Challenges Faced in Planning, Design & Construction of Grade Separator Near Apsara Border, Delhi

The paper describes the salient features of design and construction of one of the most complex three level grade separators ever constructed in the city of Delhi. The Grade Separator comprises the following major structural components:

• A 6-lane flyover at Apsara Border along the GT Road
• 2 Nos. 2 lane Underpasses along Road No. 56 and Road No. 62
• 2 Nos. RUBs constructed under extremely challenging conditions by using box pushing technique
• 2 Nos. of Foot Over Bridges across Road No. 56 and Road No. 62
• Widening of existing bridge over Major Drain & Allied Works

This is possibly the first project in India where contiguous piles in combination with prestressed horizontal anchors have been successfully used for supporting the existing ROB approach close to the proposed Underpass. This is also the grade separator with longest total length of Underpass constructed in Delhi (total length 1666m). The paper highlights the salient technical features of the project components including the design and construction issues.

Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt. Ltd., Noida

Introduction

Rapid development and urbanization of Delhi and surrounding areas coupled with the high average income of the populace (with large standard deviation) has largely eclipsed socio-cultural traits that used to represent Delhi until a few years after independence. Traffic congestion, longer travel times and high levels of air pollution are just some of the growing problems faced by the city’s residents. Rising incomes and a burgeoning middle class has seen an increase in private vehicles in the past two decades, particularly because the public transport system has not kept pace. Delhi has nearly 4.5 million vehicles, which is more than that in the three other major Indian cities of Mumbai, Calcutta and Chennai put together. The growth in the number of vehicles has had a knock on effect on the roads of the city. Traffic congestion, longer travel times and high levels of air pollution are just some of the problems faced by the city’s residents. Coupled with the above, there was tremendous pressure to improve infrastructures in the urban areas in general for the Commonwealth Games of October 2010, which led to the construction of this grade separator which was long overdue at this intersection in any case.

Need for the Project

The need for a grade separator at the Seemapuri Border near Apsara Talkies was felt for more than two decades. Long queues at the Intersection, frequent jams with traffic stuck for hours were a common sight at this intersection. Public Works Department, Govt. of Delhi had initiated this ambitious project in the year 2006, with the objective to increase road connectivity between Delhi and U.P, between Anand Vihar to Shahdara and between Ghaziabad to Maharana Pratap ISBT, Anand Vihar. Feasibility study for the project was carried out based on which, a six lane flyover is envisaged along the G T Road at this intersection connecting Delhi & U.P and two underpasses of 2 lane each are envisaged along Road No. 56 (one on either side of existing ROB) to connect Anand Vihar with Dilshad Garden.

Fig. 01 shows the key plan showing alignment of Flyover, Underpasses and location of Foot Over Bridge. Part of the six lane flyover falls in U.P side, for which PWD got the working permission from UP government. Cost of the flyover however is borne by the Govt. of Delhi.

Project Award dates

Approval for this project from Technical Committee of DDA was obtained in September 2006. DUAC approval was obtained in July 2007. The administrative approval for the project was obtained on 9th June 2008 for an amount of 226.47 crores.

The construction contract for this project was awarded to M/S AFCONS Infrastructure Limited, Mumbai for an amount of `180.2 crores. The construction period was allocated as 21 months. The salient dates for the project are as under:

• Date of Commencement of Work: 10th September 2008
• Stipulated Date of Completion: 9th June 2010
• Flyover opened to traffic on: 24th April 2010
• 1st Underpass opened to traffic: 31st October 2010
• 2nd Underpass opened to traffic: 5th January 2011

Photo P1 shows the completed Grade Separator in Google Map.

Salient Features & Components of the Grade Separator
Flyover along G T Road

The 6 lanes flyover with divided carriageways of 9m width (reduced 3 lanes) is constructed along the G T Road. The total length of the flyover is 646m with length of the stilted portion of 340m and balance 306m in solid fill with Reinforced Earth Walls. The overall width of the flyover including median is 20.2m.

The span arrangement for the stilted part of flyover comprises 3 modules of continuous span units. The central module comprise 4 spans of span lengths 40m+50m+50m+40m, totaling a length of 180m, while the end modules on either side of the central module comprise 2 span continuous structure of lengths 40m+40m each. Expansion joints are provided at Abutments and at 2 intermediate sections, 80m away from the abutments. Fig. 02 shows the General Arrangement of the Flyover.

