GEOTECHNICAL PROPERTIES OF

CEMENTED VOLCANIC SOIL

By T. D. O'Rourke,1 Member, ASCE, and E. Crespo,2 Student Member, ASCE

ABSTRACT: A volcaniclastic formation, known as Cangahua, found in the Andes of Ecuador and Colombia is focused on. The deposit is composed of moderately cemented fine sand and silt-sized particles. A pronounced linear correlation is shown between strengh, in terms of both uniaxial compressive and Brazil tensile strengths, and the dry unit weight of the deposit. The tensile strength is unusually high for soil, being between 18 and 29% of the uniaxial compressive strength. Brazil tensile, uniaxial compressive, and triaxial strength characteristics de­pend on the initial void ratio and degree of saturation. As the saturation declines from 90 to 40%, test results show a fourfold increase in tensile strength. Moreover, increasing degrees of saturation cause a shift from brittle to ductile failure. Slope failures in Cangahua develop from fractures which initiate at zones of high tensile stress. Material proper­ties such as tensile strength and fracture toughness play an important role in explaining and evaluating slope failures in this material. The strong dependence of tensile strength on the degree of saturation indicates that local moisture conditions and exposure to rainfall should be considered in stability assessments.

INTRODUCTION

Volcaniclastic deposits involve materials produced by explosive or aerial ejection and also include fragments produced in volcanic vents, fragments formed from the breakup of moving lava, and deposits which weather and erode from solidified lava flows or consolidated pyroclastic debris (Fisher 1961; Fisher and Schmincke 1984). These materials can have unusual properties which arise mainly from their microstructure. They often have porous fabrics in which particles and particle assemblages are interlocked or cemented by volcanic glass and its weathered derivatives. Volcaniclastic deposits are important in Central and South America. Stability problems occur within the steep volcanic terrain of these regions at natural slopes, road cuts, and excavations for commercial and industrial structures.

In the Interandean Depression of Ecuador and Southern Colombia a volcaniclastic formation, known as Cangahua (kon-gow'a), covers most of the landscape between elevations of 2200 and 3500 m. Many important roadways, including the Panamerican Highway, are constructed within this formation, with adjoining cut and natural slopes as high as 40 to 50 m at inclinations of 70 to 90°. During the March 5, 1987 Ecuador earth-

'Prof., School of Civ. and Envir. Engrg., Cornell Univ., Ithaca, NY 14853. 2Grad. Res. Asst., School of Civ. and Envir. Engrg., Cornell Univ., Ithaca, NY

14853. Note. Discussion open until March 1, 1989. To extend the closing date one

month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on November 11, 1987. This paper is part of the Journal of Geotechnical Engineering, Vol. 114, No. 10, October, 1988. ©ASCE, ISSN 0733-9410/88/0010-1126/$1.00 + $.15 per page. Paper No. 22828.

1126

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

quakes, slope failures in the Cangahua Formation blocked portions of the Panamerican Highway, interrupting traffic for nearly 24 hrs.

Cangahua has a high porosity, typically 40 to 50%, and a low dry unit weight, typically between 11 and 14 kN/m3. Its uniaxial compressive strength, which ranges from 200 to 600 kPa, is consistent with that of very stiff to hard clay (Terzaghi and Peck 1967). Cangahua, nevertheless, possesses distinctive rock-like characteristics such as a cemented fabric and relatively high tensile strength and fracture toughness. Cangahua is important not only because of its behavior in slopes, but because it represents a class of materials, often referred to as soft or weak rock, for which there is a growing worldwide interest (Hoshino 1981; Yoshinaka and Yamabe 1981). An in-depth study of Cangahua helps us to understand how microstructure and strength are interrelated, and extends our knowledge to materials which are not easily classified as either rock or soil.

This paper begins with a discussion of the geologic setting of the Cangahua Formation and its engineering significance. The microstructure and index properties are described next, followed by a description of the laboratory tests and test results pertaining to uniaxial compression, Brazil tensile strength, triaxial strength and deformation characteristics, and fracture toughness. Relationships among strength, deformation, and mois­ture content are discussed with reference to special tests in which the degree of saturation was varied without disturbing the natural fabric. Finally, conclusions are presented, and the implications of the test results are discussed with respect to the general performance traits of cemented soils and weak rocks.

GEOLOGIC SETTING

As shown in Fig. 1, the Interandean Depression is a structural valley between two parallel mountain ranges of the Andes of Ecuador, the Western Cordillera, and Cordillera Real. It is about 400 km long and 20-30 km wide, composed of a series of intermontane valleys at elevations of 2000 to 3500 m. Major population centers are located in these valleys. During the past 20 years, significant amounts of road, building, and other excavation works have taken place in this region, much of which have been in the Cangahua Formation.

