Watch out for Corrosive Soil!

The soils report for a project is probably one of the most important pieces of information you can have for your site! (It’s also one of the very first things you’ll need as you start your due diligence process. LONG BEFORE YOU EVEN BEGIN CONSTRUCTION!) It can provide great insight to the history of the soils that underlie your project. It will give you instructions for how to handle the soils. It will also dictate what materials you can use (and can NOT use) during construction. And one of the most important factors that dictates this is “corrosive soil”! In order to truly identify HOW corrosive soils affects us, we must dive in and ask WHAT corrosive soil is exactly.

WHAT IS CORROSIVE SOIL?

Soils become corrosive when there are significant amounts of soluble salt solutions found within the soil. The salt solutions contain ions that attack some metals and concrete.

So the two biggest things to watch out for are the presence of:

1. Chloride (which is harmful to ferrous metals)
2. Sulfate (which affects your concrete).

These two compounds contain salt solutions and become harmful when mixed with moisture.

HOW DOES IT WORK EXACTLY?

With the presence of salt, moisture, oxygen, and certain metals, (and dependent on the PH levels of the acidity), these ingredients can cause the soil to work just like a battery! (See exhibit below for schematic of a typical battery function).

When the salt solutions mix with water, the water serves as an “electrolyte” and carries the harmful ions from one metal to another via a wire or union (from the “anode” to the “cathode”). See example exhibit below where a steel pipe now acts as the anode and a copper pipe acts as a cathode! Now you have the potential for an electrical current within your soils.

Note that the anode is attacked and becomes corroded over time because it gives up its electrons to the cathode.

(PLEASE NOTE: sometimes the anode and cathode can even be part of the same metal pipe or surface! One section of the pipe can be in a low oxygen zone while the other end can be in a high oxygen zone! For example, a metal pipe in the ocean where one end is deep below the surface where no oxygen is present, and the other end is above the water but repeatedly splashed by the waves.)

WHAT CAN IT AFFECT?

The main components of construction that corrosive soils can affect is: concrete and ferrous metals. So watch out for the following:

1. COPPER SERVICES for your water laterals. (As mentioned in a previous post the residential sewer/water mainlines around here are typically PVC, which is not an issue for corrosion. BUT in other cases if you do have steel or ductile iron pipe then watch out! Refer to the soils report for further recommendations for these metals).
   1. Recommendations:
      1. Run copper pipes within PVC to prevent soil contact
      2. Cover pipes with a polyethylene sleeve
      3. Add cathodic protection (i.e. sacrificial anode bags. These anode bags are more negatively charged and are designed to absorb the dangerous electrical charges and will corrode first)
2. Ductile Iron Pipe:
   1. Recommendations:
      1. Cover pipes with a polyethylene sleeve
      2. Wax tape covering
      3. Hot tar enamel
3. Concrete (Flatwork):
   1. Recommendations:
      1. Use Type V cement, which contains a higher sulfate-resistant mix (instead of the typical Type II)
      2. Increase the concrete strength (anywhere from 3,250 psi to 4,500 psi)
4. PVC is very resistant to corrosion! Which is why it’s a common material chosen for the sewer and water mainlines. BUT in other cases if you do have steel, CML&C, or ductile iron pipe then watch out! Refer to the soils report for further recommendations for these metals

So when you receive your soils report, it is not to be taken lightly. Pay very close to attention if it contains chloride and sulfate. Corrosive soil can cost you a lot of money! Just be sure you have a good soils technician on your team and that you stick close to his recommendations (e.g. Geotek). But most importantly, make sure you vet this kind of information out during your “DUE DILIGENCE” and “PLANNING” phases of your project. Not after!