ABSTRACT
The current SH1 alignment at Bulli Point follows the eastern margin of Lake Taupo and is situated directly above the lake at the toe of a sharp ridge. The site has a history of low speed crashes due to an ignimbrite bluff overhanging the road that forces trucks to cross the centreline in order to navigate tight corners and a narrow carriageway above the lake. In addition to ongoing safety issues, several under slips have recently affected this portion of SH1 and require repair.
A design was developed to repair the under slips, that could also be extended to resolve the alignment issues. Several challenging constraints needed to be overcome to provide a feasible and effective design. The overhanging ignimbrite bluff was unable to be removed or modified for cultural reasons and due to stability concerns relating to unfavourable defect orientation of the ignimbrite that formed the bluff. In addition, encroachment into the lake was not possible due to the area being identified as “Significant Natural Area”, nearby cultural Pa sites had to be avoided, and safe access to a recreational swimming spot was to be provided. This led to the design of a 220m long, 5.5m high piled MSE retaining wall to support widening of SH1 alignment adjacent to the overhanging bluff, and a proposed 20m high benched cut slope to ease the sharp Bulli Point corner.
The geological model for the site was highly variable with variably welded ignimbrite forming bluffs and steep ridge slopes at the lake edge, and variable distribution of alluvium and colluvium between ridges. Airfall and lake deposits were also identified within the current alignment. The geological model was largely based on field mapping due to the inaccessibility of the site for intrusive investigations.
This paper will discuss some of the challenges associated with the site and how these were addressed to provide a resilient design for the alignment.
Introduction
Bulli Point is located on the eastern side of Lake Taupo approximately halfway between Turangi and Taupo (Fig. 1). The current State Highway 1 (SH1) alignment follows the eastern margin of Lake Taupo, and at the location of Bulli Point, is situated directly above the lake edge.
Waka Kotahi/New Zealand Transport Agency (NZTA) and the local community have had ongoing concerns about the safety and route security of this section of SH1. Historically there have been several traffic accidents along this section of highway due to trucks crossing the centreline and cutting inside corners to navigate the tight bends (Fig. 2). Heavy vehicles cannot pass each other at the narrowing of SH1 at Te Poporo, the overhanging bluff immediately north of Bulli Point.
Along with the tight navigation issues at the site, there has been ongoing slope instability at the lake edge. Under slips have occurred in multiple locations on this section of SH1 between the northbound lane and the lake. There has been concern these will increase in size and frequency and potentially causing a permanent lane closure. NZTA identified five sites to be investigated in 2018, these comprised of erosion and shallow instability on the inside corners at Sites 1 & 2 and under slips at Sites 3, 4 and 5 as shown below.
25km/hr corner
Slip Sites
1
3
Slip Sites
2
4
Overhanging bluff
5
Figure 1: Site Location (images sourced from Google Earth Pro 2020)
Figure 2: Rolled truck between the north bound lane and Lake Taupo (images sourced from www.stuff.co.nz)
Background
A previous curve easing study was undertaken by Opus International Consultants (OPUS) for the site in 2017. As part of the study, a cutting through the ridge that forms Bulli Point was proposed with infilling of the two bays on the north side of Bulli Point to ease the approach and widen the carriageway at the overhanging rock site, as shown in Figure 3. This option would have greatly reduced the risks associated with the current route security and geometric concerns; and would address the stability issues around the point. However, this solution carries multiple other cultural and environmental issues which are discussed in Section 5.
Figure 3: OPUS Proposed curve easing alignment
In 2018, AECOM undertook an initial options assessment where the five sites of recent instability were visited, and remedial options were presented to NZTA based on cost, constructability and residual risk. The main driver of the slope failures was interpreted as being caused by inadequate control of surface water and loose side cast fill or loose erodible tephra materials at the slope crests. Without adequate collection and disposal of water at the base of cut and steep ridge slopes, surface water crosses the carriageway and discharges over the crest of the lake edge slopes. The resultant options assessment recommendation was mass block gravity retaining walls at Sites 1 and 2, a cantilevered retaining wall at Site 3 and mass block gravity walls at Sites 4 and 5.
NZTA requested that a “future proofing” option be developed for the repair of Sites 1 and 2 to facilitate a future curve easing proposal as had been suggested by OPUS.
The “future proof” option to repair Sites 1 and 2 was ultimately approved by NZTA. At this stage the scope of the project expanded to include geometric design of SH1 from RP6.230 to RP6.870 and included further optioneering and detailed design of gravity wall retaining to support the widening of SH1 either side of the overhanging rock bluff. A geotechnical investigation was undertaken that involved the drilling of rotary cored boreholes along the northbound lane and geological mapping of the cut slopes and the surrounding area to help inform the detailed design of remedial options.