The Superstructure comprises a steel concrete composite plate girder with in-situ RCC deck slab. The girders are supported on metallic bearings. The overall depth of the superstructure deck is kept at 1.925m. 4 plate girders are provided transversely at a spacing of 2.5m for supporting each of the 3 lane carriageway. The depth of plate girders are kept as 1.7m. High strength steel of grade Fe540B conforming to IS:2062-2006 has been used. The deck slab of 225mm thick is provided in M35 concrete on top of plate girders. For the stilted portion, the two carriageways are structurally isolated and a longitudinal clear gap of 200mm is provided at the centerline of the median along the entire length.

The bearing arrangement comprises series of Metallic Free POT cum PTFE bearings under outer girders, Guided Bearings under internal girders at Free / Expansion joint piers and Fixed Bearings under internal girders of fixed piers. The combination of these types of bearings ensure transfer of vertical loads and lateral loads from Superstructure to the foundation, through substructure. Fig. 03 shows the bearing arrangement for a typical carriageway of this project.

RCC single circular pier of 2.0m diameter have been provided under each carriageway for all piers and abutments, except fixed piers P4, in which case pier diameter of 2.75m has been provided. Pier cap is cantilever type in all cases. Seismic stoppers / arrestors are provided in the transverse direction to arrest the possible dislodgement of Superstructure in the transverse direction under earthquake loads. Fig. 04 to Fig. 08 shows the typical details of various components of the Flyover.

The foundation sub-strata as per the Geotechnical Report comprise road fill or loose filled up soil upto a depth of about 2.5m, followed by silty fine sand / fine sand layers upto 10-13m depth underlain by very dense sandy strata upto the explored depth. Total of 10 number of bore holes have been taken at the project site to establish the geotechnical properties for foundation design.

Bored cast-in-situ piles of diameter 1.2m have been used for supporting the stilted portion of flyover. Total of 108 numbers of piles have been provided for the flyover. Pile capacity considered is 287 Tonnes for a length of 30m below pile cap bottom. The safe load capacity has been confirmed by conducting initial pile load tests as well as routine load tests on working piles. Number of piles provided under each foundation (for each carriageway) is as under:

• Abutments A1 & A2: 4 nos.
• Piers P1, P3, P5 & P7: 6 nos.
• Piers P2 & P6: 5 nos.
• Pier P4: 12 nos.

Photo P2 & P3 shows the completed Flyover in service.

Underpasses along Road No. 56 & Road No. 62

Two Vehicular Underpasses are provided alongside of Road No. 56 and Road No. 62 connecting Dilshad Garden and Anand Vihar. The total length of the underpasses is 840m & 826m for Delhi side and U.P side respectively. Each underpass is provided with 2 lane carriageway of width 7.5m with 0.75m raised kerb/footpath on either side. Overall clear width between inner face of walls is kept at 9.0m. vertical clearance of 5m is provided in the covered portion of Underpass. Fibre Reinforced Concrete (FRC) wearing course of 125mm thickness has been provided over the base slab. The underpass has provision of four number of sumps of 40,000 litre capacity in each underpass with two numbers of pumps of capacity 10 to 15 HP including drainage arrangement. Fig. 09 & Fig. 10 shows the General Arrangement of the Underpasses.

Structural Scheme for Ramp Portion – Open to Sky

Total length of Ramp portion, open to sky is 328m for each Underpass. Where depth of excavation from road level is less than 3m, the proposed structural scheme comprise RCC cast-in-situ U-type RCC section, with variable height, constructed bottom-up with open cut. Prestressed vertical soil anchors are connected with the base slab which takes the buoyant forces due to rising of water table. For the portion where the depth of excavation from road level is more than 3m, RCC diaphragm walls, 800mm thick are provided with top-down construction. Fig. 11 & Fig. 12 shows the typical cross section of Underpass open to sky with open excavation and with diaphragm walls respectively.

