**3.3 Finding 3: cave porosity zone height decreases with a decrease in extraction rate**

Four different scenarios were simulated with increasing velocities from 0.0001 to 0.0004 m/s by keeping the same rock mass properties (RM2). For the block cave model, the same velocities (extraction rates) are assigned at all the grid points, whereas in the panel cave model, different velocities are applied at grid points due to the inherent nature of the caving process. An increase in velocity value means an increase in material extraction rates. The simulated porosity values for different zones under different extraction rates for both block and panel cave models are shown in **Figures 8** and **9**.

In the real-world scenario, in a propagating cave, the cave height increases with increased production. It was evident from **Figures 8** and **9** that when the ore

**Figure 6.** *Porosity value profile for RM1 (left) and RM2 (right) for block cave mine model.*

**Figure 7.** *Porosity value profile for RM1 (left) and RM2 (right) for panel cave mine model.*

*Block Cave Mine Ventilation: Research Findings DOI: http://dx.doi.org/10.5772/intechopen.104856*

### **Figure 8.**

*Porosity value profile for RM2 under four different extraction rates (0.0001–0.0004 m/s) for block cave model.*

### **Figure 9.**

*Porosity value profile for RM2 under four different extraction rates (0.0001–0.0004 m/s) for panel cave model.*

extraction rate was increased, the cave zones height was increased. The mobilized zone porosity tends to be higher as it is the actively flowing region in the cave. In both block and panel cave models, the mobilized zone porosity value ranges from 0.35 to 0.40. The height of this zone is relatively high compared to the other zones due to its proximity to the drawpoints where the ore is extracted from the cave.

In the block cave model, when different velocities, from 0.0001 to 0.0004 m/s, were applied at the drawpoints, the porosity values in the mobilized zone were observed to be from 0.30 to 0.35. This could result from stagnant flow in the cave that subjected the material to re-compaction. If the extraction rate of material is not equal to the rate of cave propagation, the material will stagnate, re-compacted, and result in lower porosity.

### **3.4 Finding 4: cave porosity is affected by the fragmentation size**

Material extraction is simulated in a block cave model by opening drawpoints to extract a targeted mass of 100,000 metric tons. As soon as the drawpoint is open, due to gravity, the spheres (broken rock) will start flowing toward the drawpoints. Measurement locations are strategically placed to measure the porosities as the material flows through the cave. Porosity measurement histories are recorded at all the measuring locations (spheres) while extracting the material from the cave through the drawpoint. From the simulation results, it was found that the porosity change in the isolated extraction zone (IEZ) ranges from 0.39 to 0.56. In comparison, in the isolated draw zone (IDZ) insignificant porosity change (0.38–0.42) was observed. The random spike(s) seen on the graph shows the change in the porosity while the material is drawn from the cave.

To study the effect of particle size distribution on the porosity change, two fragmentation distributions with different mean particle sizes and standard deviations are applied for the block cave mine. **Table 1** illustrates the particle distributions considered for the simulations.

Scenario #1 is simulated by consecutively opening the six drawbells. When the target discharge mass is reached the assigned value in the system, the next drawpoint will open immediately as shown in **Table 2**.

Scenario #2 is conducted by opening the six drawbells randomly. The discharged mass is applied to control the opening of the drawbells for the random opening. A summary of discharge mass criteria and the drawbells are provided in **Table 2**.

The numerical model of porosity assessment using a discontinuum approach successfully modeled change in porosity associated with the fragmented rock mass flow in a mature cave. It was found that during material extraction, the porosity changes relatively higher in IEZ than in IDZ. These changes for IEZ range from 0.39 to 0.56 for a block cave and 0.38–0.48 for a panel cave.


The sensitivity analysis on particle size distribution concluded that fragmentation size affects cave porosity. In the case of Fragmentation #1, the change in

**Table 1.** *Gaussian particle size distributions for fragmentation #1 and #2.* *Block Cave Mine Ventilation: Research Findings DOI: http://dx.doi.org/10.5772/intechopen.104856*


**Table 2.**

*Discharge mass criteria for opening the drawpoints for two scenarios.*

porosity ranges from 0.40 to 0.48 in IEZ and from 0.38 to 0.56 for Fragmentation #2. Similarly, the draw control strategy also affects cave porosity. In the case of Scenario #1, the change in porosity ranges from 0.38 to 0.52 in IEZ and from 0.38 to 0.48 for Scenario #2 in IEZ.
