Settlement of 61 meters crane's foundation 1

[Filip_Z]

Hello guys,
I have settlement problem with my crane's foundation.
Crane's foundation consists of four reinforced CFA piles (D=0,5m; 16m height) and plile cap (slab 5,0m x 5,0m x 1,4m).

Yesterday after overhelming od excavation, we removed sheet piles. After removing sheets which was near to excavation's sheet piles (about 1,6m to pile's cap slab edge), our foundation subsided 0.013m towards sheet pile.
I need to find out reason of this incident.

Ground:
0,00m = pile cap level
from 0,00m to -14,00m: homogeneous, fine sand (Id~from 0,65 to 0,80)
from -14,00m to -15,50m: sandy clay (Il~0,4)
from -15,50m: fine sand (Id>0,90)
Groundwater level: -2,00m

I have some ideas, maybe you'll help me to solve this problem:

1. Soil in excavation was compactioned not enough (green line at attached image - Crane_fund_section.jpg). After sheet plie removing, foundation turned towards after-sheeting ground crack. But is it possible with quite small excavation, making so considerable displacement?
2. Maybe good idea would be keep sheeting in ground near to "crane-area"?
3. Groundwater level during removing sheet piles was at -4,50m in excavation and at -2,00m outside. Is it possible, that groundwater flow from out to excavation rotated my crane's foundation?
4. FEM grid of piles consists of 0,5m parts. Isn't it to loose? Maybe designer had to make smaller grid parts near to piles cap/groundwater level? Can it make considerable analyze differences?

If You have ideas for another causes of this settlement, please feel free to help me solve my problem.
At below links you can look at

http://files.engineering.com/getfil...b87e-7fa4bd147837&file=CRANE_FUND_SECTION.JPG

http://files.engineering.com/getfil...b-8e1d-96cf159f8137&file=crane_found_plan.JPG

http://files.engineering.com/getfil...8-76f730e1ae7f&file=crane_found_FEM_model.JPG

 

[oldestguy]

The units for strength may be useful to others, but I'm stuck with English. Chances are you had not only rotation of the pile cap, but also some sideways movement. However, this appears to be a soil slope stability situation that can be evaluated by a geotechnical engineer. Remember that when below the water table most soils exhibit a considerable difference between that situation and the higher location in effective unit weight and as well as resulting shear resistance. Thus, resistance to movement sideways depends on soil friction which is greatly less than if there was no shallow water table. For sands this can be about half the resistance that otherwise would be there. In addition the two water table elevations create a lateral pressure and that, in addition to the open excavation slope likely caused the movement. Deeper lateral resistance of piles to sideways push on them also is much less due to the same reasoning. The calculations will clarify this.

 

[hokie66]

Agree in general with oldestguy. But you can't rely on those skinny vertical piles to resist lateral load for that situation.

 

[Shu Jiang PEng SE]

The pile vertical capacity, per geotechnical information and pile configuration, solely depends on the friction between pile and the sand. The ground water level (water content) in sand will significantly reduce the friction coefficients. The stability of the sand will be jeopardized once the sheet piles are removed. Both these factors affect the pile capacity.

 

[chicopee]

Seems like a lots of work to remove sheet piles as you may have been better off to either use a mobile or crawler crane of adequate rating and fitted with a vibratory extractor to remove the sheet piles.

 

[bdbd]

My guess:

I am pretty sure it is about water.

You have dewatered the excavation while keeping sheet piles. Then, you finish your work and removed the sheet piles.

Water behind the sheet piles are rushed to the excavation site, since sudden drop of water level is really dangereous based on the settlement behavior, your crane settled. It should be over by now. How deep is your excavation? If it is not deeper than 10m, it will not affect the clay under sand. Settlement should be done by now. Calm down.

Also you have 3D finite element analysis for crane... Wow.

 

[Ron]

Assuming you had sheet piling around the entire crane foundation, the sequence of removing the sheet piling could have a significant effect on the soil stress distribution. This, combined with the vibration during sheet pile removal likely caused the rotation.

The sequence of removal should not have been a progression around the foundation. A sequence that moved alternately from one side to the opposite side of the foundation would have helped to allow a reasonable redistribution of stresses. The speed of removal also should have been considered. Slower is better.

 

[aeoliantexan]

I have seen ground settlement from removal of sheet piles in sand. The sand was river alluvium that commonly has some loose layers; I assume the vibration densified the sand and caused the settlement. The sand will also tend to move laterally to fill the void left by the sheet piles. A thick taxiway slab cracked at a horizontal distance roughly equal to the length of the sheet piles.

How do you plan to verify the capacity of the piles before putting the crane back in service? It would seem that the crane could become unstable if the piles settle under load.