In the Pipeline
January 27, 2009
The United States has a voracious appetite for energy, especially alternatives to costly conventional resources. To meet this demand, utilities must develop new infrastructure to transmit energy over long distances--projects that often require surveying and engineering expertise. While this trend is creating some new opportunities for surveyors, working in this field also presents some distinct challenges.
BL Companies has learned this truth firsthand. The integrated services firm, which is headquartered in Meriden, Conn., has been involved in surveying and engineering projects for utilities for the past eight years. In August 2008, BL Companies was contacted by a local natural-gas supplier to create base maps and design prints for a new three-mile-long natural-gas pipeline near Riverhead, N.Y., which would offer residents an alternative to high-priced home heating oil. It was a standard utilities project with one significant catch--the gas company needed to supply natural gas out of the new main by Jan. 1, 2009.
To expedite the project, the gas company agreed to three interim deliverables for permitting, bidding and construction rather than one final set, which would allow teams to begin work on other parts of the project before the entire plan set was completed. The permitting set of deliverables required minimal field work and only a plan-view layout of the proposed line. Early delivery of this set would allow the local jurisdiction to begin its review for a road-opening permit.
The bid set added a profile depiction of the new line and, most importantly, the expected horizontal and vertical gas-main offsets around other utilities. Offsets come at a high price when discovered in the field during construction, so documenting them in the competitive bid process would allow market forces to bring the best price to the owner. Early delivery of the bid set would also allow bidders to begin the lengthy process of determining quantity takeoffs. Finally, the construction set accurately located the utilities and refined the number and shape of the offsets so that unexpected utility conflicts would be minimized.
Most new subsurface utility projects involve research into three key areas: existing utilities, property boundaries and pavement construction. Accurately mapping existing utilities is probably the most important aspect of controlling the cost of a new subsurface utility project, so obtaining existing utility records is key. However, it can sometimes take several weeks for existing utility owners to respond to records requests. To keep the Riverhead project on schedule, the BL team began on day one to identify and contact other utility owners in the area to request their records. Meanwhile, the firm was able to begin work on nonrecord-dependent activities, such as control and photogrammetric mapping.
Aerial photogrammetry is often a necessity on utility projects because it can deliver miles or acres of topographical data in a fraction of the time required using conventional ground survey methods. However, accuracy is always a concern. Jennifer Marks, LS, director of land surveying and also a principal in the New York City office of BL Companies, generally recommends photogrammetry suitable for 1"=20' scale plotting with 2-foot contours as the preferred standard for transmission projects where a subsurface utility will be installed. “This is really the accuracy the engineers need to make detailed decisions on how to route the new line and how to avoid other objects or right-of-way lines,” Marks says. “If the client chooses to plot the drawings at a 30-foot or 40-foot scale, this can always be done provided the background photogrammetry was created at a more accurate scale.”
On fast-tracked projects, however, obtaining such accuracies is not always practical. To expedite the process for the Riverhead project, BL turned to local photogrammetrist Geomaps International Inc. of Bethpage, N.Y. Through a combination of library photography and new mapping, Geomaps was able to provide overall project topography at a 40-foot scale within several weeks. BL then used its survey crew to locate the existing utilities and achieve subcentimeter accuracy.
According to Marks, surveyors must use their best judgment when employing library maps. In urban areas, where improvements are always being made, mapping older than one year may not be appropriate. In more rural areas, such as the Riverhead region, mapping older than three years may still be suitable. “In any case, a fresh field review to supplement the library photography is always recommended,” she says.
Balancing Quality Level with Cost
The natural-gas line being installed in the Riverhead project was a 20-inch welded steel pipe. Anytime this line encountered an existing utility, it had to be vertically offset above or below the other line, and achieving these offsets required prefabricated 90- or 45-degree fittings. If these fittings were not on the job when they were needed, construction would come to a halt or fittings would have to be manufactured in the field--both highly expensive options. Accurate mapping of the existing utilities was therefore essential to minimize or eliminate unexpected utility conflicts.
As a provider of subsurface utility engineering (SUE) services (see sidebar on p. 28), BL was required to map the existing subsurface utilities in the Riverhead project in accordance with the CI/ASCE 38-02 guidelines. To meet the deadlines for the expedited phases, BL decided to provide Quality Level D data in the earlier permit and bid-set drawings and Quality Level B data for the final construction drawings.
