13. Detailed next are two alternative foundation designs for a bridge pier. Both designs are equal in their ability to support applied loads and meet other performance criteria (i.e., they are functionally equivalent). The cost for each design is provided, but the environmental impacts have not yet been quantified. Using embodied energy as an indicator of environmental impact, make a recommendation as to which foundation design alternative is the most sustainable option. In addition to the materials, consider all aspects of the construction (materials production, materials and waste transportation, equipment operations on site) when estimating the total embodied energy in each complete foundation alternative. Assume all steel is virgin steel.
Useful Information:
• Unit weight of steel = 490 pounds per cubic foot
• Unit weight of reinforced concrete = 150 pounds per cubic foot
• Liquid diesel fuel has an embodied energy of 43.0 MJ/L14, including its energy content at the lower heating value (LHV) and the upstream processing energy.
• The fuel economy of a typical heavy-duty truck is 6 miles/gallon, and all heavy-duty trucks are fueled with diesel (Note: 1 US gallon = 3.78 liters).
• A typical concrete truck carries 10 cubic yards of concrete.
• A typical dump truck for waste disposal carries 20 cubic yards of material.
• Every truck makes a round trip for every load it delivers to or takes from the site.
The two design alternatives are as follows:
Design 1:
• The bridge pier is supported by 20 piles, which are 40-feet-long driven steel H-piles, with engineered steel section HP 12 × 84.
• The steel supplier is located 150 miles from the site.
• The steel sections must be transported on a flatbed trucks, four piles per truck.
• The machinery involved in pile driving will consume a total of 500 gallons of diesel fuel to drive the 20 piles (this does not include transportation fuel to deliver the piles to the site).
• Assume no waste transportation for this design.
• Cost of this design: $220,000
Design 2:
• The bridge pier is supported by four drilled shafts, 50 feet long and 3 feet diameter (drilled shafts are cast in place, reinforced concrete piles).
• The concrete compressive strength is 4,000 psi, and it is steel reinforced.
• The concrete batch plant is located 15 miles from the site.
• The steel rebar supplier is located 150 miles from the site and is delivered on one heavy duty truck.
• This design involves disposal of the excavated soil from shaft drilling as waste. The bulking factor for excavated soil is 1.3. (Note: The volume of excavated soil increases over its volume in-situ. In this case, the bulking factor means the volume of excavated soil is 1.3 times its volume in the ground).
• The landfill for disposal of excavated soil is located 10 miles from the site.
• The machinery involved in shaft drilling will consume a total of 700 gallons of diesel fuel to construct the four shafts (this does not include transportation fuel to deliver materials to the site or remove waste from the site).
• Cost of this design: $250,000
Solution hints:
• Determine total quantities of construction materials and waste for each alternative.
• Determine total quantity of fuel for each design alternative (for material and waste transportation and site equipment).
• Use quantities to determine total embodied energy for each design (embodied energy values per unit for construction materials are in table 8.1).
• Use given information (cost) and estimated embodied energy to decide which design is best/most sustainable.
