albany conglomerate tunnelling paper

8/18/2019 Albany Conglomerate Tunnelling Paper

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Underground Construction Challenges Associated with the Albany Conglomerate in Auckland

R C Roberts1, S W Crossen2 and L J Strachan3

1. Engineering geologist, Jacobs, Auckland. Email: [email protected]

2. MSc Geology Research Student, University of Auckland, Auckland

3. Senior Lecturer, University of Auckland, Auckland

ABSTRACT

The Albany Conglomerate is a widespread geological unit found in northern areas of Auckland, New

Zealand. It has previously been described as part of the early Miocene Waitemata Group which underlies

most of Auckland. Much of the Waitemata Group comprises weak interbedded sandstones and mudstones,

but also includes conspicuous conglomeratic units such as the Albany Conglomerate. The Albany

Conglomerate comprises a well cemented mixture of hard pebbles, cobbles and boulders of igneous and

metamorphic derivation in highly lenticular beds. A number of tunnelling projects are planned for the near

future in Auckland. Recent experience has shown that the variable features associated with the Albany

Conglomerate can make the selecting the best construction methods very challenging. This paper explores

the engineering geological and sedimentological features of the Albany Conglomerate with respect to other

conglomerates within the Waitemata Group, and the potential implications on tunnelling projects in Auckland.

Consideration is given to the extent of proposed geotechnical investigations and the production of

Geotechnical Baseline Reports.

INTRODUCTION

Auckland is the largest city in New Zealand (Statistics New Zealand, 2012). Historically it has had a low

population density typical of New World cities, and as a result infrastructure was normally constructed on or

near ground level due to relatively low land costs and few space limitations. Recent strong population

growth is expected to continue, and according to the 2006 Census projections the population will reach 1.93

million by 2031, a 25% increase over current numbers (Statistics New Zealand, 2012).

This, combined with the city's location on a constrained isthmus and local government plans to minimise

urban sprawl (Auckland Council, 2012), is placing increasing demand on existing infrastructure. The

construction of tunnels and underground infrastructure is increasing across the globe as nations wrestle with

the demand of growing and increasingly urbanised populations. Auckland is no different, and a surge in

planned tunnelling projects has started.

As tunnelling works have expanded across the region new geotechnical problems are being encountered.

This paper discusses specific examples relating to the Albany Conglomerate and the implications of these

for tunnelling.


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GEOLOGICAL SETTING

The majority of Auckland is underlain by early Miocene aged sedimentary rocks of the Waitemata Group

(Edbrooke, 2001). These rocks include a number of formal stratigraphic Groups and Formations, including

the well-studied interbedded soft sandstones and mudstones of the East Coast Bays Formation (Edbrooke,

2001). This underlies the vast majority of Auckland, and is interbedded in places with harder conglomerate

including Parnell Volcaniclastic Conglomerates (Edbrooke, 2001, Shane et al, 2010), Helensville

Conglomerate (Edbrooke, 2001), and Albany Conglomerate (Edbrooke, 2001).

During the early Miocene, plate boundary reorganisation between the Pacific and Australian plates caused

rapid subsidence in the Northland region and the formation of the Waitemata Basin (Whattam et al, 2007;

Shane et al., 2010). Thought by some to be elongate and narrow (Ballance, 1974; Hayward, 1976), the basin

has been previously described as an inter-arc or intra-arc basin because remnants of calc-alkaline volcanoes

presently border its margins (Hayward, 1976). However, recent studies suggest that the calc-alkaline

volcanism post-dates the early subsidence in the Northland region (Shane et al., 2010).

Deposition following this subsidence took the form of turbidity currents, pelagic fallout, debris flows and

slumping, and resulted in the myriad of rock types that comprise the Waitemata Group (Balance 1976,

Hayward 1976). This complex sedimentary origin has resulted in a sometimes unpredictable variability

between benign and extremely challenging tunnelling conditions.

HISTORICAL TUNNEL BORING MACHINE USE IN AUCKLAND

Construction is currently taking place on a major motorway tunnel as well as a number of tunnelled sewers

and water supply pipes (eg Johns, 2013). Many more are planned including large scale projects such as a

railway line under the city centre, a new road crossing under the harbour, and numerous water and waste

water schemes across the region (eg Dougan, 2013). Tunnel Boring Machines (TBMs), including Earth

Pressure Balance Machines (EPBMs), have become the preferred method of mitigating underground

construction risks on large projects in Auckland after a number of successful applications in the past few

years (Asche et al, 2009).