Structural Scheme for Covered Portion under GT Road

For the 150m and 164m long (UP side and Delhi side respectively) covered portion of Underpass below G T Road, the structural scheme had to be such that it involves minimum disturbance to the flow of traffic since this intersection caters to a significantly high volume of vehicular traffic. Structural scheme adopted involves construction of Diaphragm Wall on either side with top-down construction using RCC solid Slab on top. Fig. 13 shows typical cross section of Underpass & Fig. 14 shows various stages of construction in this portion. Construction of this covered portion had to be taken in phased manner to ensure uninterrupted traffic flow with minimal diversions.

Structural Scheme for Open to Sky portion adjacent to existing ROB

For the construction of Underpass close to the existing ROB with high embankment, vertical cuts had to be done upto a maximum height of about 14m close to the existing ROB approach road. The ROB had to be kept functional during the construction. This was achieved by providing 1.2m diameter contiguous piles, 20m length @ 1.5m c/c along the ROB on either side of the existing road . Total 484 numbers of piles (i,e 242 nos. on either side) have been used. The piles on either side of the ROB are connected to each other by using horizontal prestressed anchors of 50T capacity each. 151 numbers of horizontal soil anchors with 4 layers of waler beam have been used in this project to provide lateral support to the contiguous piles for retaining the embankment of ROB approach with vertical cut. By retaining the earth with contiguous piles on ROB side, excavation for construction of underpass with vertical cut was possible, which helped in providing adequate working space as well in providing the thrust blocks for box pushing in the railway portion. Photo P4 shows the erection of waler beam over contiguous pile.

For the 98m long open to sky portion of Underpass, located adjacent to the existing ROB, the structural scheme proposed comprise providing 1.2m diameter RCC bored cast-in-situ contiguous piles @ 1.5m c/c towards the existing ROB side, 800mm thick RCC diaphragm walls on the other side. Excavation is done in a phased manner with application of horizontal prestressed anchors connecting the contiguous piles on either side of the existing ROB. Fig. 15 shows the typical cross section of Underpass open to sky adjacent to the existing ROB.

Structural Scheme for Covered portion adjacent to existing Rail Line

For the 200m long covered portion of Underpass, located adjacent to the existing rail line, on either side of the rail line, the height of ROB approach embankment is maximum. The structural scheme proposed comprise providing 1.2m diameter RCC bored cast-in-situ contiguous piles, 20m long @ 1.5m c/c towards the existing ROB side, 800mm thick RCC diaphragm walls on the other side of Underpass. Excavation is done in a phased manner with application of horizontal prestressed anchors connecting the contiguous piles on either side of the existing ROB, in 3 or 4 layers. Fig. 16 shows the typical cross section of Underpass. Fig. 17 shows the sequence of application of horizontal prestressed anchors with contiguous piles in this zone.

Two numbers RUB by Box Pushing Technique

The Underpasses crosses the Delhi-Howrah rail route, which is one of the busiest rail lines in Delhi. For the 50m length covered portion of Underpass below existing railway line, box pushing technique was therefore adopted, which ensured un-interrupted flow of rail traffic throughout the construction period. In box pushing technique, entire length of reinforced concrete box is divided into segments (5 segments in this case). The segments are pre-cast over a horizontal RCC Thrust Bed. Thrust bed is constructed at a convenient location, in this case closer to the Rail line and close to the embankment. The Boxes are then pushed into the soil one after another one to the desired horizontal and vertical profile with the help of hydraulic force created by jacks. The force of the jacks is transmitted to the pre-cast segments and thus it moves forward. Equal and opposite reaction is absorbed by the thrust bed. Box pushing activity essentially involves following activities:

1. Casting of thrust bed :
2. Laying screed on the thrust bed
3. Laying polythene sheets and grease over screed
4. Casting of Boxes
5. Installation of anti-drag system
6. Pushing of the Boxes
7. Soil Nailing
8. Rail Track maintenance during box pushing & Quality Control Measures
9. Control of alignment and levels during box pushing

Rail track is continuously monitored & maintained during the construction process. Train movement at controlled speed during the construction phase. Anti-drag system provided to reduce friction during the box pushing. Details of the boxes pushed are as under :

LHS Underpass (Ghaziabad Side)

1. Length of the jacked box section: 50m cast in 5 segments of 10m each
2. Dimension of clear opening: 9m wide x 5m height (Clear)
3. Thickness of top and bottom slabs: 0.90m
4. Thickness of Walls: 0.90m

