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Proceedings of the Institution of Civil Engineers

http://dx.doi.org/10.1680/geng.13.00040

Paper 1300040

Received 01/04/2013 Accepted 12/11/2013

Keywords: excavation/geotechnical engineering/temporary works

ICE Publishing: All rights reserved

Geotechnical Engineering

A soil-nailed excavation for the Brisbane

Airport Link project, Australia

Bridges and Gudgin

A soil-nailed excavation for theBrisbane Airport Link project,Australiaj1 Chris Bridges MSc, PhD, CEng, FICE, MHKIE, RPEQ

Senior Principal, Coffey Geotechnics Pty Ltd, Brisbane, Australiaj2 James Gudgin MEng, CEng, MICE, RPEQ

Senior Geotechnical Engineer, Coffey Geotechnics Pty Ltd, Brisbane,Australia

j1 j2

The Airport Link project in Brisbane, Australia, involved 15 km of tunnelling including 5.7 km of twin road tunnels,

busway tunnels and connecting ramps. The Kedron area is the location of a number of entry and exit ramps which

join the tunnel. The construction site consisted of a number of tunnels crossing each other and formed using a

number of techniques. Access was required to a maximum of 18 m below ground level to allow for the construction

of a pile-supported capping slab, which would act as a roof slab for a mined tunnel passing east to west. This paper

presents details of the design and construction of the soil-nailed walls which provided support to three sides of this

excavation. The excavation also provided access for commencement of a mined tunnel using canopy tubes headingeastward through one of the soil-nailed walls. The project constraints meant that the solution required close

coordination between the contractor and the designers of the temporary and permanent works. The excavation was

constructed successfully and has now been decommissioned. Design risks were managed throughout construction

through continuous on-site observation and a comprehensive monitoring programme.

Notationc9 characteristic effective cohesion

E elastic modulus

H height of soil-nailed wall

K constant

K0 earth pressure coefficientqu unconfined compressive strength

Su undrained shear strength

b bulk unit weight

Poisson ratio

9v effective vertical stress at centre of length of soil nail

beyond calculated failure surface

ave average ultimate bond stress

9 characteristic effective friction angle

1. IntroductionThe Brisbane Airport Link project was a complex A$4.2 billion

road tunnel project, which has provided over 7 km of, mostly

underground, new road. It was delivered using the publicprivate

partnership (PPP) model by the special project vehicle (SPV)

Brisconnections for the Queensland government. The works

were designed and constructed by a ThiessJohn Holland joint

venture (TJH) and completed in July 2012. The project required

15 km of tunnelling owing to the twin boring technique adopted

and a total of 25 new bridges for interchanges. The project links

Brisbane airport with the central business district (CBD), with

access portals at several densely populated suburbs.

The project was constructed within and beneath busy urban andsuburban areas, just north of the Brisbane CBD. At the tunnel

portals, cut-and-cover techniques were utilised to transition from

the mined tunnels onto the surface roads. It was in these areas

where the biggest interface with the public and Brisbanes busy

road network occurred.

This paper presents a case study of a temporary excavation that

enabled construction of a mined tunnel in the Kedron area of the

project. The near-vertical excavation was up to 18 m deep and

was formed using soil nails.

2. Project brief

The Kedron Park Hotel tunnel (KPHT) site is located within the

suburb of Kedron, approximately 8 km from Brisbane CBD and

10 km from Brisbane airport. The construction site was within

the complex Kedron portal area of the Airport Link project,

where the southbound and westbound tunnels cross over.

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The project requirement was to allow construction of the westboundexit ramp from the southbound tunnel. The ground conditions were

such that traditional tunnelling methods were not considered to be

viable; therefore, a cut-and-cover solution was preferred. Figure 1

shows the complex interaction of the tunnels in this area; of

particular relevance to this paper is the interaction between the exit

ramp from the main southbound tunnel (10) and the entry ramp

onto the main southbound tunnel which underlies it (11).

3. Site conditions

3.1 Geotechnical conditions

This particular area of Brisbane has complex geological conditions.

