Preliminary Geotechnical Considerations -...

SECTION 6 Preliminary Geotechnical Considerations

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6 Preliminary Geotechnical Considerations

6.1 Introduction

This section provides the key geotechnical considerations for the pipeline and associated structures based on the geotechnical study/investigations to date.

6.2 Slope Stability

Slope instability either during construction or the design life of the pipeline and its associated structures is one of the main geotechnical hazards. As part of the geotechnical investigations/study, the high-risk areas have been visited and an assessment made to identify preferred location for the pipeline/structure.

6.2.1 Slope Stability Analyses to Determine Setback Distances

To determine the setback distance for a pipeline/structure from steep slopes / relic slips, generic slope stability analyses have been undertaken using Slope/W software. Numerous combinations of the following scenarios were investigated.

• Embankment heights of 20m, 10m, and 5m.

• Slope angles of 2H:1V, 1H: 1V, and 1H:2V.

• A completely saturated slope with the water table at the surface of the slope; and a slope having a water table 4m deep (at the top of the Hamilton Ash layer)

• Static loading and a horizontal seismic load of 0.4g (1 in 2500yr event).

A generalised soil profile based on available information from the Tauranga area was used for the analyses. The profile consisted of a 1m layer of fill, overlying 3m of Rotoehu Ash, which overlies 5m of Hamilton and Older Ashes. Beneath the Older ashes is weathered Ignimbrite. The following table provides the geotechnical properties applied to the various units in the Tauranga region.

Table 6-1: Geotechnical Properties of Soil Units used for Generic Slope Stability Analyses for Setback Distance

Soil Unit Unit Weight (γ, kN/m3)

Effective Cohesion (c’, kPa)

Friction Angle (ϕ’, degrees)

Fill 17 4 28

Younger Ashes 16 5 35

Rotoehu Ash 16 5 35

Hamilton Ash 17 10 25

Weathered Ignimbrite

17 5 32



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The following is the summary of the analyses results for the worst-case scenario (fully saturated slope and seismic loading). It is clear from the table that the setback distance required is very long and may not be available in a few places. However, it should be noted that these distances are the absolute worst-case scenario and further site-specific assessment should be made during detailed design to quantify the risk to determine whether mitigative geotechnical works are required for a section of pipeline traversing the relic slips/steep slopes.

Table 6-2: Summary of Results for Setback Distances

Slope Angle Embankment Height (m) Loading Condition Setback Distance (m)

2H:1V (26.5o) 5 Static 8

2H:1V (26.5o) 5 Seismic 46

2H:1V (26.5o) 10 Static 26

2H:1V (26.5o) 10 Seismic 56

2H:1V (26.5o) 20 Static 51

2H:1V (26.5o) 20 Seismic 51

1H:1V (45o) 5 Static 7

1H:1V (45o) 5 Seismic 45

1H:1V (45o) 10 Static 23


1H:1V (45o) 20 Static 50


1H:2V (63o) 5 Static 7


1H:2V (63o) 10 Static 23


1H:2V (63o) 20 Static 50


6.2.2 Recommended Set-back Distance from Slopes

As part of the route selection/optimisation of the location of pipeline/structures, the following guidelines should be used for the ‘set back’ distance from an existing slope (natural/man-made):

• Based on historical approach (originally recommended by T&T in 1980, later endorsed by Richards et al, 2001) used in the Tauranga Region for residential development (i.e., for a 50 year design life) any pipe/structure should be located at a minimum setback distance of 2 x ht (in full, m) + 6 (m) from the absolute base of the slope. For example, for a 5m high slope standing at 1H: 1V(45o), set back from the base of the slope should be 2 x 5 + 6 = 16m.



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• Based on our generic slope stability analyses (for a 100yr design life and seismic loading conditions), any pipe/structure should be located the following minimum of setback distance from the crest of the slope:

– For a 5m high slope, 8m for static and 45m under seismic loadings.

– For a 10m high slope, 26m for static and 56m under seismic loadings.

– For a 20m high slope, 50m for both static and seismic loadings.

It should however be noted that these setback distances should be treated as preliminary, subject to detailed site-specific investigation and analysis during detailed design.

Any slope instability caused during a large seismic event (with 0.4g horizontal seismic load), can later be repaired/remedied following the event. It is not considered cost-effective to design mitigative works for the slopes for seismic events, during the construction of the project.

6.3 Groundwater

The groundwater within the Tauranga regional geomorphology plays a significant role in shaping/re-shaping the landscape by initiating landslides. Historically, many landslides in Tauranga occurred during major storm events. Several slope instabilities (eg, recent slip along the Hairini Rd on-ramp to SH2) are associated with surface water seeping/ingressing into the slopes from rooftops and overland flows through existing tension cracks near steep slopes. Therefore, groundwater considerations are very important during detailed design stage following final route selection.

