Timber Pile Anchors

[GeoGrouting]

Double timber piles are used to support the guy wires of about 30m towers about 25 years ago.

The ground (riverbed) is about 2m of clayey silt underlain by clean sand materials to a depth of 30m. The depth of anchorpiles are not known. Field inspection shows that the anchor piles are inclined 10-20 degrees towards the tower.

For geotechnical performance monitoring of the anchor piles what approaches one should take.

Appreciate your feedback in advance.

 

[BigH]

Were the piles put in butt first or tip first? The former would offer greater uplift resistance than straight shafted piles; the later would offer less uplift resistance. Tomlinson suggests that the uplift resistance would be in the order of 50% of the compression value for granular soils. As the taper may make the resistance more detrimental, you might choose to use a reduction value in excess of 50%. It might be a good idea to carry out a tension test if the pile is critical. I'd suggest googling tension capacity of timber piles. Their is a Timber Pile Manual on the net; it might have something in it. Don't have the time at this moment to find it.

 

[GeoGrouting]

BigH

Thank you for the feedback. One question is that what you mean by compression value of granular soil. Wonder if it can be translated to SPT.

Also observation of the anchor piles indicated that the horizental loads are more than the vertical loads. I am looking for a reference to see how far the piles can lean towards the applied forces and still be functioning.

 

[Geostructsparks]

Geotechnical monitoring -

I would think the loads and soil are probably about the same now versus 30 years ago (e.g. it hasn't fallen down for 30 years), its the wood pile capacity you want to evaluate/test. Or are they planning to enlarge the tower loads and need to know if it has the required capacity?

Are these a battered pair of piles, or are both vertical? If they are battered, then axial compressive/tensile capacity are more important, if not, then lateral pile capacity. Either way, you may want to talk to the wood technology people about how to estimate strength loss or decay, perhaps such as coring in to get small samples. I'm at a loss there.

 

[BigH]

Are not the piles "in line" with the guy wires? Or did they drive them vertically? See Tomlinson (or many others for piling in sands) for compression capacity. Meyerhoff proposed a direct correlation to N value. Others rely on N vs phi' to get phi' to put into capacity equations (the problem then lies with which Nq value do you use? Vesic? Meyerhoff? Berentzev? etc.)

 

[GeoGrouting]

BigH

I understand that by compression you mean bearing capacity. Am I right?

At each anchoring position the twin timber piles were presumably driven vertically 25 years ago. Now the piles appear to lean towards the tension forces over time and they have currently an inclination of 11 degrees to 19 degrees. The maximum horizontal movement of the pile tops could reach to 320 mm. One of the question is how much inclination (or horizntal movement) is allowed. How much roration in the pile is acceptable, beyond which we may want to replace the piles. Thank you again.

 

[BigH]

Yes - compression would be the "bearing in vertical loading." The way I would view this is the piles are starting to lean towards the same inclination as the guys - the tension vector. Of course as the piles "lean" they are causing the soil to shift - moving into the soil in the direction of the tower (passive) and away in the opposite direction. Given that this is over time, I doubt that one would lose much as far as the tension capacity is concerned - it appears to be a creep situation if, in fact, there is any movement. The movement should stop when in line with the guys. Why not put on movement pins - survey them from time to time and gather observational data before making any decision to replace. No need to replace unless it looks like the piles are starting to exceed the tension shaft friction.

 

[apsix]

If the piles have moved there should be a history of substantial guy re-tensioning; is this the case?

And, what do you mean by "The maximum horizontal movement of the pile tops could reach to 320 mm"? How would you know that when you don't know the pile depth?

 

[paddingtongreen]

You have another thread on this, I answered:

"Are you sure they are not in the installed position? I could visualize a case where they were battered to limit the shear component of the guy loading, that is, place more of the load in-line with the pile and less in perpendicular."

Michael.
Timing has a lot to do with the outcome of a rain dance.