Drilled piers and load testing

[structengineer]

I am writing a spec for drilled piers. The owner did not provide a geotech report, but through investigation of other structures at the site, we believe that we need to use drilled piers. We are going to require the owner to hire a geotech engineer after the contract is awarded to verify our design. Anyway, I am looking at the spec, and there is several sections relating to testing, including a Load test, Penetration test, and a Proof Test Hole. Is there a rule of thumb of how many piles should be load tested, etc. since I don't have a geotech engineer to talk to? Our job has approximately 50 drilled piers across a stretch of about a 1/2 mile.

 

[JAE]

For the 1000's of drilled piers that I've designed, I've never required a load test. Usually, the geotechnical engineer recommends bearing pressures, depths, etc. and the type of material required to be drilled to or into.

In the field, we require the geotech to be present during drilling and observe the tailings and verify that the proper material has been reached to the proper depth. This provides quality assurance that the geotech's bearing capacities can be reached.

Having said that, I do know that sometimes geotechs may require load tests if the materials they encounter via borings are highly variable, or simply weird, or if the on-site tailings are coming up very different from what was anticipated.

 

[structengineer]

This situation appears to be slightly different from your usual project in that the owner did not want to pay for a geotech so we are only working with some drilled pier assumptions and depths. As stated earlier, we are requiring a geotech to be hired by the owner to verify our design. Based on your statement above, it appears that you have usually worked with a geotech to determine the pier depths and capacities, is that correct?

As far as my specification for drilled piers, should I not include the load test, penetration test, and proof test hole? The geotech will have their own test to determine the values indicated on the drawings and this will be included in their price to the owner? I don't need the contractor to hire a separate set of tests as indicated in the specs.

 

[civilperson]

How did you decide on depth and diameter without soil parameters with the imposed loads? Assuming a low value for horizontal soil modulus will give over sized diameters/lengths and assuming low bearing values will increase the belling requirement. Perhaps you could bid a few standard sized piers and then specify which to use after receiving the true soil values.

 

[JAE]

I guess that even without initial pier design parameters, you'd still essentially be doing the same thing that I have done, namely, provide a design, require that the geotech be on site during drilling to verify the actual soil qualities/conditions.

I guess you will have a geotech report and recommendations before the drilling - yes? If so, you could place in your bid documents an explanation of how the sequence of geotechnical recommendations will come into play in your contract. This will affect the bids, and will most certainly cause some change orders if the geotech report (and its subsequent influence on the final design) come after the bids.

I would urge you to convince the owner that its in their best interest to get this info prior to the bids, not after. But in either case, a load test would only be required if you didn't get the geotech involvement into place prior to the drilling.

 

[SlideRuleEra]

In 1980, I was involved in a project (about 2000 drilled piers) where a series of load tests were performed before final substructure design. It was a very unusual situation on a green field industrial site.

Borings revealed that the underlying sedimentary limestone was riddled with water filled solution voids and channels. No individual drill pier could be relied on for point bearing, since it's tip could be just inches above an undiscovered void. The drilled piers (400 ton capacity / each) had to be designed for skin friction support only.

To determine the relevant drilled pier / rock friction coefficient, test drilled piers were installed. At the very bottom of each hole a block of Styrofoam was placed. Tubing cast into the concrete allowed a Styrofoam solvent to be injected - dissolving the Styrofoam. Then the instrumented drill piers were loaded to failure. Since there was the intentional void under the tip, skin friction provided all support - the friction coefficient could be calculated. Using this value, the length of the rock socket (area of concrete / rock interface) was determined.

http://www.SlideRuleEra.net

 

[JAE]

That's a good point - that uncertain parameters should be field tested for loaded piers.

Most of my experience involved clay soils underlain with a blue shale base so the geotechs usually had a good handle on the capacities.

For some conditions, where limestone voids are suspected, a smaller drilled hole is used to establish that there are no voids directly under the bearing. Again, this is a field activity during drilling and not a load test.

