stabilisation of a fill embankment using … of a fill embankment using soil nails hackney and...

Australian Geomechanics Vol 49 No 1 March 2014 81

STABILISATION OF A FILL EMBANKMENT USING SOIL NAILS

Greg Hackney and Chris Bridges Principals, Coffey Geotechnics, Brisbane

ABSTRACT Drilled and grouted soil nails have been successfully used to stabilise a section of fill embankment on the Warrego Highway near Toowoomba, Queensland that failed as a result of heavy rainfall in January 2011. Instability occurred due to groundwater rise within the fill materials resulting in a tension crack developing at the traffic lane edge and an approximate 300 mm displacement within the outer portion of the embankment. Reinforcement of the unstable fill slope with soil nails was the selected remediation method. The soil nails were installed through the existing granular fill materials and grouted into the fill and underlying basalt. Soil nail holes were drilled using equipment fitted to excavators to typical lengths of 10 m to 12 m with temporary casing used to prevent hole collapse during drilling. A reinforced shotcrete facing was constructed over the face of the embankment to provide the necessary nail head restraint and prevent erosion. Soil nails were installed in a prescribed sequence to manage the risk of construction plant trafficking the marginally stable fill embankment. Full time monitoring of construction was carried out in accordance with an action plan developed specifically for the site. The coordinated approach to the design and construction of the works resulted in a successful implementation of the remedial works.

Keywords: Soil nails, Warrego Highway, Stabilisation, Embankment

1 INTRODUCTION In January 2011, following a period of sustained wet weather, an extreme rainfall event resulted in significant damage to a number of sections of the Warrego Highway on the Toowoomba Range. Damage comprised failures in cut slopes above the road with erosion and instability of fill embankments. With the Warrego Highway forming a key transport link between the western parts of Queensland and Brisbane, providing an operational highway as soon as possible was critical to the region’s economy and recovery.

After initial emergency works undertaken by the Queensland Department of Transport and Main Roads (TMR) to clear the carriageways of debris, eight sites were identified as requiring remediation. Six sites comprised erosion of fill embankments on the uphill lanes and two involved embankment instability on the downhill lanes.

This paper discusses the design and construction of a soil nail remediation strategy for one site (Site 1) on the downhill lanes of the highway that suffered instability within the fill embankment.

2 BACKGROUND The Warrego Highway forms a vital transport link between the coal mining industries located west of Toowoomba and in the Surat Basin, and the agricultural industries of the Darling Downs and further west. Coal and agricultural produce is transported via rail and road to Brisbane for distribution and export. Coal is typically transported down the range by rail whilst an annual average daily traffic (2010 data) of 11,000 vehicles with approximately 15% heavy vehicles accommodates road freight under normal conditions.

The Toowoomba Range section of the highway is steep with up to 10% grades evident. The original range crossing was developed in the 1850’s. The current range crossing comprises side by side dual carriageways for the uphill and downhill lanes for the lower half of the range and grade separated dual carriageways for the upper half. Over the upper section, the current downhill lanes follow the alignment of the original range crossing which have been widened and upgraded several times since the 1930’s. The uphill lanes were constructed on the current alignment in the 1960’s.

On 10 January 2011, a rain event estimated to be in the order of a 1 in 200 year to 1 in 300 year average recurrence interval (ARI) storm event (BMT WBM, 2011) occurred producing significant surface water runoff volumes, severely damaging both road and rail lines that traverse the Toowoomba Range. The rail line was cut at several locations, whilst the Warrego Highway range crossing suffered significant damage.

With the rail link severed indefinitely it was imperative for the Warrego Highway to be made operational as soon as possible to provide a transport link for people, coal exports and produce. Truck movements up and down the range increased significantly, as the Warrego Highway formed the primary route for livestock and coal movements over the range.

STABILISATION OF A FILL EMBANKMENT USING SOIL NAILS HACKNEY AND BRIDGES


3 GEOTECHNICAL CONDITIONS

3.1 GEOLOGICAL AND TOPOGRAPHICAL SETTING Toowoomba is located on a plateau at the top of a 400 m high escarpment that, at this location, forms the slopes of the Great Dividing Range. The plateau at the top of the range comprises Tertiary aged basalt from the Main Range Volcanics, which overlies sedimentary rocks from the Jurassic aged Marburg Formation (Bathurst and Heaton, 2008). The unconformity between the volcanics and sedimentary formations is approximately half way down the range. Site 1 is located within the upper slopes of the range which are relatively steep (in the order of 30°) and locally steeper within incised gully features.

