International Journal of Mining Science and Technology 24 (2014) 883–892

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International Journal of Mining Science and Technology

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Soft rocks in Argentina

http://dx.doi.org/10.1016/j.ijmst.2014.10.0232095-2686/� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

⇑ Tel.: +54 3514555325.E-mail address: maugiam@hotmail.com

Giambastiani Mauricio ⇑National University of La Rioja, La Rioja, Argentina

a r t i c l e i n f o

Article history:Received 17 January 2014Received in revised form 9 March 2014Accepted 15 May 2014Available online 14 November 2014

Keywords:Soft rocksArgentineTunnelsDamsRock parameters

a b s t r a c t

Soft rocks are a still fairly unexplored chapter in rock mechanics. Within this category are the clasticsedimentary rocks and pyroclastic volcanic rocks, of low to moderate lithification (consolidation, cemen-tation, new formed minerals), chemical sedimentary rocks and metamorphic rocks formed by mineralswith Mohs hardness less than 3.5, such as limestone, gypsum, halite, sylvite, between the first andphyllites, graphitic schist, chloritic shale, talc, etc., among the latter. They also include any type of rockthat suffered alteration processes (hydrothermal or weathering). In Argentina the study of low-strengthrocks has not received much attention despite having extensive outcrops in the Andes and great impact inthe design criteria. Correlation between geomechanical properties (UCS, deformability) to physical index(porosity, density, etc.) has shown promising results to be better studied. There are many studies andengineering projects in Argentina in soft rock geological environments, some cited in the text (Chihuídodam, N. Kirchner dam, J. Cepernic Dam, etc.) and others such as International Tunnel in the Province ofMendoza (Corredor Bioceánico), which will require the valuable contribution from rock mechanics.The lack of consistency between some of the physical and mechanical parameters explored from studiesin the country may be due to an insufficient amount of information and/or non-standardization of criteriafor testing materials. It is understood that more and better academic and professional efforts in improv-ing techniques will result in benefits to the better understanding of the geomechanics of weak rocks.

� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

The soft rocks in Argentina have not received the deservedattention despite being present in a great part of the territory.Many hydroelectric dams, roads and railways have been built inthis type of rocks.

In order to encourage the study of the geomechanics of softrocks in Argentina for its application in engineering, the petroleumindustry has made a compilation of studies published inconferences and journals.

This work is a synthesis presented by the commission of softrocks as a preliminary report to the ISRM International Symposiumin Poland, in September 2013.

2. What are soft rocks?

From the point of view of the intact rock or rock matrix, therehas been a deep debate as regards soft rocks or weak rocks becauseit is not easy to define this concept [1]. They have used different

criteria to define them: criteria for strength deformability, durabil-ity, weathering degradation, strength-stress relationship, etc.

Finally it seems that an ‘‘agreement’’ has been reached betweenmajor International Associations (ISRM, IAEG and ISSMFE) andresearchers to use the simple compressive strength as a criterionto separate soft rocks from hard soils at the lower limit and fromhard rocks in its upper limit. The simple compressive strength isa property commonly used by professionals involved in the designof engineering projects, although it is understood that it is notpossible to use only one of the many geomechanical propertiesto classify soft rocks [1]. Broader classification systems should bedeveloped.

However, not all this systems agree on the limits that should beadopted to characterize a soft rock as shown in Fig. 1, extractedfrom the paper recently presented by Professor M. Kanji in the2nd South American Symposium on Rock Excavations in Costa Rica[2].

Fig. 1 shows the upper limits with hard rock to be between 25and 30 MPa of UCS (strength, resistance). The lower limit with soilsshows a greater divergence between different authors, varyingbetween 1 to 6 MPa of the UCS. The criteria for establishing theselimits are varied and in all cases related to geological andgeodynamics conditions in the site of projects. The geomechanics

Fig. 2. Porosity–strength relation with compaction stages in the standard sedi-mentary basins and the textures of lithification [3].

884 M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892

classification of soft rocks based in only one property like thesimple compressive strength seems insufficient considering thecomplexity of geological factors involved in the formation of thistype of rocks (soft rocks).

In general, according to geological evolution, soft rocks can bedifferentiated in: (a) primary soft rocks, and (b) secondary softrocks. In the first group the following rocks can be identified.

(1) Sedimentary clastic and pyroclastic rocks of low to moderatecompaction and lithification (sandstones, siltstone, shales,tuff, agglomerate, marl, etc);

(2) Chemical sedimentary rocks formed by primary minerals ofMohs hardness <3.5 (gypsum, sylvinite, halite, carnallite,some limestones etc);

(3) Metamorphic rocks formed by primary minerals of Mohshardness < 3.5 such as phyllites, schists, cloritecs schistsformed by chlorite, muscovite sericite, graphite with talcumpowder, in low grade metamorphic phase.

The second group includes all types of rock that have sufferedphysical–chemical alteration due to weathering and/orhydrotermalism.

