AN EXPERIMENTAL INVESTIGATION OF SINGLE 

FRACTURE FLOW BEHAVIOR UNDER NORMAL AND 

SHEAR LOADS 

Prepared for 

U.S. Nuclear Regulatory Commission 

Contract No. NRC–02–07–006 

Prepared by 

Sitakanta Mohanty 

Simon Hsiung 

Center for Nuclear Waste Regulatory Analyses 

San Antonio, Texas 

September 2011 
ii

ABSTRACT 

This report presents an experimental apparatus designed for a laboratory study of the effect of 

normal and shear loads on the coupled mechanical–hydrological response of a single, natural 

fracture (rock joint) in a relatively large-sized specimen {200 × 200 mm [8 × 8 in] fracture 

surface area}. Because of the practical difficulties in measuring shear load effects on 

permeability in natural rocks, relatively few experimental data are available in the open literature 

for such conditions on large specimens. The direct-shear test apparatus described in this paper 

is capable of measuring changes in fracture hydraulic conductivity (or permeability) using a 

variety of fluids injected in linear and radial flow configurations under normal and shear loads. 

The test apparatus enabled measurements of permeability for shear displacements of up to 

2.54 cm [1 in], an improvement over previous devices. The normal stress experiments were 

conducted using both radial and linear flow configurations and the shear displacement 

experiments were conducted using only the linear flow configuration. The linear flow 

experiments provided a better fracture area coverage during flow than the radial flow 

experiments. The experiments found that an increase in normal stress to 8 MPa [1,160 psi] 

changed fracture permeability by nearly 35 percent, and gouge production during 

shearing changed fracture permeability by 300 percent at the 5-MPa [725-psi] normal 

stress level. The shear displacement experiment results show that gouge production can be 

a significant factor affecting rock joint permeability. Experiments of this kind are valuable for 

obtaining a better understanding of which key mechanical parameters influence fracture flow 

and providing data that could be used in evaluating modeling capabilities for coupled 

mechanical–hydrological processes. 
iii

CONTENTS 

Section Page 

ABSTRACTS ................................................................................................................................ ii 

FIGURES ..................................................................................................................................... iv 

TABLES ........................................................................................................................................ v 

ACKNOWLEDGMENTS .............................................................................................................. vi 

1 INTRODUCTION ................................................................................................................... 1

2 EXPERIMENTAL APPARATUS ......................................................................................... 2-1

2.1 Mechanical Loading ................................................................................................... 2-1

2.2 Fluid Flow .................................................................................................................. 2-3

2.2.1 Theory ............................................................................................................ 2-3

2.2.2 Radial Flow Setup .......................................................................................... 2-4

2.2.3 Linear Flow Setup .......................................................................................... 2-5

3 EXPERIMENTAL PROCEDURE ........................................................................................ 3-1

3.1 Radial Flow ................................................................................................................ 3-1

3.2 Linear Flow ................................................................................................................ 3-1

4 RESULTS AND DISCUSSIONS ......................................................................................... 4-1

4.1 Normal Stress Experiments ....................................................................................... 4-1

4.2 Shear Displacement Experiments ............................................................................. 4-2

5 CONCLUSIONS .................................................................................................................. 5-1

6 REFERENCES ................................................................................................................... 6-1
iv

FIGURES 

Figure Page 

2-1 Direct Shear Test Apparatus for Flow Studies: 

 Assembly and Instrumentation Diagram ........................................................................ 2-2 

2-2 Schematic of the Linear Flow Apparatus With Normal and Shear 

 Loading Arrangements ................................................................................................... 2-5 

2-3 Schematic of the Flow Loop for the Linear Flow Experiment ......................................... 2-6 

4-1 Comparison Between Relative Change in Mechanical (Displacement) Equivalent 

Hydraulic Apertures as Normal Stress Changes ............................................................ 4-2 

4-2 Gouge Material After Being Dried To Illustrate Texture and Particle Size ..................... 4-5 
v

TABLES 

Table Page 

1-1 Selected Previous Studies on Permeability Measurement Under 

 Shear Displacement ....................................................................................................... 1-3

4-1 Equivalent Hydraulic Aperture Interpreted From Radial Flow Experiments ................... 4-2 

4-2 Shear Displacements and Corresponding Equivalent Hydraulic Aperture During Forward 

Cycles ............................................................................................................................ 4-4 
vi

ACKNOWLEDGMENTS 

This paper was prepared to document work performed by the Center for Nuclear Waste 

Regulatory Analyses (CNWRA®) for the U.S. Nuclear Regulatory Commission (USNRC) under 

Contract No. NRC–02–07–006. The activities reported here were performed on behalf of the 

USNRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. The paper is an independent product of CNWRA and does not necessarily 

reflect the views or regulatory position of USNRC. 

The authors gratefully acknowledge T. Wilt for his technical review, R. Lenhard for his 

programmatic review, L. Mulverhill for her editorial review, and L. Neill and A. Ramos for their 

administrative support. The authors also gratefully acknowledge the excellent comments 

provided by R. Fedors (NRC) and M. Nataraja (NRC). 

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT 

DATA: All CNWRA-generated original data contained in this report meet the quality assurance 

requirements described in the Geosciences and Engineering Division Quality Assurance 

Manual. Sources for other data should be consulted for determining the level of quality for 

those data. 

