Applicability Of Standard Penetration Tests To Estimate Undrained Shear Strength Of Soils Of Imphal.

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Reynard Young
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1 International Journal of Engineering Technology Science and Research March 217 Applicability Of Standard Penetration Tests To Estimate Undrained Shear Strength Of Soils Of Imphal. Ngangbam Bulbul Singh, MTech student Manipur Institute of Technology Ningthoujam Jibanchand, MTech student Manipur Institute of Technology K Rambha Devi, Assistant Professor Manipur Institute of Technology ABSTRACT Standard penetration Test (SPT) is a common field test to predict the behaviour of coarse grained soils. However, it is used widely in all types of soil due to its simplicity and cost effectiveness. This study makes an attempt to study the applicability of the existing correlations between SPT number and undrained shear strength of fine grained soils of Imphal by comparing the predicted and the laboratory values of shear strength of fine grained soils in Imphal. A correlation to predict the undrained shear strength from the SPT values for the soils of Imphal is also presented. Introduction: The SPT is a common field test originally designed to determine the relative density of cohesionless soils but is currently the most popular and economical method to empirically determine the soil strength and bearing capacity of soil. However, SPT is recommended mainly for determining strength of coarse soils and its use should be restricted to qualitative evaluation of the soil profile only in clayey soils (British Standard 27a, p). British Standards 21 states that if an empirical relationship is used in the analysis,it shall be clearly established that it is relevant for the prevailing ground condition (British Standard 21, p23). SPT is mainly useful under conditions of financial and time limitations in a project as also where obtaining undisturbed samples of soil becomes difficult. Under such conditions, various correlations of SPT number can be used to estimate the geotechnical parameters and obtain the bearing capacity of shallow foundations or carrying capacity of piles. Terzaghi was the first to realise that the penetration resistance of the split-spoon sampler could provide useful in situ test data and he, along with Harry Mohr correlated the SPT numbers with various properties of soils, including the relative density of sands, consistency and unconfined compressive strength of clays, and allowable bearing pressure on sands and clays and the first published SPT correlations appeared in Terzaghi and Peck (1948). Peck et al(1953) noted that SPT values correlated better for sands than for fine soils. Other researchers like Dé court (199), Nixon (1982), Ajayi and Balogum (1988) also correlated the field SPT with the undrained strength of fine grained soils. The influence of plasticity index on the strength was incorporated in the correlations of fine grained soils with SPT values (Sanglerat 1972; Stroud 1974; Sowers 1979; Sirvikaya & Togrol 22). The soils found in Imphal valley of Manipur, India, consists mainly of clayey or silty clay with intermediate to high plasticity(ci, CH, CI-MI). Data used in this study are collected from soil investigation reports of various project works carried in Imphal valley. The SPT values and the laboratory undrained strength obtained from laboratory tests were correlated and compared with the predicted values by various researchers. Testing method: The Standard Penetration test carried out at site is according to Indian Standard conforming to IS Standard Penetration Test Split spoon samplers attached at the ends of drill rods are lowered in boreholes at depths where the SPT is to be carried out. The sampler is driven into the soil for a distance of 4mm by blows of a drop hammer of 65kg falling vertically and falling freely from a height of 7mm. The number of blows required to penetrate every 1mm is recorded and the number of blows required to penetrate the last 3mm is recorded as SPT number N at that particular depth. The number of blows required to penetrate the first 1mm is disregarded. The SPT is carried out every 1m to 1.5m interval or whenever there is a change in the soil profile.

2 International Journal of Engineering Technology Science and Research March 217 Laboratory tests were carried out on undisturbed samples collected at various depths in order to classify the soils from sieve analysis, hydrometer tests and Atterberg's limits. The natural water content, specific gravity, density, undrained strength and consolidation parameters were also determined from tests conforming to relevant Indian Standards in all the projects. Results and Discussion: About 2 data from various projects around Imphal are used for analysis and undrained shear strength of these soils are predicted from the correlations given in Table 1 and compared with those obtained from the laboratory tests. It is observed that the observed values of s u fit best with those predicted by Terzaghi and Peck (1967),, and Sivrikaya and Togrol (22). These are shown in Figs. 1, 1 and 1(c) respectively. Table 1 : Correlations between undrained shear strength(s u ) and SPT(N) Researchers Soil type Undrained shear strength Terzaghi & Peck (1967) Fine-grained soil 6.25N Sanglerat (1972) Silty clay 12.5N Clay 1N Hara et al. (1974) Fine-grained soil 29N.72 Stroud (1974) PI<2 (6-7)N 2<PI<3 (4-5)N PI>3 4.2N Sowers (1979) Highly plastic soil 12.5N Medium plastic clay 7.5N Low plastic soil 3.75N Nixon (1982) Clay 12N Décourt (199) Clay 12.5N 15N 6 Sivrikaya & Togrol (22) Highly plastic soil 4.85N field 6.82N 6 Low plastic soil Fine-grained soil 3.35N field 4.93N N field 6.18N 6 Hettiarachchi & Brown (29) Fine-grained soil 4.1N 6 It is observed in Fig. 1 that the observed and predicted values match reasonably for low SPT numbers (less than 2) but there is substantial deviation for SPT values higher than 2. Similar observation was made by Behpoor and Ghahramani(1989). The laboratory values, however, are lower than the predicted values even for N values less than

3 Undrained strength s u (kg/cm 2 ) Undrained strength s u (kg/cm 2 ) Undrained strength s u (kg/cm 2 ) International Journal of Engineering Technology Science and Research March Terzaghi &Peck (1967) y = 4.943x R² = Sivrikaya & Toğrol (22) s u = 4.94N R² = (c) (d) Fig. 1: Comparison of undrained strength of laboratory values and from correlations with SPT values by Terzaghi and Peck (1967), Stroud (1974), (c) Sivrikaya and Togrol (22) and (d) present study 252

