INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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The paper was published in the proceedings of the 11th Australia New Zealand Conference on Geomechanics and was edited by Prof. Guillermo Narsilio, Prof. Arul Arulrajah and Prof. Jayantha Kodikara. The conference was held in Melbourne, Australia, 15-18 July 2012.

Victoria Park Tunnel – Drawdown and Settlement in the Central Business District

S. J. France1, G. Newby2 and A. L. Williams3

1Beca Infrastructure Ltd, P.O. Box 6345, Auckland 1141, New Zealand; PH (64) 300-9000; FAX (64) 300-9300; email: [email protected]. 2Beca Infrastructure Ltd, P.O. Box 6345, Auckland 1141, New Zealand; PH (64) 300-9172; FAX (64) 300-9300; email: [email protected]. 3Beca Infrastructure Ltd, P.O. Box 6345, Auckland 1141, New Zealand; PH (64) 300-9140; FAX (64) 300-9300; email: [email protected].

ABSTRACT

Construction of the Victoria Park Tunnel (VPT) project required the excavation of a 450 m long, 3 lane road tunnel through Auckland’s CBD in close proximity to the Waitemata Harbour. The tunnel, constructed using cut and cover techniques, had a maximum excavation depth of 11 m bgl (some 9 m below the groundwater table) through contaminated fill, compressible alluvium and sedimentary rock. Although the tunnel is now sealed, short term dewatering to facilitate construction was necessary. A combination of low permeability diaphragm and secant pile walls was used to retain the excavation and limit groundwater inflows; however the base of the excavation was left open for periods of up to 4 months before the floor slab was laid. In order to evaluate the potential for consolidation settlement of existing structures including nearby historic buildings, the potential for contaminant and salt water migration, and guide detailed design, numerical groundwater modelling was undertaken. Groundwater level and ground settlement monitoring was undertaken before, during and following construction, with pre-construction monitoring providing valuable records of naturally occurring large seasonal variations. Recorded drawdown, settlement and inflows have generally remained within the consented levels. Some exceedances were recorded however a pragmatic approach to monitoring and management of these exceedances allowed works to continue with minimal disruption and no adverse environmental effects. This paper presents a brief comparison of calculated effects against those which occurred, with a focus on two isolated groundwater level trigger exceedances (their causes and management).

Keywords: cut and cover tunnelling, groundwater, drawdown, settlement, groundwater modelling, monitoring

1 INTRODUCTION

The Victoria Park Tunnel (VPT) project was undertaken to address the last major bottleneck on the central motorway network between the and Newmarket, in Auckland, New Zealand. The VPT project involved construction of a 450 m long, 3-lane north-bound tunnel. The tunnel was constructed by cut and cover methods and is located to the west of the existing Victoria Park Viaduct (Figure 1) having a maximum depth to underside of floor slab of 11 m, some 9 m below the groundwater table.

The tunnel is located within the Central Business District in close proximity to businesses, residential apartments, historic buildings and the Waitemata Harbour. Materials excavated in tunnelling include contaminated fill and compressible alluvium. An understanding of the interaction between the tunnel (during construction and long term) and the groundwater system was required to inform tunnel design and assess (and as far as possible avoid), potential deleterious effects on the environment (ground settlement and spread of contaminants).

Groundwater drawdown, total settlement and groundwater inflows recorded prior to, during and following construction are presented and compared to results of numerical modelling. Two exceedances that occurred during construction, their likely causes and the approaches to monitoring and management of them to allow works to continue are discussed.

ANZ 2012 Conference proceedinga 101 New Westhaven N Auckland Zealand Marina

Pre-reclamation shoreline St Mary's Bay Western Reclamation

Cut and cover tunnel North Shore & approaches LEGEND Victoria Waitemata Harbour Park Reclamation Fill East Coast Bays Formation CBD Existing motorway 5 km ~500 m Auckland Existing viaduct 1

Figure 1: Location of Victoria Park Tunnel

2 HYDROGEOLOGICAL SETTING

The project corridor occupies generally low-lying reclaimed land (Victoria Park) of the original Freeman’s Bay embayment. East Coast Bays Formation (ECBF) interbedded sandstone and siltstone forms the bedrock (Figure 2) and outcrops along the old cliff line and foreshore. Secondary hydraulic conductivity is dominant in these weak rocks with the majority of groundwater flow along bedding surfaces and defects. Hydraulic conductivity is typically of the order of 5 x 10-7 m/s but values of up to 5 x 10-6 m/s were recorded along the alignment of inferred paleo-valleys.

