International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147

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International Journal ofRock Mechanics & Mining Sciences

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Technical Note

Thermography analysis of a roadway excavation experiment in 601 inclinedstratified rocks

W.L. Gong a,b,n, J. Wang a,b, Y.X. Gong c, P.Y. Guo a,b

a State Key Laboratory for Geomechanics and Deep Underground Engineering, Beijing 100083, Chinab School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, Chinac School of Mathematics and Statistics, South Central University for Nationalities, Wuhan 430074, China

a r t i c l e i n f o

Article history:

Received 8 March 2012

Received in revised form

23 December 2012

Accepted 29 December 2012Available online 31 January 2013

1. Introduction

Excavation stability connected with energy release has been anactive field of research for the community of rock mechanics [1]. Theexcavation damaged zone (EDZ) around the tunneling face, wherein situ rock mass properties and conditions have been altered due tothe excavation impact, can be mechanically unstable and could alsoform a permeable pathway of groundwater flow which would raisethe safety concern of the deep geological structures [2]. Themechanism, degree and extent, therefore, should be better under-stood and quantified for the creation or operation of deeply buriedtunnels. This paper presents an experimental investigation on aroadway excavation in 601 inclined stratified rocks based onphysical modeling and infrared thermography.

The excavation stability related problems have widely beeninvestigated by means of in-situ experiments usually incorpo-rated with numerical methods and acoustic emission (AE) [3–8].Meanwhile, laboratory experiments were carried out to investi-gate the underground excavation problems [9], such as the trapdoor tests [10]; laboratory experiment using a model specimen[11], and the centrifuge model test [12,13]. In recent decades asthe mining went deeper, investigations on the undergroundexcavation have been focused on the rock mass with discontin-uous or weak surfaces, including, for example, numerical simula-tions [5,14,15] and the small-scale model tests [16]. Althoughdifferent degree of success was achieved in these studies, thereare still some aspects of the excavation-induced damage in

09/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.

x.doi.org/10.1016/j.ijrmms.2012.12.047

esponding author at: School of Mechanics and Civil Engineering, China

ity of Mining and Technology, Beijing 100083, China.

6 10 15910785289.

ail addresses: gongweilicumtb@126.com, gwl@cumtb.edu.cn (W.L. Gong).

complex geological environments left uncertain, especially inthe stratified geological structures.

In order to promote better understanding of the excavation-induced damage in stratified rock masses, experiments on road-way excavation in large-scale geological physical models wereconducted in the State Key Laboratory for Geomechanics andDeep Underground Engineering (SKL-GDUE), at the China Uni-versity of Mining and Technology Beijing (CUMTB) in recentyears. Structural behavior of the differently inclined rock stratawas captured based on infrared thermographic technique. Todate, the relevant papers published include excavation in hor-izontal strata [17,18], 451 inclined strata [19], and vertical strata[20]. This paper presents the experimental results from theexcavation in 601 inclined strata. As the advancement in ourexpertise for image processes, deeper insights into the excavationdamage will be reported in this document.

2. Experiment

2.1. Rock-model material

The rock strata are constructed with a large number of smallprismatic plates, the so-called ‘‘elementary slab’’ [17], which aremade of gypsum and water. Gypsum has been recognized asa good rock-model material and a significant laboratory experi-ments on crack initiation and coalescence were conducted withthis material [21–23]. The elementary slab is made by an intimatemixture of gypsum and water and casts against a mold. All theelemental slabs were fabricated with the same surface dimension400�400 m2 and three thicknesses of 1, 2 and 3 mm [17]. Threeartificial rocks were produced using gypsum with three water–gypsum ratios, among them 0.8:1 for sandstone, 1:1 for mudstone

Table 1Real and artificial rock properties.

Unit weight (kN/m3) UCS (MPa) Tensile strength (MPa) Young’s modulus (GPa) Internal friction angle

(1)

Poisson’s ratio

Real rock Artificial rock Real Rock Artificial rock Real rock Artificial rock Real rock Artificial rock Real rock Artificial rock Real rock Artificial rock

Sandstone 27 15 64 8 5.83 0.72 26 3.22 34 32 0.15 0.13

Mudstone 26 11 44 5 5.59 0.69 21 2.62 36 33 0.13 0.12

Coal seam 14 8. 26 3 0.90 0.11 5 0.61 40 33 0.36 0.32

Weak surface

Coal-seam

Mud-stone

Fig. 1. Weak bedding surface of the rock strata simulated by the rock-model materials: (a) schematic of the rock strata and their interface, and (b) photograph of the

elementary slabs.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147 135

and 1.2:1 for coal, in terms of the mechanical and materialproperties [17,18]. Dimensional analysis was carried out todetermine such analogous factors as geometric scale factor,al ¼ 12; force scale factor as ¼ 8 and body force scale factorag ¼ as=al ¼ 8=12¼ 0:67 [17]. These scale factors were used todesign the physical model and the excavation scheme. The realand artificial rock properties were reported in Table 1.

2.2. Rock strata

Fig. 1 shows the principle for producing rock strata. A largenumber of the elementary slabs (shown in Fig. 1b) with the samemodel-rock property were used to ensemble a rock layer of thesame property and a certain number of the rock layers with thesame rock property were used to construct a rock stratum. Asshown schematically in Fig. 1a, the mudstone stratum and coalseam stratum are assembled by placing the elementary slabs inlayers with perfectly mated interfaces. A weak surface is formedbetween the two different model rock strata. This weak surface ishomogeneous along its striking at macroscopic scale, while hasmany minor asperities at ‘‘microscopic scale’’ constituted bymany small gaps between the parallel-placed elemental slabs inthe rock layer. The gaps and asperities formed by the layeredslabs also exist within the rock strata. These minor flaws can beviewed as the origin of the heterogeneity of the rock underexternal loading. Three classes of the model-rock strata wereconstructed in our test, i.e. sandstone stratum, mudstone stratumand coal stratum which create two types of interfaces or weakfaces, i.e. standstone–coal interface and mudstone–coal interface.