For clastic and pyroclastic rocks, the geological criteria that canbe used to set the difference between soft rocks and hard rocks, isthe degree of lithification reached by them. Hoshino [3] studiedsandstone and pelitic rocks in two sedimentary basins in Japanfrom samples obtained from deep drilling, up to 3000 m deep.From the correlation between petrographic characteristics,petrophysics (porosity) and mechanical properties (uniaxial com-pressive strength, module of elasticity, cohesion and propagationspeed of P waves) it was defined three stages of consolidation inpelitic sediments: dehydration, framework and cementation(Fig. 2).

In the first stage of lithification, the sediment suffers a process ofcompaction with dehydration as a predominant phenomenon. Thisstage is characterized by drastic reduction of pore spaces andmineral transformation (smectite to illite). The rock porosity variesbetween 30% and 80% and the UCS between 1 and 30 MPa. Theprocess occurs at a confining pressure that reaches 30 MPa, at atemperature of 55–60 �C for 1–2 million years (Tables 1 and 2).The study concludes that the higher the temperatures and

Fig. 1. Uniaxial compressive stre

pressures they suffer for millions of years, the higher the strengththe rocks can reach. It is important to highlight here that theseparation between two different geomechanic compartments(low strength vs high strength) in pelitic rock occurs by a changein the mechanisms of sediment lithification.

For metamorphic and chemical sedimentary rocks, the geologi-cal criteria that can be used to separate soft rocks from hard rocksis the proportion of primary minerals with Mohs hardness lessthan 3.5. Among the primary minerals that formed the soft rocksare: the sulphatic minerals (gypsum), chloride (halite, silvinite,and carnallite), phosphate, some carbonate, phyllosilicates (biotite,muscovite), clay minerals, chlorite, talc, graphite, vermiculite, ser-pentinite, etc) all of them with Mohs hardness less than 3.5. Theserocks could be considered soft rocks: salt (gypsum, carnallite,halite, etc), some phosphorites, some calcareous rocks, phyllites,schists, serpentinites, etc.

On the other hand, the weathering phenomena, hydrothermalalteration and retrograde metamorphism, are processes that trans-form the hard rock into rocks of less strength. In these cases thenewly formed minerals are minerals with Mohs hardness less than3.5. An excellent work on the mechanical properties of pyroclasticrocks (tuffs) altered by hydrothermal process, shows that in thedry state, argillic altered rocks (clays group) have strengths<25 MPa, while rocks with propylitic, potassium, Laumontite groupand Mordenite alteration, can reach strength up to 90 MPa [4]. It is

ngth (UCS) for soft rocks [2].

Table 1Mechanical properties of argillaceous rock as function of lithification in terms of compaction substages [3].

Compaction stage Porosity (%) Uniaxial strength (MPa) Viscosity (poise) Cohesive strength (MPa) Young modulus (MPa) P-wave velocity (m/s)

I A 80–50 1–5 1011–1012

B 50–30 2–30 1012–6 � 1012 1–12 1000–2000II C 20–30 20–60 5 � 1012–5 � 1013 10–25 1000 1500–3000

D 20–12 60–100 1013–5 � 1014 20–40 4000 2000–3500III E 12–5 100–200 40–80 6000 4000–5000

F 5–1.5 200–300 15000G 1.5–0 300–400

Table 2Development of lithification and consolidation of sedimentary rocks in progressive stages of compaction [3].

Porosity (%) Compactionstage

Solidificationmechanism

Binding structure Geologicalduration(106 years)

Overburdenpressure(MPa)

Geo-temperature(�C)

Void ratio

>50 I A Dehydration Immature: gel-like structure 1–2 <35 <65 >130–50 B 0.43–120–30 II C Framework 1st Lithification: mineral grains make

up framework by cemented atcontact-point of grains

6–8 35–95 75–130 0.25–0.4312–20 D 15–25 0.14–0.25

5–12 III E Cementation 2st Lithification: cementation spreadsin every pore space by replacement ofauthigenic minerals

30 95–140 130–170 0.05–0.141.5–5 F >140 0.015–0.05<5 G <0.015

M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892 885

also interesting to see that all rocks tested in saturation gotstrength lower than 25 MPa.

These two concepts outlined above (lithification processes andhardness of primary minerals or newly formed minerals) couldbe used in the future to identify soft rocks and classify rocksgeomechanically.

3. Weak rocks in Argentina

In Argentina there are many geological formations, outcroppingand buried rocks, that can be classified as weak rocks but they havenot received the deserved attention. Fig. 3 presents Argentina’sGeological Map which shows the enormous complexity in the dis-tribution of lithologic units and the structuring during geologicalevolution.

Since it is not the subject of this contribution detailing the geo-logical evolution of the region it will not be given a detaileddescription of each formation. It will just be described in greaterdetail those geologic units discussed in the analysis of thereferences.

Studies of soft rocks in Argentina have been the result of indi-vidual efforts of both professionals in geotechnical and geologicalengineering, and of a few researchers from government agenciesfor Research and National Universities. With the exception ofstudies developed by the public company Agua y Energia SE, thegovernment did not develop research projects on the issue of softrocks in the country, despite the large number of geological forma-tions that can be categorized, a priori, as belonging to this group.