RHS Underpass (Delhi Side)

1. Length of the jacked box section: 50m cast in 5 segments of 10m each
2. Dimension of clear opening: 7.6m wide x 5m height
3. Thickness of top and bottom slabs: 0.70m
4. Thickness of Walls: 0.50m

For the RHS underpass, the jacked box underpass section lies between existing ROB pile foundations on one side and abutment well foundation of Railway Bridge over nallah on the other side. Minimum clearances between the faces of the existing foundations and the faces of the boxes to be pushed were specified by the Railways with the objective of reducing effects of lateral forces on the existing foundations generated during pushing of the boxes. This made the box pushing task extremely challenging and involved use of several alignment controlling measures like soil nailing, continuous supporting of track during pushing operation, …etc. Added supervision by railway authorities 24 hrs a day had to be taken to ensure safe construction.

Fig. 18 shows the schematic cross section of Box being pushed on either side of ROB. Photo P5 shows the construction of RHS side precast boxes for puhing below the rail lines.

Foot Over Bridges

Two numbers of Foot over bridges are presently under construction (i,e in July 2011). One FOB is being constructed at Road No. 62 towards Dilshad garden side with escalator, staircase and lift. The second Foot over bridge is being constructed at Road no. 56, which is integrated with the Metro Station at Dilshad Garden and the petrol pump towards UP side. The FOB’s are constructed with prefabricated steel girders for the deck with concrete deck slab, supported on steel columns and resting on open foundation. Roofing is not envisaged for the FOBs. Photo P6 shows the erected FOB at Road No. 62.

Bridge Over Drain & other Allied works

Apart from the major work of construction of a Flyover and two Underpasses, the project also involved widening of the existing bridge over trunk nallah at the intersection of GT Road and Road No. 56. The bridge over nallah has been widened by 18m on both sides by constructing RCC box type bridge for ease of traffic at surface level.

Other allied works involved in the project includes:

• Construction of roadworks in slip roads, approaches of flyover, merging roads on the entry/exit of underpasses constructed with two layer (150mm thick each) of GSB, two layers (125mm thick each) of WMM, two layers (75mm each) of DBM and 50mm thick BC as wearing coarse.
• Construction of Rotary at Intersection and Landscaping of the Rotary Island
• Shifting of sewer line, which was detected in the alignment of the Underpass on U.P side.
• Construction of Diversion roads & barricading duing the construction
• Horticulture, Landscaping, Traffic Signage & Electrical Street Lighting.
• Painting (Anti carbonation paint in exposed concrete surfaces of flyover, RE wall and crash barrier, synthetic enamel paint on surfaces of diaphragm walls, ceiling of deck slab of underpass, inner surface of crash barrier and outer surfaces of kerb stones)
• 25mm thick cement tiles in pattern over 15mm thick cement plaster in 200m length of underpass, footpath tiles …etc.

Design & Construction Aspect of Flyover Along G T Road
Foundation & Substructure

1200 mm diameter bored cast-in-situ piles have been chosen for the foundation of the Flyover. 1.2m diameter was preferred as compared to 1.0m diameter due to following reasons:

1. Span lengths are longer (minimum span 40m), thereby vertical loads per foundation is quite large.
2. The design of foundation is governed by the horizontal forces caused by braking, seismic bearing restraint, wind etc. Larger diameter pile performs better under lateral loads.

The vertical load carrying capacity calculated based on static formula as per IRC:78-2000. Lateral load carrying capacity from geotechnical considerations is assessed based on provisions of Appendix-C of IS: 2911 (Part 1/Sec2) – 1979. Initial and routine load tests were carried out at site to confirm the safe vertical as well as lateral load carrying capacity of piles. Integrity testing / low strain dynamic testing were also carried out on randomly selected piles to check the integrity of piling works. Hydraulic operated rotary type piling rig have been used for the piling works. Photo P7 shows the piling work at LP1 location.

Pile caps of minimum thickness 1.8m (i,e 1.5 times the pile diameter) has been provided. Pile caps are designed based on bending theory. Loads on piles are assessed by considering rigid body action of the pile cap.