2.3. Geological model

The large-scale physical model was built with total nine strataincluding one sandstone, four mudstones and four coalseams andall the strata inclined at an angle of 601 with respect to thehorizontal, to simulate steeply dipped geological structures. Theconstructed geological model, as shown schematically in Fig. 2a,is 1.6 m high, 1.6 m wide and 0.4 m in thickness. The alternatingstrata are indexed with 1–9 from the left to the right with the

rock properties shown in Fig. 2d, including the thickness of thestrata and the number of layers of the elementary slabs used ineach stratum assemblage. The excavation zone was located in thecenter of the model within the stratum 4 (coal seam) and has adimension of 200 mm in height, 250 mm in width and 400 mmlong (equal to model’s thickness). An overview of the geologicalmodel at laboratory was given in Fig. 2b. The geological model isable to simulate by the geometric scale factor al ¼ 12ð Þ, anengineering rock mass with a plane dimension of 19.2�19.2 m2

and a tunneling face of 3�2.4 m. During the excavation, thevertical and lateral loads were applied uniformly on the top andtwo side boundaries of the model by the testing machine devel-oped by He [19] while the bottom of the model was fixed on thebasement of testing machine. The lateral pressure vs. verticalpressure s2/s1 was kept at an constant magnitude of 0.3 MPa/1 MPa which is equal to the lateral pressure coefficient l¼0.3,so as to reproduce the unbalanced stress state found in deepunderground mining as shown in Fig. 2a.

2.4. Excavation plan

Fig. 2c shows the total excavation volume (200�250�400 m3)which was divided into seven sub-spaces termed ‘‘rock block (RB)’’.The RBs were numbered #1–#7 corresponding to the roadwayexcavation sequence. The roadway excavation was designed astwo phases (Fig. 3); phase 1 (Fig. 3a): full-face excavation, i.e.tunneling on #1 RB until a small passage is cut through; and phase2 (Fig. 3b): staged excavation, i.e. removing one RB at eachexcavation stage. For description of the full-face excavation, theexcavated volume of the artificial materials from the roadwaytunnel was referred to as ‘‘footage’’ thereafter and for the stagedexcavation, the term ‘‘excavation stage’’ denotes removing of the RB,and total of seven excavation stages were performed step by step insequence during the phase 2 excavations. The diagram E6 meansdestruction of the #1 RB, i.e. the loss of the bearing capacity as aresult of the failure of the material. The two phase excavation plan isthe same as those of the excavation experimental results conductedin our institute in the published papers [17–20].

200

mm

400 mm

1600 mm

Infrared camera

σ1 = 1.0 MPa

Imaging area 400 × 267 mm2

400 mm

200 mm

Processor

(The excavation volume)

9

8

7

65

4 3 2 1

Video camera

400 mm

400

mm

1600

mm

Excavation volume250 × 200 × 400 mm3

1 Sandstone 440 14

2 Coal seam 140 10

3 Mudstone 120 7

4 Coal seam 250 17

5 Mudstone 150 5

6 Coal seam 60 4

7 Mudstone 140 5

8 Coal seam 60 3

9 Mudstone 240 8

Geological sections

StratumNo.

Rocktype

Thickness(mm)

Layernumber

σ 2 = 0.3 MPa

(Cross-section)

250 mm

Front view #1 RB

250 mm

A – A

#3 RB

#5 RB

#7 RB

#2 RB

#4 RB

#6 RB

Fig. 2. Constructed large-scale geological model; (a) schematic drawing of the geological model; (b) photography showing an overview of the model at laboratory; (c) the

excavation volume (3-dimensional view) and the sectional view (A–A), and (d) geological sections of the model-rock strata.

Initial stage: E0

0/5 footage 1/5 footage

Step 1: E1

2/5 footage

Step 2: E2

1/2 footage

Step 3: E3

3/5 footage

Step 4: E4

4/5 footage

Step 5: E5

Critical point

Step 6: E6

P0 P3 P4 P5 P6 P7

Break through

P1

Removing #1 RB

P2

Removing #2 RB Removing #3 RB Removing #4 RB Removing #5 RB Removing #6 RB Removing #7 RB

Fig. 3. Schematic of the two phase excavation plan; (a) the full-face (phase 1) excavation at sectional view and (b) the staged (phase 2) excavation at front view.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147136

2.5. Excavation method

The excavation of a roadway tunnel without support by meansof long-round drill and blast was simulated by an operator witha hammer and a chisel. Fig. 4 shows schematically the scenariowhere the operator performs the full-face excavation with thehammer and chisel. The excavation was started from the backside of the model and went through to the front face. The infraredcamera detects the temperature rise with an imaging area of

400�400 mm2. The hammer-chisel impact feeds an intense andpulsed pressure against the face. The cycles for blowing the chiselwith the hammer by the operator are at a slow rate with arandom manner. To be exact, the rate of the loading is stochastic,i.e. when the face was shallow, the blowing was fast (about oneblowing every second) as a result of the easy operation; when theface went deeper, the blowing was very slow (around one blowevery five seconds or more), because the operator needs to findthe right place to be excavated. Thus the excavation-induced

#3 RB#1RB#2RB#4RB#6 RB

#5RB#7RB

200

mm

400

mm

400 mm

Chisel

Hammer

Infrared camera

Large-scale geological model

Excavation zone

Imaging areaOperator

Fig. 4. Schematic illustration of the phase 1 excavation (full-face excavation); the tunnel excavation without support with dill and blast method was simulated by an

operator using a hammer and a chisel as the tunneling tools.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147 137

impact is, therefore, not periodical and a rate-dependent process buta consecutive-stochastic dynamic loading and unloading process.Since the excavation area (250�200 mm2) is much smaller than thesize of the geological model (1600�1600 mm2), the vibrationintensity caused by the impact depends on the excavation status.For example, at the beginning of the full-face excavation, the wholemodel trembled against the blowing at high-frequency with sharpstriking sound. When the face advanced deeper, the vibration waslocalized within the face and the sound was muffled to be a low-frequency noise. Over the stages excavation, the vibration existed ina localized manner. For example, removing of #1 RB caused higherintensity of the vibration than that in removing #7 (the last) RB.The intense vibration occurred at the stage 1 to stage 3 excavations.At the last two stages P6 and P7, the intensity of the vibrationswas minor.