Mon describes the geological and geotechnical conditionsencountered during the excavation of headrace tunnel, 3.500 mlong and 4.2 m excavation diameter (3.6 m internal diameter),belonging to the Dam ‘‘Las Maderas’’ located in Jujuy Province inthe north of Argentina [5]. This tunnel was excavated in lowstrength tertiary rocks (siltstones and sandstones) belonging tothe Rio Guanaco formation and the medium strength Cambrian-Ordovician shale and sandstones of the Meson Group (Fig. 4).

Paleozoic sedimentary rocks represent an alternating sequenceof pelitic (shale and siltstone) and sandy layers (quartz and

micaceous sandstones) with dark gray and black coloring, layeredcentimeter to decimeter thickness. This rock has a very definiteanisotropy due to stratification, clearly labeled. The Tertiarysediments of continental fluvial origin, form a sequence mainlysilty-sandy layers interbedded with clay, and thin conglomeraticbank casts, red and yellow colors. Materials are poorly consoli-dated and not presenting cementitious material. The apparentcohesion is given by the clay content and moisture [5]. In generalis very little bedding which prints marked homogeneous charac-teristics. Geomechanical properties of these soft rocks were pub-lished by Vendramini and Fabra (Table 3) show that the averageUCS of 3.1 MPa and static deformability modulus is approximately1000 MPa [6].

Tunnel excavation in tertiary rocks was much easier than in theOrdovician rocks of higher strength. It was observed that moresandy sequences showed greater tendency to overthrow andcollapse especially in bedding surfaces in contact with peliticmaterials. The use of blasting methods produced well-defined cutsbut because of their low self-sustaining time, often suffered col-lapse and large overbreaks. The entire section was supported byusing shotcrete and metal frames. In areas close to the contactbetween the Ordovician and Tertiary sequence, were problemswith severe deformations in the steel arc, which was later rein-forced support with concrete final lining. The excavation in theOrdovician pelitic sequence was also very problematic due to thelarge structural condition offered by the combination of subhori-zontal layering thin and subvertical discontinuity families. Thisled to continuous falls of rock slabs in the vault of tunnel especiallywhere fault zones are presented.

Vendramini and Fabra published the geomechanical propertiesof different rock related to projects undertaken by the statecompany Agua y Energia SE between 1975 and 1982 [6]. Table 3presents a summary of the mechanical properties of weak rocks.It highlights the test in the Cretaceous sedimentary rock of Hydro-electrical project Chihuído (Neuquén province), Tertiary sedimen-tary rock of hydroelectric Las Maderas and Los Alisos (Jujuyprovince) and Zanja del Tigre project (Salta province), tertiarysedimentary rocks in the hydroelectric project Condor Cliff and

Fig. 3. Geologic map of the Argentina with the location of the works discussed in this paper.

Fig. 4. Geological map and longitudinal section through of headrace tunnel of theLas Maderas dam [5].

886 M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892

La Barrancosa (Santa Cruz province) (now called N. Kirchner and J.Cepernic) and Cretaceous rocks of La Leona project (Santa Cruzprovince). All are clastic rocks with compressive strength between2 and 38 MPa and high deformability (0.2–6.4 GPa). Vendraminiand Fabra note that subandean tertiary rocks are generally homo-geneous and continuous, with high deformability (1 GPa) and lowshear strength, cohesion between 0.01 and 0.2 MPa and frictionangles between 11� and 34� [6]. Regarding the sequence of sand-stones and mudstones of Cretaceous Neuquén Group, theseauthors note that these generally have good mechanical properties(E = 10 GPa, c = 0.27 MPa, / = 56�). The intercalated mudstonespresent friction angle 20� and weaken the rock mass.

Rimoldi analyzed the geomechanical conditions of foundationsubsoil bridge over the Uruguay River between the city of

Table 3Laboratory test in weak rock published by Vendramini and Fabra [6].

Year Project Lithology Geologicalenvironment

Density(g/cm3)

Deformability modulus(MPa)

Strength test

Load Discharge UCS(MPa)

Triaxial(MPa)

Cohesion(MPa)

Frictionangle(�)

Indirecttensile(MPa)

r1 r3

1975 PH ElChihuido(Neuquén)

Red sandstone, medium-finegrain silt–clayed matrix

Sedimentarycontinental(cretaceous)

2.17 6400 7000 34.0 60.0 8.0 5 40 2.0

1976 CH LasMaderas(Jujuy)

Silstone sandy-clayedreddish and yellowish

Sedimentarycontinental(Terciary)

2.15 1000 1500 3.1 5.4 9.0 3 25

1977 Los AlisosDam (Jujuy)

Sandstone tuff 2.10 300 400 2.0 2.5 0.6

1977 Tuff 1.80 175 250 1.8 2.2 0.61977 Zanja del

Tigre (Salta)Sandstone silty 2.21 300 400 5.5 5.5 0.7 40 0.5

1978 Condor cliff(Santa Cruz)

Coarse sandstone sandstoneclayed

Patagonia australExtrandina (Tertiary)

2.15–2.24 1300–2000 1400–2400 35.0 17.0 1.5 10 40 2.5

1978 Silstone sandy-clayed 2.12 1200 1400 6.6 0.61978 Silstone tuffaceous 1.93 900 1100 7.0 0.61978 La Leona

(Santa Cruz)Medium grain sandstone 2.16 3000 3200 38.0 2.5

Table 4Geomechanics properties of Puerto Yeruá formation (Upper Cretaceous) [7].

