PRELIMINARY GEOTECHNICAL DESIGN

Transcription

1 PNCC Manawatu River Pedestrian/ Cycle Bridge PRELIMINARY GEOTECHNICAL DESIGN 1 Introduction Opus has been commissioned by the Palmerston North City Council (PNCC) to prepare a Detailed Business Case (DBC) for He Ara Kotahi Manawatu River Pedestrian/Cycle Bridge, Contract 3382, within the City of Palmerston North, as part of the government s Urban Cycleway Fund (UCF) Package. This preliminary geotechnical design report explains the basis of the geotechnical design of the bridge structure, based on the known ground conditions and estimated loads. 2 Background A preliminary geotechnical appraisal report for the He Ara Kotahi Manawatu River Pedestrian/Cycle Bridge was carried out (Opus, March 2016), using the existing geotechnical information from previous investigations undertaken as part of Powerco s Manawatu River underground cable investigations, and other available data. This preliminary geotechnical design report has been completed as part of the DBC for Manawatu River Pedestrian/Cycle Bridge. The scope of this work is to undertake foundation design for the proposed piers and abutments using loads on foundation provided by the structural designers. No specific site investigations have been undertaken during this or any previous stage of work. The ground conditions have been assumed from nearby investigations undertaken for other projects. Therefore we currently have an incomplete picture of subsoil conditions at the proposed bridge site. Soil parameters used in the preliminary geotechnical design report are inferred from the assumed ground conditions. 3 Design Loads Preliminary loads at the top of the piers have been provided by the structural designers, based on the current layout of the bridge. Unfactored dead loads, live loads and earthquake loads are listed in the table below. Refer to Appendix-A for load combinations. No water pressure or flood debris load cases have been assessed. Not all load combinations have been checked at this stage July 2016 Opus International Consultants Ltd

2 PNCC Manawatu River Pedestrian/ Cycle Bridge Table 1: Load Summary Description Dead Load Live Load Earthquake Load Pier 3300 kn 1350 kn 1200 kn Abutment 1650 kn 650 kn 1200 kn 4 Foundation Design 4.1 Soil Parameters The geotechnical design parameters used for design of the foundation are shown in table below. The unit weight of soil, drained cohesion parameter and friction angle were estimated based on the SPT results from the borehole data from geotechnical factual report for Manawatu River underground cable. All elevations used in this report are derived from site survey. The approximate bridge deck level is RL m. Table 2: Soil Parameters Material SPT N Elevation of start of layer (m) Approximate Thickness (m) Unit Weight γ (kn/m 3 ) Friction Angle φ' ( 0 ) Cohesion c (kpa) Silty Sand 0-3 RL. 26.1m 4m Gravels RL. 22.1m 9m Silty Sand RL. 13.1m 9m Sand 2-9 RL. 4.1m 10m Silt 24 RL. -5.9m Scour An indicative scour depth at river piers has been provided by the hydraulic engineers based on typical values for New Zealand rivers. A design scour depth of 8m below the current base of the river has been assumed when determining the pile founding depth. This allows for both general and localised scour around the river piles. 2 July 2016 Opus International Consultants Ltd

3 PNCC Manawatu River Pedestrian/ Cycle Bridge 4.3 Pier Foundation General The depth and diameter of the pile governed by the critical case of the horizontal seismic loading. The foundation design was completed based on the procedure listed in B1/VM4. Refer to the detailed calculation in Appendix A. Horizontal forces due to water flow and debris are not considered in the design of pile. We have therefore not checked the lateral capacity of the piles allowing for general and localised scour during a flood event, when water pressures and debris loading may be significant Pile Depth A single cylinder pile foundation has been assumed for the pier foundations. Based on the vertical and horizontal loads expected, the depth of scour of the river bed, and our current knowledge of the ground conditions, a suitable founding layer has been assumed present from about RL -5.9 m. However this is based on the final SPT reading from BH3 of the Powerco underground It is proposed that 33m long single bored reinforced concrete pile to be constructed as the foundations for the pier. The proposed pile foundation embeds ~6m (RL m) into the stiff silt layers underlying 10 m thick layer of loose sand so that the risk of settlement is mitigated and allowing for scour depth of approximately 8m. The pile depth for the land pier is calculated based on bridge manual recommendation, not relying on rock to protect the piers. Possibly that river channel may move; therefore need to found this pile at RL m Pile Diameter The maximum moment in the pile shaft based on a subgrade reaction method has been calculated to be 19,600 kn.m. A single 2.0 m diameter bored pile with 1% reinforcement has sufficient capacity to resist the moment and provide sufficient vertical capacity. 4.4 Abutment Foundation General A shallow foundation option was rejected due to the foundation rotation risk that could occur due if liquefaction of the underlying soils occurred. The depth and diameter of the pile at the abutment are governed by the intensity of the horizontal seismic loading. The foundation design was completed based on the procedure listed in B1/VM4. Refer to the detailed calculation in Appendix A Pile Depth A single 15 m long bored reinforced concrete pile has been assessed as required for the foundation of the abutment. The proposed pile is assumed to found at about RL m in medium dense silty sand. 3 July 2016 Opus International Consultants Ltd

4 PNCC Manawatu River Pedestrian/ Cycle Bridge Pile Diameter The maximum moment in the pile shaft by a subgrade reaction method is 9,522 knm. This moment has been used to determine the pile diameter. A single 1.5m diameter bored pile with 1.5% reinforcement has sufficient capacity to found the abutment. The passive soil load from liquefaction-induced lateral spread has not been allowed for in the pile design Ground Improvement The liquefaction potential near the Manawatu River was studied for the report Assessment of liquefaction and related ground failure hazards in Palmerston North, New Zealand (GNS Science Consultancy Report 2011/108, July 2011). The liquefaction ground damage potential map develop for the report shows the site has a liquefaction ground damage potential of moderate to very high. The effect of lateral spreading on the abutment foundations may be significant, as the approach embankments are likely to form a depth of non-liquefiable material, on top of loose alluvial silts that are susceptible to liquefaction-induced lateral spread. The passive soil forces imposed on the wide (8 m+) abutment would need to be resisted by the bridge structural system. To reduce these passive soil loads, ground improvement beneath the abutment area could be undertaken to reduce the severity of lateral spread. There are a number of different ground improvement types and methods that could be adopted. Stone columns, installed by a displacement method, has been adopted at this stage for costing purposes, as this is relatively economic and has been proven to be effective treatment for liquefaction in Christchurch. Based on the information available at this stage, we have assumed approximately 49 No. 600 mm diameter, 6 m long columns at 2 m spacing, arranged in a triangular pattern at each abutment. The proposed layout and general arrangement is shown in the concept design drawing. At the first pier on the city side, there is not the embankment above the liquefiable alluvial materials, and the passive soil pressure is likely to be significantly reduced from the abutment case. Should lateral spread occur on the riverbank the material is more likely to flow around the pier. We have therefore assumed the pile and pier column would have sufficient structural capacity to resist the load from lateral spreading at this location Fill Embankment Fill embankment slopes should typically be constructed with flat slopes to match the existing stopbank slopes. This will allow maintenance of vegetation on the side slopes. Prepared by: Reviewed by: Ravi Sundar Geotechnical Engineer Mark Frampton Principal Geotechnical Engineer 4 July 2016 Opus International Consultants Ltd

5 PNCC Manawatu River Pedestrian/ Cycle Bridge APPENDIX- A 5 July 2016 Opus International Consultants Ltd

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