Mechanical Properties of the Combination of Surrounding Rock and Primary Support under Impacting Load
Round 1
Reviewer 1 Report
The dynamical experiment was conducted on the the combination of surrounding rock and primary support. The influence of support thickness was studied. The following comments are listed for consideration and should be addressed before acceptance.
(1) How to make the combination of the primary support and surrounding rock? Some pictures in test are suggested to be given.
(2) How to realize the cohesive contact between the primary support and surrounding rock?
(3) The radius of the hollow excavation is 60 mm. The sum of thickness of the cement and sandstone is 50~70 mm, which is much less than 4 times of the radius of the hollow circle. The outer layer should not be named as the surrounding rock according to the theory of rock mechanics. Otherwise, the error existed in the test scheme as authors neglected the factor.
(4) Usually in dynamic test the loading rate is considered. What do the authors think of this factor on the strength? (Experimental investigation of the strain rate effect on crack initiation and crack damage thresholds of hard rock under quasi-static compression). Uncertainty analysis (A practical and efficient reliability-based design optimization method for rock tunnel support) and complex unloading stress conditions (Unloading behaviors of shale under the effects of water through experimental and numerical approaches) are important challenges to underground tunnel engineering. The relative research results could be reviewed or discussed.
(5) What is the loading rate and the corresponding condition in engineering?
(6) Please show the monitoring facilities, such as the deformation monitoring device.
Other editorial comments:
(1) Line 102: "elected"->"selected".
(2) First letters in tables should be written in capitals.
Author Response
Point 1: How to make the combination of the primary support and surrounding rock? Some pictures in test are suggested to be given.
Response 1: The specimens, shown in Figure 2, were made from the combination of cement and sandstone. The sandstone with a certain hollow circle was used to simulate the surrounding rock in the tunnel, and the cement was applied to describe the primary support. According to the suggestions of the expert, some necessary pictures in the producing process of specimen were added in the paper.
Point 2: How to realize the cohesive contact between the primary support and surrounding rock?
Response 2: The primary support and surrounding rock were bonded by cement, whose strength is 42.5 MPa in this paper. And the cementation method was unchanged whole the study.The specimens making process : the rock ring was used as the peripheral mold, and the steel pipe with a diameter of 60 mm and a length of 200 mm was selected as the inner diameter mold. Two acrylic plates ( one of which has a self-made scale ) were used as the upper and lower pressing templates. The cement mortar was poured into the mold. After natural solidification, it was closely cemented with the rock part.
Point 3: The radius of the hollow excavation is 60 mm. The sum of thickness of the cement and sandstone is 50~70 mm, which is much less than 4 times of the radius of the hollow circle. The outer layer should not be named as the surrounding rock according to the theory of rock mechanics. Otherwise, the error existed in the test scheme as authors neglected the factor.
Response 3: The sample size is mainly designed according to the actual tunnel. The actual tunnel situation is: diameter 6m, support thickness 1.2-1.5m. Therefore the ratio relationship of the radius in the simplified model of the primary support and surrounding rock in this paper was set to be 1:2.
Point 4: Usually in dynamic test the loading rate is considered. What do the authors think of this factor on the strength? (Experimental investigation of the strain rate effect on crack initiation and crack damage thresholds of hard rock under quasi-static compression). Uncertainty analysis (A practical and efficient reliability-based design optimization method for rock tunnel support) and complex unloading stress conditions (Unloading behaviors of shale under the effects of water through experimental and numerical approaches) are important challenges to underground tunnel engineering. The relative research results could be reviewed or discussed.
Response 4: According to the theory of rock mechanics, the dynamic strength increases with the increase of loading rate, and it presents strongly loading rate dependence. The testing specimens in this paper are composed of the primary support and surrounding rock, which are regard as a whole. The effect of the thicknesses of the sandstone (primary support) and the cement (surrounding rock) were considered, that do a great significance on the dynamic strength of the specimen.
Point 5: What is the loading rate and the corresponding condition in engineering?
Response 5: In engineering practice, the loading rate is caused by the external dynamic loads, such as blasting, collapse and the excavation state. Different kind of external dynamic loads may change the loading rate, which is one of the main factors influencing the dynamic strength of the primary support and surrounding rock. The loading rate in this paper was controlled by controlling the air pressure of the impact device of the testing system, so as to study the effect of which on the dynamic strength of the primary support and surrounding rock. The research results of this paper have an important theoretical reference on the tunnel engineering.
