Clay Minerals (1986) 21, 311-332

G E O T E C H N I C A L P R O P E R T I E S A N D B E H A V I O U R

O F T H E M O N A S A V U H A L L O Y S I T E C L A Y , F I J I

D . J . K N I G H T

Associate and Head of Geotechnical Engineering, Sir Alexander Gibb & Partners, Earley House, London Road, Earley, Reading RG6 1BL

(Received 11 December 1985; revised 27 February 1986)

ABSTRACT: Geotechnical properties and behaviour are described of the very wet, halloysitic, residual clay found in the dense rain forest at Monasavu, Viti Levu, Fiji, where the annual rainfall can exceed 5 m. The tropical climatic conditions have caused deep weathering of sandstones and produced a highly plastic clay with low density and a natural moisture content greatly in excess of the standard compaction optimum. This clay was found to contain halloysite which was 'amorphous' rather than crystalline. The material was used in this natural state in an 85-m high rockfill dam at Monasavu Falls as an unusually soft core, the construction of which involved unconventionally light compaction by low-ground-pressure-tracked dozers. Its resulting behaviour in terms of three-dimensional total and effective stresses, stress paths and deformations throughout the construction, impounding and full reservoir stages was closely monitored. This behaviour is examined in the light of the clay's classification, mineralogical, compaction and engineering properties determined before and during construction. Despite its unusual properties, it is concluded that the clay is a good engineering material, behaving like others containing halloysite in the more common tubular form, and, moreover, that the high natural moisture content is of positive benefit.

The purpose of this paper is to describe the geotechnical propert ies and behaviour of the residual clay material used for the core of Monasavu Dam, Fiji, and to examine this use and behaviour in the light of those propert ies determined both prior to and during

construction. Monasavu D a m is an 85-m high rockfi l l /c lay core embankment dam situated at

Monasavu Falls in Viti Levu, the main island of Fiji (Fig. 1). It was constructed for the Fij i

Electricity Author i ty between 1979 and 1982, with impounding completed in 1983 (Knight, 1984).

The dam is situated on the wet side of the Nadrau Plateau in the Central Highlands at an elevation of 670-750 m, immediately ups t ream of the substantial cliff where Nanuku Creek once formed the spectacular 126-m high Monasavu Falls (Fig. 2). This plateau is part of a

mountainous barrier forming a transit ion zone between the wet east and dry west zones of the island. Although the latter experiences a marked dry season in the winter months the annual variations of rainfall in the wet zone are less pronounced. Average monthly rainfalls at Monasavu from September 1980 to December 1984 are shown in Table 1, from which

it may be deduced that the 19-month core construct ion period experienced a total rainfall of 7536 mm.

1986 The Mineralogical Society

312

(a)

SOUTH

/ 0

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D. J. Knight

PA C IF I C

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q

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(b)

Bo Monosavu

Lautoka

. o @

- ~ - - ' ~ ~ Suva Ground levels (ft}

~;:.~ A bove El.2000 lmAbo~ EI.3ooo

FIG. 1. Location map. (a) Fiji Islands. (b) Viti Levu.

TABLE 1. Monasavu rainfalldata.

Month 1980 1981 1982 1983 1984 Period average

Jan - - 716 1072" 537 254 645 Feb - - 398 447 1302" 228 594 Mar - - 343 4285 867 724 591 Apr - - 444 320 293 457 379 May - - 166 175 208 390 235 June - - 107 305 102 510 256 July - - 45 338 158 78 155 Aug - - 200 515 477 196 347 Sept 444~ 140 145 168 59 191 Oct 435 776 176 368 155 382 Nov 430 329 841 432 429 492 Dec 147 469 429 416 354 363

TotM - - 4133 5191 5328 3834 4622

* Cyclones. ~" Start of core construction. $ Finish of core construction.

Geotechnical properties of Monasavu clay 313

\

f

ersion bnel

e e

i9.3

Spillway.

FIG. 2. Plan of dam and location of clay borrow area.

ITI

G E O L O G Y

The dam site is underlain by a flat-lying sequence o f sedimentary rocks, belonging to the Ba Group, into which is intruded a thick monzonite sill, which forms the Monasavu Falls 300 m downstream of the dam axis and constitutes the dam foundations in the river. At the dam site a conglomerate occurs immediately above the sill and this is overlain by a sequence of almost horizontally bedded sandstones of various grain sizes which form the dam abutments (Fig. 3). The sand grains consist of reworked tuffaceous material and rock fragments. Weathering of these lithic sandstones in the tropical climatic conditions has produced a weathered profile varying in thickness from 3-25 m, most of which comprises clay.

Table 2 summarizes the five zones o f differing rock qualities, o f which the RI and RI I materials represent sound rock, and RI I I to RV the overlying weathered profile. The

TABLE 2. Rock quality grades in Monasavu sandstone.

