A Rock Mass Classification Method for Tuff Tunnel Based on the High-Pressure Gas Expansion Method

. A precise rock mass grade result is crucial for directing the tunnel excavation engineering design. A novel rock mass classifcation method for tuf tunnel based on the high-pressure gas expansion method (HPGEM) was proposed, which was primarily built on feld test data previously acquired by the research team. Te main achievements are as follows: combined with feld data and the HPGEM rock-breaking theory, analyzing the rock uniaxial compressive strength, rock mass integrity index, and the relationship between the gas generator unit consumption and ftted the relevant equations. After that, the rock optimal uniaxial compressive strength (about 150MPa) and the rock integrity factor (about 0.85) were obtained. With reference to the BQ rock mass classifcation method, a new rock mass classifcation method that applied to HPGEM was proposed. Tis study flls the gap of the appropriate rock mass classifcation method requested on HPGEM.


Introduction
Te intricate urban pipeline network and complex surface/ underground building led to the adverse construction environment of a tunnel.Traditional dynamite blasting is carried out with extreme vibrations, so it is difcult to meet the demands of construction under the higher vibration limit condition.As a result, the innovative rock-breaking techniques have been developed such as gas blasting [1], mechanical excavation [2], and tunnel boring machine (TBM) [3].However, these techniques have some drawbacks on certain aspects.For instance, gas blasting has expensive equipment and complex operation requirements [4], the mechanical excavation is inefective and has a narrow feld of application [5], and the TBM equipment is expensive and necessitates large construction site [6].Tese drawbacks constrain the efcient construction of hard rock tunnel excavation near important buildings in urban construction.Te "high-pressure gas expansion method" (HPGEM) [7] was proposed to address these drawbacks.Tis is an emerging rock-breaking method, and its working principle is as follows: in the sealed condition, with the gas generating agent in the expansion pipe as the active ingredient, chemical and physical reaction occurs, and in a very short time, a large amount of high-temperature and high-pressure gas is released, which expands the surrounding rock and does the work, and then achieves the efect of rock breaking.Te HPGEM method is characterized with strong rock-breaking capacity and low vibration hazard.However, the working principle of HPGEM is signifcantly diferent from that of dynamite blasting.Te traditional rock mass classifcation method of tunnel (such as rock quality designation (RQD) [8], tunneling quality index (Q) [9], rock mass rating (RMR) [10,11], geological strength index (GSI) [12], and China national standard basic quality (BQ) [13]) is mainly based on the stability (solidity) of the rock mass; thus, the weak the quality of the rock mass, the better the blasting efect.For HPGEM, when the strength and integrity of the rock mass is higher (within certain limits), the rockbreaking efect is better.Also, when the strength of the rock mass is lower and the integrity is poorer, HPGEM cannot complete the rock-breaking work because the gas escapes through the cracks.Tis characteristic is completely opposite to dynamite blasting.Terefore, the existing method of tunnel rock classifcation is not applicable to HPGEM at all.
Te classifcation of engineered rock mass dates back to the 18th century, and the Russians categorized rock mass into fve grades based on their hardness.In the early 1960s, the Europeans further subdivided it into six categories of the rock mass [14].Te rock mass classifcation method for design purposes started to emerge at the turn of the 20th century; the most well-known ones are Vickers hardness classifcation [15], Terzaghi classifcation [16], etc.; in the middle of the century, the rock mass classifcation method for describing the stability of the surrounding rock started to emerge, including Vutukuri classifcation of the surrounding rocks [17] and Deere classifcation of the RQD index [18].In addition, the standard for engineering classifcation of rock mass (China, GB/T 50218-2014) [13] divides rock mass into fve grades primarily based on rock hardness and rock mass integrity, which is a method that combines qualitative and quantitative aspects.As science and technology have advanced in recent years, the rock mass classifcation method has benefted.For instance, Liu et al. [19] trained classifcation samples of the rock mass surrounding the tunnel using the improved BQ method while the tunnel was being built.Tey then used the trained classifcation samples to create an intelligent SVC rock mass classifcation model.A "site-specifc" classifcation method of rock mass based on entropy weight-TOPSIS-grey correlation analysis was developed by Dai et al. [20].By collecting rock feature information and training neural networks, the literature [21] is able to recognize diferent types of rock mass and grades them.Tis method overcomes the shortcomings of previous classifcation methods and provides a more accurate and reliable rock mass classifcation method for underground mining.Gong et al. [22] investigated the rock-breaking process of TBM construction and proposed a number of construction prediction models [23][24][25], which ofered theoretical guidance for the categorization of tunnel-surrounding rock.Qi and Wu [26] discussed in detail the infuence of three aspects of geological conditions, construction equipment, and on-site management on the digging speed of TBM construction and carried out a hierarchical prediction study on the rock quality of TBM construction based on the classifcation of the surrounding rock of traditional dynamite blasting construction and fuzzy mathematical methods.Alemdag et al. [27] indirectly estimated the deformation modulus by means of neural networks, neuro-fuzzy, and genetic programming, which provided assistance in determining the results of rock body classifcation.
According to the development history of rock mass classifcation methods, it is necessary to establish a "sitespecifc" rock mass classifcation method for specifc construction methods and geological conditions.Presently, there is no research on the rock mass classifcation method of HPGEM, which may cause misjudgment in construction progress.Terefore, this paper took a tuf tunnel as the engineering background, attempted to discuss the infuence of uniaxial compressive strength and rock mass integrity on gas generator unit consumption (GGUC), and on this basis, draws on the BQ method and fnally tried to propose a rock classifcation method applicable to HPGEM rock breakage in tuf tunnels.Tis study may fll the gap of the rock mass classifcation method applicable to HPGEM and lays the foundation for the wide application of this emerging method in engineering.

