By Paolo Benedetti
This is a story about a job that didn’t happen – not yet, at any rate. It all started when I was brought in to bid on a hillside swimming pool project and ended months later, with the dust still settling and the project scuttled for the moment. And nobody could have been happier with that outcome than I was.
That fact that my client and I decided to pull back and wait is not so unusual, but given the fact that the decision was driven by advanced military navigation technology makes this one of the most unusual situations I’ve ever encountered.
is that, although the project may never be built (or will at the very least be delayed for a while), I’m certain that our firm has come away with a new tool that will, when it’s appropriate, play an important role in our business and in those of other watershapers from this point forward.
Here’s what happened.
THE VIEW FROM ABOVE
I was brought in to look at the site, a single-family residence on 8.5 acres atop a hill in Palo Alto, Calif., with spectacular views of Silicon Valley to the south and the southern reaches of San Francisco Bay to the east. We’re talking a drop-dead-gorgeous location on some extremely steep and rugged terrain.
The owner had called in a variety of firms to visit his home and offer bids. Those who went in before me suggested that all it would take to support the pool was a key cut into the hillside. That’s not what I observed. In fact, as I walked around the site, warning lights and alarm bells kept going off inside my head.
For starters, there were cracks in the foundation of the home. Looking for a reason, I spotted some grading that had been done downslope in creating a fire road. What I saw down there simply could not have been done with any kind of building code in mind, and my immediate suspicion was that cutting the path for the road had effectively undermined the house and destabilized the entire slope.
I let the client know what I was thinking and let him know that, while I was willing to help, nothing should happen without complete soils and geology reports.
To my way of thinking, it’s utter insanity to build a watershape anywhere without knowing what’s going on geologically, and it’s particularly insane to dive into a hillside job without knowing exactly what’s going on beneath the surface. And anyone who shifts responsibility for knowing those conditions to the client is, to me, dodging a basic professional duty.
In this case, the extreme topography of the site also meant that getting a true and accurate picture of ground conditions also would require getting a detailed surveyor’s report in combination with the soils testing. The client may (and probably will) balk at the added costs that come with these reports, but in a case such as this one, there really was no alternative. The client agreed with me here, and we started by contacting a soils engineer who came in and started a standard array of soils tests.
The situation with surveying, however, was not nearly so simple: Because of the topography and massive trees on the site, there were no clear lines of sight to visual monuments on the nearby road or adjacent structures. To shoot the site using standard surveying methods therefore would have involved shooting dozens of extreme composite angles and taken weeks if not months to execute.
EYES IN THE SKY
Recognizing the difficulties presented by the site, the geologist referred me to a surveyor who includes GPS-based readings among his services.
I had heard of GPS technology, but I really didn’t know anything about it or its capabilities. So I did a bit of research and found out that the system had been developed for military applications and saw its first major use during the Gulf War, where it guided “smart bombs” to within centimeters of their targets.
It’s simple in concept: Orbiting the earth are 24 satellites in what are known as geosynchronous orbits, which means each satellite maintains a constant position relative to the earth. These satellites send out precise radio signals that can be picked up by special ground-based GPS receivers.
Because the satellites are in known, fixed positions, the receiver can use the signals from three or more satellites to pinpoint locations on the ground with respect to longitude, latitude and elevation. The more sources the receiver can “find,” the more accurate the reading.
Even on a tough site like ours, getting good readings is almost never a problem because the satellites are hundreds of miles above the surface of the earth and allow for easy triangulation and remarkably accurate readings. The surveyor need only move around the site, receiver in hand, to take the necessary readings.
On our job, the surveyor took readings at more than 1,200 reference points spread across four acres of the site. The data points were entered into a computer that generated a 3-D topographical map of the hillside.
WHAT WE LEARNED
The information gathered by combining the survey results with the soils report was startling, especially to the homeowner. As I suspected, the downslope grading had not only been completely out of compliance with any known code but also was contributing to the undermining of the house and the destabilizing of the hillside.
The soils report revealed that the immediate culprit was run-off from the decking and the roof. Combined with the expansive clay soil, this uncontrolled flow of moisture had resulted in differential settlement – and a dropping of one side of the house by more than two inches.
The details were forwarded to a structural engineer, who offered suggestions toward remediation. Specifically, the slope adjacent to the fire road needed to be stabilized with either concrete surface materials or a retaining wall, while a portion of the house needed re-leveling – and the installation of concrete underpinnings to a depth of 20 feet.
The ticket for all of this work will come to about $200,000, so it was no surprise when the homeowner decided to put the swimming pool on hold until it was clear that the house wouldn’t be sliding down the hill on top of it. Once the remediation work is done, the surveyor will come back, reshoot the site and certify that the house is once again both level and, we all hope, stable again.
The upshot: My client’s house will be saved, and he assures me that someday I’ll be called in to build a pool – perhaps as soon as next year if all goes well. And when it goes in, he already knows it will need the support of a system of piles and grade beams rather than a couple of keys.
As we’ve gone through this process, I’ve learned a few things about GPS technology that are both interesting and promising. For starters, I am encouraged to know that technology exists that can yield extremely accurate topographical information. The system we used on this job, for instance, has an accuracy to within 20 centimeters over distances of more than 20 miles. Given the dimensions of most sites and pool structures, this means that when I need to, I can work with extremely tight tolerances.
I also learned that GPS readings can be taken with relative ease on even the most demanding sites. And the fact of the matter is that the sites most difficult to survey using traditional methods are those that probably need it most. As a result, many jobs that should have detailed topographical maps don’t have them – even though these are situations where that kind of information is most important.
In this case, the information helped a grateful client save his home from disaster.
DOWN THE ROAD
My first experience with GPS readings was impressive, and the information proved most useful. But what has me more excited about the technology is its potential in helping me in other ways than those described above.
Once you know the exact elevations and pitch of a given slope, for example, it’s possible to generate extremely precise pile and grade-beam designs. It won’t be a matter of giving or taking a critical inch. Best of all, levels can easily be re-checked after the forms are set – and before the concrete trucks arrive.
I also see great potential in the system’s ability to pinpoint property lines and establish proper setbacks in jobs where I know the tolerances will be tight. So if I need a setback of 10 feet, I can nail it down to the centimeter and firmly verify the location with city and building officials. I won’t have to set things up with a 10-foot setback and tack on an extra six inches as a fudge factor.
Of course, the equipment and the service are still very expensive, and it’s obvious I won’t be calling in a GPS-savvy surveyor on every job. As is the case with almost everything else in our high-tech world, however, the prices of the systems are already falling, and I’d be willing to bet we’ll start seeing more and more of this technology on job sites.
It’s all reminiscent of the arrival of laser-leveling systems: At first, they seemed so exotic, high-tech – and extravagant. But today, I’m one of many builders who can’t imagine working without them. Similarly, I think the advantages afforded by GPS eventually will result in its widespread use in the watershaping industry.
For my part, I know that any time I’m confronted with a challenging site, I’ll be advising my clients to avail themselves of a GPS survey. It’s already saved one house that I’m aware of. Who knows what benefits will emerge down the line?