Effect of matric suction on pile behavior in unsaturated soils by physical modeling in FCV

Document Type : Original Research

Authors
M. mortezaee 1
S. A. naeini 2
A. eslami 3
1 Ph.D. Student, Civil-geotechnical Eng. Dept., Imam Khomeini International University
2 Prof., Civil-geotechnical Eng. Dept., Imam Khomeini International University
3 Prof., Civil and Environmental Eng. Dept., Amirkabir University of Technology
DOI: 10.22034/22.3.83
Abstract
Deep foundations are often used in a typical engineering project to transfer loads from heavy superstructures (bridges, highways, embankments and high-rise buildings) to the subsoil safely without subsidence or instability problems. In order to achieve an accurate and economical design in each of these projects, we must have a good understanding of how deep foundations behave, which can be done through analytical methods, numerical methods and experiments or experimental observations. Since a series of simplifications are usually performed in numerical and analytical methods, the use of experimental methods is selected to achieve a more desirable result. Experimental methods can be done by examining real models on site or performing experiments on a small-scale model in the laboratory (physical modeling), which due to lower cost, simplicity and reproducibility, usually physical modeling compared to Real on-site models are preferred. For physical modeling, various devices such as Simple Chamber (1 g), Calibration Chamber (CC) and centrifuge device can be used. The limitations and difficulties of these devices led to the recent introduction of a device called the Frustum Confining Vessel (FCV) for the physical modeling of deep foundations. In this device, which is in the form of an incomplete cone, by applying pressure to the bottom of the device, a linear stress gradient can be created along the central axis to be consistent with what is actually happening. Despite the many advantages that this device , it does not have a system for measuring the matric suction in unsaturated soils. Therefore, in this article, by embedding precision instruments in the sample made of this device in Amirkabir University of Technology (FCV-AUT) and implementing and loading of a pile in the soil inside it, It is possible to study the behavior of piles in unsaturated soil and the effect of moisture and matric suction on the load-displacement curve. The pile used in this research is an open-end pile which was implemented by driving method in Firoozkooh silty sand with gravimetric moisture contents of 0, 5, 10, 15 and 20% and in each case a load-displacement curve was drawn. The floor pressure applied in this research is equal to 100 kPa, Which was determined by placing several stress sensors at different soil depths and measuring the stress changes in depth. The following results were obtained after experiments:

In the tested soil, up to 5% moisture content, no significant lateral friction is observed between the pile and the soil, and the end resistance, which is the result of plugging the soil inside the pile, provides most of the bearing capacity of the pile.
From 5 to 15% moisture content, the lateral friction between the pile and the soil is gradually mobilized and while creating an initial jump in the curve of load-displacement, it increases the bearing capacity of the pile exponentially. But from 15% moisture content onwards, the initial slope decreases again due to the reduction of lateral friction.
Considering the final bearing capacity of the pile as the load corresponding to 10% of the displacement of the pile head, the effect of matric suction on the changes of final bearing capacity was investigated. It was found that By increasing the amount of matric suction, first the final bearing capacity of the pile increases and after reaching a certain value, it decreases. The cause of this phenomenon is changes in wet soil behavior in different areas of the soil-water characteristic curve (SWCC) and is affected by various factors such as grain size, initial moisture content and soil density percentage.