The ECBF is overlain by a variable thickness of strongly anisotropic Tauranga Group alluvium comprising soft or loose, unconsolidated, compressible silty and sandy sediments. In-situ testing -7 indicates a mean hydraulic conductivity of 2 x 10 m/s. A KV/KH ratio of 0.01 to 0.1 is typical.

A variety of fill types were encountered along the alignment, including hydraulic fill up to 5 m thick. Pockets of hydrocarbons and other contaminants resulting from the former industrial land use are found within the fill, as are areas of construction fill.

Recorded groundwater levels indicate a relatively flat water table across the tunnel (a gradient of < 4 %), dropping from around 5 m RL (at the southern approach) towards 3 m RL (near the northern exit).

Central Motorway Junction St Mary's Bay (to Newmarket) (to Harbour Bridge)

finished ground level Northern Portal Southern Portal

Fill groundwater level residual Alluvium ECBF finished tunnel ECBF floor level

10x vertical exaggeration

Figure 2: Schematic geological long section along main tunnel alignment

3 CONSTRUCTION METHODOLOGY

Retaining walls were installed prior to excavation to provide excavation support and a cut-off to groundwater. Both diaphragm and secant pile retaining walls were used (depending on depth to rock) to form the low permeability walls of the tunnel. Walls extended to the greater of either 4 m below underside of the base slab or 2 m embedment into rock.

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Bulk excavation between the retaining walls commenced in September 2010 with the final base slab poured in September 2011. Excavation of the tunnel was undertaken progressively from three fronts with groundwater inflows pumped from the base of excavation. The floors of the excavations were left open and drained for periods of up to 4 months before being partially sealed with site concrete or fully sealed by pouring the structural base slab.

4 ASSESSEMENT OF EFFECTS USING NUMERICAL GROUNDWATER MODELLING

Two-dimensional and three-dimensional groundwater modelling was undertaken using the programs GeoStudio SEEP/W and Visual MODFLOW Pro (Schlumberger) respectively (France, 2008). Modelling suggested that on average a maximum drawdown of up to 2.5 m might occur immediately adjacent to the tunnel walls in the ECBF. Where higher hydraulic conductivity ECBF was encountered (inferred paleo-valleys) a drawdown of 4.0 m to 7.5 m was considered possible.

Within the compressible Tauranga Group, maximum drawdown immediately adjacent to the tunnel was predicted by modelling to be 0.7 m with measurable drawdown (0.1 m) extending 200 m to 500 m depending on the geological profile and presence of higher permeability drainage channels. One- dimensional consolidation settlement analyses suggest that this magnitude of drawdown could have the potential to induce consolidation settlements of up to 20 mm immediately adjacent to the tunnel reducing to 5 mm to 10 mm at the nearest buildings.

Negligible drawdown was indicated by modelling within the fill, with groundwater flow reversal occurring over a limited extent in these near-surface materials such that both contaminant migration and saline intrusion were considered unlikely.

Modelling indicated seepage of 20 m3/d to 50 m3/d of groundwater into the excavation as a result of drawing the water table down to excavation floor level in the ECBF during construction.

5 CONSTRUCTION MONITORING

5.1 Monitoring Programme

A groundwater and settlement monitoring programme was developed on the basis of the pre- construction assessment of effects.

Piezometer pairs were installed at 10 sites around the project, each with one standpipe screened at depth in the ECBF (where the greatest drawdown was expected to occur) and the other in the shallow alluvium or fill (where there was the greatest potential for adverse settlement effects to occur). Baseline monitoring undertaken over a 12-month period prior to excavation identified the typical seasonal variation to be generally 0.5 m within the ECBF rock and 0.9 m in the Tauranga Group Alluvium or Fill. Critically, this baseline monitoring also captured a very dry summer (January to April 2010 when less than 35 % of mean rainfall fell) which identified that groundwater levels could naturally drop a further 0.3 m to 0.5 m on average, but up to 0.8 m in some locations during periods of low rainfall.

Forty-two ground and 96 building settlement pins were also installed around the project site and monitored some 12 months in advance of works to identify normal seasonal variations. At most pin locations the seasonal variation was found to be 2 mm to 6 mm, however 10 % of marker pins exhibited variations of 10 mm to 20 mm and 4 sites (including one building marker pin) were found to vary seasonally by more than 20 mm.

5.2 “Typical” ECBF Response to Excavation

All piezometers screened within the ECBF showed a lowering of groundwater level in response to excavation dewatering (Figure 3a). At eight of the ten monitoring sites drawdown ranged between 0.4 m and 2.0 m, consistent with the modelling. Time-drawdown plots for the sites closest to the excavation show a more pronounced drawdown and recovery curve in response to site activities. A

ANZ 2012 Conference proceedinga 103 clear relationship between drawdown and distance from the excavation is evident (Figure 3b) and would not generally be expected if fracturing within the ECBF had a preferential orientation.

