Stress changes ahead of an advancing tunnel

Ira. r Rock ,Mech. Min. $ci. & Geomeck. Abste. VoL 10, pp. 673-.697. [~¢llamon Press 1973. Printed in G ~

Britain

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL JOHN F. ABEL Associate Professor, Department of Mining and Geological Engineering, University of Arizona, Tucson, Arizona, U.S.A. and FtTZHUGH T. LEE Engineering Geology Branch, I.}.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, U.S.A. (Received 20 March 1973)

Abstract--lnstrmnentation placed ahead of three model tunnels in the laboratory and ahead of a crosscut driven in a metamorphic rock mass detected stress changes several tunnel diameters ahead of the tunnel face. Stress chanip:s were detected 4 dian~ters ahead of a model tunnel drilled into nearly elastic acrylic, 2.50 diameters ahead of a model tunnel drilled into concrete, and 2 diameters ahead of a model tunnel drilled into Silver Plume Granite. Stress chants were detected 7.50 diameters ahead of a crosscut driven in jointed, closely foliated i~rs:isses and gneissic granites in an experimental mine at Idaho Springs, Colorado. These results contrast markedly with a theoretical elastic estimate of the onset of detectable stress changes at 1 tunnel diameter ahead of the tunnel face. A small compressive stress concentration was detected 2 diameters ahead of the model tunnel in acrylic, !.25 diameters ahead of the model tunnel in concrete, and I diameter ahead of the model tunnel in granite. A similar stress peak was detected about 6 diameters ahead of the crosscut. No such stress peak is predicted from elastic theory. The 3-dimensional in siru stress determined in the field demonstrate that geologic structure controls stress orientations in the metamorphic rock mass. Two of the computed principal strcs.ses are parallel to the foliation and the other principal stress is normal to it. The principal stress orientations vary approximately as the foliation attitude varies. The average horizontal stress components and the average vertical stress component are three times and twice as large, respectively, as those predicted from the overburden load. An understanding of the measured stress field appears to require the application of either tectonic or residual stress components, or both. Laboratory studies indicate the presence of proportionately large residual stresses. Mining may have triggered the releaseof strain energy, which is controlled by geologicstructure.

INTRODUCTION THE interaction o f the geology and the in situ stress field within the rock ahead o f an advancing tunnel is one o f the perplexing problems in applied rock mechanics. Little is known of the behavior o f stress in this zone because inaccessibility makes instrumentation o f the zone difficult. The importance o f this zone to successful tunneling is obvious. The common contractual requirement that feeler holes be maintained ahead o f the face to determine rock conditions is testimony to its importance. The common legal requirement for feeler holes ahead and to the side o f entries advancing toward potentially dangerous accumulations o f water or methane in coal mines testifies to the dangers present in that zone. An investigation o f this problem was undertaken by the U.S. Geological Survey. The larger rock mechanics research project, o f which this investigation is a part, includes investigation of the interaction o f geology, tunnel and room excavation, and the in situ stress field. The laboratory work was accomplished at the U.S. Geological Survey in Denver, Colorado. 673

674

JOHN F. ABEL AND FI'rZHUGH T. LEE

The fieldwork was accomplished through the cooperation of the Colorado School of Mines and the use of its experimental mine at Idaho Springs, Colorado. This report contains the results of laboratory model studies under known applied stresses and the results of a field study. STRESS DISTRIBUTION AHEAD OF AN ADVANC~G TUNNEL Theoretical studies of stress conditions for cylindrical openings in an infinite elastic body have been performed [I] (pp. 9-10). However, the few previous field and laboratory measurements made indicate that in an in situ rock mass the stress distribution is at some variance with that postulated by elastic theory. GALLEand WILHOIT[2] (pp. 145-154), in their photoelastic frozen stress work on the stress distribution around a cylindrical bore, indicated that a stress and strain influence zone related to the bore (really a cylindrical tunnel) extends ! diameter into the photoelastic material ahead of the bore, ! diameter behind, and 2 diameters radially outward from the walls. This agrees with the results from a photoelastic study by AeEI. [3] (pp. 10-1 I) that indicate the influence zone extends !. 14 diameters ahead of the bore, 1.24 diameters behind, and 1.92 diameters radially outward. Most records of field studies obtained from measurements in rock masses differ from the anticipated elastic response. Neither the extent of the influence zone measured behind the face of a tunnel beading nor the extent of the influence zone measured ahead of an advancing tunnel, or longwall face, agrees with theoretical elastic or photoelastic analyses. In fact, field-deformation measurements seriously disagree with predicted elastic behavior. For example, in the Straight Creek Tunnel pilot bore [3] (p. 16) dynamic rock-mass deformations took place over a zone that was from 6 to 60 diameters behind the advancing face. Field measurements generally agree with the extent of the radial influence zone predicted elastically and as measured in photoelastic material [3] (p. 19). The distance ahead of the tunnel face in which the stress distribution is affected by the tunnel advance is essentially unknown. No measurements of rock deformation, or strain, or stress have been reported for the rock ahead of a tunnel. There are obvious practical difficulties in placing instruments in this region. A number of European investigators, however, have made convergence measurements ahead of advancing Iongwall mining faces. A longwaU face, on the order of 600-3000 ft in length and 3-9 ft in height, approximates the geometry of a tunnel in the vertical plane perpendicular to the iongwall face. The divergence of the reported results is informative in anticipating the zone of influence in the rock ahead of a tunnel where a strain sensor could conceivably detect strain.distribution changes associated with the tunnel excavation. Various investigators at the International Conference on Strata Control in Paris in 1960 reported their findings on the extent of the zone of influence as follows: POT'rS [4] (p. 252), Kolar Schist, India: Onset > 15-20 ft. DENKttAUS and Htct. [5] (pp. 247-248), quartzite, South Africa: Onset > 20 ft., Peak 15-20 ft. LEEMAN [6] (p. 308), quartzite, South Africa: Onset at 25 ft, Peak 12 ft. DE REEPEg and BnUENS [7] (p. 328), coal, Belgium: Onset at 15-90 ft. (Approx. 5 thicknesses of seam ahead of face.). CARTEg [8] (p. 471), coal, Great Britain: Onset < 216 ft, Peak > 46 ft. SPACKEt.Ea [9] (p. 483), coal, East Germany: Peak < 18 ft (probably < 12 ft). HENSN^W [10] (p. 484), coal, Great Britain: Onset 46 ft.

of C:ISI acrylic showing 34imensional boreholc probe grountrd into central grttges can be seen on entrrnal surf’xe. Am F. strain gages.

