The residual shear strength of Neogene marly soils in ... - Springer Link

The residual shear strength of Neogene marly soils in the Granada and Guadix basins, southeastern Spain N. El Amrani Paaza 7 F. Lamas 7 C. Irigaray 7 J. Chacón 7 C. Oteo

Abstract The analysis of samples taken from the Neogene and Cretaceous marly series of the Granada and Guadix basins (south east Spain) shows an inverse correlation between the clay content and the residual friction angle. It has been verified that the carbonate fraction of the soil has a significant influence on the residual friction angle; there is a threshold of 25% carbonates, above which the value of fbr increases from 20 to 307. It has been proved that the presence of smectitic clay as the dominant constituent produces a low value of residual shear strength, associated with high fragility in unconsolidated undrained tests. It has been proved that the degree of compaction of the soil and therefore the initial porosity, has a direct influence on the effective residual friction angle (fbr ). Résumé L’analyse d’échantillons prélevés dans les séries marneuses du Néogène et du Crétacé des bassins de Grenade et de Guadix (sud-est de l’Espagne) montre une corrélation inverse entre la teneur en argile et l’angle de frottement résiduel. Il a été vérifié que la teneur en carbonates de ces sols a une influence significative sur l’angle de frottement résiduel ; il existe un seuil de 25% de carbonates, audessus duquel la valeur de fbr augmente de 20 à 307. Il a été prouvé que la présence de smectite comme minéral dominant de la fraction argileuse est la cause d’une faible valeur de résistance résiduelle au

Received: 2 February 1999 7 Accepted: 31 March 1999 J. Chacón (Y) Department of Civil Engineering, Faculty of Sciences, University of Granada, Avda. Fuentenueva, s/n, 18071 Granada, Spain e-mail: jchacon6goliat.ugr.es, Fax: c34 958 243367 C. Oteo Department of Terrain Morphology and Engineering, Polytechnical University of Madrid, Spain

cisaillement, ainsi que d’une grande fragilité du matériau dans des essais non consolidés non drainés. Il a été prouvé que le degré de compacité de ces sols et, de ce fait, la porosité initiale, a une influence directe sur l’angle de frottement résiduel fbr . Keywords Residual shear strength 7 marly soils 7 Granada 7 Spain Mots clés Resistance résiduelle au cisaillement 7 Sols marneux 7 Grenade 7 Espagne

Introduction The study area is situated in the east of the Andalusian autonomous community in south east Spain. Extending around the north and south of the city of Granada, it forms part of the Granada and Guadix basins and covers an approximate area of 600 km 2 (Fig. 1). The stratigraphy of the Granada basin consists of Neogene materials. The basal unit of the sequence consists of detrital deposits of Lower Tortonian calcarenites, conglomerates, marls and silt. This is overlain by Middle Tortonian to Upper Messinian marls (Rodríguez Fernández 1982) The southern edge of the Guadix basin comprises sedimentary materials of alternating Tertiary marls and marly limestones and JurassicCretaceous dolomites and limestones (Foucault 1964). The term “marl” describes an intermediate rock between a clay and a limestone. The alteration process of the marls depends on climate, the dissolution-recrystallization process of the carbonate component (Arkin 1988) and the nature of the clayey fraction (Heraud, Restituto & Le Roux 1978; Oteo 1986). These two components of marly material create a sensitivity to weathering which results in the marls changing their soundness and mechanical properties with time. Two main groups of marls in the Granada basin can be distinguished based on the plasticity of their weathering

Bull Eng Geol Env (2000) 58 : 99–105 7 Q Springer-Verlag

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N. El Amrani Paaza et al.

Granada Basin

Guadix Basin

Eastern Sector

7

I

b a b a b

IN SPA

N E S

Messinian

W

Guadix

Granada

0

25 km

2 1

I

Pliocene

Granada

3

II

10 9

8

7 6 5 4 3 3 3 2

Group 2

4 2 1

5

Group 1

II

b a

III

5+6 5

V

III

a

A

8

IV

6

c b c b c b

B

8

IV

IV

e

10 9

V

III

C

V

Pleistocene

f

Tortonian lower upper II I

D f A a+b

Barremian

Valanginian

D

d

Lower Eocene C B a+b+c+d e

Holocene

Central Sector

1

Granada and Guadix Basins External zones of the Betic mountain chain Internal zones of the Betic mountain chain Sample sites

100

Fig. 1 Stratigraphic scheme of the Granada and Guadix Basins. (modified from Rodríguez Fernández, Sanz de Galdeano and Vera, 1991). Key of lithostratigraphic units of the Granada Basin: IcII: Depositional sequence I and II (Tortonian). III: Depositional sequence III (Messinian). IV: Depositional sequence IV (Pliocene). V: Depositional sequence V (Lower and Middle Pleistocene). 1-Quéntar Formation (calcarenite member). 2-Quéntar Formation (marly member). 3-Dúdar Formation (3: Progradant deltaic facies, changing to marls towards the inferior of the basin. 3n: Marls and conglomerates with patch reefs; locally only reefs). 4-Massive gypsum and salt. 5-Pinos Genil Formation (conglomerates and blocks). 6-Cenes-Jun silts (lacustrine lutites and marls).