Principles of HPGEM for Rock Breaking
Te HPGEM combines the high efciency of explosive blasting with the gas expansion characteristics of liquid carbon dioxide phase-change fracturing.Te apparatus (expansion pipe) for the HPGEM consists of the following three main parts [7]: the gas generator storage pipe (gas generator containers), the iron pipe (air-conducting function), and the wire (used to connect electrical triggers), as shown in Figure 1.
Te rock-breaking mechanism of the HPGEM can be divided into three stages (Figure 2).

2.1.
High-Pressure Gas Initially Impacts the Surrounding Rock at High Velocity.Te gas generator in the expansion pipe can produce a signifcant volume of gas instantaneously when cracking the rock using a HPGEM.As the expansion pipe is completely sealed by the pressed slurry material and the rock mass exerts a confning pressure on the pressed slurry material, the pressure inside the expansion pipe also rises rapidly (caused by the rapid generation of gas) and the pressure inside the expansion pipe is much greater than the confning pressure.Te compaction pressed slurry material, by virtue of its strength and the confning pressure, can resist damage in a very short period of time, which further leads to an increase in the pressure in the expansion pipe.Subsequently, the pressed slurry material is crushed by the confning pressure generated by the high-pressure gas, and this pressure is then rapidly applied to the surrounding rock.
Compared to dynamite blast [28], this stage is due to insufcient shock wave, which prevents the expansion pipe near the compression pressed slurry material and the rock mass from being crushed, but it can still directly break the rock in a small way, causing the rock to cause some small secondary fssures.Compared to other gas blasting, HPGEM in this stage is of the wider range of broken rock and more comprehensive due to the high pressure, so its break diameter increases.

High-Pressure
Gas "Wedged" into Rock Fissures.At this stage, high-pressure gas is "wedged" into the surrounding rock mass at high speed along primary and secondary fssures, thus expanding and extending these fssures.At the same time, the development of fssures makes the rock mass strength weakened.Tis point is similar to the explosive explosion generated by the explosive gas rock-breaking mechanism, but diferent from the liquid carbon dioxide phase change fracturing method, where carbon dioxide for the frst time becomes a part of the action of the rock mass facing the energy-discharging piece of the rock body, and the 2 Journal of Engineering rock body has nothing to do with the situation of the fssures.On the other hand, the high-pressure gas expansion method is to release the high-pressure gas comprehensively and uniformly into the surrounding rock mass.Terefore, even if the release time and the acting pressure are the same, the rock-breaking efect of the high-pressure gas expansion method may be more uniform, whereas the liquid carbon dioxide phase change fracturing method may have greater uncertainty.

