Impact of Ballast Fouling on the Mechanical Properties of Railway Ballast: Insights from Discrete Element Analysis
Round 1
Reviewer 1 Report
1- Very nice piece of research. I suggest to continue it also by considering the decrease in permeability of the degraded ballast. The reduction of porosity is already determined against the breakage (figure 14c). It can be the subject of the next paper.
2- In line Nr. 160, which the speed of applying the shear is mentioned, why the same rate as what was used in the real experiment wasn’t taken?
3- Some of the figure titles are shifted to the next page which I do believe is because of the Word program . Please cross check them.
Author Response
Please see attached reply.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This is an interesting paper that simulated fouled ballast direct shear behavior by decreasing inter-particle contact modulus and surface friction coefficient instead of injecting small particles. The correlations between model parameters and ballast fouling levels were established. Some comments and suggestions can be found as follows:
- Line 50-51: According to previous studies, only adjusting inter-particle friction angle could match the experimental results well. So, it is not appropriate to state that accurate results could not be achieved by adjusting one parameter such as inter-particle friction angle. Also, the references here could not support the authors’ point.
- Line 57-60: The two references only presented experimental results, so they could not support the authors’ statement that numerical results were significantly different from experimental results. It will be more convincing if the authors could add some related references.
- Line 91 and Figure 1: The characters denoting “sphere size ratio” in text and figure are not identical.
- Line 147 and Figure 5: It is mentioned that particle size in the DEM model ranges from 7.5 mm to 40 mm, but there exists a portion of particles finer than 7.5 mm according to the gradation curve in Figure 5. It will be better if the authors could clarify if they scalped the gradation or they used 7.5 mm (minimum size) particles to represent all the particles smaller than 7.5 mm.
- Table 1: The inter-particle friction coefficient is 4.1, which is not normal to exceed 1.0. It is strongly recommended to explain the selection of parameters since those parameters and VCI are correlated in this paper.
- Line 238: Can the authors estimate how much time could be saved compared to the previous methods?
- Section 9 (Line 291~309): It is highly recommended to show model parameters used for the 12 materials in Section 9. The authors obtained the shear strengths of 12 materials with different breakage indices (from 0% to 28.72%) which represent 12 different ballast degradation levels. Apparently, the 12 materials have different VCIs (fouling levels). In the previous sections, the authors have already established the correlations between VCI and model parameters, so there should be 12 sets of model parameters. In my opinion, showing the parameters could help link Section 9 to the previous sections.
Author Response
Please see attached reply.
Author Response File: Author Response.pdf
