Copper Corrosion in Soil

First, you need more info on the soil.
In addition to making soil resistivity measurements as indicated in
Soil Resistivity
thread338-81629 [I forgot to mention, you must 1st check for buried metal (except for the soil box method],

Moisture content is of course important, but it is somewhat determined by the resistivity measurement. Easy to do: weigh a sample before & after baking.

Soil pH is very, very important, and is easily measured: Mix soil 1:1 by volume with DI water, and measure pH using either a pH meter or pH test paper. Corrosion of copper is minimum at pH 9.7 according to Table 3.7 in Practical handbook of Corrosion Controls in Soils, S. A. Bradford, chap. 3, ‘Corrosivity of Soils’ (2000).

Also, measure the redox (reduction-oxidation) potential of the soil to get an idea of dissolved oxygen. Easy to measure with an ORP meter if you have a groundwater sample (take care not to expose the sample to air for a significant time). It may be difficult to make an ORP measurement of adsorbed soil moisture. I suggest boiling some DI water to make air-free, capping and cooling, then using it to mix with soil in order that there is sufficient conductivity for a measurement.

Test groundwater and/or soil for reactive ions (Cl[sup]-[/sup], NH[sub]4[/sub][sup]+[/sup], NO[sub]3[/sub][sup]-[/sup], NO[sub]2[/sub][sup]-[/sup], SO[sub]4[/sub][sup]-2[/sup], etc.) and TDS. Is salt water nearby, or is salt used as a road de-icer in this area?

ibid., p. 49-50 describes a polarization measurement method for ranking the corrosivity of different soils.

Also, check with your government, e.g., USDA Natural Resources Conservation Service, for soil classifications in your area. Virtually all soils in the Northern Hemisphere were classified long ago. Check also with local water agencies for groundwater data, and the USGS has topographical maps with depth to groundwater.

See also Effects of Soil Characteristics on Corrosion. Special Technical Publication 1013 ASTM (1989).
Don’t forget to check for stray currents from the cathodically protected pipelines.

Once you know what you are up against, then you can choose the best countermeasures.