Problem Q&A
1. As the FLAC/Slope is a 2D modeling software, to analyze the slope stability, we are not sure which side you expect us to analyze.
The orebody is assumed to be infinite in length (out of the plane dimension). It is recommended to review the concept of infinite slope to select what section of the ore body analyze.
2. What do you mean by Rock Mass Strength Variability? Does this mean we have a window to choose the material strength?
The coefficients of variability indicate that the rock mass properties are not uniform. This parameter can be considered when performing a probabilistic analysis or in selecting the design rock mass strength for deterministic analysis.
3. The dewatering cost is based on the amount of water contained above the phreatic surface. How do I assume water with a dewatering cost above the phreatic level, would not be below?
Point 3 on the guidance section of the problem has been clarified as follows:
The dewatering cost is based on the amount of water removed above the phreatic surface after excavation.
4. Is there a specific location for the pseudostatic analysis for the use of a seismicity factor?
A pseudostatic analysis is not required in the problem. Therefore, it is not necessary to know the specific location of the problem.
5. As one of the Design Acceptance Criteria was quoted: Factor of Safety ≥1.3 or Probability of Failure ≤5%. Does it mean that acceptance criteria should be factor of safety (Fs ≥ 1.3) and then we should check the probability of failure (≤5%)?
Depending on the analysis approach selected to solve the problem either deterministic or probabilistic, you may choose between one of these two acceptance criteria.
The report guidelines ask your team to define how you are going to address the rock mass variability. Having the probability of failure as an acceptance criteria may be helpful to work this section of the report.
6. In the material properties chart is the density given as the dry density or saturated density?
The density reported on the rock mass properties table is the dry density of the rock. In addition, you are provided with the porosity.
FLAC/Slope automatically calculates the saturated density when you enter the dry density and porosity of the rock.
8. Are the rock mass properties provided for the three depth intervals equal for the orebody and host rock ?
The properties for both the ore body and the host rock are the same, and vary according to depth as shown in the rock mass properties table.
9. Our team has opted to go for software that allows us to design the 3D pit. If I am going with different designs for the open-pit mine (elliptical or circular), then I'm getting different answers for my mining depth. Apart from that, the answer also depends on the bench dimension parameters used. So is it possible to have multiple answers to the problem?
The problem can be solved considering just one 2D Section that is assumed to be constant along the infinite length. However, a 3D analysis can also be performed assuming a ‘slice’ of a constant cross-section along a specific length. The assigned length will not affect the maximum depth.