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Engineering properties of loess in Algeria
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Engineering Geology 99 (2008) 85 – 90 www.elsevier.com/locate/enggeo
Engineering properties of loess in Algeria M.S. Nouaouria ⁎, M. Guenfoud, B. Lafifi Civil Engineering and Hydraulics Laboratory, Guelma University, Algeria Received 3 February 2007; received in revised form 10 November 2007; accepted 20 January 2008 Available online 16 February 2008
Abstract Loess in North Africa has been investigated using samples from Algeria. The specific gravity, Atterberg limits, grain size distribution and dry density were determined. The hydro-collapsibility properties, due to wetting under different stress levels were measured in single-oedometer tests. The results of this investigation indicate that the properties of Algerian loess are similar to those of loess from many parts of the world, such as Iowa and Libya; they can be classified as silty loess. © 2008 Elsevier B.V. All rights reserved. Keywords: Algeria; Grain size; Engineering properties; Hydrocollapse; Loess; Oedometer tests
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . Materials and experimental programme . Test results and discussion . . . . . . . . 3.1. Index properties of Algerian loess 4. Hydrocollapse characteristics . . . . . . 5. Discussion . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction The Sahara is one of the world's major sources of minerogenic dust (Prospero et al., 2002), many of the silty deposits in adjacent areas being attributed to aeolian processes. Loess is found in many parts of the worlds. In China, for example, it has been estimated that the area covered by loess exceeds 630,000 km2, of which about 60% is collapsible (Derbyshire et al., 1995). Silt constituting 50–70% by weight is the predominant grain size fraction in loess, less than 5% of particles being greater than 1 mm, and clay particles making up only a few percent. Quartz ⁎ Corresponding author. E-mail address: [email protected] (M.S. Nouaouria). 0013-7952/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.01.013
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and feldspar are the major mineral constituents of the coarser grades, while illite is the principal clay mineral. In addition, collapsible loess may contain relatively large amounts of calcium carbonate, as well as various soluble salts distributed on the surface of particles in solid or semi-solid state. Loess deposits have certain distinctive characteristics (Rogers et al., 1994) that define the material. Two major defining features are particle size distribution and the susceptibility of the soil structure to collapse when loaded and wetted. Chen (1992) stated that the defining physical characteristics of loess in its natural state are low water content and high porosity. It has been estimated that loess covers nearly 10% of the surface area of the earth (Pecsi, 1968). It is found in continental drylands on all continents, with the notable exception of Africa.
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response to wetting at different stress levels. For this purpose, undisturbed soil samples representing two types of loess were collected from two locations: Daia Bendahoua and Metlili, Ghardaia (southern Algeria). 2. Materials and experimental programme
Fig. 1. Map of Algeria showing sampling location, Ghardaia.
Scheidig's (1934) map of world loess distribution recognizes only one loess region in Africa (North Libya). However, this conclusion was based on limited information, owing much to the work of Rathjens (1928) who described loess-like materials in Libya but found Tripolitanian loess to have a grain size N 100 μm for loess. More recent work by Coudé-Gaussen (1987) has shown that North African loess may have larger modal sizes than better known loess deposits. In the Techine field-section (Matmata, Southern Tunisia), for examples, three series occur in a 17 m thick succession (Coudé-Guassen et al., 1982): an upper grey-beige loess, an intermediate series with altered reddish horizons; and a lower ochre loess. This work showed that Tunisian loess is coarser in granulometric composition than most other loess. Assallay et al. (1996) have recently justified Scheidig's classification of the North Libyan silty deposits as loess. They demonstrated that it has index properties and shows aspects of hydrocollapse behaviour similar to those found in loess from many parts of the world. They concluded that this material can be classified as silty loess in the Tripoli region and clayey loess in the Ghat area. Abdrabbo et al. (2000) have reported an extent of loess throughout the Western Desert of Egypt, especially at certain locations including Sidi Baranee, New El-America city, and El-Boustan. Despite the huge surface area of Algeria, no serious study has tackled the question as to whether or not loess is present in this vast area. In their study of Algerian duricrusts, Smith and Whalley (1982) concluded that these residual deposits contain quartz and feldspar grains, which exhibits evidence of limited aeolian abrasion and chemical weathering; they suggested a Holocene age for these deposits. Because of the potential hazards posed to engineering structures with foundations on collapsible soils that undergo high volume change, it is important to identify such soils and to understand clearly their engineering properties. This paper is a first attempt to fill the gap in knowledge of the known loess-like silts of Algeria. Given the clear need for more careful analysis of North African silty sediments as a means of determining whether they constitute true loess, the main objective of this paper is to investigate the collapsibility characteristics and behaviour of selected Algerian materials with particular reference to their
An experimental investigation was carried out to determine the index properties and the hydro-collapsibility characteristics and behaviour of the samples. Undisturbed block samples were obtained from depths of between 0.6 and 1 m, and then carefully trimmed, waxed and placed in boxes. The two samples sites lie 45 km apart in the Ghardaia region, some 600 km south of Algiers (Fig. 1), some soil assessment has already been undertaken in this region with a view to dam construction. Specific gravity, and Atterberg limits were determined according to British Standard Procedure (BSI, 1990). Particle size analysis of material passing the 63 μm sieve was undertaken using Sedigraph Technique. The microstructure of loess particles was examined using a scanning electron an X-ray scanning system (Sedigraph). The collapsibility characteristics and behaviour of the
Fig. 2. Scanning electron micrograph of loess samples showing clay-coated silt grains of natural structure of (a) yellow loess and (b) grey loess.