As shown in Fig. 2, the Cangahua Formation is found throughout much of the Interandean Depression. It is estimated that the formation covers 20,000 km2 at thicknesses of 100-120 m (Crespo 1987). It is composed mainly of an unstratified, volcaniclastic deposit of yellow-brown to gray-brown fine sand and silt-sized particles. The formation also includes coarser grained fall-out tephras that are interbedded with the Cangahua.

A prominent feature of the regional geology is the grouping of large stratovolcanoes, both extinct and active, in two parallel rows that extend from the Colombian border southward to latitude 2° 30'S. Lava flows, ash falls, pyroclastic flows, and large mudflows are located near these eruptive centers. One theory for the origin of Cangahua accounts for the material as an ashfall which accumulated during periods of volcanic activity when the interbedded tephra of larger grain size were also deposited (Sauer 1950). A

1127

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

FIG. 1. Location of Interandean Depression

different theory, favored in this work, accounts for Cangahua as a loess-like sediment (Clapperton and MacEwan 1985). Evidence suggests that these materials accumulated as wind-deposited epiclastic sediments, which were weathered primarily from pyroclastic rocks and volcanic mud flow deposits. The age of Cangahua is estimated between 12,000 and 50,000 years.

Fig. 3 shows that steep slopes of Cangahua are present in highly developed areas. The photograph provides a picture of eastern Quito where the Cangahua Formation outcrops as a prominent line of hills. In

1128

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

o°-

l ° -

50 km _ J

H CANGAHUA FORMATION

ANDESITIC VOLCANIC DEPOSITS

H STRATO-VOLCANOES

Z°-\

FIG. 2. Distribution of Cangahua Formation in Interandean Depression [Modified from Geologic Map of Ecuador (Direccion General 1982)]

this area, steep cuts have been excavated in the Cangahua and several high-rise apartment buildings have been built near the slopes.

MICRO-STRUCTURE

The determination of the mineralogical composition of Cangahua was conducted by petrological observations of thin sections and by x-ray diffraction. A simple technique for obtaining thin sections was adapted from procedures used in soil science. It involved the impregnation of the material with a low-viscosity resin. Cangahua is composed mainly of plagioclase and hornblende with minor amounts of augite, hyperstene, and a trace of quartz. Sand-sized mineral grains of plagioclase and hornblende exhibit a strong corrosion at cleavage faces.

The x-ray diffraction analysis of the clay fraction produced typical characteristics of amorphous materials. Nevertheless, a minor crystallinity

1129

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

I <•'

{

CUT SLOPE IN CANGAHUA

m:m .... * * ^ $ K * |

FIG. 3. Photograph of Cangahua-Covered Foothills of Eastern Quito, Ecuador

was revealed in two peaks at 7.5A and 10A. This observation suggests that a small amount of poorly crystallized montmorillionite or haloysite may be present.

Fig. 4 shows a representative detail of the intergranular space of

U.

A> • •»

4

\ :

'irrr^swiiAWJUAH S P A C E , )U <MATf«:< ANO SILT) "̂

5 *

: - . J ,

3

o \

FIG. 4. Thin Section Micrograph and Illustrative Diagram of Representative Inter­granular Space of Cangahua

1130

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

VI

• '-V

.J.

'• " , ' ••"• '" ~ ' - " i . 1 . . . - . « - ^ . •• , . > * • • * , ; _

• ."n : * £ ? * . - - 'U-tr* !;-.*• . • ' V f S / . . ' '• •""• V " *

i ' • "'•".•*' *. . V : ' " : * , . - ' ' " " ; "•

, • "*-'"V i - :° • v

FIG. 5. SEM Micrograph of Matrix Material, Scale Bar Indicates 1 Micron

Cangahua taken from a horizontal thin section in plain light. The figure presents several mineral grains of fine sand size surrounded by matrix material. The coarser mineral grains are not in direct contact but are embedded in the matrix material. This arrangement indicates that the strength of Cangahua is mostly a consequence of the strength of the matrix. In addition, microscopic observations of disaggregated material

1—1 :

100

80 O i

<u £ X>

L. Q) C

LL

• * —

C CD O k_ CO

Q .

60

40

20

10

SAND COARSEI MEDIUM 1 FINE

SILT OR CLAY

1.0

-

-

y^v

/-Grain size ' plots for

5 block v samples

i

0.1 0.01

Grain Size (mm) FIG. 6. Grain Size Distributions for Five Samples of Cangahua

1131

0.00I

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

have shown freshly broken grains which have remained attached to pieces of the matrix. This observation suggests that the matrix strength is greater than that of some individual particles.

Fig. 5 presents an electron micrograph of the matrix material of Cangahua. Close to the center of the micrograph is a cemented bond between several silt particles. Bonds such as this one are present around every grain, producing a cemented aggregate. Scanning electron micro­scope observations showed marked peaks of silica at intergranular bound­aries which suggest that amorphous silica is a dominant cementing agent.

