Okay, it may not be significant to engineering history, but it is a unique and interesting spot for U.S. history. Have you ever heard of the Lorton Reformatory? If not, it's definitely worth a quick look, especially in regards to the suffragettes. Essentially it's a former D.C. prison turned parks, housing, arts center, you name it. Some good places to learn more are GC9GKEV, GC9GME3, or the Workhouse Arts Center's website.
What makes this area particularly interesting in regards to engineering is the juxtaposition of the old prison facilities with the modern construction. Apartment buildings are nestled between guard towers, art galleries are inside old cell blocks, and a Lidl is being built behind some of the twenty-foot walls.
I attached a few photos of some of the remaining structures. I promise that the bolt is visible in every one, even if it does feel a bit like an I Spy...