Lateral Pile capacity in Rock

[Geosubhtech]

It is a bridge to be constructed in creek portion near sea in India. The fluctuation of water is there due to tidal variation. The bridge is a balanced cantilever.
Vertical load 1250t.
Lateral load 200t.
About the strata :
thin layer of soft marine clay
completely to highly weathered rock around 7 to 8m in center as it is varying along the length.(like while drilling the pieces i can break in hand like biscuit)
then moderately weathered rock followed by fresh basalt rock below.
Socketed to fresh rock up to 4D. (Dia = 1.5m)

So i am trying to calculate the lateral capacity of pile.
So, any suggestions for shear criteria and deflection criteria to get the lateral pile capacity.
Tried BROMS method but the variation from completely weathered to moderately is very high so couldn't take average.
Tried with P-y curves but i am thinking the models are not suitable for the strata.
Please tell any other methods which you have used in your practice.
Thank you.

 

[EireChch]

What are your RQD and UCS vales like? If it’s highly weathered and UCS <0.6MPa you should treat it as a soil. P-y curves are appropriate.

 

[fattdad]

Please also recognize that RQD is only measured on rock that cannot be broken by hand. So, if you have a 12-in long piece of intact rock core and can break it by hand, the rock will have an RQD of zero, irrespective of the length of core pieces.

You may find some use of the RMR (rock mass rating) to assign shear strength. Bienieowski (sp) also has methods for modulus, or so I recall?

f-d

ípapß gordo ainÆt no madre flaca!

 

[Geosubhtech]

The RQD values are 0 in the completely weathered rock region. and UCS values are less than 10 Mpa.
But how can i cross check my lateral capacity. As p-y curves were created by Software by many iterations, i want to compare by capacity for satisfaction purpose by both shear and deflection criteria. for that help is required.
see the core box photos.

 

[EireChch]

You can assess your lateral capacity for the marine clay and rock layers using p-y curve methods. Using L Pile you need to choose what type of rock your in "strong" or "weak" rock, below 6.9MPa is weak.

As far as i know, Broms solutions are for cohesive and non-cohesive soils. I dont think he has a solution for rock?

 

[Geosubhtech]

Yes i already did p-y curves. But any analytical method to cross verify the ultimate capacity, if available.

 

[Settingsun]

Try taking lateral bearing capacity equal to [0.33 to 0.67]*vertical bearing capacity, with a further reduction near the surface where the rock isn't confined. 0.33 for when RQD is zero and 0.67 for when RQD is high. See links below for some options on near-surface reduction. This is of course a rough method meant for order-of-magnitude checking.

This report is fairly recent and includes a literature survey that shows how slim the pickings are for rock lateral load methods. Only a couple that are specifically for rock and which are non-linear.

http://www.eng.auburn.edu/files/centers/hrc/930-425-2.pdf

This document is more recent and has another rock p-y curve that wasn't included in the Auburn Uni report. You might be able to use it for a check calculation.

https://www.rocscience.com/help/rspile/pdf_files/theory/RSPile_-_Laterally_Loaded_Pile_Theory.pdf

Note that the Lpile/Reese method gave best results when the parameters were back-calculated to match test measurements. Without a test pile, you can only hope for a lucky match so need to go with a conservative design. Lateral load testing should be specified for construction but it would be better to do a test pile during design. From the magnitude of the loads and that the structure is a balanced cantilever bridge, this seems like a major project that should make that investment for public safety and potentially economic design.