“Nothing in life is easy.”
― Louis Sachar, Holes
One of the difficulties in a transmission line designer’s life is determining how much geotechnical information is sufficient for an efficient and reliable design. When all the important stuff is beneath the surface, it is difficult to know exactly what type of soil you will encounter when you eventually drill the hole to install the structure. You really want to know the soil properties and strength and know if it is sufficient to support the intended structure.
So you have a line that runs a few miles (or a few hundred) along a corridor. You can almost be certain that the soil conditions will not be uniform throughout the length of the project. You can also assume you will have budget constraints that may limit your ability to gather all the soil data you would prefer. Someone will likely limit the number of soil borings you can obtain.
If the line requires direct embedded structures, you may resort to minimal geotechnical data due to typical, long-standing and proven “rules of thumb.” An example would be the common “10 percent + X feet” embedment guideline. Is this the best approach? Many would argue it is not, but we know it has been used for many decades with minimal difficulties.
However, you may have a line that requires concrete foundations, typically a drilled pier system. In this case the complexity increases and the need for more detailed design would really be nice.
Many factors can affect the cost of the geotechnical phase of the project. How many pole locations will you choose for soil sampling and testing? Are the sites accessible? Will specialized equipment be required? How quickly does the data need to be available to support the design and project schedule? How rigorous does the soil testing need to be to support the foundation design?
It is always a judgment call and a balance of budget and schedule. This is the difficulty in the life of a line designer.
May I respectfully suggest that it may make sense to take a boring at each pole location? No, I am not writing this at the urging of the “geotechnical lobby.” I simply believe the cost of geotechnical sampling and testing can often be offset by cost savings realized by less conservative designs.
Let’s assume a typical soil sampling/testing cost for a pole location is $5,000 (My apologies to my friends in the geotechnical market if this is a bad assumption). Also assume a 12 foot diameter pier that is 35 feet deep. Assume concrete cost at $150/cubic yard. If you can reduce the depth of the foundation by three feet because you can design for a known soil profile, the savings in concrete and installation alone more than offset the cost of the soil boring.
We know that many miles of line have been successfully designed, built and operated using a variety of geotechnical sampling approaches. At the end of the day, the line designer is responsible to use the tools available to make it all work.
Some utilities use a “sample every location” approach, and I applaud them. For those that do not, I simply encourage a thoughtful consideration of the alternative.
“I’m not stupid. I know everybody thinks I am.”
― Louis Sachar, Holes