3D laser scanner survey aids design of Rinconda Water Treatment Plant

Bill Shadel and Phuong Vu
Senior Engineering Technicians
Santa Clara Valley Water District
San José, California

The Santa Clara Valley Water District provides the wholesale water supply and flood management to the nation's top high-tech area, Silicon Valley. Of the district's three water treatment plants, the Rinconda Plant is the second largest with 80 mgd peak capacity. It was completed in 1967 at the west foothills of Los Gatos. Since the creation of the original plans there were several changes to the plant resulting in many sets of drawings. With Rinconda serving a very large area in Silicon Valley, further expansion of the plant was scheduled to take place. In conjunction with the work, district staff and consultants realized the need for a current, comprehensive plan of the plant and it was decided that the district would produce this new set of plans as 3D AutoCAD models that would lend itself to future updates. A further benefit of the creation of a 3D model of the plant was to be the utilization of the model as a training tool and also as a guide in emergency situations. This later aspect became more pronounced after September 11, when awareness of the safety of the nation's water supply received added attention.

With the water treatment plant serving a very large area in Silicon Valley, the need to keep the plant on-line and finish modifications during the winter months was of utmost importance. Work needed to be completed when redundant processes could be upgraded and brought back on-line before the summer demand would require the whole plant to be operational. With this in mind, staff felt the need to take additional steps to ensure accurate plans and reduce the chance of any delay.

The Technical Service and Design Units within the district decided to purchase a 3D laser scanner and associated software to facilitate the accuracy and timeliness of the survey. This was definitely a daring step, as 3D laser scanning as a means of surveying large facilities is a newly-emerging technology that came in vogue only in the last two to three years. It involves a 3D laser-scanner head about the size of a computer monitor, which is set on a tripod and connected to a battery pack. The laser emits a visible green beam that is moved both vertically and horizontally by a set of mirrors. The device captures the spatial location of points at 1,000 point/second. A typical scan takes 5-10 minutes and yields about 300,000 to 500,000 survey points which are displayed on the system's laptop screen as "point-clouds." The scanner is typically moved around in order to capture as much of the plant as possible from many different angles. The system's software is used to merge these separate point-clouds into one large "master" that can contain more than a million survey points.

Because points are very densely spaced, point-clouds tend to look like color photographs on the laptop's screen. However, unlike photographs, these clouds represent 3D reality with 6mm or better accuracy. Thus, one can turn around the cloud to observe the plant from different angles, take measurements from point to point or pipe to pipe, and use the software's ability to convert these point-clouds into CAD objects, such as pipes, valves, flanges, reducers, steel beams and similar. While this system is used in capturing many large objects from ancient temples to rock walls to modern highway bridges, its most effective use is in the plant area, where its accuracy allows surveyors/designers to capture and model even the smallest pipes and conduits with the greatest precision.

After the designers took a four-day training of the use of the scanner and software, they were ready to attack the project. A crew of two designers spent five days in field scanning, collecting 28 scans from the upper level and nine on the lower level of the plant. Although there were some hard-to-reach places, the crew successfully avoided any scaffolding or climbing that would have been necessary with other methods. First, they used styrofoam balls as targets to aid the assembly of individual scans. Later, they found that it was more effective to use flat targets. They also began to involve the district's surveying department in surveying targets with total stations, that helped to orient the data into the plant grid coordinates.

As it was the same crew of designers that undertook the surveying (using the 3D laser system) as well as the actual design, they had ample opportunity to revisit the plant to double-check measurement for accuracy. The crew encountered a variety of pipes from 84" mains suspended as high as 20' to very small conduits. As this was their first large scanning project, the resulting model was very detailed, registering not only major piping but also minor pipes, electrical conduits, pipe hangers, and light switches. They even modeled the face of some pressure gauges with actual numbers.

As a result of the work, the designers gained an intimate knowledge of the existing conditions. Many of the raw scans were tied into the 3D model to illustrate potential issues. They also noted several discrepancies to existing older drawings.

There were additional benefits of this unique approach: the consolidation of multiple as-built projects and modifications into one model; the ability to extract point data from scan clouds for further study; the visualization capability of the cloud; the ability to extract 2D sections from any location; and finally, the safety offered by remotely surveying dangerous areas.

With Phase 1 of the project completed and constructed, SCVWD is currently working on Phase 2 models and final documentation, again using their newly-won skills with 3D laser scanning.

To reach Bill Shadel, call (408) 265-2607 or send e-mail to w_shadel@scvwd.dst.ca.us; to reach Phuong Vu, call (408) 265-2607 or send e-mail to phuongvu@scvwd.dst.ca.us.