2.6. Infrared detection

An infrared camera detects temperature changes of the viewedsurface based on thermo-mechanical coupling effect, i.e. theinternal dissipation due to elasticity and inelasticity of thematerial under external loading [24–28]. Generally, infraredthermography is used in two modes, i.e. the passive and theactive [18]. The active thermography uses a known external heatsource for heating up the tested object and, therefore, needs tocalibrate the obtained temperature increase physically with theheat source. As the heating process is operated in a controlledway, a large temperature increment could be obtained. Thus thetemperature-induced stress or strain has larger amplitude and theobtained thermogram has higher contrast, which may contributeto identify the physical process by the subsequent image analysis.The active infrared thermography is usually used in the case ofmaterials with a relatively simple constitutive relationship suchas metals and composites [28,29]. The passive thermographydetects the temperature field induced by the external loadingbased on the coupled thermo-mechanical effects without use ofany extra heat sources. When using passive thermography, thetemperature increment may be relatively smaller and the resul-tant thermogram is often vague in contrast. Therefore, filtering ofthe raw thermograms should be implemented [18]. For 24 hearlier prior to the testing, all the instruments were placed inthe same room with the PFESA model, so that the detected IRtemperatures are the temperature variation due to the excavationimpact. TVS–8100MKII infrared thermography, working at the

mid-wave band (3.6–4.6 mm), was used in passive mode in thisstudy. More details about the technical specifications can befound in Ref. [17].

3. Thermal-mechanical coupling

Thermal-mechanical coupling effect was investigated for inter-pretation and quantification of the detected infrared sequence frommetals [26,30] and rock and rock-like materials [25,27,31] underlaboratory testing conditions. Based on the second law of thermo-dynamics and compatibility and separability of the fundamentalequations of mechanics and constitutive relations, the thermo–viso–elastic–plasticity equations were developed by Loung [26]. The finalresults of the coupled thermomechanical equation take a universalform [26],

rCv_T ¼ rrþdivðKgradTÞ�ðb : D4 : _E

eÞTþS : _E

Ið1Þ

where r (kg m�3) is the mass unit in the reference solid; Cv

(J kg�1 K�1) the specific heat at constant deformation; T the absolutetemperature and the superposed dot stands for the material timederivative; r the heat supply; div the divergence operator; K thethermal conductivity (W m�1 K�1); grad the gradient operator; bthe coefficient of the thermal expansion matrix; ‘‘:’’ the contractedproduct operator of second order tensor; D4 the fourth-orderelasticity tensor; Ee is the elastic strain tensor; EI is the inelasticstrain tensor; and S is the second Piola–Kirchhoff stress tensor. Thevolumetric heat capacity of the material C ¼ rCv is the energyrequired to raise the temperature of unit volume by 1 1C (or 1 K).

This equation delineates the four different thermal-mechanicalcoupling mechanisms represented by four terms on the right-handside of the Eq. (1) underlying rock failure processes [26], includingsources or sinks of heat in the scanning spot (the first term),thermal conduction (the second term) related to the transference ofheat by thermal conduction to re-balance the temperature distribu-tion of the stressed object, thermoelasticity (the third term)representing the thermal-elastic coupling effect which is a linearprocess and the conversion between the mechanical and thermalenergy is reversible, and energy dissipation (the last term) gener-ated by viscosity and/or plasticity involving expanding of initialfissures and joints, generating of new fractures, fracture propaga-tion and extension, pore gas desorbing–escaping, rock friction, etc.It is a non-linear process and energy conversion is irreversible.Detailed explanation of Eq. (1) can be found in Refs. [26,27].

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147138

Although the principles were made clear, but the discrimination ofthe four quite different phenomena is difficult, especially for theintrinsic dissipation, when interpreting the thermograms obtainedunder usual conditions as pointed out by [26]. In this study, imagedenoising and data reduction were used to ascertain the fourdifferent mechanisms of the thermal–mechanical coupling effectover the excavation processes.

20

30

-30-30

010

-20-10-20

-10

0

1020

0.2

0.4

0.6

0.8

1

0

30

Fig. 5. Three-dimensional representation of Gaussian high pass filter (GHPF).

4. Image processing

Real world rarely comes clean. Noise contamination problemsare commonly found in the graphical testing techniques. Filteringthe noisy image while preserving the image features such as edgesand textures is of central importance for making progress in theinfrared thermography techniques [26]. At laboratory infraredthermography is often used to detect small-scale objects, e.g. thelaboratory-sized specimen [31–34] or used in the active mode[26,28,30]. In this case, the image resolution is higher due to thesmall imaging region (defined as ‘‘pixel/ unit area’’) or the highradiant flux simulated by the extra heating source. Thus therequirements for the image processing may be lower. However, inthe case of detecting a large-scale geological model as in our testwith the infrared thermography used in the passive mode, thethermograms are highly contaminated by the noises with anarrowed dynamical range. Therefore, for the low-quality thermo-grams, image processing procedure should be properly performed.

Processing of a low-quality thermogram often involves suchprocedures as image denoising and enhancement [18]. Major taskfor the image de-noising is the estimation of the noises that mayexist in a specific physical process and the recovery of the cleanimage by the use of the appropriate filters. For laboratorymechanical experiments, the noises contained in the thermogramcan be divided into the following classes, i.e., the environmentalradiation noise, electronic current induced impulsive noise (alsosalt-and pepper noise) and additive-periodical noises. The envir-onmental radiation noises can be removed by image subtraction.The image subtraction algorithm can be given by

f k x,yð Þ ¼ f k x,yð Þ�f 0 x,yð Þ ð2Þ

where k is the frame index of the infrared sequence, f 0 x,yð Þ is thefirst frame of the infrared sequence taken when the model rockwas at intact state; x¼ 0,1,2, � � � ,N�1 and y¼ 0,1,2, � � � ,M�1 arethe pixel coordinates, and M¼120 and N¼160 are the maximumpixel number respectively for the image matrix. Subtraction of thefirst frame from the following infrared sequence will suppress thebackground radiation noises.