4 Distance (m) 0 50 100 150 3 0 0.5 2 1 1.5 1 2 0 2.5 Max Drawdown (m) DrawdownMax

Groundwater Level (m RL) (m Level Groundwater Max recorded drawdown 3 Range of modelled drawdown Dig 3 3 Digcommences Dig 1 1 Digcommences 2 Digcommences -1 ("typical" ECBF) Month 3.5 Figure 3a: Drawdown over time in ECBF piezometers 3b: Maximum drawdown in ECBF piezometers vs. distance from excavation

In two areas, Victoria Street and Beaumont St, drawdown of more than 2 m was recorded in response to activities within and outside of the VPT project site. These sites are discussed in more detail below.

5.3 “Typical” Fill / Tauranga Group Response to Excavation

Eight of the ten shallow piezometers showed a lowering of groundwater level that could be attributed to excavation dewatering (Figure 4a). Drawdown typically ranged between 0.4 m and 0.9 m, in some places slightly greater than that predicted by numerical analyses, though within recorded seasonal ranges and hence insufficient in magnitude and or duration to result in any settlement trigger exceedances. A less distinct relationship between drawdown and distance is seen in the shallow piezometers. Rather, the greatest drawdown occurs where the residual soils or hydraulic fill are more directly connected to the underlying ECBF rock (Figure 4b). For two of these monitoring sites, located near Victoria Street, drawdowns of 1.2 m and 2.1 m were recorded.

4 Distance (m) 3.5 0 50 100 150 3 0 2.5 2 0.5 1.5 1 1 0.5 1.5 Max recorded drawdown Fill / Hydr Fill 0 Max Drawdown (m) Drawdown Max TGA

Groundwater Level (m RL) (m Level Groundwater -0.5 2 RECBF

Dig 3 3 Dig commences Range of modelled Dig 1 1 Dig commences 2 Dig commences A -1 B drawdown ("typical" ECBF) Month 2.5 Figure 4a: Drawdown over time in shallow piezometers 4b: Maximum drawdown in shallow piezometers vs. distance from excavation

5.4 Groundwater Seepage

Visible groundwater seepage into the excavation was observed at several locations from fractures that day-lighted in the excavation face, however overall groundwater seepage was very minor in comparison to surface water run-off and water introduced from construction activities. Groundwater inflows were assessed to be typically of the order of 5 m3/d to 10 m3/d, though occasional peaks of up to 20 m3/d were recorded; this is at the lower end of the range of expected inflows. Small inflows are typical of the ECBF during short term dewatering, and because of the very low storativity of fractures in the ECBF, large drawdowns can result (close to the seepage face) from only a small water loss.

5.5 Ground Settlement

Ground and building settlement markers surveyed throughout the duration of the project have indicated only limited settlement that could be attributable to drawdown or other construction activities.

ANZ 2012 Conference proceedinga 104 Less than 10 % of all pins showed a measurable drop (more than 3 mm) below their naturally occurring pre-construction lowest level.

All other markers monitored as part of the project, including those where the groundwater level was lowered for up to 8 months, indicate that vertical movements and angular distortions have been within the seasonal range of the preceding year and in many cases within the accuracy of the survey.

Given that the modelled levels of drawdown occurred, and in some cases were exceeded, this suggests that the settlement calculations possibly underestimated the degree of pre-consolidation and / or overestimated the time for settlement to occur in the anisotropic, low permeability soils. This observation is consistent with monitoring from other project sites in comparable hydrogeological units (France and Williams, 2010). It is suggested that pre-construction monitoring of ground and building movements due to seasonal groundwater variations for this and other projects in the Auckland area can be better used to calibrate calculated ground settlements for more accurate assessments for future projects, to avoid implementation of unnecessary mitigation measures such as the construction of recharge wells required in this project.

5.6 Beaumont Street Trigger Exceedance

Near to where the tunnel crosses below Beaumont Street a drawdown of 2.95 m was recorded in the ECBF in piezometer S09_pz001 in November 2010 (Figure 5). At this location a small gap between two of the secant piles was observed, allowing a noticeable inflow of groundwater from behind the walls. Adjacent to the tunnel (and hydraulically up-gradient) the tunnel site adjoined a construction site not associated with the project where a deep basement car park was being excavated to about the same level as the tunnel. Exceedance of consented drawdown trigger levels was also recorded at this site, prompting both sites to undertake a period of more intensive groundwater level and settlement monitoring. Data sharing between the two projects confirmed that although groundwater levels in the rock were lowered, groundwater levels in the compressible soils were not significantly lowered and detectable ground settlement was not observed. This monitoring allowed the excavation to continue unimpeded, although measures were undertaken to accelerate the sealing of the tunnel in this section.