However, experience of tunnelling, and in particular the use of TBMs in Auckland is relatively limited, and

only one example is known to date of a tunnel encountering the Albany Conglomerate (confidential project

involving the authors). Excluding a number of small scale pipe jacking projects, the only published examples

of the use of a TBM in Auckland are:

Vector CBD Reinforcement Project. This 6.185 km long, 3.56 m diameter tunnel was constructed in

two sections. The 2.5 km section between Hobson substation towards Newmarket was excavated with a

Road Header and the remaining 6.7 km section from Penrose towards Newmarket was excavated with a

TBM (Rahman & Barber, 2002). The project was completed in October 2000 after being delayed by the poor

performance of the TBM, which was significantly hindered by the sticky nature of the ECBF when excavated

(Asche et al, 2009)


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Project Hobson. This 3 km long, 4.2 m diameter sewer tunnel was constructed with an EPBM

capable of operating in open and closed modes. The contractor finished ahead of programme in 2009 and

the project was generally considered a success. The reason for this success has been attributed to the use

of spoil conditioning agents to minimise problems with sticky spoil (Asche et al, 2009).

Rosedale Outfall. This 3 km long, 3.2 m diameter sewer tunnel was also constructed with an EPBM

similar to the one used on Project Hobson. This also performed well and finished on programme (Asche et

al, 2009).

Waterview Connection. This project, currently under construction, comprises twin 2.5 km long road

tunnels each 14.5 m in diameter. Construction is by a custom built EPBM which, at the time of writing, had

travelled 537 m (Johns, 2013).

As a result of the lessons learned on the first three of these projects, Asche et al. (2009) concluded that EPB

tunnelling can be undertaken in Auckland with a large degree of cost and programme certainty. However,

none of the projects on which this assessment was based encountered the Albany Conglomerate or other

strong conglomerates of the Waitemata Group.

ALBANY CONGLOMERATE

Distribution

First described by Bartrum (1920) from a small outcrop along Lucas Creek, the Albany Conglomerate can be

found exposed from Albany to Makarau (Fig. 1). Depicted as being a stratigraphic member of the East Coast

Bays Formation (Ballance, 1974), the conglomerate has been mapped extensively in north Auckland

(Schofield 1989; Edbrooke, 2001).

The Albany Conglomerate exists as lenticular beds of varying thickness interbedded with mudstones and

sandstones of the East Coast Bays Formation (Ballance, 1974; Johnston, 1999). Where it is best exposed in

Wainui (Figs 1 & 2) the conglomerate is of considerable thickness. Previously estimated to be up to 1.5 km

thick (Schofield, 1989), a more recent 3D gravity survey over Wainui shows that the conglomerate has a

maximum thickness of 450 m (Johnston, 1999). North of Wainui and further south; rare exposures of the

Albany Conglomerate are no greater than 5 m thick and may show normal grading from a pebble-dominant

conglomerate to a sandstone or pebbly-sandstone (Ballance, 1974).

The best outcrops of the Albany Conglomerate exist in small abandoned quarries in the Riverhead-

Kaukapakapa district where the conglomerates are used for road aggregates (Ballance, 1976). When

crushed, the conglomerate pebbles produce a high quality aggregate. Small quantities are used for concrete

aggregate, plastering and other industrial purposes from the main quarries at Wainui and Coatesville

(Christie et al, 2001).

Subsequent faulting and penecontemporaneous slumping of the Waitemata Group has resulted in isolated

structurally complex outcrops of the Albany Conglomerate that are difficult to correlate (Ballance, 1976;

Sprott, 1997, Fig. 1).


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FIG 1 - Auckland region showing mapped outcrops of Albany Conglomerate (shown in orange) and

Helensville Conglomerate (shown in purple) (adapted from Edbrooke, 2001). Outcrops of significance are

labelled; 1, Lucas Creek and 2, Oteha Valley.

Typical composition

The Albany Conglomerate consists of well-rounded pebbles and boulders in a poorly sorted sandy matrix

and can be clast- or matrix-supported (Sprott, 1997). Igneous and meta-igneous rocks comprise

approximately 80% of clasts found in the conglomerate; including diorites, micro-diorites, gabbros, basalts

and basaltic andesites (Ballance, 1974; Sprott, 1997). These clast types commonly show hydrothermal

alteration up to a greenschist facies (Sprott, 1997). Sedimentary clasts variably comprise the rest of the

Albany Conglomerate and include sandstones, mudstones, carbonates and cherts (Ballance 1974; Sprott,

1997; Johnston 1999). Much of the matrix shows strong reactions to hydrochloric acid, believed to be the

result of calcitic cement.