High-Pressure Gas Macro Rock Breaking.
After the initial high-pressure gas wedged into the rock mass, the subsequent high-pressure gas infux along the previous fssure channels and form an interconnected pressure body, which is then transformed into the quasistatic pressure acting on the rock mass; on the other hand, the initial high temperature and high pressure of the gas acting on the rock mass will derive tensile and shear stresses in the rock mass.Under the dual action of quasistatic pressure and tensile and shear stresses, the rock mass will be macroscopically damaged.

Tuff Tunnel Rock Mass Classification Method Based on HPGEM
As the engineering environment becomes more and more complex, the factors considered in the rock mass classifcation methods become more and more comprehensive, detailed, and specialized [29,30].Te mainstream rock mass classifcation standard [13] currently used in China is mainly based on the stability of surrounding rocks and is divided on the basis of geologic factors, which mainly includes the strength of rocks and rock mass integrity.Regarding the method of tunnel rock mass grading under TBM construction conditions, a majority number of scholars [31,32] take the construction efciency of TBM as its key comprehensive index.Also, the construction efciency is mainly obtained by analyzing its infuencing factors.Although there are many factors afecting the construction efciency, geological factors have always been the most critical, especially the uniaxial compressive strength of rock and the integrity of rock mass, and these two factors have a significant infuence on the results of rock mass classifcation.In view of this, this paper refers to the selection of indices for rock mass classifcation in TBM construction, and considering, at the same time, the infuence of single consumption of gas generator on the efciency of rock breaking, the uniaxial compressive strength of the rock, and the integrity of the rock mass are selected as the evaluation indices for the classifcation of HPGEM rock mass.Combined with the BQ rock mass classifcation method, we try to propose the HPGEM rock mass classifcation method for tuf tunnels.

Rock Uniaxial Compressive Strength and GGUC.
HPGEM is a rapid reaction in a confned space, generating a large amount of high-temperature and high-pressure gas, which in turn does work on the surrounding rock mass to achieve rock breaking.If the compressive strength of the rock mass is too low, it will lead to premature rupture of the rock mass in the vicinity of the expansion pipe, which may form a pressure vessel with a larger space, increase the uplift space of the individual expansion pipe, and thus lead to a lower peak pressure of rock breaking.Tis may even cause the primary fssure in the rock mass to penetrate, and the high-pressure gas will enter the external environment, resulting in failure of the rock breaking.Based on the literature [7] published by the research team, in the actual rock-breaking process, the lower the uniaxial compressive strength of the rock, the worse the efect of rock breaking, and when the uniaxial compressive strength of the rock is higher, it may achieve a better rock-breaking efect.At this time, in order to ensure the efect of rock breaking, it is necessary to reduce the hole distance and other parameters, which will lead to an increase in GGUC.
Field test data show that the cost of HPGEM is generally more than twice times that of dynamite blast without considering the cost of time.Terefore, HPGEM is recommended to be used in hard rock areas where dynamite blast is not suitable, especially when the uniaxial compressive strength of the rock is above 60 MPa, and its rockbreaking efect is signifcant.When the uniaxial compressive strength of the rock is lower than 30 MPa, the rock-breaking efect is poor, the GGUC is large, the cost is high, and sometimes, it cannot break the rock successfully.If HPGEM is used for rock breaking in hard rock tunnels, it can be observed that there is an obvious correlation between the uniaxial compressive strength of rock and GGUC.In order to fgure out the relationship between the two so as to provide a basis for the later rock mass classifcation, the feld test data were analyzed and the results are shown in Figure 3.
Based on the ftting results (polynomial curve ftting), it is found that there is an obvious binomial relationship between the uniaxial compressive strength of rock and GGUC.As the uniaxial compressive strength of the rock increases, the GGUC decreases gradually, but it has leveled of at close to 150 MPa.Terefore, it is hypothesized that HPGEM may be more suitable for application in harder areas, which not only saves material costs but also reduces time costs due to changes in the layout parameters.In addition, the derivation of the ftted equation was performed to fnd the uniaxial compressive strength of the rock at the lowest GGUC, i.e., the minimum GGUC K min � 0.48 kg•m − 3 and the optimum rock uniaxial compressive strength R c � 159.35 MPa.