Because of the cementation, the determination of grain size and grain size distribution was sensitive to the disaggregation method. Experiments were conducted to evaluate several disaggregation techniques, and the method finally adopted was a two-step procedure of mild mechanical fragmentation followed by ultrasonic treatment (20 Hz, 30p,) for a period of 30 min in 0.01% sodium hexametaphosphate solution. In addition, grain counts were performed on thin sections with a micrometer mesh. The resulting grain size distribution plots are shown in Fig. 6. The material was composed by weight of about 60 to 65% of fine sand, 10 to 15% of clay, and the remaining fraction of silt.

INDEX PROPERTIES

Samples were collected in an area east of Quito, in a series of broad hills that separate the city from the lower suburban areas of Tumbaco and Los Chillos. Five cubic block samples, approximately 0.5 m on each side, were taken from test pits at recent slope exposures. Each pit was approximately 1.5 m by 1.5 m in plan and was excavated to a depth of 1.0 m. No organic soil cover was present. Various cutting utensils, including special knives and a long hand auger, were used for removing the material from around the block and for the final trimming. Immediately after excavation the samples were covered with waxed cheese cloth and a double layer of wax to protect them against loss of moisture. The blocks were sealed in plastic bags, fitted within wooden crates, and transported to the United States as air freight. At the Cornell Geo technical Laboratory the blocks were stored in a 100% humidity room.

Index properties pertaining to the five block samples of Cangahua are listed in Table 1. The porosity and unit weight of the material were

TABLE 1. Summary of Index Properties

Sample number

(1) 4 5 7 -8

10

Partially saturated

unit weight,

7 (kN/m3)

(2) 12.8 15.8 16.6 15.8 14.9

Dry unit weight, yd

(kN/m3) (3) 11.0 13.1 14.4 13.4 12.5

Specific gravity,

G (4)

2.58 2.58 2.58 2.59 2.59

Void ratio, e

(5) 1.30 0.93 0.76 0.89 1.03

Degree of saturation, Sr

(%) (6) 32 56 52 52 48

1132

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

determined in accordance with the suggested methods of the International Society of Rock Mechanics (ISRM) (Brown 1981). There was considerable difficulty in fully saturating the specimens so that submerging under vacuum was needed to obtain 100% saturation. The porosity was found to range between 43 and 56%. This high porosity is typical of volcanic ash and loess-like deposits. The dry unit weight ranged between 11 and 14 kN/m3. The moisture content, which was determined in accordance with the procedures recommended by the ASTM (1987), was between 15 and 20%. The determination of the specific gravity of solids was accomplished in accordance with ASTM (1987) specifications. The degree of saturation, Sr, listed in Table 1 is defined as the percent of the void space which is occupied by water.

It should be emphasized that the values listed in Table 1 represent averages taken from several locations within a given block. Little variation in index properties was found for each block. Checks made during subsequent triaxial testing showed values consistent with those summa­rized in the table.

STRENGTH CHARACTERISTICS

Compressive and Tensile Strengths Uniaxial compressive tests were performed on right circular cylindrical

specimens, 35 mm in diameter and 87 mm in height. A combination of special blades and a small high-speed drill were used to trim the specimens on a rotating pedestal. A rock cut-off saw was used to trim the specimens to their final heights.

Loads were applied at a fixed rate of strain of 0.4%/min. This rate resulted in specimen failure within 5 to 10 min after initial load application. All specimens failed in a brittle fashion, with a pronounced drop in stress after attaining a peak value.

Table 2 summarizes the uniaxial compressive strength determined from specimens removed from five block samples. Three tests were performed for each block sample. The mean strength, range of strengths, and coefficient of variation are summarized for each set of tests.

The Brazil test was used to provide an indirect measure of the uniaxial tensile strength. The test was performed in accordance with ISRM specifications (Brown 1981). Special precautions were taken during these

TABLE 2. Summary of Uniaxial Compressive Strengths

Sample number

(1) 4 5 7 8

10

Dry unit weight,

7«* (kN/m3)

(2)

11.0 13.1 14.4 13.5 12.5

Uniaxial compressive strength, qu (kPa)

Mean8

(3) 360 495 664 535 452

Range (4)

314-437 462-559 648-694 496-594 442-472

COVb

(5)

0.18 0.11 0.04 0.10 0.04

"Mean of three test values. bCOV = coefficient of variation.

1133

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

TABLE 3. Summary of Brazil Tensile Strengths

Sample number

(D 4 5 7 8

10

Dry unit weight,

7rf (kN/m3)

(2) 11.0 13.1 14.4 13.5 12.5

Brazil tensile strength, qt (kPa)

Meana

(3)

68 110 207 127 101

Range (4) •

55-93 93-136

107-255 104-168 90-121

COVb

(5) 0.31 0.21 0.20 0.28 0.17

"Mean of three test values. bCOV = coefficient of variation.

tests to ensure that the load associated with primary fracture was re­corded. There was a tendency for a partially split specimen of Cangahua to seat within the loading device such that it was reinforced against additional load. As a consequence, loading continued to increase after primary cracking with the result that ultimate loads were sometimes as much as 20% higher than the primary cracking loads. All Brazil tests were accom­panied by continuous displacement monitoring, using a direct current differential transformer (DCDT). The DCDT was positioned to measure the displacement of the loading jaws. In this way, the test operator could perceive the development of primary cracking as evidenced by a marked change in slope of the load versus displacement plot. All Brazil tensile strengths reported in this work were measured at primary cracking.

