Densification effect on Liquefaction potential of Reinforced Soil by Stone Columns L’effet de densification sur le risque de liquefaction des sols renforcés par colonnes ballastées Zeineb Ben Salem Université de Tunis El Manar – Ecole Nationale d’Ingénieurs de Tunis, LR14ES03-Ingénierie Géotechnique. BP 37 Le Belvédère, 1002 Tunis,Tunisia,
[email protected]
Wissem Frikha Université de Tunis El Manar – Ecole Nationale d’Ingénieurs de Tunis, LR14ES03-Ingénierie Géotechnique. BP 37 Le Belvédère, 1002 Tunis,Tunisia,
[email protected]
Mounir Bouassida Université de Tunis El Manar – Ecole Nationale d’Ingénieurs de Tunis, LR14ES03-Ingénierie Géotechnique. BP 37 Le Belvédère, 1002 Tunis,Tunisia
ABSTRACT: Stone column is a ground improvement technique employed to increase the bearing capacity and to reduce settlements of weak soils. Installation of stone column is widely adopted to prevent liquefaction. The main improvement mechanisms of stone columns as a liquefaction countermeasure are drainage, stiffening and densification. Liquefaction potential can be evaluated in terms of factor of safety FS against liquefaction. The aim of this study was to assess the effectiveness of stone columns technique to mitigate liquefaction by considering the densification effect. Twenty four cases studies, where SPT and CPT tests have been performed before and after stone columns reinforcement, were used as a basis of the research. The results show that considering the densification effect considerably improved the assessment of liquefaction potential of reinforced soil by stone columns. RÉSUMÉ : Les colonnes ballastées représentent une technique de renforcement des sols qui permet d’améliorer la capacité portante et de réduire le tassement. Cette technique est largement adoptée pour réduire le potentiel de liquéfaction. En effet, l’augmentation de la résistance à la liquéfaction du sol initial se traduit essentiellement par différents mécanismes qui sont principalement la densification, le renforcement et le drainage. Dans le présent travail, vingt quatre études de cas ont été traitées dans le but d’évaluer l’efficacité des colonnes ballastées vis-à-vis de la réduction du risque de liquéfaction tout en tenant compte de l’effet de densification. Pour ces cas d’études, des essais SPT et CPT ont été effectués avant et après mise en place des colonnes ballastées. L’analyse du potentiel de liquéfaction à travers le calcul d’un facteur de sécurité FS montre que la prise en compte du mécanisme de densification engendre une réduction importante du risque de liquéfaction du sol renforcé.
KEYWORDS: Liquefaction, stone columns, densification 1
INTRODUCTION
Liquefaction is the most hazardous damage during an earthquake. Liquefaction damage induces sand boils, excessive settlement, lateral spread, landslides and slope failure, loss of bearing capacity, etc. Several approaches have been developed over the years to evaluate the liquefaction potential. Seed and Idriss (1971) initially proposed a procedure known as the "simplified procedure". This procedure involves the comparison between the Cyclic Shear Stress Ratio CSR caused by the design earthquake with the capacity of the soil to liquefaction resistance expressed in terms of Cyclic Resistance Ratio CRR . This procedure has been modified and improved field tests results such as those developed by Youd et al. (2001), Cetin et al. (2004) and Idriss and Boulanger (2008). To reduce the risk of liquefaction potential and its associated damages, various ground improvement methods were practiced including densification, reinforcement, grouting/mixing and drainage (Mitchell 2008). Reinforcement with granular column is the most widely adopted method to mitigate liquefaction (Mitchell and Wentz 1991). This reinforcement can mitigate the risk of liquefaction through several mechanisms by: (i) Providing drainage by which the induced excess pore-water
pressure gets dissipated almost as fast as it is generated (ii) Introducing stiff elements which can potentially carry higher stress levels causing reduction in stress levels in the surrounding soil (iii) Increasing the soil density during installation process of stone columns (Goughnour and Pestana 1998; Green et al. 2008; Rayamajhi et al. 2012; Ben Salem et al. 2016). The present paper examines the effectiveness of stone columns to prevent liquefaction focusing on densification mechanism. Twenty four cases studies based on stone column reinforcement project are considered, where SPT and CPT tests were performed before and after stone column installation. 2
LIQUEFACTION POTENTIAL
In the present paper, a focus is made on the procedure of Idriss and Boulanger (2008) for evaluating liquefaction in terms of a factor of safety defined by: CRR (1) FS CSR It is argued that a FS less than 1.0 means that liquefaction is expected to be triggered. The Cyclic Shear Stress Ratio CSR is defined using the procedure of Seed and Idriss (1971) and it is calculated a function of the peak horizontal acceleration at ground surface generated by the earthquake a max , the total and the effective
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vertical overburden stresses and the shear stress reduction factor. The Cyclic Resistance Ratio CRR is defined as a function of the magnitude scaling factor, the overburden correction factor and the corrected SPT and CPT penetration resistances defined by Ncorrected and qcorrected , respectively. 3
ANALYSIS OF LIQUEFACTION POTENTIAL OF REINFORCED SOIL
In order to study the liquefaction potential of reinforced soil with stone column, twenty four case studies using project data located in seven countries are considered. For each project, different design parameters of stone column were adopted i.e: triangular or square grid pattern, spacing, diameter, area replacement ratio, treatment depth (Ben Salem 2016). Stone columns were installed by vibro-replacement which involves the use of hollow depth vibrators that are vibrated by way of air or water to form a cylindrical cavity in the ground. Stone column reinforcement is primarily used for improving bearing capacity, limiting settlements and also for liquefaction mitigation. For establishing the effectiveness of stone column reinforcement, in situ tests were performed before and after stone columns reinforcement including 31 SPT and 33 CPT. To analyze the liquefaction potential of reinforced soil, the evaluation of densification improvement was made using SPT and CPT tests performed before and after stone columns installation.