The excavation extended primarily through a relatively thick

sequence of deeply laterised, Late Triassic aged, Aspley/Tingalpa

formation which can be found within the top few metres of the

current ground surface. Its lithology is typically recorded as a grey/

brown stiff to hard silty clay/gravelly clay, with standard penetra-

tion test (SPT)Nvalues ranging from the low 30s to refusal.

Underlying the soil strength material, and just below the base of

the excavation, the extremely weathered to slightly weathered

formation is characterised by very low- to low-strength inter-

bedded sandstone and siltstones, with occasional weathered clay

seams. The Aspley/Tingalpa formation overlays the Late Triassic

aged Brisbane Tuff formation. The highly weathered BrisbaneTuff generally has been recorded as having low to medium

strength increasing to high/very high strength as the effects of

weathering decrease. Distinctly weathered and altered tuffs have

variable strength and can be gritty and friable, due to the highquartz content of the rock.

Key geological risks identified included the possibility of sand

lenses within the upper residual soils, clay seams within the

Aspley and Tingalpa formations and the possibility of weak ash

zones within the tuff. These could lead to planes of weakness or

instability and groundwater flow paths.

Based on the boreholes undertaken within the vicinity of the

excavation, the anticipated ground conditions were

j up to 5 m of stiff clay, residual soil, overlying

j up to 15 m of hard clay, residual soil, overlying

j extremely weathered to highly weathered siltstone, expected

to be encountered at the base of the excavation.

The depth to the weathered siltstone varied across the excavation,

being shallower to the west, expected to outcrop in the walls, and

deeper to the east.

Groundwater levels were taken from standpipe readings, which

were commenced prior to award of the project. These showed a

fairly consistent depth to the groundwater table of around 10 m

below ground level.

3.2 Site constraints

The site was located within a densely populated urban environ-

ment, adjacent to Brisbanes key north south arterial road. There

1. Kedron Park Hotel2. Car park3. Moreton Bay fig tree4. Church hall5. Church6. Lutwyche Road7. Construction loading8. Deep excavations9. Tunnel through wall10. Eastwest tunnel

(exit ramp mainsouthbound tunnel)

11. Northsouth tunnel(entry ramp mainsouthbound tunnel)

8

82

3

7 9

4

11

65

10

Figure 1.Location of the Kedron site and key constraints

2

Geotechnical Engineering A soil-nailed excavation for the Brisbane


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were a number of key constraints (Figures 1and2), given below,which follow the numbering on Figure 1.

(a) The Kedron Park Hotel and car park (1 and 2) is a large

building to the north of the excavation; it is a pub housed in a

historic building of local significance.

(b) A mature Moreton Bay fig tree (3), which was protected and

could not be disturbed, is adjacent to the north wall.

(c) A church hall (4) and church (5) are adjacent to the south

wall.

(d) A major arterial road (Lutwyche Road) lies to the west of the

excavation (6).

(e) Deep excavations (8) surround the site to the north, west and

beneath.

(f) A mined tunnel (10) was to be constructed from east

to west immediately at the base of the excavation, with

the tunnel also cutting through the east wall of the

excavation (9).

(g) A tunnel (11) runs north to south going underneath the east

west tunnel was to be constructed simultaneously with the

excavation and mined tunnel.

As the design and construction process continued, a number of

additional issues arose with regard to construction loading and

processes

j proposed blasting of the mined tunnel underneath thestructure

j tower crane immediately behind the wall (1 m offset)

j other construction loads at minimal distance behind the wall

owing to the constrained site.

4. Design

4.1 Design options and concepts

The original design concept was to construct a cut-and-cover

tunnel at this location with contiguous piled external walls and

using top-down construction to form temporary and permanent

propping. The total excavation was to be 30 m deep with the

tunnel roof constructed at 20 m below ground level. The contrac-

tor explored alternative solutions, particularly focusing on soil

nailing due to perceived cost and programme benefits.

