[b] Drilling ops and foundation settlements [/b] 2

[Mccoy]

Hello everyone,
I strongly suspect that the cracks manifested in a residential building are due to drilling operations, carried out in successive phases and in a long time stretch.
Subsoil is silty sands, loose/medium loose, with sandstone boulders up to 10-12 meters of depth; bedrock is OC clays. Groundwater only seasonally present in sparse thin perched acquifers, and at the sands/clays interface.
Building is RC beam foundation, masonry structure, 3 storeys. Pattern of cracks (not serious - serviceability limit is not reached)would suggest a non-uniform settlement of the foundation beam more exposed to vibrations coming from drilling ops. Cracks are clustered in the uppermost floor.
The beam is 4 to 8 meters away from RC shafts drilled with rig & bucket,quite a few boulders have been found below 2 meters of depth. Shafts have been drilled twice (successive consolidation operations).
Cracks have started after 1st drilling phase.
4 years after start of cracks micropiles for foundation consolidation have been drilled along the beam. After that, crack pattern worsened. Successively cracks have slightly increased their extent, then stopped after 5 years from micropile drilling.
On top of that, there has been vibrations coming from dozers, and farther away from more rig machines involved in
slope consolidation and earth moving ops.
Is it possible that sands have been compacted by drilling + rig machinery?
Do you have analogous experience in similar conditions? And what about vibrations coming from micropile, drilled not thru but very close to foundation?
The slope below the building collapsed (a small landslide), but the closer tension crack was at 20+ metres of distance - and cracks started a few months after landslide, just after inception of shaft drilling.
Thanks in advance for your opinions.

 

[Focht3]

An interesting problem.

First, let's eliminate the unlikely contributors to the problem. The operation of construction equipment (dozers, trucks, pans, etc.) more than 7 meters from the damaged building is unlikely to have contributed to the distress, or affected soils to a depth of more than one meter.

Vibrations from the drilling process could have affected the soils beneath the building, depending on the foundation of the damaged structure. (You didn't describe that for us.) But I would strongly suspect a loss of ground during the drilling process is the real culprit. In other words, the drilling process has allowed soils from areas adjacent to the piers and micropiles to flow into the shaft excavation, where they are removed. This phenomenon behaves a lot like the densification of loose sands subjected to vibrations, except that the soil density decreases. CPT soundings - if possible - can help you evaluate the likely cause. That is, if you have pre-construction CPT soundings to compare with the current results.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[DRC1]

I agree that ground loss from the drilling may be the problem. I would suspect that the boulders may be part of the problem. Any time you are removeing large obstructions it is difficult not to significantly disturb the surronding soil. Depending on conditions, grouting may help.
Another problem may be the small lanslide. The slope may be slowly creeping, indicating F.S is slightly lesss than 1. This may be a like a locomotive, starting slow then increasing in speed rapidly. I would take the small landslide and the cracks as a warning sign and have a thorough investigation promptly.

 

[Focht3]

Agreed. Look at the entire area - not just the building. Slope failures in loose sands tend to move very quickly -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Mccoy]

Thanks Focht3 and DRC1.
Ground loss sounds good to justify the settlement. Since no prior soundings exist, it's just a matter of discriminating possible cause/effect relationships. Foundation is a perimetral RC beam, 2.5 ft wide, 3 feet deep (thick), apparently pretty high flexural rigidity, load is about 150 KPa.
Also creep sounds good, being a time-dependent process; consolidation of the landslide was only completed after 4 years from its occurrence by building a cantilever wall.
The two might have acted in a combined fashion. Vicinity and chronology would tend to favour ground loss, though (and boulder remotion).
Slope became unstable after excavation operations and intense precipitation, but in the area more instability events are known.
By the way I don't see the tipmaster stars I sent you, heck, I'll try again ASAP from another workstation!!