Generator Pad Design Questions

[PengStruct]

Dear readers,

I've been asked to provide a price for designing a foundation for a 5,000 lb emergency generator set which is to be installed on grade next to a apartment building with a 13.5' deep cellar. When I check the 12" thick concrete basement wall for the soil and surcharge (generator & pad) loads I'm finding that the wall may not work in bending. Any thoughts about how to bring the slab loads down to the bottom of the wall? Sonotubes? Piers with square footings? Has anyone had any luck with either of these approaches? The generator is rotating type and I would size the slab for 3x the weight as I've seen suggested elsewhere on this site.

Lastly, I'm assuming that the soil here is uncontrolled backfill and would be specifiying 12" lifts of controlled fill over an area a few feet wider that the footing while using the code minimum 1.5 tsf allowable bearing and 60 pcf lateral soil load on wall. After reading the code it seems I can do this without requiring a geotech for this job but would appreciate if anyone can advise of anything Im missing?



Chad Serman

http://www.sermanengineering.com

 

[oldestguy]

I was waiting for others to jump in. So, a question first. Which way do the first floor joists run? Can you transfer the top of wall inward tendency to the floor system, even to opposite wall.? I have done this at houses where tornadoes might move the house off the foundation. Place scabs on the floor joists from below as one way. If joists not there, how about the beams set in the wall, with angle clips to transfer the "push"?

In addition, can you hang the inboard edge of slab on the wall, as by SS angle and SS anchor bolts?

These two alone may be sufficient for that loading, along with support adjustments in case of outboard end movement through the years.

Assuming the length of time for generator operation will not bother residents if vibrations get transferred into the building. Even so, better than no power.

 

[PengStruct]

The top of the wall is actually braced by a concrete slab at the ground level so there are no floor joists. So, checking the wall simply supported with a trapezoidal lateral load with surcharge from the generator and pad will not work if the wall is minimally reinforced. I am planning to try to chip away the concrete cover to see the bar size and spacing but, assuming that there is not enough reinforcement, it seems my only option is to support the generator pad on piers at the corners at least that bring the load down to the level of the adjacent footing to avoid adding horizontal load to the wall. Just hoping to get feedback from anyone who has seen this before?

Chad Serman

http://www.sermanengineering.com

 

[oldestguy]

Mini piles may be an option. Short lengths of pipe (then joined with couplings) forced down by jack hammers can develop rather good loadings. A backhoe also can be used, with bucket doing the push. Applying proof load with heavy vehicles and hydraulic jacks is a possibility. There may be an underpinning contractor that can do the whole job relatively cheaply. With perhaps 6 mini piles, I'd bet you would have no problem supporting the slab and generator. It sure doesn't look like a necessity to bring in a big rig and all the expense with that.

 

[oldestguy]

Another option is Chance anchors. Install by a utility auger and then apply proof load. More capacity each compared to mini-piles.

Likely a local utility contractor can do the job. Lengths will depend on what is encountered as they go in.

http://www.foundationperformance.org/pastpresentations/DeardorffPresSlides-8Apr09.pdf

 

[oldestguy]

Dig around on the Internet and you an see the many variations. Considering the light load you have, you probably don't need the drilled installation of pipes.

Check this video as what I envision.

http://www.youtube.com/watch?v=-EEbnMrSwAc

Numerous options.

 

[PengStruct]

Thanks for all the info. But wouldn't it be much simpler to support the slab using (6) 12" dia. sonotube footings that go down 13' ? I'm finding that this would be plenty of bearing area at least for the gravity loads.

 

[oldestguy]

Sonotubes are fine, but getting them down there is the reason simpler things are what I brought up. Will the ground cave in to the drilled holes you need? Of course digging with back-hoe will work for this, but what a mess might develop if there is caving ground. Will you have to sheet-brace the hole to hold back loose earth? Any buried utilities in the area? Then getting the site back to normal is another cost. You then run the risk of damage due to the back-fill work,

Another option is grouted auger holes, done by injecting grout in hollow stem augers as they are withdrawn. But that takes some modification of standard test boring equipment. I saw that done once by a test boring contractor. A regular grout piling job takes a crane and the associated drilling gear and grout pumps, all pretty expensive for this small job.

I'd bet there are underpinning contractors in the area that can do these micro piles or similar at much less fuss and cost. Ask around.

 

[motorbiketocrank]

I'd try to address the problem by lessening the load rather than beefing up the foundation. Ex: Increase pad size to reduce pressures, replace a certain depth of soil with lightweight material below the pad to offset the new load, etc.

Oldestguy is right on the construction issues. No one ever wants to pay the cost of geotech investigation so they assume things will work just fine and then run into collapsing holes, buried debris, groundwater, etc during foundation installation. Also, the weight of equipment necessary to install such pier/pile systems may cause you more of a problem than your generator.

 

[PengStruct]

Thanks geobdg, That will be the approach to begin. The site is confined somewhat so the bigger I make the footing the more it encroaches on other building foundations and retaining walls (dense urban area) I'll try to get the m/e/p engineer to give me the absolute lightest loads to use for all equipment and then verify the actual wall reinforcement. Hopefully that will do it and I can base my price on that. After that I'll tell them they'll need to hire a good geotech.

 

[PengStruct]

Last question for you guys: If it turns out just using the slab is ok, can I put the slab on the uncontrolled fill? I see in the code that there is a bearing capacity for uncontrolled fill if there are no organics or debris but would you recommend removal of some of it to be replaced with controlled fill? If so, how much should be removed?

 

[oldestguy]

Sounds like a funny code. However, considering that the load spreads out in the soil below the slab, say with 60 degree from horizontal (and hangs up some on the wall, most old codes will allow 1,000 psf on almost anything. I'd guess you are well below that anyhow. Assuming the wiring is flexible, or can take some distortion, wait until the load has been there for a while (???) and any immediate settlements are gone before connecting things. Why not then make provision for adjusting the unit's position if there are changes through the years. In Long Island area, I would assume any frost action to be rather minimal. If silty ground is present, perhaps replace the top 2 feet with clean granular soil to minimize frost heaving.

As for finding re-bars in concrete, I've tried several methods and have not found any that are good. A surveyor's dip needle may do OK for those near the surface, as would a compass.

 

[aeoliantexan]

I would not trust uncontrolled fill. If it gets wet at some future time, perhaps due to some event you can't forsee, it could settle several inches.

I would favor some type of piers penetrating well into the native soil below the backfill. That sidesteps the question of lateral pressures against the wall as well as the backfill question. Chance anchors may be a good choice. They can be installed with portable equipment if access is restricted. But without either extensive experience in the vicinity plus knowledge of the geology, I could not defend a design nor the assumption that the conditions will allow it to be installed. Exploration is needed.

 

[PengStruct]

Thanks guys, we will be getting a geotech on board. Appreciate the help.

 

[kieran1]

Why not work out what depth of foundation you would be comfortable with. Ie depth at which the celler wall will work when the surcharge is applied from the foundation. You could excavate out under the pad and fill with weak mix concrete up to the foundation formation level.

Kieran

 

[BigH]

Why not look into using Elastizell as a light replacement for a 5 or 6 ft thick layer of existing soil.

 

[jayrod12]

Wouldn't it be cheaper to reinforce the inside of the wall with angles or channels or tube steel columns that are connected to the slab at the top and bottom(assuming cellar floor is slab) at some spacing. you could probably get the posts themselves for $200 a post. the installation cost is minimal as well.