ANALYSES AND CODES: No codes were used in the analyses of the work described in 

this report. 
1-1

1 INTRODUCTION 

Understanding fluid flow and transport in rock fractures (or joints) is important to many 

engineering and geological applications, including disposal of high-level nuclear wastes in 

geologic media, oil/gas exploration and production, geothermal energy exploration and 

production, dam stability, remediation of contaminated sites, understanding of diagenetic 

processes and fault sealing, and understanding the genesis of earthquakes (Nemoto, et al., 

2009; O’Brien, et al., 2003). In many of these applications, the prediction of flow and transport 

is complicated by the coupled processes that govern flow in fractures. 

In the disposal of high-level radioactive waste, flow and transport may be perturbed by 

thermal-mechanical–hydrological–chemical coupled processes over a long period by the 

radioactive waste’s decay heat (Ghosh, et al., 1994). Fractures are continuously encountered in 

many potential host rocks for a repository for the disposal of high-level radioactive waste. The 

disposal tunnel excavation, dynamic ground motion from earthquake and nearby 

underground weapon testing, and the rock thermal expansion can change the mechanical 

stress state of the rock mass. These deformations can cause the joint to dilate, close, and 

shear, leading to changes in the joint hydrological properties, thus giving rise to 

mechanical-hydrological coupling. 

Field evidence suggests significant mechanical–hydrological coupling effects. Geohydrological 

analysis in the Stripa Site Characterization and Validation Project overpredicted inflow to a drift 

by an order of magnitude (Ohlsson, et al., 1992). The lower measured value was attributed to 

potential reduced inflow due to excavation-induced stress changes. These stress changes 

cause the joints around the drift to open or close, which in turn strongly affects the hydraulic 

permeability (Rutqvist, 1995). Similarly, Loma Prieta earthquake presents an example of 

earthquake-induced hydrologic changes. This earthquake led to increased joint permeability 

resulting in the water table in the mountains dropping more than 21 m [69 ft] while greatly 

increasing the flow of springs and streams in the foothills (Rojstaczer and Wolf, 1992). 

By definition, a mechanical-hydrological coupled process in a rock joint is a two-way process in 

which the mechanical stress affects fluid flow and the fluid pressure affects the mechanical 

condition (e.g., the effect of fluid pressure buildup on rock deformation). The majority of coupled 

mechanical-hydrological process studies have focused on the former. The mechanical stress 

effects on fluid flow have been studied both numerically and experimentally at field and 

laboratory scales. Predictive models for studying ensemble behavior of fractures (e.g., scaling) 

estimate flow and transport involving networks of fractures in porous matrices. However, 

fracture network flow characteristics are governed by the characteristics of flow in a single 

fracture. Stress-transmissivity relations reflecting behavior in a single fracture are typically 

provided as input to fracture network coupled process models. Therefore, research is needed 

to better understand the stress-transmissivity relationship for a single fracture. The most 

fundamental parameter is fracture permeability. 

Most past studies have estimated fracture permeability as a function of normal stress 

(e.g., Barton, et al., 1985; Hakami and Larsson, 1996; Pyrak-Nolte, et al., 1987; Raven and 

Gale, 1985; Raven and Gale, 1985; Sundaram, et al., 1987; Witherspoon, et al., 1980) and very 

few as a function of shear stress (Bandis, et al., 1985; Makurat, 1985). Both laboratory 

experiments (Makurat, et al., 1990; Raven and Gale, 1985) and field testing (Carlsson and 

Olsson, 1986; Jung, 1989; Rutqvist, et al., 1990) have shown that the flow rate along a single 

joint can be highly sensitive to the change in joint aperture. Therefore, fluid flow through the 
1-2

fracture network system under deformation may be affected by the deformation of this fracture 

network (Rutqvist, 1995). The fracture permeability studied under joint shear is somewhat 

limited primarily because the experiment is more difficult to conduct than similar experiments 

under normal stress and the experiments are even more difficult if other coupled processes are 

studied simultaneously. Investigators have overcome some of the experimental difficulties by 

using very small specimens or limiting shear displacement. Table 1-1 shows a quick-view 

summary of the shear displacement experiments conducted by several groups of investigators. 

These experiments differ in a variety of aspects including rock material (real rock and replicas), 

specimen shape (prismatic versus cylindrical), specimen size, fluid injection pattern for 

hydrological experiments (linear versus radial), shear displacement type (translational versus 

rotational), shear displacement extent, and applied normal load during rock shear. The 

variations in some cases reflected different objectives for the experiments and also reflected 

experimental limitations. For example, while Faoro, et al. (2009) showed large shear 

displacement, an artificial fracture was used for the experiment. Likewise, though Lee and Cho 

(2002) conducted linear flow experiments on large specimens, the shear displacement was 

very limited. 

