Tunnelling through volcaniclastic grit; monitoring and ... · France, S. (2017) Tunnelling through volcaniclastic grit; monitoring and management of groundwater effects on the Waterview

France, S. (2017) Tunnelling through volcaniclastic grit; monitoring and management of groundwater effects on the

Waterview Connection Project Proc. 20th NZGS Geotechnical Symposium. Eds. GJ Alexander & CY Chin, Napier

1

Tunnelling through volcaniclastic grit; monitoring and management of

groundwater effects on the Waterview Connection Project

S J France

Beca Ltd, Auckland, NZ.

[email protected] (Corresponding author)

Keywords: groundwater, drawdown, tunnelling, volcaniclastic grit, Waterview Connection

ABSTRACT

The NZ Transport Agency’s Waterview Connection project involves the construction of 5 km of

motorway to complete Auckland’s Western Ring Route. Half of this new link is twin 14.5 m

diameter tunnels constructed by Tunnel Boring Machine with 16 sequentially excavated cross-

passages. Over most of the tunnelled length the excavation is through weak interbedded

sandstones and mudstones; however, a 150 m long zone of moderately strong volcaniclastic grit

was identified during site investigations.

This zone was characterised by steeply dipping, fractured and faulted, higher permeability rock

which might allow a direct groundwater-surface water connection to overlying Oakley Creek, and

increased the risk of drawdown in overlying contaminated and compressible soils. This zone was

identified as one of the highest risk areas in terms of potential adverse effects resulting from

groundwater drawdown and ground settlement.

Monitoring during drained maintenance stops recorded inflows to the tunnel excavation of up to

30 L/s with near instantaneous drawdown at distances of almost 200 m, and, air bubbles in the

creek coincident with the TBM location confirming the connection between groundwater and

surface water. Visual inspections and instrumented monitoring were used to regularly inform

drive parameters and negligible adverse effects were recorded as a result of the drawdown during

tunnelling. Pre-excavation fissure grouting was undertaken ahead of cross-passage excavations to

limit groundwater inflows and drawdown, and monitoring confirmed the effectiveness of this

technique in these materials. These proactive mitigation measures meant that negligible adverse

effects were recorded as a result of the drawdown.

1 PROJECT INTRODUCTION

The Waterview Connection project involves the construction of 5 km of 6-lane motorway through

and beneath Auckland’s western suburbs, linking two existing State Highways to complete a

motorway ring route around the city. Half of this new link will be in tunnels and the remaining

half comprises surface highways and approach trenches (Figure 1).

The segmentally lined mainline tunnels were constructed using a 14.5 m diameter Earth Pressure

Balance Tunnel Boring Machine (“the TBM”). The mainline tunnels are linked by 16 sequentially

excavated, nominally 6 m diameter cross-passages.

2 GEOLOGICAL CONDITIONS AND IMPLICATIONS FOR TUNNELING AND EXCAVATION

2.1 Geological Setting

France, S. (2017) Tunnelling through volcaniclastic grit; monitoring and management of groundwater effects on the Waterview Connection Project

2

The project is located in the western-central suburbs of Auckland, with the tunnel alignment

broadly following the incised valley of Oakley Creek, between the Mt Albert (Owairaka) Volcano

to the east and the Great North Road/Blockhouse Bay Ridge to the west (Figure 1).

Figure 1: Location map for the project (modified after France et al, 2011)

Tertiary age, weak sandstones and mudstones of the East Coast Bays Formation (ECBF), form

the bedrock in the project area and outcrop in the Great North Road / Blockhouse Bay Road Ridge

and locally within Oakley Creek. Compressible Tauranga Group alluvium fills the paleo-valley

of Oakley Creek and is encountered within the present-day valley of Oakley Creek and beneath

the basaltic lava flows to the east of the Creek.

Mt Albert (Owairaka) Volcano erupted onto a now buried paleo-ridge of ECBF and lava flows

in-filled the paleo-valley to the north, west and south. The current position of Oakley Creek

roughly follows the western edge of the lava flows. Uncontrolled back-filling of historic quarries

within the basaltic lava flows has resulted in pockets of landfill adjacent to the Creek.

