Commenced in January 2007
Frequency: Monthly
Edition: International
Paper Count: 32586
Design Charts for Strip Footing on Untreated and Cement Treated Sand Mat over Underlying Natural Soft Clay

Authors: Sharifullah Ahmed, Sarwar Jahan Md. Yasin


Shallow foundations on unimproved soft natural soils can undergo a high consolidation and secondary settlement. For low and medium rise building projects on such soil condition, pile foundation may not be cost effective. In such cases an alternative to pile foundations may be shallow strip footings placed on a double layered improved soil system soil. The upper layer of this system is untreated or cement treated compacted sand and underlying layer is natural soft clay. This system will reduce the settlement to an allowable limit. The current research has been conducted with the settlement of a rigid plane-strain strip footing of 2.5 m width placed on the surface of a soil consisting of an untreated or cement treated sand layer overlying a bed of homogeneous soft clay. The settlement of the mentioned shallow foundation has been studied considering both cases with the thicknesses of the sand layer are 0.3 to 0.9 times the width of footing. The response of the clay layer is assumed as undrained for plastic loading stages and drained during consolidation stages. The response of the sand layer is drained during all loading stages. FEM analysis was done using PLAXIS 2D Version 8.0. A natural clay deposit of 15 m thickness and 18 m width has been modeled using Hardening Soil Model, Soft Soil Model, Soft Soil Creep Model, and upper improvement layer has been modeled using only Hardening Soil Model. The groundwater level is at the top level of the clay deposit that made the system fully saturated. Parametric study has been conducted to determine the effect of thickness, density, cementation of the sand mat and density, shear strength of the soft clay layer on the settlement of strip foundation under the uniformly distributed vertical load of varying value. A set of the chart has been established for designing shallow strip footing on the sand mat over thick, soft clay deposit through obtaining the particular thickness of sand mat for particular subsoil parameter to ensure no punching shear failure and no settlement beyond allowable level. Design guideline in the form of non-dimensional charts has been developed for footing pressure equivalent to medium-rise residential or commercial building foundation with strip footing on soft inorganic Normally Consolidated (NC) soil of Bangladesh having void ratio from 1.0 to 1.45.

Keywords: Design charts, ground improvement, PLAXIS 2D, primary and secondary settlement, sand Mat, soft clay.

Procedia APA BibTeX Chicago EndNote Harvard JSON MLA RIS XML ISO 690 PDF Downloads 558


[1] Hanna, A. M. and Meyerhof, G. G. (1980), ‘Design charts for ultimate bearing capacity for sands overlying clays’, Canadian Geotech. J., 17(2).
[2] Thomé, A., Donato, M., Consoli, N. C. and Graham, J. (2005), ‘Circular footings on a cemented layer above weak foundation soil’, Can. Geotech. J. 42: 1569–1584, NRC Canada.
[3] Serajuddin, M. and Chowdhury, M. A. (1967), ‘Studies on Engineering Properties of East Pakishsan Soil’, Proc, First Southest Asian Regional Conference on Soil Engineering, Bangkok Thailand, pp. 9-12.
[4] Serajuddin, M. (1998), ‘Some Geotechnical studies on Bangladesh soils: A Summary of papers between 1957-96’, Journal of civil engineering, The institution of engineers, Bangladesh, Vol. CE 26, No-2, 1998.
[5] Serajuddin, M. and Ahmed, A. (1998),‘Correlation between SPT and Unconfined Compressive Strength of Cohesive Soil Deposit of Bangladesh’, Journal of CE, IEB, Vol. CE 24, no 1, 1996, pp. 69-81.
[6] Yasin, S. J. M. and Shafiullah, A. M. M. (2003), ‘Effect of Particle Characteristics on the Strength and Volume Change Behaviour of Sand’, Journal of Civil Engineering, The Institution of Engineers, Bangladesh, Vol. CE 31, No. 2, 2003.
[7] Brinkgreve, R. B. J. and Broere, W. (2002), ‘PLAXIS 2D Version 8–Manual’, Delft University of Technology & PLAXIS b. v., The Netherlands, A. A. Balkema Publishers.
[8] K. K. Sorensen & N. Okkels (2013), ‘Correlation between Drained Shear Strength and Plasticity Index of Undisturbed Overconsolidated Clays’, Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013.
[9] Mitchell, J. K. (1976), ‘The properties of cement-stabilized soils. Proceeding of Residential’, Workshop on Materials and Methods For Low Cost Road, Rail, and Reclamation Works, Australia: 365–404.
[10] Waterman, D. and Broere, W. (2004), ‘Application of the SSC model - part I, II & III’, Delft University of Technology/Plaxis BV.
[11] BNBC(2017), ‘Soils and Foundations’, Bangladesh National Building Code (BNBC), Housing and Building Research Institute– HBRI, Dhaka, Bangladesh, Part-6, Chapter 3, pp. 6-143-211.