5c6-Towards the interior of the basin, fresh-water stromatolitic limestones (5b) and lutites and gypsum, locally with turbiditic intercalations. 7-Lacustrine micrites. 8-Alhambra Formation (conglomerates with intercalated clays) (eastern border) and Conglomerates of Moraleda (central and southern sectors). 9Travertines. 10-La Zubia Formation (alluvial fans). Key of lithostratigrafic units of the Guadix Basin: A: Depositional sequence A (Valanginian). B: Depositional sequence B (Barremian). C: Depositional sequence C (Lower Eocene). D: Depositional sequence D (Holocene). a-Micritic clay with intercalated marls. b-Biomicritic grey marls (Fco. Abellán samples). c-Marly limestones. d-Grey marls (Portillo samples). e-Limestones with silt. f-conglomerates.

products (El Amrani Paaza 1996, El Amrani Paaza & Chacón 1996). The first (Group 1) is formed by the Middle and Upper Tortonian marls of the lower member of the Quéntar Formation (Rodríguez Fernández 1982). It consists of

clayey silts interbeds of sands and marls. The texture is finely stratified with the individual materials behaving as high plasticity silts (MH) clays (CH) and low plasticity silts (ML). In this paper the engineering behaviour of selected high plasticity soils of the Quéntar Formation is contrasted


Residual strength of marly soils

with that of other soils in the Granada basin and Guadix basin, all of which are low plasticity. The second group (Group 2) is formed by the more recent members of the series: the Dúdar marly Formation of Lower Messinian age (Rodríguez Fernández 1982) and the Middle and Upper Messinian clayey silts. The marly part of the Dúdar Formation has a stratified to massive texture, represented by clayey silts with amorphous or crystalline gypsum in fine bands – millimetres to centimetres in thickness. Crystallized gypsum occurs as medium to coarse grained material and forms beds up to metres in thickness. The marls of these two members behave as low plasticity clays (CL) and low plasticity silts (ML) – see Fig. 2. The experimental data obtained in the marly-clayey formations of the Guadix basin were obtained from an engineering geological study of construction materials related to the low permeability cores of earth filled dams. The dams are situated in the Guadiana river basin, which is part of the Guadalquivir river hydrographic system. They were constructed and managed by the Confederacion Hidrografica del Guadalquivir, an independent body under the Spanish Ministry of the Environment. The earth fill of the first dam (Portillo) comprises inorganic low plasticity clays (CL) with a carbonate content below 25%. The clay and silt fraction averages 63.3% by weight, of which the clay fraction represents 28.7% and the sand fraction is between 30 and 40%. The marls of the second dam (Fco. Abellán) are inorganic low plasticity clays (CL) with a carbonate content of around 34.5%. The average fines content is 83.9%; the sand sized portion being 20% and the clay content 25.4% by weight. The principal minerals in this soil are smectite, illite and kaolinite. The average proportions vary within the Neogene series of the Granada basin, being generally between 36 to 54% smectite, 33 to 47% illite and 12 to 17% kaolinite, progressively changing from the base to the upper member (El Amrani Paaza et al 1998). In the two fills from the dams studied, however, the proportion of these clay minerals varies with 12 to 52% and 25 to 63% smectite, 45 to 55% and 15 to 35% illite, 12 to 16% and 7 to 16% kaolinite respectively.

The slope movements affecting the studied materials are mainly solifluction, generated by high water tables which remain at a relatively consistent depth of 1 to 3 m throughout the year (Chacón Irigaray & Boussouf 1992) due to the low permeability soil layers below these depths. The residual strength is critical to progressive failure which is more pronounced in brittle soils (Burland 1990). The residual shear strength represents the shear strength of materials which have undergone previous movements and hence contain shear surfaces. Establishing the residual shear strength can indicate the level of risk of a progressive failure.

