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 sheet piles • Geosynethics and membranes • Case study – River Dargle • Contracts • Some ideas and recommendations www.gdgeo.com
Plastic sheet piles • 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 www.gdgeo.com
Embankments • Footprint of Embankments can be larger than desired. • Embankments need to prevent seepage during flood events • GDG has used geotextile such as Paradrain TM to reinforce cohesive materials • Geomembranes to provide cut-off in granular materials. www.gdgeo.com
Case study – River Dargle www.gdgeo.com
Ground conditions • Stability Modelling • Seepage Analysis Deemed Critical • Additional boreholes drilled and piezometers installed with dataloggers to investigate tidal influences. www.gdgeo.com
Flood wall analysis The change in Head (m) over time (hours ) 5 4 3 Head (m) 2 1 0 -1 0 6 12 18 24 30 36 Time (hours) 5.45 Flood Defence Wall 4.45 Design Flood Level 3.80 mOD 3.45 River Wall 2.45 Golf Course Road Hight Tide 1.54 mOD Swale 1.45 Elevation (m) 0.45 0 Low River Level -0.20 mOD -0.2 -0.55 0 . 2 Gravel -1.55 8 5 2 -2.55 0.4 9 1 0.2 m 0.32512 m³/days -3.55 ³ / 0.6 d Silt a -4.55 y s -5.55 -6.55 Gravel -7.55 -8.55 -9.55 0 5 10 15 20 25 30 35 40 45 50 55 Distance (m) www.gdgeo.com
Contract structures www.gdgeo.com
Design fees • Typical contracts tendered on 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 www.gdgeo.com
Impact on solutions • 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. www.gdgeo.com
Conservative design 1. 1. In Install ll sh sheet pile iles to o 22 22.5 .5m in into bedrock 2. Sh 2. Sheet pile iles over 10 100m se section Item Quantity Unit Rate Total 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) 330750 kg € 0.95/kg €315,536 Installation 2250 m 2 € 20.00/m2 €45,000 Burning piles 100 m € 20.00/m €2,000 Total cost €375,536 www.gdgeo.com
Value Engineering 1. 1. Undertake detail iled geotechnical l ass assessment 2. 2. Valu alue engin ineer ext xtent an and depth of of sh sheet pile iles to o 9m 9m 3. 3. No No ac account of of costs ass associated with ith easi asier in install llation 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 m 2 € 20.00/m2 €13,500 Burning piles 75 m € 20.00/m €1,500 Total cost € 142,661 www.gdgeo.com
Comparison between methods 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 www.gdgeo.com
Alternatives? • 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 www.gdgeo.com
Conclusions • 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 of 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. www.gdgeo.com
Thank you www.gdgeo.com
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