Stabilizing Ground in the Middle of the Ocean
By Spencer Schram
Getting a track-mounted drilling rig to the most remote U.S. military base in the world wasn’t the only problem Spencer Schram and the team at geotechnical engineering firm CTL|Thompson had to overcome when the U.S. Navy hired them to provide geotechnical expertise and engineering solutions for a satellite communication project on Diego Garcia, located just south of the equator in the British Indian Ocean Territory. But it was a big one.
Schram, geotechnical department manager of CTL Fort Collins, also had to consider seismic activity, a jungle landscape that made it difficult to mobilize, and highly complex geological conditions that were markedly different than in his native Colorado.
“The location presents a work environment unlike anything in the world,” said Schram. “But with CTL’s experience on the island and our broad range of geotechnical expertise, we were able to design a winning solution.”
Schram and his team were uniquely qualified to overcome the geotechnical challenges presented by Diego Garcia, an atoll located 1,000 miles from the nearest continent, with soil that consists of hard fossilized or eroded coral. Nearly 20 years ago, CTL’s Jeffrey Groom had consulted with the U.S. Navy and contractor San Juan Construction to design and test materials for a new runway at the naval base, which supported military operations at the height of the war in Afghanistan in 2002.
This time around, Groom stayed stateside and helped Schram manage the logistics of getting the drill rig – and all other drilling equipment – to Diego Garcia. The trip involved a truck ride from Sacramento to Oakland, where the gear boarded a container ship to Singapore. Finally, the rig and other equipment were sealifted to Diego Garcia.
Once the job site was established, Schram got to work with his soil testing. In some areas, the subsurface conditions are soft, liquefiable sand and gravel made predominantly from eroded coral. In others, they are fossilized coral, an extremely hard material. To accommodate these conditions, CTL employed a mud-rotatory technique to log the subsurface conditions and collect samples for testing. Mud-rotatory involves the use of a bentonite slurry mix that is pumped into a casing to help advance the drill bit and keep the hole from collapsing.
After clearing the coconut-tree-laden jungle, CTL teamed up with a local contractor to help lay down mud mats on the soft, saturated soils to allow access to the boring locations. Water was pumped as far as 100 yards to create the bentonite slurry mix.
CTL’s investigation included evaluating eight locations as sites for 13-meter antennas and one site for a 34-meter antenna. The antennas will be covered with radomes – structural, weatherproof enclosures that protect the antennas without attenuating the signals. The radomes are constructed on 10-foot-high ring walls.
The foundation had to withstand a range of loads. Each 13-meter antenna will have a maximum deadload of approximately +50 kips with the radomes weighing approximately +21.7 kips. The 34-meter antenna will have a maximum deadload of approximately +1,072 kips, and the radome will weigh approximately +36 kips.
Due to the loads and liquefaction potential of the softer soils, CTL recommended deep foundation systems, including auger-cast piles, driven piles and Franki piles, which are high-capacity, cast-in-place deep foundations constructed using a drop weight and casing. These arrangements were not only the best for the geologic conditions, but also deep enough to help mitigate movements due to seismic activity. The last major earthquake near the island, back in 1983, registered magnitude 7.7.
“Success on this project was only possible due to the depth and breadth of experience on the CTL team and the support from our client, the U.S. Navy. While the island conditions are extraordinary, we truly enjoyed taking on the various challenges,” said Schram.
Before heading back to the U.S., the CTL team was asked to spearhead two additional projects: a wind farm that will produce electricity for the island and a pump station to extract cold ocean water, which will act as a coolant for the naval base. CTL may be back to drill, this time on a barge along the pipeline from the pump station to the ocean.
“We hope to explore new projects on Diego Garcia that rely on past experience but bring new questions to challenge our engineering chops,” said Schram.
CTL wrapped up its work in February 2019, about eight months after it began. Even when the job was finished, logistics were a factor – the drill rig and equipment took an additional four months to make their way back to Sacramento.
Spencer Schram is geotechnical department manager of CTL Fort Collins, one of CTL|Thompson’s seven offices in the Rocky Mountain West. He is a Professional Engineer with expertise in foundation and pavement design, slope stability, heave estimates, seepage/permeability, and subsidence estimating. He has consulted for clients ranging from residential and commercial development, roadways, bridges, dams/reservoirs, landslides, onsite wastewater treatment systems, retaining walls, utilities, and more. In addition to his wide-reaching expertise, Schram helped CTL|Thompson become the first IAS accredited laboratory to test helical piles.