Geologic formation Description SUCS classification LL IP SPT Triaxial test UCS (MPa)

c (MPa) Friction angle (�)

Upper Purto Yeruá Poorly cemented whitishsand, without diagenesis

SC 20–29 8–10 50 0.082 29 1.7–6.3(0.025–0.143) (25–28)

Medium-UpperPurto Yeruá

Quartzitic sandstone andwhitish sandstone, compact

4.4–54

Medium–LowerPurto Yeruá

Medium sandstone, compact 50 0 40(39–40)

Table 5Stratigraphic column-Chihuído project [8].

Group Formation Members Lithology Thickness (m)

Neuquén Rio Limay Huincul Sandstone and mudstoneCandelero Upper red sandstone 150

Chihuido sandstone 130Chihuido sand andmudstone

25

Lower red sandstone 16Rayoso Lower red sandstone 100

M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892 887

Paysandú (Uruguay) and Colon (Argentina), with special emphasison the weak rocks of the Puerto Yeruá formation (UpperCretaceous) [7]. The geology of the bridge area is characterizedby a single stratigraphic sequence formed by unconsolidated sedi-ments of Tertiary-Quaternary, supported on a substrate of softrocks belonging to the Puerto Yeruá Formation. These sedimentsconsist of compact sandstones, pink to whitish pink and fine to

Table 6Geological characteristic-Chihuído project [8].

Lithology Grain size Matr

Lower red sandstone Medium to fine Silt–Lower red sandstone Medium to coarseChihuido sand and mudstone Medium to fine Silt–Chihuido sandstone Medium Silt–Upper red sandstone

medium grained. Generally show a marked bedding sometimescrossed denoting a possible origin fluvial eolian for these deposits.The bedding planes show clay materials, yellowish gray to graycolor and bentonitic nature. They are characterized by a markedand variable degree of density depending on the degree ofepigenetic cementation which also gives variability in geomechan-ical properties. Geomechanical properties of the sediments androcks of this formation are summarized in Table 4.

The continental origins of Fm. Puerto Yeruá reveal a markedmechanical heterogeneity with very different values of the defor-mability modulus and compressive strength. This is due to thecompositional and geometric variability of the beds, the secondaryepigenesis and the various degrees of cementation.

Malecki and Gonzalez presents the results of laboratory andfield studies developed in the foundation rocks of the of theChihuído hydroelectric project (Neuquén province) [8]. In theproject area outcrop sedimentary rocks of the Upper Cretaceousof continental origin, belonging to Groups Rayosa and Neuquén.Stratigraphic column is presented in Table 5.

ix Cement Color

clay Ferruginous-Siliceous ReddishCarbonatic Grey to greenish gray

clay Carbonatic Brown violetclay Carbonatic Reddish

Red to reddish brown

Table 7Geomechanical properties-Chihuído project [8].

Lithology Specific gravity(g/cm3)

Porosity (%) Void ratio (%) Absortion (%) Deformabilitymodulus (GPa)

Strength tests

Uniaxial(MPa)

Triaxial Cohesion(MPa)

Frictionangle (�)

Indirecttensile (MPa)

r1 r3

Lower red sandstone 2.17 14.5–18.1 14 5.8 6.4 34 60 8 5 40 2Chihuido sand and

mudstone2.31 11–15

Chihuido sandstone 2.25 12.5–14.5 16 5.65 15 63 100 8 9 42 7.7Upper red sandstone 2.31 14.6 17 5.5

Table 8In situ deformability test [8].

Lithology Plate bearing Deformabilitymodulus (GPa)

Designload (MPa)

Type Direction

Chihuídosandstone

Talobre Normal to bedding 8.87 8

Parallel to bedding 9.30 8Goffi Normal to bedding 8.03 4

Parallel to bedding 13.50 4

888 M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892

Table 6 shows the petrographic composition of sedimentaryrocks and the Table 7 summarizes the geomechanical propertiesof the intact rock.

Tables 8 and 9 presents the results of field tests of direct shearand deformability executed in Chihuído sandstone in the region ofthe project.

Table 10 shows the dynamic properties of the sandstonesobtained from the seismic refraction methods.