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Table 1 Collapse settlement criteria for Kansas–Nebraska loess (after Holtz and Hilf, 1961) Dry density (mg m− 3)
Condition
b1.28 1.28–1.44
Loess is considered loose and highly susceptible to settlement. Loess is medium dense and is moderately susceptible to settlement, particularly for critical or heavily loaded structures. Loess is quite dense and may be capable of supporting ordinary structures without serious settlement. A more general criterion has been used for earth dams: this density is used as the division between low and high-density loess or the division where special foundation treatment is required for lower densities.
N1.44 1.36
materials with particular reference to their response to wetting at different stress levels was assessed by subjecting undisturbed soil specimens (70 mm in diameter and 19 mm thick) to onedimensional compression in an oedometer. Two methods are currently used to evaluate and determine the collapse parameters, namely the single and double oedometer tests. These have been shown to be reliable in investigating collapsibility properties. According to Lawton et al. (1989), and Basma and Tuncer (1992), these techniques give similar results. The single-oedometer test consists of loading the specimens incrementally to a specific state of vertical stress and allowing the sample to come to equilibrium under the applied pressure. The sample is then flooded with water, and the deformation is measured. The deformation induced by the addition of water is divided by the initial height of the specimen, expressed in percent, from which the magnitude of the hydrocollapse can be defined. The single oedometer test method was performed on undisturbed soil specimens from Ghardaia. Grey (from Daia Table 2 Index properties of Libyan (After Assallay et al., 1996) and Algerian loess Property
Natural moisture Content (%) Specific gravity (Gs) Dry density (Mg/m3) Bulk density (Mg/m3) Void ratio
Tahala Gharyan Khoms Garaboli Grey Yellow loess loess loess loess loess loess (Libya) (Libya) (Libya) (Libya) (Algeria) (Algeria) 3 2.73
6 2.66
3 2.68
2 2.67
5 2.68
6 2.73
–
1.36– 1.42 –
1.40– 1.46 –
–
1.42
1.43
–
1.49
1.52
0.87– 0.96 –
0.84– 0.92 –
–
0.89
0.91
–
15
18
Mechanical analysis Sand (%) 10 Coarse silt (%) 52 Fine silt (%) 25 Clay (%) 13
38 30 21 11
18 39 30 13
39 31 22 8
16 51 24 9
2 61 25 12
Atterberg limits LL (%) PL (%) PI (%)
27 19 8
31 20 11
25 17 8
30 23 7
33 22 11
– –
Degree of – saturation (%)
41 24 17
Fig. 3. Plasticity properties of Algerian loess as compared with loess types defined by Gibbs and Holland (1960). (□ Grey loess; ■ Yellow loess).
Bendahoua) and Yellow (from Metlili) samples were prepared and tested in natural conditions, water contents at the time of testing being 5% and 6% respectively. 3. Test results and discussion 3.1. Index properties of Algerian loess Index properties for the Yellow and Grey Loess are summarised in Table 2. Despite the close similarity of their general characteristics, there are some slight differences in index properties, although both soils are classified as silty loess. The Grey Loess consists of 16% sand, 75% silt and 9% clay. The more silty Yellow loess consists of 2% sand, 86% silt and 12% clay. The Yellow loess has a slight higher dry density and higher specific gravity (2.73). This may be explained by the fact that, as shown by X-ray analysis, its composition is dominated by the feldspar group (Potassium Aluminium Silicate K (Si3Al)O8), and Calcium Magnesium Carbonate (CaMg(CO3)2), while Calcium Magnesium Carbonate is dominant in the Grey loess, which contains only 4% of K(Si3Al)O8) and has a specific gravity of 2.68. The specific gravity values of the Algerian samples are very similar to those for loess from many other parts of the world. For example, Assallay et al. (1996) report values for Libyan loess of between 2.66 and 2.73. Values for Chinese loess of between 2.65
Fig. 4. Particle size distribution of Algerian yellow and grey loess.
