The Design of Ground Anchors and Soil Nails in the Repair of State Highway 1

State Highway 1 between Christchurch and Picton was significantly damaged by the magnitude 7.8 Kaikōura Earthquake in November 2016. The North Canterbury Transport Infrastructure Recovery (NCTIR) is an alliance partnership, set-up to restore the road and rail networks in this area. Several projects undertaken by NCTIR have utilised ground anchors or soil nails in the repair or construction of structural assets, however the design used varied from site to site. This paper will discuss some types of ground anchors and soil nails. It will illustrate how they were used on this project, how corrosion protection was achieved and the design standards which were adopted.

1. Introduction

1.1 North Canterbury Transport Infrastructure Recovery (NCTIR)

The damage caused to the road and rail transport infrastructure along the north east coast of the South Island by the 2016 Kaikōura Earthquake was unprecedented in New Zealand. North Canterbury Transport Infrastructure Recovery (NCTIR) is an alliance partnership between the NZ Transport Agency (NZTA), KiwiRail, Fulton Hogan, Downer, HEB Construction and Higgins that was set-up to restore the road and rail networks. More than 1300 people from 100 organisations are part of NCTIR, working to restore the transport corridor along the Kaikōura coastline.

1.2 Typical Applications in NCTIR Structures

Ground anchors and soil nails have been utilised by NCTIR in the design of retaining walls, rock fall mitigation, tunnel repairs and various other uses on and around State Highway 1 and the MNL Railway. This paper will focus predominantly on the use of ground anchors and soil nails for retaining structures supporting State Highway 1 where such uses have included:

• Installing anchors through damaged existing gabion basket walls and kingpost walls to improve stability and/or wall capacity.
• Incorporating anchors into the design of new king post and bored pile walls in order to reduce the embedment required and limit outward displacement under future seismic loading.
• New gabion walls incorporating ground anchors rather than MSE reinforcing in order to minimise the excavation needed during construction.
• Soil nail walls designed to stabilise existing soil slopes and crib walls
• Stabilising steep slopes below new gravity walls with soil nails

2. Design Guidelines

2.1 Standards and Codes

For the NCTIR project the design of assets supporting the state highway has been completed in accordance with NZTA Bridge Manual 3rd Edition, 2016 (Bridge Manual). In addition to this, a Design Philosophy Report for Retaining Structures was prepared by NCTIR and approved by NZTA. Between these two documents, the following list of references specific to the design of ground anchors and soil nails was developed:

• NZTA Bridge Manual 3rd Edition, 2016, Part 6 Site stability, foundations, earthworks and retaining walls
• NCTIR Design Philosophy Report for Retaining Structures on the Existing Road Network 100001-CD- RW-DP-0001[1]
• BS8081:2015 Code of practice for grouted anchors, The British Standards Institution 2017
• BS EN 1537:2013 Execution of special geotechnical works — Ground anchors
• FHWA-IF-99-015 Geotechnical Engineering Circular No. 4 – Ground Anchors and Anchored Systems – June 1999
• FHWA 96-069r Manual for Design and Construction of Soil Nail Walls (which has since been superseded by FHWA-IF-03-017 Geotechnical Engineering Circular No. 7 – Soil Nail Walls)

In addition to the above documents, NCTIR made two applications to the NZ Transport Agency to depart from the Bridge Manual during the design of ground anchors and soil nails. In both cases these were a departure from the approach to corrosion protection required by the Bridge Manual.

2.2 Ground Anchors or Soil Nails

As defined in Table 6.4 of the Bridge Manual, soil-nailed walls are categorised as a mechanically stabilised earth (MSE) wall. Soil-nailed walls comprise concrete and steel insertions knitting together a whole block of soil to form a mass which acts under gravity to remain stable, whereas anchored walls, as defined in the Bridge Manual, are designed to transfer some of the loads on walls to the ground outside the zone of influence of the wall. The NCTIR project used either soil nails or ground anchors at over 30 sites.