Table 3 summarizes the Brazil tensile strengths determined from spec­imens removed from five block samples. Three tests were performed for each block sample. The mean strength, range of strengths, and coefficient of variation are summarized for each set of tests.

Fig. 7 shows both the uniaxial compressive strength and the Brazil tensile strength plotted as a function of the dry unit weight of the material. In the figure, the mean strengths at various unit weights are plotted, and the coefficient of determination, r2 , for each linear regression is indicated. Fig. 8 shows the mean uniaxial compressive strength plotted as a function of the mean Brazil tensile strength, with the correlated strengths pertaining to the same dry unit weight.

There are clear linear relationships between strength and dry unit weight and between compressive and tensile strength. The ratio of compressive to Brazil tensile strength varies from 3.5 to 5.5, which is substantially lower than most reported ratios for rock and cemented soils (Clough et al. 1981; Ingraffea et al. 1984).

Triaxial Strength Characteristics Drained triaxial compression tests were performed on right circular

cylindrical specimens, 35 mm in diameter by 87 mm in height, at confining pressures of 0, 60, 120, 200, and 300 kPa. Volume changes were measured by means of an air-water interface electronic transducer with an accuracy of about 10 mm3.

1134

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

.-. 800 o

c 0) w

55 > '(75 i/> <D k. Q .

£ o u

C a

c

600

400

200

Uniaxial compressive strength —v

^ r*=0.9l

Brazil tensile strength —\

Dry Unit Weight (kN/m3) FIG. 7. Relationship Among Brazil Tensile Strength, Uniaxial Compressive Strength, and Dry Unit Weight

Figs. 9 and 10 show triaxial test data for specimens taken from block samples 7 and 10, respectively. In each figure, the principal stress difference and volumetric strain are plotted as a function of axial strain. A pronounced brittle behavior is demonstrated at low confining pressures,

o

c 0)

800

</> 600 -<u >

'55 10 0) t» QL E o O "5 '* o '£ 3

400 -

200 100 200 300

Brazil Tensile Strength (kPa)

FIG. 8. Relation between Uniaxial Compressive Strength and Brazil Tensile Strength

1135

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

a a.

CD O

c CD

in in 0) t -«»—

CO

o

c

c 'a 55 o

*k_

<u E O >

1600

1200-

400 - ,

-

-

1 io

i

/ L ^ - a - 4 , 2 0 0

N *-9 120

X^4»60

Confining pressure, kPa

l l l l

0.08

Axial Strain

0.12

FIG. 9. Principal Stress Difference and Volumetric Strain versus Axial Strain, Sample 7

with the behavior becoming increasingly more ductile as the confining pressure increases. The plots of volumetric strain versus axial strain show an initial contraction, the magnitude of which increases in direct propor­tion to the confining pressure. The initial loss of volume is followed by an increase in volume. The maximum principal stress difference appears to be related to the rate of dilation. In each case, the peak values of principal stress difference coincide with the maximum slope along the dilative portion of the volumetric strain plot. The brittle-to-ductile transition of Cangahua occurs at a ratio of major to minor principal stress, cVa^, between 7.5 and 10. This ratio is similar to that shown by data for moderately cemented sands (Clough et al. 1981) but much larger than the ratio for brittle-to-ductile transition reported by Hoek (1983) for rock.

1136

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Q

I (A

I

I 1 U U

1200

1000

800

600

400

200

0.

_

-

-

-

- J - Z - §9 X o

••88 2 0 0

120

m 60

Confining

1 I l

pressure, kPa

I I

0.0 c '5 55 o

"S3 E O > •0.01

0

N ^ ^ * ^

i i

60

r*300

1 0.04 0.08

Axial Strain

0.12

FIG. 10. Principal Stress Difference and Volumetric Strain versus Axial Strain, Sample 10

Peak and Residual Strengths The peak and residual strength values for specimens from block samples

7 and 10 are plotted in Figs. 11 and 12, respectively. Strength values are shown in the form of a q-p diagram with coordinates of {u^ - a3)/2 versus (o-j + CT3)/2. Peak strength values were taken from tests performed at CT3 < 120 kPa, which is below the brittle-to-ductile transition level for the material under study. Mean tensile strength measurements were also used to evaluate peak strength parameters. The residual strength was deter­mined from triaxial data, where the stress-strain curves leveled out after the peak principal stress was achieved.