The concept design focused on initially reviewing the feasibility of

a soil-nailed excavation for the full depth of 30 m. Owing to site

constraints and constructability concerns, the excavation would

need to have near-vertical walls (.858), with absolute vertical

walls at the south-east corner, near the church hall, and at least part

of the east face where the westbound tunnel would penetrate the

wall. Stability analyses were undertaken to determine a proposed

nailing pattern for preparation of a costbenefit assessment.

N

Hotel

Inclinometer5

Piled wallsupportedexcavation

(cut-and-cover)Soil-nailed excavation

Tunneladvancethrough

wall

Proposedmin

edtunnel

Churchhall

Church

Tower cranepile supported

Inclinometer3

Proposedmined

tunnelunder A

Majorarterialroad

Figure 2.Site layout

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The adopted solution consisted of a soil-nailed excavation to thetunnel roof at approximately 20 m below current ground level

(design by the temporary works designer), below which a

temporary anchored, piled retention system would be installed

(design by the permanent works designer). This was developed in

response to concerns from the tunnelling team regarding the out-

of-balance forces that would be exerted on the northsouth

tunnel, which passed less than 5 m below the base of the full

excavation. The 1.2 m dia. piles, spaced at 1.3 m centres, would

be propped by a temporary arched capping slab and two or three

rows of ground anchors. The maximum soil-nailed excavation

depth was now 18 m in order to reach a platform level for

installation of the piles. The pile-supported excavation extended

an additional 10 m below the soil-nail wall, allowing for con-

struction of the elliptical tunnelling (Figure 3).

After installation of the piles, an arched roof would bridge

between the piles of the north and south walls. A tunnel would

then be mined beneath the arch and a permanent lining installed

between the piled walls.

The adoption of this temporary works scheme led to major

changes in the permanent works over the original tender design.

A smooth interface between the two design teams was, therefore,

crucial and a series of meetings were held between the two teams

of designers and the contractor to develop the design (temporaryand permanent) for this section of the works. These meetings

were also important in overcoming the concern of some indivi-duals in the construction team with respect to the height and

inclination of the soil-nail wall. This was due to the lack of

experience of soil nailing within the construction team and the

perception that significant deformations would result.

4.2 Detailed design

The Australian standard on earth retaining structures (AS 4678-

2002 (Standards Australia, 2002)) includes the design of soil-

nailed walls as an informative appendix only. The complexity of

this wall as well as its height, however, specifically moves it

outside the limitations of this standard. Various international

guides and standards were available at the time of design,

including BS 8006:1995 (BSI, 1995), Ciria C637 (Phear et al.,

2005), Geoguide 7 (GEO, 2008) and Geotechnical Engineering

Circular No. 7 (Lazarte et al., 2003). The authors had previous

knowledge of Ciria C637 and this guide was adopted. The design

also had to comply with the various project specifications and the

project deed, which incorporated client-specific requirements that

fell outside the specifications.

Ciria C637 uses the limit state design methodology of Eurocode

7 (BSI, 2004), where partial factors are applied to the soil

strengths and to the actions affecting the wall. The partial factors

used were as detailed in Table 8.2 of Ciria C637 and arereproduced inTable 1.

Construction sequence:

1.

2.

3.

4.

5.

Top heading of southboundtunnel constructed.Soil-nailed excavationcompleted.Bored piles installed and roofslab formed.Pilot tunnel of westboundtunnel constructed withground anchors installed asexcavation proceeded. Southboundtunnel completed.

Westbound tunnel completed.

Church hall

15mlongsoil

nailsExcavation

backfilled oncompletion ofmined tunnel

roof

Temporaryexcavation level

Mined tunnel(westbound)

2

3

4

5

3 3

4

1

4

Southboundmined tunnel

Permanentstructure

Medium-strengthsiltstone

Very low-strengthsiltstone

Extremelyweatheredsiltstone

Residual soilvery stiff tohard clay

Residual soilstiff clay

Figure 3.Construction sequence (section A)

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A summary of the geotechnical parameters adopted in the

analyses is presented in Table 2. The geotechnical parameters

were derived from the available site investigation data, including

limited laboratory testing. These parameters were agreed by the

teams designing the temporary and permanent works.