2.3 Key Guidance from the Bridge Manual for Ground Anchor Design

The following points summarise the key guidance from the Bridge Manual for ground anchor design:

• Walls that are restrained using anchors are designed to transfer some of the loads on walls to the ground outside the zone of influence of the wall.
• Anchored walls are generally rigid systems, and shall be designed to resist the full ground, groundwater and earthquakes forces on the walls.
• The anchor system shall be designed to ensure a ductile failure of the wall under earthquake overloads.
• Pull-out tests shall be carried out on trial anchors.
• On-site suitability tests shall be carried out accordance with BS EN 1537.
• On-site acceptance tests shall be carried out in accordance with BS EN 1537on all as built ground anchors.
• Long term monitoring and instrumentation should be carried out in accordance with FHWA-RD-97-130 Design manual for permanent ground anchor walls.

2.4 Key Guidance from the Bridge Manual for Soil Nail Design

The following points summarise the key guidance from the Bridge Manual for soil nail design:

• Soil nailing shall be carried out only on drained slopes that are free of groundwater
• Limited controlled block displacement may be allowed in strong earthquakes
• Pull-out tests shall be carried out
• On-site suitability tests shall be carried out on a selected number of production soil nails as per BS EN 1537(49)
• On-site acceptance tests shall be carried out in accordance with BS EN 1537(49) on at least 25% of all as built soil nails.

2.5 Corrosion Protection for Ground Anchors

The Bridge Manual states that all anchor systems shall be corrosion protected to ensure durability over the design working life of the structure. Two classes of protection are specified. Table 6.5 and Figure 6.5 in the Bridge Manual provide a decision making process based on the aggressively of soil, consequences of failure and the cost benefit. Class I includes double corrosion protection by encapsulation of the tendon or bar pre-grouted under factory conditions inside a corrugated plastic sheath. Class II has single corrosion protection such as galvanizing or a fusion bonded epoxy-coating.

2.6 Corrosion Protection for Soil Nails

The Bridge Manual states that soil nail reinforcement shall be subject to the same corrosion protection requirements as ground anchors.

3. Ground Anchor and Soil Nail Systems

3.1 Systems considered at NCTIR

Both ground anchors and soli nails can be considered as a series of structural elements (rods) protruding into the ground. The rods can have various different forms depending on the system adopted. During the design process, various types of ground anchor or soil nailing systems were considered by the NCTIR designers. These included solid bars, hollow core self-drilling bars, fibre reinforced plastic (FRP) bars or direct push anchors. The following sections provide a general description of each of these ground anchor types.

3.2 Solid Bars

Solid bar anchors or nails typically comprise steel bars installed in a drill hole and grouted. These bars can be encapsulated in a grout core which is cast within a plastic sheath to provide Class I corrosion protection. In the case of construction in weak ground where drill holes are at risk of collapse during installation, temporary casing can be provided to keep the drill holes open.

3.3 Hollow Core Self-drilling Bars

Self-drilling anchors do not require a separate process to form the drill hole. A sacrificial drilling head at the tip of the anchor rods forms and governs the diameter of the anchor hole. Grout is pumped through the hollow core of the rods which eventually form the steel reinforcement component of the anchor. If necessary the grout can also be used as a drilling fluid during installation. Self-drilling anchors cannot be encapsulated in a plastic sheath to provide double corrosion protection.

3.4 FRP Bars

Fibre reinforced plastic (FRP) bars are solid and are installed using the same methods as described above for solid steel bars. A key advantage over a steel anchor is that they are significantly lighter. A lighter bar is much safer to handle on site. The other key advantage of FRP bars is that they have better corrosion resistance characteristics when compared to steel and can achieve a 100 year design life without encapsulation or additional treatment. The disadvantage is that the failure mechanism is brittle rather than a ductile plastic failure.

3.5 Direct Push Anchors

Direct push anchors typically have a flat head on the end of a series of steel rods which is rotated at the end of installation to provide a mechanical anchor. There is no grout component to these anchors and they rely on loading of the soil at the end plate to achieve their capacity. Because the anchors are pushed, rather than drilled, they are not suitable for installation in rock. These also cannot be encapsulated in a plastic sheath to provide double corrosion protection, although the specification of more corrosion resistant materials can improve the durability of these anchors. Of the systems presented here, only these direct push anchors are only suitable as ground anchors and not also suitable as soil nails. This is due to the load transfer mechanism.