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Fig. 5. Hydrocollapse-wetting pressure relationship for undisturbed specimens from Algeria (Grey loess and Yellow Loess) tested in single Oedometer test.
and 2.70 are reported by Bell (1992), those from Kent, U.K. Vary from 2.68 to 2.70 (Derbyshire and Mellors, 1988), those in parts of the United States lie between 2.57 and 2.79 (Sheeler, 1968), while those for the loess of Iowa State ranges from 2.68 to 2.73. The microstructure studies show that both samples have an “open structure (Fig. 2). Using the collapse settlement criteria developed by Holtz and Hilf (1961) shown in Table 1, the Algerian loess can be considered as medium dense and moderately susceptible to settlement, particularly beneath critical or heavy structures. Table 2 shows comparative index properties for Algerian and Libyan loess. Loess soils are slightly to moderately plastic, the plasticity increasing with increases in clay content. Based on plasticity characteristics, Gibbs and Holland (1960) subdivided loess into three types: sandy loess, silty loess and clayey loess. They indicated that Liquid limits for silty loess varied from 25 to 35% and values for clayey loess ranged up to 45%. All plasticity values for the Algerian loess samples plot close to the A-line and below the 50% Liquid Limit on the Casagrande plasticity chart (Fig. 3), indicating that the material is inorganic soil of low plasticity. Such soils are generally very sensitive to changes in moisture content. The test results show that all of the Algeria samples considered here plot within the ‘silty loess’ envelope of Holtz and Gibbs (1951). Particles size results for both samples are shown in Fig. 4. It is evident that the dominant size fraction in these samples is silt (2–60 μm). Silt makes up some 86% of the Yellow loess and 75% of the Grey, and clay percentages are 12 and 9, respectively.
Fig. 6. Hydrocollapse test results for undisturbed grey loess flooded at different stress levels. (Pw = Wetting pressure).
appropriate in the light of the view of Lutenegger and Hallberg (1988) that this is the most common method used to identify a collapsible soil, and one with the most meaning to geotechnical engineers. The degree of hydrocollapse of each sample was then calculated as follows. Hydrocollapse ¼
δH k H0
Where δH = change in sample height due to saturation H0 = Sample height immediately prior to saturation For the double oedometer test, the change in sample height due saturation is taken as the difference between the unsaturated sample heights at any given pressure. These definitions are standard throughout the literature Rogers et al. (1994). The curves shown in Fig. 5 illustrate the effect of wetting pressure on the collapsibility of the loess, and indicate that the magnitude of the hydrocollapse increases as the applied pressure
4. Hydrocollapse characteristics Undisturbed soil specimens from Ghardaia were prepared for testing. The single Oedometer test was considered the most
Table 3 Identification of hydrocollapse problems (after Jennings and Knight, 1975) Collapse potential (CP)
Severity of problem
0%–1% 1%–5% 5%–10% 10%–20% N20%
No problem Moderate problem Trouble Severe trouble Very severe trouble
Fig. 7. Hydrocollapse test results for undisturbed yellow loess flooded at different stress levels. (Pw = Wetting pressure).
M.S. Nouaouria et al. / Engineering Geology 99 (2008) 85–90
of wetting increases up to a certain value, at which the maximum collapse will be obtained due to saturation. Based on the values given by Jennings and Knight (1975) and shown in Table 3, it can be taken that the Algerian loess considered here poses severe collapse problems. For a stress range up to 1600 kPa, the measured magnitudes of hydrocollapse of the specimens vary over a range of 1 to 21% and 0.8 to 20% for the Grey and Yellow loess, respectively, depending on the level of the vertical pressure applied to the specimens at the time of wetting. Figs. 6 and 7 show the post-wetted relationship between specimen height and applied pressure for all specimens that were flooded at different stress levels. 5. Discussion On the basis of the results presented here, it is evident that there are similarities between the loessic soils in southern Algeria and those found in many others parts of the world. This includes the demonstrable collapsibility of the Algerian soils described, collapsibility generally being predicted on the soil having void space in its natural state that is sufficient to sustain its liquid limit moisture at saturation. As far as the Algerian loess is concerned, the high void ratios of the Algerian loess result in a hydro-collapse magnitude of about 13% for both Grey and Yellow loess for a wetting pressure of 200 kPa. Based on the identification of the severity of collapsibility given by Jennings and Knight (1975), the Algerian loessic soils tested in this study are regarded as likely to pose severe problems, having a susceptibility to settlement particularly in respect of critical or heavily loaded structures. The Atterberg limits, grain size characteristics, and specific gravity of these Algerian loess samples were found to be very similar to those of loess deposits from many parts of the world. Based on the plasticity and particle size test results, and according to the definition of loess types given by Holtz and Gibbs (1951) and Gibbs and Holland (1960), the Algerian material can be classified as silty loess. The amount collapse depends on the magnitude of vertical stress at the time of wetting. As loess and loess-like sediments are always unsaturated, the reduction in matric suction is one of the major causes of collapse (Lloret and Alonso, 1980; Maswoswe, 1985). It seems vital that in parallel with the continued accumulation and comparative analysis of the bulk behaviour of loess, greater effort should be devoted to the study of the behaviour of collapsible loess in the context of the theory of unsaturated soils in which the constitutive relations for volume change in unsaturated soils could be used to explain more comprehensively collapsible soil (Fredlund and Morgenstern, 1976). 