The design was carried out using limit state design and a limit

equilibrium method of analysis. Two software packages were

utilised for design work, namely

j

Slope/W, limit equilibrium slope stability analysis programdeveloped by Geostudio

j Snailz, soil reinforcement program developed by Roadway

Geotechnical Engineering, California Department of

Transportation.

Snailz was primarily used to determine the soil nail length required

based on the design nail spacing, borehole diameter, soil properties

and facing strength. This analysis was then checked using Slope/W.

The two programs assess soil nails differently, with Snailz allowing

for different bond stresses for each soil type and including the

effect of the soil nail head in the analysis. Slope/W, however, allows

for a different bond stress for each nail and a more complex

geometry can be analysed. On this project Snailz generally gave

higher factors of safety than Slope/W, most likely owing to Snailz

considering the contribution of the nail head in the analyses. A

typical section analysed is shown inFigure 4.

The characteristc bond or skin friction (ave) was initially

estimated usingEquation 1based on equation 8.4 of Ciria report

C637

Parameter Design approach 1,combination 1 (DA1-1)

Design approach 1,combination 2 (DA1-2)

Actions (A), multiply action by partial factor given below

Permanent unfavourable 1.35 1.0

Permanent favourable 1.00 1.0

Variable unfavourable 1.50 1.3

Variable favourable zero zero

Materials (M), divide material strength by partial factor given below

tan9 1.0 1.25

c9 1.0 1.25

Su 1.0 1.40

qu 1.0 1.40

Bulk density 1.0 1.00

Resistance (R)

Overall stability 1.0 1.0

Bearing capacity 1.0 1.0

Sliding 1.0 1.0

Table 1.Partial factors used in analyses

Geological unit Description b: kN/m3 c9: kPa 9: degrees E: MPa ave: kPa K0

Residual soil Stiff clay 21 5 25 40 75 0.3 0.6

Residual soil Hard clay 21 5 28 80 125 0.3 0.6

Siltstone Very low low

strength

22 22.5 30 90 125 0.3 1.0

Note:b, bulk unit weight;c9, effective cohesion;9, effective friction angle;E, elastic modulus;ave, average ultimate bond stress; , Poisson

ratio;K0, earth pressure coefficient.

Table 2.Characteristic material properties used in analyses

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tave (K9vtan 9 c9) (kPa)1:

where,9v is the effective vertical stress at the centre of the lengthof a soil nail beyond the calculated failure surface (kPa), 9 is

the effective friction angle (degrees), c9 is the effective cohesion

(kPa) andKis a constant taken as 1.

The results of the assessment of the characteristic bond were

compared to typical values of bond stress typically experienced

for this material in South-East Queensland and appropriate values

of characteristic bond were adopted for each material. This was

confirmed during the design phase by the execution of an

ultimate load test.

In order to determine the maximum working loads on each rowof soil nails, analysis of each stage of construction of the soil-

nailed walls was carried out using a global factor of safety

approach (unfactored soil parameters) within Snailz, whereas

overall stability was checked using partial factors.

The two-dimensional finite-element analysis software, Plaxis, was

used to predict ground movements, particularly in the vicinity of

the church hall and church at the crest of the south wall. The

construction sequence considered in the Plaxis modelling consid-

ered the construction of the temporary and permanent works.

Models were generated by both the temporary works and

permanent works design teams and compared and agreed before

construction began. The construction sequence adopted was as

follows (Figure 3).

(a) Construct southbound top heading.

(b) Staged soil nail excavation to the base of excavation.

(c) Install contiguous piles from base of excavation.

(d) Construct roof slab.

(e) Construct westbound pilot tunnel.

(f) Excavate and complete southbound tunnel.

(g) Finish westbound tunnel.

In the Plaxis analysis, characteristic values were adopted and the

predicted and actual ground movements (measured by inclin-

ometers) at two sections on achieving final excavation level are

given inFigure 5.