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FIG 2 - Photo panel of representative images of the Albany Conglomerate. A - Outcrop at Wainui Quarry,

Auckand. B - Close-up of Albany Conglomerate at Wainui Quarry showing poorly defined lenticular bedding

(10 cm long compass for scale). C - Andesitic boulder (diameter of 1.4m) in Lucas Creek, Albany. D –

Typical clasts recovered during excavation in Oteha Valley (blue hacksaw blade approximately 300 mm long

for scale).

Origin and depositional environment

The Albany Conglomerate is inferred to have been deposited in a submarine fan complex by channelised

high-density turbidity currents (Ballance, 1974; Sprott, 1997; Johnston, 1999). Paleocurrent measurements

made by Sprott (1997) and Johnston (1999) show a general trend suggesting that sediment source was

derived from the northwest and flowed south - southeast.

Most clast lithologies of the Albany Conglomerate have been sourced from the Northland Allochthon (Sprott,

1997). These include the allochthonous deep-sea sediments and ophiolitic oceanic crust of the Tangihua,

Mangakahia and Motatau Complexes which were thrust onto the northern margin of New Zealand during the

Oligocene (Whattam et al., 2005). Texturally super mature igneous and meta-igneous clasts from the Albany

Conglomerate (Tangihua Complex) have undergone significant transport and reworking before being


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deposited (Sprott, 1997). These clasts are believed to have been transported in a fluvial and beach

environment prior to be deposited as polymictic Albany Conglomerate beds (Sprott, 1997; Shane et al.,

2010). In contrast; sedimentary clasts of the conglomerate are often texturally immature or deformed.

IMPLICATIONS OF THE ALBANY CONGLOMERATE ON TUNNELLINGThe significant strength contrast between the Albany Conglomerate and the surrounding rocks means that

selecting an appropriate tunnelling methodology may be challenging. East Coast Bays Formation rocks

have variable strength, but 90% of undrained triaxial compressive strengths fall in the 0.8 to 4.8 MPa range

(Kermode, 1992). Although the authors have been unable to identify any strength testing on the Albany

Conglomerate, the cobbles and boulders are dominated by diorite, but andesite and rhyolite pumice which

are typically one or two orders of magnitude stronger than the surrounding East Coast Bays Formation

sandstones and mudstones.

In Auckland this has been dealt with by changing tunnelling methodology from open face TBM in the EastCoast Bays Formation to hand excavated pipe jacking at the geological boundary with the Albany

Conglomerate (confidential project). In other examples (eg Krulc 2007, Shirlaw et al 1988) conglomerates or

boulders have been dealt with using the New Austrian Tunnelling Method.

FIG 4 - Typical face conditions within the Albany Conglomerate in Albany.


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Excavations into Albany Conglomerate material have been reported to suffer from high groundwater inflows,

particularly at the base (personal communication, Dietmar Londer). However, no documented evidence of

this has been discovered by the authors, and the tunnel in Albany did not encounter this issue.

Material Propert ies for TunnellingHunt and Del Nero (2011) suggested that the properties of most interest for tunnelling in a cobbles and

boulders are:

1. Clast frequency

2. Clast distribution

3. Clast size

4. Clast shape

5. Clast composition

6. Clast abrasiveness

7. Matrix composition

There is very little published data available on these properties for the Albany Conglomerate. The following

suggestions are based on the experiences of the authors. Given the variability of this material site specific

tests will be required to confirm these parameters as part of the design process.

Clast frequency

The frequency can be presented as a volumetric density or occurrence per unit length of tunnel. In the

project undertaken in Albany, the 683 m3 tunnel within the Albany Conglomerate encountered 534 boulders

between 200 mm and 500 mm in size, and 91 boulders greater than 500 mm in size. Approximately 5% of

the rock volume was classified as boulders, approximately equally split between those larger and smaller

than 500 mm diameter.

Clast distribution

The clast distribution can vary in conglomerates between random scatter or geologically constrained. In the

case of the channelised Albany Conglomerate the distribution will be geologically constrained, meaning that

a good geological model is valuable in assessing the risk.

