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
2) Is this approach workable and does it violates any building code?
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.
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.
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
Sounds like a lot of work, but good luck.
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.
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