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GPS lends labor, time and cost savings for Las Vegas Beltway project.The Las Vegas Beltway is a highway project surrounding the entire outskirts of the Las Vegas metropolitan area. Typically, Clark County Public Works (CCPW) has awarded segments of the beltway on a two-mile contract basis; however, for our contract CCPW combined three two-mile segments into a single award. Since it is such a large project, we determined that using GPS technology for surveying was the most efficient and cost-effective way to approach this task. What surprised us is that we ended up using GPS surveying on 80 percent of the job. It proved to us that GPS technology can significantly reduce our costs, while enabling us to stay on schedule when working on major highway contracts.
In addition to the complexities one would expect from a project this size, we had some additional challenges. On a fast track, the project had six separate contractors working at all times; the need to stay ahead of so much ongoing work was one of the factors that led to our decision to depend on GPS. The project included four plan sets of approximately 400 pages from three different design firms. Additionally, two datums were used: NGVD29 and NAVD88.
After reviewing all phases of the project with Paul Burn, PLS, survey director of G.C. Wallace Inc., Henderson, Nev., we chose Trimble (Sunnyvale, Calif.) GPS equipment with the new RoadLink software as the best approach to expedient, accurate results. The software enabled us to take design data from the client right into the field using the Trimble Survey Controller software. The RoadLink software does not create points from the alignment and export them; instead, it allowed us to take the complete alignments and templates into the field. This was useful if offsets needed to be set in the field. The station and offset for any location can also be reported if required.
For a project this size we would normally expect to use as many as six two-person field crews. With GPS, we accomplished this project with only two field crews, augmented with a third crew as needed. Specifically, we used Trimble’s GPS total station 4800 and 4700. Realistically, we feel we saved 30 percent to 50 percent in personnel costs. GPS also has a built-in time savings factor on each task because our surveyors did not need to set up conventionally and traverse.
GPS With EaseAs a complete surveying system, each GPS total station consisted of survey grade GPS receivers, GPS antennas, radio modems for transmitting data between the base station and roving GPS receivers in real time, and the TSC1 data collector with Trimble Survey Controller software. Our survey teams used the Trimble 4700 real-time systems for all the control work. One receiver is used as a base station; the other as a rover to locate and navigate to survey points. Vectors accurate to a couple of centimeters are computed between the fixed base station and the rovers, using Real-time Kinematic (RTK) GPS surveying.
The 4800 unit was used mostly for construction staking as it has all of its rover components on a single lightweight pole. This feature eliminated the need for a backpack and cables strung between the surveyor and the survey pole. For the user, the freedom of movement helped to maintain a rapid pace for laying out and setting the construction points.
One of the main advantages of using GPS is that slope staking on a larger scale is much faster than with conventional methods. Due to the user-friendliness of the TSC1 software, slope staking is easily accomplished with a few keystrokes. Additionally, we no longer had to worry about onsite control being destroyed or construction crews getting in the way of our backsight targets because our base station was set up two miles from project center! We were in constant radio contact with the base station, even if there was no visible line of sight between base station and rover.
A particularly useful technique with the 4700 is mounting the GPS antenna on the outside of a truck. In this configuration, we maintain lock with the GPS satellites as we drive from one end of the project to the other. This technique allows response to the contractor’s requirements project-wide, saving the time necessary for a new setup as would be required with conventional instrumentation.
Since the GPS system measures a relative base line expressed in the WGS84 datum, and differences are also with respect to the WGS84 ellipsoid, it was necessary to calibrate the GPS to the local site coordinate system at the start of the project. As two datums were used on the Las Vegas Beltway, two calibrations were computed. The NGVD29 calibration used only an inclined plane in the vertical, while the NAVD88 projection used a combination of an inclined plane and geoid modeling. The use of geoid modeling in field operations allowed us to achieve more accurate results in the vertical orientation, as the local surface of the gravity field was taken into account. This surface was required to accurately transform GPS ellipsoidal heights to local height datums established by gravity-based instruments such as levels. The Trimble Survey Controller software allowed a high-resolution geoid model to be taken into the field for real-time leveling with GPS.
As with all GPS applications, results are only as good as the control network projection upon which they are based. Basic horizontal control was pre-existing on the site and was checked by RTK methods. To meet the high precision required for vertical control, a level network referenced to external bench marks was established using digital bar code levels. This network included a number of selected horizontal control points located inside and outside the project area. These 3D control points were used as the basis for the site control.
Given the size of the project area and the fact that the controlling record surveys were performed by two other firms whose field methods were unknown (whether conventional or GPS), we decided a mean horizontal residual of 0.060 feet with a maximum of 0.101 on one point was within acceptable tolerances. The mean vertical residual for both vertical calibrations were also acceptable. It is interesting to note that the NGVD29 inclined plane calibration yielded a mean vertical residual of 0.055 feet using seven vertical control points. The NAVD88 calibration using the same seven control points and implementing a combined inclined plane/geoid model resulted in a mean vertical residual of 0.026 feet with a single maximum residual of 0.049 feet.
A true testimonial of our work came when a competing firm, working in the same area, checked some of our marked elevated construction stakes. They had done independent differential elevations from outside the work area and confirmed our non-control elevations at ±0.03 feet.
From Office to FieldWe used a hand-held TSC1 controller running Trimble Survey Controller software to provide system control as well as collecting and managing data from both GPS and conventional survey instruments. Using the same controller for both GPS and conventional observations gave us the same user interface in the field. It also allowed us to upload and download data to and from both types of equipment from the same software package. The Trimble Survey Controller can switch between GPS and conventional total stations in the field, so the entire job file, with all alignments and templates, etc., was transferred and work could continue using the same project file. The RoadLink software module of the Trimble Survey Office package enabled us to easily import the alignments and templates, which were then uploaded to the TSC1 for use in the field.
