Assessing the Stress–Deformation and Safety Factor against Sliding in Reinforced Deep Urban Excavations in Unsaturated Soils
Publication: International Journal of Geomechanics
Volume 24, Issue 7
Abstract
There has been limited research conducted to date on the role and significance of soil parameters in an unsaturated state on the amount of deformation and safety of reinforced deep urban excavations. This study investigates stress–deformations and static/pseudostatic safety factors against the general failure of an anchored deep excavated wall in an unsaturated soil deposit through two-dimensional finite-element modeling and limit equilibrium analysis, respectively. A suction-dependent elastic–plastic Mohr-Coulomb model is used in the analyses considering the effective stress approach in unsaturated soils. The results obtained from numerical modeling are validated against field monitoring data, including wall deformations and tensile forces in the anchors. A series of parametric studies are then performed assuming different groundwater levels, surcharge loads, and surcharge load distances from the wall crest to investigate their effects on the stability and deformation of the unsaturated soil excavation. Results are compared with those obtained from the corresponding routine analyses, in which the unsaturated state of the soil is not considered. The parametric study shows that the depth of the groundwater table is more influential on the results compared with the intensity and location of the surcharge load. The study demonstrates that unsaturated soil conditions result in a reduction of up to 37% in the maximum horizontal deformations of the excavation and increase the static and pseudostatic safety factors against general failure by 28% and 19%, respectively. In addition, taking unsaturated soil conditions into account during analysis leads to a decrease of up to 31% in the estimated tensile forces in the anchors. More importantly, it is shown that a more cost-effective stabilization plan can be developed for deep urban excavations by considering soil unsaturation effects, as demonstrated by comparing the results of the numerical analyses with the field data.
Practical Applications
Presenting a safe and economically feasible plan for stabilizing deep urban excavations is a significant challenge for geotechnical engineers. Traditionally, engineers have relied on classical methods that consider soil parameters under dry or saturated conditions. However, in practice, the soil above the water table is unsaturated, and its mechanical properties are greatly influenced by the saturation level. Contrary to common belief, rainfall or pipe leakage near an excavation site does not fully saturate the soil and eliminate suction effects. Previous studies have shown that percolation from rainfall or other factors does not completely infiltrate clay-rich soils, mainly affecting moisture content and suction at shallow depths. This study evaluates the stress–strain behavior and the factor of safety against sliding in a deep excavated wall in Tehran, Iran, by considering unsaturated soil parameters. Comparing the outcomes with conventional methodologies, the results demonstrate that incorporating unsaturated soil parameters provides more accurate results aligned with field observations. This approach also results in smaller displacements of the excavation wall and higher safety factors against wall sliding. Accordingly, incorporating unsaturated soil parameters ensures accurate safety assessments for urban excavation walls and enables the creation of cost-effective and optimized design suggestions.
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Data Availability Statement
The data sets generated or analyzed during the current study are available from the corresponding author upon reasonable request.
Acknowledgments
The first author would like to thank the Niroo Research Institute for providing him with the opportunity to contribute to this research. In addition, the authors would like to thank Mr. Mohammad Mohammadi Zahed for his helps in performing parts of numerical simulations and analytical calculations.
