I've built retaining walls with dry-stacked rocks and have used several inches of gravel under the bottom stones. Now I'm building a single-level circular retaining wall, i.e. Just one level of 8 to 12" thick stone--not mortared. Should I add gravel underneath i.e. for drainage or stability? I live in Victoria, British Columbia where the winters are not too bad. The average winter daily high and low temperatures are 8.2°C and 3.6°C, respectively. Thank you.