There’s a Volcano on our Project Site

Water is the life blood of any city, but its systems are not always pretty. So the two-million-gallon Forest Park Low Tank was embedded into the hillside to preserve the natural character of the area and leave unfettered views. However, this presented engineering challenges. Overcoming those challenges helped us win a 2017 Grand Award from the American Council of Engineering Companies (ACEC).

Wait—What’s Down There?

The subsurface conditions were quite unusual. Maps showed them as hard volcanic rock, but our geotechnical explorations discovered a new volcanic vent, as yet unmapped. Although of great interest to geologists, volcanic vents are rarely built on. A search of case histories did not find any information to guide the process. We embarked upon an exploration and laboratory testing program to determine if the 100-foot plus pile of cinders would support the tank. We determined that the cinders were fairly uniform across the area, resulting in uniform support for the tank. Our testing further determined the magnitude of loading the cinders could support. With this information we were able to design a foundation that did not require expensive subgrade improvements or pile foundations.

Our high-tech analyses confirmed a low-tech approach would work.

Burying Infrastructure to Preserve the Natural Beauty

In many places, water tanks are constructed within large cuts that many may view as eyesores and which permanently remove natural habitat. This has been accepted over decades as a necessary compromise to provide a robust water supply to our cities. However, this compromise does not need to be accepted. Much like the trend of burying power, communications, and other utilities that were once also overhead, the Forest Park Low Tank demonstrates that water infrastructure can be adapted similarly.

Making the Water Supply Safe

Water is a critical resource in any disaster that disrupts our infrastructure. It’s common knowledge that we cannot survive for more than three days without water. During any natural disaster, it is imperative that our water remain safe and accessible. We completed a site specific seismic hazard (SSSH) as part of our work, so the tank and appurtenant facilities will withstand the next “Big One.”

Defining Ingenuity

Sometimes ingenuity is not devising something new, but applying simple methods to solve a problem. We used performance-based results to guide changes in shoring design, and confirmed landslide mitigation approaches during construction. We avoided designing expensive foundation alternatives, installing bulletproof (and expensive) secant shoring walls, and over-analyzing slope stability prior to construction. And then we buried our best work.

The one thing to remember about this project is that we did not blow our top over an unexpected volcanic vent; instead, we persevered and worked with the design and construction teams to build a successful project…and then buried it out of “site.”Finished project

The Game of Thrones Wall—An Engineering Perspective

Game of Thrones Wall

Photo: HBO

The Game of Thrones (the HBO series based on George R. R. Martin’s books, A Song of Fire and Ice) features a giant wall made of ice. Seven hundred feet high. It’s an imposing structure, but it has to be, in order to keep out the terrifying dead people who inhabit the north.

According to Martin, “You could see it from miles off, a pale blue line across the northern horizon, stretching away to the east and west and vanishing in the far distance, immense and unbroken. This is the end of the world, it seemed to say.”

Such an extraordinary structure couldn’t help but draw attention from our geotechnical engineers and staff, who responded to some of the quotes from the books.

“The wall is a hundred leagues long.”

A league was supposed to be the distance that a person could walk in one hour. An English league, once upon a time was about three miles long, which would make the wall three hundred miles long.

“The wall is 700 feet high.”

This is almost as tall as the 1201 Third Avenue Building in Seattle, a 55-story building, which coincidentally has beautiful blue coloring as well. Certainly it takes a lot of work to design and build a high-rise—imagine building so many adjacent high-rises that they would stretch for 300 miles. That’s never been done.

At a height of 700 feet and a unit weight of 57.4 pounds per cubic foot (pcf) for fresh water ice, the base contact pressure on the underlying soil/rock would be on the order of 40,000 pounds per square foot (psf). (Compare that to a high-rise on glacial till at 14,000 psf).

“The wall has stood for, what, eight thousand years?”

Assuming a coefficient of secondary compression, C-alpha, of 0.02 and assuming that the base upon which the wall is built is comprised of some reasonable thickness of compressible organic muskeg (say ten feet), and assuming the wall was built over a period of one hundered years, the wall will likely have settled about three to five feet under its own weight.

