The work on vibration monitoring was between a diversion tunnel for BaiShuiJiang third class power station in YanJin county of YunNan province, and the ShiuBaYan Tunnel underpass of NeiKun Railway. The LS-DYNA explicit dynamic numerical simulation and in situ blast vibration monitoring were used over a short study distance to monitor vibration in¬ uence on the existing tunnel by blasting in the diversion tunnel.
Scope
– The work (see conclusions at end of article) has shown:
– The relationship between tunnel diameter, vibration velocity and acceleration;
– The relationship between the distance and relative position of the blasting face relative to the existing tunnel and vibration velocity and acceleration;
– The relationship between the amount of explosives, and the vibration parameters;
– The influence of rock condition on vibration parameters;
– A formula relating the above influence parameters;
– The value of on-site blasting vibration monitoring, with the importance of manoeuvrability;
– Countermeasures to reduce blasting vibration influence.
For adjacent tunnel construction by drill and blast, as a new tunnel reaches an existing tunnel, the influence on the existing tunnel should be mitigated or eliminated to insure personnel safety and tunnel security. The engineering quality and schedule of the new tunnel should also be insured. In the work presented the blasting influence on adjacent existing buildings and tunnel in a short clear distance are studied. However, the study of dynamical influence of updown crossover tunnels is comparatively less. Based on the practical situation of diversion tunnel for BaiShuiJiang power station and the underpass of the ShouBaYan rail tunnel, by using the LS-DYNA method for explicit dynamic numerical simulation and in situ blast vibration monitoring, the vibration influence on the existing tunnel by blasting was studied.
The diversion tunnel has a plane angle of 82 deg with the railway lines in the underpass tunnel. The clear distance of two tunnels is 13.06m. The rail tunnel section measures 6.55m height and clear width 4.9m. The Grade III lining is 400mm thick and made of C20 concrete. An invert (grate) layer is 200mm thick. The diversion tunnel crosses strata of limestone, marlstone, sandstone, etc and has a circular section of 7.0m i.d., 8.1m o.d. The lining is also of C20 concrete, 500mm thick.
Finite element model
To simplify the finite element model for convenience of study all simultaneous initiation dosage was condensed into one cut-hole with the diameter of the cut-hole calculated by actual dosage. In the model the front and back boundaries are all 42m away from the existing tunnel, and the upper boundary is 33.6m away from the existing tunnel. The left and right boundaries are all 34m away from the underpass tunnel, and the lower boundary is 38m away. All the six boundaries are nonreflecting.
The rock, explosives, air and lining are all simulated by SOLID164. The whole model has 131,136 elements, and 138,572 nodes in total. Explosives, air, rock and concrete lining use the ALE algorithm, which shares nodes. To simplify it is assumed that the parameters of the plug material are the same as media material.
Materials parameters
According to the design data and related literature, rock parameters, explosives, air and lining are as shown in tables one, two and three.
The tunnel wall rock media is simulated by a plastic follow-up model [Dong, Y-x et al, 2006] with the parameters in Table One.
The concrete for preliminary and secondary lining concrete is simulated by the JHC material (Zhong & Wang, 2006) as in Table Two.
The tunnel case in the project utilised high-power emulsion explosive RJ2#, with the parameters as follows:
Air is included by the void material model with the density of 0.0012g/cm3. The other six parameters have default values, and the equation of state adopts linear polynomial.
A total of 31 operating cases were calculated. In the basic operating case the tunnel wall rock is grade III; the two tunnels are circular in section; the diameter of the existing and new tunnels are both D=8m; the clear distance between at the up-down crossover is H=D=8m; the blast initiation is full face initiation, and chainage 4m. Using changed values for the new-build tunnel diameter, the clear distance between the two tunnels, chainage, blast initiation mode and tunnel wall rock grade, the 31 operating modes were developed.
The maximum influence on the existing tunnel by blast vibration from the underpass new-build underpass tunnel occurs on the crossover section of two tunnels, so nodes and elements were selected on the crown, spring and arch invert of this crossover section and reviewed.
Analysis of simulation
In cases 1-10 the cases were divided according to changes in the diameter of the new-build tunnel. The relationship between maximum vibration velocity/acceleration and diameter is shown in Figure 1.
