Case Study: Geotechnical Investigation for Upgrades To National Route 2 Section 26 From Mt Edgecombe Interchange To Tongaat Toll Plaza

Abstract

With the completion of the new King Shaka International Airport north of Durban, the South African National Road Agency Limited (SANRAL) identified the N2 section 26, between Mt Edgecombe and the Tongaat Toll Plaza as requiring capacity improvements due to the increase in traffic volumes. The improvement of this section, is estimated to cost over R800million, and will take the form of widening by adding two lanes to each carriageway with various interchange improvements. The project also includes three incrementally launched viaducts and the widening of several large embankments and cuttings. This paper will discuss the geotechnical investigation and design of various aspects relating to some of the geotechnical constraints encountered on this interesting multifaceted project including:

• Settlement and consolidation of approach fills to the Mhloti Dual Viaducts
• Slope stability analysis of embankments at Mt Edgecombe Interchange
• Pile foundations to Umhlanga River Bridges and Mhloti Viaducts
• Lateral support to various cuttings and structures.

1. Introduction

Due to the increase in traffic volumes following the completion of the new King Shaka International Airport north of Durban, the N2 Section 26 was identified as requiring capacity improvements. The capacity improvement of this section will take the form of widening by providing two additional lanes to each carriageway, either in the median, or outside of each carriageway, depending on the topography and geometric design considerations. This implies that the upgraded N2 will comprise four lanes in each direction once competed. Vela VKE was appointed by SANRAL to undertake the civil engineering design and construction supervision of the project with an estimated construction value of over R800 million and construction scheduled to commence towards the latter half of 2011.

This paper will focus on the investigations of some of the more interesting geotechnical constraints to the development of the project, as well as briefly discussing the design of the solutions developed. The paper will specifically aim to contextualize the planning and the goals of the investigation in light of the constraints identified. This includes:

• The settlement, consolidation and stability of the approach fills to the Mhloti Dual Viaducts
• The slope stability of the high fill embankment at Mt Edgecombe Interchange
• The pile foundations to Umhlanga River Bridges and uMdloti Viaducts
• and, the lateral support to various cuttings and structures.

As a point of departure the paper will briefly discuss the planning of investigations for infrastructure upgrade projects, before providing an introduction to the project and an overview of its geological setting.

2. Planning Investigations for Infrastructure Upgrade Projects

From the outset 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 new or Greenfield development;

• Firstly, there potentially exists a wealth of information comprising previous investigations, as-built drawings and geotechnical reports which can be studied to develop an understanding of ground conditions and the design of the existing infrastructure
• and secondly, there also exists performance history, sometimes formalised through maintenance records, but often simply made 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 very 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 historical information available (which is dependent on the level of sophistication of the client) and scope of the proposed upgrades, the geotechnical engineer can do significantly less fieldwork and drilling on the project and carefully focus the teams attention on particular, well defined, geotechnical problems. In the case of SANRAL projects, design reports and drawings are usually readily available and often substantial.

In general, the geotechnical engineer would thus need to target areas/aspects where:

1. There are significant changes to the infrastructure and/or imposed loads.
2. New/alternative construction methods can be considered.
3. Less conservative designs can be adopted; by using more sophisticated parameters/analysis, or a clearer understanding of ground conditions, and/or a better understanding of the performance of the existing (similar) infrastructure.
4. It is important to understand the impact of new construction adjacent to existing infrastructure.
5. Information obtained is unclear or inadequate.
6. Performance of existing infrastructure is poor, or problems have been or are known to have been experienced during construction or utilisation.

3. Site and Project Description

Section 26 of the N2 freeway stretches approximately 21km from N2-M41 interchange at Mt Edgecombe (S27.72968° E31.05661°), to the Tongaat Toll Plaza (S29.63022° E31.12157°) which is just north of the new King Shaka International Airport. The route generally follows a north easterly direction, sub paralleling the natal north coastline about 2 to 3 kilometres inland.

