INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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The paper was published in the proceedings of the 12th Australia New Zealand Conference on Geomechanics and was edited by Graham Ramsey. The conference was held in Wellington, New Zealand, 22-25 February 2015.

Waterview Connection: Environmental impacts of a deep drained trench

S.J. France1 and A.L Williams2

1Beca, Beca House, 21 Pitt Street, P.O. Box 6345, Wellesley Street, Auckland, 1141, New Zealand; PH (64) 9 300-9000; FAX (64) 9 300-9300; email: [email protected] Beca, Beca House, 21 Pitt Street, P.O. Box 6345, Wellesley Street, Auckland, 1141, New Zealand; PH (64) 9 300-9172; FAX (64) 9 300-9300; email: ann.williams@beca./com

ABSTRACT

The NZ Transport Agency’s Waterview Connection project involves the construction of 4.8 km of motorway to complete Auckland’s Western Ring Route. Half of this new link will be tunnelled; the remaining half is surface highways and approach trenches. The Southern Approach Trench (SAT) is 400 m long with a temporary excavation up to 29 m deep, through basalt lava flows, Quaternary alluvium, residual soils and Tertiary age interbedded sandstones and siltstones. The concept design considered a drained base with a grout curtain through the basalt and cut-off walls through underlying units to control groundwater inflows and limit environmental impacts such as drawdown, stream depletion and consolidation settlement. Through analysis of the hydrogeological setting it was determined that the trench could be fully drained, without a grout curtain, with manageable groundwater inflows and negligible environmental effects. This paper describes how limited inter- connection between the basalt lobe (through which the SAT is excavated) and the main basalt flow and creek was determined, and presents observations of groundwater inflows during and following construction. Structural controls (paleo-valleys and faults), combined with the low storativity of the units, result in drawdown effects up to 230 m from the trench in some areas. Despite more than 2 years of drawdown, there has been negligible consolidation settlement or stream depletion that could be attributed to construction.

Keywords: basalt, drawdown, settlement, groundwater inflows

1 INTRODUCTION

The Waterview Connection project involves the construction of 4.8 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 constructed by a Tunnel Boring Machine (TBM), the 10th largest of its kind and the largest to be used in the Southern Hemisphere. The remaining half comprises surface highways and approach trenches (Figure 1). The Southern Approach Trench (SAT), where the TBM was assembled and launched, is 400 m in length with a temporary excavation up to 29 m deep. Figure 2 shows key features of the trench and its construction.

2 GEOLOGICAL SETTING AND STRUCTURAL FORM OF THE SAT

The Waterview Connection project is situated within the Waitemata Basin, a shallow marine basin formed between 24 and 18 million years ago (Kermode, 1992). Consolidated sediments that infilled the basin formed a sequence of weak sandstones and siltstones (East Coast Bays Formation, ECBF), locally interbedded with and cross-cut by coarse volcaniclastic sandstone (Parnell Grit member). ECBF underlies the entire project area and outcrops in ridges to the south and west of the alignment. The ECBF is overlain by a variable thickness of Tauranga Group alluvium (~2 million to 10,000 years old) which has infilled a series of paleo-valleys incised in the ECBF. The alluvium is overlain by a ponded basalt lava flow from the Mount Albert volcano (30,000 to 100,000 years old) that flowed into the Oakley Creek paleo-valley. Oakley Creek then re-established itself and now delineates the western boundary of the lava flow.

The SAT is constructed through one of the lateral lobes of the basalt lava flow (Figure 2). The excavated basalt face (maximum height of 12 m) is fully drained with face support provided by rockbolts, mesh and a shotcrete facing. Horizontal bored drains are used to control groundwater at the excavation face and a subsoil drain was constructed within the Alluvium at the toe of the basalt. Below the basalt, the trench is retained by bored soldier pile walls with strip drains behind sprayed concrete arches formed between the piles. Herring-bone sub-soil drains beneath the roadway permanently drain the base slab. Oakley Creek was required to be diverted in four places in order to accommodate the trench and surface highway.

Figure 1. Location plan

Figure 2. Key features of the SAT: A) Southern portal headwall. B) Shotcreted basalt face. C) Bored pile wall retaining soils and ECBF. D) TBM assembly. E) Original alignment of Oakley Creek. F) Diverted alignment of Oakley Creek. G) Edge of basalt lava flow. H) Basalt paleo-valley.

3 HYDROGEOLOGIAL SETTING

3.1 ECBF

Water levels within the ECBF are indicative of a regional water table of low gradient, discharging to the northwest (towards the harbour) and locally discharging to Oakley Creek (Figure 3). Groundwater flow is largely defect controlled through fractures and bedding planes, although some lesser flow can also be expected through the matrix of coarser sandstone beds. Rainfall recharge occurs slowly via a series of cascading water levels; however, the main recharge for this unit is from up-gradient flow within the rock mass. The ECBF has a relatively low hydraulic conductivity and storativity.

