Testing Existing Caissons

[molerat2210]

I am interested to re-use 63 existing caissons (i.e., drilled shafts)that supported a previoius building. The shaft diamters range from 3.0 to 4.5 feet and the bell diameter ranage from 5.5 to 8.0 feet. As built drawings indicate caissons constructed on sound bedrock for an allowable bearing pressure of 50 tsf. I would like to prove capacity of each caisson to 2x design load or about 5000 tons max. I understand Statnamic tests would work for this purpose but cost prohitive for 63 tests. Any ideas for less expensive methods to prove the capacity? I will core through the center of a selected number to verify strength of the concrete and bedrock.

 

[aeoliantexan]

If you can determine the actual loads imposed by the previous building, why not consider them already tested, and design them for that load? I you want to use them for a larger load than the previous use, the structural design of the shaft may control. If my rough calculations are correct, at 50 tsf bearing, the shafts were stressed to about 2300 psi, which may have been all that the code allowed at the time they were designed. You may want to do pile integrity testing to evaluate the quality of the shaft construction.

 

[molerat2210]

Aeoliantexan,

It is a heavy concrete building. Your idea is a good one. We probably will not need 2,500 ton column loads in the future but we don't know what we want to build yet since this is the demo phase. The thinking right now is to get some feasibility information for concept development and design phase.

The problem with re-use of existing foundations has always been getting to 100% certainty (99.9% confidence isn't good enough). I would need to collect enough information at the end of the demo phase to develop the design. I am now looking at a drop hammer dynamic test (ASTM D 4945) at a reasonable cost. The drop hammer test, core sampling and 40-year load test might be enough to get me over the hump. Not sure if I can get over the hump without some sort of test.

The problem with PITs is the bells since it's not a uniform cross section.

Thanks for your input.

 

[molerat2210]

Aeoloantexan,

I had the wrong diameter in my first post. The largest shafts are 5.5 feet diameter and 8.0 foot diameter bell with allowable bearing pressure of 50 tsf. From that, the working stress in the shaft was 1470 psf. As built drawing says 3000# concrete for caissons. The buiding was constructed in 1975. I guess they allowed higher stresses in the old days?

 

[massagung]

just to add,
as we know to estimate bearing capacity there is many method, static, dynamic and others. for existing caisson to make sure the bearing capacity i suggestion to test axial static compression load test (ASTM D-1143) of course this is not less expensive than statnamic test. but if there many existing caisson pile you can mix the test using HSDT/PDA (ASTM D-4945). if you compare cost of testing and production new pile in my rough calculation its still can be acceptable lah..

M.A.

 

[molerat2210]

Thanks massagung,

I did change the testing plan to HSDT ASTM D4945. HSDT tests appeared to be limited to test loads of 2500 tons. That per test cost of statnamic test is very high and not well suited to this job since we want to test every caisson.

 

[oldestguy]

There was an expert in caisson design and construction in Chicago, named Clyde Baker. Might still be alive. He put some monstrous loads on caissons. See if you can find any of his papers on the subject. I remember some at ASCE conferences in the 60's, but have no library now. He even want so far as to core drill some as I recall. Was Engineer of the Year for Engineering News-Record some time ago

 

[Ron]

You don't need to test all 63 caissons. Test a few to validate their capacity as designed. You'll probably find that they are capable of more than the design.

If you tested 10 percent of them, it would be a statistically defensible number of tests to validate your assumptions.

 

[aeoliantexan]

Testing all the caissons would be unprecedented. If this were new construction with documented design and construction quality assurance, testing any caissons this large would be unusual. I would start by searching for the original geotechnical investigation report and the reports of construction inspection. Potential sources include the original building owner, the design firm, the geotech firm, the contractor or subcontractor, and the City building permits department. If you find those, you have as much as the original owner had available to rely on the caissons, plus a full-scale load test of every caisson! If you are not planning to exceed those previous loads, why would you want more? This assumes, of course, that there was no record or visible evidence of poor foundation performance.

Caissons on competent rock rarely fail by excessive settlement or bearing capacity failure. The failures that have occurred were usually due to some construction deficiency, such as entry of water or soil into the concrete during placement. The PIT test should be able to verify the competence of the shafts down to the top of the bell. That would not be expensive. A few cores through the bell and into the rock should give a general check on the quality of construction, including the concrete quality and the cleanliness of the bearing surface. Generally, that was better 30 or 50 years ago than today, because down-hole cleaning and inspection were common practice.

Find a well-established local geotechnical engineer and get his advice and perspective.

 

[molerat2210]

Thanks for the more good advice.

I think I have settled on the plan. The specs will require the contractor to test all 63 using ASTM D 4945 and core 5. I like the idea of getting intergrity test data above the bell. I do have the as built drawings, which are the original drawings stamped "as built". This is not sufficient to me to prove that the bells were constructed properly. I suppose to original inspectors have gone off to meet the great geotechnical engineer in the sky (and taken the records with them). The high strain dynamic load test aspect of ASTM D 4945 will service as the proof test of load-carrying capacity of each shaft (up to 2500 tons). The existence of the previous building does not necessarily prove any specific load carrying capacity for each caisson since the maximum previous load due to the variety of load combinations will never be known. I am not in the risk taking business (neither is anyone else involved), so this plan looks to me to provide the needed evidence.

Thanks for the input.

 

[Ron]

Your program is overkill and economically wasteful.

 

[molerat2210]

This program is about $150K. That's a fraction compared to costs of new foundations. I feel this is somewhat innovative. I usually see the old foundations abandoned. There is an alternate plan to test only 5 (like Ron suggests). It will be for the strutural engineer to decide.

 

[aeoliantexan]

The $150K cost seems reasonable. You team certainly is entitled to decide how risk averse they want to be. And I do commend you for choosing to use the old foundations if possible.

We will be curious to know the outcome of your testing program, if you are willing to share it.