Earlier this week we started filling the trenches with gravel. A gravel trench foundation, also known as a rubble trench or french drain, is an alternative foundation that requires an engineer’s seal to be approved in the state of New Mexico.
The trenches are lined with filter fabric to reduce the amount of mud that gets washed into the gravel. We have installed 2″ rigid foam insulation to a depth of 2 feet on the inside wall of the trenches. We are using 3/4″ gravel to fill the trench. We are also setting a 4″ diameter perforated PVC pipe at a slope of an 1/8″ per linear foot in the gravel for drainage of any water that may find its way into the footing.
Getting the slope right on the drain pipe has been a time-consuming process for us. The pipe comes in 10′ lengths, so we make sure to achieve the right slope for every 10′ section. However, when we double-checked the slope of the pipe after laying our first 80 feet, we realized that somehow the slope had gotten off and we had to correct it. It now seems pretty essential to really pack in gravel on the sides of each 10′ section once you have the slope you want, so that the pipe doesn’t lift up as you connect additional pipe lengths. We are definitely learning as we go!