Circular piers are provided with vertical grooves allround from aesthetic considerations. Base section of pier is designed for ductility with adequate confinement reinforcement. Cantilever type pier caps is provided supporting the superstructure on bearings. Pier cap is designed based on flexure theory for combined bending and torsion for the loads transferred from the deck. Photo P8 shows the concreting of Pier Cap at RP6.

Superstructure
Fabrication and Erection Scheme

The Plate Girders are fabricated in fabrication yard, located at Mundka and brought to site in pieces. Maximum length of individual piece is restricted to 12m and maximum weight of a single plate Girder is restricted to 20 Tonnes. The prefabricated girders are first assembled on ground adjacent to the span in which it is to be erected. Girders were assembled in length as per the approved construction scheme. Erected girders are in lengths of about 45m (for 40m span) and 25m (for 50m span). Shear studs are fixed on top flange. Two cranes of 75 Tonnes capacity each are used to lift the assembled girder in position (Photo P9 & P10). Erected girders are supported on bearings over pier and on temporary cribs at the cantilever overhang. After erection of all the girders in a module, the RCC deck slab is cast on top by taking support from the erected girders (Photo P11).

Structural Modelling of Superstructure and Design Issues

Superstructure is designed for following loads and their combinations

1. Dead Loads & Superimposed Dead Loads
2. Carriageway Live Loads
3. Temperature Gradient Loads (Rise and Fall)
4. Braking & Tractive Effort
5. Bearing Friction
6. Earthquake Loads or Wind Loads
7. Stresses caused by Shrinkage of deck concrete
8. Differential Settlement

For the service stage analysis of Superstructure for superimposed dead loads and live loads, a grillage model is used and the analysis carried out in software STAAD/Pro. The superstructure is modeled using discrete beam elements in orthogonal direction. Full composite action between the deck slab and the girder is assumed. Separate models have been used for live load analysis and superimposed dead load analysis since the modular ratio and section properties of longitudinal members for sustained loads and for instant loads are different (to account for creep). Precamber has been provided in the girder (at splice locations) to account for deflection of permanent loads + 75% of the live load. Live load deflection is restricted to span / 800 as per the provisions of IRC:22. Deck slab is designed based on effective width method.

Design of the Superstructure takes into account the stage by stage construction process for dead load and dead load of deck slab, wherein the statical system keeps changing till all the girders in a module are erected. Design is based on provisions of IRC:22-1986 and IRC:24-2001. Working Stress Approach has been adopted for the design of structural members.

Bearings

The bridge bearings are proprietary item, designed and manufactured by the manufacturer M/S Sanfield (India) Ltd., Bhopal. A warranty for trouble free performance for at least fifteen years and free rectification of defects / replacement, if any, during this period has been obtained from the manufacturer. Design of Bearings conforms to provisions of IRC:83 (Part 3). The types of Bearings used with design vertical and lateral loads are given below:

1. Free POT cum PTFE Bearings : Vertical Load Capacity 230T (8 Nos.)
   
   : Vertical Load Capacity 100T (16 Nos.)
   
   : Vertical Load Capacity 95T (8 Nos.)
2. Sliding Guided Bearings : Vertical Load Capacity 200T & Lateral load capacity 75T (8 Nos.)
   
   : Vertical Load Capacity 100T & Lateral Load Capacity 30T (24 Nos.)
   
   : Vertical Load Capacity 235T & Lateral Load Capacity 125T (8 Nos.)
3. Fixed Bearings : Vertical Load Capacity 230T & Lateral load capacity 125T (8 Nos.)
   
   : Vertical Load Capacity 205T & Lateral load capacity 85T (4 Nos.)

Expansion Joints:

Modular Expansion joints capable of accommodating the structures movement has been provided in the deck. Expansion joints are special type of joints, generally of the proprietary type. The Expansion joints are supplied with 15 years of replacement guarantee. The modular expansion joint system is designed for 40T bogie loading and impact in accordance with IRC:6-2000.

The modular expansion joint system consist of a double layer, box type, preformed elastomeric joint seal mechanically held in place by steel edge and separation beams. Each elastomeric sealing elements are continuous transversely and has movement capacity limited to a maximum 80mm of movement per seal. An independent support bar welded to the center beam individually supports each machined or extruded transverse center beam. These support bars are suspended over the joint opening by sliding elastomeric bearings. The modular expansion joint system provides equidistant control of the elastomeric seals.

