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Should I use the intact rock modulus or the rock mass modulus?

It really depends on the model and what you are modeling. If you are doing a simple continuum model of a rock mass, without discrete joints, then the modulus is the rock mass modulus. If you are doing a model in which a joint network has been defined using discrete joint, then the modulus is representative of the material between the joints. If the material is intact rock, then the modulus is for intact rock.

Is the shotcrete in RS2 based on cylinder or cube strength?

RS2 treats the shotcrete lining as a structural member and the support capacity curves are based on the compressive strength of shotcrete as determined in a cylinder test.

Can RS2 models include changes in material properties over time?

RS2 does not explicitly analyze the change in material properties over time and the subsequent numerical analysis of this process in relation to an excavation. This is an extremely difficult analysis to do. The only example that may be related to this is the modeling of the Yacambu-Quibor tunnel, where the GSI was reduced to model the change in material over time.

Is it possible to change a material modulus over different stages?

The modulus can be changed both at failure (to a residual modulus) and from stage to stage (hardening or softening). See the Material Properties>Staged Material Properties dialog for more information. Note that you should not drop the modulus to zero in finite element analyses, as this will cause numerical issues.

What initial element loading do I apply for backfill?

Backfill should have an initial element loading of Body Force Only.

How can I model ground improvement?

By default, RS2 resets the stresses in an element when it changes material. This is to model the removal and redeposition of the material through backfilling.

If you want to model ground improvement, and you want the stresses to remain in a particular finite element, when you change the element's material you have to disable the Reset Element Stress When Material Changes To This Material option in the Stage Properties dialog. You need to do this for the material that you are changing the element to.

Hoek Brown Failure Criterion

Are the values of mi, s, and a for intact rock or the rock mass?

This is a very good question. If you measure the GSI for the entire rock mass, and use the corresponding Generalized Hoek-Brown parameters in a numerical analysis which also includes the discrete joints, then you are basically double counting the effect of the joints. When using discrete joints in RS2, the Generalized Hoek-Brown properties should be of the rock between the joints. If the rock between the joints is intact rock, then use the intact properties for it. If you are modeling only major joints or faults in RS2 then the material between those interfaces may not be intact rock. In that case, determine a GSI value for that material between the joints but not including the major joints that are modeled discretely.

For intact rock: mb=mi; s=1.0; a=0.5

In RSData, the Use Lab Data option computes the intact strength. Setting GSI=100 sets the rock mass strength to be equal to the intact strength.

For plastic analysis, how do I estimate the residual value of the parameter mi?

On the topic of residual strength, please see the following response from Evert Hoek:

In response to your question about how Phase2 handles the post failure characteristics of rock masses I have to say that we, the rock engineering community, do not have very good models of this process yet. The process itself is illustrated on page 26 of the chapter on rock mass properties and in the attached pdf file. How you create these responses in Phase2 is presently a matter of judgement rather than science.

In general, in poor quality rock masses (RMR or GSI < 40) I assume perfectly plastic post failure behaviour which means that the rock mass retains its strength and there is no volume change. This is achieved by using the same properties (mb, s and E) for both peak and residual strength and setting dilation to zero. For better quality harder rock masses I tend to use the Disturbance factor D to achieve a strength and modulus reduction after failure and a typical D=0.7 value seems to work well in most cases. The second pdf attachment shows this process in Phase2 version 8 where I have used D = 0.7 in the GSI option when specifying rock mass properties in a model. Note that I have left the dilation = 0 since I have never found a satisfactory way of using this parameter.

A number of researchers are working on this issue of post peak behaviour and I am sure that we will arrive at better solutions in the next few years. Meanwhile, this is the best guidance that I can offer you.

There is also a paper by Cai that may be useful:

Determination of residual strength parameters of jointed rock masses using the GSI system, Cai, M., Kaiser, P.K., Tasaka, Y., Minami, M., 2006, published in International Journal of Rock Mechanics and Mining Sciences 44 (2007) 247-265 Elsevier.

The two attachments referred to can be viewed here:

Post Peak Properties

Post failure of rock masses

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