ORIGINAL PAPER

Contributions of In-Situ Stress Transient Redistributionto Blasting Excavation Damage Zone of Deep Tunnels

Peng Yan • Wen-bo Lu • Ming Chen •

Ying-guo Hu • Chuang-bing Zhou • Xin-xia Wu

Received: 10 July 2013 / Accepted: 15 March 2014 / Published online: 17 April 2014

� Springer-Verlag Wien 2014

Abstract With the background of construction of the

headrace tunnels with the deepest buried depth in China at

present, by means of carefully acoustic velocity detection

and analysis of Excavation Damage Zone (EDZ), the

contributions to damage zones made by the effect of in situ

stress transient redistribution are studied and compared

with the extent of damage caused by the explosive load.

Also, the numerical simulation was adopted to verify

detecting the results. It turned out that the in situ stress

transient redistribution during blasting has great influence

on the development of EDZ of deep tunnels. The blasting

excavation-induced damage zone of deep tunnels can be

divided into the inner damage zone and the outer damage

zone from the excavation surface into surrounding rocks.

Although this damage zone dividing method is similar to

the work of Martino and Chandler (2004), the consider-

ation of developing a mechanism of the inner damage zone,

especially the contribution of in situ stress transient redis-

tribution, is totally different. The inner damage zone,

which accounts for 29–57 % of the total damage zone, is

mainly caused by explosive load and in situ stress transient

adjustment, while the outer damage zone can be mostly

attributed to the static redistribution of in situ stress. Field

tests and numerical simulation indicate that the in situ

stress transient redistribution effect during blasting con-

tributes about 16–51 % to the inner damage zone in the 2#

headrace tunnel of Jinping II Hydropower Station. For

general cases, it can be concluded that the in situ stress

transient redistribution is one of the main contributors of an

excavation damage zone, and damage caused by in situ

stress transient redistribution effect may exceed the dam-

age caused by explosion load and become the main

inducing factor for damage with the rise of in situ stress

levels.

Keywords Deep tunnel � Excavation damage zone

(EDZ) � Explosive load � Transient redistribution �In-situ stress

1 Introduction

With the further implement of the West Development Plan

of China and the exhaustion of shallow mineral resources

at present, more and more deep transportation and mining

tunnels, or water diversion tunnels are being constructed or

planned. The increase of buried depth means higher in situ

stress and more intensive excavation disturbances. Gener-

ally speaking, the excavation disturbed zones of tunnels

could be divided into three parts from excavation boundary

to the inside surrounding rocks: the failed zone, the dam-

aged zone and the disturbed zone, respectively (Read

2004). Obviously, the Excavation Damage Zone (EDZ)

inside surrounding rocks should be considered more care-

fully than the visible failed zone on the surface. The esti-

mate of the damage extent and intensity as well as the

P. Yan � W. Lu (&) � M. Chen � Y. Hu � C. Zhou

State Key Laboratory of Water Resources and Hydropower

Engineering Science, Wuhan University, Wuhan 430072,

Hubei, China

e-mail: wbluyan@qq.com

P. Yan � W. Lu � M. Chen � Y. Hu � C. Zhou

Key Laboratory of Rock Mechanics in Hydraulic Structural

Engineering Ministry of Education, Wuhan University,

Wuhan 430072, China

P. Yan � X. Wu

Key Laboratory of Geotechnical and Engineering

of Ministry of Water Resources, Yangtze River Scientific

Research Institute, Wuhan 430010, Hubei, China

123

Rock Mech Rock Eng (2015) 48:715–726

DOI 10.1007/s00603-014-0571-3

study of mechanical characteristics of surrounding rocks

both have significant effects on safety evaluation and

optimization of supporting parameters in rock engineering

(Hsiung et al. 2005; Martino and Chandler 2004; Zhang

et al. 2012). For example, the length and layout of rock

bolts are directly determined by the range of EDZ. It has

been proved by a large number of engineering practices

that designing the support according to the extent of EDZ

has much more technological economy and social benefits

than that based on the traditional engineering analogy

method (Hsiung et al. 2005).

The major mechanisms related to the development of an

EDZ are: (a) excavation impact; (b) stress redistribution

after excavation; (c) back-pressure by rock supports; and

(d) swelling or slaking with groundwater reactions. The

first two are the main factors (Martino and Chandler 2004).