Notation
The following symbols are used in this paper:
- A
- section area;
- ah, av
- horizontal and the vertical pseudostatic seismic-induced ground accelerations;
- c′
- effective soil cohesion;
- D
- surcharge distance from the wall crest;
- D10
- soil particle diameter corresponding to 10% finer;
- D60
- soil particle diameter corresponding to 60% finer;
- Dh_cr
- horizontal displacements of the wall crest;
- Dh_max
- maximum horizontal wall displacements;
- DL
- concentrated point load;
- E
- modulus of elasticity;
- Esat
- saturated elastic modulus of soil;
- Eunsat
- unsaturated elastic modulus of soil;
- e
- void ratio;
- F
- reduction (penalty) factor of soil shear strength;
- Ff
- FoS regarding forces equilibrium;
- Fh, Fv
- equivalent horizontal/vertical static point loads;
- Fm
- FoS regarding moments equilibrium;
- FoS
- FoS;
- FoSst, FoSps
- static and pseudostatic factors of safety;
- Ft_min, Ft_max
- minimum and maximum tensile forces of the anchors;
- Gs
- specific gravity;
- g
- acceleration of gravity;
- Ixx
- major moment of inertia;
- i
- numeral;
- k
- number of data in each data set;
- kh, kv
- horizontal and vertical dimensionless seismic coefficients;
- LL
- liquid limit;
- N
- normal force acts on the base of the slice;
- NRMSE
- normalized root-mean-square error;
- n, m
- fitting parameters of Van Genuchten’s model related to pore size and the pore size distribution;
- O
- ordinary analysis type;
- PI
- plasticity index;
- qbond_ult
- net ultimate bond strength;
- qbond-all
- net allowable bond strength;
- R2
- measure of the goodness of the fittings;
- RMSE
- root-mean-square error;
- S
- surcharge load;
- Se
- effective saturation of the soil;
- Sr
- degree of saturation of the soil;
- U
- unsaturated analysis type;
- UField
- field recorded data;
- UNum
- numerical data;
- Ux
- horizontal displacement;
- u
- pore-water pressure;
- ua, uw
- pore air and pore-water pressures within the soil;
- W
- depth of groundwater table;
- WS
- slice weight;
- α
- constant parameter dependent on plasticity index of the soil to define Eunsat;
- αS
- inclination angle of the slice base;
- αv
- fitting parameter of Van Genuchten’s model depending on air entry/explosion matric suction;
- β, R, x, f, d, ω
- geometric parameters of the sliding wedge;
- χ
- effective stress parameter;
- ϕ′
- effective internal friction angle of soil;
- ϕb
- friction angle of soil related to the matric suction;
- γ
- bulk unit weight of soil;
- ν
- Poisson’s ratio;
- θ
- volumetric water content of soil;
- θr
- residual volumetric water content of the soil;
- θs
- volumetric water content of soil at zero matric suction;
- σ
- total stress;
- σ′
- effective stress;
- σn
- net normal stress;
- τ
- shear strength of soil; and
- ψ
- soil matric suction.
References
Alonso, E. E., J.-M. Pereira, J. Vaunat, and S. Olivella. 2010. “A microstructurally based effective stress for unsaturated soils.” Géotechnique 60 (12): 913–925. https://doi.org/10.1680/geot.8.P.002.
Ashrafi, H. 2006. Principles and basics of excavation and support structures. [In Persian.] Tehran, Iran: Dept. of Housing & Urban Development of Iran.
ASTM. 2016. Standard test method for measurement of soil potential (suction) using filter paper. ASTM D5298-16. West Conshohocken, PA: ASTM.
BHRC (Building and Housing Research Center). 2014. Iranian code of practice for seismic resistant design of buildings: Standard 2800. [In Persian.] 4th ed. Tehran, Iran: BHRC.
BHRC (Building and Housing Research Center). 2016. Iranian code of practice for foundation design. [In Persian.] Article No. 7. 3th ed. Tehran, Iran: BHRC.
Bishop, A. W. 1959. “The principle of effective stress.” Teknisk Ukeblad 39: 859–863.
Bishop, A. W., and G. Blight. 1963. “Some aspects of effective stress in saturated and partly saturated soils.” Géotechnique 13 (3): 177–197. https://doi.org/10.1680/geot.963.13.3.177.
Burrage, R. E., and J. B. Anderson. 2019. “Unsaturated behavior of excavations in residual soil at the Auburn University National Geotechnical Experimentation Site.” AIMS Geosci. 5 (4): 921–939. https://doi.org/10.3934/geosci.2019.4.921.
Finno, R. J., J. T. Blackburn, and J. F. Roboski. 2007. “Three-dimensional effects for supported excavations in clay.” J. Geotech. Geoenviron. Eng. 133 (1): 30–36. https://doi.org/10.1061/(ASCE)1090-0241(2007)133:1(30).
Fredlund, D. G., and H. Rahardjo. 1993. Soil mechanics for unsaturated soils. New York: Wiley.
Garakani, A. A., M. M. Birgani, and H. Sadeghi. 2021a. “An effective stress-based parametric study on the seismic stability of unsaturated slopes with implications for preliminary microzonation.” Bull. Eng. Geol. Environ. 80 (10): 7525–7549. https://doi.org/10.1007/s10064-021-02440-x.
