Posted: 3/11/2008 10:20:20 PM EDT
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I have a tri-level home with a very high (tall?) crawl space on one side (55"). I would like to dig a section of it out to over 6'. Given that I may dig around 24" anyone know how close to the supports I can dig without endangering the structural integrity? I have provided an image to make my question and situation a little more clear.  I will stay at least 4' from the outside walls. |
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Where the footer is loaded, note the footprint of the bearing surface. From there, you should be okay to dig outside a 45 degree plane or cone extending down and out from the perimeter of the bearing surface. Example: Say a 4x4 post sits on the footer. The footer width was designed to carry that load, largely without internal strains, so if the footer is 16 inches wide, the lower footing's bearing surface on the soil, associated with that load, would be 16 by 16 inches. From that 16 inch square, you could safely excavate a truncated pyramid extending down and out at a 45 degree angle. If the entire footing is loaded, as in a perimeter footing, you could excavate at and away from a 45 degree plane extending down and in from the footer's inner vertical face. Caveats: 1. The 45 degree figure is a rough working number that applies to most types of compacted soils, however, porous fill like pea gravel will require a flatter angle. 2. The 45 degree figure will allow you to excavate and leave open the newly excavated area. Any type of retaining wall structure will allow that angle to be steepened, but you need a soils engineer or CE to calculate the retaining wall's ability to resist creep and soil displacement under the footing. Personally, I build to 200 year standards if at all possible, and do not mess around with compacted soils under or immediately adjacent to footings. Period. The sole exception would be digging adjacent to an unloaded section of footer. Sometimes it's easier to cut a long trench to pour a footer into, when you only need a few bearing pads spaced along the length of the trench. In that instance, I would be ok with digging adjacent to the unloaded sections of footer, any sections with nothing resting on them. Otherwise, I'd leave it alone unless the footer showed signs of failure and needed repair. It's your house, YMMV. |
I am not an engineer, But have done this same thing before. Also I slept in one of those hotels last night. You can only dig about 4-6 linear feet out at one time, then you MUST pour a pad and build a wall for support. You can try posts for support, but if it were my house, I would go slow and reienforce often. Do not get too over eager and dig to much out at one time. More than likely the house would not fall on your head, but the stress and cracking of timber and drywall will be excesive. You could also jack the entire house up and then dig out the area. Build the walls and lower the house back down. Proablabe a lot more expensive though. Best of Luck. Expect to get a few cracks in the drywall. You should get an engineer, or at least a reputable contractor to look at the situation first. |
Jeffers and Vatopa, thanks for the sound advice. |
The 45 degree thing is exactly what I was going to say. To make it easy, if you want to dig down 24" then you need to stay AT LEAST 24 inches away from any footing. Think of a right angle triangle, if both legs of the triangle are the same length, the angle is 45 degrees. |
| I just bought a money pit----I mean a house that the guy before me dug out 3 feet down to make it an 8 foot ceiling---I am now tring to figure out what the hell I have to do to make it stable, dry and can then finish it. He dug pretty damn close to the supports---and it is an old house----3 story 7 bedrooms, so I am gonna get on it quick this summer. I am also thinking about picking up a backhoe for snow this winter, and using it when I build my garage this summer---nothing big 20x30, but I would like a small concrete underground room that runs from the garage to the basement, and I really don't think it would be all that hard to do. I used to work for a company that jacked up houses and replaced joists----better to do it roght the first time than to have to pay these morons to come in and do it.-----peoples door locks used to be anywhere from 3 to 4 inches off!---would have to jack very slowly till they were more or less lined up---hoping not to crack any windows----good luck |
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I think I am going to throw some support headers up to try to take the load off of the area near where I will be digging, then pour new footers once the digging is done. I appreciate everyone taking the time to respond . (Though I am saddened no one commented on my fancy artwork  )
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Actually, I was. What program did you use and was it free? I've done this before, in my house up in Seattle. Very sandy soil, so I stayed away from the house footings about 4 feet, IRC, went down about 4 feet. I've looked into doing it at my home here in North Alabama with it's very hard and compact dry clay soil. I could go a lot closer to the footings, but I eventually discarded the idea due to cost and the fact that I'll be moving here in a couple years. My biggest mistake while building this house was to not have a crawl space room/basement dug out at the time. Merlin |
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I didn't read the thread beyond the first post. Those channels of concrete or whatever ...... that runs across ... Dig a couple of deep wells under them. 4 feet deeper than you want to dig down the floor. Pour those wells full of concrete to form pillars. The wells/pillars should be close to the load bearing points. Do the same under the outside walls. You are installing supports piecemeal. Once the supports are in, you can dig the place out. You want to dig deeper than where you intend the floor to be. Deep enough to place a drain or two and pipe it over to the pit that is going to be for a sump pump. It's not rocket science. I'm reasonably confident that anyone with the technical prowess to make the graphic you posted, has enough upstairs to handle this. The problems you will have are that it will always take-on/seep water because you cannot install a drainage system beyond the "basement" walls you are making. If you dig under the cinder blocks (ass-uming block footings) then pouring the walls will be challenging. Now .... If you fuck this up and the place shifts or sags 'just a little' ... The counters will come away from the walls. The ceramic tiles will break. The drywall/plaster will have running cracks at the corners of all doors and windows. Doors and windows won't open and close any more; especially sliding doors. The floors will be uneven enough that anyone walking through the place will notice. It goes on and on. Not rocket science, but you had better not short cut anything. Good luck, and post pics. |
Definately let us know how it goes. I've got a 26x26 crawlspace, with close to 10' in one corner, and 4.5' in the opposite. I've started digging it out just over a year ago, but had to stop due to the dust. Now that a fan is in place to suck it out the back vent, I'll start again this summer. The basement stairs are on 28" 250lb linear actuators (12V) and rise up to almost 8', giving me access to that space. Eventually, a 20'x20' room, with 8' ceilings is the plan. Full water, drain, lighting, space set aside as a complete shelter. If that fails, a great man-cave. No Girls Allowed....unless she's cute....can't have ugly kids after the apocalypse, can we?! Please let us know how it progresses. |