4 100 10 11

3 Jan Jan 80

2 60 Daily Rainfall Excavation 1 40 Alluvium Excavation sealed commences

0 20 (mm) Rainfall Daily Water Level (RL m) (RL Level Water ECBF

-1 0 Month Figure 5: Drawdown over time in S09_pz001

A rapid setting epoxy was used to also seal the gap between the secant piles at about the time the adjacent excavation reached target depth, resulting in steady recovery of the groundwater level over the ensuing months. Monitoring identified that during periods of heavy rainfall, the groundwater level in the ECBF at both sites rose very rapidly, at times by more than 1 m almost instantaneously. This is a more rapid response to rainfall than expected and is perhaps due to the shallow depth to rock which daylights just up-gradient of both excavations.

The combination of the leak in the tunnel wall and the adjacent drained excavation (which cut off up- gradient flow) was considered responsible for the trigger exceedance. However the marked response to rainfall suggests that the drawdown may also have been exacerbated by the dry weather at that time (the preceding two months having had less than 50 % of mean monthly rainfall).

5.7 Victoria Street Trigger Exceedance

Where the tunnel crossed below Victoria Street, the density of utilities to be moved and/or protected meant that retaining walls could not be excavated from the ground surface and four 7 m long sections

ANZ 2012 Conference proceedinga 105 of the tunnel walls were constructed from within an open excavation (that is, without groundwater cut- off). Correspondingly, a pronounced drop in groundwater levels in the ECBF occurred in four monitoring wells in response to the dewatering of this excavation. Drawdown in the shallow soils can also be clearly seen in S06_pz043 (Figure 6). The recorded drawdown of 3.2 m and 2.1 m (in the deep and shallow piezometers respectively) exceeded the consented trigger levels for drawdown prompting more frequent groundwater level and settlement monitoring within 100 m.

4 100

10 11 Excavation

3 Jan Jan sealed 80

2 60 Daily Rainfall 1 Excavation 40 Fill commences Residual ECBF

0 20 (mm) Rainfall Daily Water Level (RL m) (RL Level Water ECBF -1 0 Month Figure 5: Drawdown over time in S06_pz043

As the more intensive monitoring confirmed that ground settlement had not yet been triggered, it was communicated to the consenting body that the most pragmatic approach to managing the exceedance was to accelerate the works in the area, keeping in mind the broader objective to seal the tunnel excavation as early as possible. The construction programme was reviewed and partial sealing of the walls and base slab was brought forward, resulting in a steady recovery of groundwater levels. The exceedance of the consented drawdown trigger can be directly attributed to the top-down wall construction method.

6 SUMMARY AND CONCLUSION

Monitoring prior to construction identified significant natural variations in both groundwater level (up to 1.5 m) and in ground level (up to 20 mm). Monitoring during and following construction of the Victoria Park Tunnel has indicated that groundwater drawdown and inflows to the excavations were generally consistent with the results of modelling undertaken during detailed design and consenting. Two isolated groundwater drawdown trigger exceedances occurred, but these could be directly attributed to specific construction activities, both on and off-site.

Ground and building settlement surveys indicated limited settlement that could be attributable to drawdown or other construction activities, even where the groundwater level was lowered (in some cases below trigger levels) for months at a time. This suggests that one or more of the parameters used in settlement calculations are overly conservative and that such calculations should be reviewed to assess how monitoring can be used to establish appropriate correlations with drawdown effects.

Where exceedances in drawdown did occur, they were managed with more intensive monitoring and data sharing between parties, allowing works to proceed and acknowledging the overall need to seal the tunnel excavation as early as possible to limit the potential for adverse environmental effects.

7 ACKNOWLEDGEMENTS

The writers would like to thank the New Zealand Transport Agency and the Victoria Park Alliance for permission to publish details of the project.

REFERENCES

France, S. (2008). “Victoria Park Tunnel: Drawdown, damming and contaminant migration.” Proceedings, 8th Young Geotechnical Professionals Conference, Wellington, NZ, 5-8 November 2008. France, S. and Williams A.L. (2010). “New Lynn Rail Trench: the realities of groundwater modelling.” Proceedings, 11th IAEG Congress, Auckland, New Zealand, 5-10 September 2010.

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