R..M. f.p_ 67-l]

core hole. Strain

FIG. X. Granite labontory

tunnel model showing grouted sensor. Tunnel indicate loading direction.

drillcd into rear side. Armwr

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

675

LABORATORY LNVESTIGATIONS Tests with a solid-inclusion borehole probe described elsewhere [i !] were made in the laboratory using three model materials, acrylic, concrete, and Silver Plume Granite. The purpose of these tests was twofold: (I) to provide information necessary to compare subsequent measurements obtained with the probe in the field with those postulated from elastic analyses, and (2) to detect the 3-dimensional stress changes ahead of advancing cylindrical tunnels, actually drill holes, in blocks of the three materials (Fig. l ). The three materials employed in the model testing program were chosen to represent a range of elastic properties, from an ideally elastic material (acrylic), to a heterogeneous elastic artificial rock (concrete), to an approximately elastic brittle rock (granite). These tunnel models differ from an actual tunnel in several respects. The primary difference is the absence of a semi-infinite body of adjacent material restraining the model. The absence of this restraint resulted in lateral (horizontal) tensile strains that were measured in all three models. The estimate, using elastic theory, of the lateral stress equivalent to these lateral strains is [12] (p. 474): ~'u -----Induced horizontal tension v = Poisson's ratio ~',4 = Applied vertical stress v

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in addition, the models were subjected to external loads and not to body loads. The model tunnels were smooth-walled cylindrical boreholes, unlike conventionally excavated tunnels, although perhaps similar to those drilled with a boring machine (mole). For each increment of tunnel advance in the model the stress distribution was obtained by removing the calibrated block of material from the hydraulic press, drilling the increment of tunnel advance specified, replacing the model in the press, and successively loading and unloading the model. Norm:ally two load cycles were sufficient to demonstrate both repeatability of stress distribution and lincarity in the stress-strain response in the probe vicinity. It was not physically possible to drill the model tunnels under load. Any increase or decrease in the stresses measured ahead of the advancing tunnel are related only to changes of model tunnel length, which occurred between model Ioadings. The model tunnel in the acrylic was advanced with a twist drill. The concrete and granite model tunnels required the use of a diamond drill. The drilling introduced a thermal disturbance in the model, which necessitated waiting to verify stability after each increment. The model loading was adjusted to maintain the same average stress in the model after removal of each tunnel increment. All model testing was done under room conditions. ACRYLIC MODEL A model was constructed of a single block of cast acrylic--a uniformly isotropic elastic material--19 in. high, 13 in. wide, and 12 in. deep. A 1.88-in. dia. hole was drilled into the 13 x 19-in. face in order to leave the maximum material between the walls of the hole and the boundary of the block. The model material has a tensile elastic limit of 12,000 psi, elastic modulus (E) of 0.327 x 10" psi, and a Poisson's ratio (O of 0.39. A spherical steel sensor(the measuring element of the solid-inclusion borehole probe), with E = 30-5 x l06 psi and v = 0.285, was epoxied into the acrylic model. The epoxy has E = !.55 x 106 psi and v = 0.39 [I I] (pp. CI5-C!7).

676

J O H N F. ABEL A N D F I T Z H U G H T. LEE

The acrylic model was tested under an average uniaxial stress of 1950 psi prior to drilling the tunnel. The resulting strains measured by the sensor prior to tunneling were equivalent to 2100 psi in compression in the direction of loading (vertical), 1000 psi in tension normal to the axis of the borehole in which the sensor was epoxied, and 1800 psi in tension parallel to the axis of the borehole. The 1-88-in. dia. tunnel was advanced in the direction arbitrarily chosen as N. 45 ° E. by drilling into the side of the acrylic model opposite the sensor implacement hole. The tunnel was advanced in increments; the stresses were measured after each advance. *SOC

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FIt;. 2. Ch:mges in principal stresses in acrylic model.

TAIII.E I. AVERAGE STRE.~$ES DETERMINFD AIIEAD

( }- ~ COMPRES~IVE SrRE~S; . . . . Maximum principal stress

OF TUNNEL FACE IN A('RYLIC BLOCK TrNSILE STRES.S)

Intermediate principal stress

Minimum principal stress

Dia. from

sensor

Magnitude

Bearing

center

(psi)

(')

Plunge {r )

Magnitude (psi)

4-47 3.93 3.39 2.86 2.32 2.05 1.51

~2100 i 2100 ,~2170 ~2190 -.- 22(~0 ~-2300 +2210 +2180 -! 2290 -i-2320 +2570

N. 66 W. N. 65 W. N. 65 W. N. 66 W. N. 65 W. N . 6 3 W. N. 7 0 W . N. 7OW. N. 6 9 W . N. 66 W. N. 6 5 W .

74 73 73 73 73 73 73 72 72 72 71

-1000 - 980 -IO40 -- 9t.~O - IO20 -1020 --1040 -- IOO0 - 980 .... IOOO --1030

1.24 0.97 0.70 0-43

Bearing (") S. S. S. S. S. S. S. S. S. S. S.

39 E. 36 E. 38 E. 36 E. 36 E. 36 E. 35 E. 33 E. 32E. 30 E. 25 E.

Plunge ('') 14 15 15 15 15 15 14 14 14 14 15

Magnitude

Bearing

(psi)

(')

Plunge ("1

N. 50 E. N. 52 E. N. 50 E. N. 52 E. N. 52 E. N. 52 E. N. 52 E. N . 5 4 E. N. 55E. N. 57 E. N. 62 E.

7 8 8 8 8 8 IO t0 I0 I0 12

- 1820 - 1750 -1820 -1780 -1790 -- 1780 --1930 -- 18nO -1800 - 18011 -18(0

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

677

As the tunnel in the acrylic model was incrementally advanced tov, ard the probe, the probe sensed an increase in compression in the direction of loading. The principal stress changes in the plane normal to the direction of loading were very. small. The directions ofthe principal stress changes remained essentially unchanged (Table 1, Figs 2, 3). NOfeTI.O

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I::t(;. 3. Ik:aring and ph, ngc of principal stresses in acrylic model (lower-hemisphere, equal-area projection). +~200

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J O H N F. ABEL A N D FITZHUGH T. LEE

678

Stresses estimated photoelastically by GALLE and WILHOIT [2I, in a smaller but similarly loaded plastic block, ahead of a 1.25-in. dia. bore are in good agreement with the stresses measured by the probe fi)r a 1-50 tunnel diameter interval between the probe and the tunnel face (Fig. 4). Some divergence exists, however, between the stresses measured when the face was between !.50 and 2.25 tunnel diameters. CONCRETE MODEL

A second model, of the same size and shape as the acrylic model, was constructed of high-strength concrete. This concrete has a compression strength of 7580 psi, an elastic modulus (E) of 3.3 × 106 psi, and a Poisson's ratio (v) in the stress range used in the testing of 0.26. A brass spherical sensor, which is more sensitive than the steel sensor, was employed. The elastic modulus (E) of the brass sensor is 15 ::< l06 psi and its Poisson's ratio (,,) is 0"285. The epoxy grout used was carborundum-filled and had an E o f 0 . 4 5 ;< 106 psi and a v of 0.28. The concrete model was tested under an average uniaxial stress of approximately 13 l0 psi after a tunnel of 1"88-in. dia. and 1.75 in. length had been driven. The resulting average strains measured by the sensor were equivalent to approximately 1420 psi in compression in the direction of loading, 500 psi in tension normal to both the loading direction and to the axis ofthe model tunnel, and 150 psi in tension parallel to the axis of the model tunnel. As the tunnel in the concrete model was incrementally advanced east toward the probe, the probe sensed an increase in compression in the direction of loading. The principal stress change normal to both the borehole and the direction of loading remained essentially * 5C,9

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FK;. 5. Changes in principal stresses in concrete model.

