- SPECIAL REPORTS
- THE MAGAZINE
On Nov. 23, 1923, the California Bears opened the new California Memorial Stadium at the University of California at Berkeley (UCB) with a win over visiting archrival Stanford. It was the start of a long and sometimes contentious life for the stadium.
Tucked into the hillside at the mouth of Strawberry Canyon in the Berkeley Hills, Memorial Stadium is considered one the most beautiful settings in college athletics. The stadium, which is listed on the National Register of Historic Places, is ringed by trees and hills and enjoys an expansive view of San Francisco Bay. With a capacity of 71,799 spectators, Memorial has welcomed hundreds of thousands of fans to UCB football games and other events each year. But after nearly 90 years of service, Memorial Stadium needs an update. The stadium has fallen behind the standards for services and facilities available at other large university stadiums, and local residents have long voiced concerns over noise, parking and disruptions. Even more pressing is the fact that Memorial Stadium sits directly astride the Hayward Fault, an active geologic fault that runs along the eastern side of San Francisco Bay.
After years of planning and negotiations, construction work to upgrade the stadium began in November 2010. It’s a substantial project. All seating on the west side of the stadium will be removed and the earth excavated to completely expose the stadium’s west wall and façade. A new press box and club level area will be constructed on the west side of the stadium bowl. The playing field will be lowered by four feet to improve sight lines for spectators, and seating capacity will be reduced to 62,717. Concourses will be widened and concession areas enlarged and improved. Other improvements include new lighting and sound systems, additional restrooms, and improved facilities for disabled persons.
Earthquake mitigation is a central part of the renovation, and the work on the west side of the stadium is aimed at reducing seismic risk and enhancing the safety of the building’s occupants and game day visitors. Rather than try to resist the motion of an earthquake, modern structures are designed to move with the quake. The new press box and club area sits on its own pre-stressed concrete support walls and shock absorbers that enable it to accommodate an earthquake’s coseismic motion. From a structural standpoint, it is effectively isolated from the rest of the stadium.
To further reduce potential damage from the Hayward fault, engineers identified seating sections at the north and south ends of the stadium bowl that lie directly on the fault. Two sections will be removed and rebuilt as surface-rupture blocks. The design for the surface-rupture blocks follows the convention of moving with an earthquake. At Memorial Stadium, the surface-rupture blocks must accommodate horizontal motion of up to 6 feet (1.8 meters), and 2 feet (0.6 meters) of vertical movement. The new sections will be constructed on a bed of sand 30 inches (76 centimeters) deep. A film of plastic sheeting is embedded into the sand. The result is a “slippery” surface beneath the stadium surface-rupture blocks, which allows the blocks to move independently from the rest of the stadium. A series of covered expansion joints will connect the blocks to the rest of the stadium.
Webcor has engaged an array of subcontractors and technical consultants for the stadium. To provide surveying services for the project, Webcor selected F3 & Associates, a San Francisco Bay Area surveying firm specializing in commercial and industrial projects. According to Webcor Construction Manager Victor Elliot, F3 provides a variety of services at the stadium, including construction staking, monitoring, topographic surveys and 3D scanning. Elliot describes the project as a massive retrofit in which a new interior structure will be built, with the west wall preserved as the exterior cladding. “We are basically drilling and attaching the historical wall to the new structure,” he says. “So it was critical to know exactly where that was and how it interfaced with the new design.”
The survey work began with planning to bring in local control. One of F3’s principals, Sean Finn, LSIT, says the firm decided to connect the stadium to existing state plane control on the UCB campus. For the control work, the F3 surveyors used a Trimble 5700 GPS system and Trimble 5601 DR200+ Total Station connected to Trimble Ranger Handhelds running Survey Pro Software. They established discrete targets and marks on the stadium wall and set control points around the stadium perimeter. Conventional leveling and traverses completed the vertical control. Throughout the project, the surveyors check the onsite points and marks against offsite control points.
After tying the stadium to the control and making measurements to the existing walls, the surveyors worked with design and construction teams to make a collective decision on the location of the stadium center point. An axis line, determined from as-built surveys in the stadium’s north and south tunnels, completed the definition of the horizontal coordinate system. All elevations were tied to local benchmarks. “It was complicated,” Elliot recalls. “We’re located on the fault, and obviously there has been a tremendous amount of movement. We had an existing stadium that we are tying into and it was critical that we knew the point of origin. Without Sean and all of his technology, we probably would have missed that boat.”
Finn says that keeping the scans precisely on the control network is a major part of the project’s success. Over the course of the work, F3 teams have collected more than one hundred individual scans; each scan is precisely registered into the stadium control network. The result is a single 3D point cloud of the entire stadium, consisting of upwards of a billion points. The combined data from the two scanners provides a good picture of the stadium as well as the ability to zoom in on areas where more detail or dense data is needed. “That’s the beautiful part of scanning within a project datum,” Finn notes. “There is no reason why you can’t have a global fitting near a hundredth of a foot. It just requires that you have patience and care in your control and in the setup of your scanner. You can capture something and it drops right into the point cloud. If they didn’t have the ground excavated deep enough and it’s now exposed, we can just set up and do a scan, and it drops right in where the other data was. The picture gets clearer and more detailed as the work moves forward.”