“The top wide enough for a dozen armored knights to ride abreast.”

How wide is an armored knight? Say five feet? 5 x 12 = 60 feet wide? To safely travel, there would need to be at least three feet between riders so the total width (including four feet on either side for shoulders and jersey barriers) would be 101 feet.

“The gaunt outlines of huge catapults and monstrous wooden cranes stood sentry up there, like the skeletons of great birds, and among them walked men as small as ants.”

To anchor the catapults and cranes, it is likely that the overturning and uplift forces on the catapults and cranes would control the design. The overturning forces associated with the action of the catapults and the wind loads on the structures (resulting from the unobstructed exposure to the predominant winds due to the height of the wall) could be resisted by using high-capacity drilled micropiles.

“It was older than the Seven Kingdoms and when he stood beneath it and looked up, it made Jon dizzy. He could feel the great weight of all that ice pressing down on him, as if it were about to topple, and somehow Jon knew that if it fell, the world fell with it.”

One of the Seattle-Tacoma International Airport Third Runway walls (135 feet tall) was built stepped in, in order to avoid this feeling when you stand at the base of it. However, typically, a tall wall looks shorter when looking up than when it does when looking down. Jon is a weenie.

“Eight hundred feet above the forest floor, a good third of that was earth and stone rather than ice.”

It seems that people got creative over the years, sometimes making use of on-site materials, a good practice to save cost, time, and the environment. It also makes sense, when you are building a structure with a contact bearing pressure equal to 40,000 psf, to do overexcavation and replacement with densely compacted (i.e., 95 percent of the maximum dry density, within two percent plus or minus of optimum moisture content, as determined by ASTM D1557 Test Procedure) well-graded sand and gravel with less than five percent passing the U.S. No. 200 sieve based on the minus three-quarter-inch fraction.

Have questions about the geotechnical design of other giant structures? Need a dragon or two? Contact Garry “the Hound” Horvitz.

Towering Hills for Beauty and Strength

Governors Island

Photo: Timothy Schenk

A dozen years ago an American port representative was asked how his port was preparing for rising sea levels. “Well…we aren’t,” he answered, somewhat sheepishly, because he knew they should be. Back then, the public was skeptical of the controversial topic, and frankly many ports had too many other priorities. But now public officials see the situation in a new light. They are taking advantage of waterfront development projects to make property not only more resilient to climate change, but also more beautiful and beneficial to the public.

A perfect example is the 40-acre Governors Island Park and Public Space in New York. West 8, an urban design and landscape architecture firm, transformed the abandoned former military island into a green oasis with an extraordinary 360-degree experience of water and sky that has won numerous awards. Part of the makeover involved creating four tall, dramatic hills from twenty-five to seventy feet high. This meant overcoming a major challenge involving Governors Island history.

Governors Island Park and Public Space

Pumice, or lightweight fill (the light colored material) is placed on the water side of the tallest hill. Image courtesy of West 8

From Subway Dirt to Island

Back in 1637, when a Dutch man bought Governors Island for two ax heads, a string of beads, and some nails, the island was only about 72 acres. In 1901, somebody needed a place to discard the dirt from the excavation of New York’s Lexington Avenue subway line. What better place to put it than Governors Island? The dirt widened the island by 100 acres.

Fast forward to the twenty-first century. Now that the island had been sold back to the people of New York for one dollar, it was possible to take advantage of the island’s potential views, which meant building upwards. To create the new hills, West 8 needed to add 300,000 cubic yards of new fill—enough to fill 40 Goodyear blimps. The challenge was to keep that massive amount of dirt from pushing the island built on subway fill out into the harbor.

Hart Crowser worked with the lead civil engineer to make the hills strong yet light. Twenty-five percent of the new fill is from the demolition of structures and parking lots. This made it sustainable and strong. Pumice lightened the load. Some of the fill was wrapped in geotechnical matting, and the steepest slopes used wire baskets. This allowed hills as high as seventy to be built within twenty feet of the shoreline, and allowed for varying slopes and walkways, where the public can safety enjoy the park.

Governors Island reopened to the public on May 28.