Innovative Highway Stabilisation on Rimutaka Hill Road

Selvem Raman

Opus International Consultants, Wellington, New Zealand

Keywords: Slope Stability, Soldier Pile, Ground Improvement Piles, Rock Anchors

ABSTRACT

A section of the SH2 Rimutaka Hill Road was affected by a dropout caused by storm events.

Transit New Zealand let a “design and build” tender for the design and construction of a 70 m

long retaining structure supporting the road formation to reinstate the affected traffic lane. The

instructions for tendering indicated that the specimen design which was a contiguous bored pile

wall does not meet the principal’s requirements.

A cost effective design solution consisting of a combination of unanchored and anchored (rock

anchors) soldier pile wall, and ground improvement piles was developed to provide the required

performance. This considers the varying depth of bedrock level below existing ground. A

shotcrete facing was provided to support the ground between the piles to transfer the load to the

piles.

Trench cut-off drains, sub-horizontal drainage holes, additional sumps, discharge culverts with

extended flexible hoses, weepholes and stripdrains were installed to reduce the groundwater

pressures and improve stability.

The design considered the importance of the highway as a key arterial road and also Transit’s

desire for a 50 year design life and 0.2g peak ground acceleration earthquake design.

1 INTRODUCTION

In February 2004, the Wellington Region was hit by a number of major storm events that had a

significant impact on the state highway network. The Rimutaka Hill Road on State Highway 2

suffered a number of dropouts. Of these sites, Site 7, which is located at RP 921/9.55

approximately 850 m east of the Rimutaka Summit, is a large dropout with a surface area of

approximately 850 m2. The total length of highway affected was about 70 m.

Following heavy rainfall in June 2005, a larger slip occurred immediately east of the February

2004 slip. The head scarp of this slip encroached to the edge of the road. Tension cracks within

the east bound lane developed above the slip and into the carriageway. Both slips converged

downslope leaving a large potentially unstable wedge of soil between the two.

Transit New Zealand (Principal) let a “design and build” (DB) tender, for the design and

construction of a 70 m long retaining structure and all the necessary ancillary works in

supporting the road formation at Site 7. MWH, the region’s network management consultant,

was the Principal’s Advisor, and had undertaken the site investigations and preliminary design.

Fulton Hogan (FH) approached Opus International Consultants (Opus) to be the Designer for

the project. Subsequently, FH with Opus collaboration, won the DB contract.

2 PRINCIPAL'S REQUIREMENT

A specimen design was made available in the tender document for contractor’s information.

However, the tender document clearly stated that the Specimen Design does not meet the

Principal’s requirements. The DB contractor was required to prepare and submit a new tender

design which meets all the principal’s requirements (Transit New Zealand, 2006).

The design had to be in compliance with the Transit New Zealand Bridge Manual. The design

was required to be carried out for a design durability life of 50 years. The Principal’s

requirement specified that the retaining system should be designed for a reduced seismic load

corresponding to a 100 year return period earthquake with a peak ground acceleration of 0.2g.

The reduced PGA was due to Transit’s expectation that this section of the Rimutaka Hill road

would be realigned in about 20 years time.

3 LOCATION AND SITE TOPOGRAPHY

The slip site is shown on Figure 1. The slip and site features are shown on Figure 2. The slips

are located on a north facing slope, downhill from the summit. A large soil wedge remains

intact between two north facing headscarps, immediately below the road. The site is mostly

covered with thick bush and shrubs both uphill and downhill from the road. At this section SH2

is a two lane road with a west-bound passing lane. The passing lane was closed following the

slips, and two lane access was maintained. Rock exposures can be observed on the cut batter

flanking the road on the south side.

Figure 1: Project Site Location

There are two culverts located below the existing road at both northern and southern ends of the

section to discharge the water from the roadside stormwater drain at the southern side of the

road. The culvert outlets were discharging on to the slope below the road through the use of

drainage socks. Cracking in the pavement extending approximately 2 metres into the

carriageway was noted in August 2005. Further cracking in the road pavement was observed in

February 2006 and was located along the eastbound lane/middle lane white line.

