' I ', Comparisons of deformation characteristics of rockfill materials usipg monotonic

and cyclic loading laboratory tests and in situ tests

Fill Dam Division, Public Works Research Institute, Ministry o f Construction, Asahi I-banchi, Tsuk~&l, Ibaraki 305, Japan

AND

Japan Dam Engineering Center, Mesonic 39 Mori Building, Azabudai 2-4-5, Minato-ku, Tokyo 106, Japan

Received January 4, 1993

Accepted November 10, 1993

The deformation characteristics of rockfill materials at very small strains were investigated by comparing the results of monotonic and cyclic loading laboratory tests with geophysical P- and S-wave logging data from the field. Using a precision linear variable differential transformer for displacement, the elastic moduli of rockfill mate- rials at very small strains were measured in monotonic and cyclic loading triaxial tests. The laboratory test results agreed well with the field results. The shear moduli of rockfill materials from both a monotonic loading torsional simple shear test and a cyclic loading torsional simple shear test also showed good correspondence. Furthermore, the shear modulus predicted from the in situ shear wave tests in rockfill dams corresponded reasonably well with the modulus in the large-scale triaxial tests in the laboratory.

Key words: deformation characteristics, embankment dams, rockfill materials, laboratory test, jn situ test.

Les caractCristiques de deformation des matCriaux d'enrochement ont CtC examinCes ii de trbsrpetites dCformations en comparant les rCsultats d'essais de chargements monotone et cyclique en laborato~re avec des donnCes de profilage PS gCophysique sur le terrain. Au moyen d'un LVDT (transformateur linCaire variable diffkrentiel) de prCcision pour mesurer le dCplacement, les modules Clastiques des matCriaux d'enrochement B trks petites deformations ont CtC mesurCs dans des essais triaxiaux en chargements monotone et cyclique. Les rCsultats d'essais en laboratoire concordaient bien avec les rksultats de chantier. Les modules de cisaillement des materiaux d'enrochement obtenus tant par un essai de cisaillement simple en torsion avec chargement monotone que par un essai de cisaillement simple en torsion avec chargement cyclique ont aussi montrC une bonne concordance. De plus, le module de cisaillement predit par les essais d'ondes de cisaillement in situ dans les enrochements de barrage concordaient raisonnablement bien avec le module obtenu dans des essais triaxiaux ii grande Cchelle dans le laboratoire.

Mots elks : caractCristiques de diformation, barrages en enrochement, matCriaux d'enrochement, essai de laboratoire, essai in situ.

[Traduit par la redaction]

Can. Geotech. J. 31, 162-174 (1994)

1. Introduction To analyse the behavior during earthquakes of soil struc-

tures built from coarse grained granular materials (rockfill material), such as rockfill dams and reclaimed islands, the dynamic physical properties of the rockfill materials at very small strains must be known. The evaluation of deforma- tion characteristics of in situ ground at minute strains is especially necessary for the design of soil structures (Burland 1989). However, to obtain the dynamic physical properties of rockfill materials from laboratory tests, highly precise, large-scale apparati are required for the cyclic loading tests. As a result, fewer of these tests are conducted than labora- tory tests using sandy soil.

Monotonic loading tests are conducted in the region of elastic to plastic states leading to the state of failure (axial strain E,= 0-0.15). Research using monotonic loading tests has included cyclic loading tests at very small strains as part of the test. Teachavorasinskun et al. (1990, 1991) pointed out that there is a significant correlation between the results of monotonic and cyclic loading tests at minute strains. This implies that not only the strength of a material but also its dynamic physical properties may be obtained by a monotonic loading test alone, thus simplifying laboratory tests. Printed in / Imprun& au Canada

The comparison of shear moduli at very small strains from laboratory and in situ experiments is also important in conventional, precise investigations of the deformation characteristics of the ground shear modulus. Powell and Butcher (1991) obtained such shear moduli by in situ tests and laboratory tests using undisturbed specimens from six sites with cohesive soil and reported that the moduli were almost the same.

The maximum shear modulus at a very small strain, G,,,,, (G,), derived from laboratory tests is usually smaller than the shear modulus G,,,,,, derived from in situ elastic wave veloc- ity because of inevitable disturbance of the sand and cohe- sive soil specimens at the time of sampling. Kokusho (1987) reported that this tendency increased in proportion to the increment of G,,,,,. However, Tatsuoka and Shibuya (1991) attributed the difference between the in situ and laboratory tests to the inaccurate measurement of the very small axial strain indicating that in general the deformation characteristics were defined at too large a strain, and also to the effect of bedding errors while preparing the specimen. They pro- posed a new method for measuring the axial strain and con- ducted laboratory and in situ tests using a soft sedimentary