6. Conclusion (a) The southern Algerian loess is of two types, named here Grey and Yellow loess. Lithologically and mineralogically, the two types are quiet similar, although there are some differences composition that explain the colour difference. (b) The results of this investigation indicate that the Index Properties of southern Algerian loess are similar to those
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found in loess from many parts of the world, such as Iowa (USA) and Libya. (c) Both the Grey and Yellow loess soils are rich in silt and can be classified as a silty loess. (d) The amount of hydrocollapse is dependent on the magnitude of vertical stress acting on specimen at the time of wetting. The degree of hydrocollapse increases as the applied pressure increases up to a certain level of stress, at which point maximum collapse is reached (20% and 21% for yellow and Grey loess, respectively). (e) A unique post-wetted specimen height applied pressure relation was observed in the Algerian samples tested. Flooding at any applied pressure caused the material to collapse down to a certain specimen height prescribed by the unique post-wetted specimen height/applied pressure curve. References Abdrabbo, F.M., Hansy, R.M., Hamed, K.M., 2000. Collapsibility of Egyptian loess soil. unsaturated soils for Asia. In: Toll, Leong (Eds.), Rahardjo. Balkema, Rotterdam, pp. 595–606. Assallay, A.M., Rogers, C.D.F., Smalley, I.J., 1996. Engineering properties of loess in Libya. Journal of Arid Environment 32, 373–386. Basma, A.A., Tuncer, E.R., 1992. Evaluation and control of collapsible soils. Journal of Geotechnical Engineering Division, ASCE 118, 1491–1504. Bell, F.G., 1992. Engineering Properties of Soils and Rocks, 3rd ed. ButterworthHeinemann, Oxford, U.K. (345pp.). British Standards Institution, 1990. Bs 1377: British Standards Methods of test for soils for civil engineering purposes. B.S.I., London. Chen, Z.Y., 1992. Practical solutions to problems of collapsible loess in China. Proceedings of the 7th International conference on expansive soils. Dallas, Texas, pp. 1–12. Coudé-Gaussen, G., 1987. The Peri-Saharan Loess: sedimentological characterization and paliclimatical significance. Geological Journal 15, 177–183. Coudé-Gaussen, G., Mosser, C., Rognon, P., Tourenq, J., 1982. Une accumulation de loess du Pléistocéne supérieur dans le Sud-Tunisien: la coupe de Téchine. Bulletin de la SocieÂte geÂologique de France 7 (t. XXIV), No. 2, 283–292 pp (in French). Derbyshire, E., Mellors, T.W., 1988. Geological and geotechnical characteristics of some engineering loess and loessic soils from China and Britain: a comparison. Engineering Geologist 25, 135–175. Derbyshire, E., Meng, X.M., Wang, J.T., Zhou, Z.Q., Li, B.X., 1995. Collapse loess on the Loess Plateau of China. In: Derbyshire, E., Dijkstra, T., Smalley, I.J. (Eds.), Genesis and properties of Collapsible Soils. Kluwer, Dordrecht, pp. 267–293. Fredlund, D.G., Morgenstern, N.R., 1976. Constitutive relations for volume change in unsaturated soils. Canadian Geotechnical Journal 13 (3), 261–276. Gibbs, H.H., Holland, W.Y., 1960. Petrographic and engineering properties of loess. US Bureau of Reclamation, Engineering Monograph, vol. 28 (37 pp.). Holtz, W.G., Gibbs, H.J., 1951. Consolidation and related properties of loessial soils. American Society for Testing Materials. Special Technologies 126, 9–33. Holtz, W.G., Hilf, J.W., 1961. Settlement of soil foundations due to saturation. Proceedings of the 5th International Conference on Soil Mechanics and Foundation Engineering, vol. 1, pp. 673–679. Jennings, J.L., Knight, K., 1975. A guide to construction on or with material exhibiting additional settlement due to collapse of grain structure. Sixth Regional Conference for Africa on Soil Mechanics & Foundation Engineering, Durban, South Africa, September. Lawton, E.C., Fragaszy, R.J., Hardcastle, J.H., 1989. Collapse of compacted clayey sand. Journal of Geotechnical Engineering, (ASCE) 115, 1252–1267. Lloret, A., Alonso, E.E., 1980. Consolidation of unsaturated soil including swelling and collapse behaviour. Geotechnique 30 (4), 449–477. Lutenegger, A.J., Hallberg, G.R., 1988. Stability of loess. Engineering Geology 25, 247–261.
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Maswoswe, J., 1985. Stress path for a compacted soil during collapse due to wetting. Ph.D thesis, Imperial College, London. Pecsi, M., 1968. Loess. In: Fairbridge, R.W. (Ed.), The Encyclopaedia of Geomorphology. Reinhold, New York, pp. 674–678. Prospero, J.M., Ginoux, P., Torres, M., Nicholson, S.E., Gill, T.E., 2002. Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product. Reviews of Geophysics 40, 1. Rathjens, C., 1928. Loess in Tripolitanien Zeit. Ges. Evdkunde Zu Berlin, pp. 211–228.