Rock quality grade Description

RV RIV

RIII RII RI

Soft, red-brown silty clay. (Residual clay: no trace of intact rock structure.) Firm to stiff red-orange to brown silty clay with gravel-sized rock fragments in banded and small block form. Completely weathered rock, with relic structures visible in excavations. (Dam core material.) Banded, weak clayey sandstone of variable hardness. Discoloured massive sandstone, recovered as long sticks of core from boreholes. Fresh grey-massive sandstone.

314

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FIG.

D. J. Knight

L. Embankment d SL.pillwa~ y - -[ I-

Instrumented sections Core borrow area

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. . . . . . . 7 ' ~ . . . . . . . . . . . . . / * Monzonite * ~ \ \ \ \ \ ' j , "

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I I I I I I +L00 +300 +200 +100 0 - I00 -200

Chainoge (m) eMain instrument locations

3. Geological profile, longitudinal section along dam core trench and location of instrumented sections.

-800

750

-700

- 6 5 0

600

interface between RIII and RII is sharp. The terms RI to RV, whilst specific to the site, are based iargely on standard grades.

The bulk of the weathered profile comprised RIV and RV, with RIII constituting only a small part of the total thickness above sound rock. Within the RIV zone shiny slickensided surfaces of several square metres were found in excavations. Small conchoidal features as well as larger joints exhibited the typical black staining of manganese oxide.

It is of interest to compare the RIV and RV materials with those described by Wesley (1973) in Java, Indonesia. There the pedology from the parent rock of volcanic origin to the fully residual soil shows less weathered yellow Andosols occurring above the 1000 m elevation and more weathered red-brown Latosols occurring below that otherwise arbitrary elevation. The Monasavu clays are the same as the latter in both colour and elevation.

I N V E S T I G A T I O N S

Investigations for a suitable core construction material concentrated on the RIV material as it was soon realized that it represented the only potential source of abundantly available material. The investigations comprised both field trials, involving core material selection and proving, and laboratory testing.

Core material selection and proving

Initial investigations by large trial excavations in potential sources of core material revealed the weathered succession (Table 2) to be of adequate thickness for practical working, and the RIV material, drier than the RV, was selected for proving trials. On excavation the RIV material appeared as lumps of clayey weathered rock; as expected, drying altered its properties and working affected its behaviour, so that the common classification tests were inadequate as a unique description of the material.

The results of early classification tests performed up to 1979, prior to the start of dam construction, are given below. They represent samples of potential core material found

Geotechnical properties of Monasavu clay 315

within the RIV weathering grade. These results may be compared with the measured properties of the clay in the constructed dam core (Table 5).

Natural moisture content: Air dried, average = 72% Oven dried, average -- 83%

Natural dry density: Range = 0-79-0.98 t/m 3 Average = 0.86 t/m 3

Specific gravity: Range = 2.28-2.65 Average = 2.46

Liquid and plastic limits, and particle-size analyses, were also done in considerable number but, as the testing methods were varied during this stage of the investigations, it is inappropriate to quote values.

Uncertainty existed about the feasibility of constructing a clay core with this highly plastic material having such a high natural moisture content. Trials involving various types of plant and methods were therefore made to see whether it could be satisfactorily placed and compacted. The wet climate precluded prior drying of the material to near its optimum moisture content, and it soon became apparent that it would have to be used in its natural condition. A trial embankment constructed in one of the large trial excavations in drizzly weather and wet ground conditions demonstrated that a low-ground-pressure-tracked D6-bulldozer could successfully borrow fresh material, spread and then compact it in thin layers without sinking excessively below the surface. This technique broke down the lumps and partially remoulded the RIV material into a homogeneous layer, so that the product resembled a composite of small discrete chunks (up to 100 ram) within a soft matrix and it proved physically possible to construct an RIV clay bank in these conditions. Undisturbed block samples were taken to measure permeability and determine shear strength and consolidation parameters, the values of which were subsequently verified from the main core construction described later.

Laboratory testing

The laboratory testing programmes were designed to include the following tests on disturbed and 'undisturbed' samples of the natural and recompacted soil and the clay from the as-constructed core.

Classification tests: Natural moisture content and dry density; liquid and plastic limits; specific gravity; particle-size analysis; dispersivity.

Mineralogical analyses: X-ray diffraction and scanning electron microscopy.

Compaction tests: Maximum dry density; optimum moisture content.

Engineering design tests: Consolidated-drained triaxial compression tests; consolidated- undrained triaxial compression tests with pore-pressure measurement; oedometer consolidation tests; permeability tests.

Construction control tests: Unconfined compressive strength; classification tests as above.

