Innovative Highway Stabilisation on Rimutaka Hill RoadSelvem RamanOpus International Consultants, Wellington, New ZealandKeywords: Slope Stability, Soldier Pile, Ground Improvement Piles, Rock AnchorsABSTRACTA 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 mlong retaining structure supporting the road formation to reinstate the affected traffic lane. Theinstructions for tendering indicated that the specimen design which was a contiguous bored pilewall does not meet the principal’s requirements.A cost effective design solution consisting of a combination of unanchored and anchored (rockanchors) soldier pile wall, and ground improvement piles was developed to provide the requiredperformance. This considers the varying depth of bedrock level below existing ground. Ashotcrete facing was provided to support the ground between the piles to transfer the load to thepiles.Trench cut-off drains, sub-horizontal drainage holes, additional sumps, discharge culverts withextended flexible hoses, weepholes and stripdrains were installed to reduce the groundwaterpressures and improve stability.The design considered the importance of the highway as a key arterial road and also Transit’sdesire for a 50 year design life and 0.2g peak ground acceleration earthquake design.1 INTRODUCTIONIn February 2004, the Wellington Region was hit by a number of major storm events that had asignificant impact on the state highway network. The Rimutaka Hill Road on State Highway 2suffered a number of dropouts. Of these sites, Site 7, which is located at RP 921/9.55approximately 850 m east of the Rimutaka Summit, is a large dropout with a surface area ofapproximately 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 February2004 slip. The head scarp of this slip encroached to the edge of the road. Tension cracks withinthe east bound lane developed above the slip and into the carriageway. Both slips convergeddownslope 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 andconstruction of a 70 m long retaining structure and all the necessary ancillary works insupporting 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 forthe project. Subsequently, FH with Opus collaboration, won the DB contract.2 PRINCIPALS REQUIREMENTA 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 tenderdesign 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 designwas required to be carried out for a design durability life of 50 years. The Principal’srequirement specified that the retaining system should be designed for a reduced seismic loadcorresponding 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 roadwould be realigned in about 20 years time.3 LOCATION AND SITE TOPOGRAPHYThe slip site is shown on Figure 1. The slip and site features are shown on Figure 2. The slipsare located on a north facing slope, downhill from the summit. A large soil wedge remainsintact between two north facing headscarps, immediately below the road. The site is mostlycovered with thick bush and shrubs both uphill and downhill from the road. At this section SH2is a two lane road with a west-bound passing lane. The passing lane was closed following theslips, and two lane access was maintained. Rock exposures can be observed on the cut batterflanking the road on the south side. Project Site Figure 1: Project Site LocationThere are two culverts located below the existing road at both northern and southern ends of thesection to discharge the water from the roadside stormwater drain at the southern side of theroad. The culvert outlets were discharging on to the slope below the road through the use ofdrainage socks. Cracking in the pavement extending approximately 2 metres into thecarriageway was noted in August 2005. Further cracking in the road pavement was observed inFebruary 2006 and was located along the eastbound lane/middle lane white line. Figure 2: layout of the Slip and Site Features
4 GEOLOGY & GROUND CONDITIONSThe bedrock in this area is the Esk Head belt formation of the Torlesse Supergroup (Institute ofGeological and Nuclear Sciences, 2000). This deformed rock belt consists of Greywackesandstone and argillite dominated sequence with varying degrees of deformation.The ground conditions at the site comprise generally medium dense to loose silty sandy graveland gravely silt, underlain by very dense gravely silt and silty gravel with some cobbles andboulders. The overburden soils are underlain by slightly to moderately weathered interbeddedsandstone and mudstone, weak to very strong. The bedrock depth is generally up to 13 m with adeeper section up to about 18 m at eastern side of the section. Groundwater monitoringindicated that the groundwater is at or up to a metre above the overburden-bedrock interface.5 GEOTECHNICAL ASSESSMENTThe fill slope supporting the eastbound lane was found to be of marginal stability, as indicatedby the cracking within the pavement. The slip on the slope below the road has createdoversteepened head scarps, with a large potentially unstable wedge of soil between two slipheadscarps. The site therefore is vulnerable to failure in future storm or earthquake events. Theslip is within the fill and colluvium on which the road is constructed. The slip is a shallowtranslational type failure that occurred within the fill.The slip is presumed to have occurred during prolonged and intense rainfall events thatsaturated the soils in the steep slope resulting in failure. The uncontrolled discharge from thetwo stormwater culverts, directly onto the slope may have also contributed to the erosion andundercutting of the slope. The thick loose overburden soils, the highly fractured and shearedgreywacke bedrock and a significant variation in bedrock level presented challenginggeotechnical issues.6 REMEDIAL DESIGN CONCEPTSVarious remedial design concepts were explored and discussed with the DB contractor duringthe tender stage. The involvement of the contractor helped in evaluating cost effectiveness ofdifferent 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 limitedspace 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 mThe design concept will support the road formation, and isolate the highway from any potentialfuture slope instability below. Figure 3 presents the longitudinal section along the retainingwall showing the inferred bedrock profile and the combination of different options within thewall section.