Rogers, C.D.F., Dijkstra, T.A., Smalley, I.J., 1994. Hydro-consolidation and subsidence of loess: Studies from China, Russia, North America and Europe. Engineering Geology 37, 83–113. Scheidig, A., 1934. Der Loss und seine geotechnischen Eigenschaften. Steinkopf, Dresden. 133 pp (in German). Sheeler, J.J., 1968. Summarization and comparison of engineering properties of loess in the United States. Highway Research Record (212), 1–9. Smith, B.J., Whalley, W.B., 1982. Observations on the composition and mineralogy of an Algerian duricrust complex. Geoderma 28, 285–311.
Engineering Geology 99 (2008) 85 – 90 www.elsevier.com/locate/enggeo
Engineering properties of loess in Algeria M.S. Nouaouria ⁎, M. Guenfoud, B. Lafifi Civil Engineering and Hydraulics Laboratory, Guelma University, Algeria Received 3 February 2007; received in revised form 10 November 2007; accepted 20 January 2008 Available online 16 February 2008
Abstract Loess in North Africa has been investigated using samples from Algeria. The specific gravity, Atterberg limits, grain size distribution and dry density were determined. The hydro-collapsibility properties, due to wetting under different stress levels were measured in single-oedometer tests. The results of this investigation indicate that the properties of Algerian loess are similar to those of loess from many parts of the world, such as Iowa and Libya; they can be classified as silty loess. © 2008 Elsevier B.V. All rights reserved. Keywords: Algeria; Grain size; Engineering properties; Hydrocollapse; Loess; Oedometer tests
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . Materials and experimental programme . Test results and discussion . . . . . . . . 3.1. Index properties of Algerian loess 4. Hydrocollapse characteristics . . . . . . 5. Discussion . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction The Sahara is one of the world's major sources of minerogenic dust (Prospero et al., 2002), many of the silty deposits in adjacent areas being attributed to aeolian processes. Loess is found in many parts of the worlds. In China, for example, it has been estimated that the area covered by loess exceeds 630,000 km2, of which about 60% is collapsible (Derbyshire et al., 1995). Silt constituting 50–70% by weight is the predominant grain size fraction in loess, less than 5% of particles being greater than 1 mm, and clay particles making up only a few percent. Quartz ⁎ Corresponding author. E-mail address: [email protected] (M.S. Nouaouria). 0013-7952/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.01.013
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and feldspar are the major mineral constituents of the coarser grades, while illite is the principal clay mineral. In addition, collapsible loess may contain relatively large amounts of calcium carbonate, as well as various soluble salts distributed on the surface of particles in solid or semi-solid state. Loess deposits have certain distinctive characteristics (Rogers et al., 1994) that define the material. Two major defining features are particle size distribution and the susceptibility of the soil structure to collapse when loaded and wetted. Chen (1992) stated that the defining physical characteristics of loess in its natural state are low water content and high porosity. It has been estimated that loess covers nearly 10% of the surface area of the earth (Pecsi, 1968). It is found in continental drylands on all continents, with the notable exception of Africa.
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M.S. Nouaouria et al. / Engineering Geology 99 (2008) 85–90
response to wetting at different stress levels. For this purpose, undisturbed soil samples representing two types of loess were collected from two locations: Daia Bendahoua and Metlili, Ghardaia (southern Algeria). 2. Materials and experimental programme
Fig. 1. Map of Algeria showing sampling location, Ghardaia.
Scheidig's (1934) map of world loess distribution recognizes only one loess region in Africa (North Libya). However, this conclusion was based on limited information, owing much to the work of Rathjens (1928) who described loess-like materials in Libya but found Tripolitanian loess to have a grain size N 100 μm for loess. More recent work by Coudé-Gaussen (1987) has shown that North African loess may have larger modal sizes than better known loess deposits. In the Techine field-section (Matmata, Southern Tunisia), for examples, three series occur in a 17 m thick succession (Coudé-Guassen et al., 1982): an upper grey-beige loess, an intermediate series with altered reddish horizons; and a lower ochre loess. This work showed that Tunisian loess is coarser in granulometric composition than most other loess. Assallay et al. (1996) have recently justified Scheidig's classification of the North Libyan silty deposits as loess. They demonstrated that it has index properties and shows aspects of hydrocollapse behaviour similar to those found in loess from many parts of the world. They concluded that this material can be classified as silty loess in the Tripoli region and clayey loess in the Ghat area. Abdrabbo et al. (2000) have reported an extent of loess throughout the Western Desert of Egypt, especially at certain locations including Sidi Baranee, New El-America city, and El-Boustan. Despite the huge surface area of Algeria, no serious study has tackled the question as to whether or not loess is present in this vast area. In their study of Algerian duricrusts, Smith and Whalley (1982) concluded that these residual deposits contain quartz and feldspar grains, which exhibits evidence of limited aeolian abrasion and chemical weathering; they suggested a Holocene age for these deposits. Because of the potential hazards posed to engineering structures with foundations on collapsible soils that undergo high volume change, it is important to identify such soils and to understand clearly their engineering properties. This paper is a first attempt to fill the gap in knowledge of the known loess-like silts of Algeria. Given the clear need for more careful analysis of North African silty sediments as a means of determining whether they constitute true loess, the main objective of this paper is to investigate the collapsibility characteristics and behaviour of selected Algerian materials with particular reference to their
An experimental investigation was carried out to determine the index properties and the hydro-collapsibility characteristics and behaviour of the samples. Undisturbed block samples were obtained from depths of between 0.6 and 1 m, and then carefully trimmed, waxed and placed in boxes. The two samples sites lie 45 km apart in the Ghardaia region, some 600 km south of Algiers (Fig. 1), some soil assessment has already been undertaken in this region with a view to dam construction. Specific gravity, and Atterberg limits were determined according to British Standard Procedure (BSI, 1990). Particle size analysis of material passing the 63 μm sieve was undertaken using Sedigraph Technique. The microstructure of loess particles was examined using a scanning electron an X-ray scanning system (Sedigraph). The collapsibility characteristics and behaviour of the
Fig. 2. Scanning electron micrograph of loess samples showing clay-coated silt grains of natural structure of (a) yellow loess and (b) grey loess.