Point 6: Please show the monitoring facilities, such as the deformation monitoring device.
Response 6: The strain signal is a kind of electrical signal caused by the impact load in this test, which is captured by the strain gauges, amplified and displayed by Oscilloscope, shown in Figure 3.
Reviewer 2 Report
The manuscript presented mechanical properties of the combination of surrounding rock and primary support under impacting load that could be applied to tunnel construction
IN fact, the exerimental procedure and results are interesting. However, there are no scale effect study. The physical model in 1g condition is quite small to represent the tunnel prototype.
The study cannot be consider to design cndition of real tunnel. Therefore I don't know how the result can be used in the practice. The physical model could represent the pipe, but I cannot see the real alication in this situation.
Author Response
Response to Reviewer 2 Comments
Point 1: In fact, the exerimental procedure and results are interesting. However, there are no scale effect study. The physical model in 1g condition is quite small to represent the tunnel prototype.
The study cannot be consider to design cndition of real tunnel. Therefore I don't know how the result can be used in the practice. The physical model could represent the pipe, but I cannot see the real alication in this situation.
Response 1: The purpose of this paper is to explore the mechanical characteristics of the combination of primary support and surrounding rock under dynamic loads adopting experimental method. There are some differences with the practice situation certainly in engineering. The actually physical model was simplified to be a kind of pipe model in this paper, which is similar with the tunnel engineering. The results of dynamic characteristics in this paper can give some references in engineering designing.
Reviewer 3 Report
1. This experiment has a good effect on the tunnel surrounding rock with homogeneous isotropy, but generally speaking, the tunnel failure occurs more in places with strong structure and complex surrounding rock properties, so the experiment studied in this article does not have universal adaptability to the tunnel in general conditions.
2. The article lacks the consideration of the mechanical properties of surrounding rock under the influence of original geostress.
3. The methods and means adopted in the article are more conventional and lack of innovation.
4. In order to increase the practical value of the article, it is suggested that the author supplement more general experimental conditions and increase the comparison of numerical analysis matching with the experiment.
Author Response
Response1: Considering the particularity of local rock mass, the study of this experimental focuses on the dynamic properties and failure mode of a kind of combination of surrounding rock and primary support mainly. It may not be universal, but which is useful for understanding the failure process of the tunnel during tunnel excavation. In the later study, the authors will take the local particularity of surrounding rock into account according to the engineering conditions, such as the distribution of geostress, anisotropy and heterogeneity of bottom hole rock.
Response 2: The mechanical properties of the combination of surrounding rock and primary support were studied under the uniaxial dynamic loads in this manuscript. The mainly purpose of this study is to clarify the stress-strain relationship and its failure mode under different strain rate of the external impact loads. The confining pressure caused by the original geostress will be considered utilizing numerical methods in the future study. Thanks for your suggestion.
Response 3:The study of this manuscript is to clarify the dynamic mechanical properties of the combination of surrounding rock and primary support based on the SHPB apparatus under uniaxial impact loads, which is a kind of conventional experiment. However, the main purpose of this study is reached. And, there are few relevant studies reported about the tunnel simulated by double-layer annular specimen.
Response 4:This manuscript studied the dynamic properties and failure mode of the combination of surrounding rock and primary support, the main controlling input parameter is the strain rate acted on the specimen launched by the impact velocity of the SHPB apparatus, and the effect factors includes the thickness of the sandstone and the cement. This a an experimental study, the corresponding numerical study had been finished by the team of the authors. The numerical analysis results shall be published subsequently.
Author Response File: 
Author Response.pdf
Reviewer 4 Report
The manuscript “Mechanical properties of the combination of surrounding rock and primary support under impacting load” is interesting. However, the work is limited scope. Please add some correlation work in the “discussion”. Results and discussion should be described under separate headings.
Author Response
Thank you for your recommendation. The structure of this manuscript has been adjusted, which makes the article seem more logical.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Thank you for the amendment. Additionally, please refer to the aforementioned 4th comment in the 1st round review and add the relative references in the latest research review.
Author Response
Thank you for your reminding. The relative references had added to the latest research review.
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript still associates the study with tunnel construction applications. It could be used for the pipeline, but I don't think that blast can be used in this case. It is an interesting academic result, however, it is not applicable.
Author Response
Thank you for your reminding, the scale effect will be further considered in the future study to verify the relevant reference model.
Author Response File: Author Response.pdf