Rock Mass Integrity and GGUC.
Rock mass integrity is also one of the most important factors afecting rockbreaking efectiveness.Similar to the uniaxial compressive strength of rock, the discontinuous structure of rock mass, such as primary joints and fssures, will also have a signifcant infuence on the rock-breaking efect of HPGEM and the high-temperature and high-pressure gases generated by HPGEM will be "prematurely relieved" due to the existence of primary structure of the rock mass, even leading to rockbreaking failure.In order to maintain the rock-breaking efect, it is necessary to take the same measures mentioned above, such as reducing the hole spacing and other parameters and increasing the unit consumption in exchange for the rock-breaking quality.
At present, the integrity of rock mass is divided into fve grades as follows: complete, relatively complete, relatively broken, broken, and extremely broken, mainly based on qualitative indicators such as the development degree, combination degree, type, and corresponding structure type of the rock mass structural plane [33].To study the relationship between the integrity of rock mass and the GGUC of HPGEM, a quantitative index of the integrity degree of rock mass should be introduced.In reference [13], the coefcient of integrity of rock mass is used as the quantitative index of the integrity degree of rock mass.If the coefcient of integrity of rock mass is not convenient to obtain, the volume joint number of rock mass can also be used instead.Table 1 shows the relationship between the integrity coefcient of the rock mass and the number of joints in the rock mass volume.Consistent with the analysis of rock uniaxial compressive strength, the relationship formula between rock mass integrity and GGUC was ftted by using the feld test data.Te original data and ftting results are shown in Figure 4.
Te ftting results show that the pattern of GGUC with changing rock mass integrity is similar to that of rock uniaxial compressive strength.However, the decrease in GGUC is slightly greater than that caused by the uniaxial compressive strength of the rock.Te GGUC decreases with increasing rock mass integrity, leveling of as it approaches 0.7.Terefore, it is hypothesized that HPGEM may also be more suitable for application in areas of higher rock mass integrity.Here, the minimum GGUC K min � 0.6 kg•m − 3 and the optimum rock mass integrity index K v � 0.86.

Construction of the Rock Mass Classifcation Method for HPGEM.
Combined with the abovementioned analysis, an attempt is made to construct the tuf tunnel rock mass classifcation method.
With reference to the standard for engineering classifcation of rock mass (GB/T 50218-2014) [13], it is constructed using a combination of qualitative and quantitative methods, mainly quantitative methods.Among them, the qualitative basis is mainly the characteristics such as the degree of rock hardness and rock mass integrity, while the quantitative basis is the scoring value of the basic quality indicators of the rock mass.Te rock uniaxial compressive strength R c and the rock mass integrity coefcient K v were used to complete the quantitative evaluation, and the interrelationships between the qualitative and quantitative indicators are shown in Tables 2 and 3, respectively.
Te HPGEM as an emerging rock-breaking method has no rock mass classifcation method applicable to this rockbreaking technique.Two main parameters, uniaxial compressive strength of rock and the degree of rock mass integrity (qualitative indices are the number of structural surface groups, average spacing, the degree of combination of the main structural surfaces, and the type of structure and the quantitative index is the integrity coefcient), are selected to carry out the study of HPGEM rock mass classifcation for tuf tunnels.Also, drawing on the "Engineering Rock Mass Classifcation Standard" (GB/T 50218-2014) [13] and based on the ftting formula of the feld test results, it is also classifed into I P , II P , III P , IV P , and V P , a total of fve grades, and a new rock mass grade classifcation method is constructed, and the specifc details are shown in Tables 4 and 5.In order to facilitate the diferentiation and improve the practicability, the grading interval is slightly adjusted, for example, the uniaxial compressive strength of the rock mass of 154.375 MPa is adjusted to 150 MPa for grade I and the integrity coefcient of the rock mass of 0.949 is adjusted to 0.95 for grade I and so on.At this point, the tuf tunnel rock mass classifcation method based on the HPGEM was established.