As a requirement of the project deed, a minimum surcharge of

20 kPa had to be adopted in all significant temporary works

designs. In addition, specific surcharges from construction plant

including mobile cranes and concrete trucks at the crest of thewalls also had to be considered. Proposed equipment was

discussed with the construction team during the early stages of

design development.

Soil nails of 32 mm dia. with a steel grade of 500 MPa were

nominated. These were installed in 150 mm dia. holes at an angle

of 108 below the horizontal in 40 MPa grout. The nail lengths

varied from 10 m to 15 m due to the change in excavation height,

varying load conditions around the excavation and improving

ground conditions with depth.

The soil nails were installed with a vertical spacing of 1 .2 m and

a horizontal spacing of 1.5 m centre to centre (Figure 4). The soil

nail heads consisted of a 200 mm3 200 mm 3 20 mm thick

plate placed on top of the shotcrete surface and held in place by a

nut. The shotcrete was nominally 100 mm thick with a minimum

strength of 40 MPa at 28 days and had a single layer of mesh

Base of soilnailed wall

Very lowlow-strength siltstone

Soil nails:Top row 05 m below crestSpacing 12 m V: 15 m HTen 15 m nailsFour 12 m nailsInclination 10 below horizontal

Sub-horizontal drains:Top row 5 m longBottom row 10 m longSpacing 5 m HInclination 10 above horizontal

Residual soil hard clay

Residual soil stiff clay

GWL (RL12)

RL 222

RL 5

Figure 4.Typical section

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(SL82) placed centrally throughout, with an additional layer of

mesh (1 m2) at the soil nail heads.

The structure was nominated as being required for 2 years;

however, the project deed stipulated a design life of 5 years forthe tensile elements. Given the short duration, durability was

considered through sacrificial thicknesses of steel elements.

Sheathing of the soil nails, as required for permanent soil nails,

was not undertaken.

The detailed design took account of the site constraints as

detailed in Section 3.2. The location of the Moreton Bay fig tree

immediately at the crest of the north wall, for example, meant

that the upper row of soil nails beneath the tree could not be

installed. The project arborist recommended that the cut face was

protected by plastic sheeting beneath the shotcrete so that the soil

adjacent to the tree would not dry out. The shotcrete was,therefore, thickened and heavily reinforced over this zone with

the next row of soil nails designed to take the additional load.

The location of the tower crane led to the position of the soil

nails being amended to avoid the supporting piles. Similarly, the

soil nails were also located to avoid the inclinometers behind the

wall facing.

The interaction between the wall and the proposed tower crane

was considered in detail. This included finite-element modelling

to assess the soilstructure interaction within the system. It was

critical to design the tower crane foundation within the rotation

tolerance of the crane. The relative movement of the soil-nailed

wall and the tower crane piles led to additional down-drag, or

negative skin friction, in the foundation. As a result, design loads

in the piles were estimated to be greater than that from the static

load of the crane itself.

The most significant effect on the design, however, was the

location of the future tunnel portal on the eastern face of the

excavation (Figure 6). Here, the steel nails were replaced by

glass-reinforced plastic (GRP) nails to enable the bars to be

ripped up during tunnelling. The GRP nails were 40 mm dia.installed in 150 mm grout holes. Unlike steel bars, which can

behave plastically and creep after their tensile capacity has

been reached, the GRP nails require greater deformation to

achieve their ultimate capacity, after which they fail in a brittle

manner. The GRP nails adopted had a break load capacity of

860 kN.