Geological Setting
The site is located on the edge of Lake Taupo, which lies within the Taupo Volcanic Zone (TVZ). The TVZ extends from Mt Ruapehu in the south to White Island in the northeast. The TVZ comprises multiple volcanic vents and geothermal fields, the largest of these is Lake Taupo. The current lake configuration is the product of multiple large-scale eruptions, the largest of which was the Oruanui event, which erupted an estimated 1,170km3 of material approximately 27,000 years ago (Leonard et al, 2010). As recently as 181AD Taupo erupted approximately 120km3 of material in the Hatepe eruption producing the Taupo Pumice Formation that is found extensively throughout the North Island. Ongoing volcanic and geothermal activity is still occurring throughout the TVZ.
Site Geology
The geological map of the area (Leonard et al., 2010) indicates that the site comprises non-welded ignimbrite and phreatomagmatic fall deposits, and reworked ignimbrite of the Oruanui Formation (Q3v) and the Taupo Pumice Formation (Q1v). Through geological mapping undertaken around the site and five machine drilled boreholes, site geology was interpreted, and a geological model established.
At the location of Sites 1 and 2, the road is constructed on a mix of recent alluvium, side cast fill and Oruanui Formation tephra deposits (ignimbrite, airfall ash and lapilli). Variably welded, weak to moderately strong ignimbrite is exposed in the bluff that SH1 traverses. Although typically case hardened, at the location of the bluff between Sites 1 and 2, sub-vertical jointing was evident persisting the full height of the slope. The northbound lane at Site 2 has been built on a mix of colluvium, side cast fill and tephra deposits. Colluvium comprising boulders up to 1 m³ were observed within the south bound lane cut face.
Further south at Bulli Point, the road has been formed by cutting into case hardened non-welded Oruanui Formation ignimbrite and Taupo Pumice Formation. Loose erodible tephra is exposed by the shallow drop out at the slope crest (northbound lane shoulder) (Fig. 4). Near vertical portions of the slope below the road suggest case hardened, non-welded ignimbrite as observed by the cut slope above the road.
Figure 4 Bulli Point cut slope.
Geomorphology
The wider area is characterised by a series of east/west orientated ridges with steep slopes terminating at the lake edge as ignimbrite bluffs. Gullies have formed between the ridges and have been partially infilled with slope debris (colluvium). The road has been formed by cutting into the end of the ridge, creating a bench directly east of and above Lake Taupo. The slopes between the road bench and Lake Taupo are very steep and in many places are sub-vertical (bluffs).
Design Challenges
Due to the location of the site there were several challenges that constrained and dictated the design for the site. These are outlined below.
Rock Overhang
The ignimbrite bluff that is overhanging the south bound lane comprises persistent dilated, sub-vertical (70 to 80º) and sub-horizontal jointing that forms meter scale rectangular blocks (Fig. 5). It is thought that if modifying the bluff was to occur this would lead to ongoing instability and toppling risk that may prove expensive to remediate without the benefit of improving the horizontal alignment of the road.
Due to the overhanging bluff, the new road alignment needed to extend westward towards the lake by approximately 3m to widen the carriageway and avoid the overhang so trucks can safely pass each other. In addition to the geotechnical considerations, the overhanging rock outcrop (Te Poporo) has cultural significance to local iwi and modification to the rock was not considered an option.
Figure 5: Te Porporo overhanging bluff.
Pa Site
On the ridgeline directly above Bulli Point a historical pa site is located (Site T18/119). The site is accessed via the Department of Conservation (DoC) Waipehi Track. The Pa has two distinct activity areas marked by defensive ditches, as shown in Figure 6. A site survey and walkover were undertaken during the study and many of the pa remnants were still located and identifiable.
Since the pa is located on a popular and easily accessible DoC walking track the site has a high potential for interpretation and public education. Any earthworks or slope modifications were not to encroach closer than 10m to the pa site or increase erosion and adversely affect the site.
Figure 6: Site Map of Pa T18/119 (DoC Historic Record 2012).
Lake Taupo
Lake Taupo provides a source of recreation and cultural significance for many New Zealanders. The ignimbrite bluff below SH1 at Bulli Point is ranked as one of the best swimming spots and cliff diving areas in New Zealand, and as a result attracts many swimmers every summer (Fig. 7). Parking at the site is difficult due to the very narrow shoulder width and topography of the area. Swimmers are forced to park cars further away or precariously on the road shoulder and then walk along the road to get to the swimming spot. A desired outcome of the design was to incorporate a footpath and safe access for people to be able to reach the swimming spot.
Figure 7: Bulli Point cliff diving location looking toward the overhanging bluff.
The design was also not to encroach into Lake Taupo as defined by the high-water mark for cultural and consenting purposes. This area is located within an ‘Outstanding Landscape Area’ with portions also identified as ‘Significant Natural Area’ and as such care had to be taken to not damage or significantly cause adverse effects for the environment.
