Physical Properties of Two Core 

Samples from Well 34-9RD2 at the 

Coso Geothermal Field, California 

By C.A. Morrow1 and D.A. Lockner1

Open-File Report 2006-1230 

2006 

Any use of trade, firm, or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government. 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 

1

Menlo Park, Calif. 
2

U.S. Department of the Interior 

DIRK KEMPTHORNE , Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

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not imply endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the 

individual copyright owners to reproduce any copyrighted material contained within 

this report. 
3

Introduction 

 The Coso geothermal field, located along the Eastern California Shear Zone, is 

composed of fractured granitic rocks above a shallow heat source. Temperatures exceed 

640°F (~338°C) at a depth of less than 10000 feet (3 km). Permeability varies 

throughout the geothermal field due to the competing processes of alteration and mineral 

precipitation, acting to reduce the interconnectivity of faults and fractures, and the 

generation of new fractures through faulting and brecciation. Currently, several hot 

regions display very low permeability, not conducive to the efficient extraction of heat. 

Because high rates of seismicity in the field indicate that the area is highly stressed, 

enhanced permeability can be stimulated by increasing the fluid pressure at depth to 

induce faulting along the existing network of fractures. Such an Enhanced Geothermal 

System (EGS), planned for well 46A-19RD, would greatly facilitate the extraction of 

geothermal fluids from depth by increasing the extent and depth of the fracture network. 

 In order to prepare for and interpret data from such a stimulation experiment, the 

physical properties and failure behavior of the target rocks must be fully understood. 

Various diorites and granodiorites are the predominant rock types in the target area of the 

well, which will be pressurized from 10000 feet measured depth (MD) (3048m MD) to 

the bottom of the well at 13000 feet MD (3962m MD). Because there are no core rocks 

currently available from well 46A-19RD, we report here on the results of compressive 

strength, frictional sliding behavior, and elastic measurements of a granodiorite and 

diorite from another well, 34-9RD2, at the Coso site. Rocks cored from well 34-9RD2 

are the deepest samples to date available for testing, and are representative of rocks from 

the field in general. 

Sample Description 

 Core samples of a granodiorite and an altered and healed diorite were obtained 

from well 34-9RD2 at the Coso geothermal field. The granodiorite, from a depth interval 

of 8406 to 8407 feet (B3 R3, 1b and 1c, i.e., Box and Run numbers plus sample ID) was 

light-colored and weakly foliated. The diorite, from a depth interval of 7973’ 11” to 

7974’ 7” (B1 R1, 1f and 1g) was dark colored, with many healed fractures and strong 

foliation. These samples are similar to the rocks at the target stimulation depth in the 

ESG well 46A-19RD. Detailed mineralogy is pending thin section and XRD analysis. 

Procedure 

Porosity and Density 

 Porosity and density measurements were made on two samples each of the 

granodiorite B3 R3 and the altered and healed diorite B1 R1. Cylindrical samples, 2.54 

cm in diameter and 2.54 cm long, were dried overnight at 100°C to remove residual 

water. They were then weighed to determine the dry weight, Wd, and dry density, ρdry. 

The samples were evacuated for 2 hours, and submerged in distilled water under vacuum 

for an additional 8 hours to ensure full saturation. They were then reweighed while 
4

suspended in water to determine the wet submerged weight, Ww. Connected porosity, Φ, 

is calculated from 

Φ = 1-[(Wd-Ww)/V] (1) 

where V is the sample volume. Wet density, ρwet, was calculated by adding the density of 

the water-filled voids (porosity times the density of water, ρwater) to the dry density: 

ρsat = ρdry + Φρwater. (2) 

Strength 

 Room temperature triaxial strength tests were performed in the laboratory on 

cylindrical samples 2.54 cm in diameter and 5.59 cm long. Samples were tested under 

room-dry (as received) conditions. They were jacketed in polyurethane sleeves and 

secured between steel endplugs with hose clamps. Confining pressures were chosen 

relative to the estimated in situ pressure at a target stimulation depth of 10,000 ft (3048m) 

based on an average rock density deduced from in situ density logs. 