The graphs show that the maximum vibration velocity and acceleration on arch invert are all bigger than the other two monitoring points, while the minimum response is on crown.
When the diameter of the underpass new tunnel is less than 2.5D (D reference diameter = 8m), the vibration velocity and acceleration increase as the diameter of new tunnel diameter increases, and is basically in a linear relationship. When the diameter of new tunnel is 2.5~3.5D, the vibration velocity and acceleration rapidly increase. When the diameter of new tunnel is bigger than 3.5D, the vibration velocity and acceleration tend to decrease.
Cases one and 11~23 are divided according to the clear vertical distance between the two tunnels. The relationships between maximum vibration velocity /acceleration and clear distance is in Figure 2.
When the clear distance increases the vibration velocity and acceleration decrease. Vibration influence on arch invert is the most and on crown is the least.
By linear interpolation it was found that when the clear distance of two tunnels is 5.65D for a 4cm/s vibration velocity (control criterion), in grade III wall rock. When the new tunnel diameter is more than 5.65D, the single blast advance (‘footage’) is 4m and full-face blast initiation is used, the new tunnel will not influence the existing tunnel.
In cases one and 24~26 the model variations were according excavation blast advance (‘footage’) and initiation mode. The relationships between maximum vibration velocity/acceleration on arch invert from the new underpass tunnel excavation face are shown in Figure 3.
When the blast advance (‘footage’) decreases or the full face initiation is changed into partial face initiation, the vibration velocity and acceleration decrease. This is because of the reduced explosive dose. The arch invert is most influenced by vibration velocity and acceleration and the arch crown is least influenced. When the blasting agent dose is the same, the farther from reviewed point, the lower is the vibration velocity and acceleration at this point. The vibration response is less with the distance from the excavation face to the existing tunnel (0 on Figure 3 graphs) because the medium to diffuse vibration wave before/after excavation changes.
Cases 27~31 are divided according to rock grade. The relationship between maximum vibration velocity/acceleration and rock grade is shown in Figure 4.
The maximum vibration velocity and acceleration at the arch invert are all bigger than at the other two review points. The minimum response is on crown. As the rock becomes weaker, vibration velocity increases. Compared with the other rock grades, the change of VI is less, and the curve of V has an inflection point. Vibration velocity varies little from grade I to III, increases in grade V and decreases in grade VI.
When the diameter of the newly built tunnel, and the clear distance of two tunnels, are both D, in all rock grades, and the blasthole depth (footage) is 4m, the blasting of new tunnel will influence the existing tunnel in all cases.
Vibration influence
Analysis for the influence [China Blasting Safety Regulations, 2003] by distance, simultaneous initiation dose and rock grade follows:
Influence of distance: A change of the clear distance between the two tunnels is actually a change in the distance from the monitoring point to blasting source (R). The fit curve for the relation between blasting vibration velocity and distance on arch invert is as Figure 5.
The relational expression is:
Influence of simultaneous blasting charge: The change of blasthole length (footage) and blast initiation mode is actually the change of simultaneous blasting charge (Q). The fit curve for relation between blasting vibration velocity at the arch invert and simultaneous blast charge is as Figure 6.
The relational expression is:
Influence by rock grade: The fitting curve for the relationship between blasting vibration velocity (V) at the arch invert and rock grade (I~IV) is as shown in Figure 7.
The relational expression is:
Formula (3): v=10.821N + 4883
where N is a positive integer for rock classes I-VI (Qiu, W, 2003 and also Bi, J & Zhong, J, 2004).
Deducing formula
In the reference operating mode; in formula (1), R takes 12m and in formula (2) Q is 321.70kg. The vibration velocity calculated by the two formulae should be equal, so the average value 28.85cm/s of the two results by the two formulae is taken. The compensation factor of vibration velocity by distance R), and simultaneous blasting charge Q are:
So the formula of blasting vibration velocity considering compensation factors of R and Q of tunnel construction is:
Formula (4):
NB – is the compensation factor in consideration of the rock grade. It is 3.042 when the rock grade is III. The formula considering the rock grade influence is:
Where N is the positive integer for rock glass (Qiu, W, 2003 & Bi, J & Zhong, J, 2004, refs 8 &2)
The total explosive charge is calculated according to excavation face area, unit charge and hole length (footage).
