Gautrain Viaduct V5c INTRODUCTION The Gautrain Project is a dedicated heavy-rail line which supports the op- eration of a fast transit system capable of operating at speeds of 160 km/h. The project is divided into a series of refer- ence sections and design packages. The Bombela Civils Joint Venture (BCJV) is the contractor responsible for the design and construction of the civil in- frastructure associated with the project. Aurecon was awarded Detail Design Package 6A (DD6A). The main distin- guishing aspect of this design package is Viaduct V5c. Viaduct V5c, the single longest via- 1 duct on the project, is located on the route through Centurion, south of Pretoria. The route through the Centurion CBD is fairly congested with existing infrastruc- ture, and situated on complex dolomite GAUTRAIN VIADUCT V5c geology. The elevated Centurion Station platforms were constructed on top of five spans of this viaduct. COMMENDATION – Technical Excellence category Unique engineering design and innovative solutions were required to overcome construction challenges as- KEY PLAYERS sociated with Viaduct V5c. These solu- Client Gauteng Provincial Government tions distinguish this viaduct from the Professional Team Bombela Civils Joint Venture with Aurecon as the designer others on the Gautrain project, even Main Contractors Bombela Civils Joint Venture though it has a similar appearance to Major Subcontractors and Suppliers the standardised viaducts. These unique Dura Soletanche Bachy Geomechanic Africa JV, Franki, VSL challenges are:

12 Civil Engineering | December 2010 NNthe design and construction of founda- other consultants). The track level varies supported on two bearings at each pier, tions on a geotechnical strata that is from approximately 7 m to 22 m above which prevent transverse movement. well-known for the sinkhole risk associ- natural ground level. The longitudinal The simply-supported spans are longi- ated with the dolomite bedrock, and grade varies from 0,5% to 4%. The hori- tudinally fixed at one pier and longitudi- NNthe fact that the majority of the pier zontal alignment starts on a left hand nally free at the other. foundations are not founded on bed- curve with a radius of 1 170 m leading rock, which results in possible settle- to Centurion Station, where the track is Substructure ment and/or movement of the structure straight, followed by a right-hand curve Viaduct V5c is supported on 66 rein- – this has a specific influence on the with a radius of 1 600 m thereafter. forced concrete piers. Each pier com- structural design of both the sub- and prises a 3 m x 4 m rectangular hollow superstructure. Superstructure stem with 0,3 m thick walls and a solid The deck comprises a series of simply pier head and base to suit the founding DESCRIPTION OF STRUCTURE supported spans of 44 m, 50 m and 56 m solution. The pier heights vary between General in length. Span lengths are arranged to 6,75 m and 23 m, which is achieved by Viaduct V5c is a 3,2 km long match- optimise the structural efficiency and construction of a standard pier head cast segmental constructed box girder to avoid the obstacles of infrastructure (3 250 mm), a number of standard bridge which consists of 67 simply sup- encountered along the route. construction lifts (4 000 mm) and a ported spans. Five spans of the Viaduct The deck is a prestressed (internal correction construction lift (which also accommodate the elevated station and external post-tensioning), trape- varies from 2 000 mm to 6 000 mm in platforms of the Centurion Station. zoidal concrete box with side cantilevers. 500 mm increments). The Viaduct extends from the N1 at The deck is constructed as a series of the John Vorster interchange to the precast segments temporarily supported 1 Location of Viaduct V5c in Centurion N14 Jean Avenue interchange. At these by a launching girder or gantry. The 2 Viaduct segment interchanges, the Viaduct joins Viaduct Viaduct segments are typically 10,1 m 3 Typical pier V5b and V5d (which were balanced wide and 3,5 m deep, with lengths 4 Struts supporting the elevated cantilever constructions and designed by varying from 2,1 m to 2,8 m. Each deck is Centurion Station

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Civil Engineering | December 2010 13 The standardisation of the pier head Foundation types as the transportation of the 40 t seg- and standard 4 000 mm lifts allowed rein- Four different foundation types were used ments to the span erection site. forcement cages to be prefabricated. This to support Viaduct V5c, mainly due to NNErection of the under-slung gantry on was advantageous considering the tight construction restraints and the complex pre-fitted pier brackets. This gantry construction programme. nature of the geology associated with the was used as a launching girder at Viaduct. A comprehensive discussion will each span. Centurion Station follow further down with regard to the NNThe launching girder was loaded with The majority of the Centurion Station geological conditions, geotechnical design the required number of (matching) buildings (ticket offices and foyer) are and foundation types. segments and the joints were then located underneath the viaduct with prepared with an epoxy. Epoxy elevators