Waitemata Harbour

Mt AlbertVolcano

20 m

20 m

40 m

40 m

20 m

60 m

40 m

60 m

40 m80 m

100 m

Auckland CitySH16

Waitakere City

Oakley Creek

Northern Portal[cut'n'cover]

Southern Portal[cut'n'cover]

Driven Tunnels

Phyllis StLandfill

N

0 m 400 m

Fill

TGA: Tauranga Group Alluvium

ECBF: East Coast Bays Formation Sandstone / Siltstone

AVT: Auckland Volcanic Formation Tuff

AVL: Auckland Volcanic Formation Lava

Topographic contour (20 m interval)

Major road

Groundwater spring

LEGEND

Surface highw ay

TunnellingReach 3

Northern Portal[cut'n'cover]

FillTGA: Tauranga Group AlluviumECBF: East Coast Bays Formation Sandstone / SiltstoneAVT: Auckland Volcanic Field TuffAVL: Auckland Volcanic Field Lava

Topographic contour (20 m interval)Major roadGroundwater springSurface Highway


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2.2 Characterisation of Geological Conditions for Tunnelling

The Well-Connected Alliance developed a 3D geological model from the geotechnical

investigation data (over 500 locations) and used it to generate geological long and cross sections

to assess tunnelling conditions. Six tunnelling reaches were identified based on predominant

lithology, hydrogeology and geological structure. Over most of the tunnelled length (5 reaches)

the tunnelling and cross passage excavation is through typical, weak ECBF.

However, within the ECBF are lenses of a stronger, coarse grained volcaniclastic grit (commonly

referred to as Parnell Grit). Reach 3 comprises 500 m of interbedded weak ECBF and lenses of

the stronger grit, but with a central ~150 m long zone in which the tunnel excavation was wholly

within the grit. The rock in this zone was also characterised by steeply dipping joints and folded

bedding.

2.3 Characterisation of the Grit in Reach 3

In total 155 in-situ (slug, packer or pumping) tests were undertaken in the East Coast Bays

Formation over the wider project area. The geometric mean and median of all tests is

approximately 3 x 10-7 m/s, typical of ECBF, with 85 % of all test results falling in the range of

1 x 10-8 m/s to 5 x 10-6 m/s (Figure 2).

Figure 2: Permeability test results from typical ECBF and Reach 3 grit

Whilst a broad trend between permeability and rock strength was identified, review of spatial

trends indicated that the highest permeability values were associated with the steeply dipping,

openly jointed volcaniclastic grit beds located within Reach 3.

The 41 No. in-situ tests undertaken in Reach 3 indicated permeability values of up to 1 x 10-4 m/s

(Figure 2). Pumping tests indicated peak flows (from a 250 mm diameter well) of up to 16 l/s and

sustained flows of almost 10 l/s. Recorded drawdown during the pumping testing indicated a clear

SW-NE structural control on the extent of drawdown and the potential for drawdown to extend

to distances of up to 500 m.

The locations of tests that indicated high permeability, the observed structural control on

drawdown and indications of boundary features in the pumping test all coincide with the mapped

extent of thick, channel filling grit (Figure 3).

0%

5%

10%

15%

20%

25%

30%

35%

5E-10 to1E-10

1E-9 to5E-10

5E-9 to1E-9

1E-8 to5E-9

5E-8 to1E-8

1E-7 to5E-8

5E-7 to1E-7

1E-6 to5E-7

5E-6 to1E-6

1E-5 to5E-6

5E-5 to1E-5

5E-5 to1E-4

% o

f S

am

ple

s

K Values (m/s)

Typical ECBF Reach 3 Grit


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Figure 3: Inferred extent of high permeability (k) grit within Reach 3 (labelled boreholes

are referred to in text or subsequent figures)

2.4 Implications for Tunnelling and Excavation

Defects recorded in boreholes and surface lineaments suggest that Oakley Creek likely follows

larger scale geological structures, whilst geological mapping of the creek identified that the

steeply dipping grit beds observed at the depth of the tunnel extend to the surface, potentially

allowing a direct groundwater-surface water connection to the overlying creek.

Additionally, land-use at the surface of Reach 3 comprises sports fields (Phyllis Reserve) that

have been created on a former landfill with no basal liner. Hence any drawdown from tunnelling

or excavation could result in contaminant migration, whilst settlement at the surface could allow

cracking of the landfill cap and / or ponding of surface water that might result in increased leachate

generation.

Reach 3 was therefore identified as one of the highest risk areas in terms of potential adverse

effects resulting from groundwater drawdown and ground settlement. The use of an EPB-TBM

that could be operated in a closed and pressurised mode, was expected to limit the potential for

groundwater inflows and drawdown during tunnelling. However, open drained stops were

required for planned maintenance of tail brushes, changing of cutting tools etcetera, and one such

stop was proposed to be undertaken immediately prior to entering Reach 3 to allow optimal TBM

operation through this zone.

