A new two-story residential home with a 9-ft deep basement is being planned for some property I own outside of Chicago.  Four soil borings were performed at each corner of the existing house which will be torn down.  4,000 PSF bearing soil was found starting at 3.5 ft deep at the front corners of the house which then was found to drop to 14 and 19 ft deep, respectively, at each corner of the rear of the house.  The geotech soils report I received today states:

"We have included the depths and elevations to firm soil for a 4,000 PSF soil bearing capacity in the soil boring logs and plot plan summary, which can be utilized to support the loads of a caisson foundation system.  On the basis of the field and laboratory investigation, the caissons would be designed for all load bearing exterior and interior foundations, as well as the basement and garage floor slabs on-grade."

Can you provide a measure of how many caissons are normally drilled for every 10-ft length of house foundation?  For instance, if the new home has dimensions (55Lx50W)x2=210 linear ft., how many caissons would be required regardless of depth?

Lastly, any idea on what the normal size of a residential caisson is (8-inches in diameter, etc.)?

The caisson has to support the weight of the house and the footing has to be able to span between caissons.  For a grade beam sized about 16" wide x 20" deep, we put them on 7' centers.  I said grade beam instead of footing as the bending is in the opposite direction over the tops of the caissons so two mats of steel are required.  The 20" dimension in our area allows 12" in the ground and 8" out.  For Chicago the frost depth is much more so the caissons can be spaced further apart as the grade beam will be larger.  If the house is two story frame, figure 2500# per lineal foot including the footing.  For bearing at the bottom of the shaft we will also include the weight of concrete in the caisson.  At 150#/CF the weight of concrete in a caisson 20' deep is 3000#/sq. ft. For a caisson of this depth a utility type digger derrick truck would drill a 16" hole.  That's 4200# of concrete on a 1.4 sq. ft. area.  If the caissons are at 10' we add 25,000# to that and get 29,200# on 1.4 sq. ft. for a bearing capacity of 20,850 PSF.  Clearly they cannot stop at the top of the 4000 PSF material but must penetrate deep enough to pick up the remaining load in shear.  I am not familiar with your local conditions, although I was up there for TARP work, but around here you get about 1000#/SF of shaft area, so for a 16" caisson you would gain 4200#/lin. ft.  At 35' with 15' in the 4000 material, the load is 25,000# house plus 5200# caisson = 30,000#.  Caisson resistance is 15'x4200#+ 1.4 sq. ft. x 4000PSFx1/3=65,000#.  You must reduce the end bearing to get them to work together as it picks up slower than side friction. This gives you a recommended 2:1 safety factor.  So figure 15' penetration into good material at 10' spacing along the perimeter and the inside grade beams.  Interior load bearing walls will be those supporting joists running perpendicular.

I am a Geotech. in Chicago.  You need to have a Structural Engineer review the Geotech. report and design the foundation and seal it.  Who did the Geotech. report?  FYI PSlem side frction is usually not counted on and if necessary the drilled pier would be belled at the 4000 psf level and would not penetrate beyond.  The basement wall would be designed as the grade beam spanning between the drilled piers.

I did omit the shape/depth factor for bearing at the bottom of the caisson.  This is 9 times the cohesion or 4.5 times the bearing capacity.  A poured basement wall would suffice as the grade beam.  This is the sort of problem that should be designed and sealed by a local engineer as you can get by with strip footings in front, but you need to be sure dissimilar foundations will not cause differential settlement.

I realize many questions are not simplistic in nature and appreciate not only both of your answers, but your skill that I don't have in this field.

Illinois Drilling & Testing out of Addison, IL performed the tests.

One other piece of information from the report which wasn't provide in my original question due to length was:

"Although specific depths to suitable soils are indicated in the report, the caissons may extend beyond the indicated depths due to an expected or unexpected water inflitration and the use of temporary casing.  Once the temporary casing has been installed in a caisson shaft, a sufficient depth is required below the bottom of the casing in order to bell the caisson.  Due to the amount of fill which may be placed on the site to achieve desired final grades, it is possible that the fill and underlying natural, softer/organic soils may still be susceptible to long term settlement.  Therefore, the caissons should be designed accordingly to allow for negative skin friction (i.e., "drag") on the walls of the caissons."

Thanks.

Hello Runway 911:

Just a couple of curious questions as a result of your response to comments made by respondents.

1. Are the recommendations given in the geotechnical report very meaningful to a structural engineer in designing the caissons.