STRESS CHANGES A H E A D O F AN ADVANCING TUNNEL

679

unchanged, whereas the stress change parallel to the axis of the advancing tunnel and normal to the direction of loading underwent a small tensile stress change. The directions of the principal stress changes were essentially constant (Table 2. Figs 5, 6). NORTH

WEST

IrAsT

SOUTH

Fic;. 6. Bearing and plunge of principal str~.%,u,~in concrete model llower-hemisphere, equal-area projection).

TABt.E 2. AVERAGE STRF~t;S D E T E R M I N E D A H E A D OF TUNNEL FACE IN C O N C R E T E BLOCK (-~

:= COMPREX~;IVE S T R E S S ; ~

Maximum principal stress

=

TENSILE STRESS)

Intermediate principal stress

Minimum principal stress

Dia. from

sensor

Magnitude

Ikaring

center

(psi)

C)

(')

(psi)

31 W. 30 W. 30W. 32 W. 34 W. 36 W. 42 W.

82 82 82 83 83 82 83

-140 -140 -170 - 100 - 70 -170 -220

2-66 2.00 1.45 1-18 0-91 0.64 0.37

~ 1420 -,*-1580 il570 + 1640 + 1610 +1670 +1870

S. S. S. S. S. S. S.

Plunge Magnitude

Ikaring

C)

Plunge Magnitude

(')

Bearing

Plunge

C)

()

(psi)

N. 5 W.

7

-510

N. 86E.

5

N. N. N. N. N. N.

7 6 6 5 6 4

--540 -610 -580 -020 -710 -740

N. N. N. N. N. N.

5 5 4 5 5 5

5 W. 7W. 5 W. 6 W. 6W. 7W.

86 E. 84 E. 85 E. 85 E. 84 E. 83 E.

The results from the concrete model in the direction of applied uniaxial loading (Fig. 7) agree rather closely with the elastic results obtained by GAU.E and WILttOIT[2] (p. 153) in the region less than 1 tunnel diameter to the face of the advancing tunnel. The stress-strain response of the concrete model was remarkably similar to that of the acrylic block.

680

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FIG. 7. Maximum principal stress in concrete model compared to the results o['GALLEand WILIlOIT[2] (p. I.~3). GRANITE MODEL The third tunnel model (Fig. 8) consisted o f a 2-07-in. dia. tunnel drilled into the side o f a block o f Silver Plume Granite 12 in. high, 17 in. wide and 17 in. deep. Concrete platens, 4 in. high, 13 in. wide and 12 in. deep, were epoxied to the sides o f the granite, extending its height to 20 in. overall. The concrete platens were diamond-sawed from the concrete model, the properties o f which were discussed previously (E ~ 3.3 < l0 t' psi, v = 0-26). The elastic properties o f the granite, as determined from specimens removed from the block after testing, were quite close to those o f the concrete, E -- 4-5 ;< l0 ~' psi, v := 0-23. The ell'cot o f the lower still'hess :rod higher Poisson's ratio o f the concrete platens would bc to increase the lateral tensile strain in the granite at the granite-concrete interface approximately i 1.7 p~-in/in,, or 53 psi per 1000 psi o f applied uniaxial (vertical) stress. This is in addition to tensile stress o f 295 psi per I(X)O psi applied vertical compressive stress, anticipated from the absence o f restraint for the model. The probe did not sense such an increase in latcra +200,

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FK;. 9. Changcs of principal stresses in granite model.

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STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

681

tensile stress. Therefore the lateral tensile stress in the granite must have been restricted to the immediate vicinity o f the platen-rock interface. A spherical steel sensor was employed for the probe in the granite model. The elastic modulus (E) of the steel sensor was 30.5 × l0 s psi and v ---- 0-285. The epoxy grout used was the same as that used in the concrete model, E = 0"45 × 106 and v = 0"28. The granite model was tested under an average uniaxial vertical stress of ! 155 psi. A model tunnel 2.07 in. dia. and 2 in. long was driven north into the model before the model was tested. The resulting average strains measured by the steel sensor were equivalent to approximately 1380 psi in compression in the direction of loading, 90 psi in tension 40 ° clockwise from the model tunnel axis and in the plane normal to the loading direction, and 690 psi in tension 50 ° counterclockwise from the model tunnel axis and in the plane normal to the loading direction. As the tunnel in the granite model was incrementally advanced toward the probe, the probe once again sensed an overall increase in compression in the direction of loading (Fig. 9). Both principal stresses in the plane normal to the direction of loading were tensile and their directions remained constant throughout the tunnel advance (Table 3, Fig. 10). NOmTH b

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FIG. 10. Bearing and plunge of principal stresses in granite model (lower-hemisphere,equal area projection). The stresses in the granite model within 0.75 tunnel diameter ahead o f the face (Fig. 1 I) differ only slightly from those predicted elastically [2] (p. 153). The shape of the applied loading stress curve is almost exactly the same as that obtained from the acrylic model, except in this case the percentage concentration of the applied stress ahead o f the advancing tunnel is slightly lower and this stress concentration region is not as extensive. The zone o f tunnelinduced stress concentration is limited to slightly less than 2 diameters ahead o f the tunnel

J O H N F. ABEL A N D F I T Z H U G H T. LEE

682

T A B L E 3. A V E R A G E STRESSES DETERMINED A H E A D OF T U N N E L FACE I N G R A N I T E BLOCK ( ~- ~ COMPRESSIVE STRESS; - - : TENSILE STRESS)

Maximum principal stress

Intermediate principal stress

Minimum principal stress

Dia from sensor

center

Magnitude (psi)

3"53 2"57 2"09

1"61 1"37 1"13 0"89 0"65 0"41

--1390 -1370 ,1380 - 14OO -1440 -1470 --1500 --1480 --1560

Bearing C) N. N. N. N. N. N. N. N. N.

*2200

Plunge (=)

Magnitude (psi)

Bearing C)

Plunge C)

Magnitude (psi)

81 80 81 80 80 81 80 82 84

--100 -- 90 -- 90 -- 90 -130 -140 -170 -160 -190

S. 43 W. S. 4 4 W , 5.42W. S. 41 W. S. 4 1 W . S. 4 1 W . S. 4 0 W . S. 39 W. S. 39 W.

6 6 7 8 8 7 8 5 4

-670 -690 --700 --740 -750 -760 -760 --830 -840

5 W. 5W. 6W. 4 W. 3W. 6W. 4W. 12W. 13 W.

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Bearing (°) S. S. S. S. S. S. S. S. S.