Methodology In order to characterise the marls in terms of their residual shear strength, the ring shear test has been used on 33 disturbed samples, 23 from the Granada basin and 10 from the Guadix basin. The advantage of this test is that shearing takes place at a constant volume with large shear deformations, thus the values calculated are representative of those obtained with undisturbed samples. Consolidation and drained tests have been carried out on compacted soils obtained from compaction (Proctor) tests. Increasing normal stresses of 100 to 300 kPa were used in the testing. The Coloumb failure criteria was used to present the shear strength parameters expressed by

tbr pcbr csbn tanfbr where cbr is the residual effective cohesion and fbr is the effective residual friction angle. Other tests included carbonate content (NLT-116/72 Spanish test MOPU 1986) soluble sulphate content (NLT-119/72 Spanish test MOPU 1986), Proctor compaction values and unconfined compressive strengths. The purpose of the work was to establish whether any correlation existed between the residual shear strength, carbonate and clay content and the clay mineralogy of the marls.

60

Fig. 2 Soil plasticity chart Plasticity index (%)

50 CH 40 30

CL

20 10

ML or OL

CL-CH

0

0

10

20

30

40 50 60 Liquid limit (%)

Portillo Marls Fco Abellán Marls Group 1 Marls Group 2 Marls

MH or OH

70

80

90

100

110


101


200

The index properties of the marls depend on both the carbonate content, which varies between 0 and 85%. and the type and content of the clay minerals (El Amrani Paaza & Chacón 1996). The relationships between the residual friction angle (fbr ) and the plasticity index (Ip) were established by Voight (1973), Kanji (1974), Seycek (1978 in Lupini Skinner & Vaughan 1981), Vaughan et al (1978 in Lupini Skinner & Vaughan 1981) are presented in Fig. 3. This shows considerable scatter but when Ip increases, fbr decreases. For all four lithologies analysed the fbr values fall generally within the range already established in the literature. The high fbr values obtained for low plasticity soils are grouped around the Kanji (1974) curve while the data points for the high plasticity soils with lower residual friction angles fall between the curves given by Voight (1973) and Seycek (1978). The strength obtained in the ring shear test peaks at 20% deformation for high plasticity clays and silts. The peak is at about 10% deformation for the rest of the soils, which are more brittle. At larger deformations, the residual shear strength reduces to a constant value which is taken to be the residual shear strength for a given normal stress. The residual shear strength values for a normal stress of 300 kPa vary between 65 and 70 kPa for high plasticity soils, 110 and 185 kPa for the low plasticity soils of the Granada basin and 60 and 120 kPa for the low plasticity soils of the Guadix basin (Fig. 4). The results show the influence of the inherent soil characteristics (eg plasticity and texture) on the residual shear strength. In this case the correlation between texture and residual shear strength is better than the correlation between plasticity and residual shear strength, due to the clear influence of texture on the friction between particles and on the bearing capacity. 40° Fco Abellán Portillo Group 1 Group 2 Seycek (1978) Kanji (1974) Vaughan (1978) Voight (1973)

Residual friction angle

30°

20°

10°

0°

0

20

40

60

80

100

Plasticity index (%)

Fig. 3 Correlation between plasticity index and residual friction angle

102


Residual shear strenght (kPa)

Results and discussion

Fco Abellán Portillo Group 1 (φr’ min) Group 1 (φr’ max) Group 2 (φr’ min) Group 2 (φr’ max)

180 160 140 120 100 80 60 40 20 0

0

50

100

150 200 250 Normal stress (kPa)

300

350

Fig. 4 Mohr envelope from ring shear tests

The representative points of residual shear strength, obtained for the three normal stresses with a 10% deformation presented in the graph of tbr and sbn given in Fig. 4. For high plasticity soils, the effective residual friction angle has a value between 8 and 117. Effective residual cohesion shows a minimum of 5 kPa and a maximum of 35 kPa. For soils with middle to low plasticity, the effective residual friction angle ranges from 19 to 307 and the effective residual cohesion between 6 and 13 kPa. Finally, for the low plasticity soils of the Guadix basin the effective residual friction angle is between 18 and 207 and effective residual cohesion between 7 and 10 kPa. The average values of these parameters are given in Table 1. The variation of the effective residual friction angle shows a positive correlation with carbonate content (Fig. 5 a). Although the gradient of the correlation line is low, this trend can be recognised for all the lithologies. However, the influence of the carbonate content on fbr is not very strong. There is also a clear positive correlation with the gypsum content, regardless of the nature of the soil; as can be seen from Fig. 5 b the angle of the correlation lines is similar for all the lithologies. The correlation of fbr with clay content is negative and the data show more scatter (Fig. 5 c). Figure 5 a shows an increase in the effective residual friction angle from 20 to 307 for carbonate contents higher than 25%; below this percentage the increase in fbr is less consistent. The high level of homogeneity between the fills of the Portillo and Fco Abellán dams is due to their origin as the samples were all taken from the impermeable core of the dams and would therefore fulfil the requirements set out in the different specifications for those structures. The results obtained correspond with those of other authors (Lupini Skiner & Vaughan 1981, Hawkins & McDonald 1992). According to Hawkins & McDonald, soils with a carbonate content above 25% will have a “transitional mode of residual shear” while soils with less than 25% carbonate content have a “sliding mode of shear” and for this reason will have lower values of residual friction angle.