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YASUDA AND MATSUMOTO i

\ I

'i

FIG. 1. Schematic view of large-scale triaxial testing apparatus. 1, loading rod; 2. load cell for cyclic loading test; 3, loading cap; 4, specimen; 5, support; 6, porous metal disk; 7, pedestal; 8, load cell for monotonic loading test; 9, upper platen; 10, linear vari- able differential transformer for displacement; 11, triaxial cell; 12, lower platen.

rock and reported that the shear moduli at very small strains The relation between G,,, and G,,,,, of rockfill materials in both tests showed good correspondence. was investigated. Firstly, the in situ geophysical properties

This paper describes the methods of cyclic and monotonic of two rockfill dams were investigated, and the elastic and loading tests in the laboratory, focussing on the physical shear wave velocities for each depth were determined. properties of rockfil l materials at very smal l s t ra ins . Secondly, large-scale cyclic triaxial tests, in which the defor- Comparative studies on the deformation characteristics of mation properties at very small strains are measurable, were rockfill materials at very small strains are also examined. conducted for rockfill materials brought into the laboratory

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CAN GEOTECH J VOL 31, 1994 1

I i.

TABLE 1. Main features of transducer "i

Main feature

Capacity Type Accuracy Resolution

Axial load Compression Extension

Internal axial displacement

External axial displacement

External volume change

19.6 kN Strain gauge < +. 0.5 % 0.06 kPa 9.8 kN 5 mm Differential < + - 0 . 5 % 1 X 1 0 - ' %

transformer 150 mm Differential <+. 0.5 % 1 X lo-%

transformer +- 66.5 cm3 Differential < +- 0.5 % 0.03 cm"

manometer

LINEAR VARIABLE ISPLACEMENT RANSFORMER (LV

LOADING CAP

SPECIMEN

the axial load was reduced by a bearing guide between a bearing stand and the rod. By using a compensation cham- ber for the section area of the loading rod, the rod was pre- vented from floating and the volume of penetration of the rod during consolidation could be compensated for.

Axial compression and extension loads were measured by a waterproof load cell attached to-the cap of the specimen. Axial microstrain was measured; as shown in Fig. 2, by a waterproof linear variable differ$ntial transformer (LVDT) for displacement (Matsumoto et al. 1991). Axial displacement was calculated by averaging the values measured by the LVDTs installed diagonally a t two points on the specimen cap. This procedure eliminates errors as a result of mechan- ical friction, hysteresis and the specimen's eccentricity, which appear in conventional displacement transducers, and allows minute displacements to be measured very precisely. In the monotonic loading tests, besides minute displace- ments, large strains leading to failure were measured by a large-displacement transformer set on a piston rod outside of the triaxial cell.

Analog signals from each transducer were passed through amplifiers, converted to digital signals after passing through a 10 Hz low-pass filter, and input into a personal computer at 100 Hz for cyclic loading tests and 500 Hz for monotonic loading tests. In the monotonic loading test, a strain rate of 1.0% per minute during the compression test was adopted because of the good drainage of materials and to shorten the testing time, and readings were taken at 500 Hz to the required accuracy. High-speed sampling of at least 100 Hz is necessary to measure the very small strains. The specifi- cations of the main transducers are shown in Table 1.

FIG. 2. Detailed view of linear variable differential transformer {net hod for measuring axial deformations.

from the quarry sites used for these dams. Then, the shear wave velocities obtained from G,,, in a cyclic loading test were compared with the in-situ shear wave velocities.

2. Relationship between monotonic and cyclic loading tests

7 . 1. l ~ / ~ o ~ ~ l t o ~ : \ . text apparatus The specimen in the large-scale triaxial testing appara-

I L I ~ \\.as 300 m m in diameter and 600 mm in height, as \how11 i n Fig. 1. Loading was applied using an electrohy- draulic hcrvo system, which allows for normal monotonic and cyclic loading tests while controlling the strain and \trc\h conditions. The friction around the loading rod for

2.2. Materials and testing methods Materials with a maximum grain size D,,,, = 63.5 mm were

used for the tests. Sandstone (angular material) and riverbed gravel (round material) were tested with a modified grain- size distribution so as to be parallel and similar to the grain- size distributions in the field; the maximum grain size of samples taken from the quarry site was over 500 mm. Grain size distribution curves are shown in Fig. 3, and the physical properties of the materials are shown in Table 2. Specimens were prepared in six layers, with the same gradation in each layer. The air-dried materials were spread uniformly into a mold so as to avoid segregation, then compacted using an electric vibratory hammer (950 blowslmin, weight 320 N, diameter of loading plate 29 cm) to provide the specified rel- ative densities (Dr = 70 and 90%). Two triaxial cell mem- branes of 4 mm thickness were used in this study.