The route, as is typical of the N2 along KwaZulu Natal, is characterised by a rolling, undulating topography. In general, the road is located along a narrow coastal plain that gives way to major river valleys that originate off the Drakensburg escarpment to the west. In geological time, several phases of uplift, erosion and deposition have created complex landforms. The positive features are formed by resistant sedimentary beds in the north and central part of the route and fossilised sand dunes on the coastal plan to the south.

The route crosses two significant easterly flowing river valleys; the Umhlanga River and the prominent uMdloti River. Three other less prominent streams are also crossed; a locally west flowing stream at La Lucia Interchange, a north flowing stream tributary of the Umhlanga River and a southerly flowing stream tributary of the uMdloti River. Both of the last mentioned streams parallel the freeway, in the median, and result in two prominent river valleys resulting in freeway embankments up to 15m high.

As a result of this, there are several large embankments, cutting and bridges located along this section of the route.

Of the above geological formations, this paper primarily focuses, through the discussion of the geotechnical engineering problems encountered, on the recent and Quaternary age alluvial and coastal soils encountered over the project.

4.1. Alluvial Soils

The geology of the Natal coast is characterised by the presence of deeply incised paleo-channels of various large rivers along the coast, such as the uMdhloti River. These channels, which are incised into the bedrock (locally siltstone, shales and dolerite of the Vryheid formation), have 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 is deposited. The materials are thus variable but typically very weak, highly compressible, and their consolidation rate due to excess pore pressure dissipation, is very slow. These soils were encountered at the uMdloti and Umhlanga Rivers as well as at the Mt Edgecombe Interchange.

4.2. Coastal Sands, Berea Reds

Berea Red sand predominantly occurs sub-parallel to the coastline and these deposits generally comprise quartzitic sand, which tend locally to red clayey sands. These fossilised sand dune deposits are often dense and form the relief topography along the coastal plain, with many older and/or smaller bridges being founded using conventional pad foundations on the sands. These sands can also be found scattered along the route at various locations because they were used as general fill during the initial construction of this section of the N2.

5. Approach fills to uMdloti Dual Viaduct

At the northern abutment of the Mhloti Viaduct, approach fills were widened (late 2009 – early 2010 prior to construction of the N2) to accommodate the proposed addition of the new lanes using excess materials sourced from the construction of the King Shaka International Airport. The fill comprises mostly Berea Red sands and had been placed on top of the alluvial filled paleo-channel deposits locally up to 15m thick. Shortly after Vela VKE’s appointment large cracks began to appear between the existing embankment and the widened fill,.

As discussed previously, it is known that the alluvial layers would be weak and compressible and that high settlements could be expected. And, even if minor settlement was to take place after the new lanes have been constructed, cracking of the pavement would occur, particularly between the existing and new lanes. Thus, the objective of this analysis was to determine the amount of settlement which had taken place and the amount of consolidation remaining. A stability analysis was also necessary.

To determine at what state of consolidation the approach fills were, Piezocone Penetration Probe (CPTu) Testing was used. The piezocone was initially developed for use on the highly compressible material that is prominent along the coastal region in Kwa-Zulu Natal (Jones and Rust, 1983), and has been extensively used on similar soil profiles since. The CPTu is useful in this specific instance, in that it could be used to evaluate excess pore pressures developed from the construction of the approach fills. As the CPTu provides a continuous soil profile it can also be utilised to identify any thin layers of soft clay or silts, which could occur given the geological context of the site.

Three CPTu tests were conducted at the northern abutment of the Mhloti Viaduct to evaluate the settlement potential and rate of consolidation of the approach fills. The probes were done through predrilled boreholes with temporary steel casings, which were installed to the base of the fill, as some boulders were anticipated in the fill.

Six CPTu’s were carried out, but three results had to be discarded as they provided poor/corrupted data. The remaining three, however, provided good quality data and showed that although soils underlying the embankment were compressible they were largely sandy in nature and where clays were encountered, they occurred as moderately thick bands (<4m thick) with numerous sand lenses. From this, and the evaluation of excess pore pressures from the CPTu it was determined that the consolidation of the approach fills would largely have occurred during their construction.