Figure 3. Conceptual Hydrogeological Model.

3.2 Alluvium

Water levels in the alluvium are representative of an unconfined to semi-confined aquifer system with water levels typically 2 m higher than levels in the underlying rock. Groundwater flow is predominantly through pore spaces, and the units exhibit a strong vertical anisotropy due to bedding. The anisotropy results in a variable and muted connection with the underlying ECBF. Although some seasonal variation occurs there is only a muted rainfall response observed around the SAT. The alluvium is of low hydraulic conductivity and low storativity, but is compressible and susceptible to consolidation settlement if drained.

3.3 Basalt

Water levels within the basalt lava flow are indicative of a perched, unconfined aquifer system, and are 2 m to 7 m greater than those in the underlying units. The water levels respond rapidly to rainfall suggesting a direct connection with the surface. Groundwater flow in the basalt is defect and cavity -6 controlled, with hydraulic conductivity varying over several orders of magnitude (Kh and Kv = 10 m/s to 10-4 m/s) depending on the extent and inter-connection of fractures.

Investigation drilling near the centre of the main lava flow (i.e. offline from the SAT) often encountered significant groundwater inflows that were difficult to control; however, anecdotal evidence from excavations at the margin of the flow indicates negligible groundwater inflows. For this reason the groundwater flow connection between the main part of the flows and the marginal lobes was investigated.

Contoured groundwater levels in the basalt indicate a predominantly north north-westerly flow direction, analogous to the main direction of the basalt flow, in which significant water bearing defects might reasonably be expected. There is a much less distinct gradient of groundwater flow from the centre of the basalt to the lateral lobes. Where basalt is exposed in the banks above the creek, negligible seepage is observed, suggesting the majority of discharge from the basalt occurs to the north where the basalt thins and shallows resulting in surface springs. This lack of connection to the main water-bearing flow was confirmed by short term pumping tests in the basalt which indicated that although the basalt lobe was of high permeability (i.e. 1 x 10-5 m/s or greater) it was of low storativity, with limited connection to the main basalt lava flow or the creek. As such even small rates of discharge (< 0.5 l/s) could not be maintained during testing for more than a few hours.

Simultaneous flow gauging at 7 sites in Oakley Creek allowed a semi-quantitative assessment of the connection between groundwater and surface water. The monitoring indicated that up-gradient of the SAT where the creek is incised into basalt it loses water to ground. Directly adjacent to the SAT, Oakley Creek is incised into the ECBF and the base of basalt is a few metres above creek water level. In this area the water level in the creek is comparable to that in the ECBF. Given the difference in water levels, lack of direct connection between the basalt and creek and lack of observed seepage out of the basalt, the small gain (< 5 l/s equivalent to less than 10 % of in-stream flow from the up-gradient catchment) of water recorded is considered to be largely from the ECBF.

4 GROUNDWATER INFLOWS TO THE DRAINED SAT

4.1 Expected Inflows

The volume of groundwater that flows from the basalt was a key consideration during design and construction. The reference design included for a 600 m long grout curtain, surrounding the entire SAT, in order to reduce the anticipated large flows from the basalt and creek.

Given the uncertainty around the number of grout holes that would be required, grout take, effectiveness and cost, and having considered the conceptual model described above, omission of the grout curtain was identified as a detailed design opportunity to reduce cost and accelerate the construction programme.

3D and 2D groundwater modelling, using Visual Modflow Pro and SEEP/W, indicated that peak inflows (from the entire excavation) were likely to be less than 1,000 m3/d (12 l/s) with no grout curtain. The modelling also indicated that groundwater drawdown and (and therefore ground settlement) would not be significantly greater than that calculated for a grout curtain. In fact analyses suggested that omission of the grout curtain would result in reduced differential drawdown and settlements, and therefore provide a better outcome in terms of the potential for damaging consolidation settlement.

4.2 Observed Groundwater Inflows

Total groundwater inflows experienced during construction were typically of the order of 600 m3/d to 900 m3/d (7 l/s to 10 l/s), comparing well to the results of groundwater modelling. Approximately 60 % to 70 % of this flow was sourced from the basalt, with the remaining inflow from the ECBF.

Despite the groundwater table being encountered within 2 m of the ground surface, significant groundwater inflows were not encountered until the excavation was almost 9 m below ground level. When groundwater was encountered it tended to “chase” the excavation, with water typically discharging from discrete fractures located near the base or within the floor of each excavation level. Large inflows were encountered in the base of the paleo-valley (Figure 4A).

Where groundwater was seen to discharge from the ECBF this was typically along bedding planes as can be seen in Figure 4B, where groundwater discharges at the basal contact of a bed of Parnell Grit. The flow at this location is terminated by the presence of a fault.

