Calculating shear strength of concrete to concrete interface of a concrete plug in spun pile.

[cyphos168]

Hi,

I need to calculate the capacity of a concrete plug in a spun pile to resist a push force. Tests have been conducted and written about it and published but these are actual tests to determine the shear stress at the interface by dividing the failure force kN against the surface area of the plug interface.

However I need to calculate the capacity. Anyone can offer advice as to how this can be done? I am thinking I need to take into account of the concrete plug strength and apply a reduction factor. Then use a coefficient of friction between the pile concrete and plug concrete which I can take (as per literature and discussion in forum as 0.4). Use this value and multiply that with the surface area of the plug.

Is this correct? Plug is un-reinforced.

Thanks in advance.

Al

 

[hokie66]

The practical problem with depending on that interface is removal of the laitance. How do you propose to do that? There is normally a circular segment of reasonably solid laitance, then laitance around the rest of the internal perimeter. When sections of precast piles are spliced, you can sandblast or water blast the interior near the ends, and insert reinforcement. But unreinforced? What would be the application?

If the "push force" is the right direction into a tapered pile, I can see that working, if you have enough circumferential reinforcement to prevent splitting.

 

[cyphos168]

Hi Hokie66,

I have designed a spun pile which is basically inserted into a prebored hole in hard clay. Spun pile has no pile shoe hence my design by right should just take the area of the annulus of the spun pile to calculate base capacity. I would like to calculate the capacity of the base by utilising the full surface area of the pile base. Hence the plan is to pour a 1.2m long concrete of say Grade 20 to form a plug at the base.

Spun pile unfortunately won't be tapered.

I have used eurocode 2 to determine the shear stress developed by concrete to concrete interface and would like to take a conservative approach. The idea is for the soil to exert a push force on the plug. I just need to determine the length of the plug required to resist this push force. Would taking a concrete to concrete shear stress value of 1.7N/mm2 be a conservative value? If it can be calculated conservatively then I would use this approached, else I would need to find another way of doing this.

My second approach is to infill with sand all the way up with a pile head concrete plug for connection with the pile cap. No tension force. The first approach using the base plug is to avoid having to do the sand fill. Thanks.

Regards,

Al