The basalt in the area is variably weathered, with materials ranging from residual soils to slightly weathered basalt evident in cuttings above the road. Colluvial soils to varying depths are evident on the flanking slopes of ridgelines. Site 1 is located towards the top of the range, approximately 500 m below the crest of the escarpment (Figure 1). In the vicinity of Site 1, weathered basalt is exposed in the bottom of gully features, with the eroded basalt materials being deposited further down towards the escarpment toe in scree deposits.

The steep terrain required the formation of cut to fill embankments for both the uphill and downhill lanes. The road features numerous high cuts and deep fills together with several tight curves to provide a grade suitable for vehicles. The original roadway forming the current downhill lanes was manually constructed with likely little control over construction practices.

Figure 1: Upper Section of the Warrego Highway – Toowoomba Range Crossing.

3.2 SITE CONDITIONS Site 1 is located between a ridgeline and gully feature, both trending south-west to north-east (Figure 2). At the western end of Site 1 the road embankment is entirely within fill that has been placed to build up the formation. At the eastern end of Site 1 the road transitions into a cut slope on both sides of the road. Surface water collected in the drain on the high side of the road is directed to a culvert located at the eastern end. Dense vegetation covered most of the fill embankment.

Downhill Lanes

Uphill Lanes

East Toowoomba

Escarpment

Site 1

N

Brisbane (≈120km)

0.5

0.25

0

kilometres



Figure 2: Location of Site 1.

The instability at the site involved the outer edge of the fill embankment as shown on Plates 1 and 2. At the time of the initial geotechnical assessment, movement of the failed mass was in the order of 150 mm vertically and 50 mm horizontally, increasing to 300 mm vertically and 100 mm horizontally over the ensuing week. Mapping of the area below the fill embankment revealed an old mortared stone retaining wall constructed as part of widening works in the 1930s. Bellow the central portion of the Site 1 failure the stone wall had collapsed, although it was apparent the collapse had occurred in the past and was not associated with the 2011 failure.

Plates 1 and 2: Tension Cracks and Movements of the Failed Mass

Ridgeline

Gully

Site 1

N

Culvert

0

metres

50 100



Geotechnical investigations were undertaken by TMR approximately one week after the instability and comprised boreholes and test pits from the pavement surface. From that information, survey data provided by TMR and mapping, a generalised geotechnical model was developed, as shown on Figure 3. Standpipe piezometers were installed in the boreholes to allow ongoing monitoring during remedial works.

Figure 3: Adopted Geotechnical Model.

The investigations indicated that the Sandy SILT fill materials comprising the majority of the embankment were dry at the time of the investigations. The underlying Sandy CLAY material, which may comprise fill or colluvial soils that were not removed before the Sandy SILT fill was placed, was wet. From this observation, a groundwater level at the top Sandy CLAY fill was interpreted.

3.3 INFERRED FAILURE MODE An approximate circular failure surface extending down to the firm, wet Sandy CLAY unit, triggered by an increase in pore pressure, was assessed as a likely failure mode based on the following key observations and assessment:

• The interpreted geotechnical section indicated poorly compacted, more granular, fill materials within the upper section of the embankment, overlying saturated firm clayey fill materials. Given the relatively strong nature of the underlying basalt and the high moisture content and firm consistency of the Sandy CLAY fill, it was interpreted that the basal plane for movement was likely to be within the Sandy CLAY. The dry nature of the overlying Sandy SILT fill suggested that the groundwater did not rise to within that unit during the rainfall event. Note that observations of the test pits which were excavated at the location of the tension crack on the pavement did not indicate an obvious failure plane.

• Observation of the culvert at the eastern end of Site 1 soon after the failure indicated that it was blocked. This blockage almost certainly contributed to the groundwater rise within the fill although it is not known whether the blockage occurred before or during the 2011 event.

• The partial collapse of the old retaining wall below the embankment suggests that movement within the fill embankment may have occurred or been ongoing prior to the 2011 event. It is not known whether the loss of support contributed to the instability. At very least it has resulted in a steepened slope within the embankment over a section of the failed area.

• Mapping of the area below the failed embankment did not reveal any indications of heave or rupture, although dense vegetation made observations difficult.

• The shape of the tension cracks was arcuate as evident in Plate 1.

3.4 BACK-ANALYSIS OF GEOTECHNICAL PARAMETERS Back-analysis of the failure was undertaken using 2D Limit Equilibrium Methods (LEM), considering the inferred mode of failure as a guide to failure surface geometry and the geotechnical model presented on Figure 3. A Factor of Safety (FOS) of 1 was adopted for the modelling with a piezometric surface at the top of the Sandy CLAY unit for the

Chainage (m)

Elev

atio

n (m

, AH

D)

20 40 60

610

600

590

Weathered Basalt

Fill: Sandy SILT, very loose, dry

Fill(?): Sandy CLAY, firm, wet

Partially Collapsed Mortared Stone Wall

Tension Crack



drained analysis. A total stress analysis was also undertaken to provide a basis for undrained strengths within the soil units.