The majority of the testing was carried out in Fiji, with some of the early work being done mostly in Australia. All construction control testing was done in accordance with

316 D. J. Knight

either Australian or British standards. Because of the suspected, and eventually confirmed, presence of halloysite, investigations were carried out on the effect of different drying methods on moisture content determination, involving (i) oven drying at 110~ (ii) oven drying at 80~ (iii) flame drying and (iv) air drying. For consistency and comparison purposes, all moisture contents during the construction control stage were measured by oven drying at 110~ which resulted in the greatest apparent moisture content value. Specific gravity was also found to increase with the efficiency of the drying method.

To examine the effect on the optimum moisture content (Frost, 1967; Wesley, 1973), two different methods of sample preparation for the standard compaction tests were used involving (i) natural drying to a moisture content below the expected optimum and the subsequent addition of water to achieve the required moisture content range, and (ii) natural drying ('drying back') through the testing range until the optimum moisture content was reached. In controlled comparative tests it was found that method (ii) gave average values of optimum moisture content 9% higher and maximum dry densities 2% lower than those determined from method (i).

The early engineering design testing was done on potential core material recompacted to maximum dry density at optimum moisture content, as was originally intended for the dam core. Following the radical change in specification whereby the clay was only lightly compacted at its high natural moisture content, a series of triaxial, permeability and consolidation tests was carried out on 100 mm diameter undisturbed specimens procured directly from the as-constructed core, from the bottom of 0.5 m deep pits at 16 different locations when the dam was at a height of 23 m. The results of these were used to check the parameters adopted for design. A further set of twenty 100 mm diameter samples was also tested in unconfined compression at various times up to 28 days after sampling to investigate the possibility of strength increase with time. A marginal increase was noted, but it would be inappropriate to attribute much significance to this.

The routine construction control testing was performed on a series of six 100 mm diameter core cutter samples for classification testing and an adjacent series of four 38 mm diameter unconfined compressive strength test samples.

M I N E R A L O G Y

Mineralogical analyses of the Monasavu clay were performed at different stages of the project by a variety of organizations, all principally concerned to confirm the suspected presence of halloysite, in either its hydrated or 'meta' form. The well known effects (Frost, 1967; Wesley, 1973) of this mineral on testing procedures for the routine laboratory tests, on the practicalities of construction and on the engineering behaviour of soils containing it made it highly desirable to determine its presence.

Techniques used were X-ray diffraction (XRD) and scanning electron microscopy (SEM), mostly on samples from the dam core borrow area or the dam itself, together with some others on less highly weathered rock material. Table 3 summarizes details of the six series of mineralogical analyses performed and Table 4 summarizes results of the analyses in terms of the minerals identified.

Series 1. The presence of hydrous aluminium oxides and amorphous materials was noted. It seems possible that these could be gibbsite and allophane respectively.

Series 2. Despite repeated efforts, Slansky (1978) could only obtain relatively poor XRD patterns and this he ascribed to the nature of the material rather than to experimental

Geotechnieal properties of Monasavu clay 317

factors. This would appear to indicate its amorphous nature. The measured basal spacings for the soil were 9.0 A (very broad) for air-dried and 11.5 A for glycolated specimens. After heating to 400~ two spacings were shown, a stronger one of contracted dehydrated halloysite at 7.4 A and a weaker one of halloysite still hydrated at I0.2 A.

Series 3. The report (AMDEL, 1979) noted a low abundance of crystalline minerals, suggesting the presence of allophane or another amorphous component.

Series 4. XRD indicated that the principal minerals present in the whole sample were a 7 A kaolinite or metahalloysite phase, maghemite and hematite. The clay fraction was examined to characterize the 7 A phase. Behaviour during various pre-treatments suggested that this phase was a poorly-ordered kaolinite rather than a normal metahalloysite, although the wet-state basal spacing of 10.5 A was very similar to that of a halloysite. Subsequent SEM showed no trace of the tubular morphology typical of halloysite, but rather a honeycomb texture (Fig. 4) often developed by smectitic clay minerals. However, doubts existed about the precise location of the sample, and thus whether it was the clayey sandstone parent material of the RIV clay, or rather decomposed monzonite. It was therefore decided to resolve the question of halloysite presence by further SEM work in Australia on fresh material taken from the dam clay core borrow area.

Series 5. Both XRD and SEM were carried out, the former indicating halloysite but the latter giving inconclusive data (Figs 5a~:l) with respect to 'tubular' halloysite.

FIG. 4. Scanning electron micrograph: series 4 (• 11 000).

318 D. J. Knight

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Geotechnical properties of Monasavu clay 319

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320 D.J . Knight

TABLE 4. Summary of minerals

Date of Series Sample No. Report Am

Minerals identified

Ep Gi

Halloysite

H H~ H m

1 �9 30/5/77

2 �9 2/6/78 m v m

m

m

m

3 WIII(A)I 22/6/79 WIII(A)2

WIV 1 WIV 2

WV 1 WV 2

CD - - Tr/A Tr - - CD - - Tr/A Tr

T r

Tr

Tr Tr A Tr

Tr/A A

D D D D

D D D D

Not stated

4 �9 7/3/80 *?*?

