Effective control of the explosive energy and the propagation direction of blastinduced crack and minimizing damage of remaining rock mass are the main purposes of directionalcontrolled blasting. In this paper, the experimental test on blast strain fields affected by water jet slot and blasthole wall protection material is conducted. Next, the FEM software ANSYS/LSDYNA is used to simulate the blast‐induced crack propagation and the blast stress wave transmission of different types of blasthole, and the distribution and evolution law of dynamic blast stress are also analyzed. The results indicate that the existence of blasthole wall protection material is not only beneficial to the “guiding effect” of blast‐induced crack propagation of water jet slot but also beneficial to reduce blastinduced damage of remaining rock mass. Besides, the bigger
In China, drillblasting method is the most costeffective way in the construction of underground space engineering. However, it has many disadvantages, which produces some random radial fractures from a blasthole along the unspecific directions and invites overbreaks among the rock masses with low strength or with a number of cracks, posing a serious threat to longterm stability of permanent structures including underground nuclear waste repositories and openpit slopes [
Coal and rock mass cutting using wateronly jet.
Above research studies concentrate mainly on the Vnotch directional blasting technology with mechanical grooving tool assistance and demonstrate that this method is superior to the traditional blasting technology to control the direction of blastinduced crack propagation. However, this method has a disadvantage of notching tool to be stuck and wear and tear, especially for the soft surrounding rock blasting. In order to solve this problem, the unique character for low damage, directional control cutting, and highly energy accumulation of water jet attracted the attention of researchers (Figure
Based on above studies, an attempt was made in this paper to study the dynamic evolution law of blasting strain under different conditions. An experiment was performed to investigate the evolution law of dynamic strain of different models. Then, numerical simulation for the transmission and reflection of blast stress wave and the propagation law of blastinduced crack were performed by using the FEM software ANSYS/LSDYNA.
Blast stress wave has the characteristics of wide band, high upper limit frequency, and large amplitude change, so the test instrument must have a high sampling frequency. In this paper, a multichannel dynamic analysis signal testing system produced by Donghua Testing Technology Co., Ltd., was applied; it comprises an ultrahigh dynamic strain meter with eight channels, a blast loading system, and a computer used to collect blasting data; the connection of the test system is shown in Figure
Diagram of test system connection.
Diagram of experimental test procedure.
PMMA was used in these blast tests in that it almost shares similar fracture behaviors with brittle rock under dynamic conditions [
Parameters of PMMA.
Density (g/cm^{3})  Poisson ratio  Elastic modulus (GPa)  Longitudinal wave velocity (cm/ 
Shear wave speed (cm/ 

1.2  0.31  6.1  2.3 × 10^{5}  1.26 × 10^{7} 
In order to investigate dynamic strain law under different blasting conditions, three shapes of blastholes were designed: #1 test model has one regular blasthole, #2 test model has one blasthole with two water jet slot along the horizontal axis, and #3 test model has one water jet slotted blasthole with the blasthole wall protection material mounted on the two opposite surfaces of the blasthole; PVC material was used as the wallprotected material. Each test model and its strain gauge arrangement are shown in Figure
Sketches of three different blasting models and the strain gauge arrangement: (a) #1 test model, (b) #2 test model, and (c) #3 test model.
Table
Strain values for three different blasting models.
Test model  Strain value  Values (  

Channel 1  Channel 2  Channel 3  Channel 4  
Model 1  Positive strain peak value  636.454  537.698  672.342  570.690 
Negative strain peak value  −495.945  −248.893  −722.151  −396.569  
Mean strain peak value  83.223  73.074  92.998  78.041  


Model 2  Positive strain peak value  869.829  633.962  623.167  520.278 
Negative strain peak value  −341.492  −280.124  −718.531  −388.996  
Mean strain peak value  171.153  132.421  80.121  63.676  


