Pile Load Test on Auger Pressure Grouted Displacement piles (AGPD)

[GeotechD]

We performed two static pile load tests on AGPD piles of 16 and 18 inches diameter and 75 feet long. the cage length was 40 feet and the central rebar continued to the pile tip. the subsoil conditions was typical in central Florida (Loose sands followed by silty/clayey sands and sandy clays then the limestone). these piles did not reach the limestone and they are basically friction piles. There was a fat mediun to stiff clay layer at depth 40 to 50 feet.
The 16-inch pile was able to sustain a load of 300 tons and was settled only 0.6 inch. The 18-inch pile failed to carry the same load and excessive settlement began at 240 tons even without increasing the load on the pile.
I have some thoughts and I need your comment and suggestions:

1- There is nicking at or close to tip of the cage that may be occured due to the developed positive pore water pressure through the fat clay layer under the existing overburden pressure (it is a contractive soil). The method of installation of this type of piles may cause excess lateral stresses in the undrained conditions (beccause it is fast)and if the soil is contractive, positive pore water pressure will develop and the time of installation is not enough for it to dissipate, so it may cause nicking to the fresh concrete.

2- There was a .07 inches difference between the two used dial gages, which implies that there was some type of excentricity. May be the pile is structurely damaged at the tip of the cage due to excessive bending moment resulted from the excentricity. There are some evidences supporting that assumption; the recation piles on one side were pullet up a little bit. The dial on that same side has more settlement that the other gage.

Any comments or ideas will be greatly appreciated.

 

[GeoPaveTraffic]

My assumption would be that the installation of the two piles was different, and that the 18-inch pile was not installed properly for the conditions. This illistrates the biggest problem with ACP, installation is very dependent on installation crew performance and carefull inspection.

What inspection was performed during installation of the two test piles and the reaction piles? Do the inspection records show a difference in the amount of grout placed in the pile or in different zones of the pile? Was there difficulty in placing either the center bar or the cage in any of the piles?

Given that the average compressive load in the 18-inch pile was only 1,886 psi at the top of the pile, much less at a depth of 40 feet, I think it is unlikely that the pile failed structurally as suggested. It is possible, just unlikely based on the available information. I would look closly at the installation records for the piles and compare the boring logs for the test pile locations, unless they are very close together. As for the 0.07 inches difference in the pile head movement; that amount does not seem out of character for a plunging pile. For that matter, some of that may have occured during the intial seating loads.

Beyond that, have the contractor install another 18-inch test pile, monitor the installation carefully, and see what happens. Then when production piles start make sure everyone is installed just like the test piles.

P.S. One thing that just occured to me, how much did the reaction piles move?

 

[dmoler]

Was the pile installation done with electronic monitoring of the grout pressure and torque per depth as commonly done for those?

If so, I would compare the torque between each to see if the 16 inch encountered limestone at the bottom or at other intervals that was not at the 18 inch.

Also, I would check the grout pressure to see if there was any pressure loss in the 18 inch and therefore a discontinuous pile.