But under high in situ stress conditions, the excavation

impact should be redefined. The damage caused by blast

loading is obvious when the Drilling & Blasting (D&B)

method is adopted. Excepting for the explosive load, the

in situ stress transient redistribution accompanied the

blasting process may be another important component of

excavation impact under conditions of deep depth or high

in situ stress (Lu et al. 2012).

During blasting, the adjacent blast holes would connect

with each other within tens of microseconds after detona-

tion to form a new free face driven by the blast load, and

accompanying this process, the normal in situ stress on the

new surface will release transiently, which may lead to

damage to surrounding rocks (Abuov et al. 1989). The

monitoring results during excavation of the Dong-feng

underground powerhouse carried out by Zhang et al. (1993)

indicated that the influence of blast excavation on sur-

rounding rocks includes two main parts: the effect of the

explosive stress wave and the effect of in situ stress tran-

sient release on the excavation surface. Lu et al. (2011)

studied the mechanism of in situ stress transient release by

numerical simulation and analysis of the blast excavation-

induced vibration data monitored from Pubugou Hydro-

power Station in China. The studies mentioned above on

the transient releasing or redistribution of in situ stress

during blasting mainly focused on the dynamic effect of

vibration or seismicity, and is rarely involved with the

excavation damage zone. However, in fact, this impaction

could not be ignored again with the increase of the exca-

vation depth and the level of in situ stress.

The Jin-ping-II Hydropower Station (JPII) with a total

installed capacity of 4,800 MW, located at the huge bend

of the Ya-long River at the junction area of three counties

named Mu-li, Yan-yuan and Mian-ning, respectively, in

Sichuan Province of China, has the deepest hydraulic

tunnels under construction in China at present (Zhang

2007). The station was designed to concentrate water head

with a group of long headrace tunnels, which were com-

prised of four parallel tunnels with a length of 16.7 km per

tunnel. The average buried depth of the headrace tunnels is

about 1,500–2,000 m and the maximum depth reaches

2,525 m. The measured maximum principle in situ stress

along the headrace tunnel lines both reach 70 MPa under a

depth of 2,500 m (Shan and Yan 2010). The headrace

tunnels were planned to excavate with the D&B method

mainly, and the TBM (Tunnel Boring machine) method is

only adopted in some parts. Because of complicated geo-

logic conditions and high in situ stress, the excavation

disturbances of the headrace tunnels of JPII are extremely

intensive (Tang et al. 2010). The monitored extents of EDZ

in headrace tunnels are mostly larger than 1.5–2.0 m and

the maximum extent may exceed 3.0 m in some large

deformation positions. This project provides a special case

to study the developing mechanism of EDZ under high

in situ stress conditions. The contributions of explosive

load and in situ stress transient redistribution to developing

of EDZ based on the deep headrace tunnels of JPII are

thoroughly studied in this paper.

2 Constitution of Damage Zone

In order to study the developing mechanism of EDZ under

high in situ stress conditions and the damage effect of

stress transient redistribution during blasting, the field

detecting results of EDZ are analyzed firstly, and then the

damage extents caused only by blast load are determined

and deducted from the intensive damage zone to get the

damage extents caused only by in situ stress transient

redistribution.

2.1 Detection Section

The cross section at stick number 15 ? 700 m in the

headrace tunnel #2 (DT2#) of JPII (shown in Fig. 1) was

chosen to be examined and studied carefully. As mentioned

above, DT2# is excavated with the D&B method, and the

lithology of surrounding rocks around this detection section

is T2y marble and the buried depth is about 1,054 m.

According to field tests by the hydraulic fracturing

method, the main characteristics of the geo-stress field

could be summarized as follows (Shan and Yan 2010):

(a) The maximum, middle and minimum principle

stresses all increase non-linearly as depth increases

(see Fig. 1), but r1/r3 reduces as depth increases;

(b) The azimuth angle (or trend) of maximum principle

stress is mainly within 120�–160�, which indicates

that the direction of principle stress changes a little

as cover thickness increases.

716 P. Yan et al.

123

(c) The dip angle (or plunge) of maximum principle

stress varies from 6.45� to 75.4�. This indicates that

the state of in situ stress changes from near

horizontal to near vertical self-weight stress. The

in situ stress field along the tunnel axis could be

divided into three representative geo-stress zones:

the valley stress field in the intake and outlet areas,

the gravity stress field in the middle of the tunnel and

the fault stress belt where the fault crosses or is close

to the tunnel.