Alicurá Dam on the Rio Limay (Neuquen and Rio Negro prov-ince), was built between 1979 and 1984 for the state companyHidronor. The particularity of this dam is the presence of theinterbedded mylonitized pelitic levels in the sequence of sand-stones to Las Coloradas Formation of Jurassic age. These shearedpelitic layers have received special attention in the study offoundation conditions of concrete structures because of their lowfrictional strength [9–11]. The lithology of the formation is a resultof succession polycyclic of conglomerates, conglomeraticsandstone, sandstone, clayey sandstone, sandy micaceous shales,mudstones and siltstones silty clay. Folding processes haveproduced interbedding slides located in some of the pelitic layers.The rock mass is mechanically inhomogeneous and this is particu-larly critical on the left bank of the valley where the layers dip intothe valley creating a clear instability.

Geotechnical properties of the sandstone are highly variableand the average values obtained from tests on samples of rockNX diameter are presented in Table 11.

The shale sheared layers were classified as clay as low tomedium plasticity (IP between 5 and 20), liquid limit between 20and 40. More plastic samples show about 40% clay and 25% ofthe particles through the sieve 200.

The drained strength parameters were determined by directshear tests, simple shear tests and triaxial tests on remolded

Table 9Direct shear test-Chihuído project [8].

Lithology Position Cohesion (MPa) Friction an

Chihuido sandstone Subparallel bedding 0.38 57Chihuido sand and mudstone Lithologic contact 0.04 37

0.19 36Mudstone Parallel bedding 0.23 20Lower red sandstone 0.17 55Upper red sandstone Bedding 0.36 34

samples because it was very difficult to extract undisturbed sam-ples (Table 12). The samples were compacted to a density similarto the samples undisturbed. The dry density of these shales rangedbetween 1.8 and 1.9 g/cm3 while the specific density variedbetween 2.44 and 2.68 g/cm3.

Direct shear tests were performed with controlled deformation(3.3 l/min) for normal stress between 0.1 and 0.5 MPa. The consol-idated undrained triaxial tests were performed to obtain shearstrength parameters for verification of slope stability under seismicactions. Cyclic testing of the remolded weak shales samplesindicates that the material is not susceptible to degradation undercyclic loading resistance. Creep tests (creep) indicate that theseshales have negligible creep.

Vardé published the only state of the art on the mechanics ofweak rocks in Argentina, approaching the subject from its concep-tual aspects to the presentation of several cases of work in whichthe author took part [1]. He discusses the issue of weak rocks ana-lyzing three cases: (a) poor cementing sedimentary rocks; (b)slightly lithified tuffaceous volcanic rocks; (c) decomposed igneousand metamorphic rocks.

The Chocón-Cerros Colorados is an earth dam with clay coreand gravel shoulders, currently in operation built between 1972and 1977 by Hidronor and located on the Rio Limay (Neuquén-Rio Negro Province). In the design stage great efforts were madein the campaign of investigations to characterize the soft sedimen-tary rocks of the Upper Cretaceous. These formations belong to thegroup called Neuquén and the stratigraphic sequence comprisesUpper Sandstones, El Chocón Sandstone and Lower Sandstone.The former is formed by alternating layers of sandstones and cal-carenites with siltstone and claystone lenses from 3 to 5 cm thick.Found Gypsum lenses thin and discontinuous side in both the baseand the roof of formation. El Chocón sandstones, outcrop in the leftmargin, they are massive rocks with little jointing and goodfoundation conditions. The lower sandstones lying below theelevation 290 m emerge on the site. All sequence is subhorizontal.The presence of secondary gypsum in rock discontinuities is com-mon, in the first 15 m higher, with thickness reaching to 1–2 cm.The presence of gypsum causes leakage problems and the stabilityof the dam [1].

Plate loading tests were performed in situ from exploratorygalleries in 3 levels deep. The plates of small diameter (0.28 m)and 300 kg/cm2 pressure and plate of high diameter (0.6 � 1.3 m)and pressures of 50 kg/cm2 were used (Table 13).

gle (�) Shear modulus (MPa) Shear strength (MPa) Normal stress (MPa)

4.57 3.50 2.002.23 1.30 1.502.40 0.90 1.100.17 0.38 0.52

1.20 0.700.91 0.52 0.40

Table 10Dynamic properties of sandstone-Chihuído project [8].

Lithology Wave velocity Dynamicmodulus(E) (GPa)

Poissoncoef.

Shearmodulus(G) (GPa)

L (m/s) T (m/s)

Upper red sandstone 2430Chihuido sandstone 3450 1970 20.70 0.27 8.93Lower red sandstone 2650

Table 11Design parameters-Alicurá dam [10].

Parameter Value

Density (g/cm3) 2.4Uniaxial compr. strength (MPa) 30–40Deformability modulus (GPa) 18Poisson coefficient 0.05Friction angle (o) 35–55Cohesion (MPa) 0.2–2.5Longitudinal wave velocity (m/s) 2800

Fig. 5. Plasticity chart for the fine particles of the El Chocón sandstone (In red thesheared shales of Alicurá dam; In blue the tuff of Collón Curá) [1].

Table 14Physics properties and strength parameter [1].