During construction, groundwater potentially poses problems should the excavation extend through sandy layers, with shallow groundwater table in the area. This may lead to trench instability during excavation, requiring either temporary shoring or benching, sometimes resulting in over excavations and associated claims. Further assessment should thus be made during detailed design.

6.4 Settlement/Floatation

The pipeline corridors for the route options being considered traverses through sections of wetlands, stream crossings, low-lying land infilled with recent sediments and harbour crossings. Where this occurs, care should be taken to ensure any likely settlements are estimated and where necessary, piled foundations (for structures) or pipe bridges (for pipeline stream/harbour crossings) should be used.

At low-lying or other locations where there is a shallow depth to groundwater, any buried structures (eg underground tanks) or buried pipeline will be subject to hydrostatic pressures. The structures and pipeline at these locations should thus be designed to withstand these uplift pressures. It should also be noted that during a seismic (both 1 in 500yr and 1 in 2500 yr) event, additional dynamic uplift pressures may occur.



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Based on the geotechnical investigations/study to date, the locations where settlements/floatation is likely to pose geotechnical risks have been identified and listed in the tables of geotechnical summary for pipeline and structures (Section 6.7).

6.5 Earthquake Hazards

There are no known active faults within the pipeline route corridors. The closest active faults are some 35km distant (Kerepihi Fault to the west, Matata Fault to the east and offshore faulting). Thus the seismic hazard is governed by ground shaking from crustal faults some distance from Tauranga and from deeper subduction event earthquakes.

The Tauranga region is located within Seismic Zone 2 (GNS 2000). A Western Bay of Plenty Lifelines Study (Brabhaharan et al, 2002) provides a detailed area specific assessment of earthquake hazard for the Tauranga region.

The recognised earthquake hazards are: -

• Ground shaking

• Liquefaction

• Ground deformation

The pipeline and associated structures must remain functional following an earthquake and in accordance with the Australia/New Zealand Standard for Structural Design Actions (NZS1170.5:2004) these structures are classed as importance Level 4. Based on this importance level the structures should be designed for a return period of 2500 years i.e. Ultimate Limit State. A return period of 25 years is required for a Serviceability Limit State (SLS 1) requiring avoidance of damage that prevents a structure from being used as originally intended without repair. For Importance Level 4 there is an additional more onerous Serviceability limit State (SLS2) requiring critical facilities to remain operational in 500 year return period motions.

Using AS/NZS1170.5, the expected peak ground acceleration for the 1:2500 year event is 0.36 for strong rock to 0.4g for soft soils.

The site-specific microzoning study for Tauranga indicates peak ground accelerations of 0.3-0.4g for rock sites to 0.4-0.5g for soft soil sites. We recommend that the site-specific assessment peak ground acceleration values be used for this project.

Brabhaharan et al (2002) suggests that the pipeline corridor area is likely to liquefy both during a 1 in 500yr and 1 in 2500yr events.

Liquefaction can lead to the following effects, which can cause damage:

• Ground damage – subsidence, lateral spreading, flow failure, slope failure



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• Buoyancy of buried services, tanks and chambers (manholes)

• Foundation failure due to reduction / loss of bearing capacity

• Settlement of structures on liquefied materials

Of particular note to buried structures is the increased risk of buoyancy, foundation failures, loss of bearing pressures and settlements. Areas of “localised liquefaction” can expect ground subsidence varying from none to small i.e. < 100 mm. The areas of “extensive liquefaction” will generate ground subsidence > 300 mm and possibly over 1000 mm in places.

Lateral spreading can occur along river / stream banks and coastal areas, where the liquefied ground could displace towards the free surface. Any embankments or bridge abutments built on liquefiable ground are also likely to be affected.

The following is a summary of liquefaction potential assessment based on geology:

• Estuarine Deposits: Even when the liquefaction of very soft fine-grained soils does not occur, these soft soils can undergo severe loss of strength during ground shaking and give rise to ground strains and lateral spreading.

• Volcanic Ash Deposits: The ash deposits are generally considered resistant to liquefaction due to fine-grained plastic nature and the presence of some welding effects. However, some localised liquefaction is likely to occur during strong ground shaking.

• Coastal Sand Deposits: Coastal sands are generally medium dense to dense, only localised liquefaction is likely to occur

• Ignimbrite Areas: Liquefaction of the Ignimbrites soils is unlikely.

Where the pipeline traverses through non-liquefiable to liquefiable soils (eg through Ignimbrite areas to estuarine deposits, differential settlements of the pipe are likely and may even result in pipe bursting during a strong ground shake.

The earthquakes are also known to trigger landslides. Often, relic slips can be re-mobilised following a strong earthquake and /or coupled with a significant rainfall event.

6.6 Constructability

6.6.1 Excavatability

URS experience at sites around the Tauranga region indicates that the volcanic air fall deposits and their weathering products, weathered ignimbrite materials, or alluvial soils derived from the volcanic deposits are expected to be to excavated with ease.