Expansion joints with ‘Four’ and ‘Two’ modules have been provided at intermediate expansion joint pier and at Abutment locations respectively. The expansion joints are installed after laying the wearing coat. Steps involved in installation of expansion joints are:

1. Sae-cutting of the Wearing coat to the required width. Block-out to be clean, dry, free from loose particles with deck reinforcement fully exposed.
2. Splicing of individual fabricated EJ segments by welding to achieve continuity and maintain alignment.
3. Insertion of neoprene seal into the edge beam profiles to ensure locking using lubricant adhesive & adjustment of the gaps between edge beams.
4. Levelling of the edge beam assembly and providing necessary formwork to ensure uniformity of expansion gap.
5. Welding of studs anchorages with deck reinforcement & loosening of clamp plates & nuts as required.
6. Covering of the gap between edge beam and central beam by masking tape and concreting of blockout ensuring proper compaction.

Photo P12 shows a typical 4 seal expansion joint being installed at P2 location.

Reinforced Earth Wall for the Solid Fill portion

The solid fill ramp portion of the flyover on either side of the stilted portion is provided with reinforced soil wall panels (RSWP) using galvanized MS strips. The total length of RE wall portion is 306m. 146m length is provided on U.P side while the length towards Delhi side is 160m. Maximum height of wall above ground level is 5.5m. Walls are embedded in ground by 1.0m. The Reinforced Earth wall system is designed as per the provisions of BS:8006-1995 in absence of any specific guidelines in Indian Codes. For seismic design of RE Wall, provisions of AASHTO code (Mononobe-Okabe method) has been followed since BS code is silent on seismic.

The facia panels provided in RCC grade M35. Panels are of size 1.85m (width) x 1.50m (height) with thickness of 180mm. Photo P13 shows the construction of RE wall in progress. The RSWP are anchored at 4 points with galvanized strips. Galvanized chequered steel strips, 50mm x 5mm thick and conforming to IS:2062 have been provided on the backfill, connected to the facia panel. These specially manufactured strips are hot dip galvanized as per IS:4759 having zinc coating of 1000 gm/sq.m as specified in BS:8006-1995. The coating thickness is based on 100 year design life for the galvanizing, considering mild corrosive exposure in backfill. The long-term design strength of galvanized strips is taken as 40 KN/m.

Design & Construction Aspect Of Underpass
Open to Sky Underpass Section

RCC U-type Section proposed for the open to sky portion with depth of excavation less than 3m. This portion is constructed by open excavation method. For the portion where depth of excavation is more than 3m, adequate space is not available in the area for open cut excavation, hence diaphragm wall is provided. The construction scheme in this case involves strutted excavation after constructing the RCC diaphragm wall (800mm thick) on both sides of the underpass.

Soil anchors are provided in the base raft in this zone to counter uplift forces due to buoyancy. The design water table is considered as 1m below ground level for this purpose as per clients advice.

Design of Shallow Depth Portion

Design of the shallow depth portion of Underpass is carried out by modeling the structure in 3D-frame in STAAD/Pro. Due to presence of soil anchors, which imparts high concentrated load on the base raft, the simplified 2D method of analysis was not considered adequate in this case. The support springs at the base is given in the form of ‘soil springs’, at each nodes to represent the stiffness of the soil underneath. The design of open to sky portion caters for the following loads:

1. Dead Loads & SIDL
2. Lateral Earth Pressure (Active)
3. Live Load Surcharge – One side or both side
4. Vehicular Live Load (Class A 2 lane / Class 70R / Class AA )
5. Buoyant forces
   
   

   Photo 15: Temporary Strut with
   Diaphragm Wall & Contiguous Pile
   1. Construction of Diaphragm Wall on both sides with M35 grade concrete (Photo P14).
   2. Excavation upto bottom of base slab in stages with intermediate strutting using waler beams (Photo P15).
   3. Casting of Base Slab, intregated with diaphragm wall leaving pockets for the soil anchoring to be done later.
   4. Casting of road surface/wearing coat, crash barriers over the walls, underside road kerbs …etc.
      
