When the Arecibo radio telescope collapsed on December 1, 2020, plans were already underway for a cable replacement program for the 57-year-old facility. What began in August 2020 as our engineering consult for a cable-socket failure rapidly switched to site stabilization, condition and damage assessment, post-collapse remediation and recovery, and a forensic investigation to determine the underlying causes of the collapse.
The Arecibo radio telescope comprised a spherical reflector that covers an area nearly equivalent to 11 soccer fields, with 18 cables (from towers to instrument platform) suspending an instrument array 150 meters above. In the 1990s, six of the 18 cables were added to support a 300-ton increase in instrument load. Over the years, cables were inspected and one (a back stay, which supports one of the three towers) was replaced in 1981.
In August 2020, replacement of a single cable was planned and proposals sought for the work. Just prior to notice of the award, a cable released from its socket, putting the replacement program on hold while the project shifted to emergency assessment to understand the effect of the cable loss on the entire structure.
We were initially asked to support emergency construction services by one of the contractors seeking to perform emergency repairs. At the request of the observatory we were instead retained by the observatory operator (University of Central Florida) to determine the adequacy of the damaged structure and provide design of repairs. At this point, Thornton Tomasetti became engineer of record for emergency repairs, construction engineering, and final repairs.
To design the repair, we conducted a structural assessment, refined models of the facility and factors of safety and produced construction documents. A key approach was the design of a “cable arresting system” - if a cable broke while crews were working on the structure, the arresting system would allow it to slide out slowly, rather than spring out, flail around and possibly hurt someone or damage other structures.
New cables were ordered and enroute to Puerto Rico when a second cable snapped on November 6. This dramatically increased the stress on remaining cables, making most locations of the site unsafe for workers. Our original repair plans would no longer work since the site was now more precarious.
Could Arecibo be Saved?
Collaborating with the contractors, other engineers, the University of Central Florida and the observatory owner, the National Science Foundation, we developed a long list of ideas to increase load capacity and make the structure safe enough to repair. It was a race against time, since component wires of the cables were continuing to break, further elevating the stress on remaining cables. All options to save the structure were long shots, and just before Thanksgiving we advocated taking down the telescope since wire breaks were happening more often due to the higher stress.
During the fourth week of November the project team was in constant communication, logging the number of wires broken each day and assessing the state of the telescope. Hoping for the best and preparing for the worst, we contacted D.H. Griffin, a demolition contractor, to review the situation. On December 1, as our team was driving up the mountain to the observatory, the platform collapsed.
Switching to Recovery
Thankfully, no personnel were near the dish at the time and there were no injuries largely due to the establishment of a safe zone prior to the collapse prohibiting staff in areas of potential danger.
An immediate concern was an area where soil was impacted with a few hundred gallons of hydraulic fluid, which fell 150 meters from the platform to the ground. Thornton Tomasetti, Langan, DH Griffin and CSA Group conducted this remediation, removing debris from the site to gain access to the soil and remove the impacted soil.
Starting in December, we focused on emergency clean-up activities (such as the clean-up of tons of steel that resulted from the collapse and the removal of the rest of the fallen structure), safety engineering, soil testing -- which confirmed no penetration of hydrocarbons to the underlying aquifer -- and began forensic work to determine the cause of the collapse.
The forensic report is due to the National Science Foundation and the University of Central Florida in December 2021.