lur

cim

rth Florida, Jacksonville, FL 32224-2666, USA

lting, Inc., Las Vegas, NV 89118, USA

specimens. As expected, compressive strength decreases with increasing porosity due to lithophysae in tuff and cavities in

(200spalling through web failure as the percentage of macroporosity within the specimen increased.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Tuff; Lithophysae; Porosity; Plaster; Uniaxial compressive strength; Failure modes

1. Introduction

Rocks are composed of both matrix material and

pore space; an increase in porosity reduces their

stiffness and strength. Typically, in rock, porosity is

random. The influence of microporosity on the com-

pressive strength of rock specimens has been well

documented for a variety of rock types, such as

sandstones (Dunn et al., 1973; Vernik et al., 1993;

Yale and Nieto, 1995; Palchik, 1999), dolomitesgrouped into four distinct categories: spalling, axial splitting,plaster analog specimens. Failure modes of cylindrical specimens were also investigated. The failure modes observed were

shear failure and web failure. The failure modes transition fromcDepartment of Civil and Environmental Engineering, University of Nevada Las Vegas, Las Vegas, NV, 89154, USA

Received 19 February 2003; accepted 22 January 2004

Abstract

The presence of lithophysae in some units of Topopah Spring Tuff at Yucca Mountain, Nevada, the U.S. high level nuclear

waste repository, have a detrimental effect on the engineering properties of the rock mass and its performance. The lithophysae

were formed by pockets of gas trapped within the falling volcanic ash that formed the tuff units. The porosity associated with

the lithophysae is termed macroporosity because of the large pore size as compared with traditional rock pores. In this paper,

lithophysae-rich tuff and analog models (both cylindrical and cubic) made of plaster of Paris containing artificially created

cavities were tested to assess the effect of macroporosity on both the uniaxial compressive strength and failure modes of theaDivision of Engineering, University of NobMACTEC Engineering and Consucontaining cavities

Nick Hudymaa,*, B. Burcin Avarb, Moses KarakouziancCompressive strength and fai

Topopah Spring Tuff spe

Engineering Geology 73microscopic and created by spaces between minerals

or individual grains. The pore structure within rock is

usually interconnected and the distribution of pores is

0013-7952/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.enggeo.2004.01.003

* Corresponding author. Fax: +1-904-620-1391.

E-mail address: nhudyma@unf.edu (N. Hudyma).e modes of lithophysae-rich

ens and analog models

www.elsevier.com/locate/enggeo

4) 179190(Hatzor and Palchick, 1997), dolerite (Dearman,

1974) and granite (Dearman et al., 1978).

In some rock types, porosity is also created by

cavities, vugs or vesicles that are visible to the

unaided eye. This type of porosity can be termed

macroscopic porosity or simply macroporosity. Rocks

such as vesicular basalt (Al-Harthi et al., 1999), vuggy

type analog models using a two-phase media can be

beneficial to the study of naturally occurring rocks.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190180limestone and lithophysae-rich tuff (Tillerson and

Nimick, 1984) contain this type of porosity. Previous

studies on such rocks indicate that the compressive

strength depends upon the amount of macroporosity

(Al-Harthi et al., 1999) and pore structure (Price et al.,

1994).

The size distribution of pores also affects the

strength. Luping (1986) found that a material with

low porosity but higher number of large pores may

have a lower strength than a material with a higher

porosity but a fewer number of large pores.

The engineering properties of lithophysae-rich (or

sometimes called lithophysal) tuff has attracted wide

spread attention in recent years because the U.S. high

level nuclear waste repository will be located within

the Topopah Spring Tuff units of Yucca Mountain,

Nevada, which contains lithophysae-rich portions.

Lithophysae are the cavities formed by trapped pock-

ets of gas within the falling volcanic ash. Besides the

microporosity, these cavities are the main source of

porosity in the tuff. There have been numerous studies

on the mechanical properties of Topopah Spring tuff,

such as compressive and tensile strength, elastic

modulus, Poissons ratio (for instance, Price et al.,

1994; Martin et al., 1994, 1995; Avar, 2002; Avar et

al., 2003). Other researchers have found good corre-

lations between compressive strength and porosity

(Howarth, 1987; Nimick, 1988; Fuenkajorn and Dae-

men, 1992). The porosity of lithophysae-rich tuff

differs from other porous rocks such as sandstone,

for example, where porosity is due to spaces between

grains. Lithophysae are macroscopic and they have a

large variability in their distribution of size and shape.

