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# Borehole stability/hydrofracture in rock

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## Borehole stability/hydrofracture in rock

(OP)
I was wondering if anyone knew of a calculation procedure to predict the pressure at which borehole stability can be ensured and/or hydraulic fracture would be initiated in a borehole in rock? The aim would be to determine the allowable pressure range during the drilling construction stage of a drilled and grouted pile: borehole stability analyses would provide the lower pressure bound and hydraulic fracture analyses would provide the upper pressure bound.

There are a variety of methods for calculating this for soils, but the mechanism in rock is likely different (i.e. reliant on the shear strength of rock discontinuities). I'm not looking to do any testing to confirm these pressures - I am looking to provide a quantitative basis to inform recommendations.

### RE: Borehole stability/hydrofracture in rock

My question is: Do you know of problems on real jobs where pressure is found to be insufficient? From what I have experienced the only problem noted is withdrawing the auger too rapidly causing a partial vacuum and a resulting cave-in of soil not rock. No way to measure that negative pressure down there that I know of.

### RE: Borehole stability/hydrofracture in rock

What is your definition of rock?. In all my experience with rock , I have found that the hydrostatic pressure within a borehole can be approximated at 40psi per 100 feet of depth. However this is rock with a compressive strength say 12-40,000 psi. I think some better definitions are required here

### RE: Borehole stability/hydrofracture in rock

(OP)
The purpose of the borehole stability analysis is to assess whether to use a casing (and what length it should be) or to use a drilling fluid of a specified density. The borehole stability assessment is therefore more relevant for the surficial soils, so I'm not so worried about that - there are existing methods to predict this for soil.

However, I am more concerned by the risk of hydrofracture along rock discontinuities. The method for predicting borehole stability is going to be the same as for hydrofracture (just looking at different bounds to the problem), and I'm not aware of a method to predict hydrofracture specifically for rock. I expect to do this correctly you'd have to consider the discontinuities explicitly and possibly also the rock mass strength.

The UCS ranges from approximately 1 MPa to 60 MPa, though the discontinuities will have a significantly smaller shear strength.

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