Clast size

Clast size is defined by approximate diameter or maximum length. In the Albany Conglomerate cobbles (60-

200 mm) are dominant. Boulders (clasts greater than 200 mm across) in the Albany Conglomerate are

commonly 0.2 to 0.5 m across, but boulders over a metre in diameter were encountered in a recent

tunnelling project in Albany (Fig 4). The largest boulder observed to date was 1.4 m maximum length.


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Clast shape

Shape effects fracturing by TBM cutters, passage through equipment, crushing after ingestion and abrasion

of the cutters and mucking system. Flatter clasts are generally easier to split than round clasts. Angular

clasts are generally more abrasive than rounded clasts. In the Albany Conglomerate the boulders and

cobbles have been rounded by transportation, and are normally rounded to sub-rounded.

Clast composition

Rock mineralogy, lithology, compressive strength, and degree of fracturing and weathering all effect TBM

cutters, the energy required for commutation of clasts and abrasion of the cutters and mucking system.

Mineral grain size, fabric, crystal arrangement, foliation and sedimentary sub-bedding also generally affect

the energy required for splitting and crushing as well as abrasiveness of the clasts. In the Albany

Conglomerate the cobbles and boulders are dominated by diorite, andesite and rhyolite pumice. The

shortage of testing on the Albany Conglomerate means there is currently inadequate data to give an

evidence based assessment of boulder strength. These materials commonly range in uniaxial compressive

strength from 20 MPa to 250 MPa, and in the absence of test data this range is considered an appropriate

initial estimate until site specific testing provides more reliable data.

Clast abrasiveness

Abrasiveness of the ground and wear to cutters, cutterhead, crushers, chamber and mucking system

increases as the percentage of hard minerals in the matrix soil and clasts encountered increases.

Abrasiveness can be estimated from the relative percentage of minerals of different Moh’s hardness classes

and by tests on both the clasts and soil matrix. Angularity of the clasts also increases abrasiveness. Theauthors have not identified any abrasiveness testing being undertaken on the Albany Conglomerate to date.

Matrix composition

Matrix density, strength, grain size distribution, and permeability affect ease of dislodgement by cutters, disc

cutter effectiveness, ability to push boulders aside, and stand-up time or ground improvement required for

manual drilling and splitting. Krulc (2007) noted the importance of matrix cementation and density to allow

an assessment of the viability of open face tunnelling and the risks of clasts falling from the tunnel roof,

sidewalls and face if not supported. The matrix of the Albany Conglomerate is very dense, poorly sorted

sands. The ability to push boulders aside is expected to be very limited. The permeability of the material

was shown in Albany to be relatively low, as demonstrated by low levels of groundwater inflow. However,

there are anecdotal reports of high groundwater flows which suggest that the permeability is highly variable

and should therefore be assessed on a site by site basis. The matrix appears to be cemented with calcite.

LIMITATIONS OF THESE RECOMMENDATIONS

The material properties suggested above are based on examination of sites across north Auckland.

However, close examination of the so-called Albany Conglomerate outcrops at Lucas Creek (Bartrum, 1920)

and nearby boreholes in Oteha Valley shows that these localities are not typical of the Albany Conglomerate

mapped further north (Bartrum, 1924; Hayward, 1976; Ballance, 1974; Fig. 1). These channelised deposits


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in Albany include dacite and rhyolite pumices which are not derived from the Northland Allochthon, but are

observed in core from Oteha Valley to be mixed with basalts and micro-diorite clasts which are known to be

derived from the Northland Allochthon. It is believed that these deposits represent a mixing of two sediment

sources. Since the Oteha Valley and Lucas Creek deposits are non-typical of the ‘Albany Conglomerate’ it

might be more meaningful to have them identified as a separate formation. It has been suggested that theWainui Hill serves as a better type locality for the Albany Conglomerate (Ballance, 1976).

It is therefore important to carefully describe any conglomerate encountered during a ground investigation for

underground construction in order to confirm the applicability of the recommendations made in this paper.

Two of the most commonly encountered conglomerates within the ECBF, the Helensville Conglomerate and

Parnell Volcaniclastic Conglomerate, are described here to assist with future identification.

Helensville Conglomerate

The Helensville Conglomerate is compositionally, texturally and stratigraphically distinct from the AlbanyConglomerate (Bartrum, 1924; Ballance, 1974; Hayward, 1976), and is comprised dominantly of andesite

cobbles and boulders inferred to be derived from the West Coast Kaipara Volcano with rare Northland

Allochthon derived clasts (Schofield, 1989; Hayward, 1976). Lenticular beds of Helensville Conglomerate,

up to 100 m thick are interbedded with sandstones and mudstones of the Cornwallis Formation, and are

interpreted as proximal submarine channel or canyon fill deposits (Hayward, 1992).