The variability of size and shapes of the cavities cause

large variations in strength of specimens so determi-

nation of design strength values based on porosity

becomes cumbersome.

The purpose of this paper is to investigate the

influence of macroporosity in lithophysae-rich tuff

on uniaxial compressive strength and failure modes

using both tuff specimens and rock-type analog mod-

els made from plaster of Paris, which will be referred

to as plaster throughout the paper. When testing tuff

specimens, there is no control over discontinuities,

such as cracks or fractures, present in the rock or the

sizes and shapes of the lithophysae. The use of rock-

type analog models allows investigators focus onporosity due to cavities so that not only the influenceIn this study plaster was used as an analog material

to produce both cubic and cylindrical specimens. The

cavities were created by Styrofoam (made of polysty-

rene) spheres in both cylindrical and cubic specimens,

and injecting air in some of the cylindrical specimens.

Although lithophysae are not in simple geometrical

shapes, it is not feasible to physically model complex

cavity shapes. Therefore, a simple spherical geometry

was chosen to model the cavities. Both specimens

were tested under uniaxial compression and their

compressive strengths were computed. These results

were compared to the results from uniaxial compres-

sion tests on lithophysae-rich tuff, either tested for this

study or taken from a database containing test results.

Finally, failure modes of plaster models and tuff were

investigated as an attempt to understand possible

failures of tuff during its performance in the field.

2. Specimens

The specimens used in this study include both

lithophysae-rich tuff specimens from outcrops near

Yucca Mountain on the Nevada Test Site and plaster

analogs. Each of the specimen types and materials are

described in detail below.

2.1. Lithophysae-rich tuff

The tuff specimens used in this study were cut

from large irregularly shaped samples from surface

outcrops of the upper lithophysae-rich zone of Top-of the amount of porosity but also the influence of

sizes of cavities can be studied.

There are some shortcomings when using physical

models to investigate rock properties because of the

inherent heterogeneous nature of natural rock. Rock is

often a multi-phase composite material; the matrix is

composed of different minerals, inclusions and de-

fects. The rock-type analog models used to represent

rock produced from dry plaster are two-phase materi-

als (solid and voids) and they can be considered

almost homogeneous when compared with naturally

occurring rocks. If the effect of each phase on the

overall strength of rock is assumed to be small, rock-opah Spring Tuff near Yucca Mountain on the Nevada

Test Site. Topopah Spring Tuff (Tpt) is the proposed

repository host horizon, which was formed from a

volcanic eruption that occurred about 12.8 million

years ago (Sawyer et al., 1994) and has a maximum

thickness of about 350 m in the vicinity of Yucca

Mountain (Fox et al., 1990). Petrographically, Tpt is

zoned from basal crystal poor high silica rhyolite,

with silica content of approximately 75% to a capping

crystal rich quartz latite with silica content of approx-

imately 69% (Schuraytz et al., 1989). The actual

repository will be approximately located in the middle

to lower portion of the Topopah Spring tuff. This

section is densely welded, with variable fracture

density and lithophysae-rich content (BSC, 2001).

Portions of the repository will be located within two

lithophysae-rich zones, upper lithophysal zone

(Tptpul) and lower lithophysal zone (Tptpll) (Mon-

gano et al., 1999).

Excavations within existing cross drifts at Yucca

Mountain provide useful information regarding lith-

are typically 1:1 to 5:4 with a few individual cavities

up to 3:1 locally. The vast majority of these cavities

are orientated with their major axes in a near hori-

zontal orientation. Many of the larger cavities have

irregular boundaries and appear to have formed from a

number of coalesced cavities. Vapor phase minerals

coat the interior surfaces of cavities (Mongano et al.,

1999).

Besides influencing the continuity and homogene-

ity of the rock mass, the lithophysae make coring of

the rock very difficult. In the field, the size of the

lithophysae ranges from approximately 1 to 100 cm

(Mongano et al., 1999). Since the sizes of the cavities

are on the same order of size or larger than the core

barrel, core recovery was poor (Martin et al., 1995).

