In SAICE Civil Engineering, April 2016.
The first phase of the upgrade of the N2 Freeway Section 26 saw extensive geotechnical works. A feature of this phase of the project included the widening of the dual uMdloti River viaducts. The viaducts are founded on 45m deep piles that required subcontractor KellerFranki to import a massive Bauer BG28 rig for the project. The project also included over 2km of fabric reinforced Mechanically Stabilised Earth Walls and 2km of cut retaining walls. This article discusses some of the more interesting facets of the geotechnical components of the project.
FOUNDATION ENGINEERING
The freeway expansion project involves the widening of the N2 from eMdloti Interchange to Tongaat Plaza, for the South African National Roads Agency SOC Ltd (SANRAL). The approximately 10km section has been widened by adding two lanes to each carriageway at a cost of R390 million. The project forms part of SANRAL’s strategic development of the N2 north coast route which has become necessary due to increased traffic volumes.
GEOTECHNICAL INVESTIGATIONS
Although investigations for this phase of the project were limited, it is necessary to recognise that there are two obvious differences between an investigation of a project which comprises the upgrading of existing infrastructure and that of a greenfield development:
- Firstly, there is potentially a wealth of information comprising previous investigations, as-built drawings and reports which can be studied to develop an understanding of ground conditions and the design of the existing infrastructure, and
- Secondly, there is the performance history, sometimes formalised through maintenance records, or simply by observations, which can be useful in understanding how the infrastructure has behaved given its design and the prevailing ground conditions.
The process of obtaining, sorting and studying this information can be cumbersome and implies a significant amount of work to be done during the planning stages of the project, in advance of any investigative site works.
However, depending on the quality of the information the planned investigative works can be curtailed. Additionally, that which is done can be focused on particular geotechnical problems or project optimisations. The geotechnical engineer would thus need to target the investigations on aspects where:
- There are significant changes to the infrastructure and/or imposed loads;
- New or alternative construction methods can be considered;
- Less conservative designs can be adopted;
- The impact of new construction adjacent to existing infrastructure needs to be considered;
- Information obtained from historic records is unclear or inadequate;
- Performance history of existing infrastructure is poor.
UMDLOTI VIADUCTS FOUNDATIONS
A significant feature of this phase of the project was the widening of the 281.5 metre long dual viaducts over the uMdloti River using the incremental launch method (ILM). The structural design included a deeper deck section with a continuous cast in-situ stitch to the existing structure. This implied that the new widened deck attracted more load and was more sensitive to settlement, which needed to be accommodated in the foundation design.
The foundation design was further complicated by challenging geological conditions. The uMdloti River is characterised by the presence of a deeply incised paleo-channel which has since filled with various sediments after the recovery of sea-levels after the last Ice Age. Factors such as the nature of the material in suspension and the velocity of the river, all contribute to the type of material that was deposited. Materials encountered, including running sands; boulders and Hippo Muds; are typically highly variable and very weak and can present a challenge to piling.
Foundations for the viaducts comprised 4 x 900mm diameter permanently cased Screwed-In-Casing-Augered Piles (SICAP). The SICAPs were well suited to this project and provided a number of specific benefits. Depths up to 45m were achieved using the oscillator attachment; installation was possible under high water table conditions and the pile was suited to the difficult soil profiles encountered without collapse of the pile annulus.
The pile shaft was constructed by driving an open-ended casing into the ground by means of a casing drive adaptor on the Kelly Bar. An oscillator attached to the piling rig was utilised when depths exceed approximately 20m. The soil on the inside of the casing was then augered and removed whilst the side walls were supported by the temporary casing.
INCREASED SHAFT STRESS
Although the final pile configuration did not differ from that utilised on the existing piers, the additional load and stricter settlement criteria required shaft stresses on piles to be increased from an estimated 8MPa (on existing) to over 11MPa. In so doing, the same pile configuration could be used, thereby eliminating the need for a fifth pile at each pile cap and resulting in an estimated saving of some R2 million.
The design loading of a pile is a complex relationship between the quality and reliability of the pile itself – affected by factors such as piling method, strength and integrity of concrete and quality control and supervision to name a few – and the geotechnical design and geological conditions (rock/soil strength and integrity; soil-structure interaction and stress-strain behaviour to name a few more). This influences geo-structural design aspects such as foundation settlement, rotation and stiffness which ultimately affect the design of the superstructure.
Due to the poor soils, emphasis was placed on the formation of rock sockets which need to be embedded in hard but very highly fractured rock. In the design of rock sockets, the more favoured empirical methods, relate the end bearing and shaft capacity of the socket only to the strength of the rock itself. However, authors such as Peck (1979) and Thomlinson (2014), have shown that when the bedrock is fractured, these methods can incorrectly estimate the capacity. Thus methods which consider settlement criteria with due consideration of the rock quality (fracturedness) are preferred.
