Bearing Capacity of Rock Foundation 1

[GeoGrouting]

There is a 1.2m x 1.2m founadtion on rock. The rock type is feldespatic Decite and the rock surface shows discontinuities with about 0.2m spacing. RQD about 70%. The RMR may be about 40-50. How one can assess the bearing capacity of rock. Is there any minimum bearing capacity value that I can use for design. I would appreciate if you let me know of any table or graphs that can be used for the above. Thank you.

 

[DRC1]

Check your local building code. They usually list conservative presumptive values for area soils and rock

 

[ishvaaag]

Anyway, from a book of which Peck is an author, whilst, a lookup table

RQD q allowable kgf/cm^2

100 293
90 196
75 117

50 68
25 29
0 10

with the particular clause of if the resistance to the unconfined crush qu of the rock itself is lower than the table, use q allowable=qu

http://en.wikipedia.org/wiki/Rock_Quality_Designation

 

[Star2009]

Is it according to BS standard or American standard and is it shallow foundation?

 

[Mccoy]

Geogrouting,
what you should do first is figure out your design loading.

If loads are not conspicuos, let's say above the 0.5 MPa treshold, then I believe your designation of bearing capacity gets down to a mere formal exercise.

Otherwise, you have a choice of following simplified methods, suggested by previous posters, or follow a rigorous method.

Rigorous method:

You have a preliminary assessment to do: is your rock mass isotropic or anisotropic.
if isotropic, then you can treat it as a granular medium, a soil.
Given the scale of your footing and joints spacing, your rock mass is isotropic if you'v got at least 3-4 sets of joints.

then you just figure out the resistance parameters, for example Mohr-Coulomb by RMR or Hoek-Brown or other methods.
You can relate RMR index to phi and c and Ey same stands for Hoek-Brown.

I've got a roclab software installed on this PC. As a preliminary estimate, witn 175 MPa uniaxial compression strenght, GSI = 45, no disturbance from poorly design explosive charges, you've got Phi = 37,cohesion = 11 MPa, Young modulus 2583 MPa.

You might build whatever on it. Almost.

If rockmass is anisotropic, then you should check phi an c of joints and apply specific methods for foundations outlined in the literature (Carter, Kulhawy & Carter, more I do not remember, some on Duncan Willy's book "Foundations on rock".

Something you've got also in some USACE manual freely available in the internet.

 

[ishvaaag]

A statement of allowable compression stresses for rocks in old codes by the same Peck, Hanson, Thornburn text:

 

[GeoGrouting]

Hi ishvaaag

Re: I've got a roclab software installed on this PC. As a preliminary estimate, witn 175 MPa uniaxial compression strenght, GSI = 45, no disturbance from poorly design explosive charges, you've got Phi = 37,cohesion = 11 MPa, Young modulus 2583 MPa.

What would be the c, phi values for a rock with RMR = 41, RQD = 57. Thank you.

 

[GeoGrouting]

Through research (google) in several publications I have found that for RQD = 75% the allowable bearing capacity is 12 MPa. The same rock mass has RQD = 41, which is correlated to allowable bearing capacity 1.4 MPa. The difference is 10 times. Could anybody comment on or suggest other correlations for RMR and allowable bearing cpacity. Thank you.

 

[Star2009]

do you have UCS(Unconfined Compressive Strength) values for Rock sample? if you have that u can calculate the allowable bearing pressure.

 

[GeoGrouting]

Hi Star2009

Hammer strikes indicated a UCS of about 50-100 MPa. How can one obtain the bearing capacity from this UCS value. Thank you.

 

[ishvaaag]

In Bowles 5th edition section 4-16 bearing capàcity of rock it is stated that first you reduce the UCS by multiplying by the square of the (unitary) RQD, so first, for a 75 MPa UCS you would have a notional UCS 75*(0.75)^2=42.18 MPa.

Then it is said that for RDQs uncer 75% safety factors 6 to 10 are used, whilst for high RQDs safety factors of around 3 are common. You are in the divide so maybe you could choose 5 or so as safety factor. With this insight you allowable would turn 8.43 MPa, or the allowable contact pressure for the concrete if lesser.

There's more in Bowles and if you can get a hand on it would prove to be a long lasting good reference.

 

[GeoGrouting]

Dear ishvaaag

Thank you for the data. If the bearing capacity is estimated from some other references the amout obtained by RQD would be on the very high side. Hoek-Brown theory gives 2.98 MPa for RMR of 44 and the same RQD. Is that right? What is your opinion about the diffrences and whichone should be used in design. Thank you.

 

[ishvaaag]

For rocks that are actual rocks and not debris or gravel of the same, I might use a quite high value, or optimistic one, for the design. At 75% it is likely you are in such case. For shallow foundations it is almost of no concern, given the usually imparted stresses. But if pile tip, or whatever, in piles, I could go with the higher values because the strength in the rocks is there. This would imply some consideration on the nature of the discontinuities, if complementary measures should be taken or if by whatever the cause you are lead to not be confident on the strength on the rock. You quote one 3 MPa by one evaluation but if you check pile tip strength for a sound gravel at say 15 m depth you may get that strength for pile tip strength maybe even at safety factor 6.

With rock like with wood or cables the safety factors are quite schizoprhenic, all know that the average behaviour is sound, have strength plenty to go to limit strength and yet, because of being critical or, because the risk too much deflection in wood, and, because the wary intelligence of homo sapiens (this that good can't always be true) huge safety factors appear.

The traditional way of designing foundations, allowable compressions, uses a probabilistic approach with moderate safety factors, say 2.5 to 3. Currently, the characteristic strength approach for so unreliable or variable items as rock or even wood would require far higher safety factors, say, 6 to 10. But if you do such thing you may be also seeing -maybe not precisely for rock- huge unlikely settlements that offend the sight of almost anyone that has put something on the skin of mother earth, making almost any kind of structural connection other than loose (no connections) viable. Hence I think it is sound to promote a practice that predicts in accord to the behaviour, and for the rock case, expect that the high strength is there.