Further, the cross passages were sequentially excavated, and an enlarged (~9 m diameter) cross

passage (XP12) was required in Reach 3 to form the tunnel’s low point sump. It was identified

during the early stages of the project that pre-excavation fissure grouting would be required at

XP12 (and several other cross passages) to ensure safe working conditions and reduce

groundwater inflows during excavation.

BH709

BH529 BH732

DH833

BH730

~150 m

N

XP12


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3 GROUNDWATER EFFECTS DURING TUNNELLING AND EXCAVATION

3.1 Drained Maintenance Stop

Immediately prior to entering the highest risk zone, the TBM was stopped for a planned period of

maintenance and replacement of the tail brushes, a critical tool for maintaining face pressure and

reducing groundwater inflows during tunnelling. The TBM was stopped on 15th July 2014 and

immediate peak groundwater inflows of 30 l/s occurred at the tunnel face, though these rapidly

reduced over a few days to ~10 l/s which was then maintained for the 2 week duration of the stop.

Instantaneous drawdown of 18.5 m was recorded in the nearest piezometer (BH732) located 35 m

from the face of the TBM, with drawdown approaching near steady state conditions and a

maximum 26.3 m drawdown at the end of the two week stop.

As with the earlier pumping testing, a clear structural control on drawdown was observed, with a

near-instant response in groundwater level recorded at distances of more than 200 m away,

including at BH529 located on the opposite side of Oakley Creek (i.e. across what might often be

considered a “groundwater divide”, Figure 4 and Figure 7).

Figure 4: Extent of drawdown at end of drained maintenance stop (boreholes are labelled

on Figure 3)

Recovery of groundwater levels was equally rapid, with most piezometers recovering to 50 % to

85 % of their pre-strop water levels within 7 days, and all fully recovering within 2 months. The

only exception being BH529, which some 3 years later has still not fully recovered with an

apparent permanent drawdown of 5 m relative to pre-tunnelling conditions.

3.2 TBM Tunnelling Induced Effects on Groundwater and Surface Water

Generally, where tunnelling was undertaken in both the typical ECBF and the more permeable

volcaniclastic grit, and with a closed and pressurised face, there was negligible recorded

drawdown of the groundwater level.

12.2

~150 m

N

0.8

0.9

1.4

2.1

0.94.9

0

0

1.7

6.2

10.1

10.6

10.9

10.7

0.0

2.2

2.9

8.1

9.7

3.810.8

0.0

0.0

15.214.7

26.3

2.0


6

In some areas, where there was a concern over mechanical ground movements at the surface, the

TBM was operated at a higher face pressure in order to allow optimal performance (and hence

speed) through the area of concern, while also limiting mechanical movements and groundwater

inflows. Reach 3 was identified as one such area as a result of the large groundwater inflows

during the maintenance stop, the occurrence of contaminated land fill at the surface, and the close

proximity of buildings (with known weather-tightness issues) within 30 m of the tunnel

alignment.

As this was also an area of potential connection between groundwater and surface water, twice-

daily visual inspections of the creek were undertaken, in addition to continuous stream gauging,

in order to visually assess if any irregularities in flow were occurring. During one of these

inspections air bubbles were noted rising from the creek bed (Figure 5), coincident with the TBM

location, confirming the connection between groundwater and surface water.

Figure 5: Photo of air bubbles observed in Oakley Creek during tunnelling and map

showing TBM location at time

The TBM face pressure was slowly lowered until the bubbles reduced to an acceptable level,

whilst still maintaining minimal groundwater inflows to the face or drawdown in adjacent

piezometers. Daily monitoring of piezometers in the immediate area was then used as a check

against the TBM drive parameters (that were set early in the design phase) to confirm the optimal

operational pressure.

BUILT RINGS

APPROX. CUTTERHEAD LOCATION

OAKLEY CREEK

INITIAL OBSERVATION OF BUBBLES

BUILT RINGS

CUTTER HEAD LOCATION

INITIAL OBSERVATION OF BUBBLES


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3.3 Cross-passage XP12 Excavation

Prior to cross passage excavation, narrow (76 mm) diameter probe-holes were undertaken from

the completed south-bound tunnel, to confirm geological conditions and allow measurements of

groundwater inflows to determine if pre-excavation grouting was required. The recorded inflows

to the probe-holes indicated that peak inflows to the larger cross passage excavation of 30 l/s to

80 l/s might occur, with sustainable flows of 10 – 20 l/s (i.e. comparable to that observed during

the maintenance stop).

