Strength and Deformation of Pillars during Mining in the Shaft Pillar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study presented in the paper deals with relation of strength and deformation of pillars in the shaft pilar ČSM coal mine. Investigation of coal samples with different slenderness ratios (width/height) were carried out while the modulus of deformation related to the slenderness ratio was evaluated. Finaly, safety factors were calculated for the chosen dimensions of the pillars at the particular locations.

The paper present a comprehensive study of the behaviour of coal pears in a deep mine. In  terms of novelty and technical content the paper satisfy the conditions for the publication in a scientific journal. The results have practical application in similar boundary conditions and are comparatively evaluated and discussed with studies of similar topic. The authors are advised to improve the impact of their paper by providing a more comprehensives literature review considering the topic. For example, the references indicated under the numbers 2,3,4,5 should be summarised in terms of relevant conclusions and evaluated for their relevance. Similarly, a more comprehensive overview of the effect of the slenderness ratio on the bearing capacity of pillars is needed. The authors should distinguish between the references given and declare why are they have listed them in their paper. They should  also highlight in discussion how are they relevant to the topic and why there were chosen for comparison of the results. In general, the quality of all figures need to be improved.

Author Response

• Information on important cited references has been briefly added to the article, as well as additional information about effect of slenderness ratio on the bearing capacity of the pillars, given the limited scope of the article.
• The quality of figures will be improved (will be send separately).

Reviewer 2 Report

Comments and Suggestions for Authors

This paper mainly investigates the uniaxial compressive strength of coal pillars under different aspect ratios to provide a basis for the design of coal pillars between working faces, but there are the following problems:

1. The test is based on wide specimens, while coal pillar instability often occurs in narrow coal pillars, what is the significance of the study?

2. The aspect ratio of the working face is 8.33, while the test range is 1~7.7, which does not cover the aspect ratio, and the theoretical basis is not sufficient.

3. Wide coal pillar instability is often manifested in such aspects as sheet ganging and is mainly caused by the stress concentration phenomenon due to mining activities, while the test adopts uniform load, which does not reflect the stability of coal pillars in the field, and is not in line with the author's expectation.

4. ‘Loading was carried out at a constant strain rate so that the rate of 123loading to ultimate strength was in the range of 0.5- 1 MPa.s-1’. 1 MPa.s-1", is the adopted loading rate too high?

5. R2<0.7 in Eq. 5~7 when fitting the deformation modulus is of low confidence and the correctness is questionable.

6. Figure 8 cannot show the slenderness ratio after crushing.

7. It is suggested to increase the damage mode of the specimen, from which the damage mode can roughly reflect the damage form of the coal pillar.

 

 

Comments on the Quality of English Language

Appropriate modifications are needed

Author Response

• • The research focused on testing wide samples with slenderness ratios >1 due to the application of the results to coal mining at significant depths ( >800 m). In such conditions, the slenderness ratio is always >1, as evidenced by the references cited in the paper. Narrow pillars are designed in hard rock, where their brittle instability is commonly evident. In wide coal pillars, slow plastic deformation usually occurs, and brittle deformation is manifested at the edges of the pillars, which is evident from a review of the available literature, our laboratory research and subsequent in situ geotechnical monitoring.

   

  • A smaller slenderness ratio (>7,7) was not implemented due to the disintegration of coal thin slices (< 9 mm) during sample preparation (cutting).

   

  • The research was carried out according to the information and procedures obtained by studying the available literature and the ISRM recommended laboratory procedures, where the recommended rate is 0.5-1 MPa.s-1

   

  • The determination index of the mentioned regressions is relatively low, however, it is in-fluenced by the high dispersion of the measured values of the module Ep, especially for low values of the slenderness ratio (more explained in the article).

   

  • The title under figure 8 has been corrected

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Basically meets the modification requirements

Comments on the Quality of English Language

Basically meets the modification requirements