Like most utility records, the records BL obtained from the other utility owners were fairly accurate with regard to the absence or presence of lines in the vicinity as well as the sizes of those lines. However, utility records are not intended to depict the true positions of the lines, and positional inaccuracies can range anywhere from several feet to several hundred feet. By using the physical features provided by the photogrammetry, BL was able to correlate the information from the records well enough to complete the permit and bid-set drawings. Quality Level D mapping progressed quickly and allowed BL to generate the permitting set of deliverables within two weeks of the project’s start date and the bid set within three weeks of the start date so that the permitting authority and bidders could begin their reviews.
While the Level D data allowed BL to predict the number of gas offsets around other utilities, it was not accurate enough to say exactly where the offsets would be. Such inaccuracies can change, for example, the proposed position of the new utility from one travel lane to another when the utility is located under roads, which can drastically affect the construction schedule and budget. To avoid these problems, a higher quality level was needed. BL sent utility designators into the field to apply electromagnetic and radar techniques in order to detect and mark the existing utilities. After the designators had been in the field for several days putting down paint marks, BL’s surveyors came in to locate the marks for ultimate plotting on the drawings to achieve Quality Level B.
Why not continue to Level A? One of the most important services of an SUE provider is to help the owner achieve a positive return on the SUE investment. This means that the cost of SUE should not exceed the cost (measured both in dollars and less tangible costs) of simply moving the utility that is being located. On this project, BL determined that Quality Level B mapping was accurate enough to allow the firm’s engineers to design gas offsets around existing utilities with confidence in the number of existing utilities that would be encountered in their horizontal positions. Once the gas offsets were designed and depicted on the drawing, the number of fittings and the general depth of the offsets could be taken off by the bidders. However, because open-cut construction was being used on this project, the contractor had the opportunity to uncover the existing utility crossings and measure their depth. He could then determine whether to offset the new pipe above or below the existing utility. While this option might represent a slight deviation from the design plans, the change would only be to the length of the straight pipe between the fittings as the offset would have to be either deeper or shallower. Since straight pipe is cut to length on the job anyway, this change had no effect on the schedule and little effect on the budget. Trying to control this budget effect by providing the marginal improvement of Level A data in design would not have justified the cost of improving to that quality level.
It is important to note that Level A was not viewed as cost effective on this project because offsets could be tweaked easily in the field with little overall impact. However, on many other projects, Level A is absolutely essential. Three examples are in the installation of electric transmission lines, gravity sewer systems and trenchless installations. Custom electric cables must be pulled through predesigned duct banks where bending radius and pulling tension is critical; vertical offsets are not simply “made” in the field. Gravity sewer lines must have consistent slope and cannot be moved arbitrarily in the field up or down to avoid other utilities. Trenchless installations, such as horizontal directional drilling (HDD), pipe jacking or microtunneling, push or pull new pipelines past existing utilities without exposing them in the process. The risk of damage to these existing utilities is very high on these projects, so their true locations should be surveyed accurately first, including the appropriate use of test holes (typically vacuum excavated). In these examples, the cost to the schedule and the budget of unexpected utility conflicts is a deal-breaker, and Quality Level A is a must.
The Tools of the Trade
BL’s surveyors have many tools at their disposal that allow them to measure large-scale projects accurately, quickly and cost-effectively. In the Riverhead project, establishing horizontal and vertical control and getting that information back to the photogrammetrist was the main priority. Control was also needed for the ground-surveyed field review and utility locations.
The first tool out of the box was GPS. BL’s three-person survey crew decided to run a static baseline using six main control points along the three-mile project corridor. These static baseline control points would provide permanent and accurate values that could be used later for construction. Locating them throughout the project corridor ensured that there was always a point relatively close to the work should traditional survey methods need to be employed. Tree canopy and utility congestion both played roles in the selection of sites for control points. Static observations of 30 minutes were made on these main control points with the company’s Leica 1200 series GPS equipment, and control was post processed to two different CORS as references for redundant checks on the control points. All GPS control work was completed in two days. After the main project baseline was developed, RTK GPS methodology was used to shoot the topographic detail.
BL surveyors then turned their attention to locating the utility marks put down by their SUE technicians (using RTK GPS) and editing the aerial base map. With the BL base station set on a primary control point, two crew members used rovers to locate structures such as catch basins, manholes, valves, utility paint marks and flags. The third survey crew member followed behind the GPS rovers and used a robotic total station to locate objects that were obstructed by tree canopy and could not be located with RTK GPS.