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YASUDA A N D MATSUMOTO 5

$

TABLE 2. Physical properties of the testing materials \

Sand stones Riverbed gravel (angular rock) (round rock)

Uniformity coefficient U, Specific gravity G, Water content W (%) Absorption Q (%) Maximum void ratio em,, Minimum void ratio emin

TABLE 3. Test conditions of monotonic and cyclic loading tests

Consolidation and drainage conditions

Initial void ratio e , Angular Round

Confining stress o, (MPa) Control Condition

Monotonic Cyclic

Isotropic consolidation and drainage

1 .O%/min; strain control 0.2 Hz (sinusoidal); stress control

GRAIN SIZE, D (mm)

FIG. 3. Grain-size distribution of materials tested in the laboratory.

All tests were conducted in the nonsaturated (air-dried) condition and consolidated drained state under the condi- tions shown in Table 3. Cyclic loading was carried out as shown in Fig. 4, i.e., in staged tests. Specimens were con- solidated under the specified confining stress, and after con- solidation, cyclic load was applied at 0.2 cycles per second and increased in stages (cyclic loading was repeated 12 times for each step and the data of the 10th cycle were used for the analysis). When the generated strain reached approximately 5 X the next confining stress was applied to the spec- imen and the same procedure was repeated.

In the monotonic loading tests, failure strength was defined by a continuous shearing force and occurred at up to 15% of axial strain.

2.3. Discussion Since monotonic loading tests are conducted in the range

of the elastic to plastic region (axial strain E, = 0-0.15), leading to failure, and the deformation characteristics of the cyclic loading tests are at very small strains (6, = 10-~-10-~) , then samples subjected to monotonic loading tests may also be subjected to cyclic loading tests, provided that the very small strains are accurately measured. If a relationship is found to exist between the deformation char-

CYCLIC LOADING

0 - TIME

FIG. 4. Process of cyclic loading test.

CURVE

HYSTERESIS LOOP

E a

FIG. 5. Stress and strain relationships derived from (a) monot- onic and (b) cyclic loading.

acteristics obtained from the monotonic and cyclic loading tests in the strain ranges where they overlap, testing can be simplified by estimating the deformation characteristics of the materials from the monotonic loading tests alone. The results of monotonic and cyclic loading tests were therefore com- pared with each other by focussing on the deformation char- acteristics of the rockfill materials at very small strains.

According to Kondner (1963), the stress-strain relationship (see Fig. 5 ) derived from a monotonic loading test can be approximated by the following hyperbolic curve:

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166 CAN. GEOTECH. J. VOL. 31, 1994 r \

4 w CYCLIC AND MONOTONIC TRlAXlAL TOYOURA SAND,e, =0.65

LOADING / 0.2 / 0.4 !

AXIAL STRAIN, & a

FIG. 6. Comparison of monotonic and cyclic loading tests of

A m n r z a

cn 3 +" 1000 2 2 2- 2 d

Toyoura sand.

MONOTONIC TRIAX~AL ROCKFILL MATERIAL

-

STRESS-STRAIN RELATIONSHIP AT MINUTE STRAIN

MONOTONIC TRlAXlAL ROCKFILL MATERIAL

AXIAL STRAIN, &, (X10-6)

FIG. 7. Stress and strain relationship in triaxial test at minute strains.

where ci and b are constants whose values depend on the type of soil material and axial strain rate during a monoto- nic loading compression test, and u, is the maximum prin- cipal stress and u 3 is the minimum principal stress.

Hardin and Drnevich (1972) used the hyperbolic curve ol' I I ] as a skeleton curve in modeling the stress-strain rela- tionship obtained from a cyclic loading test. As shown in Fig. 5 . both the maximum elastic secant moduli E, from the monotonic loading test and elastic secant moduli Ea at an axial strain E;, obtained from cyclic loading tests are assumed to agree with each other.

Cyclic loading at a shear strain level greater than 1 X lop3 i z accompanied by compaction of specimens and increased \.i~lucs of elastic modulus E and shear modulus G because of rhc effect of a stress history (Ishihara 1976). In the cyclic loi~cling tests conducted in this study, the maximum axial ,tri~in occurred at only around 5 X and while the void ratio hcl'ore and after the test changed by less than 0.01. Ilcncc. coinpaction of specimens by cyclic loading at small

200 J 0.2 0.5 1 .O 2.0

SHEAR STRENGTH, T'f (MPa)

FIG. 8. Relationship between shear strength and maximum elastic modulus in monotonic loading test. A, crushed angular rockfill material; 0, round graveLmateria1.

' I ' I CYCLIC AND MONOTONIC TRIAXIAL I

AXIAL STRAIN, & a

FIG. 9. Comparison of monotonic and cyclic loading triaxial tests of angular rockfill materials.

strains does not appear to influence the values of E and G. Therefore, it is estimated that rockfill materials are not affected by the stress history in either cyclic or monotonic loading tests.