Very stiff soils have “moderate difficulty” to excavate and can take longer excavation time. In addition, welded sands and ash deposits are relatively the most difficult to excavate.



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The ignimbrite deposits often form a hard weathered crust (0.5m maximum thickness). It is expected that both the case hardened crust and the underlying materials can be excavated with a toothed bucket.

6.6.2 Pipeline Trench Stability

Based on observations made during other work in the Tauranga region, we would make the following comments on pipeline trench stability.

• Any pipeline trench in the volcanic ash sequence or ignimbrite materials, where no ground water is encountered, is expected to be stable.

• The stability of the trenches in the volcanic soils may be significantly compromised if the soils are exposed to excessive rain, or if the trench develops a standing water level

• Any pipeline trench excavated to or below the ground water level is expected to be unstable. Support and dewatering will be required.

• Soft alluvial soils are expected to be unstable.

6.6.3 Seepage and groundwater effects

Groundwater and seepages have a considerable detrimental impact on the behaviour of the soils in the Tauranga region. Perched higher ground water levels and wetter soils should be anticipated if construction takes place during or shortly after wet winter months or periods of high rainfall.

Construction difficulties can be expected along the sections of the route with high ground water and alluvial soils, specifically:

• Unstable trenches (as discussed above);

• Ground water levels above the pipeline;

• Placement of sensitive soil backfills will require particular specifications, which apply restrictions to the water content of the materials to be used as backfill.

• The groundwater encountered at locations within or close to the estuary will be tidal. This will only have an impact if the pipeline placement is by open trenching. However, any structures should be designed for the highest potential water level.

• Dewatering along the portions of the pipeline and for pumping station structures where the water table is high is recommended. Well pointing may be the most appropriate technique.



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6.6.4 Reuse of excavated soils

Bedding Material

The bedding material will be required to conform to Australian/New Zealand AS/NZS 2566.1:198 and AS/NZ 2566.2:2002.

None of the materials to be excavated along the pipe trench route are suitable as bedding material. It will be necessary to import bedding material.

Backfill Material

The volcanic soils are sensitive soils. As a result, they can lose strength when reworked.

However, under certain conditions they are suitable for reuse as trench backfill. URS experience is such that backfill will achieve adequate strength and void ratios if the natural moisture content is preserved. A dramatic strength loss is observed if the soil becomes wet during remoulding.

The following backfilling requirements are recommended:

• The open trench should be free of water prior to backfilling;

• The stockpile of excavated soils should be managed to preclude increase in moisture content;

• Alluvium and slope debris is not suitable as backfill.

6.7 Preliminary Geotechnical Considerations for the Pipeline & Structures

Based on the preliminary geotechnical study/investigations carried out to date, tables of geotechnical summary were prepared with the key geotechnical issues for the pipeline and associated structures.



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Table 6-3: Preliminary Geotechnical Appraisal for the Proposed Structures

Earthquake Hazards Route Structure Geology Groundwater Flooding Slope Stability

Settlement Floatation

Liquefaction Uplift Pressures

Slope Instability

Comments / Recommendations

Central Route (A)

Maleme Street Pump Station

− Tauranga Group alluvium comprising soft to firm organic rich silts & clays.

− Available sub-soil information indicates at least 3m of soft organic rich silsts

− Groundwater level @ 2m below ground level (close to stream water level).

− Stream level & groundwater are tidal influenced

− Flooding hazard due to stream. 100 year flood levels should be assessed

− Critical structures to be designed to withstand 100yr flooding potential

− Minor instability along the stream bank was noted during the site visit.

− Any structure located close to the stream bank should be adequately designed

− Settlement of the structures should be assessed during detailed design.

− Piled foundations may be required for pump and any underground storage tanks.

− Due to high groundwater table, proximity of a stream and flood hazard, structures would need to be designed against any floatation / uplift pressures

− Localised liquefaction likely with minor subsidence (up to 100mm)

− Due to high groundwater levels some additional uplift pressures might occur.

− Provided any structures are set-back enough distance from stream banks, risk is minimal

− Feasible to construct the pump station and associated structures provided adequately engineered.

− Settlement, liquefaction and flooding hazards to be assessed

− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and structures adequately engineered.



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Slope Instability


Ila Pump Station

− Tauranga Group alluvium comprising soft to firm organic rich silts & clays

− No data available. However, the topography and the proximity of the stream indicate a shallow depth to groundwater table.

− Groundwater table and stream water levels could be tidal influenced

− Given the topography of the area (Ila Park is low lying), flood hazard may be present.

− No significant issues, provided the structures are set-back enough distance from the stream banks


− piled foundations may be required for pump and any underground storage tanks.


− Localised to widespread liquefaction is expected (up to 300mm subsidence).

− Investigation and assessment should be made to quantify the risk during detailed design stage, so as to locate the pump station in a low risk area, if possible.

− Due to high groundwater levels some additional uplift pressures might occur during an earthquake




− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and structures adequately engineered. Central

Route (A)

Receiving Chamber 1 – Stephens Place

− Pumiceous gravels, sands and silts.