(d) Around the detection section, the middle principle

stress r2 is about 34.4 MPa and basically perpen-

dicular to the tunnel axis with a dip angle of 25�, and

the minimum principal stress r3 is 29.2 MPa in the

vertical direction with a dip of 65�. So the detection

section locates in the plane of r2 and r3.

There are 5 detecting holes in the detecting section, as

shown in Fig. 2. The five detection holes, with a depth of

10 m and a diameter of 76 mm, are numbered S1 to S5

clockwise from the left to the right.

2.2 Detection Method

The detection methods of EDZ include: the contrast

detection of seismic velocity, acoustic velocity (including

P-wave and S-wave), elastic modulus and permeability rate

before and after excavation, borehole television, etc.

Among them, the acoustic velocity test method has been

widely used in engineering because of its high accuracy

and efficiency. In fact, according to the construction

specifications in China, the detecting results of EDZ with

acoustic velocity method have always been regarded as the

necessary material for the acceptance of the project owner.

So the acoustic detecting is common and daily work in

China and the detecting workload is enormous. At some

very important positions, other methods, such as borehole

television and permeability test, should be adopted in the

meantime.

In most water conservancy and hydropower projects in

China, the measurement of variation rate of P-wave (or

longitudinal wave) velocity before and after excavation,

one of the acoustic velocity detecting methods, has been

widely used and regarded as the criterion of excavation

damage, as shown in formulation (1):

g ¼ ðc0 � c1Þ=c0: ð1Þ

The c0 and c1 are the P-wave velocity detected before

(meaning undisturbed) and after (meaning damaged)

excavation, respectively. The P-wave detecting method

contains two forms: single borehole detecting and double

hole detecting. According to the construction specifica-

tions widely adopted in China over the past decades,

Construction Specification on Underground Excavating

Fig. 1 The engineering

geologic profile and in situ

stress along the tunnel line

Fig. 2 Arrangement of detecting holes in the detection section

Contributions of In-Situ Stress 717

123

Engineering of Hydraulic Structures (DL/T 5099—1999),

the disturbed rock masses, of which the P-wave velocity

is reduced by 10 %, can be regarded as being damaged

by excavation disturbance, and the damage intensity

would increase with the reduction rate (g) of P-wave

velocity.

Fig. 3 Field testing of acoustic velocity (P-wave) in JPII

Table 1 Detecting results of

excavation damage zone of

headrace tunnel 2#

Labels Extent of damage zones at different positions (m)

S1 S2 S3 S4 S5

HT Total damage zone 2.80 2.80 3.60 4.20 3.00

HI Inner damage zone 1.60 1.40 1.40 1.50 1.30

HO Outer damage zone 1.20 1.40 2.20 2.70 1.70

HI/HT 9 100 % Percent of inner damage

zone to total damage zone

57 % 50 % 39 % 36 % 43 %

Fig. 4 Detecting results of

EDZ in headrace tunnel 2# of

JPII

718 P. Yan et al.

123

The single borehole P-wave velocity method was

adopted in excavation damaged zone detecting of headrace

tunnels in JPII, and the detecting device is the RS-ST01C

intelligent acoustic instrument made by Rock Sea Com-

pany, Wuhan, China. The tests were carried out along the

detecting hole with an interval of 0.2 m from bottom to top,

as shown in Fig. 3.

2.3 Detection Results

Through the P-wave velocity obtained from field detection

and by using Formula (1), the damage extents can be

determined. The results of the damage extent are given in

Table 1 and shown in Figs. 4, 5.

It can be seen from Table 1 that the extent of EDZ

varies from 2.8 to 4.2 m according to the acoustic velocity

detecting results. Although only the acoustic velocity

method was used to determine the damage extent, the

results of the 5 detecting holes can verify each other as a

whole.

The distribution of damage extent along the excavation

contour is seriously influenced by the in situ stress field. It

indicates that the in situ stress plays an important role in

the development of the excavation damage zone.

According to Fig. 4, the P-wave velocity of surrounding

rocks changes violently, and there is obviously a low

velocity zone near the excavation surface, so the excava-

tion damaged zone of the headrace tunnels of JPII can be

divided into two secondary zones according to the variation

of acoustic velocity: the inner damage zone and the outer

damage zone. The inner damage zone features a sharp drop

of acoustic velocity of rock masses, while the outer damage

zone is characterized by a slow decline of the rock mass

acoustic velocity, and finally the level will become close to

the undisturbed rock masses. The dividing method of the

damage zone is similar to that of Martino and Chandler

(2004) to some extent, but the interpretation of the for-

mation mechanism of the inner damage zone is totally

different.