Model 3  Positive strain peak value  1177.456  810.468  350.768  295.034 
Negative strain peak value  −772.859  −348.843  −368.530  −271.644  
Mean strain peak value  224.876  189.897  27.480  12.654 
Waveform curve of dynamic strain for #1 test model: (a) #E11 test point, (b) #E12 test point, (c) #E13 test point, and (d) #E14 test point.
Waveform curve of dynamic strain for #2 test model: (a) #E21 test point, (b) #E22 test point, (c) #E23 test point, and (d) #E24 test point.
Waveform curve of dynamic strain for #3 test model: (a) #E31 test point, (b) #E32 test point, (c) #E33 test point, and (d) #E34 test point.
By comparing Figure
For #3 test model, the existence of blasthole protection material has greatly changed the distribution law and evolution law of the explosive strain wave. On one hand, the PSPV in the direction of the water jet slot gradually increases: the PSPV at #E21 test point is 36.67% greater than the PSPV at #E11 test point, the PSPV at #E31 test point is 35.37% greater than PSPV at #E21 test point, and the PSPV at #E31 test point is 85.00% greater than the PSPV at #E11 test point. On the other hand, the blasting strain values in the direction perpendicular to water jet slot, respectively, decrease to smaller values. Just in regard to the PSPV, the figure (623.167
Furthermore, by comparing the blasting strain of #2 test model and #3 test model, we can see that in the direction of water jet slot, the blast strain is much greater than that in the direction perpendicular to water jet slot. For #2 test model, the PSPV at #E21 test point sees a 28.36% decrease from 869.829
To sum up, some conclusions can be summarized from the experimental test, as follows:
Under the combined effect of blasthole wall protection material and water jet slot, the PSPV, by comparing with regular blasthole blasting, was increased by 85.00% in the direction of water jet slot, and the PSPV in the direction perpendicular to the direction of water jet slot was reduced by 70.21%.
In contrast with directionalcontrolled blasting with water jet assistance, the existence of blasthole wall protection material is not only beneficial to the “guiding effect” of blastinduced crack propagation of water jet slot but also beneficial to reduce blastinduced damage of remaining rock mass.
The FEM software ANSYS/LSDYNA and the postprocessing software LSPREPOST were applied to investigate the blasting stress wave evolution law and the blastinduced crack propagation law for different conditions.
Three simulation cases were conducted by ANSYS/LSDYNA. Due to the symmetry of the simulated object, both simulation cases applied a quasi2D simulation model. For each case, the size of rock is 150 cm × 150 cm × 0.3 cm, the diameter of blasthole is 5 cm, the thickness of the blasthole wall protection material is 0.5 cm, and the size of the water jet slot is 2.5 cm × 0.5 cm × 0.3 cm. The #3 simulation model is shown in Figure
Numerical simulation model: (a) #3 simulation model; (b) enlargement of part A.
Material type #3 of LSDYNA (
Parameters of rock.
Density (g/cm^{3})  Elastic modulus (MPa)  Tangent modulus (MPa)  Poisson ratio  Yield strength (MPa)  SRC  SRP  Failure strain 

2.50  2.25 × 10^{4}  4.20 × 10^{3}  0.22  3.00  0  0  0.06 
Material type #3 of LSDYNA (
Parameters of blasthole wall protection material.
Density (g/cm^{3})  Elastic modulus (MPa)  Poisson ratio  Yield strength (MPa)  SRC  SRP  Failure strain 

1.43  3.00 × 10^{3}  0.3  1.8 × 10^{−1}  0  0  0.1 
Material type #8 of LSDYNA (
Parameters of explosive and its EOS equation.









1.70  0.83  2.95 × 10^{4}  8.55 × 10^{5}  2.05 × 10^{4}  4.60  1.35  0.25 
Air was modelled by the material type #9 of LSDYNA (
Parameters of air and its EOS equation.