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Table 1 Collapse settlement criteria for Kansas–Nebraska loess (after Holtz and Hilf, 1961) Dry density (mg m− 3)
Condition
b1.28 1.28–1.44
Loess is considered loose and highly susceptible to settlement. Loess is medium dense and is moderately susceptible to settlement, particularly for critical or heavily loaded structures. Loess is quite dense and may be capable of supporting ordinary structures without serious settlement. A more general criterion has been used for earth dams: this density is used as the division between low and high-density loess or the division where special foundation treatment is required for lower densities.
N1.44 1.36
materials with particular reference to their response to wetting at different stress levels was assessed by subjecting undisturbed soil specimens (70 mm in diameter and 19 mm thick) to onedimensional compression in an oedometer. Two methods are currently used to evaluate and determine the collapse parameters, namely the single and double oedometer tests. These have been shown to be reliable in investigating collapsibility properties. According to Lawton et al. (1989), and Basma and Tuncer (1992), these techniques give similar results. The single-oedometer test consists of loading the specimens incrementally to a specific state of vertical stress and allowing the sample to come to equilibrium under the applied pressure. The sample is then flooded with water, and the deformation is measured. The deformation induced by the addition of water is divided by the initial height of the specimen, expressed in percent, from which the magnitude of the hydrocollapse can be defined. The single oedometer test method was performed on undisturbed soil specimens from Ghardaia. Grey (from Daia Table 2 Index properties of Libyan (After Assallay et al., 1996) and Algerian loess Property
Natural moisture Content (%) Specific gravity (Gs) Dry density (Mg/m3) Bulk density (Mg/m3) Void ratio
Tahala Gharyan Khoms Garaboli Grey Yellow loess loess loess loess loess loess (Libya) (Libya) (Libya) (Libya) (Algeria) (Algeria) 3 2.73
6 2.66
3 2.68
2 2.67
5 2.68
6 2.73
–
1.36– 1.42 –
1.40– 1.46 –
–
1.42
1.43
–
1.49
1.52
0.87– 0.96 –
0.84– 0.92 –
–
0.89
0.91
–
15
18
Mechanical analysis Sand (%) 10 Coarse silt (%) 52 Fine silt (%) 25 Clay (%) 13
38 30 21 11
18 39 30 13
39 31 22 8
16 51 24 9
2 61 25 12
Atterberg limits LL (%) PL (%) PI (%)
27 19 8
31 20 11
25 17 8
30 23 7
33 22 11
– –
Degree of – saturation (%)
41 24 17
Fig. 3. Plasticity properties of Algerian loess as compared with loess types defined by Gibbs and Holland (1960). (□ Grey loess; ■ Yellow loess).
Bendahoua) and Yellow (from Metlili) samples were prepared and tested in natural conditions, water contents at the time of testing being 5% and 6% respectively. 3. Test results and discussion 3.1. Index properties of Algerian loess Index properties for the Yellow and Grey Loess are summarised in Table 2. Despite the close similarity of their general characteristics, there are some slight differences in index properties, although both soils are classified as silty loess. The Grey Loess consists of 16% sand, 75% silt and 9% clay. The more silty Yellow loess consists of 2% sand, 86% silt and 12% clay. The Yellow loess has a slight higher dry density and higher specific gravity (2.73). This may be explained by the fact that, as shown by X-ray analysis, its composition is dominated by the feldspar group (Potassium Aluminium Silicate K (Si3Al)O8), and Calcium Magnesium Carbonate (CaMg(CO3)2), while Calcium Magnesium Carbonate is dominant in the Grey loess, which contains only 4% of K(Si3Al)O8) and has a specific gravity of 2.68. The specific gravity values of the Algerian samples are very similar to those for loess from many other parts of the world. For example, Assallay et al. (1996) report values for Libyan loess of between 2.66 and 2.73. Values for Chinese loess of between 2.65
Fig. 4. Particle size distribution of Algerian yellow and grey loess.