      The following analysis principal has been adopted for this portion of Underpass:
   1. Construction Stage Analysis
   2. Service Stage Analysis i.e. “Wished in place” structure analysis

Design of deeper open to sky portion with Diaphragm Walls

Design of the deeper portion of Underpass involving diaphragm walls is carried out using the top down construction method. Different stages of Construction are as follows:The construction stage analysis is carried out using standard software “Wallap”. The diaphragm wall is analyzed for earth pressure using “Wallap” for different stages of construction. The effect of temporary strut at intermediate level as well as effect of bottom slab is considered by adopting concrete strut and moment restraint at respective locations.

Service stage analysis is carried out using software “STAAD Pro.” All the forces have been applied on the frame model. On the active side, net pressure applied while on the passive side, the supports are idealized as springs with stiffness taken based on the soil characteristics.

Covered portion of Underpass

800mm thick diaphragm wall with M40 grade concrete have been provided in covered portion of Underpass. Depth of diaphragm wall varies from 8m to 14m in panels. Panel size is kept as 5m, interlinked with water stopper. The total length of diaphragm wall is 2060m in this project and total number of panels are 412 in both underpasses. The design of open to sky portion caters for the following loads:


6. Dead Loads
7. Earthfill on top
8. Lateral Earth Pressure (At rest)
9. Live Load Surcharge – One side or both side
10. Vehicular Live Loads as per IRC:6 on top of slab as well as at base
11. Buoyant forces

Covered portion of Underpass is constructed in following steps:

1. Construction of Diaphragm Wall on both sides with M40 grade concrete.
2. Excavation upto bottom of top slab.
3. Casting of top slab, integrated with the diaphragm wall. Traffic is allowed over the top slab when the concrete gains strength.
4. Excavate from below the top slab upto the bottom of base slab.
5. Casting of Base Slab, intregated with diaphragm wall.
6. Casting of road surface/wearing coat, crash barriers over the walls, underside road kerbs …etc.

The analysis principles are same as explained in case of open to sky portion.

Horizontal & Vertical Prestressed Soil Anchors

This is perhaps the only project in India where prestressed anchors have been used both in vertical as well as horizontal alignment. For providing vertical and horizontal anchors, PWD has engaged a specialist agency (M/S Tech9 Engineering Solutions Pvt. Ltd.) for complete technical support in design and execution.

Vertical Soil Anchors

Vertical Soil anchors of safe tensile load capacity of 40 tonnes each have been provided on either side of the underpass raft in ‘open to sky’ portion to cater for the upward water thrust, which can arise due to high water table in the area. A total of 776 numbers of vertical prestressed soil anchors, 17m in length (10m free length, 7m fixed length) have been used in this project. Longitudinal spacing of soil anchors varies from 4m to 1.4m depending upon the depth of base raft of Underpass from GL.

The various steps involved in the soil anchoring are:

STEP 1: Cutting of 15.2mm diameter 7-ply class II strands strands to required length.

STEP 2: Corrosion protection in free and fixed length

STEP 3: Applying Bond Breaker and internal grout vent fixing.

STEP 4: Drilling with TG 20 rig machine (Photo P16).

STEP 5: Homing of anchor and grouting simultaneously during extraction of casing pipe.

STEP 6: Allowing the grout to set.

STEP 7: Stressing of anchor to required load and locking - grouting of anchor pit.

The anchors are installed in a drilled hole of diameter 200mm. The drilling of the hole is carried out using temporary casing for the full depth, which is removed in stages after completion of grouting. For the design of soil anchors, the factor of safety for bond length between grout and soil is kept as 3.0 while the factor of safety for tensile stress in strand is kept as 2.0. Each soil anchor comprised of 3 Nos. of 15.2mm diameter 7-ply class II strands conforming to IS:14268 (LRPC).

The free length of the anchors is encased in plain HDPE pipe of 125mm OD. Fixed length of the anchor is encapsulated in corrugated HDPE pipe of 125mm OD. The length of fixed portion is determined based on the requirement of bond length between grout and soil or between grout and strands, whichever is higher.