Given that the excavation was to remain in place for 24 months,

0

5

10

15

20

5 0 5 10 15 20 25 30 35 40 45 50 55

Horizontal displacement: mm

Actual

Predicted0

5

10

15

20

5 0 5 10 15 20 25 30 35 40 45 50

Reducedlevel:m

AHD

Reducedlevel:m

AHD

Horizontal displacement: mm

Actual

Predicted0

2

4

6

8

10

12

14

16

18

20

0 50 100

Reducedlevel:m

AHD

Base depth(RL 68)

Base depth(RL 68)

SPT : blowsN

Residual soil stiff clay

Inclinometer 5

Base depth(RL 68)

Residual soil hard clay

Moderatelyweathered siltstone

Moderatelyweathered

siltstone

Residualsoil stiffclay

Residualsoil hardclay

Inclinometer 3

Figure 5.Comparison of predicted against actual lateral wall

movement

Figure 6.East wall during construction showing canopy tubes

set out

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the design called for specific drainage measures to be installed.This included 150 mm wide geocomposite strip drains which

were installed between the soil nails and beneath the shotcrete at

a 2.5 m horizontal spacing. In addition, the ground investigation

had identified the possibility of a perched water table within the

upper stiff clay layer. Two rows of sub-horizontal drains were,

therefore, installed: one at the bottom of the stiff clay layer and

another at the bottom of the excavation. Both rows of sub-

horizontal drains were placed at a 5 m horizontal spacing.

The project team was concerned about the impact of ground

movements on the church hall and church. Although the excava-

tion would generate some ground movement, additional move-

ments were anticipated owing to the two tunnels beneath the

excavation, as well as the general dewatering of the area due to

nearby cavern construction. A monitoring programme for the

church and church hall was implemented in addition to the

monitoring already in place around the excavation. This included

multiple targets and settlement plates, and staged trigger levels

based on the status of construction.

5. ConstructionSoil-nailed wall construction is a top-down technique which

requires a staged approach to construction. The following

construction sequence was nominated.

(a) Excavate to 0.5 m below the upper row of soil nails.

(b) Drill, install and grout first row of nails.

(c) Place strip drains and mesh on excavated face and shotcrete.

(d) Attach head plate and nut.

(e) Excavate to 0.5 m below the next row of soil nails.

(f) Repeat steps (b) to (e).

(g) Repeat to bottom of excavation, ensuring that horizontal

drains are installed as the excavation proceeds.

The construction was complicated by the need initially to

maintain an access ramp from the south wall and into the

excavation for as long as possible, until full access could beprovided from the cut-and-cover tunnel to the west of the

excavation to service the works. This meant that the nails on the

south wall could not be installed until a majority of the nails on

the north and east walls were complete. Figure 7 shows the

varying excavation depths across the structure while the access

ramp was in operation. This required close coordination between

the site and design team to ensure that an adequate width of ramp

was left in place to provide stability to the wall.

Full-time observation of the works was undertaken by a represen-

tative of the temporary works designer owing to the high level of

risk associated with this excavation. Their responsibilities in-

cluded

j mapping of the excavation to confirm that the geotechnical

conditions were consistent with design

j nomination of test nails and observation of nail testing

j raising non-conformance reports as necessary

j responding to requests for information from the

construction team

j providing design direction to allow construction to proceed

without undue delay

j reviewing the daily monitoring results.

Acceptance testing to assess workmanship was specified on

production nails at a rate of six tests per 100 nails in accordance

with Queensland Department of Transport and Main Roads

standard MRTS03 (TMR, 2010), with the test nails nominated by

the designer. This required the nails to be subjected to loads of

1.5 times the working load for three cycles with a maximum

deflection of the soil nail not exceeding 0.1% of its length.

Ultimate load testing to confirm design parameters was also

specified and six tests on short bond lengths were undertaken at

different levels within the excavation. It was decided that theultimate load tests were to be carried out within the excavation

rather than in the excavation face, which enabled the nails to be

exhumed and examined after testing as a measure of construc-

tion quality control. Figure 8 is a photograph showing one of

the exhumed nails. The exhumed soil nails showed that the

grout column around the nail remained intact after failure, with

some nails showing a clean break in the grout at the end of the

nail. This indicated that the soil nail and grout failed as one

element as it was pulled out of the excavated face. All testing

was carried out in accordance with the recommendations of

Ciria C637.