5 A 15/7/81 B

C D E

m

m

m

m

m

3

1

2 1

2

Not stated

6 �9 Between Nov. 1981 and April 1982

Details not available

Notes:

1. Mineral key

Am = Monoclinic amphibole,

Ep = Epidote. F = Feldspar. Gi = Gibbsite. H = Halloysite.

H h = Hydrated halloysite. H m = MetahaUoysite Hem = Hematite. K ~ Kaolin.

MH - Maghemite. P = Palagonite, 'used to

describe a poorly crystalline clay, probably a smectite precursor'.

Sm = Smectite (montmorillonite). Sp = Spine[, 'probably chromite

or magnesio-chromite'. V = Vermiculite.

2. Key to other symbols used

�9 Unnumbered sample. * Identification of mineral

in report or reference. - - Absence of mention in

report or reference.

Geotechnical properties of Monasavu clay 321

identified in Monasavu clay samples, Fiji.

(See note 1)

Hem K MH P Sm Sp V Remarks

Meta-halloysite 60-70% by weight of fine fraction. Hydrous aluminium oxides and amorphous materials also present, pH 6.45 of bagged bulk sample as received.

m

m

m m

m

See Slansky (1978). Report refers to hydrated halloysite as being the only crystalline clay mineral present in the soil.

Tr? Tr?

CD D CD D

Tr Tr Tr Tr

T r q

Tr

See note 3 for key to semi-quantitative abbreviations used. Data summarized from AMDEL (1979). WlV (later changed to RIV) represents the dam core material. In each column, the left-hand symbol relates to the bulk sample and the right-hand symbol to the fine fraction.

* * * 9. * h m Last remark for series 3 applied also to series 4.

m

4 m 3 m

- - 2 - - 1

2 1

3

4 3

- - Infrared spectrometry also performed on bulk - - sample. Numbers indicate probable order of - - occurrence of mineral identified. Data summarized - - from Eggleton (1981).

Details not available Halloysite present in spherical blocky 'cabbage' or cluster form rather than in tubular stick form.

3. Key to semi-quantitative abbreviations D = Dominant. Used for the

component apparently most abundant, regardless of its probable percentage level.

CD = Co-dominant. Used for two (or more) predominating components, both or all of which are judged to be present in roughly equal amounts.

A ~ Accessory. Components judged to be present between the levels of roughly 5-20%.

Tr = Trace. Components judged to be <5%.

322 D.J. Knight

(a) (b)

(c) (d)

FIG. 5. Scanning electron micrographs: series 5, sample E. (a) x2000; (b) x3500; (c) x250; (d) x 350.

Geotechnieal properties of Monasavu clay 323

Series 6. This continued inconclusiveness regarding the presence of halloysite led to the sixth and final series of tests on a sample procured by the author from the dam core during construction. This confirmed that halloysite was present, but in spherical blocky 'cabbage' or cluster form rather than in the more common tubular form.

Conclusion on halloysite presence

The lengthy testing series established that the Monasavu clay, as was always suspected by its laboratory moisture content testing behaviour, is halloysitic but of a different form to that usually found, tending to be more amorphous than crystalline.

It is of interest to note the weathering stages by which clays containing the amorphous mineral allophane change to kaolin. The first stage shows a mostly allophane presence, with some halloysite, whilst in the next stage the allophane more or less disappears and halloysite dominates. The hydrated halloysite then becomes dehydrated to metahalloysite before the final kaolin stage is reached. There are hints of some of these stages from the above-described mineralogical analyses carried out on the Monasavu clay. Frost (1967) alluded to a number of these factors in relation to the irreversible property changes involved in the pre-testing preparation of some tropical soils.

C O N S T R U C T I O N OF D A M C O R E

The design of the dam and its core specification have been described elsewhere (Knight et al., 1982; Knight et al., 1985). A radical change was made at the beginning of con- struction from working to a conventional end-product specification to a wholly method- related specification based on what was practically achievable, with a record kept of the achieved product in terms of classification data and unconfined compressive strength.

The essential points relating to the developed construction procedures for this clay core are summarized below. It is stressed, however, that the successful execution of such procedures depends much on the practical experience of the construction supervisory staff with wet core techniques.

Borrow area

Core material was taken from a large area on the right abutment constituting required excavations for the right flank of the embankment and for the spillway (Figs 2 and 3). The area drained well after rain and plant access was afforded by the sound rock benches underlying the weathered material.

