Bearing Capacity of Shallow Foundations in Homogeneous and Layered Soils under Varying Ground Water Table Conditions by using PLAXIS 2D

Abstract

This present study focuses on the numerical analysis of the UBC of shallow foundations subjected to different soil layers and GWT case. A finite element-based analysis was examined by PLAXIS 2D to study the response of foundations placed on both homogeneous and layered soil profiles. The investigation considered foundation width, soil-layer thickness, clay cohesion, friction angle of sand, and groundwater table depth as the principal variables. Homogeneous sand and clay profiles were first analyzed by considering different groundwater levels and foundation widths of 1 m and 2 m. The influence of clay strength was examined by adopting different cohesion values. In addition, layered profiles comprising sand over clay and clay over sand were modeled with different layer thicknesses and groundwater conditions to evaluate the effect of soil stratification on foundation bearing capacity. The numerical results show that changes in groundwater position, foundation width, soil strength, and layer arrangement can significantly alter the UBC. The interaction between the upper and underlying soil layers also plays a significant role in the bearing response of layered foundations. This study demonstrates the usefulness of finite element analysis for evaluating shallow foundation behavior under complex soil and groundwater conditions and provides a systematic understanding of the parameters governing ultimate bearing capacity.

Introduction

The text presents a numerical study of the ultimate bearing capacity (UBC) of shallow strip foundations under different soil and groundwater conditions using the finite element method (FEM) in PLAXIS 2D.

Background

Shallow foundations transfer structural loads to the soil beneath them. Their bearing capacity depends on several factors, including:

• Soil strength and type

• Foundation width

• Soil layering

• Groundwater-table (GWT) position

• Thickness and properties of individual soil layers

Real soil deposits are often heterogeneous, containing different layers with different engineering properties. Groundwater also affects soil effective stress and strength, which can significantly influence foundation bearing capacity.

The study therefore investigates both homogeneous and layered soil profiles under different groundwater conditions.

Objectives

The main objectives are to:

1. Study the bearing capacity of strip foundations on homogeneous and layered soils using PLAXIS 2D.

2. Examine how groundwater-table depth affects UBC.

3. Investigate the influence of foundation width using 1 m and 2 m strip footings.

4. Study the effect of clay cohesion on bearing capacity.

5. Examine how the thickness of the upper layer affects foundations on layered soils.

6. Compare the effects of soil type, layering, footing width, and groundwater depth on UBC.

Literature review

Previous research has shown that the bearing capacity of foundations on layered soils is strongly influenced by upper-layer thickness, soil strength, footing width, and soil configuration.

Studies of sand-over-clay and clay-over-sand systems have demonstrated that numerical methods are useful for understanding complex stress distributions and failure mechanisms. Previous research has also established that groundwater changes effective stresses and soil unit weight, making groundwater an important factor in foundation design.

The literature supports the use of finite-element modelling for studying shallow foundations where conventional simplified bearing-capacity equations may not fully capture the effects of soil layering and groundwater.

Numerical modelling

The study uses PLAXIS 2D with a 2D plane-strain representation of a continuous strip footing.

The numerical model includes:

• Soil domain: 20 m wide and 10 m deep

• Foundation widths: 1 m and 2 m

• Soil profiles:

  • Homogeneous sand

  • Homogeneous clay

  • Sand over clay

  • Clay over sand

• Clay cohesion values: 25, 40, and 80 kPa

• Different upper-layer thicknesses for layered soils

• Different groundwater-table depths, including groundwater at ground level and deeper locations

• A reference condition with no groundwater

The Mohr–Coulomb model is used to represent soil behaviour. Sand is assigned a friction angle of 35°, while clay is modelled with zero friction angle and different cohesion values.

The strip foundation is represented using a plate element, and progressively increasing line loads are applied to obtain the relationship between foundation load and settlement.

Key results presented

The section provided focuses particularly on homogeneous clay.

The results show a clear relationship between clay cohesion and ultimate bearing capacity. Higher cohesion produces substantially higher UBC.

For a groundwater table at ground level:

Thus, increasing cohesion from 25 to 80 kPa produces a large increase in the calculated bearing capacity.

The results also indicate that lowering the groundwater table generally increases UBC. This is because a deeper groundwater table reduces its influence on the soil stresses around the foundation. However, once the groundwater is sufficiently deep, further lowering produces relatively small changes in bearing capacity.