Discussion
Te HPGEM, as an emerging low-vibration rock-breaking technology, can replace dynamite blast for some special sections.Researchers have already analyzed it.Cui et al. [34] reported a high-pressure foam fracturing method, which is a new mild rock-breaking method between high-stress loading rate blasting and low-stress loading rate hydraulic fracturing and has the advantages of no sparks, less dust, no harmful gases, and controllable rock-breaking shapes.Xiaoqiang et al. [35] developed a rock-breaking gas generator, and by carrying out on-site rock-breaking tests, monitoring its vibration, extracting the main component of the gas-blasting signal, and analyzing the time-frequency characteristics of the diferent components of the signal, it was found that this new type of rock-breaking gas generator blasting technology has a signifcant efect of vibration reduction and damage reduction and it is suitable for popularizing the application of earth blasting projects.Te abovementioned two devices are shown in Figure 5.
Unfortunately, to the best of our knowledge, there are still no reports on the classifcation of rock mass based on this emerging rock-breaking method.If the traditional rock mass classifcation method is used to determine the excavation ability of the rock mass under the HPGEM, it will result in serious misjudgment.It has been shown that diferent methods of rock mass classifcation may yield very diferent rock mass grade results even in the same region [20].Terefore, a rock mass classifcation method applicable to the HPGEM is essential to ensure the safety and efciency of the project.
In this paper, a new method for classifying rock mass in tuf tunnels by HPGEM is constructed by drawing on the engineering classifcation of rock mass (GB/T 50218-2014)    Journal of Engineering [13] and the geological factors considered by TBM boring [33].Combining the working principle of HPGEM with the feld test results, it can be seen that there is a nonlinear relationship between the uniaxial compressive strength of the rock and the degree of rock mass integrity for the rockbreaking efect, which is diametrically opposite to the efect of traditional dynamite blast.Specifcally, when the rock strength and rock mass integrity gradually increase, the GGUC required for rock breaking shows a gradually decreasing trend (stage 1: R c : 0∼120 MP and K v : 0∼0.7), which implies that the rock-breaking efect of the rock mass gradually increases.Subsequently, the GGUC required for rock breaking starts to stabilize gradually (stage 2: R c : 120∼180 MP and K v : 0.7∼0.9), at which time the stability of the rock mass reaches a high degree, which means that the rock-breaking efect will no longer increase with the increase of GGUC (Figures 3 and 4).Finally, the stability of rock mass is very good (stage 3: R c : >180 MP and K v : >0.9), and at this time, the unit consumption of aerosol required to break a certain unit of rock starts to increase and the rock-breaking efect starts to decrease; when the stability of the rock mass is so good, any rock-breaking method has a rock-breaking difculty here.
When the strength of the surrounding rock exceeds 150 MPa, the GGUC gradually increases and the corresponding cost also rises.Since the uniaxial compressive strength of rock is closely related to its fssure state and so on, in order to minimize the infuence of integrity factors such as fssure and structural surface on the uniaxial compressive strength of rock, the test is specially chosen to be carried out on the palm face peripheral rock which has similar integrity of the rock mass, but the result of the test will inevitably be afected to a certain extent.In addition, due to the geological conditions of the test site, most of the tests in this series were selected in tuf tunnels, and the lowest uniaxial compressive strength of the surrounding rock in this series of tests was 31 MPa and the highest uniaxial compressive strength was 147 MPa, which did not cover tuf tunnels of various strengths.Te ftting equations in Figure 3 are applicable only for tuf tunnels with rock uniaxial compressive strengths of 30 MPa-150 MPa, i.e., in the region of harder and partially harder tufs.Also, when the peripheral rock integrity index exceeds 0.86, the GGUC will gradually increase and the corresponding cost will also rise, which is mainly due to the high integrity of the rock mass, leading to the high initial pressure in the expansion tube; at this time, because the area of the compression slurry materiel with lower strength becomes a weak zone, it in turn produces cracks and becomes a pressure relief zone for the highpressure gases, which afects the fnal rock-breaking volume.
In addition, we did not fnd the area with better strength and integrity in the feld, so the feld test for rock uniaxial compressive strength greater than 150 MPa and integrity greater than 0.86 will be the research work to be done in the future.In addition to the two main geological factors of uniaxial compressive strength of rock mass and integrity coefcient of rock mass, GGUC is also afected by other factors, such as the main structural surface of the rock mass, groundwater conditions, tunnel direction, and the state of geostress.For example, the main structural surface of the rock mass is in the form of a rock mass and the integrity of the rock mass is in the form of a rock mass.Also, the main structural surface of the rock mass and the tunnel direction have a certain efect on the stability of the rock mass, the direction of the expansion pipe drilling, and the direction of high-pressure gas spillage; when the surrounding rock is in a state of high geostress, the GGUC will be increased, and if the surrounding rock is a brittle, intact, hard rock, there is the possibility of rock bursting; if the surrounding rock is softer, the larger deformation will afect the efect of rock breaking and the efciency of the construction, which in turn afects the GGUC.Groundwater and the water content state of the rock mass will also afect the strength of the rock mass to a certain extent and reduce the stability of the surrounding rock.In addition, the nature of the compression slurry outside the expansion pipe is also an important factor afecting the rock-breaking efect, and these literatures [36,37] provide references for subsequent improvements.
Similarly, due to the complexity and inhomogeneity of the rock mass, it is not possible to ensure that other geological factors except rock mass integrity are exactly the same in the test, and in particular, the relationship between the integrity of the similar rock mass and its hardness is very close; therefore, the relationship between the rock mass integrity index obtained from the test and the GGUC has a reference for other similar projects to break the rock but is for reference only.Also, due to the complexity and various anisotropies of rock masses [38], other geological factors may difer.However, to some extent, the correlation between  At this point, the strength and integrity of the surrounding rock are too high, and the rock mass is difcult to be fractured by the high-pressure gas, and even if the rock breaking is successful, the GGUC required for this process is signifcantly higher When dynamite blast is not applicable, HPGEM can only be used as a reference considering the cost and duration, but it is still recommended to use as the method for rock breaking V P At this point, the strength and integrity of the surrounding rock are extremely poor and the large number of structural surfaces present in the rock mass can cause gas to escape, making it often difcult for HPGEM to be efective When dynamite blasts are not applicable, mechanical methods may be a better method of rock breaking Journal of Engineering integrity and hardness between similar rock masses (tuf) is stronger [12], so the relationship between the integrity factor of the rock mass and the GGUC obtained from the test is useful for other similar projects of rock breaking but for reference only.