The RoadLink software has some very useful features, which simplified the job. While we would normally expect two or three people in the office conducting calculations, we needed only one PLS in the office to oversee all aspects of the project for field data preparation (not to mention the political aspects of discussions and meetings).
We first assigned horizontal alignment for a road, which can be either entered by hand or imported from AutoCAD/Softdesk. Next, we based the vertical alignment input on the stationing of the horizontal alignment, which is taken directly from the profile sheets of the plans. Lastly, we constructed and assigned the design templates of the road (or channel, or anything with an alignment) at the corresponding stations. Although it takes a little time to model at first, with practice it quickly becomes easy. The training classes offered by the vendor are outstanding for bringing the operators up to speed.
A good feature of the RoadLink software is that the templates can include a varying number of elements unlike the majority of available roadwork software packages. This allows the use of templates in situations such as road gores and in channels that transition from rectangular to trapezoidal. The RoadLink software is also excellent for making changes to the original plans quickly and creating a new file for the field. No time is required to recalculate individual points for the field crews.
We found that our familiarity with construction plans, AutoCAD design files and Softdesk helped us save time with Trimble Survey Office. Softdesk’s advanced design module provided the ability to analyze various road cross sections at given stations. Close attention must be paid to all points of transition, such as the narrowing and widening of shoulders as well as stations of side slope transition. After becoming accustomed to the software, the time necessary to translate from design plans to a file ready for the field is significant. This lends greater savings as the need for the field crews to perform time-consuming onsite calculations is eliminated. For checking field work, if a question arises, all data is right in the TSC1. Back in the office we downloaded what the field surveyors staked and plotted it in AutoCAD to check the accuracy of the points. Time and again this process demonstrated the higher level of accuracy in significantly less time using GPS.
Fort Apache Road BridgeOne of the best uses of GPS on all aspects of surveying was on the structure of the Fort Apache Road Bridge. The bridge is particularly complex due to its width, vertical curve and high amount of camber. GPS was used to establish and maintain control throughout construction and surveying—from bed excavation to alignment and grade for the top of the deck. Although we double-checked using conventional measurements, we found that GPS provided horizontal locations well within tolerance, as the elevations of the points were a function of the centerline profile and only had deviations of a few thousandths of a foot within an error circle of two to four-hundredths of a foot.
GPS was used to horizontally locate 1/10 points (280 points) on the top of the web walls for deck construction. We also used it for horizontal location of additional points along tops of the web walls at 30-inch intervals for bidwell screed grades (950 points). Horizontally, these were well within tolerance, and the optical leveling was performed due to the stringent vertical tolerances. As a rule of thumb, elevations that can be established using an optical total station can usually be established using GPS, whereas elevations that require the use of a level will generally still require this approach. However, in these circumstances, the horizontal positioning is achieved much faster using GPS than conventional methods.
GPS and the Fine PointsSurveying was conducted for rough grading and slope staking of subgrade (redheads) and finish grade bluetops of gravel and cement on the mainline beltway, ramps, perimeter streets and frontage roads. The software’s ability to navigate to the catch point, as well as the designated reference (offset) point for a given station, saves a large amount of time. It really is a case of going straight to the point, in contrast with the laborious guess-and-check methods that have to be used with optical instruments.
With the RoadLink software there is no need to take multiple shots to fix the location of the slope stake as in the conventional procedure. Thus a savings of at least 30 percent is realized in the field layout time, even with a field crew half the size. The information lath set at an offset to the catch point is then given the data from the template back to the centerline. We verify the accuracy of this data, plan centerline profile grade on our cut sheets and calculate back from our observations. We have successfully used this on roads with 30-foot cuts to roads with 40-foot lifts.
Typically, we use the RoadLink software with GPS for layout of redhead and bluetop horizontal locations. Then we follow with differential leveling to obtain the precision required for this phase of construction. For many subgrade and rough grade applications, GPS elevations will be well within tolerance.
We also used GPS for storm drains and drainage channels within the project. This project has approximately six to seven miles of storm drains ranging from 24-inch round concrete pipes to 23 x 12-foot channels. We used the RoadLink software for all such alignments. This allowed us to see cut/fill information at any offset with the push of a button. We staked the channels in a different way compared to typical construction procedure. Rather than only giving centerline and cut/fill, which would be very easy with this software, we used templates that start at profile invert grade, then to subgrade centerline, then to the edge of the structure, then include a work area, and finally side slope at the given safety slope ratio to the catch at existing ground. This is fundamentally the same way as with roads. This technique saves the contractor a great deal of grade transfer time and ensures side slopes within a safety range to avoid accidents due to cave-ins.
It is not always possible to acquire GPS satellite signals in deep channels. Some channels were as much as 40 feet deep on this project. Surprisingly, we had excellent success utilizing GPS in the channels up to 25 feet in depth, but it required advance planning in regards to satellite availability. We also attribute this success to the advanced technology in our 4700 units, which reduces the multipath signal reflection from the channel walls.
At times when there was no satellite availability and in the deeper channels, the TSC1 could simply be disconnected from the GPS antenna and plugged into an optical instrument. The entire job was transferred and the work continued with the optical instrument. The TSC1 is able to interface directly to conventional total stations made by all the popular survey manufacturers. We find that this feature supports both fast survey work and seamless data flow between the field and the office. At the end of the survey, the single job file containing GPS and optical data could be downloaded into the Trimble Survey Office software, eliminating the need to use multiple download and processing software.
For utilities that are tied to the station and offset of a defined alignment such as a road, the layout is very simple. The RoadLink file for the particular alignment was used. The offset of the particular feature was entered for navigating to that exact location easily. The same approach was used for curb, gutters and traffic barriers.