4. Significant NCTIR Ground Anchor and Soil Nail Sites

4.1 The Sand Pit

4.1.1 Location and Damage

The NCTIR Sand Pit site is located on State Highway 1, just south of the Clarence River. The earthquake damage at this site consists of both fill embankment and cut slope failures along a 700 m long section of 2-lane highway. In the worst damaged area, the embankment failure destroyed the entire width of the south bound lane, as can be seen in Photograph 1. The ground conditions in this location are predominantly dune sand and the main challenges to the project were the confined nature of the site and the short time frame available for design and construction before reopening the highway. A temporary solution, comprising anchored gabion baskets, was designed and constructed to work around these constraints.

Photograph 1: Embankment failure at the Sand Pit, photograph taken January 2017.

4.1.2 The Sand Pit Wall Design

4.1.2.1 Optioneering

The design of this wall relied on the stability of the slope below the wall and analysis showed that the static slope stability factor of safety specified in the Bridge Manual was not easily achieved. It was recognised that design solutions which addressed this instability, in full compliance with the Bridge Manual, may not be achievable with in the NCTIR timeframe. However, to facilitate opening the road by December 2017, a temporary solution could be constructed. This design would have a design life of up to 5 years. In parallel, feasibility and pricing studies for a more long term solution to realign SH1 were carried out. The selected solution presents a lower cost, temporary option with a design life of up to 5 years comprising anchored gabion walls on the outside edge of the road.

4.1.2.2 Solution Overview

The advantage of an anchored solution at the Sand Pit was that it minimised the need to excavate to install a gravity or MSE solution as this would not be possible without closing the adjacent haul road which was essential to enable other critical repair works to the south. The maximum wall height was 6.0 m, and ground anchors were installed through the new gabion baskets.
The ground conditions were of Quaternary Dune Sand. Where ground anchors were installed, this was typically encountered as Sand with minor Silt or trace Gravel increasing in density with depth. Pre-construction anchor tests demonstrated that the ground was typically unstable during drilling and collapsed during open hole drilling before an anchor could be inserted and grouted. To overcome this construction issue, self-drilling (hollow core) anchors installed with a grout flush were specified. As discussed previously, it is not possible for this type of ground anchor to meet the requirements of the Bridge Manual for permanent anchor corrosion protection. A departure from the Bridge Manual was secured because these works were part of a temporary structure with a 2-5 year design life. A departure for reduced design seismic loading for this asset was also obtained because of the short design life.

During design of the Sand Pit ground anchors, a geotechnical ultimate grout to ground bond stress of 90 kPa was adopted. This bond strength was based on sacrificial anchor testing comprising three test anchors each with a bond length of 6.0 m and drill hole diameter of 0.1 m. The anchors failed between 175 kN and 255 kN. Three sacrificial direct push anchors were also installed and tested. Each direct push anchor took just 3 to 4 minutes to install and be ready for testing. The ultimate loads achieved were 66 kN, 81 kN and 90 kN for three different sizes of anchor heads. The extension recorded when loading each anchor was between 280 and 380 mm.

4.1.3 Detailing

The main design details for the gabion basket and selfdrilling anchor wall at the Sand Pit comprised:

• A backing plate installed on each anchor to transfer the load from the gabion to the anchor.
• A wedge shaped bearing plate sat in front of the backing plate to allow anchors to be installed at 15˚ below horizontal.
• The anchor head was wrapped in denso tape to protect it from corrosion.
• A debond sleeve was installed over the hollow core anchor to create the unbounded length. This comprised denso tape and a pvc sleeve.
• A PVC pipe was installed through the gabion basket and 200 mm into the fill behind the wall to protect the anchor in this section where is is most vulnerable to corrosion, water flow and abrasion.
• Anchor nuts were to be hand tightened, rather than having a lock off load applied.

Figure 1 provides further details of the Sand Pit Ground Anchors.

Figure 1: The Sand Pit ground anchor details.