Initial back-analysis parameters were selected based on the investigation information and experience, with the values adjusted until a FOS of approximately 1 obtained. Back analysed parameters are presented in Table 1. The failure surfaces considered in the back-analyses were consistent with the field observations and were confined to within the soil materials.

Table 1: Back-Analysed Parameters

Unit Adopted γ (kN/m3) Drained Undrained c´ (kPa) Ф´ (deg) cu (kPa)

Fill: Sandy SILT 18 0 28 30 to 35 Fill(?): Sandy CLAY 19 5 30 27 to 30 Weathered BASALT 21 10 35 - Note: A number of possible combinations of values of cu for the upper materials will provide a similar FOS

4 REMEDIAL WORKS DESIGN

4.1 DESIGN OPTIONS Design concepts considered for the site comprised ‘excavate and replace’ and ‘stabilise in situ’ options. Excavate and replace options required the installation of temporary support of the existing fill materials over an approximate 8 m high face below the carriageway to maintain one operational traffic lane. The new embankment would then be constructed using rockfill, a reinforced soil structure or a gravity retaining wall. The stabilise in situ option considered soil nails installed through the embankment fill. Ultimately, the potential for errant heavy vehicles entering a deep excavation directly below a single traffic lane on a downhill section of highway proved the deciding factor in favour of the soil nailed embankment option.

4.2 DESIGN CRITERIA In the absence of specified design criteria from TMR for the works and given the limited timeframe for the investigation, design and construction, design criteria was developed that provided for typical FOS under normal conditions and considered the sensitivity of the FOS to changes in the strength parameters and groundwater level. The design criteria provided for:

• FOS not less than 1.5 using the back-analysed drained strength parameters and a piezometric surface at the level assessed at the time of failure.

• Sensitivity assessment on effective friction angle up to 5° below the back-analysed value for the soil units.

• Sensitivity assessment using the back-analysed drained strength parameters and a piezometric surface at ground surface level. For this case the FOS should be ≥1.0.

Constructability was to be considered as part of the design.

4.3 DESIGN APPROACH AND OUTCOMES The design approach adopted was that of a slope reinforced with soil nails. Although the use of permanent grouted soil nails to stabilise fill embankments has been carried out overseas (Martin, 1997; Johnson et al., 2002; Perry et al., 2003; Phear et al., 2005), there appears to be little published literature detailing the successful use of the same in Australia (e.g. Seto et al., 1993). This may be due to design concerns and/or construction difficulties such as grout loss, hole collapse and plant access on an unstable slope. The authors were concerned with the possibility that a nominal bond may be available within the embankment fill and, therefore, carried out an additional assessment in the event that the soil nails acted as passive soil anchors instead of reinforcing elements.

Analyses were undertaken using a 2D LEM approach using both Slope/W and Snailz software. Each program has limitations in terms of problem geometry or output information; therefore, both methods were utilised. Ultimate bond stresses of 20 kPa for the fill and 100 kPa within the weathered basalt were assumed at the design stage with a FOS of 1.5 applied to provide allowable values. The design bond stress within the weathered basalt was confirmed by three pullout tests undertaken on site. A 20kPa load to model traffic was included in the analyses.

The final design required seven rows of soil nails installed at a horizontal spacing of between 1.5 m (upper three rows) and 1.8 m and a 2 m spacing down the slope, as shown on Figure 4.



Figure 4: Soil Nailed Embankment Remedial Design.

Soil nails were nominated as N32 deformed bars grouted into a minimum 125 mm diameter hole. Bonded lengths for the soil nails of 4 m to 5 m into the weathered basalt were required, necessitating approximately 10 m long soil nails for the lower four rows and 12 m soil nails for the upper three rows. Durability requirements for soil nails were in accordance with the TMR specifications, comprising hot dip galvanised bars grouted within a HDPE sheath. A total of 232 soil nails were nominated for the site.

In the permanent case, a shotcrete facing was provided to provide the necessary nail head restraint and to prevent erosion of the embankment fill materials. The shotcrete was continued down to the partially failed, mortared, stone wall that existed at the toe of the fill embankment. The shotcrete facing design was undertaken using an approach from Federal Highway Administration (1998) with nail head loads assessed from the Snailz outputs. The facing comprised a 110mm thick, 32 MPa shotcrete reinforced with one layer of SL81 mm reinforcing mesh. At the nail head the shotcrete was locally thickened to 200 mm with an additional layer of SL81 mesh incorporated to improve punching resistance in the event that the soil nails act primarily as anchors. The soil nail design incorporated a 200 mm by 200 mm and 20 mm thick soil nail head plate with reinforcing mesh in the shotcrete fixed behind the head plate at each nail location.