The objectives of the study presented in this report were to develop an experimental setup for 

studying coupled mechanical–hydrological process using a single rock joint and to understand 

fracture flow characteristics, especially permeability, under normal and shear loads. The study 

involved large specimens and large shear displacements. Large specimens have the 

advantage of large joint surface area and hence better spatial averaging for obtaining fracture 

permeability. The report first describes the development of the experimental technique and then 

the flow experiments (radial and linear) under combined normal and shear loads. This report 

does not make any attempt to study the scaling effects.
1-3

Table 1-1. Selected Previous Studies on Permeability Measurement Under Shear Displac

Investigators Material 

Specimen Shape and Size, 

mm [in] 

Fluid Injection 

Pattern 

Normal Stress Which 

Shear Displacement 

Was Carried Out 

Obcheoy, et al. (2011)* Artificial fracture 

made out of marble 

and granite 

Prismatic 

100 (L) × 100 (W) × 76 (H) mm 

[3.94 × 3.94 × 2.99 in] 

Radial 3.1 MPa 

[450 psi] 

Faoro, et al. (2009) Artificial fracture Prismatic 

70 × 45 × 12 mm 

[2.76 × 1.77 × 0.47 in] 

Appeared to be 

linear 

5–20 MPa 

[725–2,901 psi] 

Auradou, et al. (2006) Transparent epoxy 

casts 

Prismatic 

12.8 × 12.8 × ? mm 

[0.50 × 0.50 × ? in] 

Radial 0 MPa 

[0 psi] 

Lee and Cho (2002) Prefractured samples 

of granite and marble 

Prismatic 

160 × 120 × 120 mm 

[6.30 × 4.72 × 4.72 in] 

Linear 1–3 MPa 

[145–435 psi] 

Yeo, et al. (1998) Epoxy resin Prismatic 

200 × 200 × ? mm 

[7.87 × 7.87 × ? in] 

Radial and Linear 0.01 MPa 

[1.45 psi] normal stress 

applied after the shear 

boxes reached a 

predetermined offset 

Esaki, et al. (1995) Prefractured samples 

of granite 

Prismatic 

120 × 100 × 80 mm 

[4.72 × 3.94 × 3.15 in] 

Radial 20 MPa 

[2,901 psi] 

Olsson and Brown (1993) Natural joint of chalk Cylindrical 

60.3 (OD) × 20 (ID) mm 

[2.37 × 0.79 in] 

Radial 4.3 MPa 

[624 psi] 

Boulon, et al. (1993) Natural and 

prefractured joint of 

granite 

150 × 150 × ? mm 

[5.91 × 5.91 × ? in] 

Linear (flowing 

from center to 

both ends) (?) 

Apply normal stress 

after the shear 

displacement under 

0.0 normal stress 

Teufel (1987) Prefractured samples 

of sandstone 

Cylindrical 

47.6 (OD); 118–124 (L) mm 

[1.87; 4.65–4.88 in] 

Unknown 53–64 MPa 

[7,687–9,282 psi] 

Makurat (1985) Concrete casts Cylindrical 

150 (OD); 150 (L) mm 

[5.91; 5.91 in] 

Linear 0.82 and 8.2 MPa 

[119 and 1,189 psi] 

*

Reference details are in the Section 6, References, of this report 

Abbreviations: L–length; W–width; H–height; OD–outer diameter; ID–inner diameter; Note: “?” indicates information was not readily available from the referenced paper. 
2-1 

2 EXPERIMENTAL APPARATUS 

2.1 Mechanical Loading 

The coupled mechanical–hydrological experimental apparatus was a direct shear test apparatus 

designed for testing large-sized single rock joint specimens under combined normal and direct 

shear loads. The test apparatus was designed, fabricated, and assembled at Southwest 

Research Institute® (SwRI®) (Hsiung, et al., 1994; Kana, et al., 1991). The apparatus consisted 

of vertical and horizontal servo-controlled loading actuators, reaction frames, shear box fixtures, 

and an instrumented jointed rock specimen (Figure 2-1). 

The apparatus had three vertical actuators {0.133 MN [30,000 lb] in capacity} set at 

120 degrees about the specimen’s vertical centerline to apply normal load to the specimen. The 

bottom of each of the three vertical actuators was pinned to the base plate (Figure 2-1). The 

total normal load was controlled at a preselected value and applied to the specimen via the 

normal load frame that acts on the three normal load rollers. The normal load frame was 

designed to provide constraints to three degrees of freedom: (i) vertical translation, (ii) rotation 

about the horizontal axis in line with the shear, and (iii) rotation about the horizontal axis 

transverse to the shear. 

The horizontal actuator {0.222 MN [50,000 lb] in capacity} produced direct shear to the upper 

specimen box, which acted through a spherical coupling. This coupling allowed for slight 

misalignment in the horizontal shearing motion and elevation changes of the upper specimen 

due to vertical load, joint surface roughness, and progressive wear. The horizontal translation 

of the upper specimen along the direction of shearing was guided by three rollers between the 

top shear box and normal load frame and side rollers. The horizontal actuator could be 

operated in either load or displacement control mode with loading patterns including 

pseudostatic and dynamic input. 

A shear box housed the rock joint specimen. The bottom portion of the specimen was longer 

than the top portion. The shear box was designed to house top and bottom portions of a rock 

joint specimen separately with the maximum specimen dimension of 203 × 203 × 102 mm 

[8 × 8 × 4 in] for the top half and 305 × 203 × 102 mm [12 × 8 × 4 in] for the bottom half. Both 

top and bottom specimen parts were grouted in their respective specimen boxes by cement. 

The bottom shear box was bolted to a 1.22 × 2.13 × 0.15 m [4 × 7 × 0.5 ft] steel base plate 

for rigidity. 

Vertical displacement transducers were installed to measure vertical displacements between the 

two rock blocks in response to the applied static normal load. These were installed at four 

locations (two at each side of the specimen) near the joint interface (Figure 2-1). 

Measurements near the interface are desirable to reduce the effects of slack or lack of strength 

in the grout. Proximity (noncontacting) eddy–current sensing type transducers were used to 

allow horizontal movement of the two joint surfaces and to measure only the vertical 

displacement changes continuously. The four vertical measurement points could be used to 

resolve the rigid body displacement of the upper block relative to the lower specimen block. 