providing access to the station Existing infrastructure and services between joints is primarily for the platforms above. The standard viaduct The major part of the Viaduct V5c route corrosion protection of the internal segments were modified by the addition is urbanised and congested with existing prestressing strands and does not of external propped extensions (precast infrastructure. A large number of existing have a structural function. Segments struts) to the cantilevers. These provide services had to be moved or relocated to are temporarily prestressed together support to the precast station platforms, enable works for the viaduct construction. so that 1 MPa stress is achieved at parapets and station canopy. Precast These services included telecommunica- each segment joint. struts are connected to the modified tion, bulk water and sewerage, power NNAs per the design, the first stage perma- segment using a combination of trans- supply, road infrastructure and buildings, nent external and/or internal tendons verse prestressing and a lower steel- to name but a few. needed to be installed and stressed. pinned connection. During this operation, the loading The precast platform panels com- CONSTRUCTION METHODS of the superstructure systematically prise a combination of panels fixed The superstructure for Viaduct V5c shifted from the launching girder to the (in-situ stitch) to the struts, and fill-in was constructed using the match-cast temporary bearings. or drop-in panels between fixed panels. segmental construction method. It is NNAt final load transfer to the bearings, These platform panels are not only widely used for the construction of long the launching girder was moved for- designed for the associated commuter viaducts in other parts of the world. The ward to the next span position. loading, but also accommodate the Gautrain Project was the first project to NNAfter the launching girder had been canopy supports and parapets. implement this method of construction moved, the span was finished to comply in South Africa. with the additional stressing require- 5 Launching of girder This method was considered eco- ment, deck furniture and ultimately, the 6 Placement of segment on launching girder nomically viable on this project because ballast and deck. there are a number of viaducts on the 5 project where the launching girder DESIGN CHALLENGES could be used. The initial costs as- General sociated with the expensive launching Aurecon’s client required an optimum girder require a substantial length design within the constraints of the ap- of viaduct to ensure feasibility of the plicable codes. A consultation process method. Furthermore, this method for the optimisation of all design of construction is conducive to an in- elements was followed, which included creased construction rate: various design iterations and liaison NNThe precast deck segments allowed with the construction team and other construction of the superstructure to sub-consultants on the project. This commence before the actual comple- process was not necessarily unique tion of the piers. to the general design of Viaduct V5c. NNThe repetitive nature of the precast However, the unique challenge was the activities and span erection facilitated foundations for the optimisation of the higher quality of workmanship and 66 piers, each with different subsoil increased production rates. conditions on the dolomite strata, and NNThe use of a launching girder mini- the effect of the foundations on the en- mised disruption to the movement of tire viaduct structure. 6 traffic below the construction works in the highly urbanised environment of Design criteria and loading Centurion. Viaduct V5c is designed according to Briefly, the segmental construction the Eurocode, which was considered process entailed the following: more suitable for the precast segmental NNConstruction of foundations and sub- construction than the traditional structure at each pier position. bridge design code used in South Africa NNMatching the casting of precast seg- (TMH7). The Eurocode has a number ments at a remote precast yard, as well of advantages and disadvantages. The

14 Civil Engineering | December 2010 advantage is that it deals with a broader dolomite bedrock the highly weathered adequate full-scale settlement data did variety of train loadings, realistic load overburden, or wad, may only have stiff- not exist and the need for a full-scale combinations and gives more freedom ness in the order of 5 to 10 MPa. load test was recommended. Although to the designer to base the design on The settlement and deflection pre- a full-scale load test to determine the first principles. dictions of the substructure were of the possible settlement is not normally However, a disadvantage of this essence to ensure that track standards are done, it had two major advantages for code is that it is dependent on National maintained at all times. the BCJV: Annexes for design particulars specific to NNTesting the expected settlements local conditions. Geotechnical investigations and measures enabled them to calibrate settlement The initial stages of the design Extensive geotechnical investigations calculations. process involved the establishment of were conducted at