The existence of the impulsive noises can narrow the dynami-cal scope and make the following image processing improbablesuch as the image enhancement. The impulsive noises canbe removed by the median filter [35,36]. The median filter is anon-linear spatial filter or rank filter whose response is based onordering the pixels contained in the image area encompassed bythe filter, and replacing the central pixel with the ranking result.Given Sxy represents the structural element (or mask) centeredat the point ðx,yÞ which is actually a matrix of m� n dimension,gðx,yÞ representing the filtered image and g x,yð Þ the noisy image,then the median operation replaces the pixel at ðx,yÞ with themedian pixel of the neighborhood defined by the mask

g x,yð Þ ¼medians,tð ÞA Sxy

fg s,tð Þg ð3Þ

Performing the median filtration on the thermogram can get rid ofthe pixel with much larger magnitude in its neighborhood areaand enlarge the dynamical scope of the image. It is a necessarystep for the subsequent image enhancement operation.

Assuming that the noises left in this step is only the additive-periodical one after image subtraction and median filtering, theadditive noise-contained image gðx,yÞ can be represented by

g x,yð Þ ¼ f x,yð ÞþZ x,yð Þ ð4Þ

where f x,yð Þ is the clean image, Z x,yð Þ is the left noise, and (x,y) arethe horizontal and vertical coordinates of the image plane respec-tively and take the discrete value in the case of digital image. Forreduction of the additive-periodical noise, frequency-domain meth-ods can be used and Gaussian-high-pass filter (GHPF) is proved tobe efficacious in implementation of this task [35]. Implementationof the Gaussian-high-pass filter involves the following four steps.

(1)

Firstly, compute Fourier transform of Eq. (4), to find

G u,vð Þ ¼ F u,vð ÞþN u,vð Þ ð5Þ

where, for digital image, 2-dimensional discrete Fourier trans-form (DFT) was employed and G(u,v)¼DFT [g(x,y)], F(u,v)¼DFT [f(x,y)] N(u,v)¼DFT [Z(x,y)], DFT is the discrete Fouriertransform operator [35]; and uandv are the frequency vari-ables in the horizontal and vertical directions in unit Hz.

(2)

Let H(u,v) stand for Fourier transform of the filtering function,multiplying Eq. (5) with H(u,v) and assuming that the productof H(u,v)N(u,v) vanishes, the filtering algorithm can be imple-mented by

G u,vð Þ ¼H u,vð ÞF u,vð Þ ð6Þ

(3)

Perform the inverse Fourier transformation on G(u,v) in Eq. (6),i.e. g(x,y)¼DFT �1[G(u,v)], then the cleaned image can berecovered from g(x,y). The transfer function H u,vð Þ used in thisresearch is the GHPF (Gaussian high pass filter), which can bemathematically represented by [35],

H u,vð Þ ¼ 1�e�D2 u,vð Þ=2D0 ð7Þ

where D0 is the cut off frequency of the filter; and D is thedistance from a point (u,v) to (M/2, N/2) which is the center foran M � N image:

D u,vð Þ ¼ ½ u�M=2� �2

þ v�N=2� �2

�1=2 ð8Þ

Fig. 5 shows a three-dimensional representation of the GHPF.GHPF can filtrate the low-pass band components (located around

center of the image plane, the horizontal plane in the figure)while preserve high-pass band components. Edges and bordersthat segment different objects in an image are important image

ΔT (°C)

0.15

0.10

0.05

0

–0.05

34.7

34.4

34.1

33.7

33.4

33.1

32.8

32.5

32.1

T (°C)

)

Fig. 6. Taking the frame 587 in the sequence corresponding to E6 in the full-face

excavation being a case, illustrating the image processing effect using the algorithm

proposed in this paper; (a) the raw thermogram, and (b) the processed one.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147 139

features corresponding to the high-frequency components. Com-pared with other frequency-domain high-pass filters such as theideal high-pass filter and butterworth high-pass filter, GHPF canpreserve fine features with smoother visual effect [37].

Fig. 6 shows the image processing effect. Fig. 6a is the inputthermogram of frame 587 corresponding to E6 in the full-faceexcavation. The raw thermogram is a fully-blurred low contrastimage with narrow dynamic range as a result of the contaminationby the noises. Not any specific and clear information could bedrawn from it. Fig. 6b is the processed thermogram of the frame587 using the proposed algorithm described above. As compared tothe raw thermogram (see Fig. 6a), the borders and edges of theexcavation-induced frictional sliding bands with changed stressrepresented by the changed brightness can be distinguished moreclearly in the processed thermogram (see Fig. 6b). The imageprocessing algorithm proposed in this paper, therefore, can be saidvery efficacious in noise reduction and image enhancement. Thethermogram analysis method, taking the processed thermogram offrame 587 as an example, will be given in the next section.