Plunge (°)

47 E. 47E. 49E. 50 E. 50 E. 50 E. 51 E. 51 E. 52 E.

7 7 6 7 7 7 7 6 5

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Fu ;. I I. Maximum principal stress in granite model compared to the results of GALLE and WILHOIT [2] (p. | 53). T A n L E 4. MODEL M A T E R I A L PROPERTIF.S

Model

Modulus (psi/in. × 106) (in.)

Poisson's ratio

Compressive strength (psi)

Host material properties 0-39 18,000 0.26 7580 0-23 27,500

Acrylic Concrete Granite

0.327 3-30 4-50

Acrylic Concrete Granite

30.5 (steel) 15-0 (brass) 30-5 (steel)

Acrylic Concrete Granite

Grout properties i.55 (steel filler) 0.39 0-45 (carborundum filler) 0.28 0"45 (carborundum filler) 0.28

Sensor properties 60,000 0.285 0.285 30,000 60,000 0.285

Tensile strength (psi)

Maximum crystal or particle size (in.)

12,000 485 880

Amorphous I 0-25

STRESS C H A N G E S A H E A D O F A N A D V A N C I N G T U N N E L

683

face for the granite (Fig. I 1) compared to 4 tunnel diameters for the acrylic model (Fig. 4) and 2.50 tunnel diameters for the concrete model (Fig. 7). These differences probably result from the differences between the model materials, which are summarized in Table 4. The acrylic is very nearly ideally elastic but of low modulus. The concrete is locally heterogeneous but in bulk elastically isotropic with an intermediate modulus. The granite is elastically anisotropic but is homogeneous and of high modulus. The relative variability of the three model materials is indicated in Table 5, which shows the variation between measured and plane-strain calculations of Poisson's ratio. The concrete is the most variable and the acrylic the least. TABLE 5. AVERAGE LATERAL (HORIZONTAL) STRESSES(o u) WITH CALCULATED AVERAGE POISSON'S RATIO~ (v) UNDER PLANE-STRAIN ELASTIC CONDITIONS

Acrylic

Concrete

G ran ite

Dia. from

S¢llsor center 4-47 3"93 3"53 3"39 2"86 2.66 2-57 2-32 2.09 2-05 2-00 1-61 1"51 I "45 1"37 1"24 1"18 1-13 0.97 0.91 0"89 0.70 0"65 0.64 0"43 0"41 0"37

oH

oH

(psi)

v

-- 1410 - 1360

0.40 0.39

-- 1430 - 1380

0.40 0.39

(psi)

--320 - 1400

0-38

-- 1400

0.38 -540

- 1480

- 1440

Mean --1410 Standard deviation 30 Measured v* Difference from measured v (%) Coefficient of variation (%) 2"i

-- 385

0"22

--390

0"22

--395

0"22

--415

0"23

--440

0-23

--450

0"23

--465

0"24

--495

0'25

-515

0-25

0-18

0"25

0-28

0"26

0-38 --620

- 1400

,,

0"40 --580

- 1420

(psi)

0"40 -610

- 1430

ou

v

0"28

0.38 --710

0-30

--740

0"28

0.387

-590

0-261

-440

0.232

0.013 0-390

140

0-039 0-260

45

0.012 0'228

0-36

-0.8 3.4

~ 0.4 23.7

14.9

-~ I-8 10-2

5-1

* Poisson's ratios measured on specimens taken from models after test.

684

JOHN F. ABEL AND FITZHUGH T. LEE

The stress-strain response of the granite tunnel model was remarkably similar to that observed for the acrylic and concrete models. In the granite model, however, the intermediate and minimum principal stresses were not alined either parallel or normal to the tunnel axis. This stress orientation variation is believed to be the results of the anisotropy of the granite. EVALUATION OF MODEL RESULTS Several aspects of the test results warrant critical evaluation. These are primarily related to the differences and similarities between the stress patterns obtained from the three models. The aspects that need to be considered in detail are: (i) the maximum principal stress at the sensor exceeded the average stress applied, (2) the small stress concentration that preceded the predicted elastic stress rise, and (3) the magnitude and variation in the tensile stresses acting normal to the applied compressive stress. PROBE STRESS IN EXCESS OF AVERAGE APPLIED STRESS Prior to the onset of stress change related to the model tunnel, the maximum principal compressive stress sensed by the probe in the acrylic model was 2100 psi compared to 1950 psi average applied compressive stress; in the concrete model 1420 psi compared to 1310 psi average applied compressive stress; and in the granite model 1380 psi compared to 1155 psi average applied compressive stress. This is a 7.7 ~'i~stress increase for the acrylic model, 8-4 % increase for the concrete, and .It)-S"'/,, increase for the granite. These increases can be explained as the result o f dcstressing at the unrestrained surfaces of the models. Tile surface destressing of the acrylic model has been described [I I] (p. C20); the stress decreases approximately 30% from the center of a face to the corner of the model. GALLE and WtLnorr [21 (pp. 153, 155) showed a similar surface destressing phenomenon. The loss of load-carrying capability near the surfaces and edges of the models had to be compensated. This loss of load was obviously, and logically, picked up by the more conlincd interior of the models, where the spherical sensors were placed. The acrylic model, which hvd the lowest still'hess of the three model materials, had the lowest increase in maximum stress concentration. The granite, the stillest model material, had the greatest maximum strc:~s concentration increase. The siz:tblejump in the maximum stress concentration for the granite may be the result of greater anisotropy and dccrc:tsed linearity of the stress-strain relationship for the natural rock (granite) in comparison to the artificial rock (concrete) :lnd the elastic amorphous material (acrylic). STRESS CONCENTRATION PRECEDING TI-IE ADVANCING TUNNEI, The stress concentration ahead of an advancing tunnel in an elastic material should be confined to a zone within, at most, 2 diameters ahead o f the advancing tunnel, it can be sccn on Figs 4, 7 and 11 that the experimental data correspond reasonably well to that obtained photoelastically by GALLt~and WlLtIOST[2] for approximately I diameter ahead of the model tunnel face (Fig. 12). Between i and 3 diameters ahead of the advancing model tunnel face there is a zone of anomalously high stress concentration. The high-stress zones measured in the models cannot be ascribed to instrument error. An indication of potential instrument error can be obtained from the standard deviations o f the intermediate and minimum principal stresses. These stresses should not be primarily the result of actual tunnel-driving and related load stresses and should not, therefore, be greatly affected by the approaching tunnel. The pooled standard deviations of the intermediate and

STRESS C H A N G E S A H E A D O F A N A D V A N C I N G T U N N E L

685

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(p. 153).