The upper and lower shear boxes were sized such that when the specimen blocks were 

grouted, approximately a 25.4-mm [1.0-in] gap would be left between the box faces. The side 

plates of each half of the specimen box were slotted, so that vertical proximeter supports and 

target plates could be mounted directly onto the sides of respective specimen halves. Two 
2-2 

Figure 2-1. Direct Shear Test Apparatus for Flow Studies: Assembly and 

Instrumentation Diagram 

(1) Vertical Load Cell No. 1 

(2) Vertical Load Cell No. 2 

(3) Vertical Load Cell No. 3 

(4) Analog Summation of Total Vertical Normal Load 

(5) Horizontal Load Cell 

(6) Horizontal Displacement of Top Block Relative to 

Bottom Block Near Actuator – LVDT1 

(7) Horizontal Displacement of Top Block Relative to 

Bottom Block Opposite Actuator – LVDT2 

(8) Horizontal Displacement of Top Block Relative to 

Horizontal Load Cell – LVDT3 

(9) Specimen Vertical Relative Displacement Proximeter 

No. 4 

(10) Specimen Vertical Relative Displacement Proximeter 

No. 5 

(11) Specimen Vertical Relative Displacement Proximeter 

No. 6 

(12) Specimen Vertical Relative Displacement Proximeter 

No. 7 
2-3 

prongs that support each plate component were cemented into lateral holes drilled into the 

specimen sides. The mean elevation of these 6.35-mm [0.25-in]-thick steel components was 

set by gauge blocks during the cementing process, so that their positions relative to the box 

faces were known. Although some movement of the specimen within the grout occurred during 

loading, the side slots were large enough so no interference occurred between the support 

prongs and the box side plates. Furthermore, the heavy mounting frame for the upper box side 

rollers was slotted so that there would be no interference between the load frame and the target 

plates as the upper box displaces both horizontally due to shear and vertically due to 

unevenness and wear of the joint interface. 

Three linear variable differential transducers (LVDT), see Figure 2-1, were installed to measure 

the relative displacements between the top and bottom specimen blocks. Each half of a 

transducer was cemented directly into a hole drilled into the respective specimen block. LVDT1 

was mounted on the near side of the specimen pair and LVDT2 was in the far side to provide 

the direct shear movement of the joint interface. LVDT3 was mounted to sense displacement of 

the upper block relative to the horizontal load cell. Thus, it measured any compliance that might 

exist in the horizontal coupling and the grout for the upper block. The position of LVDT3 was 

57.2 mm [2.25 in] from the outside surface of the upper specimen box, on the same side as 

proximeters P4 and P5 (see Figure 2-1). A 13-channel data acquisition system coupled with an 

SwRI-developed software was installed to sample and record mechanical displacement data at 

preselected rates and times. 

2.2 Fluid Flow 

The apparatus was designed to carry out steady-state flow experiments after each step of 

normal and shear displacement. The apparatus permitted fluid flow in radial as well as linear 

flow configurations depending on the modifications to the specimen and the shear box. Most 

previous investigators used the radial flow configuration because it is a much less challenging 

configuration than the linear flow configuration. In this study, the radial flow configuration 

was used only for the normal loading condition. The linear flow configuration was used for 

both the normal stress and shear displacement experimental conditions. Rock specimens 

with the maximum top-half and bottom-half specimen dimensions allowable by the test 

apparatus were used for the flow experiments. The following subsections present a brief 

description of the theory for permeability determination and modifications to the apparatus for 

both flow configurations. 

2.2.1 Theory 

Joint permeability can be determined using Darcy’s law, assuming a steady flow 

L

kA P

q

μ

(Δ ) = (2-1) 

where ∆P is pressure drop, A is fracture cross-sectional area (m2) [fracture aperture 

(b) × fracture specimen width (w)], k is permeability (m2

), q is volumetric flow rate (m3/s) [flow 

velocity (v) × A], μ is dynamic viscosity of fluid (Pa–s), and L is fracture specimen length (m). 

For a smooth parallel plate representation of a fracture, k equals b2

/12. By setting up an 

experiment identical to the Darcy’s flow experiment, the smooth parallel plate equivalent 

aperture can be back calculated. The single aperture by definition is representative of the 
2-4 

average of the aperture distribution between the two plates. For linear flow, b can be 

estimated from 

3

1

1 2 ( )

12 



 





− = w P P

q L b μ (2-2) 

where P1 and P2 are upstream and downstream pressures, respectively, and for radial flow, b is 

estimated from 

3

1

1 2

1

2

( )

6 ln

























− = P P

r

r

q

b

π

μ

(2-3) 

where r2 and r1 are the external and internal radii, respectively, of the fracture. For the radial 

flow experiments in this study, the internal radius is the inlet radius discussed in Section 2.2.2. 

The external radius is the radius of a circle with an area equivalent to the joint surface area of 

the rectangular top specimen half {203 × 203 mm [8 × 8 in]}, the smaller of the two specimen 

halves. 

 

To circumvent the difficulties of a complex flow regime, it is often of interest to conduct flow 

experiments under a laminar flow regime with low Reynold’s number. Transition from a laminar 

to turbulent flow regime was inferred from the Reynold’s number. Turbulence may be 

augmented by the presence of surface roughness giving rise to eddy formation. The eddy 

formation due to surface roughness is usually represented by a relative roughness relation 

h

r D

R ε = (2-4) 

where Dh is the hydraulic diameter and ε is the mean height of the fracture roughness (Iwai, 

1976). In this study, a gas flow experiment was also conducted. For the gas flow, the influence 

of gas compressibility and the Klinkenburg effect (Klinkenberg, 1941) needs to be considered. 

