Field testing to verify allowable design bearing pressure

[phllp]

How do DCP test results correlate to verifing allowable design bearing pressure? I know you can correlate DCP blows to a N-value which is how the orginal alowable bearing pressure is based upon along with other soil properties (phi angle(friction angle), coheision value, unit weight, etc). I have heard that a DCP is 2.5 to 3 times a a N-value. Are there any papers, publications, or standards that better descibe how to verify allowable bearing pressure in both sands and clays with DCP or hand penetrometer tests? My geotech textbook only explains how to calculate bearing capacity using terzaghi and other methods to check for settlement and bearing capacity with soil boring data but there no explaination on how to verify that allowable bearing pressure has been achieved in the field during the actual construction process.

 

[fattdad]

Foundation bearing pressure is more frequently determined on the basis of performance (i.e., to control settlement). You can do the rational bearing pressure assessment AND have too much settlement. If all you have is N-value, you can correlate to soil modulus and then integrate the change in vertical effective stress to calculate settlement (accounting for varying moduli within the seat of settlement). DCP (as also for N-value) is only valuable if you extend the data throughout the seat of settlement - often taken as 2 times the width of a colum footing or 4 times the width of a strip footing. Schmertman has information on this - much of which is related to CPT data (if I recall correctly).

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

phllp - search other threads in Eng-Tips. We have discussed DCP many times - and as to many variations on what the DCP is (Miniature TRRL or Scala Penetrometer, vs Sowers penetrometer vs more robust pentest (Cdn).

 

[jdonville]

phllp,

Depending on the sensitivity and importance of the structure, full-scale footing load tests may be performed using pile testing equipment to determine the load-deformation response of the foundation media.

As fatdad states, you need to take your verification through the influence zone of the foundation.

Jeff

 

[BigH]

phllp, I think, is trying to justify saying to the building permit people that the footing of a residential foundation meets with the requirements for which it was designed - I think that many building departments require "verification" for every residence in a subdivision - although, we know that a borehole would not be put down for it. Correct me if I am wrong, phllp.
Full "load tests" would not be performed except for important structures (and this includes settlement sensitive ones) - they are costly AND one wouldn't be flatulently considering the use of a DCP in such cases. Full "load tests" would be the most positive way to go in sands or relatively free draining soils but I would be leary in cohesive due to the slow porewater pressure response below large load tests - and don't look at "standard" plate load tests as being anywhere close to an equivalent.
You might do a search on the net for something - I recall seeing some 'hits' on using a DCP for residential (and light commercial) structures.

 

[iandig]

I addition to the other comments made above, and depending on what part of the world you are in, in Europe we are going through the introduction of Eurocode 7 [BS EN 1997 in the UK]. This series of documents, and section 3 in particular, covers geotechnical design using field testing. It also defines how each test whcih can be undertaken, should be used for design purposes. At present it should have been started to be introduced from July 07, with the removal of previous BS's by 2010 [BS 5930 will be removed], although there is still some concerns over the piling design which has resulted in some delays. From memory and reference to the previous drafts, I believe that it covers the use of Dynamic Probing for design purposes, and makes reference to the various different methods. From BS 5930 the Super Heavy Dyanmic Probe most closley matchs the mass of the SPT equipment, however as mentioned above, and test which probes the ground dynamically can be referred to as Dynamic Probing. You do need to check the mass of the hammer and the cone dimensions before undertaking the correlation between DCP and SPT.

 

[Mccoy]

EC7-3, as iandig says, includes correlations for some soil parameters. Strangely enough, though, correlatiosn are given for the light and medium DCP's and not for the Super-heavy, which is, given its mass, the only one which yields relatively reliable results.

Also, it should be used with much caution, being sensitive to lateral friction with the borehole.

In sandy soils, torque measurements are taken to check the presence of any spurious situations.

In clayey soils, results are much more iffy.

 

[phllp]

My main question is for example you have a shallow foundation and the plans call for a design bearing presure of 3000 psf. What are tests used to verify that the soil condition can support 3000 psf? How do tests differ for sands or clays? What are the ASTM specs for these tests and what is the best way to inerpet the results to confirm that 3000 psf has been achieved?

 

[fattdad]

The plans should not say a design bearing pressure without first having a design-level geotechnical engineering study to confirm this parameter. During construction, there should be no need to confirm the bearing pressure other than to confirm that the foundation is bearing on the designated soil stratum(a) and the subgrade is properly prepared (i.e., free of standing water and disturbance). My preference is for the design geotechnical engineer to be the one in the field during construction.

I really hate to think that ASTM will get to far into this as it has more to do with the geology and the engineering properties at depth than just running a test on the subgrade.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Fattdad: Good points! If you have had a soils report and know the relevant design stratum, you shouldn't have to independently verify on site that it is met - the design report indicates it is. However, you do need to be able to tell if the soils encountered differ from that of the soils report (like hitting a pocket of clay that wasn't mentioned). I think my views are well known enough when I echo yours - ASTM stay to specifying test methods and stay out of the interpretation and application thereof.