Exploitation

Bulldozers worked on fairly flat slopes across the full depth of the RIV material (Fig. 6a), towards loaders and trucks waiting at the base of the slope. This ensured, as well as drainage after rain, good mixing of the material in terms both of its inherent natural variation through the depth range, and of the rock nodule/plastic matrix proportions. The material was taken directly to the dam for placement.

324 D.J. Knight

S : �84 . . . . .

(a)

(b)

FIG. 6. Clay core construction. (a) Borrow area exploitation. (b) Compaction of soft clay core with tow-ground-pressure-tracked D6 bulldozer.

Geotechnical properties of Monasavu clay 325

Placing, spreading and compaction

The fill was brought to the edge of the core by 20 tonne rear dump trucks, with spreading and compaction carried out by low-ground-pressure ('swamp') tracked D6-bulldozers (Fig. 6b). These placed the clay in 100 mm thick layers and naturally carried out much of the compaction. The final texture at depth below the grooved surface comprised discrete uncrushed RIV nodules up to 100 mm size surrounded by a remoulded clay matrix.

Surface sealing and drainage

The frequency and suddenness of rain necessitated pre-sealing of the constructed clay surface to allow early resumption of filling. Early methods involved a 10% crossfall to the watertight grooved surface, with some hand drainage; this gave way later to the use of a small loader with wide street tracks, combined with a smaller crossfall.

Limitations

As experience was gained it was found that core placement could take place under an increasing range of conditions, including mist or low rainfall. In one month, with only two rainless days and over one metre of rainfall, about 15 000 m 3 of clay were placed. Nevertheless, some limits were set. Accordingly, no filling was allowed after heavy rain until the surface had been adequately drained. Filling was stopped when the bulldozer tracks penetrated more than about 150 mm into the fill or 'floated' on it with excessive rebound.

G E O T E C H N I C A L P R O P E R T I E S O F C L A Y IN D A M C O R E

The routine construction procedures resulted in the measurement of the classification and undrained strength properties of the as-constructed dam core (Table 5).

The mean wet density of 1.52 t/m 3 is only about 75% of the value normally attained for a clay core. The mean unconfined compressive strength of 34 kN/m 2 implies a mean undrained shear strength of only 17 kN/m 2, assuming virtually saturated conditions. This demonstrates vividly the very low strength of the core at its initially constructed stage.

In addition to the routine classification testing, sixteen sets of undisturbed samples from 100 mm diameter core cutters were taken from the 23 m height of the as-built core between October 1980 and February 1981 for engineering property measurement, correlation of results with the routine testing, and checking of design assumptions for parameters and behaviour. In consolidated-drained triaxial compression a slightly curved Mohr-Coulomb envelope develops, with 4' = 30~ and c' = 15 kN/m 2, and a yield strain above 10% and sometimes 20%. In the quick undrained conditions such large strains were usual. The coefficient of compressibility m v from oedometer tests was generally within the range 0.2-0.1 m2/MN for pressures between 100 and 1600 kN/m 2. The indicated 'preconsolidation' left by the compaction process has been between 150 and 200 kN/m 2. Permeability values of 1 x 10 -8 cm/s were obtained.

326 D. J. Knight

TABLE 5. Summary of core properties as constructed.

Property Unit No. of results Mean Standard deviation

Classification Moisture content % 1750 76 7 Dry density t /m 3 1750 0.86 0.05 Wet density t /m 3 1750 1.52 - - Air voids % 1642 2-4 1.6" Passing 0.075 mm % 340 96 4 Plastic limit % 332 59 - - Liquid limit % 332 107 9 Plasticity index % 332 48 9 Liquidity index - - 0- 35 - - Specific gravity 238 2-66 0.11

Strength Unconfined compression kN/ m 2 1008 34 9

Standard compactiont (drying down) Maximum dry density (MDD) t /m 3 5 0.99 0.05 Optimum moisture content (OMC) % 5 58 6

Relative compactiont Mean dry density/mean M D D % - - 8 7 - - Mean moisture con ten t - -mean OMC % - - 18 - -

* Abnormal distribution. ~ For comparison only.

M E A S U R E D B E H A V I O U R O F C L A Y C O R E

Monitoring system

The dam core was instrumented to measure deformations, pore pressures and three-dimensional stresses during construction, impounding and operation, by means of hydraulic settlement cells, hydraulic piezometers and total earth pressure cells concentrated in three cross-sections and up to four horizons (Figs 3, 7).

Internal vertical movement of the core was monitored by a settlement cell system able to accommodate 2.3 m of differential settlement, and two deformation tubes were installed either side of the core. Electrical extensometers were installed longitudinally along the top of the core but, despite one metre of cover, the core placement process overstrained them. Surface movement was measured by means of survey and level stations on the crest and downstream slope.

