WATERVIEW CONNECTION PROJECT. Ground Settlement WATERVIEW CONNECTION TEAM

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1 WATERVIEW CONNECTION PROJECT Ground Settlement WATERVIEW CONNECTION TEAM

2 Settlement What is it? Settlement is the lowering of ground level caused by a volume change or movement within the soil Shrinking and swelling occurs naturally every year, as the ground water level changes with the seasons Tunnels will result in two types of permanent settlement: Mechanical settlement from retaining wall movement and tunnel excavation Consolidation settlement of soils associated with changing ground water levels The amount of settlement from the two types will be cumulative at any given location The settlement is not a continuing problem The pattern of settlement beneath a particular area is of greater interest than the amount of vertical movement Different amounts of settlement beneath the same building can cause damage

3 Consolidation Settlement During tunnel construction (both driven and cut and cover) some ground water will enter the tunnel. This will alter the local ground water conditions. The amount of alteration will vary with time and will depend on particular construction activity underway. We can model this using 3-D and 2-D software, and can measure it using buried instruments Ground water alteration is common to all tunneling projects: The effect lowers the local water table temporarily The lowering of the water table causes the ground to settle This results in consolidation Once the tunnel is complete ground water levels return towards their initial state but the ground level does not

4 Typical geology and typical ground water level before tunnel construction

5 Indicative Ground water level during tunnel construction

6 Ground water drawdown Measurable ground water drawdown is estimated to occur within a 400m corridor of the tunnel alignment with negligible amounts beyond this distance Ground water drawdown will also occur ahead of and behind the tunnel portals Consolidation settlement will vary with the type and thickness of soil conditions in an area Some consolidation settlement will also occur near retaining walls, in open cut areas and at some bridges

7 Mechanical Settlement Mechanical settlement occurs during excavation with the release of pressure on soil or rock as excavation proceeds For this project two sources of mechanical settlement are expected: tunnel excavation (driven tunnel only) retaining walls Both types have been modeled and studied using 2-D software

8 Mechanical Settlement Tunnel Excavation Mechanical settlement will occur above the driven tunnel Anticipated settlement zone is narrower than for consolidation settlement Settlement varies according to depth of tunnel and characteristics of the geology Settlement effects are reduced by staging excavation and by increasing temporary support

9 Mechanical Settlement Retaining Walls Mechanical settlement will occur behind retaining walls Anticipating settlement extending to 50m away from wall Characteristic of retained open cut sections and cut and cover tunnel section Settlement effects are reduced using structural props and anchors

10 Settlement of structures Uniform Settlement NOT TO SCALE Ground level moves evenly Structure moves evenly Little strain on the structure Negligible if any damage expected

11 Settlement of structures Differential Settlement NOT TO SCALE Ground moves unevenly Structure moves unevenly/tilts Large strains within structure Greater potential for damage Gradient/shape of settlement profile of greatest importance

12 Waterview Area Anticipated settlement type Possible mechanical settlement at retaining walls Mechanical and consolidation settlement associated with cut and cover tunnel and driven tunnel

13 Owairaka Area Anticipated settlement type Mechanical settlement associated with retaining walls and consolidation settlement associated with driven tunnel and tunnel portal Mechanical and consolidation settlement associated with driven tunnel

14 Effects of Settlement An assessment and classification of existing structures sensitivity to settlement and potential effects is underway. These include: Buildings: houses, community structures and commercial premises Utilities: wastewater, gas, water supply Infrastructure: roads and rail Others: old landfill areas This type of classification is used worldwide for projects where settlement has occurred. It has previously been successfully applied to local Auckland projects including: Three Kings Quarry Vector Tunnel Victoria Park Tunnel New Lynn Rail Trench

15 Summary Settlement is common to all tunneling projects Design approach aims to reduce the effects We will use well known assessment and evaluations methods We will identify the potential for damage to structures and classify these into damage severity categories Monitoring and remediation measures such as strengthening and repairs will be part of the project NZTA will repair damage resulting from project settlement