80 (a) 70
Final Ncorrected
60 50 Type of soil -Clean sand -Loose to dense sand -Slightly silty sand -Slightly clayey sand -Silt -Clayey silt -Sandy silt -Soft Clay
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3 .1 Densification effect The SPT and CPT results have been corrected. Comparative results of initial and final penetration resistance Ncorrected and
qcorrected values recorded before and after stone columns installation are reported in Figure 1 (a) and (b) respectively. As it can be seen, penetration resistance greatly increased after stone columns installation. Significant ground densification was achieved in the clean to slightly silty sands while densification of the silt soils was very limited. In sensitive clay layers, penetration resistance has decreased. The range of increase in the penetration resistance and were about 31.13% to 69.29 % and 57.10% to 318.28% respectively. The amount of improvement can be attributed to the installation of the stone columns (Choobbasti et al. 2011; Frikha et al. 2013). It has been be also noticed that factors which influence the effectiveness of the soil densification after stone columns installation include the soil type, pre-densification relative density, vibratory type, area replacement ratio and spacing between stone columns as confirmed by Mitchell (1981), Baez and Martin (1993), Shenthan et al. (2004) and Rollins et al. (2006). 3 .2 Factor of Safety In order to study the effectiveness of stone columns to mitigate liquefaction, the chosen design parameters for this analysis include an earthquake magnitude M of 7 and a peak ground acceleration a max of 0.2 g. In fact, the majority of site projects are characterized by high seismic activities with magnitude ranged from 6.2 to 7.5 and peak ground acceleration ranged from 0.1g to 0.44g. The analysis was primarily concerned with estimating the liquefaction potential of unreinforced soil and secondly, examining densification effect in the assessment of liquefaction potential of reinforced soil. The factor of safety FS of unreinforced soil is calculated via Equation (1) where the Cyclic Resistance Ratio CRR was evaluated from SPT and CPT data recording prior to stone columns installation and the Cyclic Stress Ratio CSR was estimated using the procedure of Seed and Idriss (1971).
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Figure 2. Variation of FS versus initial Ncorrected and
qcorrected before reinforcement by stone columns
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Figure 1. Variation of initial and final values of (a) Ncorrected and (b)
qcorrected
The variation of the factor of safety FS of unreinforced soil versus the corresponding initial penetration resistance Ncorrected and qcorrected is plotted in Figure 2. As it can be seen, approximately 50% and 36.48 % of factor of safety values obtained prior to stone column installation are smaller than 1.
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The analysis of liquefaction susceptibility of reinforced soil focuses on soils which are identified as being potentially liquefiable prior to stone columns installation. By considering the densification effect, the factor of safety of reinforced soil denoted FSDens. is calculated where the CRR is obtained based on SPT and CPT data measured after stone column installation (i.e. final Ncorrected and qcorrected values). Figure 3 shows calculated factor of safety FSDens. plotted as a function of the average rate of densification which is defined as rate of variation of penetration resistance Ncorrected and
qcorrected after stone columns installation in comparison with initial values. It can be observed that the factor of safety FSDens. increases with increasing penetration resistance. Approximately 83.87 % and 32.22 % of factor of safety FSDens. values are much above 1, when the CRR calculation is based on SPT and CPT data respectively. These results confirm the effectiveness of densification mechanism in reducing the potential for liquefaction of reinforced soil.
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SPT data CPT data
FS Dens.