40°

40°

Fco Abellán Portillo Group 1 Group 2 Skempton (1964) Blondeau & Josseaume (1974) Browicka (1965)

30° 30°

25°



35°

20° 15° Fco Abellán Portillo Group 1 Group 2

10° 5° 0°

20°

10°

0

20

40 Carbonates (%)

a

60

80

35° 0° 0


30° 25°

60

80

100

Fig. 6 Correlation between clay fraction and residual friction angle


10°

0°

0

5

b

10

15

Gypsum (%) 35° 30°


40

Clay fraction (%)

5°

25° 20° 15° Fco Abellán Portillo Group 1 Group 2

10° 5° 0°

c

20

0

10

20 30 Clay fraction (%)

40

50

Fig. 5 Relationship between soil mineralogy and residual friction angle

Figure 6 compares the relationship between fbr and clay content obtained in this study and by Skempton (1964), Browicka (1965 in Lupini Skinner & Vaughan 1981) and Blondeau & Josseaume (1976). According to these authors, fbr reduces with increasing clay fraction. It is noted that for the Fco. Abellán dam a number of the low clay fraction points fall below the zone produced by Skempton. For the remaining 67% it can seen that:

a) Among the low plasticity soils of the Granada basin there are two clearly defined sub-groups. In the first, which have a clay content of below 25%, the grouping is below the lower Skempton (1964) and Blondeau & Josseaume (1976) lines. In the second, which have clay contents above 25% the data fall close to the higher Skempton line. This is probably due to the different texture of the soil samples, ie the later group has a higher carbonate content. b) The high plasticity soils of the Granada basin form a clear grouping around the Browicka (1965) correlation line because their effective residual friction angle is significantly lower than that of the low plasticity soils for any given clay content. The fact that soils with a similar clay content give different residual shear strength results indicates the influence of the minerals present within the clay fraction. A meaningful correlation exists between fbr and the clay mineralogy (Fig. 7): fbr decreases with a higher percentge of smectite and illite. It is noted that there is a direct correlation with the kaolinite content, similar to that observed by Lupini et al (1981) and Hawkins & McDonald (1992). The variation of residual cohesion with soil composition was studied in order to check the results obtained previously. The cbr value increases with clay content but reduces with an increase in the proportion of carbonate and/or gypsum. The value of the residual shear strength is influenced by the nature of the silty and sandy fraction (Lupini et al 1991) and by the morpohology of the clay particles (Hawkins & McDonald 1992). The influence of the compaction (Proctor) test on the mechanical properties has also been investigated. Figure 8 shows that samples compacted on the dry side of optimum tend to be more rigid and resistant than those compacted on the wet side of optimum. It can be seen that the fbr


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Table 1 Average values of residual shear strength, effective residual cohesion and effective residual friction obtained for soils of the Granada Basin and the Guadix Basin

wbr wbr wbr wbr

High plasticity soils of the Granada Basin (Group 1) Low plasticity soils of the Granada basin (Group 2) Fill of Fco. Abellán dam Fill of Portillo dam

min. max. min. max.

Residual shear strength

Effective residual cohesion

Effective residual friction

tbr p35c0.14sbn tbr p5c0.2sbn tbr p5.7c0.35sbn tbr p13c0.57sbn tbr p7.3c0,325sbn tbr p9.7c0.37sbn

cbr p35 kPa cbr p5 kPa cbr p5.7 kPa cbr p13 kPa cbr p7.3 kPa cbr p9.7 kPa

wbr p8 0 wbr p11 o wbr p19 o wbr p30 o wbr p18 o wbr p20 o

30°

25° 25°


10° 5° 0°

0

10

20

30 40 Smectite (%)

a

50

60

70



30°

20°

15°

10°

5°


30° 0° −15

25° 20°

Fco Abellán Portillo Group 1 Group 2

5° 0° 0

b

10

20

30

40 50 Illite (%)

60

70

30° Residual friction angle

−10

−5 0 W − Wop (%)

5

10

Fig. 8 Influence of the compaction on the residual friction angle

15° 10°


80

reduces when the moisture content (w) is above the optimum moisture content (wop). The tangential deformations by alignment of the particles of the soil tend to destroy some of the differences in structure created by the compaction test (Seed & Chan 1992, El Amrani Paaza et al 1998).

