EFFECT OF BLASTING ON THE STABILITY OF LINING DURING EXCAVATION OF NEW TUNNEL NEAR THE EXISTING TUNNEL

In recent years, experimental and numerical researches on the effect of blasting pressure on the stability of existing tunnels was widely obtained. However, the effect of the blasting pressure during excavation a new tunnel or expansion old tunnels on an existing tunnel has disadvantages and still unclear. Some researches were carried out to study the relationship of the observed Peak Particle Velocity (PPV) on the lining areas along the existing tunnel direction, due to either the lack of in situ test data or the difficulty in conducting field tests, particularly for tunnels that are usually old and vulnerable after several decades of service. This paper introduces using numerical methods with the field data investigations on the effect of the blasting in a new tunnel on the surrounding rock mass and on the existing tunnel. The research results show that not only predicting the tunnel lining damage zone under the impact of blast loads but also determination peak maximum of explosion at the same time at the surface of tunnel working.


INTRODUCTION
The effects of drilling and blasting in case of excavation of new tunnels or expansion old tunnels near the existing tunnels along of the express way from North to South of Vietnam such as tunnels in the Hai Van pass tunnel project, Cu Mong tunnel, Deo Ca tunnel, etc are obtained. Traditional excavation method was widely used during excavation tunnels through the mountains in Vietnam in [1], this method has advantages for economy conditions, workers and engineers in Vietnam. In the past the researches for the stability of rock mass around transported tunnels and the assessments of the effects of tunnel blasting on the existing tunnels and buildings were carried out by not only authors in Vietnam such as in [2], [3], [4], [5], [6][7], [8][9] but also other authors in the world in [10], [11], [12], [13], and [14]. Researches in Vietnam are obtained for the stability of buildings on the surface of the ground, the assessment standard only for buildings during blasting on the open pit mining [3].
In the recent years some researches were carried out using physical models to study the effect of blasting vibration on an existing tunnel in [6][7], [8], and [9]. Smith et al presented the results of experimental studies carried out at small scale into the propagation of blast waves along straight tunnels roughened by means of different-sized roughness elements fixed along the two model tunnels sides. The results indicated that the use of rough-walled tunnels could provide an efficient protective solution to a sensitive structure. Previous publications have indicated that the

Numerical modelling
In this section, the simulation results obtained from Phase 2 (i.e. Rock and Soil 2dimensional analysis program) are presented due to it is believed to be an effective method for support design which has been widely used in mining engineering, especially in terms of the deformation mechanics of surrounding rock mass. The length of the numeric model is 113.2 m and the height 66.5 m respectively. The all boundaries of the model are applied to the fixed conditions to simulate the real application.
The supports used by rock bolts with diameter of bolts d = 22 mm; Young modulus of steel in rock bolts E = 200000 MPa; tensile capacity of bolts -0.15 MN; out of plane spacing of bolts -1.5 m; in plane spacing of bolts 1.5 m. The length of bolts can be defined based on the radius of plastic zone around tunnels, here length of bolts L = 3500 mm. Existing tunnels supported by concrete liner with the thickness 350 mm, rebar of steel d = 22 mm; spacing of rebar 250 mm.
Blasting pressures are applied in the model by using seismic loading coefficient by formula: P = Kc. (1) Here Kc -seismic coefficient;  -body forces of rock mass.
The Seismic coefficients are dimensionless coefficients which represent the (maximum) earthquake acceleration as a fraction of the acceleration due to gravity. Typical values are in the range of 0.1 to 0.3. In this study we used the range values of Kc = 0.1; 0.2; 0.3; 0.4; 0.5; 0.6 respectively.
Based on numerical method with using Phase 2 and seismic coefficient can be received model ( Figure 4) and the result of displacement vectors of rock mass around tunnel as in the ( Figure 5). Using results of numerical method can be established the relationship between the stress and displacements of rock mass on the boundary of existing tunnels and the changing of seismic coefficient, bending moment of concrete lining with other values of seismic coefficients. The results can be seen as in the (Figures 6-8).
The research results indicate that the values on the vertical axis which located in the left corner of existing tunnels, the minimum displacements located in the rightwall of the old tunnel respectively, besides the maximum values of displacements are investigated on the floor near the left corner of existing tunnels. Basing the assessment standards for the stability of tunnels can show that if the values of displacements of rock mass around tunnels and concrete lining are bigger than limited values, the concrete lining will be cracked. The results of research also show that it is necessary to use some solutions for blasting technologies to reduce blasting vibration on the existing tunnel.  The research results (Figures. [10][11][12][13][14] indicate that the values on the vertical axis which located in the left corner of existing tunnels, the minimum displacements located in the rightwall of old tunnel respectively, the maximum values of displacements are investigated on the floor near the left corner of existing tunnels. Basing the assessment standards for the stability of tunnels, if the values of displacements of rock mass around tunnels and concrete lining are bigger than limited values, the concrete lining will be cracked. The results of research also show that in case of using equivalent blasting pressure 7 MPa the internal forces in the concrete liner of existing tunnel are negligibly changed. It is necessary to use some solutions for blasting technologies to reduce blasting vibration on the existing tunnel and the weight of explosion in new tunnel will be less than 50 kg respectively.