5. Methods for image analysis

Taking the denoised thermogram of the frame 587 as anexample, our image analysis method is shown in Fig. 7. The imagecharacterization include: (i) mean value of the infrared temperature(IRT) field (Fig. 7b); (ii) spatial Fourier spectra of the image (Fig. 7aand d) and (iii) the denoised thermogram itself (Fig. 7c). Image

matrix f(x,y) is actually the surface temperature field of the testedobject. The mean of the image matrix /f(x,y) S, also represented by/IRTS, accounts for the averaged energy dissipation at a timeinstant. Thus, /IRTS represents the overall thermal response and/IRTS is computed as sample mean from the thermograms [17].Conventionally 2-dimentional Fourier transform is generally used totransform an image, but is inconvenient for a quantitative analysis.Alternatively, one-dimensional (1-D) Fourier transform can be usedfor computing the spectra from the re-sampled 1-D ‘‘spatial series’’.Two spatial series, xi i¼ 1,2, UUU, 120ð Þ and yj j¼ 1,2,UUU, 160ð Þ weretaken along the horizontal and vertical axes across the center of theimage plane (Fig. 7c). Computing discrete Fourier transform (DFT) onthe two series, horizontal spectrum F(u)¼DFT [xi] and verticalspectrum F(v) ¼DFT [yj] can be obtained where u (Hz) and v (Hz)are the horizontal and vertical spatial frequencies respectively.Variable k¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiu2þv2p

is the spatial frequency (or wave number) ofarbitrary direction and k is connected with wavelength l by therelation lp1=k [38]. Hence, propagation features of the stress wavecan be analyzed from the two spatial spectra.

The wavelength parameter in a dynamical process has a signifi-cant effect on rock damage [39,40]. In principle, the dynamical stressinduced damage on a tunnel of characteristic length D in the path ofa plane seismic wave with wavelength l is related to their relativemagnitude. If the size of the tunnel is negligible as compared to thewavelength, i.e. Dl�1approaches zero, the stress wave inducedloading on the tunnel is similar to the static loading; if the tunnelsize is infinite as compared to the wavelength, i.e. Dl�1approaches toinfinity, the theory of wave reflection of the propagating wavesapplies [39]. In this case, the adjacent rock mass will experiencedynamical impulsive stress with respect to the high frequencycomponents. In a spatial spectrum, therefore, high frequency com-ponent is the failure precursors. From Fig. 7a and d, the horizontalspectrum has higher frequency components as compared to thevertical spectrum, indicating that degree of damage is higher in thehorizontal direction than that in the vertical. The Fourier spectrum iscompletely symmetric in a mathematical sense. So either positive ornegative half plane can be used to perform the spectral analysis.Some minor distortions in the spectrum plots are due to computa-tional digital error.

Comprehension and interpretation of a thermogram rely on suchimage features as colors, borders and edges that segment differentzones in the image, as well as the temperature scale. The tempera-ture scale represents the incremental temperature relative to thebackground radiation as a result of the subtraction of the first framein infrared sequence as described in Section 4. In an infrared image,pseudo-colors are generally used to highlight the temperaturelevels. Hot (or positive) colors stand for high-level temperaturesand cool (or negative) colors for the low-level temperatures. A greatnumber of studies demonstrated that higher temperature corre-sponds to higher stress level due to friction, shearing or stressconcentration etc. and lower temperature corresponds to lowerstress level due to tensile cracking, stress release or unloading, etc.,[24–28,30,31,41]. The borders or edges that separate hot and coolcolored areas represent different modes of the rock behaviors. Forexample, in the de-noised thermogram (Fig. 7c), the red-coloredstrips formed by the scattered IRT distribution indicate the fric-tional effect and the localized cool-colored areas represent plasticdamage due to sudden expansion of the macrocracks.

6. Experimental results

6.1. Overall thermal response

Fig. 8 shows the evolution of /IRTS against face developmentwith respect to the time sequence. The capital letters E0–E6 stands

Stress release due to excavation-induced relaxation or crack

extensions

Majorcomponent

(35 Hz)

3rd major component

(72 Hz)

2nd major component

(55 Hz)

ΔT (°C)0.15

– 0.05

0 20 40 60 80 100 120 140 160

0.10

0.05

0

20

40

60

80

120

100

y, P

ixel

num

ber

(80, 60)

-800

20

40

60

80

100

|F (

u, 0

)|

Major component(33 Hz)

-600

20

40

60

80

100|F

(v,

0)|

Frequency, Hz

F (u) = DFT [xi]

F (v) = DFT [yj]

2nd major component(56 Hz)

Stressconcentration dueto frictional sliding

< IRT > = <f (x, y)>

-40 -20 0 20 40 60

x, Pixel number Frequency, Hz

-60 -40 -20 0 20 40 60 80

Fig. 7. Taking the denoised thermogram of the frame 587 corresponding to E6 as an example, the image analysis method used in our study is illustrated here: (a) the

vertical spatial spectrum F(v) obtained by 120 points DFT; (b) the mean value of the image matrix /IRTS; (c) the denoised thermogram, and (d) the horizontal spectrum

F(u) obtained by 160 points DFT. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

0

32.2

32.3

32.4

32.5

32.6

32.7

32.8

32.39 °C E5

32.59 °C

32.41 °C

32.44 °C

32.35 °C

32.31 °C

32.59 °C

32.73 °C

32.56 °C

32.53 °C

32.48 °C

32.34 °CP7

P6P5

P4

P3

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for the excavation steps over the full-face excavation (also seeFig. 3a) and P0–P7 stands for the excavation stages during the stagedexcavation (also see Fig. 3b). As mentioned above, the averageinfrared temperature field /IRTS represents the comprehensiveenergy dissipation level (overall thermal response to the excavation)of the model rocks. During the full-face excavation, /IRTS curve canbe approximated as linear elastic increase over E0–E3; the sharp dropof /IRTS immediately after the step 3 excavation represents africtional sliding event caused by the excavation; and after the IRTdrop, /IRTS resume the linear–elastic increase until E6. During the

staged excavation, /IRTS curve develops nonlinearly. At the excava-tion stages P0–P3, energy dissipated at relatively higher level. Fromthe /IRTS curve we see that removing #2 RB results in the highestenergy dissipation level and unloading effect represented by theimmediately sharp drop of /IRTS. Removing #3 RB also causedhigher level of /IRTS and a significant /IRTS drop. Thus, #2 and #3rock blocks can be regarded as the rock support in the excavationzone. At the excavation stages P4–P7, both the temperature level andthe /IRTS drops are smaller, indicating the fact that these RBs haveless effect on the excavation stability. Detail rock responses will be

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147 141

given by interpretation of the denoised thermograms in the follow-ing context.