To determine the maximum elastic modulus E, at E, = 1 X from monotonic loading tests, the relation curve of

E$(U~--U~) and E, was conventionally used. E, is obtained for a linear section of the initial stress-strain relation, but the value of Eo fluctuates greatly depending on the axial strain range. The stress-strain relationship obtained by measur- ing very small strains is a convex curve, and as Tatsuoka and Shibuya (1992) reported, this relationship is not appro- priate for calculating the maximum elastic modulus E, at very small axial strains. In particular, there must be a care- ful evaluation of the improvement of deformation charac- teristics Eo of soft rock foundations and soils with cement at very small strains, because E, of these materials is very dependent on axial strain.

Before conducting the tests for rockfill materials, the same tests were conducted on Toyoura sand to ensure accu-

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YASUDA AND MATSUMOTO 167

CYCLIC AND MONOTONIC 200 TRIAXIAL, ROUND ROCKFILL

eo=o.30 I I

lo6 2 5 lo5 2 5 lo4 2 5 lo3

AXIAL STRAIN, & a

FIG. 10. Comparison of monotonic and cyclic loading triaxial tests of round gravels.

0.008

STRESSTRAIN RELATIONSHIP AT MINUTE STRAIN RANGE

00

(XIO-5) SHEAR STRAIN, Yai

FIG. 11. Stress and strain relationship in a torsional simple shear test at minute strains.

SHEAR STRAIN, Yat

FIG. 12. Comparison of monotonic and cyclic loading tor- sional simple shear test of angular rockfill materials.

racy. The elastic modulus at small strains from the cyclic loading test coincides with the results of Iwasaki and Tatsuoka (1977). Monotonic and cyclic loading tests were compared for a larger number of sampled data (one datum per 11500 s) on Toyoura sand; the initial void ratio e, was 0.65 and the average particle size was 0.162 mm as shown in Fig. 6. By increasing the number of sample data, the elastic modulus E for the monotonic loading test was obtained up to a minute strain of less than E, = 2 X The E val-

CYCLIC AND MONOTONIC k TRlAXlAL

MAXIMUM ELASTIC MODULUS IN CYCLIC TRlAXlAL TEST, Emax (MPa)

FIG. 13. Relationship between E, and Em,, in triaxial tests.

, .- CYCLIC AND MONOTONIC TRlAXlAL

, '

RELATIVE

1 0 70

'4 - 90

CONFINING STRESS, OC (MPa)

FIG. 14. Comparison of E ,,,,, and E ,,,, at E, = 2 X lo-'.

I I I I - B 300 - . DAM - 0 SHICHIGASHUKU DAM

d' 5 o r . " ' I I I I

0.05 0.1 0.2 0.3 0.5 MEAN PRINCIPAL STRESS, Om (MPa)

FIG. 15. Relationship between Gl(2.17 - e)? ( 1 + e ) and mexi principal stress.

ues from both the monotonic and cyclic loading tests showed good correspondence in the axial strain range lo-" to 5 X

under confining stresses of o, = 0.2 and 0.4 MPa . o tests o n The results of the monotonic and cyclic loadin,

angular rockfill materials are compared below.

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CAN GEOTECH J VOL 3 1, 1994 i \

B

~bundation rock Riverbed gra

0 50 100 - (m)

FIG. 16. Cross section of Miho Dam. Elevations are in metres.

FIG. 17. Location of lines A and B at Miho Dam. Elevations are in metres.

The stress-strain relationship at microstrains in the mono- tonic loading test are shown in Fig. 7. Precise values for the elastic moduli were calculated for axial strain incre- ments of around 1 X showing a linear relationship at strains of less than 1 X

The relationship between shear strength T~(=(u, - ~ ~ ) , , , ~ ~ / 2 ) at failure in a monotonic loading triaxial test and E, is shown in Fig. 8 on a logarithmic scale. For the same shear strengths, the E, value of round gravel materials was smaller than that of angular rockfill materials. This shows that the effect of the difference in grain-size distribution cannot be neglected. Furthermore, when specimens of similar relative densities and confining stresses are compared, the shear strengths of both angular and round materials are the same, indicating that difference in particle shape affects the dif- ferences in the elastic moduli at small strains. A linear cor- relation between log T, and log E, in both round and angu- lar rockfill materials can be seen, independent of the initial void ratio and confining stress.

The relationship between elastic modulus and axial strain obtained from monotonic and cyclic loading tests for angu- lar rockfill materials and round gravel materials is shown in Figs. 9 and 10. For both materials, the elastic moduli of the monotonic and cyclic loading tests at minute axial strains show good correspondence regardless of the confining stress.

The materials were well graded, &d the top layer of the monotonic and cyclic loading test specimens in the labora- tory were prepared so as to contiin fine fragments of mate- rials. Since the cyclic loading testing is a staged test, the bedding effects for the strain-dependent curve of elastic moduli at higher confining- pressures are assumed to be greatly reduced. As shown in Figs. 9 and 10, the similar elastic moduli of both the monotonic and cyclic loading tests proves the insignificance of the bedding effects in the preparation of the specimens.