− No data available.

− SH2 cut embankments appeared dry during Nov/Dec visits.

− No issues

− Existing SH2 road cuttings are standing at 2V:1H batter showing no obvious signs of instability.

− Any structures, including pipe should be setback at least 20m from the slope.

− No significant issues expected

− No issues



− No issues, provided the structure is located well away from the SH2 cut.

− Feasible to construct the receiving chamber.

− Slope stability hazard to be assessed




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Slope Instability


Siphon Structure 1 – Maungatapu Road


− No data available. − No issues − No issues


− No issues

− Moderate liquefaction expected with some localised subsidence (100 to 300mm)



− Feasible to construct the siphon structure.

− Liquefaction hazard to be assessed


Siphon Structure 2 – Puwhairiki Road


− No data available. − No issues

− Proximity of the headscarp feature of the ancient landslide to be noted


− No issues





− Slope stability and liquefaction hazards to be assessed


Central Route (A)

Siphon Structure 3 – Matapihi



− No significant issues.

− Proximity of the existing retaining wall in private properties to be noted


− No issues





− Slope stability and liquefaction hazards to be assessed




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Slope Instability


Siphon Structure 4 – East of Phoenix Heights (between Railway line & SH2)

− Published geology indicated Pumiceous gravels, sands and silts.

− The topography of the location indicates soft estuarine sediments

− No data available. Given the elevation (RL2m) of the ground, groundwater table would be tidal affected.

− Due to the area being low-lying (RL2m), flood risk should be assessed.

− Minor instability noted during site visit.

− Proximity of Rail embankment and SH2 corridor warrants careful engineering.

− Settlement risk should be quantified and the structure should be adequately engineered

− If high groundwater table is present, flotation of structures might occur

− Significant lateral spreading and moderate to extensive subsidence (>300mm)


− Slope instability hazards should be assessed during an earthquake

− Feasible to construct the siphon structure, provided adequately engineered.

− Slope stability, settlement, flooding and liquefaction hazards to be assessed

− Physical constraints (rail and SH2 road corridor) to be noted.

− Structure to be located well away from any steep slopes and rail embankment.


Central Route (A)

Siphon Structure 5 – Te Maunga WWTP

− Fixed foredune sands

− Shallow depth to groundwater table can be expected, effected by tides

− Any flooding is likely to result from a possible breach of treatment pond embankments

− No significant issues



− Widespread liquefaction is expected

− Uplift pressures might occur during an earthquake

− Widespread lateral spreading may initiate some instability


− Settlement and liquefaction hazards to be assessed

− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and structures adequately engineered



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Slope Instability




















Eastern Route (B)

Receiving Chamber 1 – Poike Road

− Pumiceous gravel, sand, silts and peat

− Exposed soils within the toe of the existing landslides in Poike Rd church land are stiff, silts with some sands

− Springs noted within the centre of toe of the failed slope

− Soils 3m below road level are saturated

− No data available at the road level

− No issues

− Proximity of the old slips along the northern edge of the road to be noted.





− Proximity of the existing landslides to be noted.

− Slope instability could be re-initiated during an earthquake event

− Feasible to construct the receiving chamber provided adequately engineered.

− Slope stability hazards to be assessed




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Slope Instability


Ila Pump Station




− Given the topography of the area (Ila park is low lying), flood hazard may be present.



− piled foundations may be required for pump and any underground storage tanks.



− Investigation and assessment should be made to quantify the risk during detailed design stage, so as to locate the pump station in a low risk area, if possible.






Eastern Route (B)

Receiving Chamber 2 – Ohauiti Road

− Pumiceous gravels, silts and sands

− No data available − No issues

− Proximity of the old slip to the west of the road to be noted.

− Preferred location is on the eastern side of the road





− Proximity of the existing landslide to be noted.

− Slope instability could be re-initiated during an earthquake event






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Slope Instability


Siphon Structure 1 – Welcome Bay Road


− Part of the road is engineered fill embankment

− Exposure of natural soils along the Kaitemako stream bridge abutment indicates very stiff, friable silty sands to a depth of 3m below road level.

− Springs noted within the toe of the existing landslide on Maori burial ground

− No data available at road level


− Proximity to existing active landslide north of the road to be noted

− Preferred location is along the southern edge of the road as headscarp is only 2m from the kerb line



− Minor liquefaction zone, bordering with widespread zone (through the Kaitemako stream)

− Subsidence ranging from 100mm to > 300mm

− Dependant upon the groundwater levels, some uplift pressures might occur during an earthquake

− Proximity of the landslide to be noted

− Slope instability might be re-initiated during an earthquake

− Feasible to construct the siphon structure provided adequately engineered.