minimum principal stresses were 39 psi for the acrylic model, 70 psi for the concrete model. and 52 psi for the granite model. The plot o f these standard deviations on Fig. 4 for the acrylic model, Fig. 7 for the concrete model, and Fig. I i for the granite model demonstrates that these subsidiary higher stress zones owe their existence to a real stress concentration, and not to instrument errors. An onset o f stress increase at about 2-5 tunnel diameters ahead o f a model tunnel face agrees approximately with the field evidence cited earlier (p. 674). The modcl used by GALLE and WtLHOIT [2] was o f too limited an extent, because their models extended only 2-25 diameters ahead o f their model tunnel face. The onset of elastic stress concentrations occurs 2-5 diameters ahead o f the model tunnels. This stress buildup is not continuous while increasing to the elastically predicted stress of approximately 1-50 times the average, before tunneling stress. A decrease in stress occurs prior to the final stress rise at the tunnel face. In order to explain this phenomenon, one must consider confinement or decrease in the thickness o f restraining model material in the direction of tunnel advance. The decrease in confinement in the direction of the advancing tunnel would produce a decrease in stress in the direction o f tunnel advance. The increase in maximum principal compressive stress in the loading direction should result in a small additional increase in the intermediate and minimum tensile (Poisson's) stresses norm:d to the loading direction. This release o f restraint in the direction of the approaching tunnel should tend to increase the tensile strain and, therefore, tensile stress in that direction. This directional release of restraint should have little effect on the measured strain normal to the direction of the advancing tunnel. A minor increase in tensile stress in the direction of the approaching tunnel in the acrylic model accompanies a small decrease in the maximum principal compressive stress. This occurs in the zone from I to 2 tunnel diameters ahead of the tunnel (Table I, Figs 2, 3). The lateral stress normal to the direction o f the approaching tunnel shows a minor decrease in tension as anticipated from the Poisson effect. ~o~c 1016--N

686

J O H N F. A B E L A N D F I T Z H U G H T. LEE

An increase in the lateral tensile stresses occurs in the direction of the approaching tunnel and a decrease normal to the approaching tunnel occurs within the concrete model in this zone of decreased maximum principal compressive stress, from 0-75 to 1.25 tunnel diameters ahead of the advancing tunnel (Table 2 and Fig. 5). These results are similar to those obtained with the acrylic model. The tensile intermediate principal stress decreases and the tensile minimum principal stress increases in the granite model (Table 3 and Fig. 9). The average lateral tensile stress increases in a zone between 0.75 and 1 tunnel diameter ahead of the advancing tunnel. The granite model differs from the other two in that the intermediate and minimum principal stress directions are not oriented normal and parallel to the tunnel axis. The granite was anisotropic in the horizontal plane, having a specimen stiffness of 4.89 and a Poisson's ratio of 0.210 in the intermediate principal stress direction and a stiffness of 4.85 and a Poisson's ratio of 0.244 in the minimum principal stress direction. The influence of the anisotropy in Poisson's ratio could account for an elastic (v/1 -- v) difference of about 80 psi in the intermediate and minimum principal stresses in the granite. The average actual difference was approximately 590 psi, or about seven times larger than can be explained by elastic anisotropy. No obvious explanation exists for the magnitude of the nonsymmetric lateral stress response in the granite. The directions of the intermediate and minimum principal stresses, however, are in agreement with those postulated elastically. The average lateral stress determined for the granite model is elastically reasonable (Table 6). The ratio of average lateral stress to average longitudinal stress increased with the increasing longitudinal stress that accompanied the advance of the tunnel. This implies a small increase in Poisson's ratio with increasing uniaxial stress. During testing of granite specimens an increase in Poisson's ratio from 0-20 to 0"24 was measured from 0 to 10,000 psi. No change in Poisson's ratio, however, could be detected over the small stress range measured in the granite model (1370-1560 psi). TABLE 6. CHANGE IN LATERAL TENSILE s'IrRE.~g IN GRANITE

Dia from

Measured ratio

Predicted ratio"

center

av a lat./a long.

av a lat./~ long.

Per cent dilrerencet

3.53 2.57 2.09 1"61 1"37 !" 13 0.89 0.65 0-41

0.275 0"282 0"286 0.300 0-306 0-307 0.308 0.335 0.33 !

0.295 0.295 0.295 0.295 0"295 0-295 0"295 0.295 0-295

- 6.8 -- 3.4 -- 3.0 + 1.7 +3"7 +4" 1 +4.4 + 13.6 + 12"2

sensor

Mean difference = 2.9% Standard deviation = 7.5 % 95 per cent confidence interval --- -- 2-9-8"7 * "Based on absence of restraint, elastic relations and physical specimen tests. t (Mcasu~_.dd ratio-predicted ratio) × \ Predicted ratio !00.

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

687

The release of confinement, represented by the approaching tunnel, appeared to be the dominant factor in the behaviour of the acrylic model. Poisson's strain was apparently more important in the concrete model, but release in confinement was present. The response of the granite appeared to be related more to anisotropic variations in the granite than to release of restraint related to tunneling. In all three models the maximum tensile (minimum principal) stress developed in the direction of the material most capable of carrying it--in the direction of the stiffer epoxy grout in the acrylic model, in the direction oftbe lower Poisson's ratio and stiffer concrete in the concrete model, and in the direction of lower Poisson's ratio and greater stiffness of rock in the granite model. Indicated anomalous stresses cannot be ascribed to instrument error, and an anomalous condition exists that cannot be supported by available elastic theory and measurements. It can be seen from Fig. 12 that a stress peak of the type measured could not have been detected at the stress levels contoured nor were their models sufficiently large to have included this entire overstress zone. Therefore the apparent elastic contradiction is no longer absolute. These tests demonstrate that limiting geometric boundary conditions strongly influence elastic solutions. TENSILE STRESS NORMAL TO LOADING The high magnitude of induced tensile stresses that the probe sensed in the plane normal to the axis of applied loading must be explained with regard to their magnitudes and also their variation with respect to the axis of the model tunnel and implacement borehole. Magnitudes of these tensile stress changes are shown in Figs 2, 5 and 9; their bearing and plunge are shown by equal-area projection in Figs 3, 6 and 10. The approximate load-induced tensile stresses in the acrylic block, before tunneling, were 1800 psi parallel to the direction of thc implacement borehole and 1000 psi normal to the implacement borehole; in the concrete block 2"50 tunnel diameters ahead of the sensor they were 150 psi parallel to the direction of the implacement hole and 500 psi normal to the implacement borehole; in the granite block, in the zone from 1-50 to 3.50 tunnel diameters ahead of the sensor, they were 90 psi 42 ° clockwise from the axis of the implacement hole and 700 psi 49 ° counterclockwise from the axis of the implacement hole. The restraining stress necessary to prevent elastic tensile strains from developing in such uniaxially loaded isotropic elastic bodies is equal to v i i - v times the applied stress [12] (p. 474). in the acrylic block the restraining compressive stress necessary to prevent that block from deforming laterally is: 0-39 \ I (2100 psi) = 1340 psi. I -- 0.39/ In the absence of any restraining stress, the uniaxially loaded body will be subject to tensile strains in all directions normal to the loading direction. These strains will place the unrestrained material under an apparent tensile stress [I 3]. The magnitude of the tensile stress, we believe, should be the inverse of the perfect-restraint confining compressive stress calculated above. The elastic isotropic model (acrylic), without any epoxy inclusion, should be subject, at its center, to an equal tensile stress of approximately 1340 psi in all horizontal (lateral)

