This paper describes the design evolution of two NDRRA (Natural Disaster Relief and Recovery Arrangements) sites, designated as 9501 and 4551, damaged in March 2017 by Category 4 Severe Tropical Cyclone Debbie and associated intense rainfall and subsequent flooding. The sites were two of more than 300 sites to be remediated in the South Coast Region in south east Queensland, mostly located in hilly terrain and/or with steep batters. The approach adopted for the two sites’ remedial works involved stabilising the slopes by combining soil nails and gabions. The design evolution and the choice of final remediating solution are discussed with emphasis on ground conditions, site topography, environmental and time constraints. The two case histories illustrate the approach and process for developing a cost-efficient remediation design while minimising environmental impact, and highlight the collaboration between the engineering disciplines and the Queensland Department of Transport and Main Roads (TMR).
Severe Tropical Cyclone Debbie and associated rainfall and flooding caused significant damage to Queensland, Australia’s state-controlled road network, between 28 March and 6 April 2017. TMR managed the reconstruction works as part of Natural Disaster Relief and Recovery Arrangements (NDRRA) and engaged WSP Australia (WSP) to provide design solutions for more than 300 sites on TMR roads in the Gold Coast City Council, Scenic Rim Council and Logan City Council areas.
The design and construction were to be completed before the 2018 rainfall season, meaning the design and documents had to be complete within 10 weeks of engagement. The accelerated design process required close collaboration between TMR and WSP, and between design disciplines involving the road, geotechnical, pavement and drainage.
Twenty-nine of the 300+ affected sites needed geotechnical treatments. Some sites, mostly located in hilly terrain with steep slopes, heavy vegetation and varying ground conditions, presented unique geotechnical challenges. This paper discusses the design evolution of two of these challenging geotechnical sites; sites 9501 and 4551. The design geotechnical acceptance criteria adopted were TMR Geotechnical Design Standards – Minimum Requirements dated 2015. The designs also needed to work within the constraints of NDRRA guidelines to reinstate the sites rather than develop solutions that would constitute an improvement. This paper discusses the selection of final remediation solutions emphasising site constraints, geotechnical conditions, environmental restrictions and time constraints. The WSP – TMR collaboration, and those between geotechnical and other disciplines, are highlighted.
Site location and slope conditions
Site 9501 is located on an arterial road – the Mount Lindesay Highway (TMR Road No. 25B) near the border of New South Wales and Queensland, Australia. The site is a cut into the side of a hill, with a slope of about 1V:1H below the road. A landslip occurred on the slope below the road, removing the road shoulder (Figure 1). The slip size was about 30 m (L) x 7 m (H) x 3 m (D).
Immediately after Severe Tropical Cyclone Debbie, TMR engaged another consultant to assess the failed slope and provide a remediation concept. Their proposed remediation, not based on detailed survey or geotechnical investigations, was a gabion wall with a 1V:1.5H rockfill slope behind (Figure 2). The slope’s total height was assumed at about 3.5 m.
Figure 1: Damage to Site 9501
Inferred slip surface
As WSP was engaged for the detailed design, a detailed site survey was subsequently carried out to 1m grid at ±25 mm. It was identified afterwards that the failed slope was about 7 m high – much higher than the initially assumed 3.5 m.
Figure 2: Initial design concept by another consultant
It was known that the embankment contained a drainage pipe with the inlet about 3 m below the embankment. However, thick vegetation at the time of the site survey meant the pipe outlet could not be identified. Due to time constraints, TMR agreed to determine the associated work for the existing pipe outlet after it was exposed during construction.
One borehole (BH01-9501) was drilled at the site to a 15 m depth from the road level. The ground conditions were generally 0.5 m thick fill (road base), 4.5 m thick colluvium (stiff to very stiff silty clay/gravelly clay) followed by 6.6 m thick residual soil (stiff to very stiff sandy clay), underlain by extremely weathered coal/siltstone (Walloon Coal Measures) with very low strength.
The failed section sat mainly within the colluvium material (Figure 2).
Detailed design and site constraints
The survey results showed the increased embankment height (from 3.5 m to 7 m) and indicated that the proposed gabion wall solution with rockfill batter (Figure 2) meant encroaching adjacent private property which would have been an unacceptable design outcome. The road shoulder had been partly washed away during the cyclone and TMR required its reinstatement to its previous width (2.5 m wide). This was not allowed for in the proposed concept design. TMR also required the road to remain open for construction work duration with minimum traffic disturbance.