4 International Journal of Engineering Technology Science and Research March 217 The effect of plasticity of the soils on undrained strength can be observed from Figs 1 and 1(c). The correlations which include the plasticity of the soils in predicting the strength fits better with the observed values for both low and high values of SPT as is observed from these figures. A general equation to predict the undrained strength of Imphal fine grained soil without considering the plasticity of the soil can be given as s u = 4.94N (kpa) with (r 2 =.84). As the plasticity of the soil affects the undrained strength of the soil, an attempt has been made to predict the strength by categorising the soil into three groups according to their plasticity indices as those with PI<2, 2<PI<3 and PI>3. The s u versus N values of the laboratory and predicted values for PI<2 by Stroud are shown in Figs 2 and those by Sivrikaya and Togrol in 2. Similarly Figs 3 and show for 2<PI<3 and Figs 4and for PI>3. From these figures, it is seen that Stroud's relationship fits better for Imphal soils with all ranges of plasticity whereas Sivrikaya and Togrol's relationship deviates for low plastic soils. For Imphal valley soils the following correlations are proposed for three ranges of plasticity For PI<2, s u = 5.4 N (kpa) with r 2 =.88 2<PI<3, s u = 5. N (kpa) with r 2 =.747 PI>3, s u = 4.7N (kpa) with r 2 = s u =.N R² =.818 PI< Sivrikaya & Toğrol (22) PI< Fig. 2 : Relation between s u and of the laboratory and predicted values by and Sivrikaya and Togrol (22) for PI< <2PI<3 Sivrikaya & Toğrol (22) 3 2<PI<3 2 2 s u = 5.27N 1 R² = Fig. 3 : Relation between s u and of the laboratory and predicted values by and Sivrikaya and Togrol (22) for 2<PI<3 253

5 International Journal of Engineering Technology Science and Research March s u = 4.66 N R² =.878 PI> Sivrikaya & Toğrol (22) PI> Fig. 4 : Relation between s u and of the laboratory and predicted values by and Sivrikaya and Togrol (22) for PI>3 Conclusion: From this study of relationship between laboratory undrained strength s u with SPT for fine grained soils of Imphal, it is observed that existing correlations which take the effect of plasticity into account correlate better than those which do not take effect of plasticity into account. For the soil of Imphal, correlations for fine grained soils grouped into three categories based on their plasticity indices are suggested. However, these are not a substitute of laboratory tests and more data may be included to study the exactness of these empirical formulae. Acknowledgement: The authors extend their profound gratefulness to Public Works Department, Government of Manipur, for making available the borehole logs and laboratory test data of their various projects. References: 1. Ajayi, L.A. and Balogum, L.A. (1988) Penetration testing in tropical lateritic and residual soils Nigerian experience, Penetration Testing, ISOPT-1: Rotterdam: Balkema. 2. Behpoor, L. and Jahanandish, A. (1989). "Correlation of SPT to strength and modulus of elasticity of cohesive soils, 12th International Conference on Soil Mechanics and Foundation Engineering, ISSMFE, Rio do Janeiro, Brazil, British Standard (27a) Eurocode 7 Geotechnical design Part 2: Ground investigation and testing. British Standard Institution, BS EN :27, ISBN British Standard (21) Eurocode 7 Geotechnical design Part 1: General rules. United Kingdom, British Standard Institution, BS EN :24 Incorporating corrigendum February 29, ISBN Décourt, L. (199) The Standard Penetration Test, State of the Art Report, Norwegian Geotechnical Institute Publication, vol. 179, Part ΙΙ. Oslo, Norway. 6. Hara, A., Ohta, T., Niwa, M., Tanaka, S., and Banno, T. (1974) Shear modulus and shear strength of cohesive soils, Soils and Foundation., 14(3), Hettiarachchi, H. and Brown, T. (29) Use of SPT Blow Counts to Estimate Shear Strength Properties of Soils: Energy Balance Approach, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 135, No IS Method for standard penetration test for soils. Bureau of Indian Standards. 254

6 International Journal of Engineering Technology Science and Research March Nassaji, F. and Kalantari, B.(211). "SPT Capability to Estimate Undrained Shear Strength of Fine Grained Soils of Tehran, Iran," Electronic Journal of Geotechnical Engineering, Vol. 16, Bund.N, Nixon, I.K., (1982). Standard penetration test: state of the art report, Proceedings of the 2nd European Symposium on Penetration Testing, Amsterdam. 11. Peck, R. B.; Hanson, W. E.; and Thornburn, T. H. (1953). Foundation Engineering: John Wiley & Sons, New York, 41 p. 12. Sanglerat, G. (1972). The Penetration and Soil Exploration; Interpretation of Penetration Diagrams Theory and Practice, Elsevier Publishing Co, Amsterdam. 464 pp 13. Sivrikaya, O. and Togrol, E. (22) Relations between SPT-N and q Intern.Congress on Advances Civil engineering, Istanbul, Turkey, pp Sowers, G. F. (1979) Introductory Soil Mechanics and Foundations, 4 edition. Macmillan, 621, New York. 15. Stroud, M. A. (1974) The standard penetration test in insensitive clays and soft rock, Proceedings of the 1st European Symposium on Penetration Testing, Sweden: Stockholm, vol. 2(2), Terzaghi, K. and Peck, R.B. (1967) Soil Mechanics in Engineering Practice, John Wiley, NewYork

Design Example 2.2. Pad foundation with inclined eccentric load on boulder clay

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