688

JOHN F. ABEL AND FITZHUGH T. LEE

directions. Similarly, the lateral, or horizontal, concrete model stress should have been equal to about 500 psi in tension and it should have been uniform in all directions: (

0-26 ~ (1420)= 500 psi. I -- 0.26/

The granite model should be subject, at its center, to a lateral tensile stress of 395 psi: (i 0"23 ----O"231 (1320) ---- 395 psi. if such a model could not sustain the tensile stress level, the block would fail, producing fractures parallel with the uniaxial longitudinal loading direction. The average lateral stress measured by the sensor at the center of the acrylic-epoxy model, which is nonisotropic owing to the epoxy inclusion, was approximately 1400 psi in tension prior to the start of model tunneling. The tensile strength of the acrylic (12,000 psi) greatly exceeds this stress. The tensile strength of the high compressive strength concrete was estimated to exceed 800 psi [14] (pp. 5-6). Tensile tests performed on concrete cored from this model indicated a tensile strength of 485 psi. In view of the low tensile strength of the concrete the uniaxial compressive stress applied to the concrete model had to be well below its compressive strength (7580 psi) in order to prevent tensile failure of the concrete. The average lateral tensile stress measured in the nonisotropic concrete-epoxy model was approximately 325 psi at the time tunneling commenced. The average lateral tensile stress measured by the probe at the center of the granite model prior to the onset of tunneling was 395 psi, which is considerably less than the 880-psi tensile strength determined by physical specimen testing. The individual elastic tensile stresses that should be acting laterally at the sensor location in these unrestrained models can be estimated. First, the composite stiffagsses of the material must be estimated parallel to and normal to the cylindrical epoxy-filled probe hole. The composite moduli and Poisson's ratio were calculated for the directions of the applied load, for the direction parallel to the centerline of the probe hole, and for the direction normal to both the applied load and probe-hole centerline by the method presented by NICHOLSet al. Jill. For the acrylic model: Effective modulus in loading direction El = 0.330 × l06 psi. Effective modulus parallel to centerline of probe hole and normal to loading direction E2 = 0-706 × l0 s psi. Effective modulus normal to centerline of probe hole and loading direction E---0-347 × l06 psi. Poisson's ratio (all materials) ~, ----0-39. Strain in loading direction: 2100 __- 6370 ~.-in.. 0-330 m. Strain normal to loading direction: 6370 (0.39) = 2480/~-in. in. Effective tensile stress parallel to probe-hole centerline: 2480 (0.706) ---- 1750 psi.

STRESS C H A N G E S A H E A D O F A N A D V A N C I N G T U N N E L

689

Effective tensile stress normal to probe-hole centerline and loading direction: 2480 (0-347) ---- 860 psi. These calculated values compare with measured tensile stresses of 1800 psi parallel to the probe-hole centerline and 1000 psi normal to the probe-hole centerline and the loading direction in the acryfic model. A similar calculation was performed for the concrete model. The lateral stress calculations are dependent on the elastic constants for the directions of the principal stresses: Effective modulus and Poisson's ratio in the direction of the maximum principal stress Et = 3"18 × 106 psi vt ---- 0-261. Effective modulus and Poisson's ratio in the direction of the intermediate principal stress (parallel to probe centerline), E z ----- 2"00 X 106

psi

v2 = 0-269. Effective modulus and Poisson's ratio in the direction of the minimum principal stress (perpendicular to probe centerline), Ea = 3.12 × l0 s psi v3 ---- 0-261. 1420 /x-in. Strain in loading direction: - ~ = 447 "in. " Strain normal to loading direction (unrestrained condition), 447 (0.265) = 118 ~..-m.. in.

Effective tensile stress parallel to probe-hole centerline, i l8 (2.00) = 236 psi. Effective tensile stress normal to probe-hole centerline and loading direction, i 18 (3" 12) = 368 psi. These calculated values compare with measured tensile stresses of ! 50 psi parallel to the probe-hole centerline and 500 psi normal to the probe-hole centerline and loading direction. in the granite model the intermediate and minimum principal stress directions are controlled by the anisotropy of the granite, rather than by the orientation of the probe emplacement hole. C O N C L U S I O N S F R O M M O D E L STUDIES

The model studies indicate stress influence ahead of a tunnel face similar to the reported extent of stress influence in the rock ahead of the face of advancing underground IongwaU workings (see p. 674). These model studies also show the limitation of the application of photoelasticity in predicting the extent and magnitude of the stress distribution ahead of an advancing tunnel. Photoelastic model studies underestimate the extent and the magnitude

690

J O H N F. A B E L

AND F I T Z H U G H T. LEE

of the stress distribution at distances greater than 1 diameter ahead of an advancing model tunnel. The influence of heterogeneity and rock anisotropy was also demonstrated. The results show that the probe measures reasonable and reproducible stress levels over a range of known applied stresses, within three materials of widely different physical properties. The results also show the influence of restricted boundary conditions on stress distribution. The model studies indicated that confidence could be placed in results obtained from the field study of tunneling, which was subsequently undertaken. FIELD INVESTIGATIONS The Colorado School of Mines generously made available in their experimental mine at Idaho Springs, Colorado~about 20 miles west of Denver~an appropriate site for a field investigation. A previously planned 5 × 7 ft crosscut was to be driven roughly parallel to and I0 diameters (70 It) away from an existing tunnel. In situ stress information was first obtained, followed by placement of two probes in a single hole approximately 50 ft ahead of the proposed tunnel (Fig. 13).

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FIG. 13. Plan of field site, showinggeneral arrangement of workings, probesand major fault. The field-investigation site was geologically mapped in detail. The mine is in a generally conformable sequence of folded metasedimentary gneisses and metaigneous and igneous rocks of Precambrian age. Most of the gneissic rocks were probably elastic sedimentary rocks that were deformed, recrystailized, and partly reconstituted at considerable depth and at high temperatures. The metaigneous and igneous rocks were intruded during Precambrian time--but are younger than the metasedimentary gneisses--and include granodiorite, quartz

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

691

m o m , t4

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Fl~, 14. Lo~cr-hcmispher¢, equal-area plot of the poG.'~ 1o 159 joint altitudL.~ in the study area. Contoured on 3, 6 and 9 polL's Ix:r I per cent :trua. NORTH