Based on the survey and geotechnical data, geometrical and boundary constraints and shoulder reinstatement requirements, WSP revised the design concept to a combination of soil nail and gabion solution. Soil nails would restore the existing slope to its required factor of safety and could be installed on the post-failure slope surface. To reinstate the 2.5 m wide road shoulder, a gabion wall founded at the toe of the soil nail slope was designed. Embankment fill (sandy clay) was backfilled in the void between the gabions and soil nail reinforced slope (Figure 3), with appropriate geotextile separator and drainage.
Existing pipe to be grout filled
Figure 3: Sketch showing adopted design
To identify the location of the outlet of the existing pipe relative to the proposed remediation works, a temporary excavation was carried out during construction to expose the outlet. It was found that the existing 600 mm diameter pipe was damaged and silted up (Figure 4). A CCTV camera was used to identify the extent of damage and results indicated the entire pipe was damaged and needed to be replaced.
Figure 4: Existing pipe outlet exposed
The outlet of the exsitng pipe was located at about 7 m below the road surface. If the pipe was to be replaced at the current level, pipe jacking would be required to allow the road to remain open during construction.
However, TMR advised that pipe jacking was a significant cost and would also cause significant construction delay. Subsequently, WSP carried out a basic drainage study which assessed catchment area and water velocity. The solution was to install a new 750 mm diameter pipe at a higher level which would outlet onto a gabion mattress (Figure 3). The exisiting pipe was filled with grout and abandoned. This higher-level pipe reduced the excavation depth, saved construction time and significantly reduced construction risk and cost. A hydraulic study indicated acceptable flow dissipation on the gabion and mattress.
Soil nail installation started in June 2018 and gabions were subsequently installed (Figure 5). The new pipe was installed into an open trench supported by shoring box. The trench was excavated one lane width at a time, this allowed the traffic to flow using traffic lights. The slope remedation works were completed within three months.
Figure 5: Gabion wall installed
Site location and slope conditions
Site 4551 is located on Binna Burra Road (TMR Road No. 2021) in Lamington National Park, Queensland, Australia. The road at the site is approximately 5 m wide. A failure occurred on the down slope with a gradient of about 1V:1H (Figure 6).
Figure 6: Damage to Site 4551
375 mm Dia Pipe
The slumped area was approximately 8m long and had a 375 mm diameter drainage pipe protruding from the eroded soil surface just below the road level. The failure extended about 15 m vertically down the slope. The slope height was unknown (greater than 50 m) and approximately follows the same 1V:1H grade.
It was inferred that the failure was associated with erosion from high velocity surface water flow from the culvert outlet, regressed towards the shoulder and road pavement.
As the site is within the Lamington National Park, environmental approvals associated with some construction works were going to significantly impact the construction programme. Additionally, the existing steep slope below the road had limited access for personnel and machinery. Therefore, the design development needed to consider project constraints as well as the requirement to dissipate flow energy from the existing drainage pipe over the failed area.
Initial concept options
The following three options were considered at the initial concept design:
- Option 1 – No remediation
- Undertaking no remediation was discounted immediately due to the risk of further road erosion. Heavy rain and flows from the existing drainage pipe would likely result in rapid loss of further material from the slope resulting in road closure. Binna Burra Road is the only access road to the Binna Burra Lodge resort and tourism businesses. Any erosion of the already narrow pavement would have cut off access and would not have been acceptable for local communities.
- Option 2 – Dumped rockfill
- Protecting the exposed slip slope with loose dumped rockfill was initially considered as a low-cost repair option due to its apparent ease of construction. Rock would be tipped from the existing road over the failed area and this option would not require access to the slope by personnel or machinery. It was disregarded for the following reasons:
- Dumping the rock on such a steep, high slope would not allow the rock to be contained in the eroded area. The rock would likely continue rolling down the slope, leaving high uncertainty regarding rock volume and associated cost, as well as unquantifiable disturbance to vegetation – an unacceptable outcome.
- Compaction and reinstatement of the road shoulder over the dumped rock would be difficult to achieve on the steep slope without a compacted subgrade base.
- Option 3 – Gabions and an outlet structure
- A concept design for gabions and an outlet structure along the slope was prepared (Figure 7). The proposed structure would continue to about 15 m vertically from the road surface to cover the eroded area.
- This option resulted in about 130 m3 of rock filled gabions and mattress terraced into the slope at 1 m steps. Construction of the compacted and trimmed 1 m steps would require access to the slope for personnel and machinery. It required time consuming and costly clearing, environmental approvals, and post-construction rehabilitation. Furthermore, accessing such a steep slope (1V:1H) posed significant Safety in Design (SiD) risks to personnel and machinery operators.