4.1.4 Testing

Testing of the Sand Pit ground anchors was completed in accordance with the recommendations that are published in FHWA-IF-99-015 Geotechnical Engineering Circular No. 4 – Ground Anchors and Anchored Systems. During construction 5% of the anchors were performance tested and the remaining 95% were loaded for proof testing. The results are summarised in Table 1.

Photograph 2 shows the Sand Pit retaining wall during November 2017 when construction of this structure was approximately 80% complete.

Table 1: Summary of anchor test results at the Sand Pit

Photograph 2: Anchored gabion retaining wall at the Sand Pit during construction.

4.2 Wall 375

4.2.1 Wall Location and Damage

Wall 375 is located in the Hundalees, approximately 30 km south of Kaikōura. Many of the NZTA retaining wall assets in this area were badly damaged during 2016 Kaikōura Earthquake. Some walls rotated and settled, while others completely collapsed. At this particular location a significant portion of the slope supporting the road shoulder failed which left the north bound lane constrained and the location subject to a 30 km/h speed restriction. Subsequent to the Kaikōura Earthquake, heavy rainfall and concentrated stormwater flow caused further damage. Ultimately the road geometric design at this location called for a raised road level and improved stormwater drainage. Each of these factors compounded to drive the need to construct a new retaining wall at this location. Photograph 3 shows the Wall 375 site a few weeks after the 2016 Kaikōura Earthquake.

Photograph 3: Shoulder failure at the site of Wall 375.

4.2.2 Wall 375 Design

The NCTIR design for Wall 375 comprised a gabion basket wall atop a soil nailed slope. The geometry of the site meant that a soil nail slope alone could not recreate the new road height that was required for the geometric design and therefore additional gabion baskets were required to allow the shoulder to be built up. Analysis indicated that the steep slope below the proposed baskets did not provide sufficient bearing support and soil nails were designed and installed to provide a stable formation. The total height of both the gabion basket and soil nail structures is approximately 6.0 m.

The ground conditions through the Hundalees area typically comprise of Gravel fill underlain by Loess Colluvium over firm to extremely weak Mudstone of the Pahau Terrane. Sacrificial testing was undertaken in both Colluvium and Mudstone.

In Colluvium, with N60 of typically between 6 and 20, two tests were undertaken and both achieved a load of 400 kN. These tests had to be terminated before failure due to the test load reaching 80% of the bar capacity. Two further tests in Mudstone, with N60 > 50, failed at 300 and 325 kN. All of these tests had a bond length of 2 m and a drill hole diameter of 100 mm. Based on failure at 300 kN, a conservative ultimate grout ground bond strength of 480 kPa was adopted in design for both ground types in the Hundalees. A factor of 3 was applied to this value in design.

Figure 2: SlopeW output of the pseudo-static analysis of Wall 375.

The analysis of the gabion basket wall atop a soil nailed slope included a slope stability model analysed using SlopeW from GeoStudio 2016 under static and pseudo-static conditions. An extract from the analysis including a peak ground acceleration of 0.4g is shown in Figure 2.

Figure 3 provides a cross section and details of the structure which has been constructed at the site of Wall 375.
The construction sequence for Wall 375 was as follows:
1. Clear all vegetation and loose material from the slope
2. Install the soil nails inn a top down sequence and position the drainage strips and reinforcing mesh
3. Shotcrete the slope face followed by installation of the nail heads
4. Apply a final coat of shotcrete to cover the nail heads
5. Excavate for and construct the gabion basket wall section
6. Place backfill behind the gabion baskets
7. Construct road pavement, barriers, stormwater and other associated elements

Figure 3: Typical cross section through Wall 375 and details.

4.2.3 Wall 375 Detailing

As shown in Figure 4, the design details for Wall 375 include:

• Three rows of 7 m long soil nails constructed using hollow core anchors on a diamond pattern spaced at 1.0 m vertically and 2.0 m horizontally.
• A 100 mm diameter grouted drill hole. A small amount of Shotcrete was also used to completely fill the top of the drill hole and ensure complete coverage of the bar.
• A 250 mm minimum thickness of shotcrete facing reinforced with steel mesh. Such shotcrete was applied to the upper slope face and wrapped over the top of the slope to minimise water ingress (see Figure 3).
• Geocomposite drainage strips behind the shotcrete connect to pvc weepholes at 2.0 m centres to address the risk of excess pore water pressure developing behind the shotcrete facing.
• A soil nail head detail comprising horizontal and vertical bearing bars, a trimmer bar to form a mechanical connection between the reinforcing mesh in the Shotcrete face and the nail, L-bars to reinforce the final face Shotcrete, an anchor plate, a domed base plate and nuts to match the anchor bar.

