Hi all,
I have a 6 ft high crawlspace with dirt. I am planning on digging additional 2 feet down to have full basement. The foundation is only
6 feet in height. So, I plan to dig it one foot inward from the exisiting foundation and made a 3 feet high retaining wall to hold the dirt that supports the foundation.My questions are the following.
1) Has anyone done this before and willing to share some of their experiences. 2) Is this approach workable and does it violates any building code?Thanks
Well, I haven't done it personally, but I will never forget an experience a friend of my parents had when I was just a kid. I remember us going over there and seeing his house practically collapsed because he was trying to dig himself a full basement and something went bad wrong. What you are trying to do probably can be done, but man, be careful with it or you could do some serious damage to your house.
Bobby
Done properly in a cramped space, digging by hand without a backhoe, I think this job will kill you (either because of the labor or because the house falls on you). Note that the foundation wall has a poured footing that is even wider than the wall, and you have to stay away from that by a distance that a soils engineer recommends.
This sounds like a very bad idea IMHO. I highly suggest you hire an engineer to review you plans before hand. It doesnt sound safe or structurally sound. Normally I believe people who want a full basement jack up the house off the foundation and pour a deeper one under the existing foundation, and pour a floating slab for the floor. This is very costly. It would probably be cheaper to move to a house with a basement already. I am not an expert though so take this advice with a grain of salt.
Crayola
Please list me in your will if you decide to do this project without professional consultation with an engineer.
crawl
There are professionals (and some not so professional) people that do this type of thing all the time, so you might want to consult one of them. I'm assuming that the center space is clear and all of the weight of the house is resting on the concrete walls. Can't tell you about what the code says, you need to check with your local agency.
Don't worry about the nay sayers, you can do it, you just need to consider the soil conditions, know what you want the final result to be, and then carefully plan each step. There are a lot of option but your solution is one of the simplest. Just dig straight down from inside wall of the existing footing and use the existing wall, footing, and dirt as the form for the back side of your new concrete wall. And put a footing at the bottom (don't undercut the dirt under the existing footing) for your new wall. Doing it in alternate (say 6 foot wide sections) makes sense if you have friable soil. Just plan your forms so that the sections tie together when you fill between poured sections. And be sure to consider how the floor ties in and the total height needed to achieve a planned floor to ceiling height. Don't forget to plan for heating and cooling ducts.
Sounds like a lot of work, but good luck.
W>
The problem is you have no idea how stable and compact the soil is, if you dig deeper than the footing there is a slight possibility that the soil will give way even though you are are digging a foot away. That said your ciderblock retaining wall not do much other than to provide a little weight to resist any soil movement. The side pressure against the block is what you want to control and to achieve much you would have to have a retaining system which can transfer this pressure to the soil. After your basement slab is poured the slab can be used to resist this pressure. The problem is in getting to that point safely.
Using series of lolly columns to remove the weight from the wall during this process is one way. A soil bearing strength test would be a good idea since at this point you have no idea of what it will take. The original design was probably based on code and typical local conditions. This is good enough in most cases but "typical" can cause nasty surprises. You are going to be investing a lot money and time in the project and paying a structural engineer to come up with a proceedure customized to your conditions will allow you to do a better job without wasted floor space.
On solution would be a poured reinforced retaining wall at least twice as deep as the intended basement adjacent to the existing footing. This would resist the soil pressure and transfer the force to ground below. Getting that point safely is the problem. I would want to take the load off of the wall during the process. If you use temporary shoring the base of the columns should be set in holes dug to the depth of the intended basement and a temporary footing should be provided for each column. This will prevent the trench from collapsing from the temporary support columns. I would also like to connect the retaining wall to the footing with drilled in bent rebars grouted in place.
If I was challenged to do this without shoring then another method (which is labor intensive) would be to use a power auger (~6" dia.) and drill a series of deep (4') holes adjacent to the footing (one at a time widely spaced) and then drill a horizontal hole( ~6" deep) at each hole into the footing. Grout a bent rebar (#4 or bigger) into the hole with the other leg running to be bottom of the augered hole, then fill the hole with concrete. Repeat until you have built a retaining wall from a series of close spaced (~1' ctrs.) holes.
Regards,
John
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