Linear regression analyses were used to obtain the best straight line fits of the data. For block sample 7, the angle of shearing resistance, cf>, remains essentially unchanged at 39-40°, and only the cohesive intercept declines as peak strength conditions change to residual. Similarly for block

1137

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

800

o

b" I b~

CM

& Peak strength data

A Residual strength data

Block sample 7 e=0.76

-200 200 400

cr.+ o-.

600 800

(_•__!), kPa

FIG. 11. Peak and Residual Strength Data for Sample 7

1000

o 0.

bM

I

b~

800 r

600

400

200

• Peak strength data

a Residual strength data

Block sample 10 e = l.03

-200 200 400 600 800 1000

( ^ ) . k P o FIG. 12. Peak and Residual Strength Data for Sample 10

1138

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

1000

800

600

E,/p0

400

200

0.6 0.8 1.0 2.0 4.0

FIG. 13. Variation of Initial Tangent Modulus with Confining Pressure

sample 10, the angle of shearing resistance shows a relatively small drop from peak to residual, with a proportionately larger decrease in the cohesive intercept. The natural void ratios for block samples 7 and 10 are markedly different, with the smaller void ratio of block sample 7 repre­senting a greater amount of matrix material. Increased matrix material means that additional cementing bonds were present. The cohesive inter­cept is considerably higher for the denser, more strongly cemented

1 material at both the peak and residual levels. The strength data plotted in Figs. 11 and 12 show trends similar to those

delineated by Clough et al. (1981) and Sitar (1983) for cemented sands. In their study of natural and artificially cemented sands, Clough et al. (1981) demonstrate that cementing agents contribute directly to the cohesive component of strength. Both the data in this study and those for cemented sands indicate that cohesion decreases at residual levels of deformation and with diminishing amounts of cementing material.

Deformation Modulus The initial tangent modulus, Et, was evaluated from triaxial test data and

plotted on log-log scales as a function of confining pressure, o-3. The modulus and confining pressure were normalized with respect to atmo­spheric pressure, pa , and interpreted according to the expression:

1139

Block sample 7 6 = 0.76

K*479, n*-0.0l *

j^l^-^^—m— "

Block sample 10 e = l.03

J I L

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

*-*-•£)" (1)

in which K is the intercept at cr3/pa - 1 and n is the slope of the line, both for a straight line plot on a log-log scale. The relationship given by Eq. 1 has been shown to be useful for characterizing a variety of soils (Wang and Duncan 1974), including cemented sands (Clough et al. 1981).

Fig. 13 shows a plot of EJpa versus u3/pa on a log-log scale. The values of K and n are similar to those reported for partially saturated silty clay (Wong and Duncan 1974), but the n-values are much smaller than those observed for either cemented or cohesionless sands. The i^-value is markedly larger for the dense block 7 material.

FRACTURE TOUGHNESS

Slope failure in Cangahua is caused by the formation and propagation of fractures, which often start at locations of erosion and undercutting near the toe of the slope. Because slope failure depends on fracture propaga­tion, it is important to test for properties that control crack formation and growth in the intact material.

Fracture toughness was determined for mode I fractures by means of 75-mm-diameter short rod specimens. All short rod specimens were prepared and tested according to the relative dimensions and procedures described by Ingraffea et al. (1984). The short rod specimens were taken from block sample 10 and tested at the in-situ moisture content of the material. The mean value of the critical stress intensity factor, K,c, determined from measurements with eight short rod specimens, is 3.07 x 10~2 Nm"3/2 . The value is very close to the values of 4 to 10 x 10"2 NirT3'2

reported by Saada et al. (1985) for stiff clay.

STRENGTH, DEFORMATION, AND MOISTURE CONTENT

It has been observed in the field that slope failures in Cangahua occur most frequently during or after periods of intense rainfall and during periods of unusually dry and windy weather. To investigate possible links between failure frequency and moisture conditions, experiments were performed in which a large number of partially saturated specimens were prepared and tested in uniaxial and triaxial compression.

Specimens were trimmed carefully from block samples, thus preserving the in-situ structure and void ratio of the material. The specimens were sprayed with water and allowed to dry, while monitoring changes in total specimen weight until an approximate water content was achieved. The specimens then were wrapped in plastic and stored in a 100% humidity room for a minimum of 5 days. This time was necessary for the moisture to distribute uniformly throughout the specimen. Before testing, the weight of the specimen was measured, and after testing a final moisture content measurement was obtained from the failed specimen.

Figs. 14 and 15 show the uniaxial compressive strength and Brazil tensile strength, respectively, plotted as a function of the degree of saturation. In each figure test results are plotted for specimens from block samples 7 and 10.

1140

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

o a.