Another representation of the Reynold’s number is in its reduced form for representing flow in 

a fracture 

1

2

* = <<

d

vb R

e μ

ρ (2-5) 

where ρ is the fluid density and d is the typical distance between asperities and the average 

aperture must be small relative to other characteristic length scales in the problem. 

2.2.2 Radial Flow Setup 

For radial flow experiments, a vertical hole was drilled at the center of the upper block of the 

rock joint to accommodate a 6.35-mm [0.25-in] tubing. The annulus between the hole and the 

external wall of the tubing was sealed with epoxy. The injected fluid was allowed to exit at the 
2-5 

external boundary of the rock joint at atmospheric pressure. Troughs were made in the grout at 

the bottom and along all four sides of the bottom block and partitioned. Tubing was attached to 

collect and measure outflow from each face of the joint separately to permit identifying any 

potential preferential flow channels, which could be changing as a function of normal stress and 

inlet flow rate. A flow meter on the inlet line measured the constant liquid flow rate and a U-tube 

manometer monitored the fluid pressure changes at the injection point due to normal stress. 

2.2.3 Linear Flow Setup 

For linear flow experiments, the apparatus shown in Figure 2-2 was designed to inject fluid into 

the joint interface at one end and monitor outflow from the opposite end. Sides parallel to the 

overall linear flow direction were sealed to confine the flow in one direction. The grouted faces 

of the specimen (described later) were specially treated to prevent fluid loss. A precision 

positive displacement pump was used to inject liquid at a steady rate. Absolute and differential 

pressure transducers were used to measure the inlet and outlet fluid pressures and pressure 

differentials. The schematic for the flow loop for the linear flow experiment is shown in 

Figure 2-3. 

To distribute fluid uniformly across the inlet end of the specimen, a manifold was fabricated. 

The manifold consisted of 1.59-mm [0.063-in] holes 5 mm [0.2 in] apart drilled in stainless steel 

tubing blocked at both ends. Fluid enters the manifold at its center. All these holes were 

aligned and pointed toward the fracture face. The tubing was split into two halves, and one half 

was laid against the fracture at the outlet end to ensure that the fluid would be collected from 

across the outlet end of the fracture. The split tube was attached to tubing to drain the fluid. 

Figure 2-2. Schematic of the Linear Flow Apparatus With Normal and Shear 

Loading Arrangements 
2-6 

Figure 2-3. Schematic of the Flow Loop for the Linear Flow Experiment 

[1 in = 2.54 cm, 1 psi = 6.89 kPa] 

PT: Pressure Transducer 

DPT: Differential Pressure 

Transducer 

Core Assembly: Rock Joint 

Specimen Assembly 
3-1 

3 EXPERIMENTAL PROCEDURE 

The rock-joint specimens used in the flow tests were from a vitrified and densely welded tuff 

collected from Apache Leap, Arizona, (Hsiung, et al., 1994). The pores in the rock matrix hold 

water very tightly. The specimens used reflected the maximum dimensions allowable by the 

shear boxes. The surfaces were first profiled using a noncontact, surface-height-gauging 

profilometer (Hsiung, et al., 1994) before any instrumentation was affixed to the 

individual boxes. 

The specimen halves were dried at 105 °C [221 °F] for 24 hours. Afterwards, they were left 

inside the oven for cooling with the oven door closed to minimize condensation in the pores. 

All surfaces of the blocks, except the joint interface, were coated with approximately 1.59-mm 

[0.06-in]-thick silicone rubber material and left at room temperature for 48 hours to cure. After 

curing, the specimen halves were grouted into corresponding boxes according to the procedure 

described previously in Section 2.1. The grout was left to cure for 3–4 days in the oven at 

54.5 °C [130.1 °F]. Holes of different sizes were drilled onto the grouted rocks through the 

existing ports in the steel box. The side instrumentation supports, target plates, and LVDT 

mounts in both specimen halves were then installed. The gap between the hole and the 

instrumentation inserts was sealed with silicone rubber. 

3.1 Radial Flow 

Radial flow experiments were carried out as a precursor to the more elaborate linear flow 

experiments. Note that these experiments were carried out using rectangular blocks in shear 

boxes, as described in Section 2.1. The specimens were dried before starting the experiments, 

but fluid saturation was established by injecting fluid at high flow rates over a prolonged period. 

Once the system was ready, liquid flow rates were held steady at the experiments’ designated 

rates, which ranged between 8.33 and 6,660 mm3

/s [5.1 × 10−4 and 0.41 in3/s]. 

3.2 Linear Flow 

For the linear flow experiments, which required sealing arrangements to confine flow in one 

direction (Figure 2-2), the side of the bottom specimen along the height and the contact edges 

were coated with silicone rubber and a gasket made of closed cell foam and fillers was 

emplaced on the bottom box. The gasket channels were installed on the ends of the top box, 

and the input and output manifolds between channels and the rock edge were mounted. The 

top specimen sides and the channels were coated with silicone rubber. The rubber gasket was 

then installed on the upper box. Both the upper and lower gasket-sealing surfaces were coated 

with silicone grease after the silicone rubber had set. The exteriors of the upper and lower 

gasket halves were coated with silicone grease, and the gasket-retaining plates were bolted 

onto the side of the top box. 