Due to the nature of the soils encountered, no samples suitable for laboratory oedometer testing could be obtained, and the evaluation of settlements had to be done (empirically) using the Standard Penetration Tests (SPT; N-values), obtained from rotary cored boreholes and CPTu test results.

The SPT N and CPT cone resistance values were used to assign a constrained modulus value (Young’s Modulus) to each layer, and then further used to determine the settlement of each layer. By accumulating the settlement of each layer (determined from the CPTu and borehole profiles) it was found that settlements in the order of 600mm could be expected. However, as discussed above, these have already taken place.

Finally, a slope stability analysis check of the existing slopes was also undertaken using the Prokon suite of design software with the following assumptions:

1. Geotechnical analysis parameters for the alluvial soils had to be (empirically) selected based on results of CPTu and SPT results.
2. Parameters for the Berea Red fill were obtained from shear box and triaxial tests undertaken on samples that were extracted from the fill. The parameters used for the Berea Red corresponded well with previously published data (Brink, 1985).
3. The fill is up to 12 metres high and has a side slope of 1:2.
4. From the hydrological/flood analysis, it was evident that the water level for the 100 year flood would only extend approximately 3 metres above the toe of the fill. Therefore, it would be incorrect to assume that the entire fill is saturated.

Analysis of the fill produced a factor of safety above the 1.5 required.

6. Embankments at Mt Edgecombe Interchange

The proposed upgrade to the Mt Edgecombe Interchange entails shifting the position of the North Bound Carriageway (NBC) offramp to facilitate the construction of an inner ramp-loop between the offramp and the N2 freeway. This will result in the existing earth embankment having to be widened and raised to a height of 14 metres above ground level. The embankment is made almost entirely of Berea Red sand as this material was used as general fill during the initial construction of this section of the N2 and is also situated over an alluvial channel.

The investigation of the embankment included test pitting, rotary core drilling with SPT tests, CPTu and laboratory testing of the materials encountered. The borehole and the CPTu showed the alluvial soils to extend over 23m, comprising alternating layers of compressible clays, silts and sands, which become particularly dense (SPT-N values > 50) from 10m depth but alternating with weaker clayey strata below this, to depth. Therefore, due to the weak soils in the upper portion of the soil profile, there was a concern for the overall stability of the widened embankment.

A slope stability analysis of the proposed slopes was undertaken using the Prokon suite of design software to compute the Factor of Safety (FoS). The program was also used to evaluate the probability of failure of slopes using probabilistic design parameters for soils. In order to maintain the required 1:2 slope (required to minimise fill quantities and ensure that no additional land needed to be acquired by SANRAL); a rock pioneer layer placed on a high strength geosynthetic is proposed in the deeper marshy sections where poor soil conditions are present. Furthermore, it was established that if the fill height is less than 10 metres, the rock pioneer and geosynthetic layer was no longer required and the proposed slope is stable with a FoS of greater than 1.5.

SPT-N values were again used to calculate the amount of settlement and this was estimated to be in the order of 600mm. Settlement will largely occur during construction (within 3 – 6 months) and is not of concern for the embankment itself, but does imply that final surfacing should be delayed to the end of the contract.

However, the settlements are a concern for a culvert which cuts through the embankment and is also to be widened. Several solutions were explored with the assistance of ARQ Consulting Engineers including a stone column solution, but because of the high differential settlements the final solution adopted comprised a piled raft solution.

7. Pile foundations

7.1. uMdloti Viaducts

uMdloti River crossing comprises dual viaducts, each with six spans of approximately 50m and continuous, post-tensioned box girder decks. Two new viaducts are to be constructed on the outside of the existing structures and tied-in on completion.