Properties Clayed sandstone Silstone

WL 6 30% 30% 6WL 6 50% WL > 50%

Pass mesh N�40 96–99% 98–100% 98–100%Pass mesh N�100 47–65% 59–86% 82–91% 98–100%Pass mesh N�200 22–44% 39–74% 69–87% 67–73%Dry density (g/cm3) 1.98–2.05 2.06–2.24 1.94–1.99 2.0–2.26Void ratio 0.30–0.34 0.19–0.31 0.33–0.38 0.21–0.41Solid density

(g/cm3)2.66–2.67 2.64–2.67 2.64–2.68 2.65–2.70

Effective cohesion(kPa)

1.5–2.2 1.1–2.8 1.3–2.2 0.35–0.55

Friction angle (�) 44–55 41–51 39–46 35–44Deformation

modulus (GPa)1.2–7.0 1.0–6.0 1.0–2.5 0.2–0.6

Table 15Strength properties-Monte León formation [1] (MPa).

Project Lithology Uniaxial strength Tensile strength

HP N. Kirchner Sandstone 4.9 0.7Silstone 4.9 0.7Claystone 6.0 0.6

HP J. Cepernic Sandstone 4.5 0.6Silstone 6.8 0.4Claystone 7.2 0.7

M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892 889

Atterberg limits measured on powdered samples indicate thatthe fine fraction can be classified as low plasticity clay (Fig. 5).These results are similar to those shown for sheared shales to theAlicurá dam.

Laboratory tests included a large number of triaxial tests onsamples of clayey sandstone of El Chocón formation and cementedsiltstones Cerros Colorados formation. The triaxial tests were per-formed step-wise type, drained and measuring the volume change.Were also conducted direct shear test in a specially designeddevice for testing clay weak rocks, with measurement of volumechanges and pore pressure.

The results of the tests are shown in Table 14 as modified byVardé apud Bolognesi and Moretto [1].

Vardéalso analyzed the geomechanical characteristics ofsandstones and claystones of the Monte León formation (tertiary)at the site of the future site of the hydroelectric Project N [1].The stratigraphic sequence consists of a layered sedimentary rocksubstrate corresponding to the Monte León Formation of Mioceneage. Its lithology consists of tuffs, tuffaceous mudstones,sandstones and interbedded tuffaceous sandstones with thick-nesses of several meters in the upper parts of the profiles surveyed.This unit lays well stratified, subhorizontal, in beds of several deci-meters to 2–3 m thick. The contacts between different lithologiesare predominantly transitional. Monte León Formation crops outpoorly in the area of dams and is generally covered by most

Table 12Strength parameter of sheared shales-Alicurá dam [10].

Test Shear strength

Peak

Cohesion (MPa) Friction angle

Direct shear 0–0.03 22–31Triaxial CU 0–0.03 22–30

Table 13Plate bearing test from galleries-El Chocón dam [1].

Lithology Deformability modulus (GPa)

Small plate

Initial cycle Load-dischar

El Chocón sandstone 2 2.5–3.5Lower sandstone 4–6.5 5.5–10

modern sedimentary deposits and basalt flows of Pliocene age.The results of laboratory tests indicate that the rocks exhibit lowcompressive and tensile strength (Table 15)

Residual

(�) Cohesion (MPa) Friction angle (�)

0–0.01 17–210–0.01 21–24

Large plate

ge cycle Initial cycle Load-discharge cycle

0.35 1.21.80 2.5

Fig. 7. Uniaxial compressive strength vs total porosity of gypsum and anhydriticrock [13].

Table 16Triaxial test-Collón Curá formation [1].

Triaxial test Strength parameter

Cohesion (MPa) Friction angle (�)

CD 0.01–0.40 29–50CU 0.16–0.35 21–23UU 0.60–1.20 16–40

Fig. 6. Uniaxial compressive strength vs deformability modulus of gypsum andanhydritic rocks of Argentina and Brazil [13].

Fig. 8. Tensional strength vs total porosity of gypsum and anhydritic rock [13].

Fig. 9. Strength envelope for anhydritic and gypsum rocks [13].

890 M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892

It is emphasized that these rocks are highly deformable anddeformability modulus presents average values of 1058 MPa forsandstones and 938 MPa for mudstones.

Sedimentary rocks in the region of La Leona dam Project (SantaCruz Province) correspond to Fortaleza and Calafate formations,both the Upper Cretaceous. Fortaleza formation consists ofclaystones, shales, fine to medium sandstones. Calafate Formationis formed by claystone, fine sandstones, and conglomerates.According to the results of laboratory tests published by Vardésandstones exhibit average compressive strength of 22.7, 3.2 MPaof tensile strength and 1598 MPa of deformability modulus [1].

Vardé presents the results of geomechanical tests conducted inthe tuffs of Collón Curá formation (Tertiary of the Neuquén Basin)[1]. These studies were performed under the Collón CuráHydroelectric Project (Neuquén). The rocks of the Fm. CollónCurá correspond to a large explosive volcanic event developed in

Table 17Strength properties of gypsum and anhydritic rocks [13].