 

[fattdad]

I always like a vote of confidence - ha. On the matter of ASTM - ever looked at the ASTM for running a CU triaxial shear test? Interesting to note the report doesn't require the determination of effective friction angle. That is left to the engineer (as it should be). In the case of a CU, the definination of "failure" can be based on peak strength, residual strength or strength a some defined strain. There's nothing I dislike more than having someone report a friction angle on a CU test when it is truely a matter of engineering judgement. Just give me the stress strain and p-y curves and the pore pressures. I'll do the rest.

Some things urk me - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

since dcp and bearing capacity have been discusses many times on the board, i won't comment on that. but i thought i would post a link for an astm work in progress since the issue of astm and dcp has come up.

"New Soil Compaction Verification at Shallow Depths Using the 5-lb Dynamic Cone PENETROMETER--This test method covers the procedure for the determination of the number of blows required for a dynamic cone PENETROMETER with a 5-lb (2.3-Kg) drop weight falling 20 inches (508 mm) to penetrate a certain depth in undisturbed soil and/or compacted backfill. This test method can be used for the determination of compaction efforts of soils, subgrades, and granular base materials at shallow depths."

while i have used dcp to very generally describe the inferred compaction (i.e. "poorly compacted") of shallow fill, i'm interested to see how astm prescribes such a method and quantitatively describes the compaction when the test can give such widely varying results depending on the material or things contained therein.

http://www.astm.org/cgi-bin/SoftCar.../astm.org/DATABASE.CART/WORKITEMS/WK10679.htm

 

[BigH]

I think because ASTM and others want to code us and standard us to death. It would be really amazing to see if Terzaghi, Peck, Golder, Tschebotarioff, and others could have done their pioneering work under such regulations. I tend to doubt how a 5 lb dynamic cone can go through a well graded 50mm minus crushed stone base and tell us anything meaningful - 'ell, the aggregate is bigger than the cone tip.

 

[msucog]

you said it. i wonder what their correlation graph will conclude about "2,337 blows per increment"...and i don't want to be the person running that test.

depending on how it's written, i could see a spec like that being used as a scapegoat for shotty engineering. say you've got a shallow sandy silt soil fill that has #57 stone scattered throughout. Shotty Engineering Inc runs dcp and get 16 bpi and conclude that the fill is compacted to 98% modified Proctor compaction. then whatever the fill is supporting then settled 3 inches. and as it turns out, the fill was backdumped and not well compacted but got high blowcounts because of the aggregate. they could say, "we followed astm and industry standards therefore we exercised a reasonable standard of care."

i tend to agree that having an astm test method that a monkey could run does not substitute true engineering.

 

[fattdad]

Final subgrade preparation included a 1-in thick layer of concrete. DCP recorded penetrations in excess of 100 blows per increment. Verified bearing pressure exceeded required 3,000 psf.

I can see it now. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[DarthSoilsGuy]

ASTM and Humboldt will have us DCP-ing inside our proctor molds so we have something to compare with.

 

[BigH]

Well, they already have that penetrometer in a mould (D1558) which is supposed to correlate compaction penetration with moistue content. ;-)

 

[DIRTyEngineer]

I read in another Eng-Tips forum that "There is a great paper from 24th Annual Symposium on Engineering Geology & Soils Engineering called "Dynamic CPT for Test-Pit Field Investigations: Experiences with Sowers' Cone Penetrometer" by L.E. Robinson that provides several equations to correlate Sowers' DCP blows to SPT"
Does any one know where I could get a copy of this I found the Symposium on Engineering Geology & Soils Engineering web site but this volume is out of print.

 

[MRM]

It's a question I get frequently: "I got 6 blows...How much bearin' is that? How many blows does 3,000 equal?" Phllp, as others have suggested, there are many threads on this to look into further.

I wanted to echo the collective sentiments of others about standards. No matter how good the intentions are, standards, and the use of them, can range from "decent guidelines" to "things that should be obvious to even the most casual observer" to "worthless" all the way to "downright dangerous." Good judgement, while carefully considering all of the neat numbers you can get with testing, needs to prevail.

 

[Mickney]

Several grey hairs here as a principal engineer working with Piedmont residual soils. We typically verify bearing pressures in footing excavations with DCP testing. We use this because it is quick and a good tool to check the bearing conditions at each foundation location. This is done to ensure that the bearing soils between the original test locations are consistent with our recommended bearing pressure that was based on SPT/Pressuremeter/Lab testing. Surely one does not use such extensive SPT/Pressuremeter/Lab testing at each foundation location.

Anyway, to answer your original question, check out George F. Sowers and Charles S. Hodges, "Dynamic Cone for Shallow In-Situ Penetration Testing" Vane Shear and Cone Penetration Resistance Testing of In-Situ Soils, ASTM STP 399, Am. Soc. Testing Mats., 1966, p. 29. Please be aware, as stated in the abstract, that "...the penetrometer has been found useful in the inspection of footing foundations and for light field exploration where the standard penetration range of limits is generally know. The test data show that it is capable of approximating the standard penetration resistance for the virgin soils of the southeastern U.S." I hope this gives some direction. Best regards.