preliminary geotechnical design - pncc.govt.nz · pdf filepncc manawatu river pedestrian/...

PNCC Manawatu River Pedestrian/ Cycle Bridge

1 | 07 July 2016 Opus International Consultants Ltd

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.


2 | July 2016 Opus International Consultants Ltd

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. 32.08m.

Table 2: Soil Parameters

Material SPT N

Elevation of start of layer

(m)

Approximate Thickness (m)

Unit

Weight γγγγ (kN/m3)

Friction

Angle φφφφ' (0)

Cohesion c’ (kPa)

Silty Sand 0-3 RL. 26.1m 4m 17 25 5

Gravels 7-50+ RL. 22.1m 9m 19 34 -

Silty Sand 31-50+ RL. 13.1m 9m 19 36 3

Sand 2-9 RL. 4.1m 10m 18 28 -

Silt 24 RL. -5.9m - 18 35 5

4.2 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.



4.3 Pier Foundation

4.3.1 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.

4.3.2 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. – 12.12m) 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. -12.12 m.

4.3.3 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

4.4.1 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.

4.4.2 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. 15.0 m in medium dense silty

sand.



4.4.3 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.

4.4.4 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.

4.4.5 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 Mark Frampton Geotechnical Engineer Principal Geotechnical Engineer



APPENDIX- A

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