each pier position, NNIt enabled the preloading of sub-soil design criteria for the design of the via- which typically included: material to improve material properties ducts on the project. Standardisations NNPreliminary investigation comprising and reduce predicted settlements. of the design process across all viaducts rotary percussion boreholes with Jean- A further full-scale test was carried out played an important part in the finan- Lutz parameter recording, as well as at Pier P26 (where the pier was loaded cial feasibility of the segmental con- borehole radar to establish voidedness with the full expected loading of the struction. The design criteria captured or occurrence of boulders and test pits. completed viaduct and train) after the main features of the applicable NNSpecialised investigations comprising completion of the pile cap to verify the Eurocode and provided the ‘missing’ core drilling and pressure meter testing settlement calculations. details that would normally have been in non-dolomitic layers. These foundations comprise a provided in the National Annexure. NNFull-scale loading to verify and calibrate 12 m x 12 m raft or pile cap, as well as soil stiffness assumptions. This com- 20 number 600 mm diameter percussion Geotechnical design prised the placement of 2 m x 2 m x 1 m drilled piles. The piles were either 10 m Background concrete blocks on a 20 m x 20 m area or 15 m in length, and the majority of The route of the Gautrain is situated and 10 m height. piers are founded on this type of founda- on the extensively weathered Transvaal NNAssessment of grouting boreholes to tion. As the foundation is not founded dolomites (Monté Christo Formation) verify the original design assumptions directly on rock, they are considered to which are approximately 220 million of bedrock variation and the occurrence be ‘floating’. The sinkhole load case was years old. A characteristic of these of floaters. mitigated with compaction grout injec- dolomite formations is that they contain tion or void filling (20 m x 20 m area) at layers of chert and impurities, such as Foundation design the pier locations. The objective of this very light and highly compressible wad Although a number of foundation solu- procedure was to fill existing voids in the (an acronym derived from weathered tions were used for Viaduct V5c, two bedrock and overburden. altered dolomite). These layers have a specific aspects had to be addressed or The design is therefore based on major impact on the engineering behav- mitigated as part of the design process the assumption that a sinkhole will iour of the stratum. for each pier location. These were the not form in the void filled area and the Over geological time, the dis- effects of an extreme sinkhole on all the 20 m x 20 m block will be stable should solution of calcium carbonate rock structural elements and the settlement/ a sinkhole form next to this area. BCJV by small amounts of carbonic acid in deflections of the substructure. verified this assumption with finite the groundwater gives rise to complex element modelling and calculations by three-dimensional passages and caves Floating/raft foundations (total of 46) other consultants. within the bedrock. Over the design life The preliminary design for the Viaduct Settlement results for the full-scale of the structure, these caves and pas- was done by other consultants in the loading at each pier were analysed. sages may result in the development of beginning of 2006, which only included Depending on these results and the sinkholes and/or dolines. A sinkhole is piles to rock or shaft foundations. The borehole logs, either rafts or piled a large hole with vertical slopes opening above-mentioned geological composi- rafts (with 10 m or 15 m piles) were at the surface, while a doline is a local tion makes the installation of piles to implemented. Piles were introduced to area settlement of the soil. rock or spread footings on rock ex- the raft solution to reduce settlements Furthermore, the bedrock profile is tremely difficult and expensive. where the raft foundation was insuf- highly undulating, resulting in extreme Aurecon initially proposed a ficient. Pier 26 was test-loaded to the ac- variations with regard to the overburden ‘floating foundation’ solution (piled tual working load of the viaduct which thickness and deep valleys between raft or raft foundation not founded on confirmed the assumptions. bedrock pinnacles. Variations of 30 m or rock). This proposal was then further more were encountered over distances of developed and refined in conjunction Piles to rock (total of 7) 3 m during the ground investigation for with BCJV. At a number of locations, the subsoil con- Viaduct V5c. The possibility of settlement of ditions limited access for equipment (next The dolomite bedrock may be clas- a floating foundation on dolomite to some of the main access roads) and sified as extremely hard rock (uni-axial bedrock and the subsequent effect on availability of construction equipment compressive strengths of 300 MPa or the rail tolerances required accurate made the installation of piles to rock more more). In contrast to the extremely hard settlement predictions. However, the suitable than a raft foundation.