800

700

§ 600

CO

a> 55 5 0 0 w a> t -a. E O 400

o 'c 300

200

Block sample 7 e=0.76

S 8 7 " S 8 " * " V s - ^ ^- Block sample 10

e=l.03

1 \

100 0 20 40 60 80

Degree of Saturation (%)

FIG. 14. Relation between Uniaxial Compressive Strength and Degree of Saturation

The most important factor affecting both the compressive and tensile strength is the initial void ratio, with the denser specimens showing considerably greater strength at virtually all tested levels of saturation. There is a clear relationship between strength and degree of saturation for a given void ratio. Both the uniaxial compressive and Brazil tensile strengths achieve peak values at 40 to 60% saturation, which was the range of the natural water content at the time of sampling. There was a dramatic change in the Brazil tensile strength for the dense specimens. As the degree of saturation increased from 40 to 90%, there was a decline in Brazil tensile strength from approximately 200 to 50 kPa.

The influence of the degree of saturation was investigated for specimens subjected to drained triaxial compression under a confining pressure of 120 kPa. Figs. 16(a) and {b) show the principal stress difference and volumetric strain as a function of axial strain for saturation percentages increasing to and above the field saturation, respectively. The general stress-strain behavior of the specimens is similar to that shown in Figs. 9 and 10. The most significant aspect of the data in Fig. 16 is the marked change in stress-strain behavior as a function of degree of saturation. As the degree of saturation increases, there is a progressive change from brittle to ductile response. Moreover, there also is a decrease in the maximum principal stress difference.

1141

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

o a.

! CO

I N

a m

300

250

200

150

100

50

Block sample 7 e=0.76 - \ ^

Block sample 10 n p

e=l.03

0 20 40 60 80 100

Degree of Saturation (%)

FIG. 15. Relation between Brazil Tensile Strength and Degree of Saturation

Clearly, the degree of specimen saturation has a substantial influence on the uniaxial compressive, Brazil tensile, and triaxial strengths of Can­gahua. For block sample 10, the peak strength is relatively constant at low saturation percentages. As the degree of saturation increases above approximately 40 to 50%, there is a decline in strength. This behavior is shown for the triaxial test data in Fig. 17, in which the q, p coordinates at various saturation percentages are plotted relative to the peak and residual strength envelopes. As the degree of saturation increases, the maximum stresses migrate from the peak strength envelope to values consistent with the residual strength parameters. Increased exposure to water apparently softens the clay and silt matrix of Cangahua, causing a decline in cohesion and some loss of interlocking among particles and fragments of the matrix.

Increasing degrees of saturation also affect the modulus and volume change characteristics of Cangahua. By assuming that the triaxial speci­men deforms as a right circular cylinder, the volumetric strain, V, is given by:

V = ea + 2er (2)

in which ea and er are the axial and radial strains, respectively. Poisson's ratio, v, can therefore be estimated from the volumetric strain with the relationship:

1142

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

o u.

U

ren

£ *: Q

ft <i>

(/) ri a

nc

k .

n.

1000

8 0 0

600

4 0 0

200

0

JPV Sr

^#\3v y- 38 %

F/kvtv'f'48%

' /F %k*#r l 3 %

# ^ -25%

- <§

• f J Confining pressure, l20kPa 1 i i i i i

1200

8 0 0

600-

CO 400 -

2 0 0 -

-

-

. il

~ h

i

. Sr. f * \ ---48%

f***°H*-s*~ 6 3 %

\£^*&! fc*»4~ 75 %

? ^ ^ - 8 8 %

Confining pressure, l20kPa

I 1 i l I

U.UI

0'

0 01

j> Sr

y % - 2 5 %

/V"*_ 3 8 %

/fj*~- 48%

XJ*"* 13% i i i i i

J 0.04 0.08

Axial Strain

0.12 0.04 0.08

Axial Strain

FIG. 16. Principal Stress Difference and Volumetric Strain versus Axial Strain for Various Moisture Conditions: (a) 13% =s Sr < 48%; (b) 48% s Sr < 88%

o 0 .

I b

8 0 0

6 0 0

4 0 0

2 0 0

Block sample 10

e= 1.03

Peak strength

2 5 %

3 8 % 4 8 %

-Residual strength envelope J I L__

2 0 0 4 0 0 6 0 0 8 0 0

D-, + 0-3

( ^ P ) . kPa FIG. 17. Peak and Residual Strength Values in Relation to Various Moisture Conditions

1143

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

3. •3

• D O

5 o

•£ Q. en . ,§ uT r—

!§ 'E

50

40

30

20

1 r Confining pressure = 120 kPa

Range of Modulus

Poissons ratio _ - a — -40.4

- 0 . 2

X

0.6

20 40 60 80

Degree of Saturation, %

100

o 15 or

c o (/> V) o

0_

FIG. 18. Relat ion between Initial Tangent Modulus and Poisson 's Ratio as Funct ion of Moisture Content

*4M (3)

Fig. 18 shows the initial tangent modulus and Poisson's ratio plotted as a function of the degree of saturation for the triaxial test data with a3 = 120 kPa. Poisson's ratios were determined from Eq. 3 for the initial linear portions of the stress-strain plots at axial strains less than 0.01. There is a marked decline in modulus when the saturation exceeds 60%. There also is a steady increase in Poisson's ratio as saturation increases. Progressively larger values of Poisson's ratio mean that the material becomes progres­sively less contractive. This trend in reduced compressibility coincides with a loss in strength. If the strength reduction occurs primarily because of a decline in cohesion, then it is possible that the frictional and associated dilative components of strength are mobilized at lower stress differences. An outcome of this shift from cohesive to frictional strength would be a reduction in compressibility.