Figure 4: Soil nail head and shotcrete detail for Wall 375.

4.2.4 Wall 375 Soil Nail Testing

Testing of the Wall 375 soil nails was completed in accordance with the recommendations published in FHWA-IF-03-017 Geotechnical Engineering Circular No. 7 – Soil Nail Walls. 10% of production nails in each row were to be subjected to proof testing with a design test load of 180 kN. This meant a total of 2 production nails were tested. They both satisfactorily supported the maximum test load of 270kN which is 150% of the design test load. This indicates that a bond strength of greater than 106 kPa was achieved. Photograph 4 shows the first row of soil nails for Wall 375 immediately after installation.

Photograph 4: Photograph taken May 2018 showing the first row of nails at Wall 375 immediately after installation.

4.3 Wall 433

4.3.1 Wall 433 Location and Damage

Wall 433 is an example of an existing retaining wall in the Hundalees area which was damaged by the 2016 Kaikōura Earthquake. It comprised a steel kingpost wall with timber lagging. The steel posts generally settled and/or displaced out of alignment with some movement at the base of the retained height evident. Due to the rake of the wall, it is not clear if any outward rotation of the wall occurred as a result of the 2016 Kaikōura Earthquake. Photograph 5 shows the damaged wall.

Photograph 5: Damage to Wall 433 due to the 2016 Kaikōura Earthquake, photograph taken early 2017.

4.3 Wall 433 Design

To completely remove and rebuild Wall 433 would require significant excavation and back fill works to be undertaken. This would have been expensive and require a road closure during construction. It was therefore judged by the NCTIR Design Team that the optimum solution comprised a retrofit of the existing wall with ground anchors. Such ground anchors were designed to prevent any future rotation of the existing wall and improve the overall stability of the wall. In the design calculations, a minimal embedment of the kingposts was assumed due to the uncertainty around the as-built structure. The maximum retained height of the wall was 2.0m. Figure 5 shows an elevation of Wall 433 at the location of the retrofit ground anchors and walers.

Separate sacrificial anchor tests were not undertaken for this site because of the proximity to other sites where tests had already been undertaken in the Hundalees. The bond strength value presented in the previous section for Wall 375 was adopted for wall 433 as well.

For comparison, direct push anchors were installed and tested at the same sites as previously discussed. In colluvium the direct push anchor failed at 312 kN and 329 kN in Mudstone. Installation of the direct push anchor in Mudstone was possible by predrilling a pilot hole. While the direct push anchors had sufficient capacity, the anchor head dimensions needed to achieve this capacity were considered to be too large to easily install through the existing wall and therefore self-drilling anchors were specified here.

Figure 5: Extract from the construction drawings for Wall 433.

4.3.3 Wall 433 Detailing

The key components of the repair works design comprised:

• The single staggered row of hollow core ground anchors had a horizontal spacing of 1.5 m to match the exiting post spacing.
• Ground anchors had a bonded length of 3.0 m and an unbonded free length of 3.0 m.
• The ground anchors were staggered vertically to allow space for the whalers to overlap fully over each pair of kingposts.
• Whalers, comprising galvanised back to back PFCs, were designed to transfer the load from the existing posts to the ground anchors. The PFCs had stiffener plates added and backing plates provided at each anchor head.
• Custom packing wedges were provided between the posts and the whalers had to be used to ensure uniform load transfer due to the uneven post alignment. This is also why whalers were only ever designed to span across two posts.
• In addition to the backing plates, the anchor heads were also provided with a wedge bearing plate to accommodate the angle of the anchors.
• All anchor heads were wrapped in two layers of denso tape to protect them from corrosion.
• A debond sleeve was installed over the hollow core anchor to create an unbounded free length of 3.0 m. This debond section also comprised denso tape and a pvc sleeve. It extended through the whaler to provide protection to the otherwise exposed section of new ground anchor.
• Anchor nuts were to be hand tightened, rather than having a lock off load applied.