Pore pressures were monitored by high-air-entry hydraulic peizometers installed across the core width, and stresses measured by sets of three total earth-pressure cells. One cell measured vertical stress, a second horizontal stress in an upstream-downstream direction, and a third horizontal stress in a cross-vaUey direction. Adjacent hydraulic piezometers permitted effective stresses to be deduced. All instrumentation was read regularly.

Reservoir impounding began in April 1982, after which the reservoir rose steadily through 70 m height before the first spilling in March 1983 (Fig. 9). The final 8 m was considerably hastened by cyclone Oscar (Table 1). The following sections summarize and review the measured behaviour throughout construction, impounding and the first year of

Geotechnical properties of Monasavu clay

750 7~s Fu,t sopp,y Lo~, I 1-3

g [ ~ 2 / Rockfill "-- 700 Cloy blonket 1 ~ / f

UJ

/ M o n z o n i t e 650 �9 �9

Key to instruments (exaggerated for clority)

�9 HydrQulic piezometer I Hydrou~ic settlement ce~ �9 Group of eorth pressure cells

(meesuring in 3 directions) - - - - -Deformct ion tube

~-~ 75 2 (includes settlement olEowance]

Vories

680 a m

M o n z o n i t e

FIG. 7. Dam cross-section and instrumentation at Ch. 175.

327

�84

.700

650

reservoir operation. They thus amplify and extend the information up to initial impounding given by Knight (1982), and summarize the data from Knight et al. (1985).

Deformations (internal and external)

Most of the settlement was accomplished by the end of impounding, and comparative settlements for the three cross-sections showed good consistency. The largest internal settlement occurred as expected at the centre of El. 705 at Ch. 175, where the measured settlement exceeded 1.8 m and continued slowly thereafter. This cross-section's composite behaviour at the centre of the core is shown on Fig. 8a, and indicates a consistent pattern of settlement with elevation. Behaviour of its mid-height zone is shown in more detail on Fig. 8b to both natural and exaggerated scales, illustrating the proportionate vertical movement of the core in relation to its width as well as the differential vertical displacements across it. Differential settlement is indicated at the downstream edge reflecting the differing depths of clay to the interface, and possible hang-up on the much stiffer filter. Overall the soft clay core appears to have squeezed down satisfactorily over about at least a 12 m horizontal width.

The maximum crest settlement was 0.33 m and, in the upstream-downstream direction, the maximum external horizontal displacement 0.1 m, occurring half way down the downstream slope, both at Ch. 175.

Construction pore pressures, dissipation and impounding

The general pore-pressure pattern for all core piezometers is typified on Fig. 9, showing an initial increase of piezometric level with dam fill, followed by slow dissipation of maximum construction values. Pore pressures increased again with impounding, rising suddenly to reflect the remaining 8 m of rapid filling, since when they either slowly dropped or remained steady. Piezometric level contour patterns within the core cross-sections were satisfactory.

The maximum construction stage pore pressure ratio (ru) was 1.14, where ru is the ratio

328

(a)

D. J. Knight

750- P ~ Q I 1983 Mar 1984

730

680 y

0 1 2 Settlement (m)

Hydraulic settlement ceils _ / _ ~ i n cross section (see F i g . 7 } ~

Core "Filters and - - Downstream rockfi II drain 0~ May1981

Mar 1984 Natural scale I I

E v 0" May

1981

Sept 1 1981

U3 Mar 1982 Exaggerated Mar 1983 vertical scale Mar 1984

2 20 upstream ~. downstream 20

Distance from Dam C E (m)

FIG. 8. Settlement behaviour at Ch. 175. (a) Settlement vs. elevation. (b) Settlement profile at El. 705.

of porewater pressure at a particular point to its total overburden pressure. With dissipation r u fell, to average values just before impounding within the range 0.43-0-55. Just before being affected by the rising reservoir these values had dropped to between 0.08 and O. 30, suggesting a relatively rapid consolidation of the clay core.

Total and effective stresses

The unusually soft core was always regarded as potentially advantageous, in being unable to resist applied shear stresses sufficiently to permit arching and hang-up between the rockfill shells on each side, and unable to sustain cracks during deformation which

,,~ 750i

u 715-

680 // .1980

Geotechnical properties of Monasavu clay

Fill l e v e ~ f ~ ~ "~

198 19%2 J ~9%3 19~54

FIG. 9. Piezometric level vs. time at Ch. 175.

329

would induce hydraulic fracture. Sherard (1981) has for long advocated this philosophy and has given a summary of its use (mainly in Europe).

Hydraulic fracture is avoided when seepage pressures are everywhere exceeded by the minimum total stress (approximately the lower of the two horizontal total stresses). This requires positive effective stress throughout, preferably increasing with time. The safest condition is that the minimum horizontal total stress should exceed the full reservoir head at the particular elevation. The purpose of the earth pressure cells was to verify these predictions and to quantify the behaviour.