2. How much did you spend on the investigation and report.

3. What specifically did you request the company to undertake

4. Would the design of the caissons be undertaken by a structural firm to be provided additional data by the geotechnical company

5. Would there be additional charges by the geotech company for providing such additional information or is the structural company expected to determine the required information from the geotechnical report.

The reason for asking these questions results from past discussions regarding geotechnical reports, scope of work etc.

Regarding the concept of end bearing and skin friction, it is genearlly accepted that to derive full end bearing the base of the pile has to move a considearble amount and that skin friction would be first mobilized. However, many practice the concept that end bearing should be used for belled piles. I think one needs to examine the loads that will be applied and settlement concerns to make a judgement on whether one or both factors are applicable.

In the recommendation of designing the caisson to allow for drag load the concept would be related to the structural integrity of the pile i.e that the pile is sized so that it does not fail as a structural member. This is probably hardly likely to occur for the problem at hand and requires an unyielding base, other than that settlement would be the issue.

From a personal viewpoint I rather see these recommendations in a geotechnical report with more discussion so that the structural engineer understands fully the implications. Only my thoughts. Also is price a factor.


     

I have a 64 baht question to put forth.

You indicate that it will only be a two storey house and you have a 9ft (3m) basement to be made.  Now the 9ft is well below the 4000psf bearing pressure level.  With the basement, you are removing quite a bit of loading - like a bouyant foundation.  Housing loads are not all that great so the fundamental question I pose is:  

     Why do you need to support the house on caissons??

Can't you simply support the house on spread and strip footings (maybe use a strip grid raft between columns, etc) and make them wide enough to reduce the loadings to 4000psf (taking this as the gross allowable bearing pressure based on shear - rather than the allowable bearing pressure based on settlement which should be neglible for a 9ft basement removal.  This seems to me that it would be more cost effective than bringing in drilling equipment and putting in drilled shafts.

Now if you are in Texas (Focht3 territory) the question of expansive soils would be raised; but I doubt this is needed in Chicago.

In response to the questions - my answers are in the parenthesis:
1. Are the recommendations given in the geotechnical report very meaningful to a structural engineer in designing the caissons? (Haven't yet been able to contact a structural engineer.  I planning on having a builder/architect coordinate.)

2. How much did you spend on the investigation and report?
(The boring and report are all included in the total cost which was $330 per boring (4 holes drilled) and $15 per foot for every foot they had to go below the first 15 feet to find bearing soil.  Total cost was $1,650.00.)

3. What specifically did you request the company to undertake?  (Determine quality of soil and bearing depth - it's said a creek used to run through the area until the town moved it in the 1890's or so.)

4. Would the design of the caissons be undertaken by a structural firm to be provided additional data by the geotechnical company? (I believe the formal report constitutes all the geotech. company would provide unless they were contracted for more services.)

5. Would there be additional charges by the geotech company for providing such additional information or is the structural company expected to determine the required information from the geotechnical report? (See answer to #4. above.)

For the other question:  Why do you need to support the house on caissons?

The front of the property appears to have good bearing soil at 3.5-ft depth, but it drops dramatically in about a 37-ft distance to a 14-ft depth and 19-ft depth.  I guess the geotech. firm is saying that the slope of the bearing soil from front to back of the house is so dramatic that the 9-ft deep basement would fail to sit on bearing soil within such a short distance that caissons would be needed for probably 3/4 of the foundation.  Perhaps they want to prepare me for the most expensive solution in the event the soil really is this poor.

Thanks!

Okay - I understand now that the "good" soil slopes - but still, the existing soil (or fill???) has sat there for a long time, likely.  What is the "soil/fill" above the 14ft or 19 ft depth but between it and the 9ft depth?  If it isn't "loon shit" (the only word you can use of its ilk in the geotech argot in front of important clients) - I'd still say that you might be able to found okay - maybe dig out 3 to 4 ft more in the low area (and in this you need to "follow" the actual "good" soil level through the basement) - then backfill with good crushed . . . - anyway, there are other options than drilled shafts (caissons) and you should get your structural guy, when you get him, to give you the various options vs cost.
     

Go with caissons. I have seen too many settled houses where somebody tried to wing it trying to "bridge" over organic silt or peat to save a nickle.  Don't put any $ in an underpinners pocket, you'll be sorry!  They have truck rigs up here that can blow in caissons probably cheaper than using a mat or undercutting with  virtually no risk.  Besides it  is your house (you like to sleep at night?, I do!)