From your description of this mysterious problem, my thinking centered about a few possible problem sources, none of which are easy to prove unfortunately.

The first question that came to mind is “Where did the plumbing repair contractor get the idea that the leaks could be due to “electrical current going through the copper pipes?” Can they substantiate this claim with any published or otherwise well-documented information? Is it possible that they may be basing that comment on problems they have encountered or heard about that relate to electrochemical corrosion of steel (not copper) pipes or underground tanks? I found an interesting article on Cathodic Protection (a method of protection against electrochemical corrosion) and in that article they stated that “Generally, copper piping does not require cathodic protection.“ Here is a link to that article:

http://www.cpdesigncenter.com/propanetext.pdf

The second question was, “were the houses on either side of you that experienced the same type of leaks, built by the same contractor and/or at the same time?” If so this may point to installation/workmanship issues or perhaps even a bad batch of copper piping. If your neighbors have not had their pipes fixed yet, and there is common desire to get to the bottom of this (I don’t know how expensive these repairs are), I would suggest they keep close watch on the repair and ask for the replaced piping sections…perhaps visual examination of the damaged pipes by an experienced eye will narrow down the possibilities. Also, it may be useful, during the excavation of the pipes, to inspect the areas of the pipe leaks to see their proximity to the ‘tunnel ditch’ you spoke of or perhaps other objects which may be causing mechanical abrasion.

Lastly, I found a very interesting article titled “Conditions Contributing to Underground Copper Corrosion” at the following address:

http://www.copper.org/resources/properties/protection/underground.html

I strongly suggest a strong cup or coffee (or maybe even an entire pot) and a comfortable chair for reading this one…as you will see some of the discussion gets rather technical, and they present a variety of possible causes. For the short version, here is their excellent summary:

“Copper water tubing has an outstanding history of corrosion resistance in most underground environments. Copper does not naturally corrode in most clays, chalks, loams, sands, and gravels. Certain aggressive soil conditions, however, can cause it to corrode. The basic prerequisite for corrosion is the presence of appreciable amounts of moisture. Other factors that can facilitate the corrosion process include soils having: (1) elevated concentrations of sulfate, chloride, ammonia compounds, or sulfide; (2) poor aeration, which supports anaerobic bacteria activity; (3) large amounts of organic or inorganic acid; and (4) large oxygen or neutral-salt (especially chloride) differentials.

Copper is also subject to corrosion by stray DC electricity. It can apparently be adversely affected by certain conditions in the AC system to which it might be grounded. Underground copper is reportedly susceptible to thermogalvanic effects and dissimilar material corrosion. Copper water tubes can occasionally experience corrosion fatigue.

Fortunately, in those rare instances in which copper corrodes in the underground environment, the cause of the deterioration can be identified. Once this is accomplished, corrosion can be economically mitigated by a variety of techniques.”

Depending on what evidence you are able to gather, you may discover a simple answer, or this may end up in the “cold case files”. I hope you have success!