 

[BigH]

I think that you are barking up a dangerous tree without any geotechnical input before you start your design. If, as you have intimated, there are other structures at the site, you should, in my view, get the owner to allow you to contact the geotech(s) of record on the structures to obtain their preliminary thoughts on the various aspects given that a geotech will not provide recommendations that can be construed to be relied on in fact based on nearby boreholes off the proposed building footprint. This is a standard and well deserved caveat in geotechnical reports. Secondly, I would doubt any geotech would "confirm" an allowable drilled pier capacity based on his observation (only) of the cuttings without actual input into investigation. There may be exceptions based on geology and experience but a geotech should still be rather concerned about it.
As for load tests, all bored pile (drilled shaft) designs on a recent project in South Asia for each bridge required load tests. First, there was a initial test before construction and then working pile tests during construction.
There are other methods of testing - dynamic testing using a large hammer set-up (see GRL). Also, you may wish to do integrity testing (putting in three or four 'holes' in the pile in which you can place geophones and a 'hammer' to see if the pile has voids or has necked (Cross-hole sonic logging). North Carolina DOT even has a drawing indicating the arrangement of such 'holes'.

 

[Zambo]

I have a feeling that you should have one pile load tested. Unfortunately I cannot back up this statement as I am used to working on large projects where between 1 in 200 or 1 in 400 are tested. What are the penetration and proof test holes relating to? I do not think bored piles (drilled piers?)

It is necessary to have a geotechnical report in order that the piles can be designed. Your site is not large so 5 boreholes would probably be sufficient (I'm sure someone on the forum can advise on the required number) Once you have the geotechnical report you can advise the owner on an economical foundation design.

Why not get a quotation for a geotechnical report. It will definitely be worth the money spent.

 

[UcfSE]

Due to the liability involved, we won't sign and seal commercial projects for which we have no geotechnical report. This is for shallow footings in sandy soils most often, very easy and very consistent results in Florida. It seems like assuming you need drilled piers and assuming a capacity might be dealing outside of your expertise unless you are also a geotechnical engineer. That is something to think about, imo. Working with clients and trying to help them out is one thing, but you have to cover your rear along with your company's.

 

[sundale]

I would simply quote the Owner chapter and verse.
IBC 1802.2.2.4.
"Pier and pile foundations shall be designed and installed on the basis of a foundation investigation and report as specified in ...."

I would be very very careful in showing a deep foundation system without a geotechnical report. I would also be very careful of an Owner/Client that is too cheap to pay for a soils report for a structure requiring deep foundations; this may not be a good client on a number of levels. This requires Special Inspection per Chapter 17 too (I am pretty sure).

I have had load tests on helical piers and driven piles, but never a concrete drilled pier.

 

[Zambo]

sundale, why no load test on a concrete drilled pier (bored pile). In some cases you may find that the assumed skin friction is not obtained and shaft grouting may be required.

 

[d2y]

It is not that owners are too cheap, they are sometimes ignorant of the value of geotechnical investigations. They possibly would not hire a structural engineer, if not required.

Geological formations various approaches to design/specifications. The geotechnical engineer should make the call whether load tests are appropriate or not.

During my past projects, only one site with a high water table and silty/sandy soils required load tests, because auger cast piles were recommended.

 

[PEinc]

For testing drilled piers, check out Loadtest, Inc. They are located in Gainesville, FL and in Baltimore, MD. They can do O-Cell (Osterberg Cell) tests to determine the skin friction and end bearing for a drilled pier.

http://www.loadtest.com

 

[henri2]

sundale, you are correct. The IBC does require special inspection for most cases. IBC special inspection provisions for pile foundations and pier foundations (in structures assigned to SDC C-F) are 1704.8 and 1704.9 respectively.

structengineer, IBC-03 Sections 1808.2.8.1 through 1808.2.8.3.1 and 1808.2.10 stipulates when load tests on piles/piers are required. For fequency of testing read Section 1808.2.8.3