Figure 6 shows further detail of the ground anchor head detail that was constructed at Wall 433. Photograph 6 shows Wall 433 during construction of the NCTIR ground anchor retrofit works.

Figure 6: Plan of the whaler and anchor head details for Wall 433.

4.3.4 Wall 433 Ground Anchor Testing

Testing of the Wall 433 ground anchors was completed in accordance with the recommendations published in FHWA-IF-99-015 Geotechnical Engineering Circular No. 4 – Ground Anchors and Anchored Systems. During construction one anchor of the 12 installed was performance tested and the remainder were proof tested. All ground anchors satisfactorily supported the maximum test load of 153 kN. This indicates that a bond strength of greater than 160 kPa was achieved. Creep testing recorded displacements of up to 0.65 mm between minute 1 and minute 10 of the test.

Photograph 6: Photograph of Wall 433 before anchor heads were installed, taken May 2018.

5. Testing Summary

Table 2 and Table 3 summarise the sacrificial testing discussed in this paper. Table 2 presents results undertaken on hollow core ground anchors, while Table 3 presents results from direct push anchor tests.

Table 2: Anchor test results from testing of hollow core ground anchors

Table 3: Anchor test results from testing of direct push ground anchors

6. Conclusions

In each of the case studies presented in this paper, hollow core self-drilling type ground anchors or soil nails were constructed. These were found to be significantly faster to install than the alternative solid bar solution given the collapsing ground conditions encountered on all three of the sites. Hollow core anchors do not provide Class I corrosion protection as required in the Bridge Manual for permanent structures. In these particular situations, departures were granted by NZTA to allow the use of hollow core anchors. In each case the size of the hollow core bar specified was greater than required. This was in order to give the desired capacity at the end of 100 year design life assuming a loss of material through corrosion over time. In the disaster recovery environment, the advantages afforded by speed of construction was significant and hollow core bars proved much faster to install than solid bars in the conditions encountered.

Direct push anchors were considered and tested during design. In Loess colluvium and very weak Mudstone, the capacity of the anchor was similar to drilled anchors. However in sand the capacity was much lower. Direct push anchors were very fast to install and test, but significant deformation of the anchor has to occur in order to engage the soil to provide a reaction load.

One of the key debates throughout the design process was classifying whether retaining wall assets were to have soil nails or anchors. Construction, installation and detailing often looked similar, but the two systems differed in terms of assumed behaviour during the life of the asset, the design analysis and the testing requirements.

Much attention was given during the NCTIR project to detailing for corrosion protection. The most vulnerable section of the anchors and nails is assessed in many published references and by the NCTIR Design Team as being just behind the head. This area is more likely to be exposed to air and water attack, which in many cases was exacerbated by the presence of salt. This section of the anchors and nails may also be subject to changing loads or twisting as the wall facing and components could displace in the future. These issues were addressed in a number of ways as detailed in the case studies presented in this paper, including continuing the debond sleeve to the back of the nuts, wrapping vulnerable elements in denso tape, providing pvc sleeves and encapsulating soil nail heads in Shotcrete.

The testing regimes recommended by the FHWA for ground anchors and soil nails are very different. Typically all ground anchors are subject to some level of as-built testing, whereas only up to 25% of soil nails may be tested. This is assessed by the author as a reflection of the different levels of criticality of any given bar in each system.

NCTIR initially operated in an environment where much of State Highway 1 was closed. This meant that passing traffic volumes were low or absent and closing the road was relatively easy. Once the road reopened to the public immediately prior to Christmas 2017, it became much more difficult to form large excavations which impacted the road and enable the construction of gravity or MSE walls. In this restricted post December 2017 environment, installing ground anchors or soil nails was preferred over excavation.

A cost comparison of the two methods was undertaken and construction costs were found to be comparable. However, with due consideration of the whole life cost, ancho