The relationship between shear strain and shear stress in the monotonic loading test using a large-scale torsional simple shear testing apparatus is shown in Fig. 11. The torsional sim- ple shear test can be used to simulate the stress and strain conditions in the field under earthquake loading. From an engineering standpoint, the in situ stress and strain condition can be simplified by assuming that the magnitude of the shear stress changes due to the earthquake loading with (i) rotation of the principal stress axes, and (ii) cyclic simple shear defor- mation, which is distributed uniformly in the vertical direc- tion, such as shear strain. The static stress-strain relation in the dam body is a plane strain condition in which the strain in the direction of the dam axis is zero and the angle between the principal stress at the centre of the dam body and the hor- izontal banking surface is not in a constant state (Matsumoto et al. 1990). The cylindrical specimen used in the torsional simple shear test had an outer diameter of 800 mm, an inner diameter of 400 mm, and a height of 800 mm. Gabbro from the quarry site of the Sagurigawa Dam completed in 1992 was used. The relative density of the specimen was 85%, with the grain-size distribution indicated in Fig. 3. The relationship between stress and strain at very small strains was obtained with high precision for each confining stress and was a convex curve for strains of less than 4 X lod5. The relation between shear moduli G and shear strain in torsional simple shear test y,,, derived from both cyclic and monotonic loading tests, is shown in Fig. 12. There is some correspondence at shear strains of less than 1 X

Teachavorasinskun et al. (1991) reported the correspon- dence of shear moduli for monotonic and cyclic loading tor- sional simple shear tests using Toyoura and Hamaoka sands.

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SHEAR WAVE ARRIVAL TIME (ms) %

50 100 150 200

5

10

15

E - x 20 k W cl

25

30

35

40

FIG. 18. Response amplitude and relative shear wave travel times in Miho Dam.

I MlHO DAM

40 -

50 -

- 60 -- IN SlTU TEST ---- SAWADA ET AL. 1977

700 200 400 600 800 S-WAVE VELOCITY (mls)

round rockfill materials. The figure shows that E, and Em,, are almost equal in the specimens of rockfill materials regardless of particle shape and the level of compaction. Figure 14 shows Eo and Em,, for monotonic and cyclic load- ing tests at an axial strain of 2.0 X respectively; Em,,, and E,,,, are normalized at E, and E,,,, for each confining stress. Angular rockfill specimens with relative densities of 70 and 90% were used. As the confining stress increased, the - effect of stress history became greater, and E,,,, became larger than E,l,o,o.

As mentioned above, at minute strains, the elastic moduli from monotonic loading triaxial tests correspond well with those from cyclic loading triaxial tests. This was also con- firmed by the shear moduli from a torsional simple shear test. Therefore, regardless of testing method and particle shape, the elastic moduli for cyclic loading can be estimated with reasonable accuracy from the elastic moduli of mono- tonic loading tests at very small strains.

FIG. 19. Distribution of shear wave velocity with depth in 3. Comparison of laboratory tests and in situ Miho Dam. geophysical tests

Figure 13 shows the relationship between the maximum 3.1. Shear wave velocity by laboratory tests elastic modulus Eo from monotonic loading tests and the Tests were conducted with the large-scale cyclic triaxial maximum elastic modulus Em,, from cyclic loading tests at testing apparatus on specimens of 300 mm in diameter and relative densities between 70 and 90% for both angular and 600 mm in height, as shown in Fig. 1. The grain-size dis-

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170 CAN. GEOTECH. J . VOL. 31, 1994 r \

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TABLE 4. Data from six earthquakes at Miho Dam for comparison with those from1 laboratory

Name of 1st natural Shear Reduction of earthquake Date Magn~tude frequency strain shear modulus

1 Izu Peninsula Off 29 June 1980 6.7 1.95 6.37 X 0.91 2 East Yamanashi 14 April 1981 4.5 1.86 5.40 X lo-' 0.82 3 West Kanagawa 8 Aug. 1983 6.0 1.81 1.63 X lo-' 0.78 4 Chiba West Off 17 Dec. 1987 6.7 2.00 4.00 X lo-' 0.95 5 Hakone 5 Aug. I990 5.1 1.86 5.35 x 0.82 6 Tokyo Bay 2 Feb. 1992 5.7 2.05 2.22 x lo-' 1 .OO

tribution of the specimens was modified so as to be parallel to the gradation of the rock materials used for each embank- ment dam. The maximum grain size was determined to be one-fifth to one-sixth of the diameter of the specimen. Specimens were prepared and tested as previously described.