PRACTICAL APPLICATIONS

Slope failure in Cangahua results from fracture propagation inward and subparallel to the slope face as illustrated in Fig. 19. Slope deterioration often starts from an initial condition of undercutting and progresses to the failure of an overhanging block (Crespo 1987; Crespo and Stewart 1987). Similar failure mechanisms have been observed for volcaniclastic deposits in Japan (Yamanouchi et al. 1970) and cemented sands in California (Clapperton and MacEwan 1985; Sitar 1983). Because many slopes are very high, large volumes may be involved in the eventual soil collapse. This failure mechanism does not conform to the shear failure models used in conventional analyses of slope stability. Indeed, analytical studies (Crespo 1987; Crespo and Stewart 1987) indicate that the shear capacity of Cangahua is generally sufficient to resist driving shear forces, even under earthquake loading.

1144

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

lv 'J W /

FIG. 19. General Pattern of Slope Failures in Cangahua Formation: (a) Toe undercutting; (b) spalling; (c) block failure

Material properties, such as tensile strength and fracture toughness, are directly related to the slope failure mechanisms observed in the field. These properties are especially important when evaluating slope stability under earthquake loads. Earthquakes are accompanied by horizontal accelerations which can induce tensile stresses and increase stress con­centration at the fronts of existing fracture surfaces. The test results summarized in this work provide a reference base for evaluating the tensile strength and fracture toughness of Cangahua and permit comparisons with similarly cemented soils of volcanic and other origins.

Of primary significance for engineering applications is the strength variation of Cangahua as a function of density and degree of saturation. This study has shown a strong dependence of compressive and tensile strength on degree of saturation. For example, an increase in saturation from 40 to 90% has been shown to result in a 75% reduction in Brazil tensile strength. This implies that stability evaluations for slopes in Cangahua should take rainfall and water infiltration into consideration. An effective means of accounting for possible strength reductions is to test specimens at saturation levels consistent with the maximum water expo­sure likely to occur on a seasonal basis.

CONCLUSIONS

This study has focused on the geotechnical properties of a volcaniclastic formation, known as Cangahua, found in the Andes of Ecuador and Colombia. The deposit is composed of cemented fine sand and silt-sized particles and is characterized by a high void ratio, moderate compressive strength, and relatively high tensile strength and fracture toughness.

A pronounced linear correlation exists between strength, in terms of both the uniaxial compressive and Brazil tensile strengths, and the dry unit weight of the deposit. The Brazil tensile strength is unusually high for soil, being between 18 and 29% of the uniaxial compressive strength.

1145

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Compressive strength characteristics are similar to those of moderately cemented sands, as described by Clough et al. (1981). The material exhibits a brittle failure mode at low confining stresses with a transition to ductile failure at higher confining stresses. A principal cementing agent in the soil is amorphous silica. The cementing agent contributes directly to the cohesion of the material, with loss of cementation bonds at low strains and mobilization of the frictional strength component at larger strains.

There is a clear relationship among density, degree of saturation, and strength. The amount of cementing agents increase in direct proportion to the density, so that a material with a lower initial void ratio will have a larger cohesive component of strength. The uniaxial compressive, Brazil tensile, and drained triaxial strength characteristics depend on the degree of saturation. Increasing degrees of saturation cause a shift from brittle to ductile failure at a constant confining stress.

Slope failures in Cangahua develop from fractures which initiate at zones of high tensile stress and do not conform to the shear failure mechanisms used in conventional analyses of slope stability. Similar failure modes have been observed elsewhere in volcaniclastic and ce­mented soils (Clough et al. 1981; Sitar 1983; Yamanouchi et al. 1970). Material properties such as tensile strength and fracture toughness play an important role in explaining and evaluating slope failures in this material. The strong dependence of tensile strength on the degree of saturation suggests that local moisture conditions and exposure to rainfall should be considered in stability assessments.

ACKNOWLEDGMENTS

Support for the research summarized in this work was provided by the Organization of American States and the National Center for Earthquake Engineering Research with headquarters in Buffalo, N.Y. Special thanks are extended to D. E. Karig, A. R. Ingraffea, and H. E. Stewart. A. Avcisoy drafted the figures and L. Mayes typed the manuscript.