Rock Braziliantest (MPa)

Uniaxialstrength (MPa)

Deformmodul

Alabaster gypsum 0.5 ± 0.1 4.8 ± 1.0 3.3 ±Laminar gypsum 4.5 ± 0.9 25.2 ± 9.9 19.7 ±Massive gypsum 3.5 ± 1.0 44.7 ± 6.1 32.5 ±Banded gypsum 2.9 ± 0.2 24.3 ± 0.6 30.7 ±Nodular gypsum 3.4 ± 0.5 40.0 ± 4.5 36.7 ±Auquilco anhydrite 6.8 ± 2.5 102.3 ± 11.3 69.2 ±Muribeca anhydrite 6.9 ± 2.0 62.3 34.6

the western part of north-Patagonian Massif occurred in themiddle Miocene. Andesitic-dacitic tuffs and dacitic-rhyolitic pyro-clastic flows were deposited. The sequence presented in thick sub-horizontal banks whitish color, brown and gray. Geomechanicalproperties of the tuffs tested have the following average values:

� Uniaxial compressive strength = 4 MPa� Deformability modulus = 314 MPa� Tensile strength = 0.7 MPa� Porosity = 40%� Density = 1.5 g/cm3

� Sonic velocity = 1800 m/s

The rocks are of very low resistance and high deformability.Also they present loss strength by saturation and variable weather-ing reaction. The petrographic analysis showed the predominanceof matrix (60%) is composed of predominantly volcanic glass andmontmorillonite clay in varying proportions. Atterberg limitsobtained indicated that it is high plasticity silt (Fig. 7). Triaxial testswere carried out whose results are presented in Table 16.

Giambastiani et al. and Giambastiani presented the results oflaboratory tests of sulphatic rocks (gypsum and anhydrite) in Braziland Argentina (Fm Auquilco-Lower Jurassic) [12–13]. Gypsum andanhydrite rock present great variability from the standpoint oftexture and petrographic feature. This variability exerts stronginfluence on geomechanical properties. The dry density and sonicvelocity show consistency with mineralogical purity of rocks andthe values reported in the literature, from about 2.30 g/cm3 for

ationus (GPa)

Poissoncoefficient

Plasticitytension (MPa)

Dilatationtension (MPa)

1.3 0.30 ± 0.26 3.3 ± 1.4 3.5 ± 1.512.3 0.21 ± 0.18 7.7 ± 3.0 16.8 ± 8.70.5 0.26 ± 0.03 13.0 ± 10.6 43.0 ± 6.24.5 0.20 ± 0.04 15.6 ± 5.9 23.2 ± 1.35.2 0.21 ± 0.02 10.7 ± 3.8 29.4 ± 5.82.3 0.27 ± 0.03 40.6 ± 6.0 89.2 ± 8.7

0.32 10.0 50.0

Table 18Failure criterion gypsum and anhydritic rocks [13].

Rocks Mohr–Coulomb Drucker–Prager Kim–Lade Hoek–Brown

c (MPa) / (o) K a a g1 m m rc

Alabaster gypsum 1.10 26 1.3 0.20 5.23 1.3 � 104 1.15 8.4 3.8Laminar gypsum 7.68 17 9.5 0.12 30.30 5.0 � 104 1.08 2.2 21.3Massive gypsum 7.17 26 8.9 0.19 46.22 9.0 � 104 1.20 5.2 25.2Banded gypsum 9.69 23 12.2 0.17 34.91 1.6 � 105 1.22 3.9 26.7Nodular gypsum 6.93 42 7.9 0.32 35.99 1.1 � 106 1.32 5.7 36.0Auquilco anhydrite 11.24 47 11.7 0.38 71.55 1.1 � 105 0.89 11.2 74.8Muribeca anhydrite 12.63 35 15.6 0.27 69.60 7.3 � 105 1.15 4.4 75.8

M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892 891

gypsum to 2.90 g/cm3 for anhydrite. The total porosity of thedifferent varieties of gypsum rocks varies from medium to lowporosity while the anhydrites present low to very low porosity. Itis estimated that the mesopores were formed by dissolution whilethe micropores may be due to microcracks. With respect to con-ventional geomechanical properties, the gypsum is less resistantand more deformable than anhydrite. The uniaxial compressivestrength of gypsum varies between 4.8 and 44.7 MPa (very lowto moderate strength). Furthermore anhydrite rocks can be consid-ered medium to high strength (UCS between 62 and 102 MPa).Table 17 summarizes the geomechanical properties of these rocks.

Fig. 6 shows the relationship between the uniaxial compressionstrength and deformability modulus of gypsum and anhydriterocks using the classical diagram of Ratio Modulus. It can be seenthat there is an interesting coincidence between the results ofthe studied rocks with data published in the literature for similarrocks. Both sulphatic rock types may be classified as high to med-ium modulus but gypsum falls in the field of low to very lowstrength while anhydrites are medium to high strength.

Fig. 7 shows the inverse relationship between the uniaxial com-pression strength and the total porosity. However, this relationshipis not very clear between the tensile strength to the total porosity(Fig. 8).

Fig. 9 shows the experimental data plotted in a diagram ofprincipal stress and in Table 18 the strength parameters of variouscriteria including the strength criterion Mohr–Coulomb, Drucker–Prager, Kim & Lade and Hoek & Brown.