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[1]Poulos HG, “Pile behaviour-theory and application”, Géotechnique, vol. 39, Sept. 1989, pp. 365-413.
[2] Oloo, S.Y., Fredlund, D.G. and Gan J.K-M. (1997). “Bearing capacity of unpaved roads”. Canadian Geotechnical Journal. 34(3): 398-407.Olson, R.E. and Denis, N.D. 1982
[3] Mohamed, F.M.O. and Vanapalli, S.K. (2006). “Laboratory investigation for the measurement of the bearing capacity of unsaturated soils”. In Proceedings of 59th Canadian Geotechnical Con. Vancouver, BC. pp. 1-4.
[4] Oh, W.T. and Vanapalli, S.K. (2009). “A simple method to estimate the bearing capacity of unsaturated fine-grained soils”. In Proceedings of 62nd Canadian Geotechnical Con. Halifax, pp. 234-241
[5]Vanapalli, S.K. and Taylan, Z.N. 2011. Estimation of the shaft capacity of model piles ina compacted fine-grained unsaturated soil. In Proceedings of 14th Pan-Am Con. On Soil Mechanics and Geotechnical Engineering and 64th Canadian Geotechnical Conf.
[6]Vanapalli, S.K. and Taylan, Z.N. 2012. Design of single piles using the mechanics of unsaturated soils. International Journal of GEOMATE, 2(1): 197-204.
[7]Vanapalli SK, Eigenbrod KD, Taylan ZN, Catana C,Oh WT, Garven E, “A technique for estimating the shaft resistance of test piles in unsaturated soils,” in Proc. 5th Int. Conf. on UNSAT, 2010, pp. 1209-1216.
[8]Hokmabadi, A., Fatahi, B., and Samali, B. (2015). “Physical modeling of seismic soil-pile-structure interaction for buildings on soft soils.” International Journal of Geomechanics., Vol. 15, Issue 2
[9] McCabe, B., and Sheil, B. (2015). “Pile group settlement estimation: Suitability of nonlinear interaction factors.” International Journal of Geomechanics.,Vol. 15, Issue 3
[10] Zhuang, Y., and Cui, X. (2015). “Case studies of reinforced piled highspeed railway embankment over soft soils.” International Journal of Geomechanics., Vol. 16, Issue 2
[11] Kog, Y. (2015). “Axially loaded piles in consolidating layered soil.” International Journal of Geomechanics., Vol. 16, Issue 1
[12] Eslami, A., Veiskarami, M., and Eslami, M. M. (2012). “Study on optimized piled-raft foundations (PRF) performance with connected and non-connected piles-three case histories.” Int. J. Civil. Eng., 10(2),100–111.
[13] Zare, M. and Eslami, A. “Study of deep foundations performance by Frustum Conning Vessel (FCV)”,International Journal of Civil Engineering (IJCE),12(4), pp. 271-280, Tehran, Iran (2014).
[14] Horvath, R.G. & Stolle, D. (1996). “ Frustum For Testing Model Piles,” Canadian Geothechnical Journal, Vol. 33, No.3, June, pp. 499-504.
[15]Sedran, G. (1999). “Experimental and Analytical Study of a Frustum Confining Vessel” .Doctoral Thesis, McMaster University
[16]Mullins, G., Dapp, S., Fredrerick, E., and Wagner, R. (2001) “Pressure Grouting Drilled Shaft Tips Phase I Final Report”, Final Report Submitted Florida Department of Transportation, Florida.
[17] Karimi, A., Eslami, A., Zarrabi, M., Khazaei, J. (2017). “Study of pile behavior by improvement of confining soils using frustum confining vessel”, Scientia Iranica, 24(4), pp. 1874-1882.
[18] Zare, M. A. Eslami, A. Abrang, and A. Asadi. 2013. “Frustum confining vessel (FCV) for investigation of deep foundation performance”. Proceeding of 38th Annual International Conference on Deep Foundation, Phoenix, AZ, USA, 545–553.
[19] Amirhossein Mohammadi, Taghi Ebadi, Mohammad Reza Boroomand(2020). “Physical Modelling of Axial Compressive Bearing Capacity of Instrumented Piles in Oil-Contaminated Sandy Soil”. Iranian Journal of Science and Technology, Transactions of Civil Engineering 44:2, pages 695-714.
[20] ASTM D422. 1999. Standard test method for particle size analysis of soils. Annual Books of ASTM Standards, West Conshohocken, PA.
[21] ASTM D854. 1999. Standard test method for specific gravity of soils. Annual Books of ASTM Standards, West Conshohocken, PA.
[22] ASTM D4253. 2006. Standard test methods for maximum index density and unit weight of soils using a vibratory table. Annual Books of ASTM Standards, West Conshohocken, PA.
[23] ASTM D4254. 2006. Standard test methods for minimum index density and unit weight of soils and calculation of relative density. Annual Books of ASTM Standards, West Conshohocken, PA.
[24] ASTM D5298-16, Standard Test Method for Measurement of Soil Potential (Suction) Using Filter Paper, ASTM International, West Conshohocken, PA, 2016,
[25] Fredlund DG, Xing A(1994). “Equations for the soil-water characteristic curve”, Can Geotech J;31:533–46
[26] ASTM D1143. 2007. Standard test methods for deep foundations under static axial compressive load. Annual Books of ASTM Standards, West Conshohocken, PA
[27] Das BM (2016) Principles of Foundation Engineering, 8th edn. PWS Publishing Company, Boston, MA, USA
[28] Lu, N., Wu, B. and Tan, C.P. (2007) ‘’Tensile strength characteristics of unsaturated sands’’, J. Geotech. Geoenviron. Eng., 133, 144-154.
Modares Civil Engineering journal
Volume 22, Issue 3
May and June 2022
Pages 83-96