NUMERICAL MODEL IN CASE OF EXPANSION BLASTING THE AUXILIARY TUNNEL NEAR THE OLD TUNNELS IN HAI VAN PASS PROJECT OF VIETNAM
Hai Van pass project located between Thua Thien Hue and Da Nang provinces of Vietnam, main tunnel was finished in 2005 with the total length of tunnel 6280 m with twin parallel tunnels, 4 lanes in each tunnel, and two ventilation shafts. Main tunnel has total length 6280 m, span of tunnel 13 m, the distance between main tunnel and auxiliary tunnel is 30 m, area of auxiliary tunnel before expansion is 15,5 m 2 , the height of auxiliary tunnel 4.5 m, span 5.3 respectively, View 3D for Hai Van tunnel project can be seen as in (Figure 15). In this study numerical a method will be used for analysis. Because of the difference of the geological conditions along tunnels to model for research is chosen the range of quality of rock mass with RMR ratio in the catalogues of Bieniawski RMR = 60 -80. Base on the results of investigation for rock mass conditions and profile of the auxiliary tunnel with span 5.3 m and height 4.5 m. In the main tunnel concrete lining has the thickness 35cm, rebar with diameter d = 22 cm, and space of rebar reinforcement a = 20 cm. Using numerical method PHASE 2 can be received model in this case as in the (Figure16). In the case study using the dimension of external boundary of model is 125 m and height 90 m, number of nodes and elements in model 14438, 24710 respectively, equivalent blasting pressure in this model can be defined by formula: P = 0,00337v 2 (2) Where: P -Blasting pressure, psi;  -Density of explosion; v -Velocity of explosion, fps.
Its assumption that the blasting pressure in the holes of drilling pattern in the auxiliary tunnel before expansion from existing profile to the design profile of tunnel are replaced from 7 to 20 MPa as in the ( Figure 17).   Figures.19-21). be received optimal blasting pressure during blasting in the auxiliary tunnel. Results can also show that, in case study the values of blasting pressure 7 MPa is optimal. Base on this value should have to control blasting solution to reduce blasting pressure on the boundary of auxiliary tunnel will be less than 7 MPa.

NUMERICAL MODELING THE STABILITY OF CONCRETE LINER IN THE EXISTING RAILWAY IN CO MA PASS PROJECT
In this section present the effects of blasting vibration on the stability of concrete liner of railway tunnel in the geological conditions of Co Ma pass project. Railway tunnel was built 100 years ago by France, tunnel located Km 1234+464 in Dai Lanh commune, Van Ninh district, Khanh Hoa province. Height of railway tunnel 4.0 m, span of tunnel 4.33 m respectively. Site plan of railway tunnel N 0 24, Co Ma transported tunnel and profiles of them can be seen as in the ( Figures  22-24). Railway tunnel was supported by concrete liner, the properties of rock mass around railway tunnel and support liner can be seen such as [9]: Properties   Blasting pressure 10MPa Blasting pressuer 8MPa Blasting pressure 6MPa Blasting pressure 4MPa Results in (Figures 27-29) can show that, the values of internal forces in the concrete liner of existing railway tunnel are the same as these values of this concrete liner in case of without new transported tunnel, the values of blasting pressures will be optimal. By research results can be received the optimal blasting pressure in this case 4 MPa, the equivalent weight of explosion at the same time during excavation new tunnel approximately 42 kgs explosion of P3151.

EXPERIMENTAL INVESTIGATION OF THE RESULTS
In the experiences excavation tunnels contour and smooth blasting methods can be used to reduce effects vibration of blasting during expansion the auxiliary tunnel on the stability of concrete lining in the existing tunnel in Hai Van pass project in Vietnam. In this project recommended using characteristics of drilling and blasting parameters of drilling pattern such as in (Tables.2-3).
Experimental blasting in Hai Van pass project shows that, when using optimal blasting pressure 7 MPa on the contour of the auxiliary tunnel, the values of internal forces in concrete lining of existing tunnel in Hai Van are not change. In the case study the density of explosion in each hole is less than 1kg, total density of explosion at the surface of tunnel less than 45 kgs. The results of explosion of project can be shown as in figs.30-32. Without blasting in the new transported tunnel Blasting pressuer 12MPa Blasting pressure 10MPa Blasting pressure 8MPa Blasting pressure 6MPa Blasting The length of explosive in the holes Le 0.6 m 7 The uncharged length in the cut holes Lu 0.7 m 8 The length of explosive in the holes in the contuor holes

CONCLUSION AND DISCUSSION
The above research results can show that, blasting vibration during excavation new tunnels has influence on the stability of rock mass around existing tunnel such as the stability of support lining of existing tunnel. Blasting vibration can be replaced by equivalent blasting pressure in the numerical modelling which impact on the stability such as displacements, stress on the boundary of old tunnel, and the values of internal forces in the support liner of existing tunnel.
Based on the determination of the optimal values of equivalent blasting pressures can be explained that the existing tunnel will be stable in case of drilling and blasting in the new tunnel has not influence on the values of internal forces in support liner of existing tunnel.
Experimental blasting in Hai Van pass project shows that, when using optimal blasting pressure 7 MPa on the contour of the auxiliary tunnel, the values of internal forces in concrete lining of existing tunnel in Hai Van are not change. In case study the density of explosion in each hole is less than 1kg, total density of explosion for surface less than 45 kgs and using 10 delay times for other holes group. The vibration of process blasting and drilling in the auxiliary tunnel have not influence on the stability of old linings.
The results of experimental blasting in Co Ma pass project in case of excavation new transported tunnel near the railway tunnel can indicate that, using optimal blasting pressure 4 MPa on the contour of new tunnel, the drilling and blasting in new tunnel has not influence on the stability of old railway tunnel. The weight of explosion at the same time in new tunnel over 40 kgs.
Received results also show that in the practice when building new tunnels near the existing tunnel to reduce effects of vibration of blasting can be used other blasting solutions such as using many delay times between holes, types of electrical detonators, direction controls of vibration ways, arrangement of cut holes in the surface, using empty holes etc. In other words, appropriated drilling and blasting parameters must be controlled and used, both models for smooth and contour blasting are recommended in the contour of the auxiliary tunnels.