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692

J O H N F. A B E L A N D F I T Z H U G H T. LEE

diorite, and biotite-muscovite granite. A prominent structural feature in the study area, fault 2, is shown on Fig. 13. Figure 14 shows the orientation of the joints mapped in the study area, fault 2, and the crosscut axis. The pervasive foliation of the metamorphic rock, which trends approximately N. 70 ° E. and dips approximately 55 ~ NW., is the most striking structural feature (Figs 13, 15). I N S I T [ / S T R E S S FIELD The 3-dimensional stress field was determined by the strain-relief method at five sites. three approximately 50 ft northwest of probes 9 and 10 and two approximately 90 ft south. The composite results of these in s i t u principal stress determinations show a consistent clustering of orientations (Table 7 and Fig. 16). NORTH "--'T'--

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FI(;. 16. Orientation of principal in sit, stresses (lower-hemisphere, equal-area projection). These clusters o f preferred principal stress orientations each contain five principal stress determinations. Cluster ~,,~ contains three maximum and two intermediate principal stress determinations; cluster ,7a contains two maximum, two intermediate, and one minimum principal stress determination, and cluster ~c contains four minimum and one intermediate principal stress determination. The stress variation, both in orientation and in magnitude, is an indication o f the geologic variation present in the metamorphic rock mass. The 95 per cent confidence interval for the orientations of cluster ~'a covers the full variation of the poles o f the foliation present in the study area. The average values o f the other

STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

693

two principal stress clusters, ~4 and c,c, lie on and adjacent to the trace o f the average foliation plane (Fig. 15). The variations in the magnitudes o f the principal stresses included in the individual clusters are probably related to variations in stiffness between foliation layers and to joints normal to the foliation. The five determinations were necessary because no single measurement could be assumed to be representative o f the average background stress field. The influence of anisotropic changes in modulus across foliated lithologic contacts or across and along faults and joints apparently caused large local variations in the background stress field. The probable accuracy o f about 3 per cent for the clusters o f stress orientations is indicated by the independent check for orthogonality shown on Fig. 16. TABLE 7. PRINCIPAL in $#R STRE.qSES DETERMINED IN FIELD-STUDY

Bearing

Mean

A B C

N. 39 E. S. 35 E. N. 73 W.

To

N. 5.8 E. N. 72"2 E. S. 2-0 E. S. 68.0 E. N. 43.0 W. S. 77.0 W.

(psi)*

95 % confidence interval

95 ~ confidence interval From

Magnitude

Plunge (°)

(~)

Principal stress cluster

AREA

Mean From 19 36 44

11'4 17"1 23"9

95 % confidence interval

To

Mean From

To

26"6 57.9 64. I

1085 1015 650

1587 1567 863

583 463 437

* All stresses are compressive. These principal stress clusters are the best estimate o f the stress field acting on the rock in the vicinity o f the tunnel investigation. The 95-per cent confidence intervals also indicate the magnitude o f expected local variations in the in situ principal stresses. The magnitudes and orientations o f the principal stresses and principal stress clusters cannot be explained on the basis o f the 350 ft o f overburden present. Overburden can account for only 385 psi of vertical stress and 100 psi o f horizontal restraining stress. The absence of seismic activity in the Idaho Springs, Colorado, region reduces the probability of large or unequal tectonic boundary stresses. Six-inch-diameter core samples o f gneiss were taken from the Idaho Springs Formation in the study area, placed in the temperature-humidity controlled room, gaged, and overcored. Residual compressive strain releases were measured by six overcored strain gages. The magnitudes ranged from + I to + 5 4 4 0,-in/in.) over a period o f 1050 hr after overcoring. The mean residual compressive strain release was + 175 (~,-in./in.), with a standard deviation of 133 (~,-in/in.) The indicated equivalent compressive stress release is from 8 to 4352 psi. Residual strains present in the Idaho Springs Formation are capable of developing the stress magnitudes measured by in situ stress relief overcoring and of" providing the driving stresses indicated by the decompression during stress monitoring. The dispersion o f magnitudes o f the residual strains would also favor a nongravity-oriented in situ stress field. The principal stress cluster, ~m, in the study area lies approximately normal to the foliation (Fig. ! 5). The other principal stress clusters, ~'A and Oc, lie in the average foliation plane and are also approximately normal to other joint sets.

694

J O H N F. A B E L A N D

FTI'ZHUGH

T. LEE

The apparent control of the present stress field orientation by much older geologic features, foliation and joints, is indicative of the important influence of geology on the orientation of the in situ stress field. ,Although faults are the most important local control on the stress field at other locations in the mine, foliation appears to be the controlling geologic feature in the study area. S T R E S S CHANGES OBSERVED DURING EXCAVATION

The central crosscut was driven N. 36 ° W. (Fig. 13). Probes 9 and I0 were installed ahead of the advancing excavation and monitored from 28 July, 1967, following the excavation increment of 15 July, 1967, until they were destroyed by the advance of the central crosscut. These probes permitted a field study of the stress changes occurring ahead of an advancing tunnel, in this case the central crosscut. This situation is similar to the laboratory model tunnel studies. The incremental (round-by-round) stress change histories for probes 9 and 10 are shown on Fig. 17. These stress changes show an initial increase in compressive stress followed by a subsequent much larger decrease in compressive stress. The magnitude of the stress changes near probe 10 was consistently less than that of those near probe 9. This is probably because probe 9 was closer to the zone of stress concentration and more directly alined with the advancing tunnel and, thereby, more nearly normal to strike of the foliation than probe 10. -4000

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STRESS CHANGES AHEAD OF AN ADVANCING TUNNEL

695

Stress changes associated with the advancing tunnel apparently were already taking place before the probes were installed. This behavior can be seen from the initial shape of the curves in Fig. ! 7, which indicates that the onset occurred more than 7 diameters ahead of the advancing tunnel face. Photoelastic analysis [2] predicts the onset of stress change at I tunnel diameter ahead of the tunnel face. in the elastic acrylic model the onset occurred more than 4 tunnel diameters ahead of the model tunnel: in the heterogeneous isotropic concrete model the onset occurred at more than 2.50 tunnel diameters: and in the homogeneous anisotropic granite model at more than 2-00 tunnel diameters. The mining studies previously referenced (p. 674) indicated that the onset of stress influences related to Iongwall mining could be detected from five to more than 25 seam thicknesses ahead of an advancing face. A compressive stress peak was measured, by probes 9 and 10, between approximately 6 and 6"50 tunnel diameters ahead of the advancing tunnel face (Fig. 17). No similar stress peak is predicted from elastic theory. A similar compressive stress peak was observed in each of the tunnel models, at 2 tunnel diameters for the acrylic model, at !.25 tunnel diameters for the concrete model, and at approximately 1 tunnel diameter for the granite model. The mining field studies indicate a peak from less than four seam thicknesses to more than eight seam thicknesses ahead of the advancing face. The absolute stress magnitude of this peak is not known, because probes 9 and 10 sensed stress changes relative to the initial reading. The results of this field study are in approximate agreement with the available field data, in partial disagreement with the laboratory results, and in outright disagreement with elastic theory. The orientation bfthe maximum principal background stress cluster, ~A, in the study area is flat-lying and trends northeast, and the orientation of the intermediate principal background stress cluster, ~s, is gently plunging and trends northwest (Table 7). Because the maximum and intermediate stress changes near probes 9 and I0 were also relatively fiat Trojectories of maximum WincSpai stress Trojeclorles of Intermediate print,pal stress Oecompreesion Compression

