Retaining Order

South of Boston, contractor Julian Crane designed a new facility to store and repair their heavy construction equipment. The superstructure is a prefabricated steel frame. They hired me to design the foundation.

I proposed a stem wall with footing below the four exterior walls of the building. Every project is unusual in some way, and for this facility it was a sloping site that dropped off into a wetland. No sitework was allowed on the wetland itself, but my client wanted to build the facility as close as possible on the higher ground. So I proposed that the foundation wall double as a retaining wall on two sides. The wall would have a combined loading of vertical forces from the building itself and horizontal forces from the soil behind it.


Retaining wall (top) | Frost wall (bottom)

One of the more insidious forces that engineers must consider is live load surcharge. When you stand in a precarious spot, like atop a pile of dirt or a snowy ridgeline, your weight pushes the ground material outward. Too much force and the dirt collapses or the snow avalanches. Your weight is the live load (because it can move around), and surcharge means it’s an extra force that increases the material’s tendency to spread. At the Julian Crane facility, my retaining wall needs to hold back not only the soil itself, but also a surcharge from the weight of any heavy construction equipment inside the building.

I specified an 8-foot retaining wall under the two sides of the facility where the ground slopes down to the wetland, and a 4-foot frost wall with minimum reinforcement under the other two sides for economy. My client decided to go deluxe and had the contractor use the 8-foot wall detail on all four sides, with my approval. I also specified a minimum bearing strength for the soil below the foundation footings, which the contractor achieved by filling and compacting new soil.


Inside the steel-frame facility.

Construction proceeded over the last year, and (pending some final sitework including guardrails and paving) the facility is substantially complete.


Outside the new facility.

China Week: The Great Wall

He who does not reach the Great Wall is not a true man.
-Mao Zedong

I saved the best for last. The Chinese call it 万里长城, pronounced “wànlǐ chángchéng” (literally “the 10,000-mile-long wall”), or simply 长城 “chángchéng” (“the long wall”). Archaeological surveys place the total length of all sections of the wall between 5500 miles and 13,000 miles. Thus, even though the number “10,000” is meant to be figurative (like “myriad”), it turns out to be an accurate description.


Doug traverses an unrestored part of the Great Wall.

Semantics aside, the Great Wall is truly great. The oldest parts were built nearly 3000 years ago, but the parts still standing today were largely built or rebuilt during the Ming Dynasty of 1368-1644. The wall traverses some of the most rugged terrain imaginable. Guard towers spaced by sightlines allowed sentries to keep an eye on the empire’s entire northern border, and enabled rapid communication. The wall aimed to stop nomadic invaders long enough for China’s armies to mount a defense. It didn’t always work.

Today’s wall is the product of tens of millions of laborers. Typically each side is a double course of brick sandwiching rubble. Two colors of brick are used, with the tan parapet standing out from the grey bulk. Here’s a new vocabulary word for you: crenellation. The Great Wall’s parapets have a crenellated, or toothy shape, rather than being solid. The low parts are called crenels, and they’re useful for shooting an arrow at an invader or for admiring a view.


A cross section of the parapet.

The 4- to 5-meter-wide interior is filled with rocks and earth, and paved over on top with stones. Touristy sections of the wall have stairs, while crumbling unrestored segments are slippery smooth. The stairs include periodic channels carved out for drainage. Guard towers typically have window openings and a terrace on top accessed by narrow steps.

A handful of restored wall segments near Beijing are the most visited. These segments charge admission fees and offer refreshment and souvenir stations, as well as amusements (for an additional fee) like chairlifts and toboggan rides. One may traverse the natural parts of the wall between restored segments, and overnight point-to-point trips are popular with backpackers.

I imagine the builders would be proud that their life’s work survives as not only China’s most iconic monument, but also the world’s first long distance hiking trail. It’s a pretty simple one to follow.


This concludes China Week. Please read the first six parts if you missed them: Architecture, Business, Food, Pollution, Bridges, and Transportation.


Louisiana got besieged by a 500-year flood a couple weeks ago. I received a firsthand account from my cousin, who lives in Baton Rouge. He was one of the lucky ones – flood waters didn’t reach his house – and he has spent days helping neighbors clean out.

An unnamed, weeklong storm dropped more than 20 inches of rainwater over a third of the state. Rivers reached record flood stage, mixing with mud and sewage as the water submerged low-lying land. Outsiders tend to assume all of Louisiana is low-lying, but many affected houses are not officially in a floodway and many residents do not have flood insurance. Even for those who do, the federal provisions are messy and limited to $100,000 of personal property. In homes where the water has receded, the stench is almost unbearable. Cleanup involves removing personal items from the house, then gutting and replacing everything below the high-water line… and often, because of wicking moisture, well above that line too.

How can you design or retrofit a house to minimize the damage a flood would cause? LSU College of Agriculture developed this proposal, which divides walls horizontally in two. A chair rail hides a gap in the drywall and a partition between lower and upper insulation. Thus, any flood water that infiltrates the house can’t damage anything above the chair rail. The concept is to eliminate “water bridging”, much like a Passive House avoids thermal bridging to keep heat in.


LSU proposal for floodproofing. For more information, click the first link in the paragraph above.

To further protect the lower half of the house, LSU recommends water-resistant flooring and wall panels, along with insulation that doesn’t absorb water, such as rigid foam. Electrical wiring is as high as possible. None of the details seem overly expensive to install, other than requiring a like-minded contractor to do the work correctly. I’d recommend all homeowners in a floodway, and even outside of it (500-year floods do happen), to consider protecting their property with details like these.

Of course, you could also literally elevate your house, or surround your property with a cofferdam, or (if your house is on wheels) drive it to higher ground when heavy rain threatens. But most folks who take action at all will probably go for the LSU ideas.

To help rebuild the affected communities, you can send funds to the Baton Rouge Area Foundation or the Louisiana Red Cross.