All the soil nails passed the acceptance tests; however, there were

issues with poor-quality centralisers being used, which collapsed

under the weight of the steel bars. This product was quickly

replaced with one of acceptable quality. The ultimate load tests

confirmed the design parameters used with calculated ultimate

Figure 7.Access ramp into excavation

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bond stresses up to 250 kPa in the hard residual clay. Some tests

were cut short as failure had not occurred when the load had

reached 80% of the ultimate tensile capacity of the nail, beyond

which it was not safe to continue the test.

On completion of the excavation, the permanent works began

with piles being installed along the toes of the north and southwalls and part of the east wall. In addition, 114 mm dia.

canopy tubes, 12 m to 15 m long, were installed in the east

wall in preparation for the mined tunnel. Once the piles were

installed, fill was placed inside the excavation in order to

provide a form for the tunnels arched roof. The reinforced

concrete roof arch was cast (Figures 9 and 10). The tunnel was

then mined and permanent lining was placed beneath the arch.

Upon completion of tunnelling, the excavation was backfilled

with flowable fill.

During construction the issue of rock blasting within the tunnels

during mining was raised. An independent specialist assessed the

effect of blasting on the soil nailing and concluded that there

would be no effect. The design team reviewed this assessment

and reanalysed selected wall sections in Snailz using unfactored

parameters and a horizontal coeffiecient of acceleration of 0.2,

while targeting a factor of safety of greater than 1.

6. PerformanceWall movements were monitored by reflector survey prisms

which were fixed to the shotcrete wall at two levels as the

excavation proceeded. In addition, lateral movements were also

monitored by inclinometers which were located around the

excavation, along with ground settlement monitoring markers to

measure vertical deformations. Trigger levels were established for

all monitoring points and the results of all monitoring were

distributed around the design and construction teams. The wall-

monitoring system, with the exception of the reflector survey

prisms, was unchanged from that planned for the original piled

wall.

Two monitored sections of the excavation are presented in Figure

Figure 8.Exhumed nail following ultimate load testing

Figure 9.Reinforcement for arch capping slab

Figure 10.Aerial photograph during arch construction

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5, which shows the predicted and actual lateral ground movementat completion of the full depth of excavation. The encountered

ground conditions at inclinometer 3 comprised approximately

4 m of stiff residual clay over 8 m of hard residual clay, overlying

moderately weathered siltstone. At this location the measured

lateral ground movements towards the excavation reached 31 mm

at completion of the excavation, against a predicted movement of

40 mm. The prisms were installed as the excavation proceeded

and could only measure deflections from the time of installation.

They would not, therefore, show the total movements of the wall,

but would be indicative of the deformations that were occurring

and could be checked against the inclinometers, which were

approximately 2 m back from the wall face. At completion of the

excavation the prisms at this section had moved 28 mm at 18 .2 m

AHD (Australian height datum) and 12 mm at 13.4 m AHD, 2 m

and 6.8 m below the wall crest, respectively. The upper prism

compared well to the inclinometer data, but the lower prism

showed much less movement, which may be indicative of the

delay in placing the prism after excavation.

At inclinometer 5, the ground conditions comprised approxi-

mately 4 m of stiff residual clay over 10 m of hard residual clay,

overlying moderately weathered siltstone. At this location the

measured ground movements towards the excavation at the

inclinometer reached 15 mm at completion of the excavation

against a predicted movement of 50 mm. The prism on thissection showed movements of 16 mm at 17.3 m AHD (1.6 m

below the wall crest) and appears to be in agreement with the

inclinometer. The ground surface movements were approximately

11 mm at 2 m behind the wall crest, reducing to 6 mm at 8 m

from the wall face and 0 mm at 13 m.

The ground movements at the church hall and church were less

than 10 mm at the completion of the excavation and no signs of

damage or distress were noted. Similarly, at the hotel measured

ground movements were less than 5 mm and again no damage

was noted.