Assuming excavation diameter is D, footage is L, the clear distance is H, unit dose is q, when the new-build tunnel is blasting just under existing tunnel, it should be:
Taking R and Q into Formula (4):
Formula (6):
In engineering applications the maximum vibration velocity on the existing tunnel as influenced by the new-build tunnel excavation blasting is calculated according to factors in project practice, as well as to judge the rationality of blasting parameters. From vibration velocity control criterion, the charge blasthole length (footage) and the clear distance between the tunnels, which have no influence, can be calculated.
A: ‘Footage’: When the vibration velocity control criterion ([V]) is confirmed, transform Formula 6, and the ‘footage’ formula is given as follows:
Formula (7):
B: Clear distance between two tunnels: When the vibration velocity control criterion ([V]) and blasthole charge length (footage) are confirmed, transform Formula 6 and the clear distance when no influence occurred between two tunnels is given as follows:
Formula (8):
Validation
To validate the numerical modelling with formulae results, substitute upward parameters of the various calculation for operating mode into Formula 6, and take the result to compare with the numerical simulation result as in Table 4.
The ratios between the numerical simulation results and formulation results are mostly from 90 to120 per cent, which demonstrates that the two results are comparable.
In practice, Formula 6 can be used to accurately calculate the blasting vibration velocity in similar projects. Comparing monitoring vibration velocity with calculation result in the project case, the distinction is little, demonstrating the applicability of the deduced formula.
Countermeasures
In conclusion, the influence of blasting vibration on an existing tunnels decreases when the new-build tunnel diameter and blast advance (footage) decreases, when initiation mode changes (full face into subsection blasting) and when the clear distance between the two tunnels increases. In consideration of these factors, the possible countermeasures are as follows:
Damping technique:
– The best option is to try to increase the clear distance between the two up-and-down crossover tunnels;
– Minimise the excavation advance (footage) of the new-build tunnel within reason;
– Minimise the total blasting charge; that is to adopt sub-section excavation, so subsection blasting;
– Adopt interference damping blasting technique [Gong L, Qiu W & Cao Y, 2006, and the China Blasting Safety Regulations 2003].
– Adopt mesothyrid (central) burn cut, notch cutting or small-section TBM partial excavation, and so on.
Principle to choose countermeasure: Conventional methods of reducing blast excavation advance (footage) and using subsection blasting have priority.
When these methods cannot meet needs, methods of interference damping blasting technique, excavating an antivibration ditch, circumferential damping holes and notch cutting are adopted.
Blasting test: In stretches with no blasting vibration influence the principal blasting parameters can be tested.
Based on blasting vibration monitoring results, blasting parameters are adjusted.
Conclusions
1. For full-face excavation and D0<2.5D, vibration velocity and acceleration at the existing tunnel induced by underpass tunnel blast construction increases as D0 increases and in a near-linear relationship. For 2.5D<D0<3.5D vibration velocity and acceleration increases rapidly. For D0>3.5D, vibration velocity and acceleration does not increase, but decrease as D0 increases.
2. Vibration velocity and acceleration decrease as the distance between the two tunnel increases. For the same distance, the influence of blasting vibration before the excavation face reaches existing tunnel is bigger than the influence after the excavation face passes by. For the same blasting parameters, the influence of blasting vibration decreases as distance between the tunnels increases. Vibration influence increases as the explosive charge increases. Vibration velocity increases and acceleration decreases as the surrounding rock becomes worse.
3. The blasting vibration velocity formula has been deduced, considering parameters of excavation diameter, blast excavation advance (footage), the clear distance of two up-and-down crossover tunnel and blast charge. Compared with on-site blasting vibration monitoring, the formula applicability and numerical simulation correction have been proved accurate.
4. The countermeasures of subsection excavation, subsection blasting and interference damping blasting technique to reduce the influence of vibration are given together with the principles of choice. The significance and manoeuvrability of a blasting test and vibration monitoring are also then indicated