To conduct fully water-saturated experiments, the rock specimen was evacuated in situ for 

2 hours at 744 mm [29.29 in] Hg below atmospheric pressure with a vacuum pump (with a 

dehydrator to prevent water vapor from entering) and then water was allowed to be drawn into 

the specimen under the existing vacuum (Figure 2-3). Excess water was collected at the outlet 

from the flow loop in a flask that separated the vacuum pump and the specimen holder. The 

system was then left under a positive pressure head for several days to reach pressure 

equilibrium. The positive pressure was obtained by shutting the downstream-end valve off and 

maintaining the fluid reservoir at the inlet end 3 m [10 ft] above the specimen holder. 
3-2 

The fluid injected across the entire cross section using a pump was collected from the entire 

cross section of the opposite end. The pressure drop (∆P) across the specimen was measured 

by using a differential pressure transducer. To determine air permeability, experiments were 

conducted using nitrogen (N2) (because it was readily available) when the specimen was 

completely dry. N2 was injected into the dry rock joint specimen at a constant rate from a 

high-pressure supply tank. The flow rate was controlled using a pressure regulator. The ∆P at 

the inlet end was measured by using a U-tube manometer, and the N2 flow rate was measured 

using rotameters and digital bubble flow meters. 

The downstream end was connected through a tube to a collection chamber (a large-capacity 

flask) placed so that there was at least 450 mm [17.7 in] of water remaining above the level of 

the fracture plane. This process ensured that the fracture would remain saturated even when 

no flow took place. This water head also ensured that air would not enter into the fracture when 

the mechanical loading system was deactivated and during the shearing process. At least 

0.4 MPa [57.7 psi] pressure was applied to compress the rubber seal before the two rock faces 

came in contact with each other. The inlet tubing had sufficient water height {450-mm 

[17.7-in]-long tubing} to compensate for the increase in void space, keeping the joint specimen 

constantly saturated when changing the loading conditions. 

To prevent the liquid head in the outlet tubing from falling below its entry level to the collection 

chamber during the shearing process (resulting in additional void due to dilation), a three-way 

connector was attached to the inlet end of the specimen holder. In addition, a water reservoir 

was connected via a valve and was maintained at such a height so that water would flow at a 

pressure of 3 m [10 ft]. 
4-1 

4 RESULTS AND DISCUSSIONS 

The suite of experiments involved many normal stress tests and only a limited number of 

shear displacement experiments with Apache Leap Tuff specimens because the shear 

displacement experiment is a destructive process. For normal stress experiments, both radial 

and linear flow configurations were used, but only a linear flow configuration was used for shear 

displacement experiments. 

4.1 Normal Stress Experiments 

To conduct flow experiments under the normal stress condition, the normal stress was 

increased at a preset loading rate. After the maximum normal stress {8 MPa [1,160 psi]} was 

reached, unloading (decreasing stress) took place at the same rate as the loading rate. At 

preselected load stress levels, flow was monitored. 

For the radial flow configuration, the dimension of the upper block defined the outer equivalent 

radius in the radial flow equation because the upper block is shorter than the bottom one. Low 

fluid flow rates required to maintain laminar flow for Darcy’s Law to apply resulted in 

low-pressure gradients, which were difficult to measure. Therefore, glycerol-augmented water, 

a higher viscosity fluid made of a mixture of water and glycerol {kinematic viscosity of 

16,470 mm2

/s [16,470 cSt] at 22 °C [71.6 °F] and dynamic viscosity of 1.396 × 10−2 Pa-s 

[13.96 cP]} was used to increase the pressure drop at a gravitational average fluid velocity. 

Table 4-1 presents the equivalent hydraulic apertures calculated at various normal stresses for 

the radial flow configuration, using the three fluids (air, water, and glycerol-augmented water) 

separately. Liquid breakthrough was observed on all but one side. The liquid breakthrough was 

typically one stream per side. The stream width varied slightly with the normal stress. 

Averaged over the entire test, 74.5, 18.4, and 7.1 percent of the total injected fluid was collected 

from the three sides. Despite a fourteen fold increase in the viscosity of the flowing fluid (i.e., 

glycerol-augmented water), the whole joint face could not be completely saturated nor was a 

noticeable change observed in the flow pattern. The rock specimen was saturated by flowing 

Table 4-1. Equivalent Hydraulic Aperture Interpreted From Radial Flow Experiments

Normal 

Stress 

Glycerol-Augmented Water Air 

Loading Unloading Loading Unloading Loading Unloading 

0 MPa 

[0 psi] 

0.3457 mm 

[0.0137 in] 

0.3288 mm 

[0.0129 in] 

0.2729 mm 

[0.0107 in] 

0.2725 mm 

[0.0107 in] — — 

1 MPa 

[145 psi] 

0.3148 mm 

[0.0124 in] 

0.2747 mm 

[0.0108 in] 

0.2447 mm 

[0.0096 in] 

0.2369 mm 

[0.0093 in] 

0.2532 mm 

[0.0100 in] 

0.2427 mm 

[0.0096 in] 

3 MPa 

[435 psi] 

0.2789 mm 

[0.0110 in] 

0.2541 mm 

[0.0100 in] 

0.2097 mm 

[0.0082 in] 

0.2091 mm 

[0.0082 in] 

0.2175 mm 

[0.0086 in] 