The majority of groundwater presently discharging to the SAT comes from two bored drains, one on either side of the trench and located within or close to the deepest part of the basalt paleo-valley that bisects the trench.

Figure 4. Examples of groundwater inflow: A) Base of basalt paleo-valley B) Along bedding plane at basal contact of Parnell Grit bed (terminated at fault trace).

5 EFFECTS OF THE DRAINED SAT

5.1 Groundwater Drawdown

From the commencement of basalt excavation and dewatering, the water table in the basalt and underlying alluvium dropped coincident with the excavation level. A more gradual, smaller drop in level was observed in the weathered and unweathered ECBF beneath the alluvium. For a period of time before the ECBF was reached, the water table in the basalt and alluvium was lower than that in the underlying ECBF. An example of groundwater levels in a piezometer immediately adjacent to the excavation is shown in Figure 5.

50 40 Mon. Tues. Wed, Thurs. Fri. Sat. Sun. 9am 9am 35 m) 7am (RL

Level

(AVL)

40 30 Groundwater 4pm 3pm Basalt 4pm m)

25 (RL 12/11/12 13/11/12 14/11/12 15/11/12 16/11/12 17/11/12 18/11/12 19/11/12 Date 30 Level

A EW

Groundwater

20

ECBF baslat

excavation

excavation

of

commences Basalt Base reached commences ECBF Maximum excavation depth reached 10 Piling commences Jul‐12 Nov‐12 Mar‐13 Jul‐13 Nov‐13 Mar‐14 Jul‐14 Date SPZ003.i ECBF SPZ003.ii ECBF SPZ003.iii EW SPZ003.iv AVL

Figure 5. Groundwater levels in piezometer SPZ003 since the start of excavation.

Pile excavation and the resulting dewatering around and adjacent to the main excavation, though of small volume and limited duration, had a marked and immediate effect on groundwater levels in the ECBF (Figure 5). A clear pattern of drawdown of the water table is seen on week days, with recovery overnight and in weekends (when pumping was not occurring). This drawdown was recorded in piezometers up to 100 m distance from the pile locations. This rapid response to pumping indicates low storativity in fractures within the ECBF. Similar responses to pile dewatering have been observed elsewhere in ECBF (Namjou and Pattle, 2006).

When the base of basalt was reached in the main excavation, the water table in the basalt attained a steady state condition. From commencement of excavation in the ECBF, the water table in the ECBF began to drop rapidly, stabilising when the excavation reached maximum depth. Groundwater levels in all units have been steady since the maximum excavation depth was reached in June 2013.

Outside of the construction zone the greatest drawdown has been found to occur at the approximate location of the paleovalley that bisects the SAT (Figure 6). The drawdown is offset (to the west) from the main axis towards the steepest side of the paleovalley. Drawdown of up to 3 m has been recorded in the ECBF at a distance of 230 m from the trench. Drawdown has also been observed in the alluvium and basalt, but is of a lesser magnitude. As with piezometers closer to the trench, groundwater levels outside the construction zone rapidly reached steady state and have been steady, allowing for seasonal fluctuations, since June 2013.

Figure 6. Groundwater drawdown in ECBF as at September 2014

5.2 Consolidation Settlement

The excavation has now been dewatered for over 2 years and groundwater levels have been stable for at least the last year. Survey monitoring of ground and building settlement marks has indicated negligible ground movement that could be attributed to consolidation settlement. Less than 5% (5 No.) of the settlement marks installed around the SAT have shown a measurable drop (i.e. more than 3 mm) below their naturally occurring pre-construction lowest level (Figure 7). No spatial trend is evident to link the observed settlement with a cause, and the settlements are not located in the areas of greatest drawdown or thickest alluvium. The two marks that have shown the greatest settlement (of 15 mm and 21 mm) are not located in an area of significant drawdown, nor are they located in proximity to retaining walls that might explain the marked difference in settlement. The cause of those movements is not, therefore, considered to be related to SAT construction. No damage to any building, service or other asset has been identified in the area.

The magnitude of settlement recorded is not dissimilar to a rough rule of thumb of “approximately 1 mm of surface settlement for every 1 m of drawdown” proposed by Harding et. al. (2010), with respect to dewatering around the Three Kings quarry where a similar thickness of alluvium overlies or underlies an Auckland basalt sequence. The settlement is also comparable to the magnitude of settlement recorded on other deep excavation projects in Auckland such as Britomart (Namjou and Pattle, 2006), New Lynn Rail Trench (France and Williams, 2010) and Victoria Park Tunnel (France et. al., 2012).

