Divided by Two

November 20, 2000
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Hilly subdivision is a challenge to engineers and surveyors.

Figure 1. Final proposed siteplan showing proximity of existing developments and golf course.
At first glance the Pines Subdivision located in Prescott, Ariz., looked like just another mass graded patio home project. With an average lot size of about 6,000 sq. ft., the Pines has 216 lots on 52 acres of land for a density of 4.2 units per acre. Not too bad if the site was relatively flat. In fact, this kind of housing concept has been successful in the Phoenix metropolitan area.

Prescott, however is much different than Phoenix. Located in Northern Arizona, Prescott is a very hilly, and at times mountainous, city. Elevations hover around a mile above sea level (which is?); the Pines is no exception. Ranging from a high of 5,298 feet and a low of 5,170 feet, the Pines has a not-so-subtle 128-foot elevation drop. That doesn’t leave a lot of room to make up grade between the individual pads for each lot. Compounding this problem is the proximity of several existing developments: three golf course fairways, an apartment complex, a patio home subdivision and two major roadways, all of which border the project (see figure 1). The Pines had to work in seamlessly with all of these existing and proposed developments. These challenges made it important to have engineering software.

Figure 2. Plan view of profile to cut.

Crew #1: The Engineering Crew

When 216 individual pads have to tie together with no greater than a 3:1 slope between each of them, it is important to visualize their relationships to one another during the design process. With GEOPAK, we were able to graphically show finish contours without creating elements within the CAD file itself. This greatly increased the speed at which changes could be made. When modifying pads horizontally, vertically, or in size, GEOPAK immediately updated the graphical contours with each change. This was imperative during the early stages of planning and design. If a pad is moved too high or too close relative to a nearby pad, you immediately see the fill slopes chasing onto the other pad or onto a nearby fairway. This was a very powerful tool when dealing with the developer of the project. When given the all too familiar “what if” question, it was simple to sit in front of the screen and show them the answer. Raise this pad and you chase onto the apartment project; lower that pad and you cut into the golf course grading. By going pad by pad or in groups of pads, I was able to show how the project would look when finished. Eventually, the developer began to rely on GEOPAK, not planners, to dictate the final layout of the project.

Just as important during the design of the Pines was the amount of earthwork involved. At the end of the day over 250,000 cubic yards of dirt were moved on this project for an average of 1 cubic yard moved per 1 square yard of land. Needless to say, the balancing of the project became very important. After every change was made to a pad or road, a new earthwork quantity was calculated within seconds. Quite often was the case where the developer said, “That’s perfect. Don’t change it.” We then responded, “Sure, but now you’re short 20,000 yards.” Working simultaneously between modifications of pads and calculating of volumes the site eventually balanced, while pleasing the client.

Another great feature of GEOPAK is its ability to perform dynamic slopes. We used this on several occasions when a pad required that a cut slope go from a 4:1 to a 3:1, and then maybe back to a 5:1. A good example of this would be when the developer liked the top of the cut slope to match the boundary of an adjacent subdivision. This is impossible to accomplish on a hillside subdivision unless you are capable of designing dynamic slopes. By varying the angle of the cut, you are able to dictate where the top of slope will fall. Granted, in the field it is simple to just stake a line showing the limits of cut for the contractor. For the client it was very important to know how that cut would look for landscaping and selling purposes.

A tool we found very useful in GEOPAK was the “Site Profile” tool. Quite often the client asked for a profile spanning several lots at a time to get a better understanding of the amount of cut or fill taking place (see figure 2).

GEOPAK allows you to extend a single line or several lines linked together showing the finish and existing ground profile as you go. This also proved to be a great asset for the contractor. We were asked several times to produce profiles at key locations in the project. These profiles helped the contractor decide where to attack a large cut or fill section.

The most useful tool, albeit one of the simplest, we used was the “Elevation Differences” tool. Basically used to produce a cut-and-fill grid, it was repeatedly utilized by the contractor for the planning of the mass grade portion of the project.

Figure 4. Typical perimeter line between cut and fill areas. We would stake this "daylight line" for the contractor during the early stages of construction.
With the largest cut on the project at 28 feet, and the largest fill at 27, it was important that the contractor knew exactly how much he had to move and where (see figure 4).

Crew #2: The Surveying Crew

When the time came to actually build the project, a meeting was held between the engineer, surveyor and contractor. This was not a normal pre-construction meeting, but one that focused on “how the heck are we supposed to build this?” Looking at 216 terraced pads averaging 6,000 sq. ft. in size it is easy to understand the contractor’s concern. In addition, the equipment could not stray past the boundary of the project and we had three months to do the mass grade portion.