To streamline the amount of office work required, the field crew used automatic line coding. This technique allows the office software to do a preliminary plot of the surveys automatically, including connecting points that represent lines and using the appropriate symbols for point and line features. BL has developed an extensive CAD standard specific to utility surveying. All crews use line coding in the field, so a nearly complete picture is produced when the data are imported into the CAD environment. This capability greatly reduces drafting requirements for long, linear projects such as Riverhead, where many of the utilities run the length of the project.
Field crews returned to the office over the weekend and uploaded all the utility locations to BL’s server so that drafters could begin utility drafting. While this drafting was under way, field crews returned to the field to review nonutility-related features. Thanks to the combination of these techniques, all field locations were completed in roughly seven field days.
For the final deliverables, BL employed yet another timesaving measure. The firm used AutoDesk’s AutoCAD Civil 3D Land Desktop Companion 2008 software to create a digital terrain model (DTM) or “surface” from the points provided by the photogrammetrist. Because the gas company’s specifications required plan and profile drawings, BL then used the surface to generate a profile of the ground over the proposed natural-gas main. Later, when the gas company decided to move the proposed main farther from the edge of the pavement than originally planned, BL simply generated a new profile through the surface at the new pipeline location. Creating a surface is tedious, but it can be a tremendous timesaver later on, especially for projects where changes are anticipated.
Surveying for the utility industry today is fast-paced. In order to keep up and provide great service, today’s surveyor must be well equipped with the ability to map aerially; use the latest survey techniques, including GPS and robotic total stations; and take advantage of modern software, including surface modeling. To protect themselves and provide the best possible product to their client, surveyors should understand SUE and form an alliance with a SUE provider. And finally, surveyors must be good project managers, communicating with their clients in order to fast-track projects and keep several critical paths in play at once.
By following these strategies, BL Companies was able to complete the work for the Riverhead utilities project on time and on budget. “The utility industry gives us an opportunity to apply many of the lessons we’ve learned over the years both in surveying and project management,” Marks says. “It provides a great deal of satisfaction for us and our clients.”
Sidebar: Subsurface Utility Engineering (SUE)Subsurface utility engineering (SUE) is an engineering process that combines civil engineering, surveying and geophysics to acquire, characterize and manage the below-ground utility information that is required for excavation plans. The process originated with several state departments of transportation in the 1980s but has changed considerably over the past few decades--perhaps most notably in 2002 when the American Society of Civil Engineers (ASCE) published and distributed “CI/ASCE 38-02 Standard Guidelines for the Collection and Depiction of Existing Subsurface Utility Data.” This standard formally defined SUE and established basic guidance for collecting and depicting information regarding the quality and reliability of utilities on maps and excavation plans.
Under the CI/ASCE 38-02 SUE standard, utility mapping is categorized into four distinct Quality Levels designated as A, B, C and D. These are typically presented in reverse order because they build on each other. As the mapper moves up the quality-level ladder, the map reader gains confidence in the horizontal and, eventually, vertical position of the existing utility line drafted on the map.
- Quality Level D – Existing utilities are depicted solely from utility records. No field work is done to locate evidence of the utility.
- Quality Level C – Surface utility hardware (valve covers, manholes, etc.) is located by the surveyor and plotted on the map. The utility records are then “reconciled” by the surveyor with the hardware locations so that the lines pass through the hardware in order to produce a more accurate map than Quality Level D.
- Quality Level B – Surface geophysical methods of detecting the subsurface utility lines are employed to locate the utility lines between the hardware. These locations are marked on the ground with paint or flags in a process known as “designating.” The surveyor then reconciles these paint marks or flags with the data from the prior two quality levels to further improve the accuracy of the map.
- Quality Level A – Test holes are excavated at strategic locations to physically uncover the existing utility lines. SUE providers typically excavate test holes with vacuum excavation, a minimally invasive technique that digs a small-diameter hole (usually smaller than 12 inches) straight down to the desired utility. (Vacuum-excavated test holes are much less expensive than traditional backhoe-excavated holes and are smaller, which minimizes pavement damage.) After the existing utilites have been uncovered, the surveyor locates their true horizontal and vertical position and performs the final reconciling and plotting operations.
In 1999, FHWA commissioned Purdue University to perform an independent study of the effectiveness of SUE. The study found that, on average, SUE returns $4.62 to the owner for every $1 spent on this process. It is likely that the ASCE standards have further increased this value proposition. More information can be found on FHWA’s Web site at www.fhwa.dot.gov/programadmin/sueindex.cfm.