APPENDIX I. REFERENCES

ASTM. (1987). "Soil and rock; building stones." Ann. Book ofASTM Standards, Vol. 04.08, Philadelphia, Penn.

Brown, E. T., ed. (1981). Rock characterization testing and monitoring: ISRM suggested methods. Pergamon Press, Oxford, U.K.

Clapperton, C. M., and MacEwan, C. (1985). "Late quaternary moraines in Chimborazo Area, Ecuador." Artie and Alpine Res., 17(2), 135-142.

Clough, G. W., Sitar, N., and Bachus, R. C. (1981). "Cemented sands under static loading." J. Geotech. Engrg., ASCE, 107(GT6), 799-817.

Crespo, E. (1987). "Slope stability of the Cangahua Formation, a volcaniclastic deposit from the Interandean Depression of Ecuador." Thesis presented to Cornell University, at Ithaca, N.Y., in partial fulfillment of the requirements for the degree of Master of Science.

Crespo, E., and Stewart, H. E. (1987). "Stability of cut slopes in Ecuadorian volcaniclastic deposits." Proc, 8th Panamer. Congress on Soil Mechanics and Foundation Engineering, Cartagena, Colombia, Vol. 3, Aug., 39-50.

Direccion General de Geologia y Minas. (1982). Geologic map of ecuador, 1:1000,000 scale. Instituto Geografico Militar, Quito, Ecuador.

Fisher, R. V. (1961). "Proposed classification of volcaniclastic sediments' and rocks." Geological Soc. of Amer., Bull. 72, 1409-1414.

1146

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Fisher, R. V., and Schmincke, H. U. (1984). Pyroclastic rocks. Springer-Verlag, New York, N.Y.

Hoek, E. (1983). "Strength of jointed rock masses." Geothecnique, 33(3), 187-223. Hoshino, K. (1981). "Consolidation and strength of soft sedimentary rocks." Proc.

Int. Symp. on Weak Rock, K. Akai, M. Hayashi, Y. Nishimatsu, eds., Vol. 1, Tokyo, Japan, Sept., 155-160.

Ingraffea, A. R., Gunsallus, K. L., Beech, J. B., and Nelson, P. P. (1984). "A short-rod based system for fracture toughness testing of rock." ASTM Symp. on Chevron-Notched Specimens: Testing and Stress Analysis, ASTM Special Tech­nical Testing Publication 855, J. H. Underwood, S. W. Freiman, and F. I., Baratta, eds., Amer. Soc. for Testing and Materials, Philadelphia, Penn., 152-166.

Jaeger, J. C , and Cook, N. G. W. (1979). Fundamentals of rock mechanics, 3rd Ed., Chapman and Hall, New York, N.Y.

Saada, A. S., Chudnovsky, A., and Kennedy, M. R. (1985). "Afracture mechanics study of stiff clays." Proc, 11th Int. Conf. Soil Mechanics and Foundation Engineering, San Francisco, Calif., 2, 637-640.

Sauer, W. (1950). "Contribuciones para el Conocimiento del Cuaternario en el Ecuador." Anales de la Universidad Central del Ecuador, LXXVII(328), 326-364.

Sitar, N. (1983). "Slope stability in coarse sediments." Special Publication on Geological Environment and Soil Properties, R. Yong, ed., ASCE, New York, N.Y., 82-98.

Terzaghi, K., and Peck, R. B. (1967). Soil mechanics in engineering practice, 2nd Ed., John Wiley and Sons, Inc., New York, N.Y.

Wong, K. S., and Duncan, J. M. (1974). "Hyperbolic stress-strain parameters for nonlinear finite element analysis of stresses and movements in soil masses." Geotech. Engrg. Rep., Dept. of Civ. Engrg., Univ. of California, Berkeley, Calif., July, 90 pp.

Yamanouchi, T., Taneda, S., and Kimura, T. (1970). "Damage features in 1968 Ebino earthquakes from the viewpoint of soils engineering." Soils and Founda­tions, X(2), 129-144.

Yoshinaka, R., and Yamabe, T. (1981). "Deformation behavior of soft rocks." Proc. Int. Symp. on Weak Rock, K. Akai, M. Hayashi, V. Nishimatsu, eds., Vol. 1, Tokyo, Japan, 87-92.

APPENDIX II. NOTATION

The following symbols are used in this paper:

c = cohesive intercept; Ej = initial tangent modulus; K = experimental constant;

KIC = mode I stress intensity coefficient; pa = atmospheric pressure;

n = experimental exponent; r2 = coefficient of determination; Sr = degree of saturation; V = volumetric strain; ea = axial strain; e r = radial strain; o"! = major principal stress; CT3 = minor principal stress;

<j> = angle of shearing resistance; and v = Poisson's ratio.

1147

J. Geotech. Engrg. 1988.114:1126-1147.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TE

LA

VA

L o

n 07

/04/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.