To reduce the driving force on the failed mass and to enable the soil nailing rig to reach the lower row of nails, a temporary excavation was undertaken at the embankment crest, supported at the carriageway edge by a concrete barrier retaining wall.

Site constraints and limited construction time necessitated the use of excavator mounted drilling rigs for the soil nail holes. Necessary construction load limitations affected the choices of plant for the works, resulting in a novel approach to the soil nail installation. To achieve an acceptable FOS during construction, a small excavator rig was nominated to install the upper three rows of soil nails. The small excavator was able to reach ahead of itself to drill holes, by moving across the site and standing on an area where nails and a temporary facing system comprising reinforcing mesh pinned beneath the soil nail head plates had been installed. A minimum grout strength requirement of 20 MPa was required before the small excavator could traffic the area directly above the soil nail. To achieve the necessary reach to install the lower rows of soil nails, a larger (60 T) long reach excavator was required. The larger excavator was allowed to traffic the site after the completion of the permanent shotcrete facing over the upper three rows of soil nails.

A heavy duty pavement was nominated by TMR for the completed embankment. As a risk management measure, a biaxial geogrid was included to reduce the risk of reflective cracking in the pavement between the existing and replaced sections of fill embankment.

A series of horizontal drain holes were nominated in the design to assist in controlling water levels within the slope. Other improvements to drainage at the site included an upgrade to the culvert passing beneath the carriageway and permanent outlet works and interface drains at the edges of the new section of pavement.

The design was reviewed and approved by TMR before construction commenced.

Soil Nails (N32 Bars) with double corrosion protection

Shotcrete facing, locally thickened around soil nail heads, reinforced with SL81 galvanised mesh

Temporary Excavation to provide access for construction equipment and reduce load on failed mass

Temporary retaining system

Traffic barrier

25m long, min. 100mm diameter sub-horizontal borehole drains



5 CONSTRUCTION

5.1 SOIL NAIL INSTALLATION AND SHOTCRETE FACING Construction works were undertaken over a six month period with the majority of that time comprising a 24 hour per day operation. The traffic on the highway was constrained to one lane past the work site, with very limited night-time only road closures for activities such as forming the temporary excavation at the crest of the embankment.

Soil nail holes were drilled using air-flush drilling systems. Water-flush systems were not permitted due to the risk of saturating the embankment fill materials and re-initiating instability. Due to the need to maintain a grout/soil bond within the fill, holes were drilled using a temporary steel casing to prevent hole collapse. The HDPE liners, centralisers and grout lines were installed into the casing and rock socket. Soil nail bars were sourced as continuous 14 m lengths to avoid the need for joining within the hole. Bars were fitted with centralisers and grout lines and installed into the pre-placed HDPE sheath using a small crane working from the excavated bench. Grouting inside and outside the HDPE sheath was undertaken simultaneously. The temporary casing was removed during the grouting operation with top-up grout placed continuously during this process to ensure the necessary bond was achieved.

All soil nail holes were logged by field staff to ensure consistency with the design. The vast majority of nails were installed to the design lengths, with one small area of the site requiring longer nails (up to 14 m) to achieve the necessary bond lengths in the weathered basalt.

All work on the slope was undertaken using rope access for safety. Short (1.5 m) lengths of casing drill rods were employed to reduce manual handling risks and all casing was lifted in and out using the small crane.

Plates 3 and 4: Installation of HPDE Sheath and Soil Nail Bar.

During the early stages of construction, progress was slow as a result of imposed limitations including no drilling within two nail spacings of a soil nail grouted 24 hours earlier and the minimum 20 MPa strength requirement for trafficking above installed soil nails. This situation was improved by the use of high early strength grout which was demonstrated by testing to achieve the necessary strength within six hours of installation. Once the upper three rows of nails and shotcrete facing were installed, more cost effective GP cement based grout was used for soil nails.

The shotcrete facing was installed from a man-cage suspended from a mobile crane due to the difficulties in handling shotcrete lines on the slope. The facing was placed in panels as the works proceeded. Strip drains were installed behind the shotcrete facing with weep holes through to facilitate drainage.

5.2 OBSERVATIONS AND QUALITY CONTROL The potential for ongoing movement of the failed mass, particularly early in the construction, necessitated a carefully managed monitoring programme for the site. A monitoring plan was developed comprising:



• A full-time surveyor who continuously monitored prisms for movement.