According to the result of Martino and Chandler (2004),

the inner damage zone is caused by blasting load and the

outer damage zone is attributed to the redistribution of

in situ stress. But after studying the extent resulting from

blasting load only (see Sect. 3 of this paper), it can be

found that the transient unloading effect of in situ stress

during blasting is also an important factor for developing

the inner damage zone beyond blasting load. The distri-

bution of the inner damage zone along an excavation

contour is the direct evidence. If the inner damage zone is

caused by blasting load only, the extent of it at every

location should be homogeneous provided the smooth

blasting technique is adopted, because all the contour holes

are initiated at the same moment and are loaded with the

same amount of explosive and the same charge structure

during smooth blasting.

The subordinate damage zones have been shown in

Fig. 5 and Table 1. It can be seen that the extent of the

inner damage zone is almost above 1.0 m and accounts for

40–50 % of the total damage extent. The in situ stresses in

the cross section plane of DT2# are r2 and r3, and the

values are 34.4 and 29.2 MPa, respectively. Although the

two principle stresses in the detection section are very close

to each other, it can also be found that the damage extent at

the position of the right-up arch of the detection section is

much larger than that of other parts, and this area is the

stress concentration area.

According to Martino and Chandler (2004), the extent of

the inner damage zone of the TSX tunnel is less than 0.3 m

and the outer damage zone varies between 0.4 and 1 m

depending on the location around the tunnel and the mea-

surement method. The r1 direction of TSX tunnel is par-

allel to the tunnel axis and the value of r1 is 60 MPa. The

r2 and r3 are 45 and 11 Mpa, respectively, in the cross

section plane of the TSX tunnel. The rock lithology of the

TSX tunnel is Lac du Bonnet granite. For comparison,

Martino and Chandler (2004) also presented the extent of

damage zones of BDA tunnel with similar geometry and

similar orientation to in situ stress of the TSX tunnel, as

shown in Fig. 6. The major principle stress (r1) at the BDA

tunnel is 26 MPa, and the r2 is 16 Mpa and r3 is 12 Mpa.

The extent of the inner damage zone of BDA tunnel is also

within 0.3 m and the outer damage depth is within 0.5 m. It

can be seen that the extent of the inner damage zone of

TSX tunnel is larger than that of the BDA tunnel and the

maximum extent of the inner damage zone along excava-

tion contour locates in the top-right corner in the cross

section, the stress concentration area, according to the

in situ stress field. It indicates that the distribution of the

inner damage zone along the excavation contour is

Fig. 5 Damage zones of the detection section in headrace tunnel #2

of JPII

Contributions of In-Situ Stress 719

123

evidently influenced by the in situ stress field around and

another causing factor for inner damage zone is ignored,

but this situation had not been studied thoroughly yet by

Martino and Chandler (2004).

3 Damage Induced by Explosive Load

The damage induced by explosive load only during exca-

vation should be decided first to determine its contribution

to the inner damage zone. The numerical simulation and

field testing methods are adopted for this work.

3.1 Numerical Simulation

The 2# headrace tunnel is excavated by D&B method, and

the blast design of the upper part of the tunnel is shown in

Fig. 7.

It can be seen from Fig. 7 that the smooth blasting holes

and the breaking holes are all arranged in the shape of

semicircles or gates according to the blast design, and all

the holes located in the same layer are designed to detonate

in the same microsecond delay.

In order to estimate the range of the damage zone caused

by the explosion load more accurately, numerical simula-

tion was carried out based on the blast process of DT 2#

15 ? 700 section. The numerical model shown in Fig. 8

includes 59,472 grids and 60,751 nodes. The simulation

strategy adopted in this paper features no grids and nodes

in the tunnel area and the explosive loads are loaded on

excavation boundaries. This method is very efficient

because it avoids the complicated simulation of the deto-

nation process, and the simulation accuracy also meets the

requirements of damage simulation (Lu et al. 2011).

During the blasting excavation process, the explosive

energy of cut holes and breaking holes are mainly used to

crush rock masses excavated, and the damage of reserved

rock masses (or surrounding rocks) is mainly produced by

the contour blasting. To simplify the simulation, only the

Fig. 6 Comparison of the extent of the inner and outer damage zones

around TSX and BDA tunnels [Martino and Chandler (2004)]

Fig. 7 The blast design diagram of headrace tunnel

Fig. 8 The numerical model of blast excavation of headrace tunnel

Fig. 9 Curves of blasting load vs. time (Lu et al. 2011, revised)

720 P. Yan et al.

123

damage effect caused by the smooth blasting is considered

during simulation. The explosion load used in simulation is

shown in Fig. 9. The Pb0 is the peak of explosion load in

the diagram, tr is the rising time of explosion load, and td is

the duration of explosion load.

