ABSTRACT

This case study describes the design and construction of an MSE (Mechanically Stabilised Earth) wall in the geological environment of the Taupō Pumice Formation, a non-welded ignimbrite and reworked deposits from the 181 AD Taupō eruption. The engineering properties of the pumice and the causes of the washout event will be described followed by the design of the wall that was adopted to reinstate the damaged waterfront. The case study will show site photographs of the washout event, photographs taken during construction and the finished repair as it stands now. It will describe and illustrate various challenges during construction and the design modifications completed as construction proceeded.

INTRODUCTION

WSP was engaged by Taupō District Council to undertake the detailed design of a wash-out underslip that occurred on Lake Terrace in Taupō between Ferry Road and Roberts Street in July 2019.

Figure 1: Burst in the water main that caused the underslip (source www.stuff.co.nz, photo supplied by James Hunt)

The erosion of site material was caused by a burst watermain which led to the failure of the slope, pavement and subsequently the wastewater pipe. The compounded effect of the high-pressure water and wastewater flow scoured a significant volume of soil material in to the lake.

SITE LOCATION AND DESCRIPTION

The scour site was located on Lake Terrace, a part of Thermal Explorer Highway passing through town of Taupō between Ferry Road and Roberts Street. The wider area of the lake front is characterised by 60 – 80º. steep bluffs partially covered in low growing vegetation and bush, with some larger trees at the base of the slope, on the lake edge. The main cause of the scour was a water main break on 02/07/2019 just after 2pm causing a large washout through the dispersive pumice soils. About an hour later the footpath collapsed and took out a wastewater pipe causing a wastewater spill in to the lake. The washout is shown in Figure 2.

Figure 2: Full extent of the washout shown on UAV imagery

The affected area contained several utilities services including the affected water main, wastewater, stormwater, telecommunication and power cables.

Geology of the site

The site is located on the edge of Lake Taupo, which lies within the Taupo Volcanic Zone. This zone extends from Mt Ruapehu in the south to White Island in the north east and comprises multiple volcanic vents and geothermal fields; the largest being Lake Taupo itself. The current lake configuration is the product of multiple large-scale eruptions, the largest of which was the Oruanui event, which occurred about 27,000 years ago.

With reference to the published 1:250,000 scale geological map of the region (Leonard, G.S.; Begg, J.G.; Wilson, C.J.N. (compilers) 2010) the site is located on the Taupō Pumice Formation, a deposit from the 181AD Taupo eruption that produced approximately 120km³ of material.

The Taupō Pumice Formation is a very low-density pumice and non-welded ignimbrite. Site observations along the lake front indicate that both reworked pumice deposits and non-welded ignimbrites are present in the vicinity of the site.

DESIGN

Since the design was a part of emergency works it was important to start work on the design as soon as possible. Investigations completed included the following: site inspection undertaken by WSP Engineers, unmanned aerial vehicle (UAV) inspection of the area and survey work consisting of Total Station and UAV collected data.

Ground profile

As seen during the washout inspection, the material of the Lake Terrace plateau is formed out of clean pumice sand and gravel layers to the depth of approx. 9.5 m and non-welded ignimbrite at depth. The stable angle of this kind of material can exceed 70º with no major global stability issues. Modes of failure in these kinds of materials are usually face-erosion/vegetative cover, dispersion/rilling, root jacking, toe erosion (lake slotting), stormwater overtopping the cliff/running off the roadside verge, seismic events.

The cliff face adjacent to the washout site is heavily vegetated. To the north west, there is a section of cliff with reported minor long-term instabilities and slight overhangs extending for approximately 20m. Visible exposed pumice bluffs to the east are near-vertical and show no signs of major instability. To the west of the site is an old benched portion of cliff with ponga retaining walls that is in poor state of repair. The benching is understood to be a man-made landscape feature for community use rather than for slope stability and there is a dilapidated concrete staircase down the middle of the benches.

The Taupo Formation ignimbrite is a massive non-welded ignimbrite unit comprising pumice sand and gravel in a partially cemented fine grained matrix. Based on site inspection it is anticipated that the designed remediation of this site will be founded in partially-cemented to non-welded ignimbrite/pumice rock. Groundwater was expected to be controlled by the lake water levels.

General considerations

The task at hand was to design a support structure to remediate the washout that had occurred. After discussing several options that would be suited for a safe, rapid and permanent solution for the remediation of the site, it was decided to proceed with a design of an MSE wall with a vegetated face in order to provide permanent support whilst matching the visual character of the surrounding lake front. The narrow base section of the washout was to be filled with no fines concrete in order to rapidly fill the part of the void which was very narrow and steep and posed a risk to workers constructing MSE at that level. The lower part was protected with a rip-rap coarse rock bund. Rip-rap at the base was top-soiled, seeded and then covered with turf reinforcement matting (Cirtex T-RECS) and planted with native local plants.

