Hello All,
I’m desperate for a lead on info, case studies, and/or other research for a project proposal to add(dig) a sub-basement below an existing basement for a townhouse in the mid-Atlantic area.
In a nutshell, we love the house, love the area, love the commute, love the neighbors, etc… just would also love to have another floor of space in our home and down is the only way to go since our HOA would not approve a high-rise townhouse among the others! I gotta have my workshop somewhere, right?
How realistic/feasible would it be to dig a sub-basement below the existing house? Can anyone point me somewhere for further research please? All I can seem to find are commercial projects.
Thanks much,
Jcd
Replies
I have read that this sort of structure is popular in some areas of Germany. It seems that some basements may be 3 or 4 layers deep. Story, from an Army publication in the form of a whoops file, was that during armor maneuvers a tank crew tried to recreate the impressive scene of a tank breaking through an abandoned house. The 40+ ton vehicle made it to the center of the house before the floor gave way. They fell 2 or three stories but were largely unhurt. After the training a earth moving crew had to dig a slope down to their level so the armored vehicle could drive out with the help of the winch on a M88 recovery vehicle. A picture of this operation was provided.
I suppose a basement this deep would be possible if the ground was dry and solid enough. A deep layer of stable and well drained bedrock would seem to be a good starting point. I would start by consulting with an engineer familiar with deep excavations in your area.
There are a number of things you'll need to consider. A few that come to mind are:
1. You say its a townhouse, so you have party walls. (You share a common wall with your neighbors) These walls will need to be underpinned. This will need to be done by an expert in this field and designed by a structural engineer. Also your property line probably goes directly down the middle of the walls, so you will need to get written approval from your neighbor.
2. Consult a local Geo-tech engineer to determine where the seasonal water tables are and what is the general geology of the area. Are you going to have to excavate rock? A costly issue.
If all those look good and your not going to lose to much on resale vale for the money invested, go for it. Good luck.
before investing so much on studies and engineering, I would make sure that it is legally possible. You could find that you are doing something that will make your neighbors less than enchanted with you. It will be a very expensive undertaking, per square foot.
Excellence is its own reward!