This Minisode shows the completed rubble trench as well as a separate drainage system to ensure water does not get into the cob cabin no matter how epic the downpour.

An additional benefit of a properly built rubble trench relates to earthquakes. This house, as other structures that use a rubble trench, will not be attached or fastened to the earth in any way. There is no need for this as the house is going to weigh so incredibly much that there is zero chance it could ever be blown over or away. In the event of a significant earthquake, this "floating" action of the house on top of the rocks in the rubble trench allows the house to move as a whole, even if just a tiny bit, thus reducing the amount of stress on the structure and significantly reducing the chance for type of cracks or damage to develop. 

The rubble trench is lined with landscaping cloth to ensure dirt does not move into the cracks of the rocks and inhibit water flow. Landscaping cloth is designed so water can still flow through it.

I used a 2 inch PVC pipe for my drainage. This will be plenty to move water out and away from the structure.