Conclusions
Based on the feld test data of the high-pressure gas expansion method in the early stage of the research team, this paper proposes a new rock mass classifcation method for the high-pressure gas expansion method in tuf tunnels, which flls the gap of the "appropriate" rock mass classifcation method in the process of its application, combining with the typical rock mass classifcation.Te following conclusions are obtained: (1) Combined with the feld test data of the highpressure gas expansion method, it is found that the unit consumption of gas generator tends to level of with the increase of uniaxial compressive strength and rock mass integrity, which means that the rockbreaking efciency will no longer increase when the uniaxial compressive strength and rock mass integrity increase to a certain degree.(2) Te equations between rock uniaxial compressive strength, rock mass integrity, and gas generator unit consumption are ftted, and the optimal ranges of rock uniaxial compressive strength (about 150 MPa) and rock mass integrity (about 0.85) for the application of high-pressure gas expansion in tuf tunnels are given by combining the ftted equations with the principle of the high-pressure gas expansion method.(3) With reference to the Chinese engineering classifcation of rock mass (GB/T 50218-2014), a new method of rock mass classifcation for tuf tunnels applicable to the high-pressure gas expansion method is proposed, which classifes the surrounding rock into fve grades and gives suggestions for the engineering application of diferent surrounding rock grades.

Figure 4 :
Figure 4: Relationship between rock mass integrity and GGUC.

Table 1 :
Rock mass integrity index K v and rock volume nodule number J v .

Table 2 :
Relationship between R C and the degree of hardness.

Table 3 :
Relationship between K v and the degree of rock integrity.

Table 5 :
Description and recommendations of tunnel rock mass classifcation for HPGEM.At this point, the strength and integrity of the surrounding rock are slightly lower, the GGUC required for lower strength of the surrounding rock decreases, while the GGUC required for poorer integrity needs to increase When dynamite blast is not applicable, it is recommended to use HPGEM rock breaking, which can efectively improve the rock-breaking efciency and reduce the cost compared with other alternative methods III

P
At this point, the strength and integrity of the surrounding rock is low although the strength is low, but the integrity is poor and will still lead to a poor rock-breaking efect When dynamite blast is not applicable, HPGEM rock breaking can be considered and mechanical methods of rock breaking can be used as well IV P