Confining Pressure % of Confining Pressure at Target Depth

unconfined 0 

13.0 MPa 25 

26.0 MPa 50 

52.1 MPa 100 Target Depth 

78.2 MPa 150 

 For the first three pressure conditions listed above, an axial load was applied to 

the samples at a rate of 1 mm/sec for 5 mm of axial displacement (shortening), during 

which time the samples failed and then slid on the newly formed shear fractures. For 

samples at the two highest confining pressures of 52.1 and 78.2 MPa, axial and transverse 

strain gauges were affixed to the sides of the cores to observe strain behavior during 

testing. For these samples, deformation proceeded in two phases. First, confining 

pressure was increased slowly to the desired level, then reduced by 10 MPa, and 

increased back up by 10 MPa to the initial pressure. In this way, the bulk modulus could 

be determined (during the pressure cycling portion of the loading) without the effects of 

sample hysteresis from the initial loading. Next, the samples were sheared at an axial 

shortening rate of 1 mm/sec until the axial load reached a value of roughly half of the 

estimated failure strength. The load was then decreased by 100 MPa, and increased by 

100 MPa back to the initial value. The samples were then removed from the vessel. The 

stress cycling under axial load enabled us to calculate Poisson’s ratio, Young’s modulus 

and the stiffness of the samples, as before, without the effects of sample hysteresis. In 

the second phase of these higher pressure experiments, the samples were disconnected 

from the strain-measuring apparatus, re-pressurized to the appropriate confining pressures 

and sheared at 1 mm/sec for 5 mm of axial shortening, in the same fashion as the first 

three lower-pressure experiments. Pressures and displacements were maintained by a 

computer-controlled servomechanism and recorded by a data acquisition system once per 

second. Accuracy for the confining pressure was ± 0.05 MPa; for σ1

, ± 0.1 MPa; for the 

displacement, ± 5 mm; and for strain, ± 0.5%. 
5

Results 

Porosity and density 

Porosity and density values are given in Table 1. Because the granodiorite was a 

fairly homogeneous material, both test samples gave similar values of around 1% 

porosity, a dry density of 2.64 to 2.65 g/cc, and a wet density (assuming the pores are 

filled with water) of around 2.65 to 2.66 g/cc. There was considerable sample variability 

in the diorite, causing porosity to range from 0.6 to 1.0%, dry density from 2.81 to 2.88 

g/cc, and wet density from 2.82 to 2.89 g/cc. 

Strength 

 Differential stress (measured load minus confining pressure) versus axial 

displacement is shown at the five desired pressure conditions for the granodiorite and 

diorite samples (Figure 1). For the granodiorite, the failure stress increased in a 

systematic way with pressure, consistent with the overall homogeneity of the rock. Most 

samples experienced a total stress drop after failure. Friction sliding on the newly 

fractured surface stabilized quickly for the 13 MPa run, but required more than a 

millimeter of displacement to evolve at the higher confining pressures. 

The diorite samples were overall weaker than the granodiorites (Figure 1b), as 

well as being less systematic in their failure strength, owing to considerable sample 

heterogeneity. For instance, at confining pressures of 26 and 78.2 MPa the failure 

strength was markedly low relative to the trend of the other pressure runs. In addition, 

the stress drops were not total at these two confining pressures. Residual strength of the 

diorites stabilized quickly, with the exception of the sample at 52.1 MPa. Peak failure 

stress versus confining pressure (Figure 2 and Table 2) illustrates these failure trends for 

the two rock types. 

 Fracture angles for all samples ranged from 16 to 25° measured from the vertical 

axis. In cases where more than one fracture formed, the principle fracture angle was 

recorded. From these angles and the recorded stresses of each sample, the shear and 

normal stress at failure resolved on the fracture surface and the coefficient of friction can 

be determined by the relations 

σΔ = axial load – Pc

 (3) 

θ = fracture angle (4) 

τ = (σΔ/2)(sin(2θ)) (5) 

σn = Pc

 + (σΔ/2)(1-cos(2θ)) (6) 

μ = τ/σn. (7) 

where σΔ is the differential stress, Pc is the confining pressure, τ and σn are the shear and 

normal stresses respectively and μ is the coefficient of friction. Failure data are listed in 

Table 2. 