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Fig. 5. Hydrocollapse-wetting pressure relationship for undisturbed specimens from Algeria (Grey loess and Yellow Loess) tested in single Oedometer test.
and 2.70 are reported by Bell (1992), those from Kent, U.K. Vary from 2.68 to 2.70 (Derbyshire and Mellors, 1988), those in parts of the United States lie between 2.57 and 2.79 (Sheeler, 1968), while those for the loess of Iowa State ranges from 2.68 to 2.73. The microstructure studies show that both samples have an “open structure (Fig. 2). Using the collapse settlement criteria developed by Holtz and Hilf (1961) shown in Table 1, the Algerian loess can be considered as medium dense and moderately susceptible to settlement, particularly beneath critical or heavy structures. Table 2 shows comparative index properties for Algerian and Libyan loess. Loess soils are slightly to moderately plastic, the plasticity increasing with increases in clay content. Based on plasticity characteristics, Gibbs and Holland (1960) subdivided loess into three types: sandy loess, silty loess and clayey loess. They indicated that Liquid limits for silty loess varied from 25 to 35% and values for clayey loess ranged up to 45%. All plasticity values for the Algerian loess samples plot close to the A-line and below the 50% Liquid Limit on the Casagrande plasticity chart (Fig. 3), indicating that the material is inorganic soil of low plasticity. Such soils are generally very sensitive to changes in moisture content. The test results show that all of the Algeria samples considered here plot within the ‘silty loess’ envelope of Holtz and Gibbs (1951). Particles size results for both samples are shown in Fig. 4. It is evident that the dominant size fraction in these samples is silt (2–60 μm). Silt makes up some 86% of the Yellow loess and 75% of the Grey, and clay percentages are 12 and 9, respectively.
Fig. 6. Hydrocollapse test results for undisturbed grey loess flooded at different stress levels. (Pw = Wetting pressure).
appropriate in the light of the view of Lutenegger and Hallberg (1988) that this is the most common method used to identify a collapsible soil, and one with the most meaning to geotechnical engineers. The degree of hydrocollapse of each sample was then calculated as follows. Hydrocollapse ¼
δH k H0
Where δH = change in sample height due to saturation H0 = Sample height immediately prior to saturation For the double oedometer test, the change in sample height due saturation is taken as the difference between the unsaturated sample heights at any given pressure. These definitions are standard throughout the literature Rogers et al. (1994). The curves shown in Fig. 5 illustrate the effect of wetting pressure on the collapsibility of the loess, and indicate that the magnitude of the hydrocollapse increases as the applied pressure
4. Hydrocollapse characteristics Undisturbed soil specimens from Ghardaia were prepared for testing. The single Oedometer test was considered the most
Table 3 Identification of hydrocollapse problems (after Jennings and Knight, 1975) Collapse potential (CP)
Severity of problem
0%–1% 1%–5% 5%–10% 10%–20% N20%
No problem Moderate problem Trouble Severe trouble Very severe trouble
Fig. 7. Hydrocollapse test results for undisturbed yellow loess flooded at different stress levels. (Pw = Wetting pressure).
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of wetting increases up to a certain value, at which the maximum collapse will be obtained due to saturation. Based on the values given by Jennings and Knight (1975) and shown in Table 3, it can be taken that the Algerian loess considered here poses severe collapse problems. For a stress range up to 1600 kPa, the measured magnitudes of hydrocollapse of the specimens vary over a range of 1 to 21% and 0.8 to 20% for the Grey and Yellow loess, respectively, depending on the level of the vertical pressure applied to the specimens at the time of wetting. Figs. 6 and 7 show the post-wetted relationship between specimen height and applied pressure for all specimens that were flooded at different stress levels. 5. Discussion On the basis of the results presented here, it is evident that there are similarities between the loessic soils in southern Algeria and those found in many others parts of the world. This includes the demonstrable collapsibility of the Algerian soils described, collapsibility generally being predicted on the soil having void space in its natural state that is sufficient to sustain its liquid limit moisture at saturation. As far as the Algerian loess is concerned, the high void ratios of the Algerian loess result in a hydro-collapse magnitude of about 13% for both Grey and Yellow loess for a wetting pressure of 200 kPa. Based on the identification of the severity of collapsibility given by Jennings and Knight (1975), the Algerian loessic soils tested in this study are regarded as likely to pose severe problems, having a susceptibility to settlement particularly in respect of critical or heavily loaded structures. The Atterberg limits, grain size characteristics, and specific gravity of these Algerian loess samples were found to be very similar to those of loess deposits from many parts of the world. Based on the plasticity and particle size test results, and according to the definition of loess types given by Holtz and Gibbs (1951) and Gibbs and Holland (1960), the Algerian material can be classified as silty loess. The amount collapse depends on the magnitude of vertical stress at the time of wetting. As loess and loess-like sediments are always unsaturated, the reduction in matric suction is one of the major causes of collapse (Lloret and Alonso, 1980; Maswoswe, 1985). It seems vital that in parallel with the continued accumulation and comparative analysis of the bulk behaviour of loess, greater effort should be devoted to the study of the behaviour of collapsible loess in the context of the theory of unsaturated soils in which the constitutive relations for volume change in unsaturated soils could be used to explain more comprehensively collapsible soil (Fredlund and Morgenstern, 1976). 