During the excavation of headrace tunnel of JPII, the

diameter of smooth blasting charge is 25 mm, the explosive

density is 1.1 g/cm3, and the explosive detonation velocity is

4,000 m/s. From the study of Lu et al. (2011), it can be cal-

culated that the peak of explosion load Pb0 acting on the wall

of blast holes is 122.3 MPa, and the duration td � 8.0 ms.

To avoid the influence of in situ stress on the excavation

damage zone, only the blasting load shown in Fig. 9 is

loaded on the inner boundary of the numerical model. The

constitutive relation of rock masses adopted in simulation

is the plastic follow-up model, and the influence of strain

rate on rock strength is considered in the Cowper-Symonds

model LSTC (Miklowitz 1960, LSTC 2003). The physical

and mechanical parameters of rock masses of the detection

section is shown in Table 2, and the strain rate parameter C

and P are obtained through numerical matching of the rule

of material dynamic strength vs. strain rate.

The simulation results of the damage zone caused only

by the blasting load are shown in Table 3. The damage

zone caused only by explosive load during excavation of

the 2# headrace tunnel of JPII is only about 0.8 m. The

calculated result needs to be verified by field tests.

3.2 Field Verification

In order to measure the damage extent resulting only from

blasting load and reducing the effect of in situ stress to the

minimum level, the shallow part of DT2# from tunnel

length 100–800 m was chosen. The selected part of DT2#

is totally in T2Z strata, and the lithology of surrounding

rocks is the Zagunao group marble with average uniaxial

compressive strength of about 85 MPa. The levels of in situ

stresses are shown in Fig. 7. As mentioned above, the

direction of the maximum principle stress r1 is basically

parallel to the tunnel axis with a small dip angle, and the

middle principle stress r2 and the minimum principle stress

r3 are both in the cross section plane of the tunnel. The

direction of r3 is nearly vertical.

The damage detecting holes are arranged in the side

walls of DT2# at a height of 1.5 m from tunnel bottom, and

the results of the extent of EDZ is shown in Fig. 10.

It can be seen from Fig. 10 that the damage zones

increase with the buried depth as a whole. With the tunnel

(a) In situ stress versus tunnel length

(b) Measured extents of damage zone

Fig. 10 In situ stresses and measured damage zone extent

Table 2 Parameters of rock

mass of the detection section

adopted in simulation

Elastic

modulus

(GPa)

Density

(kg m-3)

Poisson’s

ratio

Uniaxial

compressive stress

(MPa)

Tangent

modulus

(GPa)

Parameter

C

Parameter

P

15.0 2650 0.22 70.0 8.0 40 2.2

Table 3 Extent of damage

zone induced by explosive load

only

Labels Extent of damage zones at different

positions (m)

S1 S2 S3 S4 S5

Hb Simulated extent of damage zone induced only by

explosive load (blast load induced damage zone)

0.78 0.81 0.79 0.85 0.78

HI Detected extent of inner damage zone 1.60 1.40 1.40 1.20 1.30

Hb/HI 9

100 %

Percent of blast load induced damage zone in inner

damage zone

49 % 58 % 56 % 71 % 60 %

Contributions of In-Situ Stress 721

123

length from 100 to 800 m, the maximum principle stress r1

paralleled to the tunnel axis is within 10–25 MPa, and the

r2 and the r3 in the tunnel cross section are both less than

10 MPa. While, after stick number 0 ? 800, the value of

in situ stresses in the tunnel cross section increases quickly

to 10–25 MPa.

It can also be found that the extents of damage zones of

the DT2# with tunnel length between 100 and 800 m are

almost \1.0 m, but after 0 ? 800, the extents of damage

zone increase significantly. It clearly indicates that the

value of the in situ stress exerts an important influence on

the excavation damaged zone.

Because the values of in situ stresses are low and the

maximum principle stress is parallel to the tunnel axis, the

excavation damaged zone with tunnel length from 100 to

800 m is considered to be caused only by the blast load,

and the range of the damage zone extent is about 0.5–0.8 m

excepting three detecting extents over 1.5 m.

