Drilled piers and load testing
Drilled piers and load testing
(OP)
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.






RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
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.
www.SlideRuleEra.net
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
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'.
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
RE: Drilled piers and load testing
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.
RE: Drilled piers and load testing
RE: Drilled piers and load testing
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