 From equations (5) and (6), we can construct Mohr’s circle diagrams (Figure 3) 

for the granodiorite and diorite samples. The systematic trend of failure in the 

granodiorite (Figure 3a) clearly contrasts with the uneven spacing of the Mohr’s circles 

for the diorite samples (Figure 3b) due to the less systematic failure of the diorites. The 
6

tangent to these circles give the failure envelopes for each rock type, which we show in 

Figure 4 using data from Figure 3 at an enlarged scale for clarity. The slope of the line is 

the coefficient of internal friction, μi, 1.32 for the granodiorite and 1.29 for the diorite. 

The Y intercept gives cohesion, Co, 46.5 MPa for the granodiorite and 31.9 MPa for the 

diorite. Using equations (6) and (7) gives the angle of internal friction, φi

, and produce 

similar values of 52.8° and 52.3° for the granodiorite and diorite, respectively (Table 2). 

The two anomalously low-strength diorites were not included in these calculations. 

In addition, the residual coefficient of friction at 5 mm of displacement was 

calculated (Table 3). These values represent the rock-on-rock coefficient of friction of 

the fractured surfaces. Values ranged from 0.81 to 1.19 for the granodiorite and 0.73 to 

0.90 for the diorite, a bit higher than typical values from Byerlee’s Law of 0.6 to 0.85. It 

should be noted that some samples fractured along surfaces that intersected the steel 

endplugs. In this case, calculation of residual friction will be too high since part of the 

shear stress is still supported by intact rock. 

Although these tests were conducted at room temperature, strength results of Coso 

samples obtained by TerraTek (unpublished data) indicate little sensitivity of dry strength 

to temperature in the range of temperatures reasonable for the Coso Geothermal Field. 

Elastic properties 

 Elastic properties were determined from tests run at the two highest confining 

pressures of 52.1 and 78.2 according to the following relations: 

Bulk Modulus B = ΔPc

/Δεvol =ΔPc/(Δεax + 2Δεt) (8) 

Young’s Modulus E = σ1/εax (9) 

Poisson’s Ratio υ = Δεt/Δεax (10) 

Stiffness k = σ1/Δl (11) 

where Pc is confining pressure, εvol is volumetric strain, εax is axial strain, εt is transverse 

strain, σ1 is axial stress and l is the length of the sample. Bulk modulus, the change in 

confining pressure with change in volumetric strain, was determined from the 10 MPa 

unloading and subsequent reloading of the confining pressure, which generally had a 

different slope than the initial pressure loading curve. For calculations of Young’s 

Modulus, the change in differential stress with axial strain; Poisson’s Ratio, the change in 

axial strain with transverse strain; and Stiffness, the change in stress with displacement, 

the 100 MPa unloading and reloading of the differential stress was used. These curves 

also displayed a different slope than the initial stress loading curve. All these data are 

shown for the granodiorite in Figures 5a-c and 6a-c, and for the diorite in Figures 7a-c 

and 8a-c. Elastic properties, determined from equations 8-11, are given in Table 4. In 

addition, standard stress-strain curves are shown for the two rock types in Figures 5d-8d. 

In these plots, the failure stress (determined in a subsequent experiment), is included as a 

single data point for comparison. The axial strain at failure was estimated from the 

known displacement at failure and the slope of the loading curves before pressure 

cycling. 

Summarizing the strain tests, we see that the Poisson’s Ratio of the two rock types 

was fairly independent of pressure, at around 0.27 and 0.31 for the granodiorite and 

diorite, respectively. Other elastic properties (Bulk Modulus, Young’s Modulus and 
7

Sample Stiffness) were pressure dependent, although not always in a consistent manner 

in the diorite samples. Also, the diorite samples had uniformly higher values for each of 

