Wind Induced Settlement of Piles

[Madziwa]

Does anyone know of any work in connection with wind causing vibration that led to piles settling through loose to medium dense silty sands?

We have just withstood a storm that has led to some settlement damage on a house on a very exposed coast. The house was designed in accordance with the local code for wind sped, but the actual wind was about 1.5 times as great. This seems to have led to settlement of a braced pile.

Its almost a case of dry liquefaction caused by the sides of a house vibrating causing the piles to vibrate and this leading to settlement of the sand around the pile. The diagonal brace is connected to the pile that has settled, almost at ground level and we are considering if the wind action has been enough to use the brace almost as a jack-hammer to push the pile down.

Gareth Williams
Geotechnical Engineer,
Auckland New Zealand

 

[SlideRuleEra]

IMHO, the most likely cause of settlement is piling that do not have minimum embedment (typically 10 to 20 feet, depending on soils). Considering that the pile loads on most (residential?) houses is very low, I doubt if significant settlement would have happened for another reason.

Properly driven piling in other structures that are subject to vibration (such as bridges) rarely settle even though they are carrying substantial loads.

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[Madziwa]

Thanks fr the reply. There is a layer of loose to medium dense sand about 4m thick that our slope stability analysis indicated did not have an acceptable FOS (i.e. < 1.5)The piles extneded to approx 8m bgl.

As the sides of the building rise approx 6m above the ground, there woudl have been significant wind loading and this woudl not have been constant, hence we are wondering if this vibration has led to densification of the sand around and below the pile. The action of the brace possibly has intensified the effect on the pile. The structure is suported by a number of other piles and none of these show signs of settlement (at least to the degree that this one does), but none of the others are braced either.

Gareth Williams
Geotechnical Engineer,
Auckland New Zealand

 

[Doc08]

Madziwa, the scenario you are discribing is plausible, however, I have to agree with slideruleera that had the pile been with enough embedment in competent soil, I doubt you would have had this settlement.

It is hard to give a constructive opinion without additional data. We know that the upper 4m are made of loose-medium sand, what about the next 4m, what kind of SPTs do you have there? Your statement above indicates setllement of the soil around the pile hence "downdrag" which is physically different than the "sinking" induced by liquifaction. I guess what I am trying to convey is that you might have only downdrag on the pile induced by the settlement in the upper 4m and this would be the likely cause if the lower 4m of the pile are embedded in competent soil. However, if the lower 4m are not dense to v.dense sand, you would expect the settlement to come from the overloading due to down drag and the loss of carrying capacity of the soil in the lower 4m as well due to liquifaction.

 

[DRC1]

Vibratory hanmmers operate at 16-2400 Hz. I sasincerely doubt that any storm could generatea resonate frequecy to sink a pile. I would suggest the timber piles were driven to length based on a test pile and one pile did not fetch up completely. During the storm, wind pressure increased to near desisgn values and the pile was overloaded. If wave action was associated with the storm, high water in a sandy soil will reduce effective stresses, further lowering capacity. I would suspect that pile for one reason or another had marginal capacity and was overloaded during the storm.