The deeply incised paleo-channel of the uMdloti River is a significant feature of the local geology. The channel has been incised into bedrock; dolerite at the southern abutment and mudstone, siltstone and sandstone of the Vryheid Formation over the remainder of the viaduct. The depth to bedrock is in the order of 31 metres at the three central piers and the channel has been found to be predominantly filled with clays and some sands of variable consistency (ranging from very soft/loose to dense/stiff). A boulder layer (approximately 1m thick) was also encountered in some boreholes on the contact between the alluvial deposits and the bedrock. The depth to bedrock shallows towards the north abutment, here it is in the order of 18m.

Pile foundations are envisaged for all foundations except the southern abutment which is situated on a prominent dolerite sheet which dips into the uMdloti River valley. The poor ground conditions described present a challenge to the construction of the viaducts with regards to deep piling through weak, sometimes granular soils under the water table as well as through the boulder layer into bedrock.

The foundations are thus to be bored oscillator piles with permanent casings, a pile diameter of 900mm and shaft stress of 10MPa (Franki, 2008). Additionally, as much of the pile capacity will be derived from end-bearing on the rock at some 30m, the piles will be founded at least 1.5 times the pile diameter into bedrock.

The above design does not differ significantly from the original design and for this reason, except for issues raised with regards to the northern abutment, as discussed above, no additional drilling or investigation was undertaken. However, a detailed testing program which would include the integrity testing of the pile shaft and toe as well as testing of the quality of the underlying rock is to be incorporated during construction of the foundations.

The viaducts are also to be incrementally launched from the southern abutment. A number of factors contributed to this decision, but one of the primary reasons was the poor geotechnical conditions encountered at the northern abutment which implied additional deep piling and lateral support of excavations in poorer soils to support the deck casting yard.

7.2. Umhlanga River Bridge

Umhlanga River crossing comprises dual bridges, each with two spans and simply supported decks. Upgrades to the bridges are in the form of widening by 5m in the median.

As discussed previously, the significant feature of the local geology is the presence of the deeply incised paleo-channel of the Umhlanga River. Previous boreholes drilled showed depth to bedrock is in the order of 18 metres and that the channel has been found to be predominantly filled with sands and some clays of variable consistency (ranging from very soft/loose to dense/stiff). A boulder layer (approximately 1m thick) has also been encountered on the contact between the alluvial deposits and the bedrock.

From a comparison of the previous borehole logs and the actual pile depths from as-built drawings it appeared as if some of the pile foundations, which had comprised the driven-cast-in situ (“Franki”) type had been placed on the boulder layer rather than on bedrock, as these were unable to penetrate the boulder layer. This is generally not desirable, but given that the bridge is at sea level it could be argued that the scour depth would not exceed 8m and this could be considered acceptable. Additionally, there have also been no reported problems with the existing bridges and the new widening is relatively minor.

With the same foundation design adopted and the historic information obtained being sufficient to base the design, no additional investigations were undertaken.

8. Lateral support and retaining structures

8.1. Mt Edgecombe Interchange

The upgrading of the interchange involves a new north bound on-ramp loop, between the “north”-western spill through abutment and the first pier requiring the spill through to be cut and supported vertically. Test pits generally showed the in situ materials to comprise a red brown, medium dense, silty fine sand (Berea Red sands). Some voided dolerite layers were encountered and it is known (from the construction of the similar south bound on-ramp loop) that substantial grout losses occurred into the east abutment approach fill.

The spill through abutment must be provided with lateral support. The existing abutment has not been designed to accommodate lateral loads or horizontal displacement, and support measures were designed to ensure that these are in no way transferred to the structure.

Soil nails, two rows of nails 2 metres apart of approximately 16m length each at 1,5m spacing, are to provide the permanent lateral restraint required and will be of the self drilling injection type to overcome the previous problems encountered at the southern abutment.

8.2. V-shaped median valleys

Two prominent V-shaped valleys, with embankment heights of up to 15m, occur in the median along the N2 Section 26; one just north of the Mhloti River and one at Blackburn, just north of Mt Edgecombe. Widening of the road without some form of retaining structure would imply a narrow sliver construction along the side of the embankment and would also imply the encroachment of works into the stream, which is an environmental concern.