This also means that the typical NX sized cylindrical

specimens would not contain large cavities that are

seen in the field. For this reason, it was decided to use

cubic rock specimens rather than cylindrical ones for

uniaxial compression testing. Fig. 1 shows photo-

N. Hudyma et al. / Engineering Geology 73 (2004) 179190 181ophysae within the tuff. In the upper lithophysal

zone, lithophysae generally comprise 25% to 40%

of the rock but as much as 60% locally. In the lower

zone, there is approximately 5% to 30% lithophysal

porosity.

The lithophysae shape varies from gash like to

ellipsoidal to approximately spherical. Aspect ratiosFig. 1. Examples of cubgraphs of several of these cubic tuff specimens.

It is difficult to differentiate the components of

total porosity (microporosity and macroporosity) of

individual tuff specimens because of its heteroge-

neous nature. Otto (2003) performed analyses on 47

core samples in an attempt to determine the micropo-

rosity and rock particle densities of tuff from the uppere tuff specimens.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190182and lower lithophysal zones. He determined the mi-

croporosity for three different locations within the

matrix material: matrix ground mass, lithophysal

cavity rim, and spots (similar to rims but without

cavities). The microporosity of the rim and spots were

indistinguishable from each other and were grouped

together. He found that the upper lithophysal zone

average microporosity values were found to be 10.3%

and 28.8%, respectively.

Test data from eight cubic specimens of tuff from

outcrops of the upper lithophysal zone, which were

tested by Avar (2002), are used in this study. Each

specimen was roughly cube shaped with edge dimen-

sions between 10 and 15 cm. The size of the lith-

ophysae seen on the surface of the specimens ranged

between approximately 0.1 to 5 cm. The total porosity

of the tuff specimens, including both microscopic and

cavity porosity, ranged between 17% and 32%.

Test data from Sandia National Laboratories and

their subcontractors was also used to augment the tuff

data. This data was from two sources, the North Ramp

Geotechnical (NRG) boreholes NGR-6 and NGR-7/

7A (Martin et al., 1994, 1995) and the upper litho-

physal zone of the Topopah Spring tuff (Price et al.,

1984). The specimens from the North Ramp Geotech-

nical boreholes had length to diameter ratio of 2:1

(10.16:5.08 cm). The total porosities of these speci-

mens ranged between 12% and 24%. The specimens

tested by Price et al. (1984) were large diameter cores,

26.7 cm, and had a length to diameter ratio of 2:1.

These specimens had total porosities ranging between

30% and 40%.

2.2. Plaster models with cavities

Dry plaster has been used alone or mixed with

other materials such as sand, clay or aggregates as a

rock-type analog material (see Stimpson, 1970, for a

summary of modeling materials in rock mechanics).

Lajtai and Lajtai (1975) modeled cavity collapse

using gypsum plaster. Leite and Ferland (2001)

mixed polystyrene spheres with sand, plaster and

water to produce artificial porous rock to determine

compressive strength and Youngs modulus. In this

study, plaster was used as an analog model to

stimulate tuff matrix material. Spherical Styrofoam

inclusions were mixed with plaster in order to repre-sent cavities, similar to lithophysae in tuff. TheStyrofoam is stiffer than air, which fills the lithophy-

sae, but is much less stiff than the plaster. As seen in

Fig. 1, lithophysae are not exactly in spherical shape,

therefore using spherical Styrofoam inclusions in the

analog models is only an approximation for simulat-

ing such cavities.

A plaster paste was produced using 2 parts plaster

to 1 part water and the required volume of inclusions.

The waterplaster paste was poured into a mold and

allowed to dry overnight. The next day, the mold was

removed and the specimen was weighed daily to

monitor specimen drying. Once a constant weight

was reached, the loading sides of the specimens were

then ground flat to enable a uniform load distribution.

Further details regarding the plaster specimens are

presented below.

2.2.1. Cubic specimens

Fourteen cubic plaster specimens were produced in

an aluminum mold with sides of approximately 15 cm.

The specimens contained macroporosities between 5%

and 35% were produced using spherical Styrofoam

inclusions. Six different diameters of spherical Styro-

foam inclusions, ranging between 2.5 to 10.2 cm, were

used in the specimens. Different sizes and numbers of

Styrofoam spheres were mixed with plaster paste and

poured into the mold (see Avar, 2002 for details). This

technique was meant to obtain a random distribution

of the inclusions in the specimen.