The ground conditions encountered were generally consistent

with the design model. It is considered that the inclinometers

measured less movement than predicted as the surcharges consid-

ered in the analyses were never fully achieved on site. In addition,

the determination of deformations in Plaxis relies upon soil shear

strength and soil stiffness parameters. As no direct measurement

of stiffness was undertaken during the investigation and testing

programme, the stiffness was derived from a conservative estima-

tion based on undrained shear strength using empirical calcula-

tions and local experience. This adoption of conservative values,

which was due to the concern of impact on adjacent structures,

may have led to an underestimation of soil stiffness and, hence,

less movement than anticipated. This effect is noted more in the

results of inclinometer 5, which has the greater soil thickness.

The greater movement at inclinometer 3 can, to some degree, be

explained by the presence of a laydown area at the crest of wall,

which would have added a surcharge throughout the constructionperiod on this section of the wall. In addition, the wall at

inclinometer 3 was higher than that at inclinometer 5.

The following is a general summary of the wall behaviour.

j Horizontal wall movements were approximately 0.2% of the

wall height.

j Vertical wall movements were between 70% and 100% of the

horizontal movements.

j The measured wall movements were consistent with case

histories (Figure 11).

j The inclinometers identified lateral movement at the residual

claysiltstone boundary.

j Measured wall movements were less than those estimated by

Plaxis.

j There was no visual evidence of significant wall movement.

j There was no identifiable damage to the church and church

hall.

7. Summary and conclusionThe authors believe that this is one of the deepest soil-nailed

excavations to date in Australia. During the design stage, there-

fore, there was concern by the main contractor regarding the

feasibility of the soil-nailed solution. The reasons for this concern

were many and included

j lack of experience of the construction and verification teams

and general lack of Australian experience with soil nailing

j the large depth of excavation

j a perception of greater ground deformation resulting from the

use of soil nails

j proximity of adjacent structures

j complicated construction sequence involving two tunnels to

be constructed beneath the final excavation.

The temporary works designers were able to demonstrate to key

personnel within the construction and verification teams thefeasibility of the solution, and early involvement of the temporary

works design team with both the contractor and permanent works

design team enabled a smooth interface between the two compo-

nents of the work. The agreement of the design approach and

parameters to be used in design was particularly important and

allowed for alignment of the permanent and temporary analytical

models. This led to an agreed set of monitoring trigger levels for

each monitoring point around the excavation.

The temporary works team remained fully involved with the

construction team throughout the excavation and installation of

the soil nails, including having full-time representation on site.

This was crucial as the authors have seen many occurrences of

over-excavation and subsequent wall movement during soil-nailed

wall construction, due to contractors inexperience. This also

ensured that quality issues such as the use of inappropriate

centralisers were identified and corrected quickly. The contrac-

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tors respect for the risks involved with this element of the works

was the key to its success.

Locating the instrumentation within a metre of the wall facerequired the use of an experienced survey team, which enabled

the soil nails to be placed such that they did not impact on the

inclinometers or crane piled foundations. The results of the

monitoring indicate that the movement of the wall was within

those typically expected (Figure 11).

Ultimately, over 1500 soil nails, steel and GRP were installed in

this excavation. Ground movements were less than anticipated,

with no damage occurring to adjacent structures, and the soil-

nailing option reduced the programme time by 3 months, com-

pared with the original piled wall scheme.

AcknowledgementsThe authors would like to acknowledge the Thiess and John

Holland joint venture for their support in the preparation of this

paper.

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0

0020

0040

0060

0080

0100

0120

0140

0 5 10 15 20 25 30 35

Lateralwallmovement,

:m

h

Wall height, : mH

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Clough and ORourke (1990)

Thompson and Miller (1990)

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Shen (1981)et al.

Airport link project

h/ 05%H

h/H 04%

h/H 02%

h/H 01%

Figure 11.Lateral wall movement plotted against wall height (comparison with case histories (seeBruce and Jewell, 1986;Clough and

ORourke, 1990;Durgunogluet al., 2007a,2007b;Ho et al., 1989;Shenet al., 1981;Stocker and Riedinger, 1990;Thompson and

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