Subterranean Room Addition

[flyingvan]

    There were a few problems with our home.  There was a drainage issue solved by grading the yard, but not before it had actually washed out a hole under the stemwall in the livingroom. The basement floor would soak.  Also the floor in the livingroom was springy because they spanned 16' with 2x12's (I didn't build this one).  Furthermore, our trampoline was set on the flattest spot that was still a significant grade. 
     So, I cut a hole through the cinderblock wall on the low side of the house.  There was 30" of clearance that decreased to 10".  Hunched over digging, two buckets at a time, dug out a 10'x14' room.  The dirt was used to level out the area for the trampoline.  Encountered one boulder the size of a full size bed so had to dig down real deep to bury it.
    With the same two buckets, backfilled the ground with gravel and dug a trench for a gravity drain all the way to the street (over 100').  The bottoms of the concrete stemwalls for the livingroom were 20" up from the gravel, then it was just a soil wall running straight down.
    I cut pressure treated stock into 2x2's and screwed bolts into the back side of them.  I poured a footing in the shape of the room and closet.  Then I ran those 2x2's from the existing floor joists down to the footing. 
    Next, after running conduit for outlets and re-directing the furnace ducts, 14" melamine got screwed to the 2x2's to make a slip form.  Lots of 5/8" rebar.  (One side of this was the outside of the basement wall, under the livingroom.  I drilled into that wall and epoxied the rebar in to tie it all together).  I thought draping visqueen over the soil walls was a good idea for better concrete and an intergrated moisture barrier...Then, same two buckets, carried concrete in and dumped it behind the slip forms.  After it would set up, unscrewed the melamine and moved it up, screwing it into the 2x2's.  Those bolts in the backside of the 2x2's anchored the wood into the concrete.


   Once the wall was built up higher than the existing stemwall, concrete got dumped behind the new wall and flated up to the washed out portion.  I kept going until it was 12" higher than the existing wall.  The new wall is 14" thick. 
   The 2x2's then became the nailing strips for the interior siding.  I went with this plastic tongue and groove stuff that said 'great for craft rooms and workshops!'  I couldn't figure out why it wasn't suitable for bedrooms----it didn't offgas anything toxic.  The first night we tried the room out though, we put our electric fireplace in there for heat and ambiance---that siding creaked and popped a lot until it reached a stable temperature.
   So, drainage fixed.  The 2x2's support the livingroom floor from below, reducing the greatest span to 10' which really stiffened the floor.  Trampoline levelled.  New guestroom too. 
   
    The drystack wall for the trampoline area was all from stones dug up from under the house.
     I would never do this again---digging underground with little ventilation all hunched over with no room for a digging bar was unfun. 

[rick91351]

I feel your pain.  Really I do.  We had a partial basement in our house in town until one morning.  One of those mornings when you just wake up and you just have to do it.  I packed 22 yard of dirt up the stairs in buckets to my waiting pick up over a couple months.  One bucket got wore out the other I kept as a memento just in case I had another brain fart.  Worked too until I lost the bucket and then did the next thing one the strong back - weak mind bucket list.  BTW the concrete came in through a window we put in.