25°

Conclusions

20° 15°

It has been shown that the carbonate content has a direct influence on the residual shear strength of the marly formations studied. In the ring shear test the internal 5° residual friction angle of 20 to 307 was observed for samples which had approximately 25% carbonate content. 0° 0 10 20 30 40 50 60 70 The study confirms the role of lime-sand fraction for those soils which are more inert and inactive in the process of Kaolinite (%) c resisting shear. Below 25% carbonate content the clay fracFig. 7 tion and the percentage of smectite within that fraction Relationship between clay mineralogy and residual friction tend to govern the behaviour of the soils causing an angle increase in the value of the residual cohesion up to 35 kPa. 104

10°




It has been observed that the degree of rigidity of the soil, Hawkins AB, MacDonald C (1992) Decalcification and residual shear strength reduction in Fuller’s Earth Clay. Geotechnique with its higher or lower plasticity, has a direct bearing on 42 : 453–463 the values of the residual friction angle.

References Arkin Y (1988) Disintegration of marl slopes in Israel. Environm Geol Water Sci 11 : 5–14 Blondeau F, Josseaume H (1976) Mesure de la résistance résiduelle en laboratoire. Rapport du Département des sols des fondations. Laboratoire Central des Ponts et Chaussées Burland JB (1990) On the compressibility and shear strength of natural clay. Geotechnique 40 : 329–378 Chacón J, Irigaray C, Boussouf S (1992) Caracterización geotécnica de la ubicación de un vertedero comarcal de residuos sólidos urbanos en la depresión de Granada. In: Proceedings of the III Geological Congress of Spain & VIII Latinoamerican Geological Congress, Salamanca 2 : 415–419 El Amrani Paaza N, Chacón J (1996) Altérabilité, gonflement et Propriétés d’indices des marnes néogènes de la dépression de Grenade, Espagne. Bull Int Ass Eng Geol 54 : 123–135 El Amrani Paaza N (1996) Caractérisation géotechnique des sols cohésifs néogènes du bord sud oriental de la dépression de Grenade Espagne. (Thesis), University of Abdel Malek Essaadi, Tetouan, Morroco, 312 pp El Amrani Paaza N, Lamas F, Irigaray C, Chacón J (1998) Engineering geological characterisation of Neogene marls in the Southeastern Granada Basin, Spain. Eng Geol 50 : 165–175 Foucault A (1964) Sur les rapports entre les zones Prébétiques et Subbétiques entre Cazorla (Prov de Jaén) et Huescar (Prov de Grenade). Geol en Mijnb 43 : 268–272

Heraud H, Restituto J, Le Roux A (1978) Les marnes de Limagne. In: Proc Int Congr of Ass Eng Geol 2 : 179–190 Kanji MA (1974) Shear strength of soil rock interfaces. Thesis, University of Illinois, Urbana Lupini JF, Skinner AE, Vaughan PR (1981) The drained residual strength of cohesive soils. Geotechnique 31 : 181–213 MOPU (1986) Normas de Ensayo del Laboratorio del Transporte y Mecánica del Suelo “José Luis Escario”. Dirección General de Carreteras, Madrid, Spain Oteo C (1986) Las arcillas expansivas en Espan˜a: Distribución y propiedades. In: Curso sobre cimentaciones en Terrenos metaestables, colapsales y expansivos 2 : 1–20 Rodríguez Fernández J (1982) El Mioceno del sector central de las Cordilleras Béticas. (Thesis). University of Granada, Spain, 224 pp Rodríguez Fernández J, Sanz de Galdeano C, Vera JA (1991) The Granada Basin. In: Report No 14. Andalusian Geological Institute, Granada, Spain, pp 1–20 Seed HB, Chan CK (1992) Structure and strength characteristics of compacted clays. ASCE. J Soil Mech Found Div 85, No SM5 : 971–988 Skempton AW (1964) Long term stability of clay slopes. Geotechnique 14 : 77–101 Voight B (1973) On the functional classification of rocks for enginneering purposes. In: Proc of Symp of Rocks Mech, pp 131–135


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