16 Civil Engineering | December 2010 Large diameter pile foundations com- of the unique founding solutions, the prised four or six number 1,3 m diameter design of Viaduct V5c was influenced piles (typically 25 m long), underneath by the soil-structure interaction and a 7,2 m x 7,2 m pile cap. The extreme the rail-structure interaction. The fact sinkhole risk and deflection aspects that the subsoil conditions necessitated were considered as part of the conven- the implementation of mainly floating tional design of the pile foundations. It foundations and the installation of a is important to note that the sinkhole continuous welded rail on top of the load cases were considered critical and structure, affected the overall design resulted in a substantial increase in rein- approach and methodology. forcement quantities in the piles. The floating/raft foundations resulted in much larger structural Shaft foundations (total of 7) movements than piers founded on Shaft foundations were only installed rock. These structural movements where the founding depth was above have a direct influence on the stresses the water table, access restraints or in the structure as well as in the rail. limited space existed but construction The behaviour of the complete system, equipment was available. The shaft including the foundation conditions, foundation comprises a 7 m diameter under longitudinal traction/braking shaft with 500 mm thick walls, filled and temperature effects, is essential to with compacted soil. Again, the extreme the overall structural performance of sinkhole risk and deflection aspects the viaduct. were considered as part of the conven- The effect of the extreme sinkhole tional design of the shaft foundations. event on the piers founded on rock is The shaft foundation is considered a significant to the structural perform- very effective solution to mitigate the ance on the foundation and the overall effect of a sinkhole on the structure. integrity of the viaduct. These piers and associated foundations were therefore Spreadfootings (total of 6) designed to accommodate the extreme Spreadfootings were considered feasible sinkhole event without jeopardising the at locations with overburden less than structural preference of the viaduct. 5 m and a fairly horizontal or predictable Aurecon’s detail design team analysed bedrock profile. Typically, these footings these complicated effects which re- were 6 m x 6 m square founded directly sulted in an optimum solution that on the bedrock. complies with the design codes and satisfied BCJV’s requirements. Structural design The effect of the extreme sinkhole General event on the piers founded on The structural design of the typical via- CONCLUSION duct included: The Gautrain project plays an important rock is significant to the structural N NDesign of a conventional post-tensioned part in the economic development of performance on the foundation concrete deck and the reinforced con- the Gauteng Province. The route of this crete piers. rapid-rail project passes through the con- and the overall integrity of the NNGeneration of segment catalogues as gested Centurion urban area with existing shop documents for the construction infrastructure and is situated on complex viaduct. These piers and associated of the superstructure. These cata- dolomite geology. foundations were therefore designed logues included all the relevant details Although a number of similar pertaining to the span- and segment viaducts exist on the project, Viaduct to accommodate the extreme geometry, inserts, prestressing and re- V5c is the single longest viaduct on sinkhole event without jeopardising inforcement details. the project. The viaduct has a unique NNSpecial provisions for the segmental design and overcame exceptional con- the structural preference of the construction method and other struction challenges. These included viaduct. Aurecon’s detail design construction related inserts, which the design and construction of founda- included the accommodation of tem- tions on a geotechnical stratum that is team analysed these complicated porary brackets on the piers to accom- well-known for the sinkhole risk associ- effects which resulted in an modate the launching girder. ated with the dolomite bedrock, as well as the majority of the pier foundations optimum solution that complies Unique Viaduct V5c design that were not founded on bedrock, with the design codes and In addition to the general viaduct resulting in the provision for possible design requirements and as a result movement of the structure. satisfied BCJV’s requirements

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