Cetin, K. O., Seed, R. B., Der Kiureghian, A., Tokimatsu, K., Jr, H. L. F., R.E., K., and MossR.E.S. (2004). “Standard penetration testbased probabilistic and deterministic assessment of seismic soil liquefaction potential.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 130(12), 1314–1340. Choobbasti, A. J., Zahmatkesh, A., Noorzad, R., Choobbasti, A. J., Zahmatkesh, A., Noorzad, R., Choobbasti, A. J., Zahmatkesh, A., and Noorzad, R. (2011). “Performance of Stone Columns in Soft Clay: Numerical Evaluation.” Geotechnical and Geological Engineering, 29(5), 675–684. Frikha, W., Bouassida, M., and Jean, C. (2013). “Observed Behaviour of Laterally Expanded Stone Columnin Soft Soil.” Geotechnical and Geological Engineering, 31(2), 739–752. Goughnour, R. R., and Pestana, J. M. (1998). “Mechanical behavior of stone columns under seismic loading.” 2nd International Conference on Ground Improvement Techniques, Ci-Premier Pte Limited, Singapore, 157–162. Green, R., Olgun, C., and Wissmann, K. (2008). “Shear Stress Redistribution as a Mechanism to Mitigate the Risk of Liquefaction.” Geotechnical Earthquake Engineering and Soil Dynamics IV, D. Zeng, M. T. Manzari, and D. R. Hiltunen, eds., ASCE, Sacramento, California, United States, 1–10. Idriss, I. M., and Boulanger, R. W. (2008). “Soil Liquefaction During Earthquakes.” Monograph MNO-12, Earthquake Engineering Research Institute, Oakland, CA, 261. Mitchell, J. K. (1981). “State of the art-Soil improvement.” Proc 10th Mitchell, J. K. (2008). “Mitigation of Liquefaction Potential of Silty Sands.” From Research to Practice in Geotechnical Engineering, Geotechnical special publication 180, J. E. Laier, D. K. Crapps, and M. H. Hussein, eds., ASCE, New Orleans, Louisiana, United States, 453–451. Mitchell, J. K., and Wentz, F. K. (1991). “Performance of improved ground during the Loma Prieta earthquake.” Report No.
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Densification Improvement -50
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Figure 3. Variation of FSDens. versus the average rate of densification
CONCLUSION
To study the effectiveness of using stones columns in reducing the liquefaction potential, twenty four cases studies, where SPT and CPT tests have been performed before and after stone columns reinforcement, were used as a basis of the research. This assessment focuses on the mechanism of densification using field test results. The liquefaction potential is evaluated in terms of a factor of safety. Based on this liquefaction analysis, the main findings are summarized as follows: - The installation of stone columns significantly increased soil density. Significant ground densification was achieved in the clean to slightly silt sands while densification of the silt and clayey soils was very limited. - Densification effect considerably improved the assessment of liquefaction potential of reinforced soil by stone columns. - It can be concluded that field tests performed before and after stone column installation are recommended for liquefaction analysis and densification effect should be considered for the design of stone column to mitigate liquefaction. 5
Center,
Rayamajhi, D., Nguyen, T., Ashford, S., Boulanger, R., Lu, J., Elgamal, A., and Shao, L. (2012). “Effect of Discrete Columns on Shear Stress Distribution in Liquefiable Soil.” GeoCongress 2012: State of the Art and Practice in Geotechnical Engineering, D. H. Roman, A. Athanasopoulos-Zekkos, and N. Yesiller, eds., ASCE, Oakland CA, 1908–1917. Rollins, K., Price, B. E., Dibb, E., and Higbee, J. B. (2006). “Liquefaction Mitigation of Silty Sands in Utah Using Stone Columns with Wick Drains.” Procs. GeoShanghai Intl. Conf.,
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Average rate of Densification (%)
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Research
REFERENCES
Baez, J. I., and Martin, G. R. (1993). “Advances in the Design of Vibro Systems for the Improvement of Liquefaction Resistance.” Proceedings of the Symposium of Ground Improvement, Vancouver Geotechnical Society, Vancouver, B.C., 1–16.
Geotechnical Special Publication 152, Ground Modification and Seismic Mitigation, A. Porbaha, S.-L. Shen, J. Wartman, and J.C. Chai, eds., ASCE, Shanghai, China, 343–348. Ben Salem, Z. (2016). “Etude du potentiel de liquéfaction d’un sol renforcé par colonnes ballastées.” (Doctoral dissertation) Ecole Nationale d’Ingénieurs de Tunis. Ben Salem, Z., Frikha, W., and Bouassida, M. (2016). “Effect of Granular-Column Installation on Excess Pore Pressure Variation During Soil Liquefaction.” International Journal of
Geomechanics,10.1061/(ASCE)GM.1943-5622.0000516 04015046.
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Seed, H. B., and Idriss, I. M. (1971). “Simplified procedure for evaluating soil liquefaction potential.” Soil Mechanics and Foundations,Div., ASCE, 97:SM9, 1249–1273. Shenthan, T., Nashed, R., Thevanayagam, S., and Martin, G. R. (2004). “Liquefaction mitigation in silty soils using composite stone columns and dynamic compaction.” Earthquake Engineering and Engineering Vibration, 3(1), 39–50. Youd, T. L., Idriss I .M, Andrus R. D., Arango I., C. G., Christian J. T, Dobry R., Finn W. D. L., H. L. F. J., Hynes M. E., Ishihara K., Koester J. P., L. S. S. C., Marcuson W. F III, Martin G. R., Mitchell J. K., M., Y., Power M. S., Robertson P. K., Seed R. B., S. I. K., and H. (2001). “Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils.” Journal of Geotechnical and Geoenvironmental Engineering, 127(10), 817–833.
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