There are many things we put underground that serve important functions –unseen and protected from above-ground human activities.  What exactly is underground other than just lots of dirt and rocks?  Well, there are tunnels for trains and vehicles, fuel pipelines carrying natural gas, crude oil, and refined fuel products, ‘dry’ utility lines (electrical power, cable TV, telephone, fiber optic cables), ‘wet’ utility systems (storm drainage, sewage, potable water, and reclaimed water), compressed air and steam lines, and there are even pipes for orange juice (no joke).

We know that all of these conduits, pipes, tunnels, and systems are subsurface (underground) so why should we care.  We care because we are about to expand our campus, build a subway system, construct our solar farm, or just put up a new fence.  Every time we excavate, meaning grade, trench, dig, drill, tunnel, scrape, plow, or drive, we run the risk to hitting an existing subsurface installation potentially damaging this system and even causing personal injury or loss of life.  And, we care because it is the law contact Underground Service Alert (DigAlert) before any excavation.

Now that we understand the importance of mapping subsurface utility data it is critical to understand the techniques used to create these maps. In fact, there are quite a bit of new technologies and equipment that can gather, depict, and manage existing utility information.  The American Society of Civil Engineers (ASCE) have even quantified the quality and reliability of utility information shown on construction plans from Quality Level A (best) to Quality Level D (worst).[1]  The use of quality levels allows project owners and engineers to certify their level of utility location accuracy.

The four quality levels of utility depiction are:

  • Quality Level D – Information is obtained from existing utility records only.
  • Quality Level C – This level adds field survey of visible above-ground utility facilities including fire hydrants, valve boxes, catch basins, utility poles, etc. resolved with existing utility records.
  • Quality Level B – This level builds upon Quality Level C through the use of surface geophysical hunting techniques to confirm the existence and horizontal position of underground utilities.
  • Quality Level A – The highest level of accuracy that now uses potholing at critical points to determine the exact horizontal and vertical of the underground utilities.

It is obvious that the higher the quality level the more upfront cost will be incurred.  This does not translate into higher overall costs.  In fact, a little extra money spent at the front end of a project can translate into a lot of money saved at the back end.  For example, an engineer performs a level C investigation for a new bridge project where driven, concrete piles are planned and an existing high-pressure gas line is built 2-foot off from where plans show it.  Just think of the chain of possible problems after the contractor hits this gas line while driving one of the piles.  This inaccurate utility information can result in costly damage, delays, service disruptions, redesigns, lawsuits, and even injuries and death.

How to choose the proper quality level for your project?  This needs to be done in consultation with your project civil engineer.  The engineer needs to advise the project owner regarding potential effects their project may have on existing subsurface utilities, the different utility quality levels, and the data’s reliability.  This discussion should include the costs and benefits associated with obtaining the quality levels.  The cost of obtaining level B throughout a project with level A in sufficient locations adds about 1% while saving 10% to 15% to the typical project budget.[2]  Now this is a discussion worth having.

If you have any further questions or comments about subsurface utility research and mapping, then please feel free to email or give me a call.

John M. Cruikshank, PE

jcruikshank@jmc-2.com

310-241-6550 (x228)

www.jmc-2.com

[1] ASCE, “Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data,” 2003, p. 4-6.

[2] Ibid, p. 11.

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