It can be seen that the measured results agree with the

simulated results very well. The results prove that the

simulation is reliable.

3.3 Constitution of the Inner Damage Zone

The damage zone caused only by blast load and the inner

and outer damage zones of section DT2# 15 ? 700 are all

given in Fig. 11 and Table 3.

The result indicates that the inner damage zone induced

by the D&B excavation process is not completely caused

by explosion load. The extent of the damage zone caused

by explosion load accounts for only 49–71 % of the inner

damage zone (shown in Table 3), with an average of 59 %.

Therefore, there is obviously another load contributing to

the development of the inner damage zone. This conclusion

is totally different from the result of Martino and Chandler

(2005).

From the above analyses, the transient redistribution of

in situ stress accompanying rock fragmentation process

during blasting excavation may be the answer. Certainly, the

effect may also influence the development of the inner dam-

age zone, but it needs to be verified by more monitoring data.

4 Damage Caused by Stress Transient Redistribution

4.1 Damage Mechanism of Stress Transient

Redistribution

In Miklowitz (1960), studied the dynamic redistribution of

the circumferential stress rh (r, t) induced by the process of

quickly punching (takes about 2.5 ls) a round hole in the

middle of a tight tension circle plate using Laplace trans-

form method, as shown in Fig. 12a. The result indicates

that the tangential stress wave is induced by the fast

unloading of the normal stress on the hole wall during the

process of punching. Compared with the static redistribu-

tion tangential stress, the induced dynamic tangential stress

on the wave front is obviously larger. The dynamic process

can be considered as one of the important inducements

leading to generation and expansion of micro-cracks

around the hole punched.

In the process of excavation of a deep buried tunnel with

the excavation D&B method, the cracks preferentially

expand in the blast holes connection direction, and the

adjacent blast holes interpenetrate with each other in a very

short time (about several ms), then the rock fragments are

thrown out quickly to form a new excavation face. The

normal constraint between the excavated rock mass and the

reserved rock mass will instantly disappear, and the in situ

stress on excavation face releases transiently in the mean-

time. This process will arouse transient unloading stress

wave in surrounding rocks, just like the results given in

Fig. 12b. The transient unloading stress wave has been

proved to be one of the factors for generation and expan-

sion of cracks, which finally results in damage to the sur-

rounding rocks (Lu et al. 2011, 2012; Miklowitz 1960).

4.2 Excavation Loads Induced by Transient

Redistribution

In order to study the damage extents caused by in situ stress

transient redistribution during blasting process quantita-

tively, the numerical simulation based on the calculation

model shown in Fig. 8 has been conducted.

First of all, the transient redistribution path and time of

in situ stress on the excavation face need to be determined

during blasting. According to the stress continuity condi-

tions on excavation boundary, only when the cracks around

the blast hole and between adjacent blast holes completely

interconnected with each other and the pressure on the

excavation contour Pb0 decays to a level equaling the

Fig. 11 The excavation damaged zone of section DT2# 15 ? 700

722 P. Yan et al.

123

in situ stress of surrounding rock mass ri, the release of

in situ stress begins, as shown in Fig. 13. The ti is the start

time of in situ stress unloading. When the Pb reduces to the

atmosphere, the release of in situ stress completes (Lu et al.

2011). The start and stop time and variation law of in situ

stress transient releasing coupled with explosion load

during blasting are determined by the stress level on the

newly formed excavation face and variation process of the

blast load. The detailed calculation process can be found in

the work done by Lu et al. (2012).

During the blast excavation of headrace tunnels of JPII,

the short-hole blasting scheme is always adopted. The

diameter and depth of boreholes are 42 mm and 1.5–5.0 m,

respectively, and the spacing between adjacent smooth

blasting holes is 0.8–1.2 m. The 2# rock emulsion explosive

with a detonation velocity of 3,500–4,500 m/s is adopted.

The buried depth of the 2# headrace tunnel at stick

number 15 ? 700 m is about 1,054 m. As mentioned

above, the in situ stresses in the cross section plane of

DT2# are 34.4 and 29.2 MPa, respectively (see Figs. 5,

11). It can be estimated from the research of Lu et al.

(2012) that the in situ stress releasing time during blasting

excavation of headrace tunnels of JPII is 2–5 ms. This

estimated value is consistent with the field high-speed

photography material of rock blasting (Felice et al. 1993).