Parnell Volcaniclastic Conglomerate

Parnell Volcaniclastic Conglomerate (PVCs) beds are distinct from both Albany and Helensville

conglomerate types (Shane et al., 2010). PVCs are interbedded with sandstones and mudstones of both the

East Coast Bays and Pakiri formations (Edbrooke, 2001). PVCs are polymictic, normally graded, up to 15 m

thick beds composed predominantly of Ocean Island Basalts, with minor sedimentary, meta-igneous and

pumice clasts (Shane et al., 2010). These beds are highly lenticular and amalgamated, and are the coarse

grained deposits of high density turbidity currents (Lowe, 1982) that formed highly sinuous turbidite

channels.


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FIG 3 - Photo panel of representative cores of the key rock types described in this paper. A – Typical ECBF

mudstone from Albany. B - Albany Conglomerate from Oteha Valley. C – Parnell Volcaniclastic

Conglomerate from Newmarket, Auckland.

RECOMMENDATIONS

Targeted Ground Investigation

The inferred depositional environment of a submarine fan complex with channelised high-density turbidity

currents (Balance, 1974; Sprott, 1997; Johnston, 1999) means that the Albany Conglomerate occurs in

unpredictable locations and in relatively narrow lenses. In the case of Oteha Valley the channel was inferred

to be less than 200 m wide. Although the proposed influence of the orientation of an ancient channel or

canyon may result in a preferred alignment of these deposits along a northwest to southeast axis (Sprott,

1997), this cannot be assured on a local scale. As a result, a low-density ground investigation is likely to

miss these deposits. Given the very serious implications of encountering these material with an

inappropriate machine, consideration should be given to more comprehensive investigation, particularly in

areas where there are known outcrops of similar rocks.

Specific tests are recommended on rock material to assist with TBM specifications, including Uniaxial

Compressive Strength, Point Load, Brazilian tensile and Cerchar abrasivity. Permeability tests, which may

include both rising and falling head, are also recommended.


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Description and Classification of Conglomerates

The material properties recommended in this paper are applicable to the Albany Conglomerate as observed

in Wainui Quarry and Oteha Valley. However, the Oteha Valley rocks are not typical of the Albany

Conglomerate mapped further north. Any ground investigation should carefully assess the mix of clasts to

ascertain the likely source and the potential for any mixing of sources before applying regionally applicable

material properties.

Geotechnical Baseline Report

A Geotechnical Baseline Report (GBR) establishes a contractual understanding of the subsurface site

conditions using a series of ‘baselines’ which describe the geotechnical conditions anticipated to be

encountered. This associated with the conditions consistent with or less adverse than the baselines are

allocated to the Contractor, and those materially more adverse than the baselines are accepted by the

Owner (Essex, 2007). A good GBR requires an in depth understanding of construction methods, contractual

mechanisms and commercial issues.

A GBR was utilised on the tunnelling project described in Albany to overcome the issue, described by Asche

et al (2009), where a GBR was not used and as a result a highly competitive bidding environment allowed

the winning bidder to make overly optimistic assumptions about the ground conditions. When actual

conditions encountered were more adverse than the interpretation, significant claims were submitted, in

effect elevating the owner’s risk. The GBR should, where possible, specify the expectations for all of the

characteristics described earlier.

CONCLUSIONS

The current understanding of the depositional environment in which the Albany Conglomerate formed is still

being developed, and as a result our understanding of the distribution of these materials is limited. When

tunnelling in ground containing boulders, knowledge of the concentration and location of boulders is of

utmost importance (Hunt and Del Nero, 2011). The current lack of information about the extent and

distribution of the Albany Conglomerate means that this risk is significant for future tunnelling projects in

north Auckland where known outcrops exist.

A combination of high quality ground investigation, insitu and laboratory testing, and a carefully written

Geotechnical Baseline Report can be used to mitigate these risks, and the authors recommend their use on

future underground construction projects.

ACKNOWLEDGEMENTS

This paper would not have been possible without the support and encouragement of a number of individuals

and organisations. The authors would particularly like to thank Dietmar Londer for his reviews and comments

and Mark Thorn for his support throughout. Jacobs Engineering provided the financial and moral support

essential to complete the paper.


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