2.2.2. Cylindrical specimens

Twenty cylindrical specimens, approximately 10.16

cm in length and 5.08 cm in diameter, were also

produced. Ten of the specimens were produced with

Styrofoam inclusions. The Styrofoam inclusions were

spherical with a nominal diameter of approximately 6

to 8 mm. Plasterwater paste was mixed with the

number of Styrofoam inclusions to produce the re-

quired porosity and then poured into plastic containers.

Macroporosity of cylindrical specimens ranges from

approximately 7% to 37%.

The cavities in the remaining 10 specimens were

created by injecting air into the plaster paste. To

produce these specimens, the bottom third of the plastic

cylinder was filled with plaster and a graduated syringe

was used to inject air bubbles of a certain volume into

the plaster. Similarly, the middle and top thirds of thespecimen were filled and then air was injected from a

each specimen was calculated using the following

equation:

solid plaster volume. The plaster matrix, although

very porous, is assumed to be solid.

axial load and displacement data were collected using

a data acquisition system.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190 183/ 1 cdryGscwater

1

where /, Gs, cwater, cdry are the macroporosity, specificgravity, unit weight of water and dry unit weight of

solid, respectively. One drawback of this method is

that calculated porosity does not distinguish between

the microporosity due to spaces between grains in the

matrix, and the macroporosity due to lithophysae,

microcracks and microfractures. Thus, the porosity

of the tuff specimens is termed total porosity. The total

porosity for the lithophysae-rich tuff ranged between

approximately 12% and 35%. Based on the work of

Otto (2003), it is appropriate to assume a presence of

lithophysae in specimens containing greater thansyringe. Attempts were made to produce specimens

with between 5% and 20%macroporosity, based on the

volume of air-injected into the specimen. However,

only 4% to 8% porosity in terms of injected air were

successfully produced. The shapes of cavities were

typically ellipsoidal with varying orientations.

Several solid plaster cylinders were tested to obtain

an average 0% macroporosity value of compressive

strength. The dimensions of cylindrical specimens

were approximately 10.16 cm in length and 5.08 cm

in diameter. The solid specimens were produced by

filling bottom third of the plastic cylinder molds,

tapping the sides of the molds to remove air bubbles

and similarly filling and tapping the middle third and

top thirds of the cylindrical molds.

3. Porosity calculations

The porosity of tuff specimens and the plaster

specimens were calculated in two different ways.

For the tuff specimens, the porosity was computed

on the assumption that the tuff is made up of both

solids and voids. However, the voids could be in the

form of microporosity within the matrix, macroporos-

ity due to lithophysae, and microcracks and micro-

fractures. In order to determine the porosity, the

specific gravity of the tuff matrix and the dry unit

weight of the tuff specimen were determined accord-

ing to ASTM D854 (2002) and the total porosity ofapproximately 10% total porosity.The cylindrical plaster specimens were tested using

a small, 50 kN load frame at the University of North

Florida. The load was measured using a proving ring/

electronic dial indicator combination whose signal

was input into a personal computer. Axial strain was

measured with an electronic dial indicator. The nom-

inal strain rate used for tests was 5 10 4. Theexperimental data is presented in tabular form in

Appendix A.

5. Porosity versus compressive strength

5.1. Plaster specimens

Previous investigations have shown that there is

only a small dependence on specimen shape andThe unit weight of the specimens containing either

air-injected cavities or Styrofoam inclusions was also

determined using the same procedure. The difference

in unit weights was attributed to the presence of either

the large air bubbles or Styrofoam inclusions and the

corresponding macroporosity was computed. The

weight of the Styrofoam inclusions was neglected in

the calculations. The macroporosity for plaster speci-

mens ranged between approximately 4% and 38%.

