Replies

1. 
   Remodeler | Jan 28, 2003 04:35pm | #1

   How long a trench? How far from wall?  I recently did a long 30' trench in a slab-on grade for bathroom addition, took bulk of material out with jackhammer and fine-tuned with air hammer off compressor.  You should be able to tell when you reach footer if you're close to wall and hitting it won't undermine it unless you're hammering through it or pounding on it continually.  But between the two tools you can be precise enough to cut just off the top in the slab.  

   remodeler

1. 
   Remodeler | Jan 28, 2003 04:38pm | #2

   Suppose I should have read my post a little closer.  Jackhammer the whole thing, don't cut it with a saw - expensive, dusty, time-consuming.  A foot off the wall you'll be plenty clear of the footer and will hit pea gravel 4" down into the slab for your pipe run.  When you dig your sump pit, make sure you don't have groundwater flows that can undermine the foundation - i.e. a high water table can carry silt away from beneath footings and deposit it in your new pit.  A post-hole digger works nice for a narrow pit like you are digging.

   remodeler

1. 
   ckul | Jan 28, 2003 04:56pm | #4

   Thanks remodeler. As I am unsure what I will discover once I dig, what should I do if I see groundwater flowing? I was planning on digging out the sump pit first then the rest. I'm making a 30 foot pipe run. Any additional info is appreciated. Thanks

1. 
   Remodeler | Jan 28, 2003 07:19pm | #5

   If you do have groundwater flowing, be aware of it and what its potential is to do.  I excavated on the outside of a block foundation wall for a slab on grade and had water pouring through the wall, kind of freaked me out because the volume was so strong.  but if you do have it make sure your put is water-tight to its flow so that silt can't dissolve in it, stay suspended in the water, and then be pumped out by your new pit.  Your job's not as bad as it seems, but it sure makes a lot of noise.  An electric jackhammer works pretty good for it.  They'll charge you extra to rent more points on it if you're doing the rental supply house thing, if you have a grinder you only need one.  I would bed the pipe in your run completely in sand so that the patched concrete doesn't lay in contact with it.  I would also pour water over the sand bedding before you concrete, it will compact the sand and; run it into any corners under the edge of the slab that gravel fell out of, and also help the concrete in curing. 

   I don't think the footer could possibly come anywhere close to where you're trenching, here a 16" wide footer and 8" basement walls are common so it would be 4" past the edge of the wall.  This is the lip that anchors the basement slab.

   remodeler

2. 
   xMikeSmith | Jan 28, 2003 04:42pm | #3

   try a two foot sample first... you want to know how far your footing extends under the slab before you decide wether to be 12"   or 10"   or 16"  or whatever...

   another thing.... you may want to cut two lines so you can remove the material betwween the two instead of everything between the wall and your offset line..

   suppose your footing extends 8" under your slab... maybe you cut one line 6" off the wall and a 2d at 18" off the wall... then your concrete patch will go easier, and partially bear on the footing..

   if you only go 12" off the wall, you could end up with not enough room betweeen the footing and your line to get pipe and stoine in....

   so do a little exploring first

   Mike Smith   Rhode Island : Design / Build / Repair / Restore

1. 
   JohnSprung | Jan 29, 2003 04:10am | #6

   If you go with parallel saw cuts as Mike suggests, you might be able to save yourself the jackhammer and compressor rental.  4" x 12" isn't all that difficult to break up with an ordinary 16 pound sledge.  Or at most an electric rotary hammer. 

   The hard part is getting the first little bit broken up and dug out.  Thereafter, you can get under it with a long bar and lever it up a little.  When it's no longer supported by the ground below, it breaks real easy with a sledge.  Start in the middle to leave yourself room for the long bar and reduce the amount you have to lever up. 

   Also, as long as you have the saw, put in some perpendicular kerfs to make things easier.  Put a bunch in the middle for the start, and then every foot and a half or two feet.  If it breaks cleanly enough at the kerfs, you may even come out of this with usable stepping stones instead of rubble to throw in the dumpster. 

   -- J.S.

1. 
   billyg83440 | Jan 29, 2003 11:43pm | #7

   Very good ideas.

   If he's willing to deal with some dust it sounds like a winner.

   Billy

1. 
   xMikeSmith | Jan 30, 2003 12:05am | #8

   dust ?.....one guy on the hose, one on the demo saw...keep it weeeeettttt

   try to rent a big electric one... the gas fumes are way beyond brutal in a closed spaceMike Smith   Rhode Island : Design / Build / Repair / Restore