Garakani, A. A., S. M. Haeri, D. Y. Cherati, F. A. Givi, M. K. Tadi, A. H. Hashemi, N. Chiti, and F. Qahremani. 2018. “Effect of road salts on the hydro-mechanical behavior of unsaturated collapsible soils.” Transp. Geotech. 17: 77–90. https://doi.org/10.1016/j.trgeo.2018.09.005.
Garakani, A. A., S. M. Haeri, C. S. Desai, S. M. H. S. Ghafouri, B. Sadollahzadeh, and H. H. Senejani. 2019. “Testing and constitutive modeling of lime-stabilized collapsible loess. II: Modeling and validations.” Int. J. Geomech. 19 (4): 04019007. https://doi.org/10.1061/(ASCE)GM.1943-5622.0001386.
Garakani, A. A., S. M. Haeri, A. Khosravi, and G. Habibagahi. 2015. “Hydro-mechanical behavior of undisturbed collapsible loessial soils under different stress state conditions.” Eng. Geol. 195: 28–41. https://doi.org/10.1016/j.enggeo.2015.05.026.23.
Garakani, A. A., A. Pirjalili, and C. S. Desai. 2021b. “An effective stress-based DSC model for predicting the coefficient of lateral soil pressure in unsaturated soils.” Acta Geotech. 16 (12): 3813–3830. https://doi.org/10.1007/s11440-021-01376-6.
Garakani, A. A., H. Sadeghi, S. Saheb, and A. Lamei. 2020. “Bearing capacity of shallow foundations on unsaturated soils: Analytical approach with 3D numerical simulations and experimental validations.” Int. J. Geomech. 20 (3): 04019181. https://doi.org/10.1061/(ASCE)GM.1943-5622.0001589.
Garakani, A. A., and K. A. Serjoie. 2022. “Ultimate bearing capacity of helical piles as electric transmission tower foundations in unsaturated soils: Analytical, numerical, and experimental investigations.” Int. J. Geomech. 22 (11): 04022194. https://doi.org/10.1061/(ASCE)GM.1943-5622.000258528.
GeoStudio. 2012. Geostudio manual. Calgary, AB, Canada: GEO-SLOPE International.
Ghaffaripour, O., G. A. Esgandani, A. Khoshghalb, and B. Shahbodaghkhan. 2019. “Fully coupled elastoplastic hydro-mechanical analysis of unsaturated porous media using a meshfree method.” Int. J. Numer. Anal. Methods Geomech. 43 (11): 1919–1955. https://doi.org/10.1002/nag.2931.
Gholampour, A., and A. Johari. 2019. “Reliability-based analysis of braced excavation in unsaturated soils considering conditional spatial variability.” Comp. Geotech. 115: 103163. https://doi.org/10.1016/j.compgeo.2019.
Google Maps. 2017. “Mirdamad Boulevard, Tehran.” Satellite image. Accessed March 28, 2023. https://www.google.com.sg/maps/@35.7626346,51.4106594,349m/.
Haeri, S. M., A. A. Garakani, and M. Kamali Zarch. 2021. “Unsaturated 3D column method: New method for evaluation of stability of unsaturated slopes subjected to vertical steady-state infiltration and evaporation.” Int. J. Geomech. 21 (10): 04021177. https://doi.org/10.1061/(ASCE)GM.1943-5622.0002125.
Haeri, S. M., A. A. Garakani, A. Khosravi, and C. L. Meehan. 2014. “Assessing the hydro-mechanical behavior of collapsible soils using a modified triaxial test device.” Geotech. Test. J. 37 (2): 190–204. https://doi.org/110.1520/GTJ20130034.
Haeri, S. M., A. A. Garakani, H. R. Roohparvar, C. S. Desai, S. M. H. S. Ghafouri, and K. S. Kouchesfahani. 2019. “Testing and constitutive modeling of lime-stabilized collapsible loess. I: Experimental investigations.” Int. J. Geomech. 19 (4): 04019006. https://doi.org/10.1061/(ASCE)GM.1943-5622.0001364.
Haeri, S. M., A. Khosravi, A. A. Garakani, and S. Ghazizadeh. 2017. “Effect of soil structure and disturbance on hydromechanical behavior of collapsible loessial soils.” Int. J. Geomech. 17 (1): 04016021. https://doi.org/10.1061/(ASCE)GM.1943-5622.0000656.