Figure 6: Geological cross section through XP12 (grey line indicates canopy tube, red line

indicates probe drilling). NB: ECBF comprises grit (steeply dipping out of page).

During probe-hole drilling, and canopy tube drilling, immediate drawdowns of up to 1.5 m were

observed in the ECBF at a distance of 14 m from probe-holes (BH732, Figure 7), with detectable

drawdown (0.5 m) recorded at a distance of more than 200 m (DH833 and BH529).

Figure 7: Observed drawdown at select monitoring locations in Reach 3 (distances are

relative to outer edges of XP12)

BH732

Oakley Creek regional GWL

perched GWL

~15 m

WEST EAST

ECBF

TGA

FILL

0

5

10

15

20

25

30

Jun-14 Aug-14 Nov-14 Mar-15 Jun-15 Aug-15 Nov-15 Feb-16 May-16

Dra

wd

ow

n (

m)

rela

tive

to

1st

Ju

ly 2

01

4

BH732 (14 m away) BH730 (65 m away) BH709b (95 m away) DH833 (195 m away) BH529 (210 m away)

drained stop

probe drilling @ XP12 & XP13

1st TBM passage

canopy tube installation

@ XP12

excavatation @ XP12

fissuregrouting@ XP12

2ndTBM passage

data loggermanual reading


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A cementitious grout, targeting larger aperture joints was selected (Maclean et al, 2017) with

grouting carried out by a specialist grouting contractor, through grout ports pre-installed in the

segmental lining. Having observed the connection between TBM pressure at depth and Oakley

Creek, care was taken to limit grouting pressures to reduce the risk of grout being ejected at the

surface, with visual inspections of the creek and piezometer monitoring undertaken to confirm.

The cross passages were excavated in a series of short advances with spraying of shotcrete for

temporary support. Prior to shotcreting the excavated face was logged by an Engineering

Geologist and a visual estimation of seepage was made. Pre-excavation grouting was observed

to be highly effective in reducing groundwater inflows. At XP12, groundwater was mainly

observed as isolated seepage from occasional joints with the flow estimated to be less than l/s.

The grouting was also effective in limiting the magnitude of drawdown at distance (Figure 6),

which, for the 9 m diameter excavation (open and dewatered for 5 months), was not significantly

greater, and in some cases was less (BH709b) than observed during short term drilling and testing

of the probe-holes. The magnitude of drawdown was also less than observed during the drained

maintenance stop and flow gauging of Oakley Creek confirmed no impacts on creek flows.

4 CONCLUSIONS

Site investigations and observations from tunnelling through a discrete zone of moderately strong,

jointed volcaniclastic grit, identify that significant groundwater flows and groundwater drawdown

can propagate rapidly through the high permeability, but low storativity joints and fractures that

often accompany these materials. The extent of drawdown was strongly structurally controlled,

with drawdown occurring preferentially in a SW-NE direction, at distances of greater than 200 m

and beneath the opposite side of Oakley Creek.

Although no impact on base flows in Oakley Creek was noted, connection to the creek was

confirmed by the propagation of air bubbles to the surface. Visual inspections and instrumented

monitoring were used to regularly inform TBM drive and grouting parameters to reduce the

effects at the surface.

Peak groundwater inflows of 30 l/s and sustained inflows of 10 l/s occurred in open excavations

through the grit. However, monitoring confirms that fissure grouting was successful in

significantly reducing groundwater inflows and controlling the magnitude of drawdown at

distance.

The monitoring results confirm the need for monitoring, and proactive management of

groundwater effects, when grit is encountered in deep excavations or tunnelling projects.

5 ACKNOWLEDGEMENTS

The writer would like to thank the NZ Transport Agency and its Well-Connected Alliance

partners for permission to publish details of the project, and also Ann Williams for her review of

this paper.

REFERENCES

France, S.J., Williams, A. & Cammack, E. (2011) Western Ring Route – Waterview Connection

Driven Tunnel: Assessment of Groundwater Effects. Proceedings of the 14th

Australasian Tunnelling Conference. The Press, London.

Maclean, H.J., Cartwright, S. and Giauque, A. (2017) Fissure grouting and rock defect

characterisation for the Waterview cross passage tunnels. in prep.

Documents

Tunnelling through volcaniclastic grit; monitoring and ... · France, S. (2017) Tunnelling through volcaniclastic grit; monitoring and management of groundwater effects on the Waterview