In addition, to achieve the increased shaft stress, the piles were provided with a 5mm thick permanent casing and constructed using 45MPa concrete. The permanent steel liner was provided to protect the wet concrete from flowing groundwater and soil influx. In the long term, the liner would also protect from debris impact and chemical attack from coastal conditions.
SANRAL’s construction manual advocates that when piles are raked, a thin walled casing be used as the soil face would be prone to collapse during extraction of a temporary casing. However, the casing can also fulfil a structural function and the pile can be designed as a composite element allowing larger shaft stresses to be placed on the piles. The casing thickness when purely used to prevent influx would need to be some 3mm thick for the uMdloti site which would in all likelihood corrode in 130 years. Casings on the site were however much thicker, providing additional capacity where it was required.
Each pile was also equipped with 4 x 80mm diameter steel tubes through which concrete integrity Cross-Hole Sonic Logging (CHSL) and base integrity core testing were conducted under direct full-time supervision.
RETAINING WALLS
Two final aspects of the project which are worth noting are the approach fills and cuts to the uMdloti viaducts.
A prominent V-shaped valley, with embankment heights of up to 15m, occurs in the median just north of the uMdloti River. Widening of the road without some form of retaining structure would imply narrow sliver construction along the side of the embankment and would also imply the encroachment of works into a local stream, which was an environmental concern.
The investigations confirmed the embankments had been constructed with Berea Red sands. Below this fill, weathered products of either shale or sandstones were present followed by the respective rock. A geosynthetically reinforced Concrete Block Reinforced Wall (CBRW) was utilised to support the widened freeway. The walls were designed with assistance from ARQ Consulting Engineers. The final structures comprised two CBRW walls each about 0.5km long on the northbound carriageway, up to 3.5m in height, constructed on top of the existing embankments of 15m.
On the southern approach to the uMdloti viaduct, the N2 goes through an area where a large amount of cut had to take place to accommodate the widened road. Adjacent to the North Bound Carriage (NBC), a 25m deep cutting occurs in dolerite while adjacent to the SBC, a 10m deep cutting occurs in dolerite and shale.
A detailed study of the NBC cutting was undertaken by Davies Lynn & Partners in 1983 during construction of the existing freeway following two slip failures. The report concluded that due to unfavourable joint orientations, a pre-existing failure plane and the risk of a high water table developing in the cut that this should be cut back at approximately 1:3.
Additional investigations were undertaken to determine whether the cut toe could be stabilised using a vertical retaining wall. Investigations showed the upper portions of the cut to comprise very poor silty and clayey soils of residual dolerite. These soils were poor both in terms of their shear strength and suitability for use in engineered layerworks. Below this hard rock dolerite occurred along the toe of the cut. The dolerite varied from a dense weathered gravel to a highly fractured hard rock dolerite. No ground water was encountered.
The good quality interlocking rock with occasional gouge implied that a soil nail wall could be utilised and resulted in a R10 million saving in earthworks. Two rows of nails of approximately 6m length each at 1,5m spacing, were utilised with provision being made for some hollow self drilling injection anchors where more highly fractured dolerite was present. The soil nail wall was cladded with a precast panel to provide an aesthetic facade to the cutting.
CONCLUSION
By obtaining and studying previous records, an understanding of ground conditions and the structural design and performance of the existing infrastructure was developed. The benefits were that the ground conditions were well understood and the investigations could be focused. As an example, a deep cutting previously cut back at 1:3 was provided with a conventional, vertically installed soil nailed retaining wall with a considerable cost saving in earthworks.
At the same time, for the piling to the uMdloti viaducts, little additional investigation was necessary, but the design could be optimised by using contemporary design theories in close collaboration with structural engineers. The increase of design shaft stresses and loading resulted in the elimination of a fifth pile at each of the piers.
However, caution must be exercised when adopting such an approach. This can only be done where there is adequate quality control and direct full time supervision. It also requires continued involvement of the design teams throughout construction. Piling data and integrity testing from each pile was analysed by the design team during construction before approvals were provided and where necessary, remedial actions were recommended and implemented.
ACKNOWLEDGEMENTS
The authors would like to thank SANRAL for its kind permission to publish this article and for its support during the project, specifically Project Manager Mrs Zandile Nene. The contributions of Mr William Martin (Chief Technical Principal Structures), Dawie Erasmus (Functional General Manager, Roads and Highways) and Stuart Anderson (Resident Engineer), all of SMEC, as well as Mr Alan Parrock of ARQ are also acknowledged.
