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Structures

Abstract

This paper presents the building challenges and final design of the Cliffsend Underpass being built under the railway near Ramsgate in south east England. Network Rail are particularly sensitive about this line as it carries the new Javelin trains which service the high speed link to St Pancras International. The challenge was therefore not only to devise a scheme which met the Client, Kent County Council’s requirements, but also to develop a technique which minimises ground settlement.

The underpass is part of the new East Kent Access Road. It is 23m wide and is 125m long. For a structure of this magnitude and subject to the rail constraints highlighted above, a special box jacking concept has been developed. The jacking proposal belongs to a group of concepts based on jacking and skidding. IABSE Working Group 6 on bridge deck installation is studying the safe use and benefits of these techniques.

The design and construction of Cliffsend Underpass

Cliffsend Underpass see Figure 1, is the major design and build element of the East Kent Access Phase 2 contract, promoted and funded by Kent County Council (KCC) and, in part, by the Department of Transport. The contract was awarded to the VolkerFitzpatrick HOCHTIEF JV in the summer of 2009. The structure provides a route for the new dual carriageway, which sits in a 15m deep cutting either side of the railway. The proximity of local properties on both sides of the railway meant that an over-bridge solution is not viable. The overall geometry and physical location of the new structure is further constrained by significant existing statutory undertaker apparatus, under a minor road and level crossing above.

The resulting structure presented for design and build development comprised a 126m long, 23m wide and 7m high twin-cell structure. The deep approach cuttings would be formed of large diameter contiguous piled approach walls up to 15m deep, on either side. The upper deck level was dictated by the location of a large diameter sewer pipe above resulting in a 6m overburden from the top of the box deck up to the railway above.

Design

The challengeThe challenge for the designers was to develop a structural solution which would allow the jacking of manageable sections

of the structure whilst having no impact on the operation of the railway above. The new road layout and high skew of the railway makes this underpass one of the longest box jacked structures in the world.

Preliminary designs considered a conventional approach, utilising a pair of large box sections jacked into position. The major challenge of this proposal was the length and, ultimately, the required jacking capacity and control during installation. In addition, the extent of open face that would have resulted from these large box sections gave cause for concern amongst the geotechnical consultants, due to the likely extent of ground and associated railway track settlement that would have resulted.

It became apparent that the scale of the whole structure required that a different approach is considered.

Site investigation showed the main geological strata to be a lightly weathered chalk with few flints or boulders, overlain with up to 6m of (mainly) Thanet Sand strata. On this basis it was anticipated that the majority of excavation would be within the chalk, with Thanet Sand encountered over a short central length of the underpass in the top 1m of the face. Analysis suggested that the excavation slope at the shield face would be stable, without the need for ground treatment.

Introduction

Mark PritchardCivil Engineer

Hochtief (UK) Construction Swindon, England

Tomasz KuckiSenior Engineer

Atkins

Jan de BoerCivil Engineer

DeBoerDC / Mammoet UK

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Figure 1. Cliffsend Underpass under construction

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The technical solutionThe Tender scheme, developed in conjunction with specialist box jacking consultants (JSG SA, from Geneva) utilised multiple jacked boxes to form abutments, see Figure 2. These abutments incorporated a slide path along which the main deck sections would be jacked into position.

After contract award, a series of design review meetings were held between designers, contractors and specialist jacking consultants, to further improve on this concept.

The final solution, see Figure 2 – presented to and accepted by the Client – involves the construction of a pair of 3.05m internal diameter temporary tunnels, bored 8.50m below the railway along the lines of the underpass side walls. From within these two tunnels, mini piling rigs are used to construct rows of closely spaced reinforced concrete piles taken through the base of the tunnel and founded in the underlying chalk strata. These piles are ultimately exposed and faced to form the lower sections of side walls.

Construction sequence

Pilot tunnels and retaining wallsAn earth pressure balance tunnel boring machine was employed to drive two 126m long temporary tunnels, see Figure 3. Track movement was continuously monitored by a set of robotic total stations detecting a total vertical settlement of less than 3mm. The tunnels were completed, within an 8 week programme, in December 2010 without any disruption to train operations or the need to apply any Temporary Speed Restriction (TSR).

