near the pylons. Two sets of scaffoldings will be used: one on each side of the river. Thanks to this arrangement, main span construction can start as soon as first lateral span is completed; which withdraws other lateral spans construction from the critical path.

The main span segments of 3,5 m length, will be cast in situ using two sets of travelling formworks, and pylons using climbing formworks.
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