6. Conclusion (a) The southern Algerian loess is of two types, named here Grey and Yellow loess. Lithologically and mineralogically, the two types are quiet similar, although there are some differences composition that explain the colour difference. (b) The results of this investigation indicate that the Index Properties of southern Algerian loess are similar to those
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found in loess from many parts of the world, such as Iowa (USA) and Libya. (c) Both the Grey and Yellow loess soils are rich in silt and can be classified as a silty loess. (d) The amount of hydrocollapse is dependent on the magnitude of vertical stress acting on specimen at the time of wetting. The degree of hydrocollapse increases as the applied pressure increases up to a certain level of stress, at which point maximum collapse is reached (20% and 21% for yellow and Grey loess, respectively). (e) A unique post-wetted specimen height applied pressure relation was observed in the Algerian samples tested. Flooding at any applied pressure caused the material to collapse down to a certain specimen height prescribed by the unique post-wetted specimen height/applied pressure curve. References Abdrabbo, F.M., Hansy, R.M., Hamed, K.M., 2000. Collapsibility of Egyptian loess soil. unsaturated soils for Asia. In: Toll, Leong (Eds.), Rahardjo. Balkema, Rotterdam, pp. 595–606. Assallay, A.M., Rogers, C.D.F., Smalley, I.J., 1996. Engineering properties of loess in Libya. Journal of Arid Environment 32, 373–386. Basma, A.A., Tuncer, E.R., 1992. Evaluation and control of collapsible soils. Journal of Geotechnical Engineering Division, ASCE 118, 1491–1504. Bell, F.G., 1992. Engineering Properties of Soils and Rocks, 3rd ed. ButterworthHeinemann, Oxford, U.K. (345pp.). British Standards Institution, 1990. Bs 1377: British Standards Methods of test for soils for civil engineering purposes. B.S.I., London. Chen, Z.Y., 1992. Practical solutions to problems of collapsible loess in China. Proceedings of the 7th International conference on expansive soils. Dallas, Texas, pp. 1–12. Coudé-Gaussen, G., 1987. The Peri-Saharan Loess: sedimentological characterization and paliclimatical significance. Geological Journal 15, 177–183. Coudé-Gaussen, G., Mosser, C., Rognon, P., Tourenq, J., 1982. Une accumulation de loess du Pléistocéne supérieur dans le Sud-Tunisien: la coupe de Téchine. Bulletin de la SocieÂte geÂologique de France 7 (t. XXIV), No. 2, 283–292 pp (in French). Derbyshire, E., Mellors, T.W., 1988. Geological and geotechnical characteristics of some engineering loess and loessic soils from China and Britain: a comparison. Engineering Geologist 25, 135–175. Derbyshire, E., Meng, X.M., Wang, J.T., Zhou, Z.Q., Li, B.X., 1995. Collapse loess on the Loess Plateau of China. In: Derbyshire, E., Dijkstra, T., Smalley, I.J. (Eds.), Genesis and properties of Collapsible Soils. Kluwer, Dordrecht, pp. 267–293. Fredlund, D.G., Morgenstern, N.R., 1976. Constitutive relations for volume change in unsaturated soils. Canadian Geotechnical Journal 13 (3), 261–276. Gibbs, H.H., Holland, W.Y., 1960. Petrographic and engineering properties of loess. US Bureau of Reclamation, Engineering Monograph, vol. 28 (37 pp.). Holtz, W.G., Gibbs, H.J., 1951. Consolidation and related properties of loessial soils. American Society for Testing Materials. Special Technologies 126, 9–33. Holtz, W.G., Hilf, J.W., 1961. Settlement of soil foundations due to saturation. Proceedings of the 5th International Conference on Soil Mechanics and Foundation Engineering, vol. 1, pp. 673–679. Jennings, J.L., Knight, K., 1975. A guide to construction on or with material exhibiting additional settlement due to collapse of grain structure. Sixth Regional Conference for Africa on Soil Mechanics & Foundation Engineering, Durban, South Africa, September. Lawton, E.C., Fragaszy, R.J., Hardcastle, J.H., 1989. Collapse of compacted clayey sand. Journal of Geotechnical Engineering, (ASCE) 115, 1252–1267. Lloret, A., Alonso, E.E., 1980. Consolidation of unsaturated soil including swelling and collapse behaviour. Geotechnique 30 (4), 449–477. Lutenegger, A.J., Hallberg, G.R., 1988. Stability of loess. Engineering Geology 25, 247–261.
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Rogers, C.D.F., Dijkstra, T.A., Smalley, I.J., 1994. Hydro-consolidation and subsidence of loess: Studies from China, Russia, North America and Europe. Engineering Geology 37, 83–113. Scheidig, A., 1934. Der Loss und seine geotechnischen Eigenschaften. Steinkopf, Dresden. 133 pp (in German). Sheeler, J.J., 1968. Summarization and comparison of engineering properties of loess in the United States. Highway Research Record (212), 1–9. Smith, B.J., Whalley, W.B., 1982. Observations on the composition and mineralogy of an Algerian duricrust complex. Geoderma 28, 285–311.