Axial strain was measured by LVDT for displacement with 5 mm measuring length and stress by a 19.6 kN load cell. Shear strain and maximum shear modulus G,,,, were obtained from axial strain E, and maximum elastic modu- lus E,,,,,, through Poisson's ratio v as shown in [2] and [3]:

From the test results, the relationship between G,,, and mean principal effective stress u k of rockfill materials from the Miho and Shichigashuku dams is expressed by the fol- lowing equations (see Fig. 15):

( Shichigashuku Dam)

where e is the void ratio of rockfill materials from the dam, and Pa is atmospheric pressure. Quality-control tests con- ducted during the construction of each dam indicated that the value of e was 0.25 for the Miho Dam and 0.27 for the Shichigashuku Dam. If the deformation of a dam body is assumed to be two-dimensional with plane strain condition, then ub,, which represents the stress at depth D, can be expressed by

where K is the principal stress ratio, g is gravitational accel- eration (9.8 mls') and p, is the density of wet soil (2.3 t/m3 for Miho Dam and 2.1 t/m3 for Shichigashuku Dam by quality-control tests).

Shear wave velocity V, was calculated from G,,,, using the following equation:

The relationship between depth D and V, can thus be cal- culated from G,,,,, derived from laboratory tests.

d I CYCLIC TRlAXlAL (AFTER 0

YASUDA 1992) AND OBSERVED EARTHQUAKES.

I :* I

SHEAR STRAIN, Y

FIG. 20. Relationship of GIG,,,, and shear strain obtained from cyclic loading tests and recorded earthquake waves in Miho Dam. Circled numbers refer to earthquakes listed in Table 4.

3.2. In situ geophysical tests and discussion

3.2.1. Miho Dam The Miho Dam is a 95 m high rockfill dam with a central

impervious core as shown in Fig. 16. The elastic shear wave velocity was measured by small-scale blasting at 2.5 m intervals, utilizing the holes for measuring the internal ver- tical movement and through reception points at 2.5 in inter- vals along lines A and B on the slope surface as shown in Fig. 17. The length of line A was 250 m along the stream axis, and line B measured 200 m along the dam axis. The holes for measuring the internal vertical movement used for small-scale blasting were located at elevations 288 and 116 m from the upper end of line A. Figure 18 shows the typical shear wave signals and travel times, and the travel- time curve was obtained from this figure. The resulting velocity distributions within the dam body classified by depth are shown in Fig. 19. Since the reservoir is at the upstream side of the dam body, the V, distribution could not be obtained for upstream points. The V, deeper than 40 m was estimated from the relationship of the velocity of the primary compression wave V, and V, within 40 m depth and the observed V, deeper than 40 m. Also shown here are average values of V, for Japanese rockfill dams reported by Sawada et a1.(1977) which were slightly higher than those derived from this study. The shear wave veloci- ties from triaxial tests in the laboratory are also shown in this figure and correspond well with the results of in situ geo- physical tests. VslV, was 0.48 and independent of depth. Poisson's ratio calculated from [8] was 0.35 for the Miho Dam.

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YASUDA AND MATSUMOTO r \

. . I , . ',.

Low water v - X - n 6

-- ---T_-,,' r----. --- ----

Tuffs 1 - .I I -. .-------- Altered andes!e_-,I ,,-. '. ' ,

//' Tuffs , \ \ ''I I '!,if Tuffs [ Terrace deposits Andesites Terrace deposits

FIG. 21. Cross section of Shichigashuku Dam.

MEASURING SYSTEM

DISPLAY

DATA PROCESSOR MEMORY APPARATUS

CSHOTMARK FOR s WAVE SHOTMARK FOR P WAVE

3-COMPONENT GEOPHONES PNEUMATIC PACKER (RUBBER TUBE)

TUBE FOR NITROGEN SUPPLY

FIG. 22. Schematic view of P- and S-wave logging in Shichigashuku Dam.

1-2(Vs l V p ) 2 events was calculated based on the 1st natural frequency [XI v = for the Tokyo Bay earthquake f,,, by which very small shear

2[1-(V, / V p ) 2 ] strains were generated in the dam body. The reduction of shear modulus GIG, is expressed by the following equation:

The earthquake is one of the large-scale in situ tests. Fifty 2 earthquake events were recorded between 1976 and 1992 at the NIiho Dam. The strain dependency of shear modulus [,I L=[$) G, in the Miho Dam was investigated using six events that had larger acceleration records. The reduction of 1st natural fre- where G, is shear modulus obtained from the Tokyo Bay quency f, in the frequency response function for the five earthquake.

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CAN. GEOTECH. J . VOL. 31, 1994 i \

NORMAL WATER LEVEL 9.A r r c , c l e ----- 'i

3rd MEASUREMENT a a u IVILMGUREMENT + 293.0--- -- - 286.0 2nd MEASUREMENT 2nd MEASUREMENT

273.4/ 't;/ / \ \ - 1 - 1st MEASUREMENT

POLYVINYL CHLORIDE TUBE FOR GEOPHONES

FIG. 23. Measurement (m) of elastic waves at four elevations during the banking in Shichigashuku Dam.

SHEAR WAVE ARRIVAL TIME ( m s )

I I , I I

FIG. 24. Response amplitude and relative shear wave travel times in Shichigashuku Dam.