Conclusion

Based on the numerical analyses performed using PLAXIS 2D and the analytical calculations for homogeneous and hetrogeneous soil profiles under different GWT conditions, the conclusions are presented below: 1) The UBC of homogeneous clay increases with increasing groundwater depth and cohesion. For clay with C =40 kPa, the bearing capacity increased from 284.03 to 295.23 kPa for a 1 m footing and from 305.54 to 328.34 kPa for a 2 m footing as the GWT was lowered from ground level to 6m below ground level. 2) The presence of groundwater has a more pronounced effect on the bearing capacity of sand than on clay. For a 1 m footing, the PLAXIS 2D bearing capacity increased from 169.4 kPa at groundwater level at ground surface to 379.4 kPa without groundwater, while for a 2 m footing it increased from 313.6 to 684.25 kPa. 3) An increase in footing width from 1 m to 2 m resulted in higher UBC for both homogeneous and layered soil profiles. The magnitude of increase, however, varied with soil type, layer configuration, and groundwater condition. 4) For sand over clay, the UBC was strongly influenced by the thickness of the topmost sand layer. For the investigated case, the PLAXIS 2D values for sand thicknesses of 0.5, 1.0, 1.5 and 2.0 m were 141.8, 152.6, 155.36 and 159.95 kPa for GWT at GL case respectively, indicating that the variation in bearing capacity with increases in sand layer thickness. 5) For clay overlying sand, the UBC was governed by the thickness and strength of the top clay stratum together with the contribution of the underlying sand layer. The results indicate that the bearing capacity response of a layered foundation cannot be evaluated solely from the properties of the uppermost soil layer. 6) The comparison of PLAXIS 2D and IS 6403 for homogeneous soils showed comparable trends with respect to groundwater depth and footing width. For groundwater at ground surface, the bearing capacities for the 1 m footing were 169.4 kPa and 212.07 kPa, and for the 2 m footing were 313.6 kPa and 424.16 kPa, from PLAXIS 2D and IS 6403, respectively. 7) The comparison of PLAXIS 2D with the Meyerhof–Hanna method for layered soils showed generally similar trends with changes in layer thickness, footing width, and groundwater condition, although differences in the magnitude of UBC were observed due to the assumptions and simplifications adopted in the analytical formulation. 8) The study demonstrates that groundwater depth, soil strength, footing width, and layer thickness and arrangement of soil profile are the major parameters influencing the ultimate bearing capacity of shallow foundations. The results also indicate that the use of numerical analysis along with analytical methods provides a useful basis for assessing foundation behaviour in both homogeneous and layered soil conditions.

References

[1] G. G. Meyerhof and A. M. Hanna, “Ultimate bearing capacity of foundations on layered soils under inclined load,” Canadian Geotechnical Journal, vol. 15, no. 4, pp. 565–572, 1978. [2] Bureau of Indian Standards, IS 6403:1981: Code of Practice for Determination of Bearing Capacity of Shallow Foundations. New Delhi, India: BIS, 1981. [3] J. E. Bowles, Foundation Analysis and Design, 5th ed. New York, NY, USA: McGraw-Hill, 1996. [4] B. M. Das and N. Sivakugan, Principles of Foundation Engineering, 9th ed. Boston, MA, USA: Cengage Learning, 2018. [5] S. A. H. Mandeel, H. M. Mekkiyah, and A. F. I. Al-Ameri, “Estimate the bearing capacity of full-scale model shallow foundations on layered soil using PLAXIS,” Solid State Technology, 2020. [6] R. B. J. Brinkgreve, S. Kumarswamy, and W. M. Swolfs, PLAXIS 2D Reference Manual. Delft, The Netherlands: PLAXIS [7] G. G. Meyerhof, “Some recent research on the bearing capacity of foundations,” Canadian Geotechnical Journal, vol. 1, no. 1, pp. 16–26, 1963 [8] Meyerhof, G. G. (1955). “Influence of Roughness of Base and Ground-Water Conditions on the Ultimate Bearing Capacity of Foundations.” Géotechnique, 5, 227–242. [9] G. G. Meyerhof, “Ultimate bearing capacity of footings on sand layer overlying clay,” Canadian Geotechnical Journal, vol. 11, no. 2, pp. 223–229, 1974. [10] M. Hanna and G. G. Meyerhof, “Design Charts for Ultimate Bearing Capacity of Foundations on Sand Overlying Soft Clay,” Canadian Geotechnical Journal, vol. 17, no. 2, pp. 300–303, 1980.

Copyright

Copyright © 2026 Kuchimanchi SVRSRSV Subbalakshmi, Dr. K. Ramu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.