Haeri, S. M., S. Soleymani Borujerdi, and A. A. Garakani. 2023. “Effects of initial shear stress on the hydromechanical behavior of collapsible soils.” Acta Geotech. 18 (11): 6051–6076. https://doi.org/10.1007/s11440-023-02034-9.
Huang, A., Y. Zhu, L. Wang, S. Ye, and G. Fang. 2023. “Three-dimensional stability of unsaturated soil slopes reinforced by frame beam anchor plates.” Int. J. Geomech. 23 (6): 04023068. https://doi.org/10.1061/IJGNAI.GMENG-8173.
Hinton, E., and D. R. Owen. 1979. An introduction to finite element computations. Swansea, UK: Pineridge Press.
Jafarzadeh, F., A. A. Garakani, and M. M. Shahrabi. 2019. “Assessing the functionality of different constitutive models in predicting the behavior of deep stabilized excavations.” In Proc., 16th Asian Regional Conf. on Soil Mechanics and Geotechnical Engineering. Pathum Thani, Thailand: The Association of Geotechnical Societies in SouthEast Asia (AGSSEA).
Jafarzadeh, F., and S. Yoosefi. 2014. “Analysis of longitudinal and transverse cracks in crest of doroodzan earth dam and left abutment leakage.” In Vol. 2 of Proc., Int. Symp. on Dams in a Global Environmental Challenges, 352–361. Jakarta, Indonesia: The Indonesian National Committee on Large Dams (INACOLD).
Johari, A., and A. R. Kalantari. 2021. “System reliability analysis of soldier-piled excavation in unsaturated soil by combining random finite element and sequential compounding methods.” Bull. Eng. Geol. Environ. 80 (3): 2485–2507. https://doi.org/10.1007/s10064-020-02022-3.
Khalili, N., and M. Khabbaz. 1998. “A unique relationship for χ for the determination of the shear strength of unsaturated soils.” Géotechnique 48 (5): 681–687. https://doi.org/10.1680/geot.998.48.5.681.
Khalili, N., and S. Zargarbashi. 2010. “Influence of hydraulic hysteresis on effective stress in unsaturated soils.” Géotechnique 60 (9): 729–734. https://doi.org/10.1680/geot.09.T.009.
Khoshghalb, A., and N. Khalili. 2013. “A meshfree method for fully coupled analysis of flow and deformation in unsaturated porous media.” Int. J. Numer. Anal. Methods Geomech. 37 (7): 716–743. https://doi.org/10.1002/nag.1120.
Konrad, J.-M., and M. Lebeau. 2015. “Capillary-based effective stress formulation for predicting shear strength of unsaturated soils.” Can. Geotech. J. 52 (12): 2067–2076. https://doi.org/10.1139/cgj-2014-0300.
Lee, J. 2015. “Analysis and design of temporary slopes and excavation support systems in unsaturated piedmont residual soils.” Ph.D. thesis, Dept. of Civil, Construction, and Environmental Engineering, Univ. of North Carolina.
Lu, N., and W. Likos. 2004. Unsaturated soil mechanics. Hoboken, NJ: Wiley.
Lu, N., and W. J. Likos. 2006. “Suction stress characteristic curve for unsaturated soil.” J. Geotech. Geoenviron. Eng. 132 (2): 131–142. https://doi.org/10.1061/(ASCE)1090-0241(2006)132:2(131)22.
Marcuson, W. 1981. “Moderator's report for session on earth dams and stability of slopes under dynamic loads.” In Vol. 3 of Proc., Int. Conf. on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Rolla, MO: Missouri University of Science and Technology.
Morgenstern, N. R., and V. E. Price. 1965. “The analysis of the stability of general slip surfaces.” Géotechnique 15 (1): 79–93. https://doi.org/10.1680/geot.1965.15.1.79.
Ng, C. W. W., G. Zheng, J. Ni, and C. Zhou. 2020. “Use of unsaturated small-strain soil stiffness to the design of wall deflection and ground movement adjacent to deep excavation.” Comput. Geotech. 119: 103375. https://doi.org/10.1016/j.compgeo.2019.103375.
Oh, W. T., S. K. Vanapalli, S. Qi, and Z. Han. 2016. “Estimation of the variation of matric suction with respect to depth in a vertical unsaturated soil trench associated with rainfall infiltration.” E3S Web Conf. 9: 15003. https://doi.org/10.1051/e3sconf/20160915003.