6.2. Heat sources and thermal conduction

The coupled thermomechanical Eq. (1) provides a consistenttheoretical framework for interpretation and comprehension ofthe obtained thermograms. As pointed out by Luong [26,27], thedetected temperature change resulting from quite different phe-nomena represented by the four terms at the right-hand side ofthe Eq. (1), i.e. the heat sources, the thermal conduction effect, thereversible thermo-elastic coupling and intrinsic dissipation, mustbe correctly discriminated by such means as particular testconditions and/or specific data reduction which is the maindifficulty when interpreting the thermal images obtained fromexperiments under the usual conditions [27].

The first term on the right hand side of Eq. (1) represents theexistence of heat sources. The heat sources in our test come from theloading platen and the excavation method. Since the imaging zone isfar away from the loading platen and boundary load was keptconstant during the excavation, the generated undesirable heat bythe loading platen that may obscure the intrinsic dissipation isminor. As for the excavation method (see Section 2.5 and Fig. 4),the excavation tool will affect the infrared radiation temperaturefield at some specific excavation status, which will be analyzed later.

The volumetric heat capacity of the material C ¼ rCv is theenergy required to raise the temperature of a unit volume by 1 1C(or Kelvin). The second term governs the transference of heat bythermal conduction in which the heat passes through the materialto make the temperature uniform and ratio of the thermalconductivity to the heat capacity a¼ k=C, termed the thermaldiffusivity, will become the governing parameter in the dynamicalprocess [26]. Considering that both the thermal conductivity anddiffusivity for the rock materials is smaller than metals [27] andexcavation impact is transient, the heat conduction influence isnot global but only localized and transient.

6.3. Characterization of full-face excavation

For discrimination of the four different heat-generationmechanisms in the coupled thermomechanical equation, differ-entiating the thermo-elastic coupling and intrinsic dissipation areof central importance in comprehension and interpretation of aninfrared image. Although various test conditions were developedin the past decades, a marked advancement in this aspect may bedifficult especially in the community of RSRM (Remote SensingRock Mechanics). In this study as suggested by Luong [26], theimage processing based method was used for the differentiationof the thermoelasticity and thermoplasticity effects in the de-noised thermograms.

Fig. 9 shows the denoised thermograms (the second column)for the full-face excavation along with the horizontal (the thirdcolumn), the vertical (the fourth column) Fourier spectra, and theexcavation diagrams (the first column). Thermogram E0 corre-sponds to the intact state of the model and the E1 corresponds tothe step 1 excavation. From the two thermograms we can see thatthe IRT, i.e. the hot-colored and cool-colored areas, distribute witha scattering-random manner indicating the elastic response of themodel rocks. The components in the horizontal and verticalspectra distribute with a narrow band and very small amplitude.The thermoelastic response can be identified by these thermaland spectral features. The temperature change within elasticrange in a stressed solid is known as thermo-elastic couplingeffect [27] which is manifested by the third term on the right-hand side of Eq. (1). Within the elastic range, a material experi-ences a reversible conversion between mechanical and thermal

energy causing it to change the temperature under the conditionof adiabatic conditions.

Given that the material is isotropic and linear elastic, theintegral representation of the thermoelastic term is given by [34]for a plane stress state:

DT ¼�brCv

TD s1þs2ð Þ ð9Þ

Here, T is the surface absolute temperature of loaded solid (K), DT

is the temperature increment (K), b is the factor of linearexpansion (K�1), r is the solid density (kg m�3), Cv is the thermalcapacity of solid at normal atmosphere (J kg�1 K�1), and s1 ands2 are the two principal stresses of rock surface (MPa). Asanalyzed above, the IRT distribution pattern depicted in thermo-grams E0 and E1, i.e. the scattering-random distribution of IRT, canbe viewed as a typical manifestation of the thermoelastic beha-vior of the rock.

As illustrated in Fig. 1, the rock layer was constructed withelemental slabs and rock stratum was constructed with rocklayers. Excavation impact will cause static friction between theslabs, rock layers and rock strata. Thus the friction is a multi-scalephenomenon like in the loaded rock; friction action existsbetween mineral molecules, grains, joints, fissures and fractures[27,34]. Although /IRTS curve (Fig. 8) indicates the thermoelas-ticity for the comprehensive thermal response with respect toE0–E3 and E4–E6, the denoised thermograms present the detailedstructural information. That is, thermograms E0 and E1 representthe pure thermoelasticity manifested by the scattering-randomIRT; thermograms E2 represents the thermo-elastic behavior withweak static friction indicated by scattered IRT directionallydistributed as stratum-paralleled strips with low contrast; andthermograms E3–E6 represent the thermo-elastic behavior withstrong static friction illustrated by the higher contrast for the IRTstrips.

These findings demonstrate the heterogeneity and anisotropyof the steep-inclined stratified rocks. At the same time, themicroscopic damage induced by the static friction took place atthe very early excavation phase where no macroscopic displace-ment of the surrounding rock mass occurred since the excavation-induced tangential force not exceeding the critical resistanceforce as defined by Amontons’ law [42]. Fourier spectra alsocharacterize the degree of the excavation-induced damage by theenergy dissipation level represented by the amplitude of zero-frequency component (also known as ‘‘direct-current compo-nent’’) and major wave component (i.e. the distinct peaks in themid to high frequency bands). Higher amplitude for the direct-current component indicates higher-level of the thermal energyproduced by the thermal-mechanical coupling effect. More wavecomponents in the higher frequency bands denote the severeexcavation damage.