Figure 3: Longitudinal Section of the Wall7 DETAILED DESIGN7.1 Tender AwardFH sumitted the tender based on the final remedial concept. Based on the robust and cost effectdesign 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) andexternally (by an independent consultant), and approved for construction.7.2 Detailed Design7.2.1 Soldier Pile WallThe 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 mThe soldier piles are reinforced concrete bored piles socketed into bedrock. The soldier pilessupporting more than 3 m height of overburden (above bedrock) were tied-back with a rockanchor at 1.5 m below the top of the wall. Figure 4 shows a typical detail of the anchored soldierpile. A shotcrete facing is provided for the upper 2.5 m height of the soldier pile wall belowroad level to support the ground between the piles and to transfer the local earth pressure to thepiles. Figure 4: Typical Detail of the Anchored Soldier Pile Wall7.2.2 Rock AnchorsThe 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 thepile. The rock anchors were installed through the bored piles.
The rock anchors comprise of 500 MPa multiple Strand Anchors. The anchors were designed totransfer 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, toconfirm the grout-rock bond capacity. The anchors were designed for a grout-rock ultimatebond capacity of 900 kPa based on the pull-out test results. All the anchors were subjected toacceptance testing prior to lock-off. Figure 5 shows a typical detail of the rock anchor installed. Figure 5: Typical Rock Anchor Details7.2.3 Ground Improvement PilesGround inprovement piles were installed in the section where the bedrock depth is greater than13 m. Ground improvement piles are 600 mm diameter reinforced concrete bored piles installedin a triangular grid pattern. The ground improvement piles were chosen due to the deep bedrocklevel. A soldier pile solution would have required larger piles with higher capacity multiplelevel anchors to withstand the large loads. Figure 5 shows a schematic section through theground improvement piles. Figure 5: Typical Ground Improvement Piles DetailsThe ground improvement piles were found to be more cost effective and practical than the largediameter soldier piles and multiple anchors. Soil-pile interaction was considered in the design ofground improvement piles. The reinforcement in the piles improved the bending capacity of thepiles and provided confinement to the concrete so that the piles will behave in a ductile mannerand withstand displacements, even in a earthquake events larger than the design earthquakeevent.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 thesection strengthened by ground improvement piles, to allow for differential movement duringearthquake.7.3 Drainage MeasuresVarious drainage measures were provided to reduce the groundwater pressures on the retentionsystems and improve the overall stability of the slope below. Strip drains and weep holes wereinstalled behind and through the shotcrete face. A trench cut-off drain was installed at the uphillside 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 mspacing in the slope to penetrate into bedrock at a level about 5 m below the road level. Thesewill intercept the groundwater flow above the bedrock interface. The sub-horizontal drainageholes were installed with alternate holes having different inclinations to target different depthswhere the drains enter bedrock. The discharge from the culverts flow through flexible hosesconnected to the outlets and secured down the slope using warratah stakes.8 CONSTRUCTIONThe 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 stagewere executed to ensure that the design requirements are met.A new “W” section guardrail was installed and connected to the existing guardrail at the uphillend (Upper Hutt end) and terminates in a terminal end at the downhill end (Featherston end) ofthe wall. The slope immediately below the road section was hydroseeded to minimise furthererosion and discharge of sediments down the slope. The road within the wall section wasresurfaced prior to completion of the works.The completed wall is shown in Figure 6. Figure 6: Photograph of the Completed Wall9 CONCLUSIONSThe innovative design concepts provided a cost effective and robust solution to the dropoutrepair at Rimutaka Hill Road. The DB procurement method adopted facilitated closecooperation between the contractor (FH) and designer (Opus) during tender design, detaileddesign and construction. The design recognised Transit’s performance expectations for thissection of highway and provided an innovative, practical and cost effective solution.REFERENCESInstitute 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.