Figure 7: Gabions along the slope
Grouted rock pitching
- Option 3 was abandoned due to unacceptable SiD risks, constructability issues, environmental impacts and time constraints.
Further concept option development
Following the initial concept design stage, it was realised that the concept options were not appropriate to remediate the site and a more robust solution was required. Additionally, it was assessed that a geotechnical investigation to collect geotechnical data and site survey were required as inputs in the development of concept options.
Geotechnical investigation and site survey
One borehole (BH01-4551) was drilled from the road level to an 8.6 m depth below the ground surface. The encountered ground conditions were generally 2.5 m thick fill, 3 m thick extremely weathered basalt, which is underlain by moderately weathered to fresh basalt (Beechmont Basalt) with medium to high strength. An outline of the ground conditions interpreted following the geotechnical investigation is presented in Figure 8 (page 6).
The site survey identified that a Telstra optic fibre cable was located at about 0.7 m depth near the edge of the failed slope. TMR discussed the cable with the asset owner, Telstra, to explore the possibility of relocating the optic fibre cable. Telstra’s initial advice indicated that a total of 3 km of cable would have to be relocated resulting in significant cost. Subsequently, design options were developed to protect the Telstra cable.
No fines concrete
Temporary bored pile
Permanent bored pile
310 UC steel column
Telstra optic fibre cable
Design options to protect Telstra cable
To protect the Telstra optic fibre cable and restore slope stability, two additional concepts were developed (Options 4 and 5).
- Option 4 – Piled wall
- The piled wall option was considered and a typical cross section for this option is shown in Figure 8. The wall selected was an H-pile (UC310 steel column) set into bored piles socketed into the medium to high strength basalt.
- Precast concrete sleepers would then be guided into place and contained by the H-piles to form the wall facing and retaining structure.
- To construct the permanent bored piles for the H-pile wall, a piling rig would need to be positioned on the edge of the carriageway immediately above the scoured face. The weight and vibration of this machine would cause further collapse of the scour face. To manage this risk, a temporary bored pile wall could be constructed just behind the face of the scour to support the piling rig.
Figure 8: Piled wall option
- The existing Telstra optic fibre cable would be located by vacuum excavation and carefully exposed. The exposed cable could be protected in a split conduit during construction and locally relocated.
- Option 5 – Realignment of Binna Burra Road
Option 5 was to realign Binna Burra Road and move further away from the slip face. This would involve excavation on the high side of the road on the existing cut batter between about 2 m and 5 m high at the realignment. The length of work would be approximately 200 m and at a nominal 2 m away from the scour. The cost of traffic management, earthworks, pavement and drainage for a “like for like” carriageway standard would be substantial and would not allow for protection of the Telstra optic fibre cable located within 1 m from the scarp.
Furthermore, the risk of subsequent erosion regressing into the pavement remained and scour protection construction would still be required. The cost of this option was assessed as substantially higher than Option 4 and as a result, was abandoned.
Design options with Telstra cable relocated
Option 4 – piled wall was identified as the solution meeting the geotechnical performance criteria. TMR did a cost estimate for this option and the result was about $1.3 million, which was substantially higher than what was budgeted for.
A collaborative approach between TMR, design engineers and the contractor was initiated to develop a cost- efficient solution. It was identified that the Telstra fibre optic cable was the major restriction on the site and relocating the cable should be reinvestigated. An alternative cost-efficient slope restoration, such as soil nailing, was put forward.
TMR contacted Telstra to discuss the relocation of the cable. After careful investigations and deliberations, Telstra advised that the cable could be permanently relocated to the other side of Binna Burra Road but only about 75 m of cable could be relocated.
Two additional design concept options (Options 6 and 7) were then developed based on the assumptions that 75 m of the Telstra cable could be relocated.
- Option 6 – Reinforced soil structure
A typical cross section of this option is shown in Figure 9. It involves the excavation of the upper part of the slip slope down to a suitable founding material (i.e. extremely weathered basalt) and the installation of a Reinforced Soil Structure (RSS).
Temporary soil nail
Telstra optic fibre to be relocated
A Terramesh system together with geogrid was selected for this option.
A temporary soil nail wall would be installed to support the excavation for the permanent RSS wall.
Figure 9: Reinforced soil structure
- Option 7 – Gabion wall
- WSP was advised that the RSS (Terramesh System) in Option 6 was not approved by TMR but the temporary soil nail wall was. In Option 7, a conventional gabion wall, together with the temporary soil nail wall were adopted.
