Posted by Billy NY on December 05, 2008 at 06:17:28 from (205.188.117.74):
In Reply to: OTdigging a basement posted by Fudpucker on December 04, 2008 at 16:57:15:
Used to do a lot of those when working for a site work company, most of the issues have been mentioned already, but I'll add my 2cents worth :)
If one was using a large rubber tire'd backhoe instead of an excavator, one that has enough reach for the intended depth, you really have to plan out where the spoil will be stockpiled, starting with the removal and stockpiling of topsoil. As you excavate, those piles will get large and close to the cut, making the sidewall of the cut potentially dangerous. Remember that when you excavate naturally compacted earth, it will fluff up 15%-20% in volume, increasing the space you need to stockpile.
You can quantify what will come out of the hole, and if you have an idea of the topography around the site, be it on your building plans site drawing or not, you may have areas that can be filled, don't fill any wetlands or similar.
A lot of things depend on your soil conditions, around here you can make cuts and or dig trenches without shoring or benching back for an angle of repose at 45 degrees, go a few miles or to some different areas, you can go from this to fine sand which can be very dangerous, a person was buried several weeks ago, in a trench and in those sand conditions, result was fatal. Here we have a mix of gravel, clay, loam etc. trench walls are stable, but you still need to use a trench box or shore it up to comply with safety requirements. I've been in a large trench collapse, 17'-0" deep and over 20 feet wide, in sand, back when I was in my early 20's and have also hand dug another person out in a different collapse, he was lucky, you can absolutely never trust the wall of any cut, more so within a trench type cut, even a shallow one.
Layout: It would be advisable to have your approved plans available to a surveyor and have the surveyor, layout the lot lines and building lines, providing you with offset stakes for your actual building lines, so you have good reference points to layout the foundation cut. You also have to remember that this cut must provide enough room to work on the footings, drainage and anything you need to install on the outside of the foundation wall, until inspected and approved for backfill, if you are subject to those requirements. In addition, the walls of the cut must be stable or you must bench down or cut at a 45 degree angle of repose from the bottom of the cut(basement). You need offset stakes, set back from the actual building line for reference until you have your foundation footing and wall are forms set, if they are too close to the building you lose them during excavation, reason why we call em offset stakes, this provides you a reference point to locate the building.
Elevations: Your approved building plans should contain a site plan with the topography, and building elevations. Your surveyor should provide an elevation or bench mark(s) for your reference, so that when excavating you can check grade with an optical level, which will make your foundation hole level, and also prevent you from over excavating, you need to check periodically, taking a reading while someone holds the stick, when you get close you take your time to grade the hole, once that hole is graded, the entire foundation hole excavated, you can move in and do the footing excavation, with soil conditions here you can trench pour those, other times you have to form them. If you dig to subgrade of basement slab, to natural undisturbed, you may also want to consider that the natural material will be suitable for subgrade, or you may want to excavate that out and replace with gravel, crusher run etc. something that is more stable and will compact. Top or basement slab - minus thickness of slab, - minuse thickness of subgrade material, can be bottom of the hole. This needs to coordinate with the footing detail, so you can excavate and or form that out, top of footing is usually even with the bottom of the hole, subgrade, and drainage materials above, then the slab, all this would be coordinated on your building plans, usually in section view of the foundation as detailed by your designer.
With an established layout, you as an operator need to look at the site and plan or map out exactly how you will perform the excavation work, know where the machine will be set and where it will reach so you do not excavate your way out of an area you still need to reach later. You cannot set the machine on air or too close to the cut. You work within the reach of the machine and complete areas as you move, which is why you need a benchmark and have the ability to check the depth of your hole from a level line to the elevation that matches the height dimension of your foundation walls, your top of foundation and finish floor elevations relative to the proposed or existing grades are most important, you want to make sure enough of the foundation is below grade, but enough revealed so that your finsish floor elevations for the building are above grade, to keep water out etc. All that should be coordinated on your site plan or within your approved drawings.
Utilizing a rubber tire'd backhoe certainly can do the job, with an experienced hand in the seat, I know I could do it if it was all that I had. However, I agree with the others, you will be double and triple casting the excavated material, (spoil) as well as making use of the front bucket to push or haul away the spoil to another fill or stockpile area away from the hole. If this is a tight site with little room, it will be even more of a headache, you may need to have trucks available for loading and hauling out if you can't get rid of the spoil on site.
If you can understand the above, can figure out the layout, both building lines and elevations, know how to work in the soil conditions you have, (this time of year sites can get real messy) and can safely work with the machine, it is possible to be done, but again you really have to keep enough distance from the cut and still be able to get the reach you need to finish areas before you move the machine, you might dig awhile, then have to move spoil, it will be a lot of work for the a rubber tire backhoe, it had best be in good working order and be able to take some hard hours.
If it were me, I'd want a medium or large excavator, larger if the site allows, I'd do the above and work my way around the hole, checking elevations, and casting the spoil far enough from the hole you don't have to deal with it until backfilling the foundation, using it elsewhere on site, or loading excess out onto trucks.
Another unforseen thing to deal with is water or rock, if you have water within the cut, at the elevations you need to excavate to, this changes things, you have to dewater until that foundation is complete, you may have to raise the elevation of your finish floor in the basement to avoid a pressurization under the slab and water seeping or forcing it's way in. Rock, depending on the type, you may need to use a breaker on the end of the hoe, use more agressive methods to excavate, or raise elevations so less rock has to be excavated. Both of these will drive site work costs up dramtically, unforseen conditions must always be considered when doing any site work.
If you can understand this work, it's not all that difficult to do on your own, better to have an excavator, large enough to do the job, say from a rental house, enough knowledge on what is described above, rent an optical level and know how to use it, and have the surveyor provide your layout, offset stakes, lot lines and benchmarks to work from. When doing the mass excavation you can learn to run the machine, but would be best to have an experienced operator if available, especially for grading out the hole, you can learn to load the bucket and do the main "hogging out" of the fill, but when you get to the finess work, close to the elevation you want, grading skills are necessary, if you don't have those skills, this is where things can get messy and you have to put back material, compact same to 95% so things don't settle, you want to excavate to an elevation, level across the foundation.
You can save a substantial amount of money in many situations when doing site work, by doing the job yourself, if you have the knowledge and ability, and can rent the right equipment, but this is real intimidating, time is of the essence, meaning your construction schedule is aggressive and have no time to waste, or you don't have enough time yourself, coupled with inexperience, it can become a costly mess.
Safety, you must be aware of all work safety aspects, of this work and make sure to call the one call center for ALL UNDERGROUND UTILITY location BEFORE any excavation work commences.
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