4. Uniaxial compressive strength testing

Uniaxial compressive strength testing of all tuff

and cubic plaster specimens was conducted according

to ASTM D2938 (1995). The lithophysal tuff speci-

mens and the cubic plaster specimens were tested at

the Materials Testing Laboratory at the Nevada Test

Site, Mercury, NV, using a 4448.2 kN capacity MTS

load frame. During the uniaxial compression testing,To determine the macroporosity of the plaster

specimens, the unit weight of the solid plaster speci-

mens was determined through weight and volume

measurements. This unit weight corresponded to

specimens with zero macroporosity. The distribution

of microporosity was assumed to be the same in anyspecimen strength. Mansur and Islam (2002) found

the strength of cubic specimens was only slightly

higher than cylindrical concrete specimens with a

2:1 height to diameter ratio. Andreev (1995) also cites

references that report the ratio of peak strengths

between circular and rectangular rock specimen sec-

tions is 1:0.91. Based on these findings a common

regression curve was generated for both cylindrical

and cubic specimens.

The relationship between macroporosity and com-

pressive strength of all of the plaster specimens,

regardless of specimen shape, is shown in Fig. 2.

The best-fit regression curve is:

rc 12:618e0:0415/ R2 0:80 2

where rc is the uniaxial compressive strength and / isthe macroporosity, in percentage, due to the spherical

Styrofoam inclusions and R2 is the coefficient of

determination. The specimens containing the spherical

Styrofoam inclusions range in macroporosity from

4.6% to 37.6% and closely follow the best-fit regres-

sion curve.

The average compressive strength of the solid

plaster cylinders which contains 0% macroporosity

is 16.67 MPa. The air-injected plaster cylinders have a

very narrow macroporosity range, approximately 4%

to 7.8%, but a large variation of compressive strength,

approximately 6.5 to 13.4 MPa. This indicates that

factors other than macroporosity are affecting the

compressive strength, possibly cavity sizes, cavity

locations and cavity shapes. A spherical cavity shape

is the stiffest shape among the ellipsoidal shapes

(Kachanov et al., 1994) thus specimens containing

spherical cavities should yield higher compressive

strengths.

5.2. Lithophysal tuff specimens

The relationship between total porosity and uniax-

ial compressive strength of the cubic and cylindrical

N. Hudyma et al. / Engineering Geology 73 (2004) 179190184Fig. 2. Relationship between unconfined compressive strength and macroporosity for plaster specimens.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190 185 lithophysal tuff specimens is presented in Fig. 3. The

complete data set was used to determine the best-fit

regression curve.

The tuff specimens have total porosities ranging

between approximately 17% and 49%. Fig. 3 shows a

wide spread of data associated with the tuff speci-

mens. The best-fit regression curve is:

rc 49:36ln/ 189:35 R2 0:62 3

5.3. Normalized compressive strength

In order to compare the plaster and tuff uniaxial

compressive strengths, the strength values of both

plaster and tuff were normalized with respect to zero

macroporosity (plaster) or total porosity (tuff) com-

pressive strength values. For both cubic and cylindri-

cal plaster specimens, the data was normalized with

respect to the average compressive strength of the

solid plaster specimens. The compressive strengths for

Fig. 3. Relationship between unconfined compressive strength and total po

are roughly cubic with average dimensions of 10 to 15 cm. Tuff specimens

cm; those from Price et al. (1984) are cylindrical with diameters of 26.7 cthe tuff specimens were normalized with respect to an

estimated 0% porosity strength value through a re-

gression analysis (Eq. (3)). All of the normalized data

is presented in Fig. 4 along with a best-fit regression

curve.

The best-fit regression curve is:

Nrc 0:954 0:15/2 R2 0:90 4where Nrc is the normalized compressive strength.

Fig. 4 contains some interesting characteristics.

Plaster specimens with less than 10% macroporosity

have a wide spread in normalized compressive

strength. Normalized compressive strength of plaster

specimens with greater than 10% macroporosity do

not exhibit such a large variation in normalized

compressive strength. Normalized compressive

strengths of tuff are more dispersed than those of

plaster specimens. Most of the tuff data plots below

the regression curve, whereas the plaster data plots

mostly above the regression line. Some of the reasons

rosity for lithophysal tuff specimens. Tuff specimens from this study

of Martin et al. (1994, 1995) are cylindrical with diameters of 5.08

m.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190186for such behavior may include the presence of non-

spherical lithophysae, larger range of lithophysae

sizes within the tuff than the size of cavities within

the plaster, and the presence of unreported micro-

cracks and microfractures within the tuff that are not

present within the plaster.