Temporary soil nail
- At the existing pipe location, a new junction box drop structure would be constructed (Figure 10). This would allow drainage flows to be discharged at the ground level after substantial flow energy was dissipated within the drop structure. This reduced the risk of future scour at the outlet.
- This option was assessed as cost efficient and time efficient and was adopted as the final design.
Figure 10: Gabion wall (final design)
The construction started in August 2018 with the relocating of the Telstra optic fibre cable. Excavation was carried out and temporary soil nails were installed. Gabions were constructed and drop structure was cast in situ (Figure 11 – page 8). The construction was completed, and the road opened to public in December 2018 (Figure 12 – page 8).
Figure 11: During construction
Figure 12: Construction completed
Soil nail wall
The lessons learned from the design and construction of the two NDRRA sites are summarised below.
- Slope stabilisation works can have many constraints such as topography, environmental restrictions, property boundary, site access, live traffic and time (including an approaching rain season).
- These constraints should be identified early in the process and considered when selecting appropriate remediation options.
- Slope stabilisation works with many site constraints require ongoing collaboration between TMR, the asset owner, the design engineer and contractor.
- The following collaboration proved to be beneficial:
- Design Engineer with TMR: share information and knowledge, identify key risk areas, accelerate approval, develop a cost-efficient solution
- Design Engineer/TMR with asset owner: save construction time, decrease cost, reduce environment impact, improve safety in design
- Design Engineer/TMR with contractor: share project experience, understand construction methods and associated constraints, develop a cost-efficient solution.
- Geotechnical remediation works require collaboration between the geotechnical designer and other engineering disciplines. The following collaborations can be beneficial:
- Geotechnical and road design: fit road geometry and road furniture into remediated slope
- Geotechnical and drainage: identify impact of drainage on geotechnical design, fit drainage structure into remediated slope, design erosion mitigation, develop a cost-efficient solution
- Geotechnical and pavement: develop pavement details over remediated slope.
- Creative combination of well-known local remediation methods (and familiar to contractors) can offer cost-efficient design to a heavily constrained site.
- A combination of soil nail wall and gabions was adopted at both sites, each for different reasons. At site 9501, a permanent soil nail was adopted to stabilise the existing slope with the gabions used to reinstate the road shoulder. At site 4551, a soil nail wall was adopted for the temporary excavation and gabions were adopted as the permanent retaining structure.
- Detailed site survey should be carried out early to understand site constraints and features.
- Experience from site 9501 indicates that without a survey, design solutions can be misguided. At site 4551, the Telstra optic fibre cable was found during site survey (and later in design development) and became a governing constraint of remedial solutions.
- It is recommended that a detailed survey be carried out to extend beyond the failed area. If a detailed survey is not available, simple survey tool such as dumpy level and good engineering geology mapping can be deployed to obtain the approximate geometry, identify failure mechanism and assess slope failure risk. Any underground services should be accurately located at the early stage of the design development stage.
- Geotechnical investigations are consistently beneficial to design solution and reduce overall risk for unforeseen ground conditions.
- Geotechnical investigations consistently provide site specific subsurface condition information which forms the basis of option analysis and reduces potential for over conservative design assumptions leading to costly remediation design.
- Underground services are to be relocated where possible and negotiated with the asset owner.
- The Telstra optic fibre cable at site 4551 was a governing constraint to remediation solutions. Relocating the cable greatly reduced the construction risk and allowed a cost-efficient remediation to be implemented. It is essential to engage and collaborate with asset owners during this process.
- Geotechnical slope remediation works for NDRRA sites are subject to many constraints, including site topography, environmental restrictions, property boundaries, asset owners, safe site access, live traffic and time. These constraints are to be considered when selecting appropriate remediation options. Remediation works require collaboration between TMR, consultants, asset owners and contractors. Detailed site survey and geotechnical investigations are to be carried out to identify site topography, subsurface conditions and underground services. A combination of well-known and locally accepted geotechnical remediation measures may be adopted to produce a constructible and cost-effective solution without compromising safety while reducing environmental impact. The geotechnical remediation works require collaboration with other disciplines such as road, drainage, and pavement.
The author thanks the Queensland Department of Transport and Main Roads (TMR) for permission to publish this paper. The author acknowledges the individual efforts of all personnel involved in the design and construction outlined in this paper. In particular, the author would like to acknowledge TMR’s NDRRA delivery staff, Daryl Johnson (WSP), Darrin White (WSP), David Rankin (WSP), Vincent Blanchet (WSP) and Eric Lo (WSP). The author wants to thank Li-Ang Yang, Lianne McKenzie and Bronte Seidel for reviewing and editing the paper.