the elastic properties than the granodiorite. 
8

Table 1. Porosity and Density 

 Sample volume, 

 cc 

dry weight, 

 g 

wet weight, 

 g 

porosity, Φ dry density, 

 g/cc 

wet density, 

 g/cc 

granodiorite 

 B3R3 1c 13.13 34.688 21.694 0.010 2.642 2.652 

B3R3 1c 12.71 33.745 21.162 0.010 2.654 2.664 

diorite 

B1R1 1f 13.06 37.699 24.711 0.006 2.886 2.892 

B1R1 1f 11.80 33.228 21.535 0.010 2.815 2.825 

Table 2. Failure Data 

 Sample Pc, 

 MPa 

 σ1, MPa 

 Peak 

 σΔ, MPa 

 Peak 

sample 

failure 

angle,° 

 σn, 

 MPa 

 τ, 

 MPa 

 μi, 

internal 

 φi, 

internal 

failure 

angle,° 

 Co, 

MPa 

granodiorite 

B3R3 1b 0.0 240.2 240.2 18 22.9 70.6 

 B3R3 1c1 13.0 387.1 374.1 23 70.1 134.6 

 B3R3 1c2 26.0 596.6 570.6 16 69.4 151.2 

 B3R3 1b1 52.1 770.0 717.9 22 152.8 249.4 

 B3R3 1b2 78.2 945.2 867.0 19 170.1 266.9 

 1.32 52.8 46.5

diorite 

B1R1 1g1 0.0 186.0 186.0 18 17.8 54.7 

 B1R1 1g2 13.0 301.0 288.0 20 46.7 92.6 

 B1R1 1g3 26.0 323.3 297.3 20 60.8 95.6 

 B1R1 1f1 52.1 633.9 581.8 18 107.7 171.0 

 B1R1 1f2 78.2 581.8 503.6 25 168.2 192.9 

 1.29 52.3 31.9
9

Table 3. Stresses and Residual Friction at 5 mm Displacement 

 Sample Pc, MPa σΔ, MPa τ, MPa σn, MPa μ residual 

 granodiorite 

 B3R3 1c1 13.0 88.2 31.7 26.5 1.20 

 B3R3 1c2 26.0 131.6 34.9 36.0 0.97 

 B3R3 1b1 52.1 269.5 93.6 89.9 1.04 

 B3R3 1b2 78.2 285.7 88.0 108.5 0.81 

 

 diorite 

 B1R1 1g2 13.0 54.8 17.6 19.4 0.91 

 B1R1 1g3 26.0 87.4 28.1 36.2 0.76 

 B1R1 1f1 52.1 182.1 53.5 69.5 0.77 

 B1R1 1f2 78.2 252.7 96.8 132.3 0.73 

Table 4. Elastic Properties 

 Sample Pc, MPa Poisson's 

 Ratio 

Bulk Modulus,

 GPa 

 Young's 

Modulus, GPa 

 Stiffness, 

 MPa/mm 

granodiorite 

B3R3 1b1 52.1 0.277 26.9 74.1 1314 

 B3R31b2 78.2 0.270 36.2 75.4 1342 

diorite 

B1R1 1f1 52.1 0.311 49.1 105.1 1880 

 B1R1 1f2 78.2 0.313 53.8 100.7 1778 
10

0

200

400

600

800

1 103

0.0 1.0 2.0 3.0 4.0 5.0

Coso Granodiorite Failure Strength

unconfined

Pc= 13.0 MPa

Pc= 26.0 MPa

Pc= 52.1 MPa

Pc= 78.2 MPa

Differential Stress, MPa

Displacement, mm

240.2

374.1

570.6

867.0 MPa

717.9

Figure 1a

Figure 1. Differential stress as a function of axial displacement for (a) granodiorite 

samples from a depth of 8406 ft, and (b) diorites from a depth of 7974 ft. 
11

0

100

200

300

400

500

600

0.0 1.0 2.0 3.0 4.0 5.0

Coso Diorite Failure Strength

unconfined

Pc= 13.0 MPa

Pc= 26.0 MPa

Pc= 52.1 MPa

Pc= 78.2 MPa

Differential Stress, MPa

Displacement, mm 

581.8 MPa

503.6

297.3

288.0

186.0

Figure 1b
12

0

200

400

600

800

1 103

0.0 20.0 40.0 60.0 80.0 100.0

Peak Failure Stress

Granodiorite

Diorite

Differential Stress, MPa

Confining Pressure, MPa

Figure 2

Figure 2. Peak failure stress of the granodiorites and diorites as a function of confining 

pressure. 
13

Pc= 78.2 MPa

52.1

26

13

unconfined

-500

-400

-300

-200

-100

0

100

200

300

400

500

0 100 200 300 400 500 600 700 800 900 1000

Granodiorite Mohr's Circle

Shear Stress, MPa

Normal Stress, MPa

Figure 3a

μ

i

= 1.32

Figure 3. Mohr’s circle diagrams for (a) granodiorite samples and (b) diorites. Shear and 