0.2141 mm 

[0.0084 in] 

5 MPa 

[725 psi] 

0.2661 mm 

[0.0105 in] 

0.2547 mm 

[0.0100 in] 

0.2003 mm 

[0.0079 in] 

0.1954 mm 

[0.0077 in] 

0.2057 mm 

[0.0081 in] 

0.2036 mm 

[0.0080 in] 

7 MPa 

[1,015 psi] 

0.2510 mm 

[0.0099 in] 

0.2395 mm 

[0.0094 in] 

0.1923 mm 

[0.0076 in] 

0.1923 mm 

[0.0076 in] 

0.1972 mm 

[0.0078 in] 

0.1772 mm 

[0.0070 in] 

8 MPa 

[1,160 psi] 

0.2432 mm 

[0.0096 in] 

0.2432 mm 

[0.0096 in] 

0.1908 mm 

[0.0075 in] 

0.1908 mm 

[0.0075 in] 

0.1916 mm 

[0.0075 in] 

0.1916 mm 

[0.0075 in] 
4-2 

water through the fracture over 2 days. The radial flow experiment provided clear evidence that 

most flow took place in preferential flow channels. 

For the linear flow experiments, at each normal stress setting, steady-state fluid flow was 

established, and the ∆P across the system was recorded from differential pressure transducers 

at various flow rates. At each flow rate setting, steady state was allowed to be reached before 

∆P was measured. A flow rate of 66.6 mm3

/s [4.1 × 10−3 in3/s] was chosen for all normal stress 

tests on the basis of the Reynold’s number requirement to maintain laminar flow. The 

experiment required precision measurements, and although fluctuations were observed in the 

absolute pressure measurements, our interest was in measuring the differential pressure in 

which there was significantly less fluctuation. 

A q-∆P plot for the linear air-flow experimental results showed a linear behavior up to a flow rate 

of 26.6 m3

/s [939.4 ft3/s] corresponding to a ∆P of 4.13 kPa [0.6 psi], beyond which deviation 

from linearity was observed, indicating a non-Darcian flow regime. 

A typical variation in mechanical and equivalent hydraulic apertures with the normal loading and 

unloading conditions for a joint specimen using the linear water-flow configuration is presented 

in Figure 4-1. The changes in hydraulic and mechanical aperture are presented as a percentage 

of the total span of change recorded between 0- and 8-MPa [0- and 1,160-psi] normal stress. 

The change in hydraulic aperture shows a hysteretic behavior similar to that observed in the 

change of mechanical aperture data with a slight shift at intermediate stresses. The large 

hysteretic behavior occurred at small normal stresses. 

4.2 Shear Displacement Experiments 

For shear displacement effect studies, only linear flow experiments were conducted. The shear 

displacement was introduced in steps. At the end of each displacement step, flow data were 

collected for 4 minutes. Flow data were collected with the top rock block sheared in both 

directions (forward and reverse). The same rock block was used for forward and reverse 

displacements at three different normal stress conditions. Table 4-2 shows the normal stresses 

for which the shear was performed and the displacement steps at which water flow data were 

collected. In addition, Table 4-2 presents the estimated equivalent hydraulic apertures and 

permeability reduction factors associated with the various normal loading and shear 

displacement conditions. A permeability reduction factor greater than 1.0 implies a permeability 

increase in response to shear displacement in a given step and a permeability reduction factor 

less than 1.0 imples a decrease. The data presented in Table 4-2 are from one joint specimen. 

The results of several joint specimens had to be discarded because of fluid leakage at high 

normal stress conditions. 

At the end of the final displacement cycle at the 5-MPa [725-psi] normal stress, a maximum 

change in permeability of approximately 300 percent was observed. Because the rubber shoe 

at one end of the upper rock mass was suspected of blocking the outflow channel and thus 

raising the constricted flow effect, the upper rock was not allowed to come back to the zero 

position during reverse shear. The rubber shoe was later proven not to cause the constricted 

flow effect. During testing under 4- and 5-MPa [580- and 725-psi] normal stress, serious leaks 

occurred at the final displacement step of forward shearing and the initial step of the reverse 

shearing; hence, those data were discarded. Constricted flow effects were also observed at 
4-3 

Figure 4-1. Comparison Between Relative Change in Mechanical (Displacement) and 

Effective Hydraulic Apertures as Normal Stress Changes. Water Was Used as the 

Flowing Fluid. [1 MPa = 145 psi] 

several displacement steps, which were likely caused by the production and accumulation of 

fine particles. The migration of fine particles under a steady-state flow condition was clearly 

observed in the transparent tubing. Attempts were made to dislodge the blockage of flow 

through reverse flushing. This procedure helped clear the flow paths in only a few instances. 

Also, at the end of the steady-state experiment at each displacement step, particle accumulation 

was removed by lowering the exit end of the outlet tube, tapping the tube, and flowing water 

from the reservoir to dislodge the accumulated fine particles. 

At the end of the experiment, the apparatus was dismantled to visually inspect the changes in 

the rock joint. A large amount of fine deposits in the form of a thick paste remained on the joint 

surface. The amount of fine deposits migrating during the flow experiment accounts for a small 

Normal Stress 

(MP )
4-4 

portion of the fine deposits generated because of the shear. A total sum of 38.1 g [8.4 × 10−2

 lb] 

of fine solids (including the fines in paste and the fines migrated in the flowing fluid) were 

dislodged from the rock specimen at the end of the final displacement cycle . Figure 4-2 

illustrates the texture and size of the collected fine particles. 