Maximum shear strain in the dam body was calculated from the maximum displacement at the crest dm,, and the dam height H. The value of d,,,, was obtained from the maximum acceleration at the crest a,,, as follows:

dmax 1 a max [ lo] H H '0:

where w, = 27rf, is 1st natural circular frequency. Figure 20 shows the relationship between the reduction

of shear moduli and shear strains based on data f rom

observed earthquakes (Table 4) and cyclic loading triaxial tests using rockfill materials from the Miho Dam in the lab- oratory. The reduction of GIG,,, from the laboratory test is independent of the confining stress (Yasuda 1992). As the shear strain increased, shear moduli from earthquakes decreased and agreed well with those from the laboratory test. The increase of the stiffness of the dam body by the effect of consolidation after the completion was not con- sidered here because the characteristics of earthquakes were different from each other and similar earthquake conditions could not be established again.

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YASUDA A N D MATSUMOTO , 1

3.2.2. Shichigashuku Darn The Shichigashuku Dam is also a 93 m high rockfill dam

with a central impervious core, as shown in Fig. 21. The elastic shear wave velocity was measured through vertical observation holes on the upstream and downstream slopes of the dam body along the depth direction. The plank ham- mering method was used to provide the secondary wave in which a thick board was positioned next to the top of the observation holes and hit strongly. To measure the primary compression wave velocity, a heavy weight was dropped as illustrated in Fig. 22. A pick-up with geophones made up of three components was lowered from the upper end of the observation holes and measurements were taken at every 1 m in depth. The pick-up was fixed to the hole wall by inflating a rubber tube inside, using an external compressed air source. Four series of measurements were conducted at various stages of the embankment (see Fig. 23). Figure 24 shows the geophone response amplitude and relative shear wave travel times from P- and S-wave logging. V , values classified by depth for the third and fourth tests on the downstream slope are shown in Fig. 25. The fourth test was conducted after completion of the embankment, and the value of V, was larger because of the raising of the embank- ment at the inner part of the dam body. These values at shallow points were larger than those reported by Sawada et al. (1977). In Fig. 25 the shear wave velocities in the cyclic loading triaxial tests are also shown (a solid line), and these closely correspond with results of in situ tests at deep points.

The torsional simple shear test is considered to simulate the in situ stress condition very well. For the mean principal effective stress, Matsumoto et al. (1990) showed that the shear modulus obtained from a cyclic loading torsional sim- ple shear test was 10% larger than that from a cyclic loading triaxial test. Therefore, the curves for the laboratory tests in Figs. 19 and 25 should be shifted slightly to the right.

Poisson's ratio in the Shichigashuku Dam varies from 0.27 to 0.35, increasing with depth.

4. Conclusions In an effort to simplify test procedures, the relationship

between monotonic and cyclic loading tests was investi- gated with triaxial and torsional simple shear specimens.

By setting up a precision linear variable differential trans- former on the cap of the specimen, the elastic moduli at very small strains between lop6 and were obtained through high-velocity data sampling. Focussing on the defor- mation characteristics at very small strains, the results of monotonic loading tests were compared with those of cyclic loading tests for Toyoura sand prior to the tests of rockfill materials, and the elastic moduli of the two tests agreed well. Elastic moduli for rockfill materials in monotonic and cyclic loading tests were also similar at very small strains of less than 1 X lo-'. However, the strain dependency of the elastic modulus was evident in the monotonic loading tests and indicated a larger reduction in monotonic loading tests than in cyclic loading tests as the strain increased.

This indicates that it is possible to estimate the value of physical properties at very small strains for dynamic analy- sis from the results of a monotonic loading test alone with- out conducting cyclic loading tests. However, more data are required to confirm this, the methods of measuring minute strains in laboratory tests need to be standarized.

I I I

200 400 600 800 S-WAVE VELOCITY (mls)

FIG. 25. Distribution of shear Give velocity with depth in Shichigashuku Dam.

For rockfill dams, the deformation characteristics obtained by in situ tests (geophysical P- and S-wave logging) were similar to those estimated from laboratory tests. The similarity between the construction conditions of the rockfill dams and the method of preparing specimens in the laboratory tests (which use reconstructed materials) might explain the high degree of correspondence between the respective test results. The reduction of shear moduli with the increase of shear strains from the observed earthquakes coincides well with those from the laboratory tests. Conclusively, the large- scale cyclic loading test in the laboratory can accurately predict the deformation characteristics of soil structures. However, the similarity of deformation characteristics between monotonic and cyclic loading tests and between laboratory and in situ tests is a feature of the construction of soil struc- tures as rockfill dams and reclaimed lands.

Burland, J.B. 1989. Ninth Laurits Bjerrum Memorial Lecture: "small is beautifulm-the stiffness of soils at small strains. Canadian Geotechnical Journal, 26: 499-5 16.

Hardin, B.O., and Drnevich, V.P. 1972. Shear modulus and damp- ing in soils: design equations and curves. ASCE Journal of the Soil Mechanics and Foundations Division, 98(SM7): 667-692.