The scans at Memorial Stadium must serve many users, and F3 uses the scanners’ flexibility to provide an array of deliverables. Much of the information goes to contractors and other trades for use in manufacturing rebar cages and other components that tie into the historical wall. The various trades develop their own engineering and shop drawings based on the data from F3. For example, several sections of the stadium wall contain windows that are more than 25 feet (7.6 meters) high. To fabricate new glass for the windows, glazing contractors need precise dimensions of the window openings. When the wall is fully exposed, the tops of the window openings can be 60 to 70 feet (18 to 21 meters) above the ground. The F3 crews use the Trimble FX to scan each window opening from the ground, eliminating the need for scaffolding or other access to measure the windows. F3 technicians then create 3D models of the window openings and prepare fully dimensioned 2D drawings for the glass fabricators.
Memorial Stadium illustrates how F3 combines technologies to produce speed and precision. For example, the firm has created a procedure that uses total stations as a quality check for the scanning. On each scanning task, F3 crews use a total station and direct reflex measurements to measure a number of discrete points in the scanned area. After processing the scanning data, the office technicians bring the total station data into Trimble RealWorks to conduct additional checks and quality control.
In addition to supporting the scanning, F3’s total stations play a key role in safety. Because the seating and facilities have been removed from the interior of the stadium, the west wall of the stadium will be almost completely exposed. As a result, the wall must be supported and monitored throughout the construction process. A temporary network of steel columns and beams was placed atop the SAHPC to support the walls during the construction. F3 uses a Trimble 5601 Total Station to conduct weekly precise monitoring measurements to make sure the walls are not moving or leaning.
The original contract for surveying at Memorial Stadium did not include scanning. But after Finn spent time talking with his clients about the surveying requirements for the historic stadium walls, the need for scanning became obvious. In addition to the various contractors and fabricators, data from the scanners goes to structural, mechanical and architectural consultants. Some of F3’s clients are capable of working directly in the 3D point cloud, while others request F3 to handle the modeling needed to produce 3D objects (including surfaces, solids and polylines) for CAD systems. And it is still common for some clients to request 2D drawings in paper or electronic format.
The scanning data has a long lifetime. Following the initial deliverable, it’s not unusual for a client or subcontractor to request additional information or detail. Rather than sending a survey crew back to the site, Finn can simply turn to the point cloud and extract new information from the existing models. It’s an important time saver and can be especially valuable in providing information about objects that have been affected or concealed by subsequent construction. Elliot recalls an occasion when an architect needed additional information related to clearances for egress and requested some 3D polylines. “They asked for very specific locations and elevations of existing conditions,” Elliot says. “The beauty of the scan was that Sean had the ability, if we were looking for more information, to just pull it up from his scan rather than have to revisit the site.”
F3 clients are quickly discovering the importance and value of the 3D information, and Finn believes that scanning will soon become a common tool for many surveying organizations. It’s already a strategic part of F3’s service. The company has made significant investments in the computer systems to efficiently handle the large volumes of data, and Finn expects that revenues from scanning and modeling services will continue to grow. Because surveyors have expertise in measurement, positioning and data management, they are in a good position to grasp the opportunity to provide scanning services. “The combination of technology and surveying is the way it should be,” Finn concludes. “Surveyors can orient a scan to a specific location on the earth. It takes a surveyor to tell the scan data where to go, where it needs to be. In the end, I believe that scanning should and will become an everyday part of survey life. I fully expect the scanner to become just another tool on the survey truck.”
How do the designers know which sections of Memorial Stadium should be built into surface-rupture blocks?The Hayward Fault is one of the most heavily studied fault lines in the U.S. Geophysicists, seismologists and engineers have conducted surveys and studies on the fault for decades, and its location is known with good confidence. According to Dr. Peggy Hellweg, an associate research seismologist at the University of California at Berkeley (UCB), the location of the fault has played a role in campus planning for years.
In 1939, George Louderback, a geology professor at UCB, mapped the fault in preparation for construction of a new dormitory on the campus. He suggested a revised location for the building based on the location and expected movements of the fault. More recently, as part of the planning for the stadium renovation and construction of the SAHPC, consultants made additional studies on the fault. While the stadium straddles the Hayward, the consultants confirmed that the SAHPC would lie entirely on the west side of the fault.
Because the fault creeps about 4 millimeters (0.01 foot) per year, it provides ample surface evidence of its location, including cracks and offsets in curbs, sidewalks and other surface structures. Hellweg said that more than 35 centimeters (14 inches) of creep has accumulated in the stadium, evidenced by distress in walls and offsets in expansion joints. Years of data, including GPS observations and total station trilateration across the stadium bowl itself, provides further confirmation on the location of the fault.