Access to historical geotechnical data reduces alignment risk for transmission lines.
By Vic Donald, P.E.
Transmission lines — those beautiful catenaries traversing our country — deliver the energy we need to live, work, and play. Strange thing about transmission lines: They push and pull and sway and require a lot from the ground that allow them to suspend elegantly above our heads. These large compressive, uplift, and lateral loads must be resisted, ultimately, by the ground they touch.
The ground is where variability comes into play. If it weren’t for the ground that they must touch every quarter mile or so, structural design of these essential systems would be straightforward. No other structural foundation system is subject to such variability. A transmission line foundation may require deep piles at one structure location and rock removal at the next.
Variability in ground support conditions has a great impact on the cost of transmission lines. Choosing between alignment options for a new transmission line should take this into consideration. Thus, subsurface exploration is necessary to understand and develop even a conceptual foundation design. Such exploration is expensive and time-consuming. It requires land access and often entails land disturbance. Unfortunately, designers often make alignment decisions with little or no subsurface information.
Therein lies the risk. Mitigating this risk is necessary, and conservatism is usually the best approach to the risk of the unknown. As alignment decisions are made, designers frequently use simplified assumptions about subsurface conditions. At the preliminary stage, designers must plan for the worst and hope for the best. This approach has served us well for decades. We finalize our alignment, proceed with a traditional geotechnical exploration, design the foundations, and revise our cost estimates to reflect the actual needs.
But the rules have changed. To keep pace with required demand and to remove power transmission development barriers, the Federal Energy Regulatory Commission (FERC) issued Order No. 1000 in July 2011. This precipitated the use of an Engineer, Procure, and Construction (EPC) model. With this model, complete proposals incorporating all engineering, financing, and construction costs are submitted prior to project award. Now the EPC offer moves forward with a complete and final offer of design and construction costs, without the benefit of necessary subsurface investigations used to understand foundation design needs.
The risk of these high-variability subsurface conditions, without the ability to confirm prior to proposing, creates a major challenge in this highly competitive scenario. Requisite conservatism incorporated into foundation designs by all bidders results in excessive foundation expenses.
A better understanding of subsurface geotechnical risk prior to submitting a proposal is essential. Fortunately, we have been here before. Literally, we have been here before; we have explored the hills and valleys the new transmission line will traverse. We can take advantage of a century’s worth of exploration data to reduce this risk, provisions for conservative assumptions, and excessive foundation costs. In the end, the consumer pays less.
Connecting data for utilities
Historical geotechnical data, typically in the form of soil borings, are valuable, lasting assets for predicting subsurface conditions with an accuracy not possible in the past. This vast data lies hidden in paper reports. No digital source of locating geospatial information exists. Even if the reports can be discovered, the information is on paper only. Digital statistical evaluations and new project location condition predications are difficult to perform.
Consider this example: In 1970, when geotechnical investigations were performed for a planned highway bridge in close proximity to a future transmission line alignment, our predecessors had little appreciation for the value of geotechnical data for future projects. Because of our inability to find prior exploration locations and old reports, the 1970 data becomes invisible during the 2019 transmission line design. Imagine the value of this data; why didn’t we think about this back in 1970?
When paper becomes data
For the last 10-plus years, geotechnical engineers have been collecting location information of soil borings, allowing easy access through GIS systems. In addition, we use consistent, standardized database systems that detail data points associated with each boring location. This has become a vital, digital component of predictive analytics. Consider the value of knowing the Standard Penetration Resistance, undrained shear strength, or plasticity index of a given X, Y, and Z point below the ground surface, stitched together with millions of points of companion site data. This data analytics process provides an enormous potential to apply machine learning concepts and drill a “virtual boring” at new project sites.
Terracon has preserved historical exploration data dating back to the firm’s inception more than 50 years ago. With the conduct of more
than 150,000 geotechnical projects during the last 10 years, we have amassed a large amount of subsurface information spanning the entire country that has been georeferenced and digitized to provide a basis for predictive analytics. This data is available to perform statistical evaluations, predict average conditions, variability, etc.
We are also rediscovering our historical data. We utilized and georeferenced every report during the last 50-plus years in our GIS system. Our combination of historical information with newer digital data allows us to make well-informed predictions of subsurface conditions at any project site.
It’s more than data
The geotechnical engineering profession relies heavily on collected data to define subsurface conditions, as well as the professional experience of local engineers who have designed foundations for roads, bridges, transmission lines, and other structures. Experienced geotechnical engineers, armed with vast and ever-increasing datasets offered in public domain and historical exploration reports, can render a valuable opinion of subsurface conditions, variability in those conditions, and a confidence level in the predictions. Certainly, those 100-mile-long transmission lines — designed and priced before subsurface exploration — will benefit tremendously from the opinion formed with our vast historical database.
Using an online portal called Stage1 GeoReport, Terracon has developed a means to deliver data quickly from the public domain, historical exploration data, and the opinion of local geotechnical engineers. The Stage1 GeoReport provides a prediction of geotechnical conditions and a confidence estimate related to the prediction. The confidence estimate is derived from the geotechnical engineer’s review of data density and consistency in the vicinity of the project site. With sufficient confidence, the geotechnical engineer can even predict estimated foundation parameters using LPILE computer software, moment foundation analysis and design (MFAD), as well as other industry standards. These foundation parameters are invaluable while estimating foundation costs, often the largest unknown cost and risk to a transmission project. For larger projects, the Stage1 GeoReport helps identify alternative corridors with lower geotechnical risks or foundation costs.
Reduced — not eliminated — risk
Even with today’s methods and tools, not all risk can be eliminated. The good news is that we can develop predictions of subsurface conditions, access vast public domain resources and historical databases, and use engineering experience to eliminate inherent risks in the development of final pricing. We will never have the benefit of geotechnical data at each transmission line structure, so an EPC procurement process will always have to address this risk until foundations have been constructed. Mother Nature doesn’t comply with ANSI standards, and we tend to discover that every time we reach into the earth to construct the next great catenary support.
However, bringing this information and expert opinion to the design provides a major step forward in development of realistic designs for more precise (and less conservative) pricing models. This approach certainly reduces risk and informs the design team earlier than ever before.
We have been here before. Let’s take advantage of the fact that we probably already know what the soil boring is going to tell you.
Vic Donald, P.E., is Terracon’s (www.terracon.com) national director of geotechnical services. He has more than 35 years of experience in geotechnical and geo-environmental engineering, working with clients and geotechnical engineers across the country.