Pile in Limestone

[Valkyrie]

I would like to know is it common practice to ignore base bearing pressure when using micropile in limestone (to avoid cavities) ?
This i noticed, will very much increase the socket length of the pile, which drilling is costly. [sig][/sig]

 

[mikeh]

I do not know if it is common practice or not but..

In order to achieve some sort of end-bearing resistance for piles you must have a clean base consisting of undisturbed soil/rock which would be extremely difficult to achieve and confirm with mini-piles.

You may wish to explore pressure grouting or using expansive grout to increase your skin friction. [sig][/sig]

 

[Ron]

Not designed for end bearing in limestone. Due to the erratic deposition nature of limestone and the potential for weak roofed cavities, there would be a tendency of end loaded mini-pile to "punch" through the limestone. [sig][/sig]

 

[Mccoy]

If the building area is in karst country and occurrence of underground cavities is common, then probably the best thing would be to carry out an exploratory survey of the building site (georadar is allegedly good for cavities. Drilling exploration boreholes by DTH hammer is relatively fast and cheap.
The survey may be even be made simply by drilling some deeper micropile boreholes, and see if cavities are intercepted. If not, we have a minimum thickness of rock diaphragm which can be used as an input for calculations of shear punching. The hole can then be filled with cement grout, which, when set, can reach compressive strenghts equal to sandstones. Loss of cement circulation may warn in advance of cavities or large fracture openings.
Pressure grouting tends to consolidate the rock socket itself,and I think end bearing may be used, using a safe range of compressive strenght for the cement (in function of water/sand content,eventual additives and other relevant factors).

 

[ayfarm]

Ignoring the end bearing resistance is not common practice, beside being negligent. Depending on the limestone classification, location & properties, there are number of methods to install a pile economically in common practice.