Geotechnical Investigation
The geotechnical investigation that was undertaken was heavily constrained by several factors at the site. Drilling investigations had to be completed in the north bound lane and could not be undertaken along the proposed alignment due to the steep slope between the north bound lane and the lake. Accessing the proposed base of the retaining wall from the lake and barge was prohibitively expensive, and thus the bulk of the geotechnical investigation is based on geological mapping undertaken in the area. Field mapping identified a number of geological units and their variable distribution throughout the site.
Design
Considering all of the factors discussed in previous sections, the final design option that was presented to NZTA was split into two sections. Each section was independent of the other and could be constructed alone with only minor adjustments to the design. One section incorporated the retaining wall north of Bulli Point that extends out to the edge of Lake Taupo to widen the carriageway and move it away from the overhanging bluff. The other section is the formation of a cut through the ridge that forms Bulli Point. This realignment removes the tight 25km/hr corner and creates a safer parking location for swimmers (Fig. 8).
Figure 8: Proposed design.
Retaining Wall
The retaining wall (Fig. 9) was designed based on the adopted geological model for the site. The proposed wall needed to be constructed in the closest proximity to the lake boundary as possible, and with the potential for geological change and uncertainty during construction, a design that is easily adaptable was sought.
The agreed design was a piled StoneStrong wall with paraweb reinforcement. The wall is to be built with a vertical face allowing it to be built proximal to the lake boundary. Given the limited intrusive investigation, variability of volcanic geology and geological uncertainty, position of the wall in relation to the highwater level and the toe slope (up to 30 degrees), it was decided to use a piled foundation for the retaining wall. Driven steel H piles were recommended due to the ease of procurement and the expected driveability of the founding materials.
Figure 9: Typical retaining wall cross section
The design was carried out in accordance with the StoneStrong design spreadsheet, NZTA Bridge Manual, MSEW, AllPile and SLIDE.
The wall has a maximum design height of 5.5m and is 220m long with varying toe slopes. It has been designed so that during construction one lane of the existing SH1 can remain open and functional under stop/go traffic management.
Cut Slope
Figure 10: Proposed cut slope alignment.
The existing sharp bend at Bulli Point was proposed to be re-aligned by forming a 20 m high cut slope (Fig. 10). The cut was to be formed in non-welded ignimbrite materials and benched for stability and construction access. As with the overhanging rock and retaining wall, the site is of cultural significance with the Waipehi Track and Pa site at the crest of the ridge. The land is administered by the Department of Conservation and the steep ridge slopes are currently covered in native bush.
Intrusive geotechnical investigation to confirm cut slope materials proved difficult due to the cost associated with access as well as the cultural and environmental significance of the site. Ground conditions were determined by geological mapping and rockmass characterisation from exposed ignimbrite outcrop. The mapping results were used to carry out kinematic and rock fall analysis to assess the stability of the cut and feasibility of the proposed cut. Much of the geological outcrop revealed non-welded ignimbrite that was case hardened where exposed but with very few discontinuities.
The proposal addresses several of the NZTA objectives. By realigning, the existing curve is eased and navigation of the sharp bend improved. By retreating the existing cut, the carriageway was to be moved away from slip Site 3. Access and parking are provided for recreational users of the area.
Conclusions
While construction cost of the project is not expected to be prohibitive, the project is an example of the challenges associated with considering cultural, environmental and social aspects in geotechnical design. Ultimately, a design was adopted that addresses the safety and resilience issues associated with this portion of SH1, while minimising the effects on the environment and the nearby cultural sites. Design complexity was increased by the potential variability and uncertainty of ground conditions, constructability issues associated with working within a narrow portion of SH1 and the steep topography. This design is currently in the proposal stage and no commitment to construction has been made.
Acknowledgements
We would like to thank Waka Kotahi/New Zealand Transport Agency for the opportunity to undertake this work and our colleagues at AECOM for the technical reviews and discussions around possible design solutions for the site.
References
AECOM (30 January 2018) Bulli Point SH1N RP726/6.230-6.870 Options Assessment Report.
AECOM (23 April 2018) Bulli Point SH1N RP726/6.230-6.870 Geotechnical Factual Report.
AECOM (31 May 2018) Bulli Point SH1N RP726/6.230-6870 Geotechnical Assessment Report.
AECOM (20 September 2019) SH1 – Te Poporo (Bulli Point) Realignment Design Report (Sites 1, 2 and 3)
Google, Inc. 2020, Google Maps, viewed April 2020.
Leonard, G.S.; Begg, J.G.; Wilson, C.J.J (2010) Geology of the Rotorua area: scale 1:250,000. Lower Hutt: Institute of Geological and Nuclear Sciences Limited.
Opus International Consultants Ltd (28 August 2017) Bulli Point Curve Easing Feasibility Study, Taupo, ref 2-V0338.00.
Simmons & Associates Ltd (March 2019) Final Archaeological Assessment of State Highway 1 Proposed Improvements at Te Porporo / Bulli Point, Lake Taupo.
www.stuff.co.nz, viewed April 2020