Porosity values determined by Otto (2003) could

conceivably be used to estimate the portion of

macroporosity from the total porosity of the tuff

specimens used in this study. The effect would be

to shift the tuff data presented in Fig. 4 to the left

and the regression line presented would have a

steeper slope. However, for a heterogeneous mate-

rial like tuff, it is probably better to measure the

total porosity and microporosity for each tuff spec-

imen and then calculate the macroporosity for each

specimen. This was not conducted as part of this

study. One should note that microporosity is an

intrinsic characteristic of tuff and tuff with zero

microporosity does not exist in Paintbrush strati-

graphic units at Yucca Mountain (Martin et al.,

Fig. 4. Relationship between normalized compressive strength and ma

specimens.1994; Mongano et al., 1999). The authors believe

that performance of lithophysae-rich units is con-

trolled by macroporosity.

6. Relationship between porosity and failure

modes

Failure modes of rock are generally studied under

different loading conditions, such as uniaxial and

triaxial compressive loading, depending on the

expected in-situ stress-state conditions of the rock

mass. One of the concerns of the repository tunnels

in Yucca Mountain is that portions of the lithophysal

tuff rock mass may breakout under the in-situ stresses

and thermal loads during its long service life. Uniaxial

compression testing of plaster and tuff specimens may

provide helpful information on failure modes of the

tuff rock mass which are exposed in the tunnel

surfaces and which may contain a substantial volume

of cavities.

croporosity for plaster specimens and total porosity for four tuff

6.1. Cylindrical plaster specimens

Four failure modes were identified during testing

of the cylindrical plaster specimens: spalling, axial

splitting, shear failure and web failure. The failure

mode nomenclature, photographs and macroporosity

ranges for the three failure modes are shown in Fig. 5.

The failure modes depend upon the macroporosity

of the specimen. There is a transitional change be-

tween the failure modes where combined failure

modes are present. For instance, a specimen exhibit-

ing shear failure may also contain elements of both

spalling and axial splitting but the dominant failure

mode is shear failure.

Spalling occurred in specimens with less than 5%

that small cracks grow from pores in brittle porous

solids under compression. There is also, as shown in

the figure, some spalling of the specimen surface.

For both the spalling and axial splitting, fractures

and failure occur parallel to the maximum principal

stress orientation.

Shear failure, shown in Fig. 5-c, occurred for

specimens containing between approximately 10%

and 20% macroporosity. This mode of failure

occurs with the webbing between the Styrofoam

inclusions and the specimen fails along an inclined

shear plane.

The fourth failure mode observed has been termed

web failure, which is shown in Fig. 5-d. In this

failure mode, there is very little external expression

for the cylindrical plaster specimens.

N. Hudyma et al. / Engineering Geology 73 (2004) 179190 187macroporosity. The pieces that spalled from the

specimen were generally thin, on the order of 25

mm thick, ran almost the entire length of the speci-

men and covered on the order of 1/5 to 1/10 the

circumference of the specimen. This type of failure is

also known as tensile surface splitting and peeling

(Andreev, 1995).

For specimens containing approximately 5% to

10% macroporosity, axial splitting was the dominant

failure mode. As part of this failure mode, conjugate

fractures formed at the top of the specimen. These

fractures develop because of friction created between

the top of the specimen and the specimen loading

platen. Failure for the axial splitting specimen gen-

erally occurs along one main fracture that appeared

to initiate around one or more large cavities, as

shown in Fig. 5-b. Sammis and Ashby (1986) state

Fig. 5. Failure modes identifiedof damage and the specimen does not fail abruptly or

violently. The photograph in Fig. 5-d has the surface

cracks highlighted. This type of failure occurred in

specimens containing above approximately 20%

macroporosity. It is assumed that the webbing or

plaster between the Styrofoam inclusions constitutes

such a small part of the specimen that it simply

crumbles under the failure stresses and deforms

plastically. This type of failure is akin to pore

collapse that is often seen in high porosity sedimen-

tary rocks such as chalk.

6.2. Cubic plaster specimens and cubic lithophysae-

rich tuff

The failure modes of cubic specimens do not

show a strong relationship between failure modes

on lithophysae-rich tuff and cylindrical and cubic

plaster specimens containing cavities created by using

show a similar non-linearly decreasing trend with

increasing porosity (R2 of 0.90).