The first plan was to stake a cut-and-fill grid across the entire project. This would have worked great on a large project containing only a few pads and constant slopes. GEOPAK is capable of exporting a finished TIN file, which could then be imported into our survey instruments. This would have allowed us to stake the finish elevation at any point on the project. Unfortunately, the pads were so small that the grid needed would have made the site look like a forest of lathe and ribbon, not to mention that with one pass of a scraper or bulldozer, an entire re-stake would be needed. The second idea put on the table was the use of a modified slope staking method similar to that used on a roadway. This was quickly discarded when we realized that the stake would need to be 8’ tall to show all the data needed to cut across a hammerhead cul-de-sac.

Our final solution was a combination of the two ideas. Large cut-and-fill areas would be staked on the perimeter only during the first stages of construction. This would give the contractor stakes to use as bench marks while being set far enough away as to not get wiped out during construction (see figure 5). Once the contractor thought he was close, within a foot or two of the final pad elevation, our crews then set slope stakes on common lot corners, supplying data for each pad on either side of the stake. After a few days the contractor was able to tell us more optimal locations for stakes based on the dozer or scraper operators perspective.

There was a constant “tweeking” of methods during the project to improve efficiency. This was accomplished through constant communication between the project superintendent and the survey crew chief.

Since this was our first large-scale job using GPS, many days were spent checking control and comparing results with total station data. The Pines is a small development in a much larger master-planned community that measures 1,200 acres in size. Our crews had previously set up an extensive and very tight control network throughout the project.

Nevertheless, our first priority was to establish a control network around the perimeter of The Pines using total stations and an automatic level. This provided a network for the GPS system to localize on and a means to replace control within the jobsite when needed. As it turned out, approximately 12 control points were compromised or outright removed during the course of construction.

Our major concern with The Pines was the staking of vertically sensitive areas of the project with GPS. Besides the mass grading portion being critical, very flat sewer mains were designed throughout the site with slopes at 0.40 percent and lying over 20 feet in depth. The key to this task was to check bench marks on the job frequently while carefully monitoring the satellite geometry throughout each day of staking. In addition, we regularly checked the almanac from the GPS receiver for scheduling purposes. This provided a timetable of when we could and could not expect to stake accurately with GPS. After a short time of comparing stakes set by GPS and those set by a total station, we had no doubt that GPS could be used to stake any improvements on the project.

The two-man survey crew worked a total of 220 hours. Their equipment list for The Pines project is shown below:

  • Leica TCA1800 Auto-tracking Total Station (Leica Geosystems, Norcross, Ga.)

  • Topcon GTS800 Total Station (Topcon America, Paramus, N.J.)

  • Wild NA2 Automatic Level (Leica Geosystems, Norcross, Ga.)

  • Topcon GP-DX1 Dual-Frequency GPS with OTF for RTK (Topcon America, Paramus, N.J.)

    TDS Husky MP2500 with Survey Pro+ GPS Topo V.4.5.02 (TDS, Corvallis, Ore.)

    Pre-development condition.

    Joining Forces: The Crews Meet

    Earlier we mentioned how GEOPAK helped the contractor schedule the mass grade portion of the project. The Pines contained several areas of both large cut-and-fills. Since time is money, especially for a contractor, we were asked to help the superintendent by computing isolated earthwork quantities in specific areas. For example, there was a large cut located in the center of the project, so large that the entire cut section could not be hauled to one fill location on the project. GEOPAK allowed us to isolate a cut or fill section by placing a fence or element around that area. It then computed only the cut-and-fill quantities within that fence or element. By doing this, we were able to give the contractor cut stakes to produce just enough dirt for certain fill areas. Although this sounds simple enough, the contractor can save a great amount of time if he knows they know the haul schedule for a project before the first stake is put in the ground—especially one as complex as The Pines.

    Post-development condition.
    The Pines Subdivision currently has 2 of the 4 phases of construction complete, including roadways and utilities. All phases of construction have been mass graded to completion. In retrospect, there are not many aspects of the design process that we would do differently the second time around. Now that I know more of the small intricacies of GEOPAK we would be more efficient while designing the site. After all, the longer you use a software application the more you learn the way it “thinks.” GEOPAK has released an upgrade since the Pines project, which would have helped during the design. But, when all said and done, the Pines was a great lesson in new mass grading techniques.

    The many staking methods used on The Pines were useful, and as it turned out, required. No one method could have been used to build The Pines, and it was fortunate that everyone involved understood this from the beginning. Very quickly our crews became dependent on GPS, as did the contractor. In some cases, our ability to put stakes in the ground was tripled. The contractor actually gave the nickname “Rocketman” to our GPS technician because of the speed at which he could put stakes in the ground. With GPS we saved in some cases two or three instrument set ups per day since we could move quickly through the project while maintaining accuracy.

    Note: The GEOPAK features and tools we talked about only scratch the surface of a very powerful program. In fact, they are some of the simplest to use within the software. The portions of GEOPAK we talked about are those that were the most important to all parties.

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