SLIDE 1
Overview
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- Alternative solutions for Flood Defence Schemes
- Plastic sheet piles
- Geosynethics and membranes
- Case study – River Dargle
- Contracts
- Some ideas and recommendations
in in reducing the cost of f fl flood defence schemes Paul - - PowerPoint PPT Presentation
in in reducing the cost of f fl flood defence schemes Paul - - PowerPoint PPT Presentation
The role of f geotechnical engineering in in reducing the cost of f fl flood defence schemes Paul Quigley and Paul Doherty pquigley@gdgeo.com and pdoherty@gdgeo.com Overview Alternative solutions for Flood Defence Schemes Plastic
SLIDE 2
SLIDE 3
Plastic sheet piles
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- Suitable where limited cut-off required.
- GDG has used this solution to provide low cost
flood protection
- Driven piles or excavate trench and backfill
around sheet pile. Need to be careful about the back fill materials.
- Used successfully in UK
- Guidance provided in paper on durability
SLIDE 4
Embankments
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- Footprint of Embankments can be
larger than desired.
- Embankments need to prevent
seepage during flood events
- GDG has used geotextile such as
ParadrainTM to reinforce cohesive materials
- Geomembranes to provide cut-off in
granular materials.
SLIDE 5
Case study – River Dargle
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SLIDE 6
Ground conditions
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- Stability Modelling
- Seepage Analysis
Deemed Critical
- Additional boreholes
drilled and piezometers installed with dataloggers to investigate tidal influences.
SLIDE 7
Flood wall analysis
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- 1
1 2 3 4 5 6 12 18 24 30 36 Head (m) Time (hours)
The change in Head (m) over time (hours)
- 0.2
0.2 0.4 0.6 1
Low River Level -0.20 mOD Gravel Design Flood Level 3.80 mOD Gravel Silt Swale River Wall Flood Defence Wall Golf Course Road Hight Tide 1.54 mOD
0.32512 m³/days . 2 8 5 2 9 m ³ / d a y s
Distance (m)
5 10 15 20 25 30 35 40 45 50 55
Elevation (m)
- 9.55
- 8.55
- 7.55
- 6.55
- 5.55
- 4.55
- 3.55
- 2.55
- 1.55
- 0.55
0.45 1.45 2.45 3.45 4.45 5.45
SLIDE 8
Contract structures
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SLIDE 9
Design fees
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- Typical contracts tendered
- n lump sum for Stage I
and II & percentage of the construction costs for Stages III to V
- Percentage banded on
contract value
- High risk for designers
Plot from Engineers Fees, 2017
SLIDE 10
Impact on solutions
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- Consider 75m to 100m long section
where sheet piles may be required
- Bedrock at 22.5m below ground
level
- Made ground to 4m bgl over glacial
deposits.
- Assume that the reinforced
concrete wall on the surface costs the same.
SLIDE 11
Conservative design
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Item Quantity Unit Rate Total Mobilisation
- f plant and
labour 1 Sum €8,000.00 €8,000 Site moves 2 Sum €2,500.00 €5,000 Piles (AZ 26- 700 Z) 330750 kg €0.95/kg €315,536 Installation 2250 m2 €20.00/m2 €45,000 Burning piles 100 m €20.00/m €2,000 Total cost €375,536
1.
- 1. In
Install ll sh sheet pile iles to
- 22
22.5 .5m in into bedrock 2.
- 2. Sh
Sheet pile iles over 10 100m se section
SLIDE 12
Value Engineering
www.gdgeo.com Item Quantity Unit Rate Total Additional GI 1 Sum €20,000 €20,000 Mobilisation of plant and labour 1 Sum €8,000.00 €8,000 Site moves 2 Sum €2,500.00 €5,000 Piles (AZ 26-700 Z) 99225 kg €0.95/kg €94,661 Installation rate 675 m2 €20.00/m2 €13,500 Burning piles 75 m €20.00/m €1,500 Total cost €142,661
1.
- 1. Undertake detail
iled geotechnical l ass assessment 2.
- 2. Valu
alue engin ineer ext xtent an and depth of
- f sh
sheet pile iles to
- 9m
9m 3.
- 3. No
No ac account of
- f costs ass
associated with ith easi asier in install llation
SLIDE 13
Comparison between methods
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Capital costs Design costs Total cost Conservative solution €375,536 €13,144 €388,679 Optimised solution €142,661 €4,293 €146,954 Difference €232,875 (€8,851) €241,725
SLIDE 14
Alternatives?
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- Look at how road projects are procured
and designed?
- Split contracts after Stage II?
- Consider Design and Build options to drive
capital costs down?
- Design effort driven towards providing
value to both Employer and Contractor
- More oversight of the design process
- Optimise programme to complete
schemes quicker to save money
- Do GI and GIR early in project to identify
issues and potential solutions
SLIDE 15
Conclusions
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- Geotechnical Engineers can deliver optimised Flood Defence
solutions – retaining walls, embankments, seepage analyses. Time and effort spent on design can be very rewarding.
- The value of a useful ground model is evident in optimising design
- solutions. Ground model must be developed to understand the
inherent variability in the ground. Best to undertake GI in advance
- f preliminary design.
- Value Engineering options available for Flood Defence schemes.
- Current contract structure results in squeezing design fees and
design effort, risky for consultants and rewards expensive design solutions.
- Design and Build has potential to provide robust solutions, reward
innovation and deliver lower capital.
SLIDE 16
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