The investigations were primarily focused on confirming that the embankments had been constructed with Berea Red sands and at determining shear strength parameters. The investigations confirmed the embankment to be constructed using granular materials, varying from silty sands (Berea Red Sands) with some plasticity to dolerite gravel. Below this fill weathered products of either shale or sandstones were present followed by the respective rock. The existing embankments were thus confirmed to have been constructed over a relatively shallow bedrock profile.

A geosynthetically reinforced Concrete Block Reinforced Wall (CBRW) is thus proposed to support the widened freeway – these walls were designed with assistance from ARQ Consulting Engineers. Reinforced Earth® was considered as an alternative, but this was more costly and there would be no benefit as a result of the improved aesthetic appearance as the wall would not be visible off the road. For this same reason, the CBRW will also not having any cladding.

8.3. Cuttings approaching uMdloti River

For approximately 1km prior to the uMdhloti dual viaduct, the N2 goes through an area where initially a large amount of cut had to take place. Adjacent to the NBC, a 25m deep cutting occurs in dolerite while adjacent to the South Bound Carriage (SBC), a 10m deep cutting occurs in dolerite and shale. These cuttings need to be steepened to accommodate the freeway widening.

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 with the cutting adjacent to the SBC cut back at 1:1.5.

Additional investigations were undertaken to determine whether the toe could be steepened using a vertical retaining wall or whether the cutting would need to be widened using a slope angle similar to the existing one.

Investigations showed the upper portions of the cuts to comprise very poor silty and clayey soils of residual dolerite. These soils are poor both in terms of their shear strength and suitability for use in engineered layerworks.

However, the additional investigations also showed hard rock dolerite (or in the case of the SBC partly shale) to occur a few metres up the toe of the cuttings i.e. behind any vertically installed wall. The dolerite varied from a dense weathered gravel to a hard rock dolerite leading to refusal of the TLB at shallow depth. However, for the most part the dolerite presented as a highly fractured hard rock excavating as and resembling a densely pack gravel with rough to jagged interlocking faces.

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 earth works. Two rows of nails of approximately 6m length each at 1,5m spacing, are to be utilised with provision being made for some hollow self drilling injection nails where more highly fractured dolerite is present. The soil nail wall is also to be cladded with a precast panel to provide an aesthetic facade to the cutting.

9. Conclusion

By obtaining and studying, previous geotechnical reports, as-built and other construction and maintenance records obtained for the N2/26, an understanding of ground conditions and the design and performance of the existing infrastructure was developed. This process was cumbersome and resulted in significantly more work during the planning stages of the project, but resulted in less field work and drilling.

However, the benefit was that the ground conditions were well understood prior to the commencement of drilling and the investigation could thus be focused, including the use of more sophisticated in situ and laboratory testing on well defined, geotechnical problems. As an example, the consolidation of the Mhloti Viaduct Approach fills was shown to already have taken place and 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 Mholti Viaducts and Umhlanga River Bridge; little or no additional investigation was necessary, as such investigations would not substantially have changed the understanding of ground conditions or resulted in a change in the foundation type or depth.

References

Brink, A.B.A. 1985. Engineering Geology of Southern Africa, Volume 4. First Edition. Pretoria.
Council for Geoscience. 1988. 1:250 000 Geological Sheet 2930 Durban. Pretoria.
Davies Lynn & Partners. 1983. Geotechnical Investigation Founding conditions of Northern
Approach Fill to Umhloti Viaduct, N2/26 (Report 13). Ref 1988/PD. March 1983
FRANKI 2008. A guide to practical geotechnical engineering in Southern Africa, 4th Edition. Byrne G and Berry A.D.
Jones, G.A. and Rust, E. 1983. Piezometer probe (CPTu) for soils identification. International symposium on in-situ testing, Paris.