Total vertical stresses (Fig. 10) rose steadily throughout construction, and continued to rise through impounding for El. 680 and El. 705. The corresponding total horizontal stresses showed a similar pattern but at lower absolute values, and exceeded the reservoir head except at El. 680 where, however, the local steady seepage pore pressure was exceeded. This was particularly encouraging considering the unusually low bulk density of the core material. A high and reasonably constant ratio, 0.8, of total horizontal to vertical stress was obtained, denoting proximity to uniform all-round stress conditions.

1000

800 Ch.210 c o n s t r u c t i o n

600 j 8

/

I~0 1981

Ch. 125 Impoundir & Et.680 C h . 1 7 5 ~ - End of I ' -" ~ ' - "

i

EI.735

1983 1984 FIG. 10. Total vertical stress vs. time.

330

Z v

I

v

100-

50-

D. J. Knight

(Mobilised angle of shearing 1 ~ / ~ , resistance) I l

Effective I ,~/~,- ~ J I Toter / st resses / . Z ' ,7 .fl.). I st resses /

J ~ " End of construction , /

Pore pressure

0 200 400 600 (O-v§ and (0- v § h ) /2 ( kN /m 2) CIv=Vertical stress

Oh= Horizontal stress

NOTE.Pressure readings below lOOkN/m 2 considered to be of low accuracy

FIG. 11. Stress relationships for Ch. 175 at El. 705.

The measured effective vertical and horizontal stresses throughout showed a generally steady increase, whilst the ratio of effective horizontal to vertical stress with time generally increased consistently to between 0.6 and 0-7.

Stress paths

Total and effective stress paths were monitored throughout and after the entire im- pounding period (Fig. 11, which in effect treats the entire core as an 80 m high triaxial test specimen). These show stress paths in a �89 v - ah) against �89 v + ah) space. A reasonable approximation is that o v represents the major principal stress and a h the minor principal stress. Steadily increasing values of equivalent deviator stress are shown for each case, as well as the variation of the mobilized angle of shearing resistance r The latter is initially just below the available value of 30 ~ (or more) indicated by triaxial tests, but subsequently drops below 20 ~ The consistency of the measured stress behaviour is again demonstrated, as well as the occurrence of expected stress redistribution. The ability of the low-strength clay core to yield appears to have reduced hang-up in the upper regions, allowing the self-weight of the core to be supported within itself rather than by shear and arching to the shells.

D I S C U S S I O N A N D C O N C L U S I O N S

Properties

High values of liquid limit, plastic limit and natural moisture content imply in many soils a low drained strength and difficult working (Terzaghi, 1958). For the Monasavu clay, and other similar tropically weathered soils, these characteristics have no such dire portents, and to that extent conventional classification systems can be misleading.

The clay's natural moisture content was unquestionably high in relation to the material's optimum moisture content, and thus was early recognized as being difficult or impossible to

Geotechnieal properties of Monasavu clay 331

compact by conventional means. The impracticability of drying led to its use at its high natural moisture content, necessitating placement and compaction with very light plant. Even so, excessive trafficking could overwork the material by releasing water from its micro-pores into the soft matrix surrounding the relic nodules (cf. Vaughan, 1982).

The very low dry density of 0.86 t/m 3 led to the low bulk density of 1.52 t/m 3, and to the possibility that it would produce unacceptably low total horizontal stresses. However, the achieved stresses were adequate, and thus the low density did not in the end become a problem.

Behaviour

The essential behaviour of the core during construction, impounding and early operation is summarized as follows.

A maximum vertical deformation exceeding 1.8 m occurred in the middle of the highest section, as anticipated. Other settlements were self-consistent, showing that the main central zone of the soft core continued to settle well in relation to the stiff rockfill shells. Crest settlement was modest, being only about 0.4% of the dam height, indicating fairly rapid consolidation.

Despite the fairly rapid consolidation, common with such residual soils and causing some pore pressures at the start of impounding to be less than the steady seepage values, the pore pressure values and patterns were satisfactory.

Measured stresses were satisfactory, showing a steady rise of effective stresses throughout construction and impounding. Total stresses, accompanied by settlement, continued to rise after the end of construction. At all except one location the lowest total horizontal stress exceeded the full reservoir pressure at the particular elevation and in all cases exceeded the seepage pore pressure. There is thus good evidence that arching is small and thus of the absence of conditions conducive to cracking and hydraulic fracturing.

The overall stress behaviour is consistent not only within itself but also with measurements from other instruments.

In order that the core's long-term behaviour can be measured, a separate trial bank of similar material to the core was constructed in the reservoir. It is hoped that this will, when reservoir conditions permit, indicate whether the clay is subject to any time-hardening process not attributable solely to consolidation, such as cementation.