Ishihara, K. 1976. The basis on soil dynamics. [In Japanese.] Kajima Press, Tokyo, Japan.

Iwasaki, T., and Tatsuoka, F. 1977. Effect of grain size and grad- ing on dynamic shear moduli of sands. Soils and Foundations, 17(3): 19-35.

Kokusho, T. 1987. In-situ dynamic soil properties and their evaluations. The state of-the-art paper. In Proceedings of the 8th Asia Regional Conference on Soil Mechanics and Foundation Engineering, July 20-24, 1987. Japanese Society of Soil Mechanics and Foundation Engineering, Kyoto, Japan. Vol. 2. pp. 215-240.

Kondner, R.L. 1963. Hyperbolic stress-strain response: cohe- sive soils. ASCE Journal of the Soil Mechanics and Foundations Division, 89(SM1): 115-143.

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Matsumoto, N., Yasuda, N., Ohkubo, M., and Kinoshita, Y. 1990. Shear strength and dynamic deformation characteristics of gravels. [In Japanese.] Proceedings of the Japanese Society of Civil Engineers, No. 424 111-14, pp. 95-104.

Matsumoto, N., Yasuda, N., Yoshioka, R., and Ohkubo, M. 1991. Cyclic and monotonic triaxial tests of gravels in micro-strain range. [In Japanese.] In Symposium on Triaxial Testing Methods, Japanese Society of Soil Mechanics and Foundation Engineering, pp. 169-172.

Powell, J.J.M., and Butcher, A.P. 1991. Assessment of ground stiffness from field and laboratory tests. In Deformation of Soils and Displacements of Structures, Proceedings of the 10th European Conference on Soil Mechanics and Foundation Engineering, May 26-30, Florence, Italy. Associazione Geotecnica Italians, A.A. Balkema, Rotterdam. Vol. 1. pp. 153-156.

Sawada, Y., Takahashi, T., Sakurai, A., and Yajima, H. 1977. The distribution characteristics of the material properties and the dynamic behaviors of rockfill dams. [In Japanese.] Electric Power Central Research Institute, Abiko, Japan. Report 377008.

Tatsuoka, F., and Shibuya, S. 1991. Relationship of the defor- mation properties of soils and rocks between the triaxial tests. [In Japanese.] In Symposium on Triaxial Testing Methods, Japanese Society of Soil Mechanics and Foundation Engineer- ing. pp. 39-84.

Tatsuoka, F., and Shibuya, S. 1992. Deformation characteristics of soils and rocks from field and laboratory tests. Report of the Institute of Industrial Science, University of Tokyo.

Teachavorasinskun, S., Park, C.S., Katoh, S., Shibuya, S., and Tatsuoka, F. 1990. Deformation and strength characteristics of sand in monotonic and cyclic torsional shear. [In Japanese.] In Proceedings of the 25th Japan National Conference on Soil Mechanics and Foundation Engineering, June 13-15, Okayama, Japan. Japanese Society of Soil Mechanics and Foundation Engineering, Tokyo, Japan. pp. 461-464.

Teachavorasinskun, S., Shibuya, S., and Tatsuoka, F. 1991. Stiffness of sands in monotonic and cyclic torsional shear. In Proceedings of the Geotechnical Engineering Congress 199 1, June 10-12, Boulder, Colo. Edited by F.G. McLean, D.A. Campbell, and D.W. Harris. Geotechnical Engineering Division of ASCE, New York, N.Y. Geotechnical Special Publication 27, Vol. 2. pp. 863-878.

Yasuda, N. 1992. Behavior of embankment dams during earth- quakes and dynamic deformation characteristics of rockfill materials. [In Japanese.] Journal of Japan Society of Dam Engineers, No. 6, 43-59.

List of symbols D depth

D m , x maximum grain size Dr relative density

, \ ' I ,

maximum displacement t t the crest elastic modulus elastic modulus at axial strain E, maximum elastic modulus in monotonic tri- axial test maximum elastic modulus in cyclic triaxial test elastic modulus in cyclic triaxial test elastic modulus in monotonic triaxial test initial, minimum, and maximum void ratio 1 st natural frequency 1st natural frequency obtained from Tokyo Bay earthquake shear modulus shear modulus obtained from Tokyo Bay earthauake maximum elastic modulus in monotonic load- ing test maximum shear modulus in cyclic loading test shear modulus by in situ geophysical test specific gravity ,. gravitational acceleration dam height ;. principal stress ratio atmospheric pressure absorption velocity of p6mary compression wave velocity of secondary shear wave water content maximum acceleration at the crest axial strain shear strain shear strain in torsional simple shear test wet density mean principal effective stress Poisson's ratio shear strength shear stress (axial-tangential) in torsional simple shear test 1st circular frequency maximum principal stress minimum principal stress confining stress constants

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