6.4. Characterization of staged excavation

For better understanding of thermal responses during the stagedexcavation, it is necessary to go back to Section 5 to talk furtherabout the /IRTS curve in Fig. 8. During the full-face excavation,tunneling was confined within the #1 RB with a small face area andno macroscopic displacement occurred in the adjacent rocks. Whileduring the staged excavation, the face was expanded in size asremoving of each of the RBs caused the macroscopic displacementsin the country rocks. This is why the time-marching scheme for/IRTS is multi-linear over the full-face excavation and non-linearover the staged excavation. The multi-linearity and non-linearity ofthe /IRTS profile are divided by point E6 at which #1 RB wasdestructed for the first time. Note that in the most cases /IRTS levelover the full-face excavation is higher than that over the staged

E0: intact state of the model; scattering-random IRT distribution represents the thermoelastic behavior; wave components in the Fourier spectra existed in narrow bands with very small amplitudes also indicating the thermoelasticity. .

E1: step 1 excavation;: the face was shallow; the scattering-random IRT distribution represents the thermoelasticity; the black arrow denotes the excavated zone from the back.

E2: thermo-elastic behavior with weak static friction by scattered IRT directionally distributed as strips with low contrast

E3: strip of the scattered IRT becomes clearer representing the thermo-elastic response with localized static friction.

E4: high contrast for the strip of the scattered IRT indicates the strong static frictional effect and the localized cool colored

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area indicates the excavation damage due to the displacement of #1 RB.

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horizontal spectrum 9F(u, 0)9 in the third column; and (d) the vertical spectrum 9F(0, v)9 in the last column; the black arrows denotes the excavation area from the back.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147142

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excavation. It is primarily due to the combined two factors includingthe intrinsic dissipation and the influence by the manned tunnelingoperation, and interpretation of which will be given in what follows.

The last term on the right-hand side of the coupled thermo-mechanical Eq. (1) manifests the intrinsic dissipation generatedby viscosity and/or plasticity [26,27]. The heat productionmechanism of internal energy dissipation caused by plasticdeformation was experimentally identified by research workssuch as in the Refs. [17–20,25–27,31–34]. It is generally acceptedthat in thermo-elastic-plasticity, not all the mechanical workproduced by plastic deformation can be converted to the thermalenergy in a solid, i.e. a portion of the work is spent in the changeof material microscopic structure [25]. The plastic deformationrelated dissipation involves fracturing and friction in our test.As production of new fractures, extension of the existed fractureas mode I crack need to consume energy, thus the temperaturechange for this part of the dissipation is always negative depictedin the thermogram as cool colors. Friction between grains, jointsand fractures of different scales as mode II cracks constitutes aprincipal part of the plastic deformation in the stressed rock. Thefriction-produced heat is always positive and depicted in thethermogram as hot colors.

During the staged excavation, the macroscopic displacement-induced frictional sliding involves:(i) the elemental slabs; (ii) therock layers and strata; and (iii) newly produced fractures of asmode II cracks when the local stress exceeding the elastic limit ofthe material. Detailed heat patterns of the plastic damage can beseen in the denoised thermograms shown in Fig. 10. ThermogramP0 depicts the instant when a small passage was firstly cut throughin #1 RB. For drilling a hole through, the operator needs to blow thechisel at his best at the back side of the model until the chisel wentthrough to the front side as an intense heat source which isdetected by the infrared camera. This strong heat source narrowsthe dynamical scope of the surface temperature field. In thethermogram P0, therefore, the cutting-through area was seen as ared-colored dot and elsewhere as deep blue colored. However, thereare still deeper-blue colored IRT strips seen in the thermogram P0

indicating the frictional effect. The direct-current wave componentand high-frequency components have high amplitude accountingfor high dissipation level and degree of the damage.

The plastic deformation, whereby the excavation-inducedfrictional sliding causes permanent changes globally or locally,is one of the most effective heat-production mechanisms. Ther-mogram P1 and P2 depict the thermal response with respect toremoving of #1 and #2 RBs. Higher level of the energy dissipation(the highest for P2) is the common feature for the two thermo-grams accounting for the higher degree of the plastic damagecaused by the excavation. The borders of the slip bands are not soclear, which denotes that the excavation-induced tangential forcedid not attain the critical value of the static frictional resistancedefined by Amontons’ law. Note that there is a minor dynamicsliding immediately after the excavation stage 2 (see Fig. 8).Whereas in thermogram P3, the borders of the slip bands are verysharp indicating the dynamic friction between rock layers definedby Coulomb’s law. In accordance with the removing of #3 RB, theexcavated face was expanded so large as to the relative sliding ofthe rock layers was initiated, which caused a significant drop inthe thermal temperature as illustrated in Fig. 8. The high ampli-tude wave components distribute in the broader band accountingfor the high-degree excavation damage induced by removing of#2 and #3 RBs (referred to as the ‘‘critical RBs’’).

In thermograms P4, the slip bands have more clear bordersindicating the dynamical frictional slipping effect of the rock layers.From the thermal patterns of the analyzed thermograms, it is clearthat with regard to the damage mechanism for the steeply dippedrock strata, the slip-induced plastic deformation is dominant.

In thermograms P5 and P6, the borders of the slip bands are notvery clear and the IRT distribution tends to become even, indicatingthe less degree of the excavation-induced damage. In thermogram P7,the IRT distribution come back to the scattering-random patternindicating the fact that the thermal–mechanical coupling effect forremoving of #7 RB (the last RB) is minor and the stress redistributionin the surrounding rocks was over. From the Fourier spectra, theprogressive development of the excavation-induced damage can alsobe identified according to the energy dissipation level (amplitude ofthe direct-current component) and the amplitude and number of themajor wave components in the broader frequency bands.

7. Discussion

Up to now, the published papers concerning the thermovision-based excavation experiments in the large-scale geological modelsin our institute include the excavation in horizontally inclinedstrata [17,18], in 451 inclined strata [19], and in vertically inclinedstrata [20]. The distinct structural responses under excavation forthese differently inclined strata were revealed by the thermogramanalysis. Considering the paper length, two typical excavationstatus are compared here including the step 2 tunneling duringthe full-face excavation and the stage 3 excavation during thestaged excavation. Fig. 11 shows thermograms and Fourier spectrawith respect to step 2 tunneling during the full-face excavation. Thecommon features of the thermal responses for these four geologicalstructures are the scattering-random IRT distribution exhibiting thethermoelasticity; and the corresponding Fourier spectra also denotethe thermoelasticity with small amplitude of the direct-currentcomponent (zero frequency component) and narrow band distribu-tion of the wave components.