− Slope stability and, liquefaction hazards to be assessed


Kaitemako Road Pump Station & Siphon Structure


− No data available


− Proximity of the existing cut embankment for the sports field to be noted

− Retaining walls may be required



− Moderate liquefaction with subsidence up to 300mm

− Additional uplift pressures might occur during an earthquake event

− Proximity of cut slopes to be noted. Engineering design may be required to stabilise during an earthquake event




Eastern Route (B)

Waitaha Road Pump Station


− No data available.

− Given the alluvial group soils, shallow depth to groundwater can be expected

− Flooding potential to be assessed

− Proximity of the existing slope along the eastern side of the Waitaha Road to be noted

− Preferred location for structures is at least 20m away from the top of the slope

− Settlement issues expected given alluvial soils

− Some floatation problems could be expected

− Widespread liquefaction resulting in significant lateral spreading and large subsidence

− Additional uplift pressures could result during an earthquake event

− Proximity of the slope east of Waitaha road to be noted


− Settlement, flooding and, liquefaction hazards to be assessed




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Slope Instability


Receiving Chamber 3 – Welcome Bay Road

− Waiteariki Ignimbrite

− Exposure along road cuts indicate up to 4m ash cover over very stiff pumiceous silt and sand (Ignimbrite)

− Springs noted on the filled gully east of the structure location.

− No groundwater observed along road cuttings west of structure location


− Proximity of the filled gully to the east to be noted.

− MSE wall may exist on the northern face of the filled slope





− Proximity of the filled embankment to be noted





− Waiteariki Ignimbrite (very stiff pumiceous sandy silts)

− No data available


− Proximity of the natural slope west of Welcome Bay road to be noted





− Proximity of the filled embankment to be noted





− Terrace deposits comprising ash (silts and sands)

− Geotechnical information available indicates ashes (very stiff silts with varying contents of sands)

− Groundwater deeper than at least 3m


− Proximity of the head gully northeast of the structure location to be noted



− Moderate liquefaction with subsidence up to 300mm

− Additional uplift pressures could result during an earthquake event







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Slope Instability


Eastern Route (B)

Siphon Structure 4 - Te Maunga WWTP













Western Route (E)





















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Slope Instability


Receiving Chamber 1 – Oropi Road

− Unwelded Ignimbrite

− Soil exposure within the walkway through an existing slip indicates stiff to very stiff sandy silts

− Springs noted within the toe of the old landslide

− No data available for the road levels

− No issues

− Oropi Rd is built with sidling cut to partial fill operation alongside the headscarp of an existing landslide to the west of the road.

− Currently, there are no obvious signs of instability

− Preferable location for the structure is at least 20m away from this headscarp (eg, in front of 11 Oropi rd)





− The existing old landslide may be re-initiated during an earthquake event




Western Route (E)

Siphon Structure 1 – Fraser Street / Harrier Street Intersection

− Unwelded Ignimbrite − No data available − No issues

− Relic landslide along Fraser St, bordering Esk St.

− Fully developed, currently inactive







− Minor slope stability hazards to be assessed




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Slope Instability


Siphon Structure 2 – Fraser Street / Eighteenth Avenue Intersection

− Unwelded Ignimbrite − No data available − No issues

− Relic landslide south of the structure location, along the Fraser St

− Fully developed, currently inactive







− Minor slope stability hazards to be assessed


Memorial Park Pump Station

− Recent sediments of Tauranga Group alluvium comprising organic rich silts and clays

− Likely shallow groundwater table, effected by tides

− Flooding risk to be assessed

− Proposed pump station is located next to a 13m high cliff, standing at 1H:1V.

− No obvious signs of instability noted. Well-established trees protecting the slope.

− Settlement hazard to be assessed in detail.

− Piled foundations may be required for some structures

− Floatation of structures under hydro-static conditions could occur







Western Route (E)

Receiving Chamber 2 – Matapihi Road






− No issues





− Liquefaction hazards to be assessed




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Slope Instability


Siphon Structure 3 - East of Phoenix Heights (between Railway line & SH2)













− Settlement, slope stability, flooding and, liquefaction hazards to be assessed




Western Route (E)
















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Slope Instability




− Available sub-soil information indicates at least 3m of soft organic rich silts
















Receiving Chamber 1 – Marshall Avenue

− 4m of Younger Ash (firm to stiff clayey silts)

− Overlying 6m of Older Ash (dense sands)

− Overlying Waiteariki Ignimbrite (loose to medium dense silty sands)

− No data available − No issues

− Proximity of the road cutting should be noted



− Moderate liquefaction with up to 300mm subsidence

− Dynamic uplift pressures may occur during an earthquake

− Adjacent road cutting stability may be affected




Road Route (K)

Seventeenth Avenue Pump Station

− Recent deposits of Tauranga Group alluvium, comprising soft to firm organic rich silts and clays

− Pump station located close to Koporererua Stream.

− Very shallow groundwater, tidal affected

− Flood risk due to Koporererua Stream.

− Pump station located within the flood risk zone

− No significant issues, provided structures located well away from stream banks

− Significant settlement issues expected given soft soils.

