|Fig. 1: The Great Wall of China. (Source: Wikimedia Commons)|
During the recent presidential election, the issue of illegal immigration once again came to the forefront of the U.S. policy discussion. As part of the solution, the idea of a wall resurfaced and was popularized by the current president-elect Donald J. Trump. Building a wall is a time-tested and popular strategy for keeping others from entering. This strategy has often been used for small private venues; however, building a large wall or fence along the border is not unprecedented (see Figs. 1 and 2). With Mr. Trump taking office in 2017, the construction of a US-Mexico border wall may be up for more serious consideration. Although the nature of the wall and the overall price tag is still uncertain, some crude estimates of the cost for building an actual wall already exist. Most of these estimates focus on the direct economic cost of the wall and ignore the environmental costs. Those that do consider the environmental effects take a qualitative approach.  In this work, I develop a rough energy-cost estimate to parallel the economic estimates. In these computations I only estimate the cost of creation of the wall but not the costs associated with the necessary land preparation and road construction, so my figures likely understate the energy-cost of the wall. Furthermore, I am simply estimating a one-time, creation cost, which ignores longer term maintenance and changes in the ecosystem.
|Fig. 2: The beach on the Pacific Ocean at the U.S.-Mexico border from the Mexican side. (Source: Wikimedia Commons|
At this stage, not much is known about the details of the wall. In an interview with MSNBC, Mr. Trump, suggested that the wall would be 1,000 miles in length and 35-40ft in height.  However, in other instances he has suggested that the wall will be much taller. [3,4] Ali Rhuzkan has estimated the amount of concrete and rebar needed for a 30 ft tall and 1,900 miles long wall in The National Memo.  These numbers can be adopted to estimate that a 1,000 miles long wall with a height of 35ft and a 6ft foundation would require just over 8 million cubic yards of concrete.  Portland Cement Association (PCA) estimates the energy cost of mixing precast concrete to be around 2.63 GJ/m3 (for the least energy intensive mix analyzed) with an additional 0.96 GJ/m3 for aggregates, transportation and plant energy. 
|Energy to Produce Concrete||=||8.0 × 1016 yd3 × (||0.91 m
|)3 ×||3.59 GJ
|=||2.2 × 1016 J|
|Fig. 3: Left) Location of U.S. steel factories produced using google maps with "steel", "steel factory" and "steel mill" as the search keywords. Right) Location of U.S. concrete and cement factories. Green, red and blue correspond to precast concrete, concrete, and cement, respectively. (Source: Armin Pourshafeie)|
This construction would also produce 2.3 billion kilograms of CO2, 4 million kilograms of SO2, and many other environmental toxins. (See Marceau et al. for CO2 and SO2 production per m3 of concrete and for other byproducts. ) These estimates do not account for the possible effects due to change in demand.
The Washington Post also predicts that the wall would require approximately 1.6 million tons of reinforcing steel (rebar).  The total energy cost of steel is dependent on the amount of recycled steel used. Here I will take the value for steel with 28% recycled content as it is the least energy intensive value reported in Pirquet et al.  This puts the total energy cost at 13.2 MJ/lb.  Therefore, the required steel would add approximately 4.2 × 1016 J to the energy budget.
Finally, I approximate the cost for transporting the material to the construction sites at the border. Based on a computer search of the U.S. precast-concrete, concrete, and cement factories, I estimate an average of 100 miles transportation distance for the concrete. A similar search shows far fewer steel factories close to the border, so I estimate a 300 mile transportation distance for the steel. At 6 mpg this results in 
|Fuel Consumed||=||1.6 × 106 tons ×||2000 lb
|+ 8.0 × 106 yd3 ×||4000 lb
|≅||2.7 × 107 gallons|
or approximately 4 × 1015 J.
Overall, producing and transporting these ingredients consumes over 6.8 × 1016 J. This figure makes up close to 6% of the reported total energy consumed by all the U.S. agencies and governmental sources in the 2010 fiscal year.  While this figure represents a significant portion of energy used by the governmental agencies, it is still insignificant compared to the more than 1020 J of energy consumed by the U.S. in that year. 
© Armin Pourshafeie. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Owens, "Trump's Border-Wall Pledge Threatens Delicate Desert Ecosystems," Nature 536, 260 (2016).
 A. Brand, "Trump Puts a Price on His Wall: It Would Cost Mexico $8 Billion," MSNBC, 9 Feb 16.
 J. Preston, A. Rappeport, and M. Richtel, "What Would It Take for Donald Trump to Deport 11 Million and Build a Wall?" New York Times, 19 May 16.
 G. M. Graff, "Donald Trump's Army on the Border," Politico, 18 Jul 16.
 A. F. Rhuzkan, "An Engineer Explains Why Trump's Wall Is So Implausible," The National Memo, 21 Sep 15.
 M. L. Marceau, M. A. Nisbet, and M. G. VanGeem, "Life Cycle Inventory of Portland Cement Concrete," Portland Cement Association, 2007,
 P. Pierquet, J. L. Bowyer, and P. Huelman, "Thermal Performance and Embodied Energy of Cold Climate Wall Systems," Forest Prod. J. 48, 53 (1998).
 S. C. Davis, S. E. Williams, and R. G. Boundy, "Transportation Energy Data Book: Edition 25," Oak Ridge Nationnal Laboratory, ORNL-6992, Ocbober 2016, Ch. 5.
 "Annual Energy Review 2011," U.S. Energy Information Administration, DOE/EIA-0384(2011), Septmber 2012.