‐15 Excavation Maximum excavation

(mm) commences depth reached

‐10 Minimum ‐5

0 Preconstruction 5 Adopted

10 from

15

20 Deformation

+ve reading indicates settlement / subsidence, ‐ve reading indicates rise 25 Jul 2012 Oct 2012 Mar 2013 Jul 2013 Nov 2013 Mar 2014 Jul 2014 Date

Figure 7. Survey monitoring of all settlement marks around the SAT

All other markers monitored as part of the project, including those where the groundwater level has been permanently lowered in the alluvium, 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.

During detailed design, a comparison of values of coefficient of volume compressibility (mv) derived from oedometer tests, CPT and in-situ seismic dilatometer testing (SDMT) at both the Southern and Northern Approach Trench suggested that the oedometer tests over-estimate the average compressibility (by up to a factor of 2 to 3). Although the laboratory results accurately reflect weaker horizons in the profile, there appears to be a sampling bias because geotechnical engineers tend to select samples from weaker horizons for testing, which may not be representative of the soil mass as a whole (Figure 8).

Modulus (MPa) 0 25 50 75 100 0 Lab. consols (M = 1/mv) Scatter of all traces 25th percentile of all traces Design line used

(m) Better fit to observed data

(for selected points) unit

of

top 5 below

Depth

10 Figure 8. Schematic representation of data used to determine coefficient of volume compressibility.

A trace that approximates the lowest strengths of the 25th percentile SDMT for Constrained Modulus (M) was used to assess the volume of compressibility (M = 1/mv) that was in turn used for calculation of consolidation settlement (Figure 8). The 25th percentile was chosen, in part to provide an upper bound assessment of effects to test a pessimistic case for building damage (red line Figure 8). However, review of the measured settlement indicates that a less pessimistic fit through the 25th percentile better reflects the recorded magnitude of settlement of < 5 mm (green line Figure 8).

It has become commonplace (for deep excavation projects in the Auckland Region) to provide an upper bound estimate of effects to test tolerances for building damage and to envelope the magnitude and extent of environmental monitoring undertaken during construction. However given that this practice tends to give overly conservative (and potentially alarmist) estimates of effects, we suggest that it is more helpful to test a more realistic assessment and set trigger levels for monitoring that will advise if these “expected” limits are approached, long before serviceability tolerances are reached.

5.3 Oakley Creek

The drained SAT is in places less than 30 m laterally from the creek (Figure 2) with the drained base slab some 9 m below the creek level. Continuous flow gauging in the creek undertaken prior to construction, during and following construction has not identified any significant change in baseflow as a result of groundwater diversion to the trench. This is as expected given that most of the groundwater inflows to the SAT are sourced from the basalt which has no direct connection to the creek, and the component of baseflow that is derived from the ECBF is small relative to in-stream flow and run-off.

6 CONCLUSION

Through the careful development of a detailed conceptual hydrogeological model, supported by site investigations, it was possible to identify a series of perched and cascading water levels, with muted connection to each other and only limited connection to surface water bodies. This was critical for demonstrating that the environmental effects of the approach trench were negligible. Critically this allowed design optimisations such as full drainage of the 29 m retained height, which significantly reduced structural demands on the walls, and omission of a proposed grout curtain both of which resulted in cost savings for the project. Water level and settlement monitoring prior to, throughout and following construction has confirmed that there are significant structural controls on groundwater in the area, but potential effects such as consolidation settlement and stream depletion have been negligible.

7 ACKNOWLEDGEMENTS

The writers would like to thank the NZ Transport Agency its Well-Connected Alliance partners - Fletcher Construction, McConnell Dowell Constructors, Parsons Brinkerhoff NZ, Beca, Tonkin and Taylor, and Obayashi Corporation for permission to publish details of the project. The writers would also like to thank Bevan Hill for permission to publish the paleo-valley photo shown in Figure 4 and Gavin Alexander for his review.

REFERENCES

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. France, S.J., Newby, G. and Williams A.L. (2012). “Victoria Park Tunnel – Drawdown and Settlement in the Auckland Central Business District.” Proceedings, 11th Australia New Zealand Conference on Soil Mechanics (ANZ 2012), Melbourne, Australia, 15-18 July 2012. Harding, B.C., Pattle, A., Harris, M. And Twoose, G. (2006). “Groundwater Response to the Dewatering of a Volcanic Vent“. Proceedings, 11th IAEG Congress, Auckland, New Zealand, 5-10 September 2010. Kermode, L.O. 1992: Geology of the Auckland urban area. Scale 1:50,000. Institute of Geological & Nuclear Sciences geological map 2. 1 sheet + 63p. Institute of Geological & Nuclear Sciences, New Zealand. Namjou, P. And Pattle, A. (2006). „Post-Audit of a Numerical Groundwater Flow Model developed for Britomart Transport Centre, Auckland, New Zealand“. Proc. NZWWA 48th Conference, Christchurch 2006.