Pile Seating

[harpoon]

We are installing steel pipe pile on sloping bedrock. Is there any way to ensure that piles do not slip? Also the length of piles is 3m with the overburden bring silty clay. Will this cause problems with lateral stability of piles? What can be done to enhance lateral stability?

Appreciate your responses

 

[SirAl]

What method of installation are you using? Can you describe the bedrock?

 

[harpoon]

We are driving piles to basalt bedrock.

 

[Focht3]

Where is the site? Any special conditions to consider? Are the shortest - or longest - piles 3m long? Why the concern about lateral forces and slipping?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

Use special tips for sloped rock interfaces. Respect lateral effects you may try to isolate the piles from the clay overburden of the rock. Since 3 m deep to the rock, you may put an encased shearwall per pile as a shield, if required tied down to anchors to the rock taking any likely movement of the slope downwards. This may be difficult and sturdy but if sliding movements are feared things like this may help. Other lateral loads you imagine from its situation in a slope will be lesser, and yet be enough for what the ordinary engineering requirements are. Again, what for some is excessive, for others is just enough.

 

[SirAl]

I would like to know the strike & dip of the bedrock surface as well as the topographic relief. Also, what method of installation do you anticipate using? A 3m depth is fairly shallow. Without knowing any limitations associated with your project, I am offering a few cursory suggestions:

1)I would at least consider socketting and grouting the piles into the rock.

2)Alternatively, perhaps a pad and pier arrangement would also work. At least, with this approach, you will expose the rock for evaluation. This would also allow for preparation of the bearing surface.

With typical driving equipment, I doubt that you will obtain any significant embedment into sound basalt rock. I would also be concerned with the post-driving condition and contact area of the pile toe resting on the bedrock bearing surface.

 

[ecanuck]

One way I have overcome the potential problem of pile "kick-out" from an underlying sloping bedrock was to use a well drill rig. This can be a cheap option where you can socket into the rock.

The other way is to use pile shoes or pins to "bite" the rock; that is, if the rock is biteable. If it has a steeply sloping surface then it is probably quite dense.

 

[foremanfred]

the piles must be restrained within the rock, in order to do this a rock socket must be formed using a DTH drill bit under compressed air. The pile can only be designed using shaft adhesion as it is impossible to ensure that the bottom of the hole is clean.

Should you wish to increase the moment capacity of the pile to resist any slope movements, then it will have to be reinforced with either a steel cage or a thick wall steel tube.

at a predesign stage it is usual to allow for 2/3 of the pile to be in competent ground whilst 1/3 is retaining. if the rock is of good quality this can be slightly reduced.