Interpreting strain gauge data in bored piles

[whollycow]

Hello,

I used strain gauges on two recent CFA pile jobs. This was my first time using strain gauges. Upon converting the strain gauge data into loads and plotting my load vs depth graph, something seems off with the data. The lines on my graph "zig-zag" down the depth of the pile which indicates that the load in the pile INCREASES with depth during certain depth intervals, which does not make sense. I've looked at data from load tests on 6 piles now and this phenomenon has occurred in all piles so far.

Has anybody experienced this before?

Thanks!

 

[EireChch]

Post the graphs, load test info, depth of strain guages etc.

 

[GeoPaveTraffic]

Most likely the problem is with the area and/or modulus of the pile. Remember you are measuring strain and are calculating load based on an assumed cross-sectional area and modulus of elasticity. The modulus is probably pretty constant, but depending on the soils and how the piles were installed; the cross-sectional area can varry a lot. First thing I would do is compare the loads to the soils and the installation logs to see if there are any correlations.

Mike Lambert

 

[ATSE]

You have provided far too little information.
How many strain gages do you have, and at what depth? Also, you need to show a soil profile (or just a cpt qt and fs). Are you using sister bars with VWSG?

You need to show strain vs a complete timeline. You should expect strain variation with no applied load, based on thermal and shrinkage variances. And residual load (usually increases with time, and with depth), which is a non-trivial component of the internal stresses.

If you are new to this, I suggest digging into DFI publications. In particular, anything from Fellenius, or Tim Siegel. Both have written a lot, and explain how incorrect assumptions in interpretation of the strain data can provide incorrect results for pile behavior and capacity.

 

[whollycow]

Info for two different load tests on the same site are attached. Subsurface conditions are only provided for test pile #2 but conditions are relatively uniform throughout the site. 6 vibrating wire sister bar strain gauges (GEOKON) were used for each test pile, equally spaced along the depth of the pile, one at each level.

The loads interpreted from the strain gauge data are relative to the initial "zero" strain reading taken at the start of the load test before any load was applied. The loads shown in the graphs that I've attached were calculated using a non-uniform modulus of elasticity (modulus varies with strain) in accordance with methods described by Timothy Seigel in Load Testing and Interpretation of Instrumented Angered Cast-in-Place Piles (DFI Journal Dec. 2010).

I did not account for residual load in the piles. Load tests were performed 10 days after pile installation. Seigel states that residual loads developed within 7-10 days of installation are relatively small. I have looked into residual load effects briefly (Seigel and Fellenius) and I couldn't find any methods of accounting for residual load without taking strain gauge readings frequently starting immediately after pile installation and right up until the start of the load test, which was not done and is obviously too late to do at this point.

Any insights would be greatly appreciated!

 

[whollycow]

I think only one document got uploaded in my last post, here is the other document.

 

[ATSE]

wcow: I agree, the results pages don't make sense. And I have run many instrumented pile load tests myself, and never experienced this.

Based on the soil profile, I would expect a bit more end bearing. Test Pile #2 instrumentation shows almost no end bearing.
But more troubling is the pinching between 30ft and 40ft.
Why the results are wrong is a tougher question to answer, but I would be embarrassed to present these results as final without correction or a good explanation.

Now your detective work begins. Is the secant modulus off? Are residual stresses more than anticipated? Mixing sister bars? None of these seems likely, to be honest - but those are the kinds of "first principles" questions I would start asking. Geokon's technical sales rep should be helping as well.
Keep up posted.

 

[GeoPaveTraffic]

For TP1 the legend is unlabeled so there is not way to tell what each of the curves represents. I assume the legend should be the same as TP2.

Based on this assumption the curves make pretty good sense to me. The only thing that looks off is the load inversion in TP2 at 175 to 200% design load at ~56 feet. That looks off and I'm not sure what happened there.

As for the rest, here is my interpretation. The pile area is greater in the organic silt/clay and clayey silt layers than in the fill, silty sand and till/rock. As a result the strain gauges give a lower strain resulting in a lower load unless you account for the increase in cross-sectional area. The installation log showing crowd and rotary pressure doesn't help. What you really need to know is the amount of grout pumped in each zone.

As for the amount of load that reached the pile tip. You do not indicate how long each load increment was held or provide any data on how the strains changed with time under constant load, so not much anyone can do to help you understand that aspect of the test. But, I'm not surprised that the load left the pile in friction in a till/rock layer that had what I assume are blow counts of 63 to 100.

Mike Lambert

 

[Ron]

These are deep piles for CFA. You indicated you only have one strain gage at each depth. I will assume your piles are in the 20-24 inch diameter range. To get meaningful data, you should have gages on four sides of the pile at each depth. That allows you to assess bending in the pile under load and allows you to evaluate elastic rebound from the loading. If your piles are smaller in diameter than I assumed, the problem is compounded as you likely are seeing some curvature in the pile.

More gages would also allow you to correlate the soil conditions with the anomalous readings.

As someone else noted, if these measurements were taken right after or contemporaneous to loading, they are probably not accurate as strain relaxation will temper some of those anomalies.

 

[ATSE]

Mike makes a good point regarding varying diameter / area.
However, in order to account for the changes in strain (indirectly force), there would have to be a significant cross sectional area differences. Possible, I suppose.

Also, it appears that there is a bust in the units, showing both ft and m for depth. I am assuming feet is correct.