1118 malcolm barker maroon dam nzsold 2013 final

7/23/2019 1118 Malcolm Barker Maroon Dam NZSOLD 2013 Final

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Maroon Dam Performance and

Upgrade Design using Percentage

of Acceptable Flood Capacity –

Arewe there yet?

Malcolm Barker | GHD Principal Engineer – Dams

lireza Mojami | GHD Senior Engineer – Dams

Colin Thompson | Seqwater Senior Project Manager



Maroon Dam NZSOLD 2013

Presentation Contents

- General Description

- Issues- Geotech Investigation

- Risk Analysis

- Acceptable Flood Capacity %




AFC Definition

The minimum requiredacceptable flood capacity

(AFC) all proposed and

existing referable dams in

Queensland must be able

to safely pass.

• Fallback (PMP-DF)

• Risk basis satisfy ALARP

Cost/Benefit Ratio <1.0




• Slope failure right abutment during construction

• Low strength clay horizons (8º friction - 13º J.Barry-Cooke)• Pore pressure uncertainty for high floods

• Low slope stability factors of safety Normal and Flood

loading

• Inadequate spillway capacity (36% to 51% AFC)

• Abutment piping Normal and Flood loading

• Spillway – back erosion into reservoir

The Issues




What did we do - Winston Churchill

True Genius resides in the capacity for theevaluation of

uncertain,

hazardous and

conflictingInformation”




Embankment Boreholes and Cross Sections


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Clay Occurrences

Three main sources of clay: – Landslide Clay – thicker up to 10 cm

– Shear zones – thinner up to 2 cm

– Extremely low strength claystone or clayey siltstone



Claystone



Investigation findings – Clayey

Layers

Detailed examination (samples pulled apart)

• Weathered fissured/shattered siltstone or

claystone.

• Some polished surfaces and/or slickenslided

surfaces.

• Striations on some surfaces.

• Little effort is required to break down the

shattered/fissured stiff clay fragments to a softsilty clay.



Clay Variation along Potential Failure Plane

Potential Failure surface

Zone Lower Strength

weighting

Expected Strength

weighting

Upper Strength

weightingLandslide 30% 20% 17%

Clay Zone 60% 50% 36%

Shear Zone 10% 30% 47%



Strength Test Results

50 200 400 800




Risk Analysis

Be thankful for the problems for if

they were less difficult, someoneelse with less ability would have

your job




Risk Analysis Flood Hydrology and %AFC Approach




Risk Analysis Failure Modes

Flood Normal Failure Mode Description

F1 Flood overtopping

F2a F3a Downstream stability main dam

F2b F3b Downstream stability left abutment

F2c F3c Downstream stability right abutmentF4a F5a Piping through embankment dessication cracking

F4b F5b Piping through embankment poorly compacted layer

F4c F5cPiping through embankment poorly compacted layer along

conduit

F4d F5dPiping through embankment cracking induced byfoundation movements

E5dPiping through embankment cracking induced by foundation

movements during earthquake

F6 F6 Piping through foundation right abutment

F7 F7 Spillway Back Erosion




Existing Risk




Existing Risk Contributions

Piping Foundation 68.3%

Slope Stability 29.2%

Overtopping 1.5%




Proposed Solution

Stage 1

Stage 2




Risk Contributions after Stage 1 Upgrade

Slope Stability 81.1%

Overtopping 10.3%




AFC Schedule for Upgrade Works

Tranche Required minimum flood

discharge capacity

Date by which the required

minimum flood capacity is to be in

place for existing dams

1 25% of AFC or at least

1:2000 AEP for erodible dam

embankments (whichever is

the bigger flood)

1 October 2015

2 65% of AFC 1 October 2025

3 100% of AFC 1 October 2035

AFC Fallback Compliance




Cumulative Risk Contribution 95% at RL 218.65m




Reservoir Start Level for Probabilistic Analysis




Percentage AFC Compliance (Fallback)

Description Start Reservoir Level

Initial water level 217.5 m AHD

AFC guideline start level

215 m AHD

probabilistic analysis start levelPeak water level 218.65 m AHD 218.65 m AHD

Peak inflow 516 m³/s 1,075 m³/s

Peak outflow 252 m³/s 252 m³/s

Hydrograph duration 6 hours 6 hours

Percentage AFC 15.7% 32.7%




Risk Analysis Flood Hydrology and %AFC Approach

Reduction25%




AFC % with Risk Analysis URS 25%




URS 25% Compliance Flood Frequency data

25%




Staging Compliance

AFC requirement 25% AFC 65% AFC 100% AFCDate for Compliance 2015 2025 2035

Flood frequency data

URS <25%

Abutment upgrade

65% OK <100%

Slope stability upgrade

GHD OK

60% AFC

<65%

Abutment

upgrade

<100%


Overtopping upgrade

Probabilistic analysis

existing spillway

RL 217.5 m

OK

60% AFC

<65%

Abutment

upgrade

<100%


(overtopping upgrade

may not be required)




Conclusions

• Maroon Dam upgrade required by 2015.

• Extensive geological modelling and analysis was used to

evaluate the nature and extent of low strength clay.

• Probabilistic flood frequency used for Risk Analysis and

AFC compliance analysis.

• The percentage AFC compliance using the risk analysis

approach was successfully used to confirm the proposed

staging of the works

•

This approach has also identified the potential fordeferral or elimination of part of the final stage of the

upgrade works in 2035 on a risk basis.




Stage 1 by 2015

• Abutment grouting and pressure relief systems to EL 219.5 m

• Stability upgrade with a 33 m wide berm using material

borrowed from the spillway chute

1118 malcolm barker maroon dam nzsold 2013 final

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