Ou, C. Y. 2021. Fundamentals of deep excavations. Boca Raton, FL: CRC Press.
Ou, C.-Y., D.-C. Chiou, and T.-S. Wu. 1996. “Three-dimensional finite element analysis of deep excavations.” J. Geotech. Eng. 122 (5): 337–345. https://doi.org/10.1061/(ASCE)0733-9410(1996)122:5(337).
Pardoen, B., S. Levasseur, and F. Collin. 2015. “Using local second gradient model and shear strain localisation to model the excavation damaged zone in unsaturated claystone.” Rock Mech. Rock Eng. 48 (2): 691–714. https://dx.doi.org/610.1007/s00603-00014-00580-00602.
Pereira, J. M., O. Coussy, E. E. Alonso, J. Vaunat, and S. Olivella. 2010. “Is the degree of saturation a good candidate for Bishop’s χ parameter? Unsaturated soils.” In Proc., 5th Int. Conf. on Unsaturated Soils, 913–919. Rotterdam, Netherlands: CRC Press/A.A. Balkema.
Petersen, C. 2012. Stahlbau: Grundlagen der berechnung und baulichen ausbildung von stahlbauten. [In German.] Dordrecht, Netherlands: Springer.
Pirjalili, A., A. A. Garakani, A. Golshani, and A. Mirzaii. 2020. “A suction-controlled ring device to measure the coefficient of lateral soil pressure in unsaturated soils.” Geotech. Test. J. 43 (6): 1379–1396. https://doi.org/10.1520/GTJ20190099.
Puller, M. 2003. Deep excavations: A practical manual. London: Thomas Telford.
Roboski, J. F. 2004. “Three-dimensional performance and analyses of deep excavations.” Ph.D. thesis, Dept. of Civil and Environmental Engineering, Northwestern Univ.
Sabatini, P., D. Pass, and R. Bachus. 1999. Geotechnical engineering circular No. 4: Ground anchors and anchored systems. No. FHWA-IF-99-015. Washington, DC: FHWA.
Saleh, M., and S. K. Vanapalli. 2022. “Analysis of excavation support systems considering the influence of saturated and unsaturated soil conditions.” J. Geotech. Geoenviron. Eng. 148 (6): 04022034. https://doi.org/10.1061/(ASCE)GT.1943-5606.0002788.
Sigari, S. Y., H. Barghemadi, A. A. Garakani, and C. L. Meehan. 2023. “Performance of three-tiered geogrid reinforced soil wall with modular concrete facing block: A case study.” In Proc., 8th World Congress on Civil, Structural, and Environmental Engineering. Orleans, ON: International ASET Inc.
Tsiampousi, A., L. Zdravković, and D. M. Potts. 2013. “Variation with time of the FoS of slopes excavated in unsaturated soils.” Comp. Geotech. 48: 167–178. https://doi.org/110.1016/j.compgeo.2012.1008.1005.
Vanapalli, S., and W. Oh. 2010. “A model for predicting the modulus of elasticity of unsaturated soils using the soil-water characteristic curve.” Int. J. Geotech. Eng. 4 (4): 425–433. https://doi.org/410.3328/IJGE.2010.3304.3304.3425-3433.
Van Genuchten, M. T. 1980. “A closed-form equation for predicting the hydraulic conductivity of unsaturated soils.” Soil Sci. Soc. Am. J. 44: 892–898. https://doi.org/810.2136/sssaj1980.03615995004400050002x.
Yanamandra, G., and W. T. Oh. 2021. “Influence of excavation time, tension crack, and rainfall on the stability of unsupported vertical cuts in unsaturated soil.” Int. J. Geomech. 21 (10): 04021199. https://doi.org/10.1061/(ASCE)GM.1943-5622.0002160.
Yates, K., and A. R. Russell. 2023. “The unsaturated characteristics of natural loess in slopes, New Zealand.” Géotechnique 73: 871–884. https://doi.org/10.1680/jgeot.21.00042.
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Published In
International Journal of Geomechanics
Volume 24 • Issue 7 • July 2024
Copyright
© 2024 American Society of Civil Engineers.
History
Received: Mar 28, 2023
Accepted: Jan 16, 2024
Published online: May 6, 2024
Published in print: Jul 1, 2024
Discussion open until: Oct 6, 2024
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