A MSE type “Duramesh” reinforced soil wall was adopted as the preferred option. Facing panel elements were backed with a 200mm wide mix of topsoil and grass seed (the same grass mix as present on site) and slow release fertiliser, with bio wool matting on the front face. The “Duramesh” reinforced soil wall system was chosen since it provides a stable support option while maintaining a vegetated face of the wall. This was especially of interest as the structure is visible from the lake which is a popular tourist destination and maintaining a consistent visual result was a key stakeholder requirement.

Geometry of the scour site

Since the site was considered to be contaminated due to the wastewater spill it could not be inspected close up in the early days after the event. UAV data point cloud was used to obtain a rapid 3D model of the area and generate design cross sections.

Figure 3: Cross sections and corresponding plan views obtained from a UAV point cloud using Global Mapper Software. Cross sections from top to bottom: North tie-in, overhang, highest/critical, south tie-in

The UAV survey was flown the morning after the event and processed some hours later providing a rapid, high quality basis for emergency work design, planning and hazard management to the wider project team.

Taking into consideration the fact that AutoCAD software was known to be problematic with 3D models containing overhang geometry and delivering the design was a time sensitive task, cross sections were instead quickly obtained using Blue Marble Geographic’s Global Mapper software.

Four key cross sections were extracted for the purpose of design: tie ins to the north and the south; the cross section with the greatest overhang and the highest/steepest/critical section. These are shown in Figure 3. The UAV data point cloud also recorded tree tops and shrubbery which were subsequently filtered from the point cloud as part of data processing.

The base of the washout was rather narrow, and the shape was difficult to make out, being concealed by greenery. The point cloud was filtered using the software’s native algorithms to remove the vegetation and the shape of the base became visible as presented in Figure 4.

Benched cliff with ponga retaining walls

Staircase

Cliff top

Bottom of the washout – lake level

Debris and collapsed pipes

Foot path

Figure 4: Cross section of the washout parallel with lake shore obtained from a UAV point cloud using Global Mapper Software. Shape of the bottom and debris of collapsed wastewater and water pipes clearly visible.

MSE wall calculations

The MSE wall needed to reinstate the void and provide adequate internal and external resistance supporting the traffic lane and footpath. Geogrid tensile resistance, pull-out resistance, direct sliding, limiting eccentricity and bearing resistance were required to be assessed. Additionally, the global stability and compound stability of the MSE wall required assessment considering the height of the wall and the geometry of the slope in front of the MSE wall towards the lake front.

The MSE wall design was performed using the Allowable Stress Design (ASD) methodology consistent with the slope stability assessment. Three design scenarios including static, seismic and high groundwater cases were evaluated. The targeted factor of safety (FoS) for the static case was adopted from the NZTA Bridge Manual 3rd Edition. As there was no deep geotechnical investigation data on this site at the design stage, and all of the input information was obtained from mapping, experience and literature, a more conservative approach was adopted in assessing seismic analysis results; hence the use of target FoS of 1.20 was adopted for the seismic case.

The scour reached from the crest/ upper surface to the beach level (9.1m overall). A layer of no-fines concrete 1.8m in thickness was installed in the void beneath the MSE wall which reduced the modelled height of the MSE wall to 7.3m. The face of the MSE was modelled as 70º to match the surrounding cliffs. The no-fines concrete provided adequate bearing capacity at the toe of MSE.

The inspection identified that the soil to be retained by the MSE wall comprised approximately 1.3m pumiceous SAND and 6m thick pumiceous GRAVEL. In the MSE wall modelling, we adopted retained soil parameters of effective friction angle 38 degrees and unit weight of 16 kN/m³. The reinforced soil was modelled as AP40 with an effective friction angle 36 degrees and unit weight of 16 kN/m³; and the foundation soils comprising of no-fines concrete and pumiceous GRAVEL with inferred soil parameters of effective friction angle 40 degrees, cohesion 5 kPa and unit weight of 16 kN/m³. A surcharge of 12kPa traffic load and an additional 5kPa live load representing lawn maintenance load on the crest of MSE wall were considered in the MSE wall design. The reinforcement adopted was uniaxial polyester geogrid with a long-term design strength no less than 57.3 kN/m for a design life of 50 years.

MSE wall design calculations utilised the MSEW 3.0 software by ADAMA Engineering Inc to determine the internal capacity of geogrid strength, pull-out and direct sliding and external capacity of sliding, bearing capacity and eccentricity. The global stability of the MSE wall was performed using Geostudio’s computer program SLOPE/W adopting the Morgenstern-Price limit equilibrium slope stability method and considered the optimised non-circular failure model to be suitable for the site geotechnical conditions.

