Waterview Connection: Environmental Impacts of a Deep Drained Trench

INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 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.

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