The maximum increase and decrease in the permeability of the fracture during 25.4-mm [1-in] 

shear displacement was observed to be 30 and 30 percent, respectively, with two exceptions at 

the 5-MPa [725-psi] normal stress (Table 4-2). It is interesting to note that, in this set of 

experiments, shear displacement alone does not appear to have a large effect on permeability 

change. The shear-displacement-induced joint dilation does result in an increase in 

permeability, but the joint dilation effects may be reduced by the formation of gouge materials 

during shearing (i.e., there are competing effects). The noticeable change in permeability that 

may be attributed to joint dilation could be observed between forward shearing {shear 

displacement of 3.61 mm [0.14 in]} and reverse shearing {shear displacement of 16.79 mm 

[0.66 in]} at the 4-MPa [580-psi] normal stress. During this period, the joint permeability 

increased 50 percent [a change in permeability reduction factor from 1.3 to 0.8 (see Table 4-2)]. 

Another large joint dilation effect—a 300-percent increase in permeability (a change in 

permeability reduction factor from 3.7 to 0.7)—could be observed at the 5-MPa [725-psi] normal 

stress. The large reduction in permeability (permeability reduction factor greater than 1.0) during 

Figure 4-2. Gouge Material After Being Dried To Illustrate Texture and Particle Size 

Table 4-2. Shear Displacements and Corresponding Equivalent Hydraulic Aperture During 

Forward and Reverse Cycles 
4-5 

Movement 

F: Forward 

R: Reverse Normal Stress

Specified Shear 

Displacement Hydraulic Aperture

Permeability 

Reduction Factor 

F 2 MPa 

[290 psi] 

4.32 mm 

[0.172 in] 

0.12 mm 

[0.47 × 10−2

 in] 1.0 

10.57 mm 

[0.416 in] 

0.14 mm 

[0.55 × 10−2

 in] 0.9 

16.66 mm 

[0.656 in] 

0.14 mm 

[0.55 × 10−2

 in] 0.9 

22.71 mm 

[0.894 in] 

0.15 mm 

[0.59 × 10−2

 in] 0.8 

R 2 MPa 

[290 psi] 

16.08 mm 

[0.633 in] 

0.14 mm 

[0.55 × 10−2

 in] 0.9 

9.80 mm 

[0.386 in] 

0.13 mm 

[0.51 × 10−2

 in] 0.9 

3.58 mm 

[0.141 in] 

0.10 mm 

[0.39 × 10−2

 in] 1.3 

F 4 MPa 

[580 psi] 

3.61 mm 

[0.142 in] 

0.09 mm 

[0.35 × 10−2

 in] 1.3 

R 4 MPa 

[580 psi] 

16.79 mm 

[0.661 in] 

0.15 mm 

[0.59 × 10−2

 in] 0.8 

10.95 mm 

[0.431 in] 

0.13 mm 

[0.51 × 10−2

 in] 0.9 

F 5 MPa 

[725 psi] 

7.11 mm 

[0.280 in] 

0.04 mm 

[0.16 × 10−2

] 3.7 

R 5 MPa 

[725 psi] 

18.42 mm 

[0.725 in] 

0.18 mm 

[0.71 × 10−2

 in] 0.7 

17.25 mm 

[0.679 in] 

0.15 mm 

[0.59 × 10−2

 in] 0.8 

11.40 mm 

[0.449 in] 

0.04 mm 

[0.16 × 10−2

 in] 2.9 

shearing observed at the 2-MPa [290-psi] and 5-MPa [725-psi] normal stresses, on the other 

hand, may be postulated as resulting from substantially more gouge materials being 

produced at these specific shearing steps than at other shearing steps, resulting in a blockage 

of water flow. 
5-1 

5 CONCLUSIONS 

The change in rock-joint permeability under normal stress and shear displacement was 

investigated using large specimens. The apparatus demonstrated a capability for measuring 

the effect of up to 25.4 mm [1 in] of shear displacement on permeability, a substantial 

improvement over previous studies with linear flow experiment capabilities. The equipment 

design described in this report is such that a shear displacement of up to 50.8 cm [2 in] can 

be studied. 

Results from testing a large rock-joint specimen showed up to a 35 percent change in fracture 

permeability in response to an increase in normal stress to 8 MPa [1,160 psi]. Loading and 

unloading the normal stress showed a definite hysteresis in the equivalent hydraulic aperture, 

with a similar trend being observed from the mechanical aperture (normal displacement) 

measurements. As expected, the equivalent hydraulic aperture showed a nonlinear trend with 

the imposed normal stress. 

Performing shear load experiments was more difficult compared to the normal load 

experiments, due primarily to the extreme conditions under which the fluid-sealing mechanism 

functions. This difficulty is also complicated by the fact that shear testing is a destructive 

method of testing. Damage of the rough surface is clearly evident by the significant amount of 

fine particles produced. Shearing resulted in gouge material production and joint dilation. 

Gouge materials tend to reduce permeability, while joint dilation increases permeability. Based 

on the limited experimental results obtained in this study, it appears that gouge production plays 

a dominant role in permeability changes at high normal stress conditions. The maximum 

reduction in permeability (300 percent) was observed at the 5-MPa [725-psi] normal stress. 

However, in many instances of displacements, especially at lower normal stresses, permeability 

may not change significantly because of the competing effects of fracture dilation and gouge 

formation. 
6-1 

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