− Piled foundations may be required.

− Given the shallow depth to groundwater all underground structures should be designed for hydro-static pressures

− Widespread liquefaction expected with extensive lateral spreading and large (>300mm subsidence)

− Dynamic uplift pressures will occur during an earthquake event







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Slope Instability


Receiving Chamber 2 – Matapihi Road






− No issues





− Liquefaction hazards to be assessed


Siphon Structure 1 – East of Phoenix Heights (between Railway line & SH2)













− Settlement, slope stability, flooding and, liquefaction hazards to be assessed




Road Route (K)











− Feasible to construct the siphon structure

− Settlement, flooding and, liquefaction hazards to be assessed.




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Table 6-4: Preliminary Geotechnical Appraisal Along Pipeline Routes

Earthquake Hazards Route Approximate Chainage

(m)

Geology Groundwater Flooding Slope Stability



Slope Instability


0-100 Maleme Street (includes the Maleme Pump station site)






− Pipe to be designed to withstand 100yr flooding potential


− Any pipe located close to the stream bank should be adequately designed

− Not expected to be an issue.

− Differential settlements expected at stream bank crossings and changes in geological units.

− Due to high groundwater table, proximity of a stream and flood hazard, pipeline should be designed against any floatation / uplift pressures

− Widespread liquefaction is likely.

− Ground damage is expected to be moderate (i.e. subsidence > 300mm) to major (significant lateral spreading and subsidence)

− Due to high groundwater levels some additional uplift pressures might occur. This will only impact the pipeline if the pipe is below the ground water table.

− Provided any pipelines are set-back enough distance from stream banks, risk is minimal

− Feasible to construct the pipeline.


− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and pipeline is adequately engineered.

Central Route (A)

(all chainages from figure 1A of Hydraulic Design Report Rev D)

100-300

Maleme Street

Volcanic Ash sequence of unknown thickness overlying Te Ranga unwelded Ignimbrite

− No local soils information is available for this area

− Not known

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Te Ranga unwelded Ignimbrite.

− Localised high level perched water tables may exist

− Not expected to be an issue

Relic head scarp for old landslide has been identified.

Pipeline should be located to avoid this feature.



− Liquefaction is unlikely except for localised areas in a large earthquake.

− Ground damage is expected to be localised minor subsidence.

− Localised effects if any due to perched water tables.

− Relic slips may be reactivated in a large earthquake. pipeline Set back distances from known head scarps should be considered



− Pipeline should be located away from known slope instability. Use set back distances.

− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and pipeline is adequately engineered



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(m)




Slope Instability


300- 1000

Oropi & Poike Rd



− Not known

− Expect groundwater level @ 2m below ground level (close to stream water level).

− Stream level & groundwater may be tidal influenced




− Settlement of the pipe is not expected to be an issue.









− Detailed geotechnical investigations required prior to detailed design to ensure all hazards are quantified and pipeline is adequately engineered. Central

Route (A)


1000-2400

SH29

− Volcanic Ash sequence of silts and clays including Rotoehu Ash (Pumice Sands) and Hamilton Ash.

− Overlying Matua Subgroup of fluvial origin. Pumiceous sands, silts and gravels,


− Not known.

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Matua Subgroup.


− Not expected

− No relic landslide head scarps identified along this section of the route.

− However, potential for slope failure exists on steep slopes in very wet conditions.

− Risk to be minimised by locating pipeline on other side of the road.





− Localised effects only due to perched water tables.

− Existing slope failures may be reactivated by large earthquake event.






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(m)




Slope Instability


2400-2900

SH29 (includes the Ila pump station)




− Given the topography of the area (Ila park is low lying), flood hazard may be present.


− Settlement potential to be assessed

− Differential settlements expected at stream bank crossings and changes in geological units

− Due to high groundwater table, proximity of a stream and flood hazard, pipeline may need to be designed against any floatation / uplift pressures



− Provided the pipeline is set-back enough distance from stream banks, risk is minimal




2900-6100

Sh29, Stephens Place, Hairini St, Maungatapu Rd


− Overlying Matua Subgroup, fluvial origin pumiceous sands, silts and gravels, medium dense.

− Not known.



− Not expected

− Relic landslide head scarps identified on along pipeline route.

− Risk to be minimised by locating pipeline on other side of the road



− Minor to widespread liquefaction is expected.

− Ground damage is expected to be minor to moderate subsidence i.e. up to 300mm.






Central Route (A)


6100-6400

SH2/29

− Engineered Fill (causeway fill comprising of rock boulders and possible ash fill) overlying soft estuarine sediments

− Tidal waves influenced

− Marine environment. If built on causeway/bridge abutments, adequate erosion protection required.

− Erosion controlled.

− Likely settlement hazards to be assessed during detailed design

− Floatation risks to be assessed.

− Widespread liquefaction is expected in exposed sediments.

− Ground damage is expected to be subsidence up to 300mm

− Uplift pressures due to earthquake will only be a problem if uplift pressures exceed overburden pressures. i.e. pipeline is too shallow.