Failure modes are also related to macroporosity for

the cylindrical plaster specimens. The failure modes

of the plaster cylinders transitioned from spalling to

axial splitting to shear failure to web failure. Although

the investigation of failure modes of lithophysae-rich

tuff was not fully documented and the stress-state of

the specimens during testing may not represent the in-

situ stress state, the data still provides clues about the

possible in-situ failure of tuff with the repository

Acknowledgements

The authors would like to thank Mr. V.

Thummala and Mr. C.D. Herrington of Bechtel

Table A1. Cubic tuff specimen uniaxial compres-

N. Hudyma et al. / Engineering Geology 73 (2004) 179190188either injected air or spherical Styro-foam inclusions.

From these tests, the relationships between macro-

porosity and compressive strength and failure modes

have been determined.

The plaster specimens were characterized by their

macroporosity (the porosity due to the Styrofoam or

injected air inclusions) assuming the plaster portion of

the specimen to be solid. The macroporosity for the

plaster specimens ranged between 0% and approxi-

mately 38%. The uniaxial compressive strength de-

creased non-linearly with increasing macroporosity

(R2 of 0.80). There did not appear to be a specimen

shape effect on the compressive strength of the plaster

specimens.

The tuff specimens were characterized by their

total porosity (the porosity of both lithophysae and

microscopic pores). The total porosity for the tuff

specimens ranged between approximately 8% and

40%. The uniaxial compressive strength decreased

non-linearly with increasing total porosity, however,and porosity. The specimens failed via a combina-

tion of the previously mentioned failure modes.

Local failures occurred within the zones that

contained high cavity concentrations. Crack propa-

gation usually initiated at cavities and crossed them.

The main difference in failure between plaster and

tuff specimens is that more localized failures oc-

curred in tuff, most probably because tuff speci-

mens have larger nonspherical cavities that interface

with the outer surface of the specimen, whereas

plaster specimens contained spherical cavities sur-

rounded by the plaster matrix creating a stiffer

structure. Local failures were in the form of severe

cracking that caused irregularly shaped pieces to

break off the specimen at locations with high

concentrations of lithophysae. Specimens continued

to carry load after these localized failures. Such

failures may occur in the field during the lifetime

of the repository tunnels that are under not only the

in-situ stresses but also thermal stresses.

7. Conclusion

Uniaxial compression tests have been performedthere was a wide spread to the data (R2 of 0.62).sive strength and porosity data (after Avar, 2002)

Tests performed at the Materials Testing Laborato-

Specimen

number

UC strength

(MPa)

Total porosity

(%)

1667 15.5 31.6

1668 6.1 28.6

1669 27.5 28.3

1670 14.5 32.9

1671 39.5 30.6

1674 14.3 25.9

1675 52.4 19.3

1676 44.9 17.1Nevada Material Testing Laboratory, Mercury, NV,

who performed specific gravity tests on tuff speci-

mens and helped during compression testing of tuff

and cubic plaster specimens. Ron Price from Sandia

National Laboratories in Albuquerque, NM aided

the authors in finding supporting data on lithophy-

sae-rich tuff. The authors would also like to thank

Dr. Vicki Moon for numerous helpful and insightful

comments from her review of the manuscript.

Appendix A. Experimental datatunnels at Yucca Mountain.Normalized compressive strengths of all specimensry at the Nevada Test Site.

compressive strength and porosity data (after Martin

et al., 1994, 1995)

Table A3. Unconfined compressive strength and

porosity data from Topopah Spring Tuff (after Price

et al., 1985)

Table A4. Cubic plaster specimen uniaxial com-

pressive strength and porosity data (after Avar, 2002)

Tests performed at the University of North Florida.

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Compressive strength and failure modes of lithophysae-rich Topopah Spring Tuff specimens and analog models containing cavitiesIntroductionSpecimensLithophysae-rich tuffPlaster models with cavitiesCubic specimensCylindrical specimens

Porosity calculationsUniaxial compressive strength testingPorosity versus compressive strengthPlaster specimensLithophysal tuff specimensNormalized compressive strength

Relationship between porosity and failure modesCylindrical plaster specimensCubic plaster specimens and cubic lithophysae-rich tuff

ConclusionAcknowledgementsExperimental dataReferences