Figure 2: layout of the Slip and Site Features

Project Site

4 GEOLOGY & GROUND CONDITIONS

The bedrock in this area is the Esk Head belt formation of the Torlesse Supergroup (Institute of

Geological and Nuclear Sciences, 2000). This deformed rock belt consists of Greywacke

sandstone and argillite dominated sequence with varying degrees of deformation.

The ground conditions at the site comprise generally medium dense to loose silty sandy gravel

and gravely silt, underlain by very dense gravely silt and silty gravel with some cobbles and

boulders. The overburden soils are underlain by slightly to moderately weathered interbedded

sandstone and mudstone, weak to very strong. The bedrock depth is generally up to 13 m with a

deeper section up to about 18 m at eastern side of the section. Groundwater monitoring

indicated that the groundwater is at or up to a metre above the overburden-bedrock interface.

5 GEOTECHNICAL ASSESSMENT

The fill slope supporting the eastbound lane was found to be of marginal stability, as indicated

by the cracking within the pavement. The slip on the slope below the road has created

oversteepened head scarps, with a large potentially unstable wedge of soil between two slip

headscarps. The site therefore is vulnerable to failure in future storm or earthquake events. The

slip is within the fill and colluvium on which the road is constructed. The slip is a shallow

translational type failure that occurred within the fill.

The slip is presumed to have occurred during prolonged and intense rainfall events that

saturated the soils in the steep slope resulting in failure. The uncontrolled discharge from the

two stormwater culverts, directly onto the slope may have also contributed to the erosion and

undercutting of the slope. The thick loose overburden soils, the highly fractured and sheared

greywacke bedrock and a significant variation in bedrock level presented challenging

geotechnical issues.

6 REMEDIAL DESIGN CONCEPTS

Various remedial design concepts were explored and discussed with the DB contractor during

the tender stage. The involvement of the contractor helped in evaluating cost effectiveness of

different proposals. The following concepts were considered during tender stage:

• Contiguous bored piles with 2 levels of rock anchors (specimen design),

• Soldier piles with rock anchors,

• Soldier piles with deadman anchors,

• Soil nailing of the slope below the road and

• 3 rows of concrete columns along the edge of the road.

The final remedial concept was chosen to accommodate large variations in the bedrock level

(from existing ground level to 18 m depth), and allow construction from within the limited

space available. The final remedial design concept comprised a combination of:

• Unanchored Soldier Pile wall, where depth to bedrock is up to 3 m

• Anchored Soldier Pile wall, where depth to bedrock is 3 m to 13 m

• Ground Improvement Piles, where depth to bedrock is greater than 13 m

The design concept will support the road formation, and isolate the highway from any potential

future slope instability below. Figure 3 presents the longitudinal section along the retaining

wall showing the inferred bedrock profile and the combination of different options within the

wall section.

Figure 3: Longitudinal Section of the Wall

7 DETAILED DESIGN

7.1 Tender Award

FH sumitted the tender based on the final remedial concept. Based on the robust and cost effect

design concept, the DB contract was awarded to FH with Opus as its designer. Subsequently,

Opus developed the detailed design which was reviewed both internally (within Opus) and

externally (by an independent consultant), and approved for construction.

7.2 Detailed Design

7.2.1 Soldier Pile Wall

The soldier piles comprised:

• 600 mm diameter piles at 2 m centres, for bedrock depths of up to 8 m

• 900 mm diameter piles at 2 m centres, for bedrock depths between 8 m and 10 m

• 1200 mm diameter piles at 2 m centres, for bedrock depths between 10 m and 13 m

The soldier piles are reinforced concrete bored piles socketed into bedrock. The soldier piles

supporting more than 3 m height of overburden (above bedrock) were tied-back with a rock

anchor at 1.5 m below the top of the wall. Figure 4 shows a typical detail of the anchored soldier

pile. A shotcrete facing is provided for the upper 2.5 m height of the soldier pile wall below

road level to support the ground between the piles and to transfer the local earth pressure to the

piles.