Conclusions

The following conclusions are drawn: 1. The tropically weathered clayey sandstone used as the core for Monasavu Dam is a

good engineering material, despite its very high natural moisture content, high plasticity and low density. This conclusion is very similar to the one drawn by Wesley (1973) for some halloysitic and allophanic clays in Java, Indonesia.

2. The clay is halloysitic but apparently tending to be more amorphous than crystalline, with the halloysite present in a spherical blocky 'cabbage' or cluster form rather than as the more usual tubular sticks. This difference does not, however, appear to affect the

geotechnical engineering behaviour of the clay in comparison with that experienced with the tubular morphology.

332 D. J. K n i g h t

3. The c lay possesses a high intrinsic s t rength and displays very sa t i s fac tory behav iour

both during and after cons t ruc t ion .

4. The mater ia l is capable o f successful use within an e m b a n k m e n t d a m core in its

natura l wet state. Such a state is, moreove r , not only capable o f prac t ica l use, but also has

cons iderable benefits in c o m p a r i s o n with a lower mois ture con ten t state, in terms of its

s t ress - s t ra in behaviour .

A C K N O W L E D G E M E N T S

The author thanks the Fiji Hectricity Authority, its General Manager, Mr D. S. Pickering, CBE, and its Consulting Engineers Gibb Australia for their kind permission to publish this paper. He is also most grateful to his colleagues Mr N. M. Worner of Gibb Australia, for access to detailed construction notes, and to Mr G. L. Smith, FSCET, of Sir Alexander Gibb & Partners for assistance with some of the data research involved. The dam was constructed by the Special Projects Division of the Fiji Hectricity Authority. The first three power projects for which it was constructed were funded by, inter alia, IBRD, EIB, ADB, Australian Development Assistance Bureau as well as from Government grants, internally generated funds and local and overseas commercial loans.

Soils testing during the investigation stage was carried out by Longworth and McKenzie Pty Ltd of Sydney, Australia; a limited amount of construction stage soils testing was carried out by Golder Associates of Suva, Fiji. Mineralogical testing was shared between: Department of Mines, Geological Survey of New South Wales; The Australian Mineral Development Laboratories, Frewville, Adelaide, South Australia; The Australian National University, Canberra, ACT. Some mineralogical testing was also arranged through Dr P. R. Vaughan and Dr H. Shaw of Imperial College of Science and Technology, London.

R E F E R E N C E S

THE AUSTRALIAN MINERAL DEVELOPMENT LABORATORIES (1979) Clay mineralogy of six soil samples. Unpublished Report GS Q4797/79 for Sir Alexander Gibb & Partners Australia, 22nd June 1979.

EGGLETON A. (1981) Fiji clay investigation. Unpublished Report for Sir Alexander Gibb & Partners Australia, 15th July 1981, The Australian National University, Canberra, ACT.

FROST R.J. (1967) Importance of correct pretesting preparation of some tropical soils. Proc. 1st Southeast Asian Regional Conference on Soil Engineering, Bangkok, 43-53.

KNIGHT D.J., WORNER N.M. & McCLUNG J.E. (1982) Materials and construction methods for a very wet clay core rockfill dam at Monasavu Falls, Fiji. Proc. 14th Int. Cong. on Large Dams, Rio de Janeiro, 4, 293-303.

KNIGHT D.J. (1982) Discussion, question 55: Materials and construction methods for embankment dams and cofferdams--testing methods and quality control. Proc. 14th Int. Congr. on Large Dams, Rio de Janeiro, 5,598-604.

KNIGHT n.J. (1984) Monasavu Dam in Fiji. Introductory note, ICE/BNCOLD, London, March 1984, 1-6. Also discussion paper, 'Monasavu Dam, Fiji', BNCOLD News and Views, 27, 12-13.

KNIGHT D.J., NAYLOR D.J. & DAVIS P.D. (1985) Stress-strain behaviour of the Monasavu soft core rockfill dam: prediction, performance and analysis. Proc. 15th Int. Congr. on Large Dams, Lausanne, 1, 1299-1326.

SHERARD J.L. (1981) Building embankment dams in areas of high rainfall. Syrup. on hydro-electric development in the Amazon Region, Sdo Paulo, Brazil.

SLANSKY E. (1978) Clay mineral analysis of two specimens from Fiji (for Longworth and McKenzie Pty Ltd). Geological Survey Report No. GS 1978/153 (unpublished). Geological Survey of New South Wales, Department of Mines.

TEaZAGm K. (1958) Design and performance of the Sasumua Dam. Proc. Inst. Civil Engineers 9, 369-394. VAUOHAN P.R. (1982) Design and construction with wet fills. Special Lecture to Associacdo Brasileira de

Mecdnica dos Solos, Sdo Paulo, April 1982. WESLEY L.D. (1973) Some basic engineering properties of halloysite and allophane clays in Java, Indonesia.

Geotechnique 23, 471-494.