From within the tunnels 530 No, 450mm diameter, bored cast in situ piles have been constructed. The piles are 14m long and reinforced for the top 8m. They

were constructed using Klemm KR 702 limited height rigs. At peak productivity 4 No. piles were installed per rig per shift. Piling was completed in April 2011. Then the reinforced concrete pile caps were constructed within the tunnels, following which a low-friction slide track was set onto the pile cap, installed to very high geometrical tolerances.

Deck unitsThe underpass roof, or deck, sections comprise 6 abutting units, each 23m wide, 22m long (max) and 1.80m deep heavily reinforced concrete slabs with end down stand sections. These are being cast on site 150m west of their final location, see Figure 4. Each section of deck will be jacked up to the underpass portal and engaged at the front end onto the slide tracks extended from within the tunnels (as described previously). The deck sections are being constructed using a single reusable shutter. RMD formwork is being used to support the deck sections during concreting. The internal shutters to the legs are hinged for striking while the outside shutters are removable to allow fixing of reinforcement.

Each deck unit contains 1040m3 of concrete and 240 tonnes of reinforcement. The majority of the reinforcement is in a double layer of 50mm diameter high yield bars in the bottom of the slab. The high reinforcement density is a result of the

loads applied from 6m of overburden. 80% (by weight) of the reinforcement has been designed to allow prefabrication ‘as beams’ which are lifted into place by the 20 tonnes twin hook gantry crane which services the casting yard, see Figure 4.

Box jacking

Tunnelling shieldThe front, or lead, deck unit incorporates a mining shield and cutting edge designed to provide intimate support and protection at the excavation face. The shield, see Figure 5 is divided into 8 cells, each of which will contain excavation plant and miners. The shield is added to the front unit once it had been jacked into the storage area. The sheets of the anti-drag system are dispensed through slots in the front of the mining shield.

Figure 2. Cross-section of the Tender and post Tender solution

Figure 3. Western headwall with entrance of the pilot tunnels

Figure 4. Casting facilities deck units

Figure 5. Impression of the Tunnelling Shield

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Structures

Jacking arrangements

The deck units will be jacked under the railway using a total jacking force of 35,000t divided over 5 jacking stations, each with a capacity of 7,000t, see Figure 6.

The most important is the Rear Jacking Station (RJS), which consists of two groups of six 600t cylinders with a stroke of 2.20m, see Figure 7. Together with packers the RJS has an overall stroke of 22m, which reflects the maximum length of a single deck section. The RJS is first used to bring the lead unit into the embankment. It is then retracted to create space for the installation of the next deck section. This operation is repeated five times. Intermediate Jacking Stations (IJS) are located in the joints between the deck sections. Each IJS has a capacity of 7,000t and a stroke of 100mm. The front IJS has a stroke of 300mm.

Forward progression follows a caterpillar movement principle. The lead unit moves forward first, is followed by the next sections; the last section is moved forward by the RJS.

All jacking stations are powered by a central PPU with a flow of 72litre /minute at 450 bar. The measurement is made using digital stroke sensors. The theoretical forward movement speed varies between 370mm / hour and 140mm / hour, dependent on the number of operative IJSs.

The jacking pitThe RJS is installed in a jacking pit that acts as an abutment for the RJS. In the jacking pit, each deck section is transferred from the sliding system used in and out of the prefabrication yard onto the main sliding track system which is taken through the pilot tunnels. Resistance to the rear jacking loads is provided partly by ground anchoring of

the slide path in the jacking pit area and also by mobilisation of the tunnel / ground interface friction. The ground anchoring comprises 28 25m long multi strand systems, installed at 30º to the vertical and pre-stressed to a working load of 1220kN, see Figure 8.

Slide tracks and bearing systemsMammoet are providing two separate sliding systems.