• Lines of surface monitoring pegs that allowed simple visual observation of movement.

• Daily monitoring of groundwater levels in piezometers retained from the original investigation.

An action plan with defined roles and responsibilities in the event of movement was implemented and observed throughout the construction.

The quality of soil nail construction was demonstrated through acceptance testing of production nails. Acceptance testing was undertaken in accordance with the TMR specifications. Acceptance testing is a measure of quality of workmanship with soil nails selected randomly for testing. Loads were applied in increments of 20% of the nominated working load up to the full working load and held at each increment for one hour for each of three loading cycles. At each load increment deflection measurements were made. A total of 15 acceptance tests were undertaken with all tests passing the nominated requirements.

Grout cube and concrete cylinder testing was nominated and undertaken to verify the strength of the grout and shotcrete respectively. Records of grout volumes for each nail hole were taken. Over the course of the construction the frequency of testing was reduced, as the confidence in the mix and consistency of the products increased.

All work was undertaken in the full time presence of Geotechnicians acting as the designers’ representative which allowed adjustments to the design to be made quickly when unexpected conditions were encountered. The Geotechnicians also provided quality control for the overall works.

Plate 5: Completed Soil Nailed Fill Embankment

6 PROJECT MANAGEMENT AND COORDINATION The remedial works were delivered by a team comprising representatives from the local TMR offices, TMR Major Projects, Coffey Geotechnics (designer) and Fulton Hogan (contractor). The broad objectives of providing a reliable solution as quickly as possible underpinned the philosophies of the project team:

• To make decisions on the best knowledge and engineering possible at the time.

• Review those decisions during the construction process and amend the design and/or construction process to accommodate conditions encountered.

• Adopt solutions that are based on good engineering practice and industry accepted design criteria.



• Adopt solutions that provide the best judged outcomes with respect to safety, timeliness, cost and construction practicality.

The strong working relationships and level of trust developed within the project team allowed issues to be raised quickly during both design and construction and solutions to be developed. This approach was critical to the successful completion of the works.

7 CONCLUSION A failed fill embankment has been successfully stabilised using soil nails to reinforce the slope without the need for excavation and replacement of the embankment. The technique was achieved using a grid of soil nails typically 10 m to 12 m in length and a reinforced shotcrete facing to provide the necessary nail head restraint and reduce erosion potential. Soil nails were installed into holes drilled using excavator mounted rigs from the top of the embankment, using temporary casing to support the holes prior to grouting. The movement of construction plant over the site was carefully managed to reduce the risk of further instability due to construction loads. A soil nail installation sequence was prescribed and followed, with progressively larger equipment allowed on the site to install lower rows of soil nail as the stability was improved. Full time geotechnical presence on site coupled with the quality control testing of project components provided the confidence in the construction. The project management philosophy adopted by the project team provided a framework that allowed non-standard remedial options to be considered and implemented with a high degree of confidence in the constructed product.

8 ACKNOWLEDGEMENT The authors thank Sanjay Ram from TMR for permission to publish this paper and the work during the project by staff from TMR Major Projects and the TMR Toowoomba regional office.

9 REFERENCES Bathurst, R. and Heaton, G. (2008), Toowoomba Bypass – Range Cross with Tunnels, Queensland Roads, Edition 5,

March 2008. BMT WBM (2011), Technical Report on the Toowoomba Flood of January 2011, Prepared for Local Government

Association of Queensland Ltd, Ref: R.B18317.001.02.doc. Federal Highway Administration (1998), Manual for Design and Construction Monitoring of Soil Nail Walls,

Publication No FHW A-SA-96-069R. Johnson, P.E., Card, G.B. and Darley, P. (2002), Soil nailing for slopes, Report 373, Transport Research Laboratory,

Crawthorne. Martin, J. (1997), The design and installation of soil nail slope stabilization using “Snail” Proceedings Third

International Conference on Ground Improvement Geosystems, pp398-406. Perry, J., Pedley, M.J. and Reid, M. (2003), Infrastructure embankments – condition appraisal and remedial treatment,

C592, 2nd Ed., CIRIA, London. Phear, J., Dew, C., Ozsoy, B., Wharmby, N.J., Judge, J. and Barley, A.D. (2005), Soil nailing – best practice guidance,

C637, CIRIA, London. Seto, P., Won, G.W. and Choi, K.Y. (1993), Use of soil nailing in stabilisation of a freeway embankment. Proceedings

of the International Symposium on Earth Reinforcement Practice, pp537-542.

stabilisation of a fill embankment using … of a fill embankment using soil nails hackney and...

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