During this transient process, a large amount of elastic

strain energy would release in several milliseconds with a

high speed accompanying transient releasing of stress,

which will arouse significant transient unloading stress

wave in the surrounding rock masses, and cause damage to

surrounding rocks.

Fig. 12 The stress wave induced by quickly punching a hole in a circle plate in tension (Miklowitz 1960)

Fig. 13 Curves of blasting load and transient release of in situ stress

vs. time (Lu et al. 2011)

Contributions of In-Situ Stress 723

123

4.3 Numerical Simulation and Discussion

There are three kinds of calculation conditions which are

simulated as follows: (1) the extent of damage zone induced

only by the quasi static redistribution of the in situ stress after

tunnel excavation; (2) the damage zone caused by the cou-

pling effect of explosion load and static redistribution of

in situ stress and (3) the damage zone caused by the explosion

load coupling with transient unloading of in situ stress. The

simulation results are shown in Fig. 14, and the specific

damage zone data are listed in Table 4.

It can be seen from Fig. 14 that, the extent of

damage zone and its distribution along the excavation

boundary are strongly influenced by the in situ stress

field. The simulation results given in line HBT of

Table 4 mean that the extents of the total damage zone,

which is caused by the coupling effect of explosive

load and transient redistribution of in situ stress, are

very close to the monitoring results given in Table 1

excepting the position of S1. It indicates that the results

of numerical simulation are reliable. So the simulation

results can be used to study and compare the damage

Fig. 14 Damage zone schema

after blast-excavation of upside

part

Table 4 The simulation results

of the extent of damage zones

caused by blast excavation of

the upside part of the headrace

tunnel

Labels Extent of damage zones at

different positions (m)

S1 S2 S3 S4 S5

HS Damage extent induced by quasi static

redistribution of in situ stress

2.72 1.65 1.55 1.85 1.41

HBS Damage extent induced by coupling effect of

explosive load and static redistribution of

in situ stress

3.17 2.62 2.50 2.89 2.57

HBT Damage extent induced by coupling effect of

explosive load and transient redistribution of

in situ stress means in the simulated total

damage zone

3.63 2.80 3.90 4.09 3.20

HB = HBS-HS HBS subtracts HS, equals the damage extent

caused by blasting load, HB

0.45 0.97 0.95 1.04 1.16

HTR = HBT-HBS HBT subtracts HBS, equals the damage extent

caused by in situ stress transient unloading,

HTR

0.46 0.18 1.40 1.20 0.63

HI = HBT-HS HBT subtracts HS, equals the damage extent

caused by coupling effect of blast load and

in situ stress transient redistribution, the

simulated inner damage zone, HI

0.91 1.15 2.35 2.24 1.79

HB/HI 9 100 % Percent of blast load induced damage zone in

inner damage zone

49 % 84 % 40 % 46 % 65 %

724 P. Yan et al.

123

extent induced by explosive load and in situ stress

transient redistribution.

Comparing line HBS with line HS, it can be found that

the blast load makes the extent of damage zone increase by

nearly 1.0 m (except for the 0.45 m monitored form

detecting hole S1), and the distribution of increased extent

along the excavation boundary is nearly uniform. The

fourth line of Table 4 marked by HB means damage caused

by the blast load alone, and the results are slightly larger

than the monitoring result 0.78–0.85 m, as given in

Table 3. The reason for the difference may be the coupling

effect of blast load and redistributed stress.

The fifth line of Table 4 marked by HTR means damage

induced only by in situ stress transient unloading or tran-

sient redistribution, while the sixth line of Table 4 marked

by HI means damage caused by the coupling effect of the

blast load and in situ stress transient adjustment, which can

be regarded as the inner damage zone in field results. It can

also be concluded from the numerical simulation results

that the inner damage zone is constituted by a damage zone

caused by blast load and the one induced by transient

unloading of in situ stress.

The seventh line of Table 4 marked by HB/HI means that

the sixth line is divided by the fourth line, which equals the

rate of damage extent caused only by the blast load in the

inner damage zone acquired from the numerical simulation.

The results shown in the seventh line are very close to the

monitoring results shown in Table 1. Thus, the explosive

load almost contributes 49–84 % to the inner damage zone,

while the transient adjustment of in situ stress accounts for

the remaining 16–51 % of the inner damage zone.