The HT strands in the free length is covered by flexible HDPE tube of 20/22mm ID as a double protection measure. The thickness of the tube is kept as 2.5mm thick. The annular space between strands and the HDPE tube is filled with grease. The greased HDPE pipes encasing the strands are further encased in 125mm dia plain HDPE pipe in the free length portion, which is cement grouted. The portion outside this HDPE pipe and 200mm diameter bore hole is also cement grouted, which gives 3rd level of protection to the strands against corrosion.

The treatment of the HT strands in free length includes cleaning followed by application of a coat of primer of minimum 40 micron DFT. As soon as the primer coat dries up, three coats of epoxy based paint is applied sequentially. For the portion of strands in the fixed length portion, the HT strands are first pre-treated by thoroughly cleaning using thinner. First coat of epoxy formulation is uniformly applied on the strand and it is allowed to dry for a period of 2 to 3 hours. The second coat is applied thereafter and is allowed to dry for 24 hours. The surface is next made rough by manually rubbing the top surface with sand paper and the third coat of epoxy based paint is applied uniformly. While third coat is still tacky, quartz sand is sprinkled over it to increase the bond.

Fig. 19 shows the schematic details of Vertical Soil Anchor adopted

Horizontal Soil Anchors

The horizontal soil anchor system has been adopted in the existing ROB to hold the contiguous piles installed on either side of existing ROB together. Steps involved in the horizontal soil anchoring are :

STEP 1: Drilling horizontally from both sides.

STEP 2: Fabrication of horizontal anchors

STEP 3: Installation of anchors into drilled holes.

STEP 4: Waler beam erection on either side of the approach carriageway.

STEP 5: Stressing of anchors simultaneously from both ends.

STEP 6: Grouting of anchors.

Photo P17 shows the completed Underpass, RHS side.

Conclusion

Construction of grade separator at Apsara Border was a daunting task, which was accomplished with exemplary quality of workmanship and team effort. The project not only involved constructing a 6 lane flyover in one of the busiest intersection in Delhi at Seemapuri Border, of NCR but it also involved conceptualization, planning and execution of 2 underpasses in an extremely challenging working conditions with restricted space between existing ROB with 10m high embankment on one side and a nallah on the other side. To add to the complexity of the problem was the challenging task of railway box pushing between the two existing structures with very limited space in between. The challenge posed brought out number of innovative solutions, both in design as well as in execution, which had never been tried before. Credit for successful completion of this project goes to the excellent team work and understanding between the Client (PWD), Proof Consultant, Contractor & the Design Consultant.

Quantities of Major Items in this project

1. Cement : 35,034 MT
2. Reinforcement : 9277 MT
3. Structural Steel, Superstructure : 1811 MT
4. Structural Steel, Waler Beam : 430 MT
5. Concrete : 92,500 cu.m
6. Bitumen : 857 MT

Acknowledgments

The author wish to place on records his appreciation for the cooperation received from the authorities of Delhi PWD (NCTD) during the entire duration of this project and also in writing this paper. The cooperation extended by Shri U C Mishra (Project Manager, PWD), Shri Kailash Narain (EE,PWD) are noteworthy. Author is also grateful to the unsung heroes from the Proof Consultant, Contractors as well as from PWD, whose deep involvement and untiring efforts has helped to complete such a complex project in reasonable time.

Credits

• Client: Public Works Department, NCT Delhi
• Proof Consultant: M/S B&S Engineering Consultants Pvt. Ltd.
• Contractor: M/S AFCONS Infrastructure Limited, Mumbai
• Design Consultant: M/S Crafts Consultants (I) Pvt. Ltd.
• Quality Assurance: Delhi Technological University (Formerly Delhi College of Engineering)

Mr Alok Bhowmick is the Managing Director of one of the reputed Structural Engineering firm, namely B&S Engineering Consultants Pvt. Ltd., Noida. The highlights of his carrier of 30 years include designing bridges, flyovers, Underpasses, Aqueducts, Industrial structures and other structural engineering works. His experience has been gained mostly working in various consultancy organizations. He is an active member of several technical committees of Indian Roads Congress (B-1 : General Features of Design Committee, B-2 : Loads & Stress Committee & B-4 : Reinforced, Prestressed and Composite Committee). He has been given the responsibility by IRC to draft the Explanatory Handbook and Commentary on Limit State code for Bridges. He is also a member of National Advisory Committee, National Information Centre for Earthquake Engineering (NICEE).