Swelling and creep under uniaxial compression tests were alsocarried to study long term geomechanical properties of gypsumand anhydritic rocks [13]. The results showed that the time-depen-dent behavior of rocks of gypsum and anhydrite is related to twophysicochemical processes which can occur in nature coupled:(a) chemical transformation of gypsum and anhydrite and (b)

Fig. 10. Uniaxial compressive strength vs deform

creep. They together or separately can justify the order of magni-tude of the slow deformations observed in some underground inEurope.

In Swelling tests the performed anhydritic rocks presentincreased volume mainly by the precipitation of gypsum crystalson the surface of the sample and not by the mineralogicalconversion anhydrite-gypsum. This would be an isovolumetrictransformation and not isomolar as most authors sustains. Expan-sion rates obtained in laboratory vary from 0.6% to 1.1% per year.

Creep tests under uniaxial constant show clearly sections ofprimary or transient creep and secondary creep or stationary.The rheological model that best represents the creep process isthe Burger model. All rocks had a boundary between the primaryand secondary creep that flow approximately coincides with thestress to critical energy release (damage start). This suggests thatthe mechanisms of creep would be related to the processes ofinitiation and propagation of microcracks. Creep tests show thatthe gypsum, as expected, has higher viscoelastic and irreversibledeformation than anhydrites. Strain rates for both types of rockin the secondary creep vary from 1 � 10�5/day to 1 � 10�8/day[13].

Fig. 10 shows the relationship between the uniaxial compres-sion strength and deformability modulus of all samples presentedby Vardé and Vendramini and Fabra [1,6]. It is observed that mostrocks considered low to very low strength have a low to mediumratio modulus and a considerable amount of samples have strengthless than 7 MPa. No clear trend is observed by type of rock and itsgeological age. Probably the physical and chemical processes(decompression and weathering) suffered by the rocks during theirexhumation may have altered its mechanical properties.

Another factor that could affect the mechanical properties atlaboratory scale is the degree of saturation. Fig. 11 shows that

ability modulus of weak rocks of Argentina.

Fig. 11. Water saturation vs uniaxial compressive strength of weak rocks ofArgentina.

Fig. 12. Porosity vs uniaxial compressive strength of weak rocks of Argentina.

892 M. Giambastiani / International Journal of Mining Science and Technology 24 (2014) 883–892

there is a coarse inverse relationship between rock watersaturation and the uniaxial compressive strength.

The porosity, marked by several authors as the property indexstrongly affects rock strength, especially in clastic rocks, does notshow a clear relationship on the rocks work published in Vardé(Fig. 12) [1].

4. Conclusions

Soft rock or weak rock or low strength rock, is a still fairlyunexplored chapter in the area of rock mechanics. By the fact thatmaterials with intermediate mechanical properties of soils androcks, their approach is not easy. Included in this category areclastic sedimentary rocks and pyroclastic volcanic rocks of low tomoderate lithification (consolidation, cementation, new formedminerals), chemical sedimentary rocks and metamorphic rockformed by minerals with Mohs hardness less than 3.5 as somelimestone, gypsum, halite, sylvite, between the first and phyllites,graphitic schist, chloritic shale, talc, etc., among the latter. Theyalso include any type rocks that have suffered alteration processes(hydrothermal or weathering). They have common properties fromthe standpoint of physics and geomechanics. We are convincedthat an approach from the geological history, considering thegenetic and geodynamics transformation processes, will identifya priori presence of soft rocks. Clastic and chemical sedimentaryrocks, pyroclastic rocks and low-grade metamorphic rocks, aresuspected of being soft rocks, thus confirming its nature will bevaluable to guide field and laboratory investigations for the project.

In Argentina the study of low-strength rocks have not receivedmuch attention despite having extensive outcrops in the Andes andgreatly impact in the design criteria as discussed in the precedingchapters in various works and projects.

Correlation between geomechanical properties (UCS,deformability) to physical index (porosity, density, etc.) haveshown promising results to be better studied. There are many greatprojects in Argentina in soft rock geological environments, somecited in the text (Chihuído dam, N. Kirchner dam, J. CepernicDam, etc.) and others such as International Tunnel in the Provinceof Mendoza (Corredor Bioceánico), which will require the valuablecontribution of this discipline of rock mechanics.

The high complexity of these materials, both in intactconditions as part of the rock mass, involves making a great effortto give studies to increase geomechanical understanding of theirbehavior in front of the engineering works.

The lack of consistency between some of the physical andmechanical parameters explored from studies in the country maybe due to an insufficient amount of information and/or non-standardization of criteria for testing materials. It is understoodthat more and better academic and professional efforts in improv-ing techniques will result in benefits to the better understandingthe geomechanics of weak rocks.

In times where engineering projects should be carried outunder more restricted budget and schedule, usually in areas wheregeological conditions are worse, investment in the study of theserocks is essential.

Acknowledgment

Financial support for this work, provided by the University of LaRioja, is gratefully acknowledged.

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