normal stresses at failure (red data points) for each confining pressure are shown along 

the perimeter of the corresponding Mohr circle. 
14

Pc= 78.2 MPa

52.1

26

13

unconfined

-300

-200

-100

0

100

200

300

0 100 200 300 400 500 600 700

Diorite Mohr's Circle

Shear Stress, MPa

Normal Stress, MPa

Figure 3b

μ

i

 = 1.29
15

0

50

100

150

200

250

300

0.0 50.0 100.0 150.0 200.0 250.0 300.0

Granodiorite Failure Envelope

y = 46.545 + 1.3183x R= 0.99538 

φ

i

 = 52.8°

μ

i

 = 1.32

x intercept = -35.3 MPa

C0

 = 46.5 MPa

Shear Stress, MPa

Normal Stress, MPa

Figure 4a

C0

Figure 4. Failure envelopes for (a) granodiorites and (b) diorites. For the diorites, the 

two anomalously weak samples were excluded. 
16

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Diorite Failure Envelope

y = 31.904 + 1.2928x R= 0.99999 

φ

i

 = 52.3°

μi = 1.29

x intercept = -24.6 MPa

Shear Stress, MPa

Normal Stress, MPa

Figure 4b

C0 C0

 = 31.9 MPa
17

-5

-4

-3

-2

-1

0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Bulk Modulus

Granodiorite, Pc= 52.1 MPa

axial strain

transverse strain

volumetric strain

millistrain

Confining Pressure, MPa 

-y = -0.97958 - 0.012115x

y = -1.05 - 0.012939x

Figure 5a

Figure 5. Strain data for the determination of elastic properties of the granodiorite at a 

confining pressure of 52.1 MPa: (a) Bulk Modulus, (b) Young’s Modulus and Sample 

Stiffness, (c) Poisson’s Ratio, and (d) the stress-strain relation, with the failure strength 

and estimated strain at failure shown as a single point. 
18

0

100

200

300

400

500

-10

-8

-6

-4

-2

0

2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Young's Modulus

Granodiorite, Pc= 52.1 MPa

differential stress

axial strain

Differential Stress, MPa

Axial Strain, millistrain

Displacement, mm

y = -616.82 + 3811.5x

-y = -2.1981 - 5.1432x

Figure 5b
19

-9

-8

-7

-6

-5

-4

-3

-2

-1

-2

-1.5

-1

-0.5

0

0.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Poisson's Ratio

Granodiorite, Pc= 52.1 MPa

axial strain

transverse strain

Axial Strain, millistrain

Transverse Strain, millistrain

Displacement, mm 

y = -2.1981 - 5.1432x

y = -1.4512 + 1.4248x

Figure 5c
20

-8

-6

-4

-2

0

2

0

100

200

300

400

500

600

700

800

0.00 -2.00 -4.00 -6.00 -8.00 -10.00 -12.00 -14.00

Stress-Strain

Granodiorite, Pc= 52.1 MPa

volumetric strain

transverse strain

differential stress

Volumetric, Transverse strain, millistrain

Differential Stress, MPa

Axial Strain, millistrain

failure stress 717.9 MPa

estimated strain -13.55

Figure 5d
21

-7

-6

-5

-4

-3

-2

-1

0

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0

Bulk Modulus

Granodiorite, Pc= 78.2 MPa

axial strain

transverse strain

volumetric strain

millistrain

Confining Pressure, MPa 

-y = -1.1095 - 0.0085607x

y = -1.7509 - 0.010494

Figure 6a

Figure 6. Strain data for the determination of elastic properties of the granodiorite at a 

confining pressure of 78.2 MPa: (a) Bulk Modulus, (b) Young’s Modulus and Sample 

Stiffness, (c) Poisson’s Ratio, and (d) the stress-strain relation, with the failure strength 