The results mentioned above indicate that the damage

zone caused by transient unloading of in situ stress is one

of the main components of the excavation-induced damage

zone, and the distribution of the transient unloading

induced damage zone is intensively influenced by the

redistributed stress field. Therefore, in situ stress transient

unloading inducing rock damage cannot be ignored, and it

should be studied as a surrounding rock damage factor

equaling explosive loads. It is also worth mentioning that

the measuring and simulation results given in this paper are

obtained based on the in situ stress level at 29–35 Mpa, and

the damage effect of in situ stress transient unloading is

expected to be more intensive under much higher stress

conditions.

5 Conclusion

The conclusions are as follows:

1. Due to the coupling effect of the high level in situ

stress and the excavation action, the blasting

excavation induced damage zone of deep tunnels can

be divided into the inner damage zone and the outer

damage zone from excavation surface into surrounding

rocks.

2. The inner damage zone of deep tunnels excavated by

the D&B method is mainly caused by the explosive

load and in situ stress transient redistribution, while the

outer damage zone is mostly attributed to the static

redistribution of in situ stress. The inner damage zone

accounts for 29–57 % of the total damage zone extent.

3. Field tests and numerical simulation indicate that the

in situ stress transient unloading effect accompanying

the blasting process contributes about 16–51 % to the

inner damage zone of deep tunnels and the in situ

stress transient adjustment/redistribution is one of the

main contributors, and the damage caused by in situ

stress transient adjustment effect may exceed the one

caused by the explosion load and become the main

cause for damage.

Acknowledgments This work is supported by the Chinese National

Science Fund for Distinguished Young Scholars (No. 51125037), the

Chinese National Natural Science Foundation (Nos. 51179138,

51279135, 51279146 and 51009013). The authors sincerely express

their thanks to all the supporters.

References

Abuov MG, Aitaliev ShM, Ermekov TM et al (1989) Studies of the

effect of dynamic processes during explosive break-out upon the

roof of mining excavations [J]. J Min Sci 24(6):581–590

Felice JJ, Beattie TA, Spathis AT (1993) Face velocity measurements

using a microwave radar. In: Preece DS, Evans R, Richards AB.

Coupled explosive gas flow and rock motion modeling with

comparison to bench blast field data[C]. Proceedings 4th

International Symposium Rock Fragmentation by Blasting.

Vienna, Austria, pp 239–246

Hsiung SM, Chowdhury AH, Nstsrss MS (2005) Numerical simula-

tion of thermal–mechanical processes observed at the lkdrift-

scale heater test at Yucca Mountain Nevada USA [J]. Int J Rock

Mech Min Sci 42(5/6):652–666

LSTC (2003) LS-DYNA keyword user’s manual [M]. Livermore

Software Technology Corporation, Livermore

Lu WB, Yang JH, Chen M et al (2011) An equivalent method for

blasting vibration simulation. Simul Model Pract Theory

19(9):2050–2062

Lu WB, Yang JH, Yan P et al (2012) Dynamic response of rock mass

induced by the transient release of in situ stress. Int J Rock Mech

Min Sci 53:129–141

Martino JB, Chandler NA (2004) Excavation-induced damage studies

at the underground research laboratory [J]. Int J Rock Mech Min

Sci 41(8):1 413–1 426

Miklowitz J (1960) Plane-stress unloading waves emanating from a

suddenly punched hole in a stretched elastic plate [J]. J Appl

Mech 27:165–171

Read RS (2004) 20 Years of excavation response studies at Aecl’s

underground research laboratory [J]. Int J Rock Mech Min Sci

41(8):1251–1275

Contributions of In-Situ Stress 725

123

Shan ZG, Yan P (2010) Management of rock bursts during excavation

of the deep tunnels in Jinping II hydropower station. Bull Eng

Geol Environ 69:353–363

Tang C, Wang J, Zhang J (2010) Preliminary engineering application

of microseismic monitoring technique to rockburst prediction in

tunneling of Jinping II project. J Rock Mech Geotech Eng

2(3):193–208

Zhang CS (2007) Study on technical cruxes of headrace tunnels of

Jingping II hydropower prjoect on Yalong River. China Inv Des

8:41–47 (in Chinese)

Zhang ZY, Liu Y, Zhang WX (1993) Research to the vibration effect

induced by blasting excavation of the main power house in

Dongfeng hydropower station [A]. Proceedings of Engineering

blasting [C], pp 299–305 (in Chinese)

Zhang CQ, Feng XT, Zhou H et al (2012) Case histories of four

extremely intense rock bursts in deep tunnels. Rock Mech Rock

Eng 45:275–288

726 P. Yan et al.

123