The MSE wall assessment results indicated that the required geogrid length was 7m to 16m horizontally and the geogrid spacing was 0.6m vertically as shown in Figure 5.

Obstructions in the Reinforced Soil Mass

There were two manholes 3.0m in diameter and 0.6m round pipe embedded the reinforced soil mass. The strength loss from three layers of geogrid that were cut due to the presence of these obstructions was compensated for by two additional measures.

DD manhole

Lake front

MSE wall body – geogrid reinforced fill

Lake Terrace road

Foot path

MSE wall Face

No Fines Concrete

Rock bund

Figure 5: Typical cross section of MSE wall with the strengthening measures applied

The first measure was the extension of geogrid to the back face of the excavation. The inclusion of geogrid here stabilises the soil behind the manholes and consequently reduces the lateral earth pressure on the MSE wall.

The second measure is the inclusion of additional geogrid layers which reduced the geogrid spacing to 0.3m vertically near the face of the wall. These two measures aid the overall structure, replacing the strength lost by cutting the geogrids to accommodate the manhole. The detailed construction drawing is presented in Figure 5.

CONSTRUCTION AND CHALLENGES

Construction work on the MSE wall began as soon as possible and the Detailed Design report for the MSE wall was delivered nine days after the washout occurred.

During the subsequent weeks the design was modified in real time to accommodate the changes due to the addition of a downstream defender (DD) unit that was placed within the MSE wall zone – the scour had provided the client with opportunity to increase the resilience of water assets along the foreshore and the new downstream defender located in the scour formed part of this upgrade. Further changes were adopted with the use of clean site-won pumiceous gravel material in the upper sections of the MSE wall in place of AP40.

A brief overview of the construction process is presented in the site photographs taken during site inspections and by on-site WSP engineers.

Figure 8: Excavated access ramp to the bottom of the washout site, and the excavated wider area for the MSE wall placement. Numerous services visible on site.

Once the access ramp excavation was complete and the contaminated debris removed, work commenced on placing the rock for the rip-rap bund and the placement of the no fines concrete in the foundation area of the MSE wall. As visible in Figure 9, the excavated slopes in natural pumice are stable even as steep as 75º from horizontal.

Figure 9: Final placement of the no-fines concrete in the base of the MSE wall. Temporary excavation of the pumiceous material at a steep angle

Figure 10: Work on the MSE wall, steel mesh face element placed and geogrid visible

Figure 11: Construction of the MSE wall around mid-height utilising quarry materials, downstream defender installation within the body of the MSE wall. Challenges with compacting the material and installation of the geogrid layers.

Figure 7: Completion of the MSE wall to full height with the use of site won pumiceous gravel material in the upper sections.

The construction of the MSE wall structure was completed on 23/09/2019, 74 days after the initial version of the design was delivered to the site team.

CONCLUSION

This case study presented a real-time emergency response design for a repair of a washout site in pumiceous materials at the lake Taupō waterfront. The remedial solution of an MSE wall with a vegetated face was selected in order to provide permanent support whilst matching the visual character of the surrounding lake front. With no geotechnical investigation on site and only UAV spatial data of the washout geometry, the early design had to rely on the WSP experience with the pumiceous materials and local knowledge in the region. Appropriate software was used to extract vital information about the site geometry and to perform necessary calculations in order to optimise the proposed repair solution. The design was delivered within nine days of the washout event and then further amended in real time without delaying the construction program.

Figure 12: View of the completed MSE wall in November 2019 with the green face partially established

REFERENCES

Bridge manual, SP/M/022 Third Edition, Amendment 3, Effective from October 2018, © NZ Transport Agency

Edbrooke, S.W. (compiler) 2005: Geology of the Waikato area: scale 1: 250,000. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences 1:250,000 geological map 4. 68 p. + 1 folded map

Leonard, G.S.; Begg, J.G.; Wilson, C.J.N. (compilers) 2010: Geology of the Rotorua area: scale 1:250,000. Lower Hutt: Institute of Geological & Nuclear Sciences Limited. Institute of Geological & Nuclear Sciences 1:250,000 geological map 5. 102 p. + 1 folded map

Sigurnjak, M; Follett, B (2019) Third Issue, Taupō Waterfront Washout Repair, MSE Wall – Geotechnical Design Report

Website: www.stuff.co.nz, 02/07/2019 Article: “Taupō residents warned not to flush toilets as council scrambles to fix pipe break”

WSP Taupō Office, Lake front Emergency Work, Photo documentation

Software: Global Mapper v 19.1.0 2002-2018 – Blue Marble Geographics

MSEW 3.0 by ADAMA Engineering Inc,

SLOPE/W, Geostudio