Major differences of the thermal behavior for the four strata atstep 2 tunneling during the full-face excavation depicted in thethermograms include:

�

Horizontal strata in 1st row [17]: the scattered IRT in the mid-upper part of the thermogram representing the heterogeneousof the thermal response. That is, the horizontal strata behavelike an isotropic continuum solid subject to the external loadsand the heterogeneity comes from the stress concentration insuch microscopic structures as natural flows and joints.. � 451 inclined strata in 2nd row [19]: weak anisotropy of the

thermal behavior was indicated by the strata-paralleled IRTdistribution, but the IRT strip is not persistent showing thestatic frictional effect are not strong.

� 601 inclined strata in 3rd row: strong anisotropy of the thermal

behavior was illustrated by the persistent IRT strips.

� Vertically inclined strata in the last row [20]: strong anisotropy of

the thermal behavior was illustrated by the persistent IRT strips.

In simple word, at the state of thermoelasticity during the earlyexcavation phase, the horizontal strata showed weak heteroge-neous, the 451 inclined strata showed a weak anisotropy by theinconsistent static frictional bands, and the 601 inclined strata andvertical strata showed a strong anisotropy by the consistent staticfrictional bands.

Fig. 12 shows thermograms and Fourier spectra with respect tostage 3 excavation during the staged excavation. Main features of thethermal behaviors for the four physical models can be compared as:

�

horizontal strata: localized-heterogeneous plastic damagearound the face; � 451 inclined strata [17]: localized plastic damage with weak

slip bands indicated by the localized IRT distribution and non-persistent IRT strips;

Fig. 10. The denoised thermograms and Fourier spectra for the staged excavation: (a) excavation diagrams in the first column; (b) thermograms in the second column; (c) the

horizontal spectrum 9F(u, 0)9 in the third column; and (d) the vertical spectrum 9F(0, v)9 in the last column; the black arrows denotes the excavated area from the back. (For

interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147144

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in the first row; 451 inclined strata (after [19]) in the 2nd row; 601 inclined strata in the 3rd row, and vertical strata (after [20]) in the last row.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147 145

�

601 and vertically inclined strata [19]: strong slip bands con-stitute the major damage pattern represented by the persis-tent IRT strips parallel to the rock layers; � the strongest wave propagation occurred in the 601 inclined

strata, the second is in the 451 inclined strata, the third is inthe vertical strata, and the horizontal is the weakest [20]. Theintensity of the stress wave propagation corresponds to theseverity of the excavation-induced damage.

8. Conclusions

The procedure for processing of a thermogram was presentedinvolving removal of the environmental radiation noise by imagesubtraction, suppression of the salt-and-pepper noise by using amedian filter, and eliminating of periodical noise by using a Gaussianhigh pass filter. Borders and edges of the excavation-inducedfrictional sliding bands with changed stress represented by thechanged brightness can be distinguished clearly in the processedthermograms by utilizing the developed algorithm. The de-noisedthermograms are helpful to better understand different thermalbehaviors of the rock under excavation and the proposed algorithmmay be applicable in processing other noise-contained thermograms.

Such image features as means of the temperature field /IRTS,horizontal and vertical spatial Fourier spectra and de-noisedthermograms were extracted. Based on analysis of these features,different thermal-mechanical coupling processes as the reversiblethermo-elastic coupling and intrinsic dissipation for the 601inclined strata under excavation are discriminated, that is:

�

pure elastic represented by scattering-random IRT distributionwith no major wave components such as E0 and E1; � elastic with weak static friction indicated by scattered IRT strip

with dim contrast with minor wave components such as E2;

� elastic with strong static friction depicted by higher contrast of

the IRT strips with major wave components such as E3–E6;

� plastic with static friction at critical state represented by high

/IRTS with a sharp drop, localized IRT distribution and dimslip bands with more major wave components such as P2;

� dynamic frictional sliding indicated by the highest /IRTS with

a significant drop, and brightness of the slip bands, with mostnumber of the major wave components such as P3;

� localized dynamic frictional sliding depicted by the low /IRTS,

and brightness of the slip bands, with less number of the majorwave components such as P4;

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1st row; the 451 inclined strata (after [19]) in the 2nd row; the 601 inclined strata in the 3rd row, and the vertical strata (after [20]) in the last row.

W.L. Gong et al. / International Journal of Rock Mechanics & Mining Sciences 60 (2013) 134–147146

�

these thermal precursors can be used for the assessment of theexcavation damage and prediction of the deterioration of therock strength.

Comparison of this research on the thermal responses to thetunnel excavation with the published excavation experimental studiesin our institute were made and the major results, for example, can beconcluded as: (i) for step 2 tunneling during the full-face excavation,the horizontal strata [17] behaves like continuum with some weakheterogeneity; the 451 inclined strata [19] shows weak anisotropy bythe inconsistent IRT strips; the 601 and vertical strata [20] exhibit astrong anisotropy by the persistent IRT strips; (ii) for stage 3 duringthe staged excavation, the horizontal strata has a localized excavationaround the face; the 451 inclined strata has a weak slip bands withlocalized damage around the face; and the 601 inclined and verticalstrata have strong slip bands as the major damage form.

Acknowledgments

Financial support from the Special Funds for the National NaturalScience Foundation (Key Project) under Grant no. 51134005 is

gratefully acknowledged. The authors are so grateful to ManchaoHe, who developed the experiment concerned in this paper, for hisvaluable comments and suggestions, which guided us in the accom-plishment of the first draft. Last but not the least, we would like toexpress our sincerest gratitude to the anonymous reviewer for theirvaluable modification suggestions on the significant improvement ofthis article.

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