− Not an issue


− Settlement, flooding, erosion and, liquefaction hazards to be assessed

− Pipeline design and depth needs to mitigate potential hazards.




6-25


(m)




Slope Instability


6400-6900

SH29/2

− Soft Estuarine sediments

− Available data from the Maungatapu bridge foundations indicates 8-14m of silt sands, overlying pumiceous sand and silts to a depth of 22m (only one data point). Below this are further estuarine muds. Hard ground was not encountered.

− Paleo-channels exist within the sediments

− Sea Level.

− Shallow portions of the estuary near the coast are exposed at low tide.

− Flooding may result in higher water levels in the estuary. Not expected to impact the buried pipeline.


− Settlement potential to be assessed

− However, differential settlements within the paleo-channel and at the coast should be examined at detailed design stage.

− May be a problem if pipeline is shallow


− Ground damage is expected to be subsidence up to 300mm


− Not an issue


− Settlement, flooding, erosion and, liquefaction hazards to be assessed



6900-10,500

Matapihi Rd, Paper Rd,SH2



− Not known.



− Not expected

− Large relic head scarp identified at approx chainage 7000-7300.

− Pipeline should be located using an appropriate set back distance fro the head scarp.



− Moderate liquefaction is expected.

− Ground damage is expected to be limited with subsidence of 100-300mm.




− Slope stability, flooding and, liquefaction hazards to be assessed


Central Route (A)


10,500-12,600 TeMaunga wetlands




− Typically flat low-lying land in this area. Not expected to be an issue.

− Settlement of the pipe should be assessed during detailed design.



− Widespread liquefaction is expected.

− Ground damage is expected to be moderate to major with significant lateral spreading and subsidence.

− Due to high groundwater levels some additional uplift pressures might occur







6-26


(m)




Slope Instability





















Eastern Route (B)


100-300

Maleme Street



− Not known


















6-27


(m)




Slope Instability


300- 1000

Oropi & Poike Rd



− Not known
















Eastern Route (B)


1000-4200

Poike Rd, Ohauiti Rd, and Welcome Bay Rd



− Not known.


− Springs are observed at the Church property on Poike Road about 8-10m below road level.


− Not expected

− Two active landslide headscarps identified at Poike Road.

− Essential that the pipeline be placed on other side of the road from the headscarps.

− Other relic head scarps identified along the route, e.g., Maori burial ground

− Risk minimisation by avoidance.






− Existing slope instability may be reactivated in the event of a large earthquake.






6-28


(m)




Slope Instability


4200-4300

Welcome Bay Rd-Kaitemako Stream crossing



− Not known






−





− Ground damage is expected to be major to extensive (significant to extensive lateral spreading and subsidence)






4300-5100

Kaitemako Rd, Waipuna Park



− Not known.



− Not expected


− However, potential for slope failure exists on steep slopes in very wet conditions.






− Slips may be reactivated or formed in a large earthquake.

− Set back distances from known head scarps should be considered




Eastern Route (B)


5100-5300

Welcome Bay Rd - Otumanaga Stream crossing



− Not known










− Ground damage is expected to be major (significant lateral spreading and subsidence)








6-29


(m)




Slope Instability


5300-5900

James Cook Dr



− Not known.



− Not expected


− However, potential for slope failure exists on steep slopes in very wet conditions



− Localised liquefaction is expected.

− Ground damage is expected to be localised to minor with subsidence of less than 100mm.







5900-6300

Waitaha Rd & Welcome Bay Rd



− Not known










− Ground damage is expected to be moderate (subsidence greater than 300mm)




− Settlement, slope stability and, liquefaction hazards to be assessed


Eastern Route (B)


6300-6500

Welcome Bay Rd



− Not known.



− Not expected

− Ancient relic landslides exist on the steeper slopes in the area.


− Minimise risk by avoidance.




− Ground damage is expected to be moderate with subsidence of 100-300mm.









6-30


(m)




Slope Instability


6500-7000

Welcome Bay Rd


− Overlying Waiteariki Ignimbrite

− Not known.



− Not expected



− Minimise risk by avoidance




− Ground damage is expected to be localised to minor with subsidence of 100mm.







7000-7100 Welcome Bay Rd-Waiorako Stream crossing



− Not known
















Eastern Route (B)


7100-7600

Welcome Bay Rd



− Not known.

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Waiteariki Ignimbrite.


− Not expected
















6-31


(m)




Slope Instability


7600-7700

Welcome Bay Rd – stream crossing



− Not known
















7700-8300

Welcome Bay Rd


− Overlying Papamoa Ignimbrite

− Not known.

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Papamoa Ignimbrite


− Not expected














Eastern Route (B)


8300-9500

Welcome Bay Rd



− Not known.

