geotechnics of tuff
DESCRIPTION
Geotechnics of tuff soilsTRANSCRIPT
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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: [email protected] (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
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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
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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
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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