and estimated strain at failure shown as a single point. 
22

0

100

200

300

400

500

-10

-8

-6

-4

-2

0

2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Young's Modulus

Granodiorite, Pc= 78.2 MPa

differential stress

axial strain

Differential Stress, MPa

Axial Strain, millistrain

Displacement, mm 

y = -609.78 + 3849.3x

-y = -2.8027 - 5.1084x

Figure 6b
23

-10

-8

-6

-4

-2

0

2

-2

-1.5

-1

-0.5

0

0.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Poisson's Ratio

Granodiorite Pc= 78.2

axial strain

transverse strain

Axial Strain, millistrain

Transverse Strain, millistrain

Displacement, mm 

-y = -1.697 + 1.3799x

y = -2.8027 - 5.1084x

Figure 6c
24

-10

-8

-6

-4

-2

0

2

0

200

400

600

800

1000

0.00 -2.00 -4.00 -6.00 -8.00 -10.00 -12.00 -14.00 -16.00

Stress-Strain

Granodiorite, Pc=78.2 MPa

volumetric strain

transverse strain

differential stress

Volumetric, Transverse Strain, millistrain

Differential Stress, MPa

Axial Strain, millistrain

failure stress 867.0 MPa

estimated strain -15.88

Figure 6d
25

-2

-1.5

-1

-0.5

0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Bulk Modulus

Diorite, Pc= 52.1 MPa

axial strain

transverse strain

volumetric strain

millistrain

Confining Pressure, MPa 

y = -0.11149 - 0.0050013x

-y = -0.26874 - 0.0076758x

Figure 7a

Figure 7. Strain data for the determination of elastic properties of the diorite at a 

confining pressure of 52.1 MPa: (a) Bulk Modulus, (b) Young’s Modulus and Sample 

Stiffness, (c) Poisson’s Ratio, and (d) the stress-strain relation, with the failure strength 

and estimated strain at failure shown as a single point. 
26

0

50

100

150

200

-3

-2.5

-2

-1.5

-1

-0.5

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Young's Modulus

Diorite, Pc= 52.1 MPa

differential stress

axial strain

Differential Stress, MPa

Axial Strain, millistrain

Displacement, mm

y = -323.47 + 4115.2x

-y = -0.54244 - 3.9151x

Figure 7b
27

-3

-2.5

-2

-1.5

-1

-0.5

0

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Poisson's Ratio

Diorite, Pc= 52.1 MPa

axial strain

transverse strain

Axial Strain, millistrain

Transverse Strain, millistrain

Displacement, mm 

-y = -0.62894 + 1.2176x

y = -0.54244 - 3.9151x

Figure 7c
28

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0

100

200

300

400

500

600

0.00 -1.00 -2.00 -3.00 -4.00 -5.00 -6.00 -7.00 -8.00

Stress-Strain

Diorite, Pc= 52.1 MPa

volumetric strain

transverse strain

differential stress

Volumetric, Transverse Strain, millistrain

Differential Stress, MPa

Axial Strain, millistrain

failure stress 581.8 MPa

estimated strain -7.02

Figure 7d
29

-3

-2.5

-2

-1.5

-1

-0.5

0

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0

Bulk Modulus

Diorite, Pc = 78.2 MPa

axial strain

transverse strain

volumetric strain

millistrain

Confining Pressure, MPa

y = -0.41344 - 0.0046126x

-y = -0.48194 - 0.0069808x

Figure 8a

Figure 8. Strain data for the determination of elastic properties of the diorite at a 

confining pressure of 78.2 MPa: (a) Bulk Modulus, (b) Young’s Modulus and Sample 

Stiffness, (c) Poisson’s Ratio, and (d) the stress-strain relation, with the failure strength 

and estimated strain at failure shown as a single point. 
30

0

50

100

150

200

250

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Young's Modulus

Diorite, Pc= 78.2 MPa

differential stress

axial strain

Differential Stress, MPa

Axial Strain, millistrain

Displacement, mm 

y = -358.2 + 4098.1x

-y = -0.68034 - 4.068x

Figure 8b
31

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Poisson's Ratio

Diorite, Pc= 78.2 MPa

axial strain

transverse strain

Axial Strain, millistrain

Transverse Strain, millistrain

Displacement, mm

-y = -1.0009 + 1.272x

y = -0.68034 - 4.068x

Figure 8c
32

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0

100

200

300

400

500

600

0.00 -1.00 -2.00 -3.00 -4.00 -5.00 -6.00 -7.00

Stress-Strain

Diorite, Pc= 78.2 MPa

volumetric strain

transverse strain

differential stress

Volumetric, Transverse Strain, millistrain

Differential Stress, MPa

Axial Strain, millistrain

failure stress 503.6 MPa

estimated strain -6.30

Figure 8d