Monumental DC – A series where I’ll be documenting the many memorials in DC that we pass by frequently, but rarely seem to stop and pay notice to. Follow on twitter with #monumentalDC
What: The Jefferson Memorial
When: Tuesday November 8, 2011
Where: Tidal Basin, 900 Ohio Drive SW
Here I am, finally catching up on a round of memorial visits stretching back nearly a month. After riding by it many times to get to the Mount Vernon Trail, I had never stopped to look at The Jefferson Memorial up close – and this was going to be my day. As I was walking around the memorial I recalled the recent construction, and wanted to share that with you – especially since it centered around the foundations used to support the structure (my specialty).
North Face of Jefferson Memorial.
The memorial broke ground in 1938, and was completed on April 12, 1943, what would have been Thomas Jefferson‘s 200th birthday. In the ensuing decades, the plaza around the memorial has shifted and sank considerably, resulting in construction in 2009/2010 to shore up the seawall and plaza between the memorial and the Tidal Basin. What caused this?
The L'Enfant Plan didn't even show the Tidal Basin area - presumably it was considered too often wet to be inhabitable. The Tidal Basin and East/West Potomac Park were built in the early 1900s.
Well, the District is commonly known as having been built on a swamp – though in reality the area was more of a tidal plain. While what defines a swamp v. a tidal plane differs, the end result is a thick accumulated layer of soil and decomposing organic matter that is built up over time (either seasonally, or as a river floods). As the architects and planners built up our city, they added soil to raise land or create dikes to hold back the waters of the Potomac river. Even still, much of the area close to the river remained in the flood plain. When this soil (called “fill”) was added to the area, the swampy ground was not excavated and removed, creating a very soft compressible layer of very wet soil trapped between the fill and the bedrock. Imagine a really tasty pie for a moment…
DC Geology Lesson
The fill material is a lot like the crust at the top of the pie – firm enough to support a little weight (like ice cream), but not strong enough to support heavy loads (like packing it on the bottom of a bag of groceries). The delicious pie filling is soft and sticky, and oozes out of the pie – a lot like the soft tidal soil deposits. The filling can’t really support any weight. The pie tin keeps it all together, providing a firm base – just like the bedrock in the area.
When you build a structure in the former tidal plain of DC you usually need to take the weight of the structure and transport it down to the firm bedrock, since the soils aren’t strong enough on their own. Under heavy loads, these soils will settle, or could actually catastrophically fail and topple a building. Well, for the Jefferson Memorial, the original designers supported the 32,000 ton Memorial on a series of piles and caissons which extend all the way to the rock. Piles and caissons are for all intents and purposes the same thing to most people, the engineering and construction are different though. However, the designers supported the lighter loads of the plaza and seawall on shorter (estimated to be 65-75 feet long) timber piles which ended in the soft pie filling, not in the bedrock. Timber piles are a great foundation solution when you have light loads – they are economical and fast. However, if you don’t extend the piles to bedrock, you really need to know about the soils and their consolidation properties (consolidation is a way of describing the way soil particles interact and settle over time). Since the structure was built in the early 1940s, it is likely that a sufficient engineering study had not been undertaken, as the science of geotechnical engineering was still in its infancy at that time).
Memorial Under Construction in 1940 (Source:american-architecture.info)
And that’s how the plaza settled up to and over 3 feet in some areas!
So how did they fix it? Well, the quick and simple answer is that they built what they knew already worked at the monument itself… more piles and caissons. In reality, multiple engineering studies were conducted, and design alternatives were considered for cost effectiveness, practicality to implement, time to construct and many other factors. In the end, the option they selected was to support the main loads of the plaza/seawall on concrete caissons that extend 10 feet into the bedrock. In addition, a series of batter piles intersect these caissons at an angle to help provide additional load carrying capability. The contractor installed a cofferdam(basically steel wall to block out the water) from around the waters edge, carefully deconstructed the existing stonework, drilled the piles and caissons, then reinstalled the stonework on a new foundation. I sketched it out to show you:
How they fixed it
Take a look through this presentation from the Deep Foundations Institute that shows some pictures of the settlement and resulting construction to shore up the plaza.
Pretty neat, huh?
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