1.20 × 10^{−3}  0.00  0.00  0.00  0.00  0.40  0.40  0.00 
Lagrange algorithm has the priority of less computation time and accurately describing the boundary movement of structure, so it is always used to analyze the explosive detonation. However, it has a big disadvantage of causing element distortion and even leads to the termination of calculation when dealing with the large deformation numerical calculation. In order to avoid the above problem, fluidsolid coupling algorithm was adopted for the analysis of the explosive detonation, of which, ALE algorithm was used for explosive and air and Lagrange algorithm for rock and protecting pipe. The material derivative equation and the governing equations of the ALE algorithm can be expressed as follows [
At the same time, meshes of explosive and the air were joined with common nodes and so as to rock and protecting pipe. Then, the fluidsolid coupling was defined between the meshes of the explosive, air, rock, and protecting pipe by the keyword. In addition, according to the characteristics of blasting process, the time step of the simulation is 0.67, and the computation time is 0.004 s.
The Postprocessing software LSPREPOST was used to draw the diagrams of the evolution law of explosion stress wave and crack propagation of different simulation cases, as shown in Figures
Diagram of blastinduced crack propagation and stress wave transmission of #1 simulation case: (a)
Diagram of blastinduced crack propagation and stress wave transmission of #2 simulation case: (a)
Diagram of blastinduced crack propagation and stress wave transmission of #3 simulation case: (a)
In general, for regular blasthole blasting, the explosion stress uniformly acts on the wall of blasthole, and the shape of blast stress wave is round, and then the blastinduced crack forms and expands under the blast loading. Due to the characters of blast loading, the gas pressure acting on blasthole wall in all directions can be considered as equivalent, so the welldistributed blastinduced cracks generate near the blasthole, which is consistent with the classical blasting theory.
However, by comparing #1 simulation case, the blastinduced crack propagation law and the transmission law of explosion stress wave of #2 simulation case and #3 simulation case were greatly changed under the effect of water jet slot and blasthole wall protection materials. On one hand, blasting energy focus forms at the initial stage of the explosion, which promotes the initial microcrack produced at the direction of water jet slot. And nothing but air is in the water jet slot, which caused the formation of lowpressure zone, so detonation products have priority to flow along the direction of water jet slot. Then, the highpressure and highvelocity gas products wedge into the slot, further leading to crack initiation and extension along the desired direction, as shown in Figures
To further understand the transmission law of blast stress wave and the change regularity of blasting strain under different conditions, monitoring points were set in the numerical simulation model, and its schematic diagram is shown in Figure
Schematic diagram of monitoring point layout.
Figures
PT curve of different monitoring points of #1 simulation case: (a)
PT curve of different monitoring points of #2 simulation case: (a)
PT curve of different monitoring points of #3 simulation case: (a)
For #3 simulation case, when
Figures
Effective stress curve of different monitoring points of #1 simulation case. (a)
Effective stress curve of different monitoring points of #2 simulation case. (a)
Effective stress curve of different monitoring points of #3 simulation case. (a)
Moreover, by comparing Figures
In summary, some conclusions can be summarized as follows.
The experiment test on dynamic blast strain was conducted, and the results indicated that under the combined effect of blasthole wall protection material and water jet slot, the blasting strain fields were changed by comparing with the regular blasthole blasting. In the direction of water jet slot, the PSPV was increased by 85.00%, and in the direction perpendicular to the direction of water jet slot, the PSPV was reduced by 70.21%.
Numerical simulation results suggest that the existence of water jet slot and blasthole wall protection material can affect the distribution and evolution law of explosive stress wave and let the stress concentration occur at the tip of water jet slot, which promotes blastinduced crack propagation along the specific direction and minimizes the blastinduced damage of remaining rock mass. Moreover, the bigger
Both the experiment test result and the numerical simulation results indicated that the existence of blasthole wall protection material is not only beneficial to the “guiding effect” of blastinduced crack propagation of water jet slot but also beneficial to reduce blastinduced damage of remaining rock mass.
Future research work will be focused on the applications in practical engineering of this approach.
Readers can access the data supporting the conclusions of the study by sending a mail to the corresponding author (email:
The authors declare that they have no conflicts of interest.
Financial support for this work was provided by the Natural Science Foundation of Southwest University of Science and Technology (18zx7124). In particular, the authors thank M. E. D. Y. Li and M. E. F. W. Yan for their kind help in experimental investigation and sincere suggestion.