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696

JOHN F. ABEL AND FITZHUGH T. LEE

lying, a plan view contaning these nearly horizontal principal stress-change vectors was constructed (Fig. 18). The stress-change vectors were used to construct stress-change trajectories. These stress-change trajectories suggest: (1) the influence of local variations in the foliated and faulted metamorphic rocks on stress orientation, (2) the onset of tunnelexcavation-induced stress response at least 7 diameters ahead of the tunnel, (3) the minor asymmetry of the stress trajectories ahead of the advancing tunnel which probably resulted from the interaction between stress changes induced by mining and the maximum principal background stress cluster (~4), (4) that the tensile stress changes increased progressively in all directions as the tunnel approached the probes, and (5) that the recompression of probe 10, after 6 September, 1967, as the tunnel approached, probably indicated increased tangential stress at the tunnel wall. The extent and magnitude of the decompression indicate the ability of the rock mass to reflect, at a rather great distance, any potential stress relief presented by the advancing tunnel. The departure of the probe measurements from photoelastic and elastic predictions and laboratory model measurements suggests that foliation and faults control the rock-mass response. The central crosscut was driven at right angles to the strike of the major geologic weakness in the rock mass, the foliation. The foliation, and its associated jointing, presents a ready avenue for tensile strain relief toward the advancing crosscut. Apparently the rock expanded preferentially perpendicular to foliation joints, toward the crosscut face. This rock-mass weakness provides the potential for tensile strain relief parallel to the crosscut axis and normal to the plane of foliation. The influence of faults on stress changes in this general area of the mine has been noted previously [15]. The location of fault 2 with respect to probes 9 and 10 is shown on Fig. 13. At the time probes 9 and 10 were operative, fault 2 had already been exposed in the lower right corner of the face of the central crosscut. Fault 2 was completely exposed by the 8-ft round excavated on 5 August, 1967. The face of the crosscut was entirely within the footwall of fault 2 after August 5. The initial increase in compressive stress in the footwali at probes 9 and 10 was accompanied by a decrease in stress in a distant hanging-wall stress-change probe [15]. The in situ stress measurements show a decrease in compressive stress from the hanging wall to the footwall of fault 2, a condition that is difficult to model in the laboratory. An initial compressive-stress build-up occurred on the lower stressed (footwall) side of the fault as the crosscut advanced. The advance of the excavation toward probes 9 and 10 began providing an avenue for the release of stress from the higher stressed hanging wall to the lower stressed footwail when the tunnel had approached to within 6 tunnel diameters of the face. The presence of this stress-release avenue dominated the response of probes 9 and 10 once the central crosscut had reached to within 4 tunnel diameters of the probes. The overall effect of tunnel advance was to decompress the rock ahead and to the side of the tunnel. CONCLUSIONS This investigation of the rock-mass response ahead of an advancing tunnel has demonstrated the divergence of real materials from idealized elastic predictions. An advancing tunnel can induce stress changes at great distances ahead of the tunnel face. It can be inferred from the laboratory study that the extent of this zone of stress change is inversely related to the perfection of elasticity of the rock mass. Geologic features controlled the orientation of the 3-dimensional in situ stress field. The in situ stress field cannot be assumed to be gravitational. Foliation, the major geologic

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feature in the area, controls the orientation of the in situ stress field. The average horizontal stress components and the average vertical stress component are three times and twice as large, respectively, as those predicted from the overburden. The stress field appears to have both tectonic and residual stress components, as suggested by laboratory and field information. Mining triggered the release of strain energy, the magnitude and direction of which were controlled by geologic structure. REFERENCES l. OneRT L., DUVALLW. |. and ~,]ERRILLR. H. Design of Underground Openings in Competent Rock. U.S. Bureau of Mines, Bull. 587 (1960). 2. GALLE E. M. and WILHOrr J. C. Stresses around a wellbore due to internal pressure and unequal principal 8¢ostatic stresses. Soc. Petrol. Engr J. 2 (2), 145-155 (1962). 3. ABELJ. F. Tunnel mechanics. Colo. Sch. Mines Q. 62 (2), (1967). 4. PoT'rs E. L. J. [Discussion of] "The Conditions of the Ground Around Excavations in Hard Rock at Great Depth', by H. G. DENKHAUSand F. G. HILL, Third International Conference on Strata Control, Paris, 1960, p. 252 O961). 5. De.~KHAUSH. G. and HILL F. G. The Conditions of the Ground'Around Excavations in Hard Rock at Great Depth, Third International Conference on Strata Control, Paris, 1960, pp. 243--251 0961). 6. LEEMANE. R. Measurement of Stress in Abutments at Depth, Third International Conference on Strata Control, Paris, 1960o pp. 301-311 (1961). 7. REE~ER F. J. M. De and RRUENSF. P. Measuring the Loads on Roadway Supports by Means of Strain Gauges, Third International Conference on Strata Control, Paris, 1960, pp. 321-337 (1961). 8. CAR~B W. H. N. A Review of Strata Control Experience in Longwall Working in Great Britain, Third International Conference on Strata Control, Paris, 1960, pp. 471-482 (1961). 9. SPACKELeRG. [Discussion ofl 'A Review of Strata Control Experience in Longwall Working in Great Britain," by W. H. N. CARTER,Third International Conference on Strata Control, Paris, 1960, p. 483 (1961). I0. HENSHAWH. [Discu.~ion of] "A Review of Strata Control Experience in Longwall Working in Great Britain," by W. H. N. CARTER, Third International Conference on Strata Control, Paris, 1960, p. 484 (1961). I I. Nzc,oi~ T. C., AriEL J. F. and LEE F. T. A solid-inclusion borehole probe to determine three dimensional stre~ changes at a point in a rock rna.~q;. Bull. U.S. Geol. Sure. 1258-C, CI-C28 0968). 12. OnERr L. and DUVALLW. I. Rock Mechanics and the Design of Structures in Rock, Wiley, New York (1967). 13. BROWS E. T. and TROLLOPSD. H. The failure of linear brittle materials under effective tensile stress. J. Int. ~1c. R~K'k Mech. 5. (4) 229-241 (1967). 14. PORTLANDCE.q~Nr ASSOClAI"ION.Dc.vil~nand Control of Concrete Mixtures, IOth edn, Portland Cement Ass. (1952). 15. LE£ F. T., NU'HOLS T. C. and An~:L J. F. Some Relations Between Stress, Geologic Structure, and Underground Excavation in a Metamorphic Rock Mass West of Denver, Colorado, in Geological Surcey Research 1969, U.S. Geological Survey, Prof. Paper 650-C, CI27-CI32 (1969).