Figure 4: Typical Detail of the Anchored Soldier Pile Wall

7.2.2 Rock Anchors

The rock anchors were installed at 1.5 m below road level to enable practical construction,

without having to excavate too far below the road and also to minimise bending moments in the

pile. The rock anchors were installed through the bored piles.

The rock anchors comprise of 500 MPa multiple Strand Anchors. The anchors were designed to

transfer the wall loads to the bedrock through a fixed bond length in the slightly weathered rock.

Three anchor pull-out tests were undertaken before installation of production anchors, to

confirm the grout-rock bond capacity. The anchors were designed for a grout-rock ultimate

bond capacity of 900 kPa based on the pull-out test results. All the anchors were subjected to

acceptance testing prior to lock-off. Figure 5 shows a typical detail of the rock anchor installed.

Figure 5: Typical Rock Anchor Details

7.2.3 Ground Improvement Piles

Ground inprovement piles were installed in the section where the bedrock depth is greater than

13 m. Ground improvement piles are 600 mm diameter reinforced concrete bored piles installed

in a triangular grid pattern. The ground improvement piles were chosen due to the deep bedrock

level. A soldier pile solution would have required larger piles with higher capacity multiple

level anchors to withstand the large loads. Figure 5 shows a schematic section through the

ground improvement piles.

Figure 5: Typical Ground Improvement Piles Details

The ground improvement piles were found to be more cost effective and practical than the large

diameter soldier piles and multiple anchors. Soil-pile interaction was considered in the design of

ground improvement piles. The reinforcement in the piles improved the bending capacity of the

piles and provided confinement to the concrete so that the piles will behave in a ductile manner

and withstand displacements, even in a earthquake events larger than the design earthquake

event.

A shotcrete wall was installed along the middle pile for the road above to be supported.

Movement joints were provided in the shotcrete wall between the soldier piled section and the

section strengthened by ground improvement piles, to allow for differential movement during

earthquake.

7.3 Drainage Measures

Various drainage measures were provided to reduce the groundwater pressures on the retention

systems and improve the overall stability of the slope below. Strip drains and weep holes were

installed behind and through the shotcrete face. A trench cut-off drain was installed at the uphill

side of the highway and connected to the existing culverts to intercept the large seepage flows

from the hillside into the road formation. Sub-horizontal drainage holes were installed at 4 m

spacing in the slope to penetrate into bedrock at a level about 5 m below the road level. These

will intercept the groundwater flow above the bedrock interface. The sub-horizontal drainage

holes were installed with alternate holes having different inclinations to target different depths

where the drains enter bedrock. The discharge from the culverts flow through flexible hoses

connected to the outlets and secured down the slope using warratah stakes.

8 CONSTRUCTION

The construction of the remedial works was carried out between April and November (2007).

Sufficient quality control measures proposed for wall elements during the detailed design stage

were executed to ensure that the design requirements are met.

A new “W” section guardrail was installed and connected to the existing guardrail at the uphill

end (Upper Hutt end) and terminates in a terminal end at the downhill end (Featherston end) of

the wall. The slope immediately below the road section was hydroseeded to minimise further

erosion and discharge of sediments down the slope. The road within the wall section was

resurfaced prior to completion of the works.

The completed wall is shown in Figure 6.

Figure 6: Photograph of the Completed Wall

9 CONCLUSIONS The innovative design concepts provided a cost effective and robust solution to the dropout

repair at Rimutaka Hill Road. The DB procurement method adopted facilitated close

cooperation between the contractor (FH) and designer (Opus) during tender design, detailed

design and construction. The design recognised Transit’s performance expectations for this

section of highway and provided an innovative, practical and cost effective solution.

REFERENCES

Institute of Geological and Nuclear Sciences (2000) Geology of the Wellington area, scale

1:50,000. Geological Map 22. Prepared by Begg, J.R. and Johnston, M.R.

Transit New Zealand (2006) Contract 494PN, SH2 Rimutaka Dropout Site 7 Remedial Works.

Contract Documents.

Transit New Zealand (2003) Bridge Manual. Prepared by Opus International Consultants.