Bearing Capacity - Rock

[ONENGINEER]

I am designing a foundation on rock with a UCS = 50 MPa.

The RQD is about 55% and RMR is about 30. When I refer to Bienawski Table for Rock Classification it gives:
C = 150 kPa
Phi = 20 deg

for the rock mass. These give an allowable bearing capacity of 200 kPa.

When I refer to Goodman Rock Mechanics book, it suggests:

C = 0.25 (UCS) = 12.5 MPa
phi = 30 deg

for the rock mass. These give an allowable bearing capacity of 5000 kPa.

I cannot understand why the results are so much different.

It would be appreciated if you let me know where is the problem. Thank you.

 

[ONENGINEER]

Corrections:

I am designing a foundation on rock with a UCS = 50 MPa.

The RQD is about 55% and RMR is about 30. When I refer to Bienawski Table for Rock Classification it gives:
C = 150 kPa
Phi = 20 deg

for the rock mass. These give an allowable bearing capacity of 1100 kPa.

When I refer to Goodman Rock Mechanics book, it suggests:

C = 0.25 (UCS) = 12.5 MPa
phi = 30 deg

for the rock mass. These give an allowable bearing capacity of 2500 kPa.

I cannot understand why the results are so much different.

It would be appreciated if you let me know where is the problem. Thank you.

 

[BigH]

Check out Tomlinson's Foundation Design and Construction book - good presentation. Rock is sometimes a very strange bedfellow for something one thinks is cake.