By David Einstein
Photo above by Orestis Stavropoulos/Getty Images Assignment

Ground is just ground, right? To build on it, you use bulldozers to make it flat and level. To build beneath it, you bring in a tunnel-boring machine.

If only it were that simple. The fact is that soil, rock, and groundwater conditions pose some of the most daunting challenges in big construction jobs. In projects around the world, Bechtel must overcome soft clay, fractured rock, liquefiable soil, and sinkholes, just to name a few. And that’s just the natural stuff. Many projects also must deal with archaeological finds, historical buildings, and even unexploded ordnance.

Unearthing these problems and quickly solving them helps lay the groundwork for a successful undertaking. “Every project we construct must have a stable foundation, and that’s what we ensure,” says Mike Lewis, geotechnical discipline lead for Bechtel’s Geotechnical & Hydraulic Engineering Services group (G&HES).

Bechtel geotech experts work behind the scenes (and beneath the scenes), performing geotechnical engineering, hydrology, hydraulic engineering, engineering geology, and seismology for Bechtel projects worldwide. In fact, G&HES has a hand in every major Bechtel project, solving design and construction problems created by soil, rock, and water—and minimizing financial risks.

G&HES was originally formed in the early 1960s to help with designs of hydroelectric projects. But it soon expanded to include seismology services for nuclear power projects and underground transit systems, and environmental services for large remediation projects.

Today, G&HES boasts about 100 experts in two organizations: a corporate group and one dedicated to projects for the U.S. government. They work on every type of project, from tunneling jobs like the Channel Tunnel Rail Link to U.S. Department of Energy sites in Tennessee, South Carolina, and Nevada.

Although it uses advanced tools and technology, G&HES has a straightforward set of goals, says Lewis: a) Understand the subsurface conditions; b) Know what the project aims to achieve; and c) Keep solutions simple. “We can’t forget that we must build what we design within a budget and a schedule.”

Pat Ryan, manager of G&HES, says that while geotechnical services may not grab headlines, “We can mean the difference between a successful project and one with costly problems.”

Following are a few examples where geotech expertise helped projects succeed.

Limestone and Land Mines

The land on which Bechtel built Croatia’s first modern highway is composed largely of dissolving limestone. The condition, called karst (after the region of Slovenia where it was discovered), creates sinkholes, caves, and even underground rivers. Sometimes a sinkhole announces itself with a crack in the ground. But not always. “While we were cutting earthworks, we couldn’t always predict when we’d hit an underground cavern or sinkhole,” says Bechtel Field Engineering Manager Mike McGarvey. It’s too costly and time-consuming to divert a road around a sinkhole. Instead, reinforced concrete beams are used to shore up the ground.

Crews on the Croatian Motorway also faced a far more sinister danger in the form of unexploded mines left over from years of war in the region. To answer that threat, dogs were used to sniff out ordnance, bulldozers were armored, and drivers wore Kevlar-equipped clothing. Unfortunately, unexploded ordnance has been a real threat on more than one project in the recent years.

Antiquities and Athenian Schist

The ground beneath Athens is largely composed of schist—rock that’s been folded and fractured over millions of years. When you dig a tunnel in it, there’s a chance that the ground above may settle, damaging historical buildings. At the same time, you must be on the lookout for old water wells, buried structures, and irreplaceable artifacts from early Greek civilization.

So how did Bechtel build the new Athens Metro subway system? Carefully. To prevent ground-settling during tunnel construction in high-schist areas, work teams first carved narrow tunnels and supported them with metal arches and sprayed concrete, then used special machinery to widen the tunnels.

Because Athens is 3,000 years old, much of its history is buried beneath the surface. So Greek archaeologists preceded construction of every tunnel shaft and station with a survey. When artifacts were discovered—either during the surveys or during construction—work stopped until the finds could be retrieved. “Archaeology is a major challenge when you’re constructing in any city, but it’s especially true in Athens,” says Bechtel geologist Steve Walthall, who was a part of the project team.

Blue Clay and Soil Mixing

Perhaps no project has ever faced more different subsurface conditions than the Central Artery/Tunnel Project in Boston, also known as the Big Dig. “The soil beneath the city has a little bit of everything,” says Bechtel geotech engineer Joe Baka. That’s an understatement. A typical section of the new underground expressway traversing the city is 26 meters deep and had to be dug through four distinctly different types of soil—fill at the surface, followed by organics (silt, sand, etc.); a marine soil known as Boston “blue clay”; and finally a layer of boulders, gravel, and clay (glacial till) sitting atop the bedrock.

A particularly large and deep deposit of the blue clay complicated proposed excavations to construct tunnels connecting the Ted Williams Tunnel, the new Central Artery tunnels, and the Massachusetts Turnpike. The problem: even small, shallow excavations would slump and collapse. The solution: combine the clay with cement, a “deep soil mixing” technique developed in Japan that makes the soil harder and easier to excavate and makes it act as a buttress. Even that was not simple. To access the areas where soil needed to be improved, existing structures needed to be moved. In some cases, that meant replacing long bridges with new ones.

An even more elaborate approach came into play constructing four tunnels (ranging from 46 to 116 meters in length) beneath operational railroad tracks in downtown Boston. First, tunnel boxes were fabricated in 24x12-meter sections inside large excavated pits next to the tracks. Then workers broke through the walls of the pits and dug the tunnels a little at a time—after first freezing the soil so it wouldn’t settle. Finally, they used hydraulic jacks to push the tunnel boxes into place beneath the tracks.

Other Projects, Similar Issues

Croatia, Athens, and Boston aren’t unique. Around the world, Bechtel confronts similar geotechnical and archaeological issues. On the island of Trinidad, for instance, extensive soil mixing was required to stabilize the ground for a massive liquefied natural gas facility. And the Channel Tunnel Rail Link turned into the largest archaeological project in the history of England as construction crews discovered everything from Anglo-Saxon ruins to bones of a prehistoric elephant.

The location of a project often dictates the nature of geological issues. In areas near water, for example, the ground may be too sandy to support major facilities such as power and petrochemical plants. At the Rijnmond Energy Center project in the Netherlands, 4,400 concrete piles, each 25 to 30 meters long, had to be sunk for a stable foundation. And at the CSPC Nanhai Petrochemicals Complex along the South China Sea, some 18,000 piles were needed.

When the last pile is driven, or a tunnel is completed, or a new road opens, little if any evidence points to the hard work that went into identifying, analyzing, and resolving geotechnical issues. But without the effort of Bechtel’s geotechnical experts, many projects couldn’t be built.

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