The first one is used for the transport of the six 2,500t heavy deck sections from the casting to the jacking pit area, see Figure 9.

This system consists of hydraulically interconnected short stroke vertical jacks supported on PTFE pads running on a stainless steel slide track. The track has been laid to a tolerance of ± 2mm. Variation of load in the PTFE pads, as a consequence of varying track levels, is accommodated in the hydraulics which ensure that load in each jack remains constant at all times.

Figure 6. Long section with jacking stations 5 x 7,000t

111 The design and construction of Cliffsend Underpass

Figure 7. Rear jacking station with 12 cylinders 600t

Figure 8. Thrust block with ground anchors

Figure 9. 3.200t sliding system on prefab yard

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The second system operates from the jacking pit through the temporary tunnels. Here the loads are much higher as a consequence of the 6m of overburden. The total vertical load applied to this main sliding system is more than 50,000t. The system here consists of 41mm thick elastomeric bearings fixed to the underside of the deck legs. A layer of PTFE is bonded to the bottom face of the bearing. This slides on a stainless steel track on the tunnel pile caps. The only flexibility in the system is in the bearing and this is limited to approximately 2mm within the working load range of the bearing. To ensure that loads remain within design tolerance the bearing face and slide track are to be set to a tolerance of ± 0.5mm.

The first system is 175m long, and the second one is 155m, with an overlapping section of 25m in the jacking pit.

Anti-drag systemTo prevent migration of overburden material in the direction of deck movement and to reduce the friction between the deck unit and the overlying ground during jacking, there is an arrangement of continuous steel sheets that lie on the top of the deck units and are anchored to the west headwall, see Figure 10. The sheets will be dispensed through slots in the front of the mining shield from rolls suspended on the soffit of the front deck unit. Because the sheets are anchored to the west headwall, they will remain stationary relative to the overburden as the deck moves forward. This will reduce the risk of horizontal movement of the rail embankment and ensure that most of the frictional forces are generated at the interface of the top of the deck units and the anti-drag sheets. This interface will be lubricated by the injection of bentonite or high viscosity gels through grouting manifolds cast into each of the deck units.

OperationsThe process of jacking (see Figures 11 and 12) through the embankment can be divided into several activities taking place simultaneously:

• Mining and excavation at the shield: the success of the whole project depends largely on the accuracy of the mining taking place. The mining determines the speed of progress, settlement of the road / railway above and the alignment of the deck.

• Jacking of the deck units: the operators activate the hydraulic systems to achieve forward movement and to correct possible misalignments.

• Anti-drag devices and lubrication of the sliding contact surfaces: this not only reduces friction but also guarantees stability of the embankment.

• Surveying and monitoring: box jacking requires an attentive reaction to the slightest signal of a possible misalignment or deformation of the overgoing road. Failure to treat immediately will result in the process going out of control.

At Cliffsend the jacking process is programmed as a 24 hours a day, 7 days a week operation. After one deck section of 22m has been installed the RJS has to be temporarily removed in order to position a new deck section in the jacking pit. This operation will take 2½ days. In total the jacking of the six sections of this 126m long underpass will last approximately 10 weeks.

ConclusionsBox jacking is a well tested technique for building tunnels and underpasses with minimal disruption to existing road and rail infrastructure.

At Cliffsend we have devised a scheme to cater for a project of great magnitude. Because of the length of the underpass (126m) and the capacity of the applied pushing equipment (35,000t) this is believed to be the most impressive example of jacking within the last ten years.

By designing a structure of these dimensions with only a roof structure and slide tracks in bored pilot tunnels, we have responded to the challenge to build Taller, Longer, Lighter structures as set by th IABSE-IASS London 2011 symposium where this paper was published.

The jacking operation has been successfully completed on 24/08/2011. The installation of all six deck units has taken 64 days and over 4000 man-hours.

Figure 10. Anti-drag system with anchored steel sheets

Figure 11. Rear jacking station during installation of the first deck unit operations

Figure 12. Inside the underpass during the jacking operation