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Waiteariki Ignimbrite


− Not expected
















6-32


(m)




Slope Instability


9500-9600

Welcome Bay Rd – stream crossing



− Not known
















9600-10,700

Welcome Bay Rd & Asher Rd

− Volcanic Ash sequence of silts and clays including Rotoehu Ash (Pumice Sands)

− Overlying fluvial Terrace Deposits post dating the Hamilton Ash

− Local soils report indicates top 1.5m to be sandy Silt.

− Not known.

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the underlying ignimbrite


− Not expected − Not an issue − Not expected to be an issue



− Ground damage is expected to be limited subsidence (100-300mm. ) and extensive lateral spreading along the coastal edge.


− Steep slopes may become unstable in the event of a large earthquake



10,700-12700

Rangataua Bay Crossing


− No local data available for this crossing.

− Expect up to 15m of soft sediments and paleo-channels to exist within the sediments

− Sea Level.




− Settlement of the pipeline may not be an issue.




− Ground damage is expected to be 300mm subsidence


− Not an issue







6-33


(m)




Slope Instability


Eastern Route (B)


12700-13000

Te Maunga − Fixed foredune sands














Western Route (E)























6-34


(m)




Slope Instability


100-300

Maleme Street

− Volcanic Ash sequence of unknown thickness overlying Te Ranga unwelded Ignimbrite


− Not known
















300-1300

Oropi Rd


















Western Route (E)


1300-3300

Oropi Rd & Fraser St



− Not known

− Expect a relatively deep ground water level at the contact between the volcanic ashes and the Te Ranga Ignimbrite.


− Not expected









− Relic slips may be reactivated in a large earthquake.








6-35


(m)




Slope Instability


3300-3500

Fraser St-estuary/wetland crossing

− Available information indicates causeway fill overlying 15m of soft organic rich estuarine sediments

− Not known






− Settlement hazard to be assessed.

− Differential settlements expected at estuary edge crossings and changes in geological units.









Western Route (E)


3500-5400

Fraser St

Volcanic Ash sequence overlying Te Ranga unwelded Ignimbrite

− Local soils information indicates a sequence of 4m thick young ashes including Rotoehu Ash (loose sand), 2m of Hamilton Ash (stiff clayey silt), 8m of Older Ash sequence (loose silt, clayey silty and sandy silt) overlying unwelded ignimbrite.

− Ignimbrite is clayey silt grading to silty and gravely sand. Ignimbrite encountered at 14m bgl to >35m bgl.

− Local soils report indicates Localised high level perched water tables between 0.4 and 8m bgl.

− In addition a relatively deep ground water level at 13.5-13.8 just above the contact of the older ashes and the Te Ranga Ignimbrite.

− Not expected







− Ground damage is expected to be minor subsidence less than 100mm.

− Unlikely


− Relic slips may be reactivated in a large earthquake. Set back distances from known head scarps should be considered






6-36


(m)




Slope Instability


5500-7100

Memorial Park/Estuary crossing


− Available data from specific investigations of the crossing indicates 10->25m of soft silts and sands

− A paleo-channel is inferred to exist within the sediments

− Sea Level.




− Settlement of the pipeline may not be an issue.






− Not an issue





7100-11,100

Matapihi Rd through to SH2



− Not known.



− Not expected

− Large relic head scarp identified at approx chainage 7000-7300.

− Pipeline should be located using an appropriate set back distance fro the head scarp.










Western Route (E)


11,100-13,000

TeMaunga wetlands

















6-37


(m)




Slope Instability


0-500

Maleme St



















Route (K)


500-2000

Maleme St & Marshall Ave



− Not known











− Relic slips may be reactivated in a large earthquake. Set back distances from known head scarps should be considered







6-38


(m)




Slope Instability


2000-8300

Route K Expressway

− Tauranga Group alluvium comprising soft to firm organic rich silts & clays and peats.

− Local soils reports around chainage 8200 indicate at least 10m of soft soils.

− Local soils report indicates ground water at 1.3m bgl near chainage 8200.

− Stream level & groundwater are tidal influenced at northern end of pipeline. Tidal influence expected to diminish to the southwest.



− Any pipe located close to the stream bank/steep slopes should be adequately designed

− Settlement hazards to be assessed.




− Ground damage is expected to be extensive lateral spreading and subsidence.






8300-10,600

17th Ave & Grace Rd


− 3m of Younger ash overlying 4m of Older ashes

− Not known












− Provided any pipelines are set-back enough distance from steep slopes, risk is minimal




Route (K)


10,600-12,700

Estuary Crossing


− Available data from specific investigations of the crossing indicates 2-15m of soft silts and sands.

− Competent material is inferred to be 5-20m deep.

− A paleo-channel is inferred to exist within the sediments

− Sea Level.




− Settlement hazards to be assessed.






− Not an issue







6-39


(m)




Slope Instability


12,700-16,800

Matapihi Rd through to SH2



− Not known.



− Not expected













Route (K)


16,800-18500

TeMaunga wetlands















Preliminary Geotechnical Considerations -...

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