Introduction
The bearing capacity of soil is one of the most critical factors in foundation design, particularly where sand overlies clay. While dense sand offers relatively high load-bearing strength and effective drainage, underlying clay soils are often weaker, compressible, and more prone to long-term settlement. This layered interaction poses challenges for shallow foundations, where the apparent strength of the sand may mask risks of failure or excessive movement driven by the clay beneath.
Traditional design approaches have relied on simplified empirical methods, supported by field testing such as the standard penetration test (SPT) and cone penetration test (CPT), along with safety factors to establish an allowable bearing pressure. However, these methods often overlook the complex three-dimensional behaviour of layered soils.
Recognising this gap, Douglas Partners’ engineers Sean Goodall and Richard Merifield explored the problem in depth through advanced finite element limit analysis. Their study offers new insights into the three-dimensional bearing capacity of square and rectangular footings on sand over clay, providing practical design tools for industry.
Bearing Capacity of Sand
Sand generally provides a moderate to high bearing capacity, making it one of the most reliable foundation materials; however, loose sand or loose soils can have significantly lower capacity and higher settlement risk compared to medium dense sand or dense sand. Well-compacted sand can support loads of around >600 kPa, depending on factors like relative density, particle size, and footing depth. Its excellent drainage properties reduce the risk of water-related settlement, which is why sand is often preferred for shallow foundations. However, loose or poorly compacted sand can pose stability issues, particularly under heavy or dynamic loads, as it tends to shift or densify when subjected to vibrations. Engineers must ensure proper compaction and account for factors such as groundwater level, which can reduce the effective stress and, consequently, the bearing capacity
Bearing Capacity of Clay
Clay, on the other hand, presents a far greater challenge due to its low shear strength and high compressibility. The bearing capacity of clay typically ranges from 50 to 100 kPa, significantly lower than that of sand. Its performance is highly sensitive to moisture content, making it prone to swelling during wet conditions and shrinking when dry, which can lead to differential settlement and long-term foundation movement. Under undrained conditions, clay exhibits low short-term strength, which is critical during construction when rapid loading occurs. In layered profiles where clay underlies sand, the initial stability offered by sand may be deceptive, as the underlying clay can govern long-term settlement and ultimate stability, requiring careful analysis and design adaptation.
Insights from Bearing Capacity Predictions of Sand over Clay
The analysis of sand-over-clay foundations using limit equilibrium methods reveals that layered soils behave differently from uniform profiles. The study shows that the bearing capacity increases significantly as the thickness of the sand layer grows, but this improvement is non-linear and influenced by the clay’s undrained shear strength. Thin sand layers provide only marginal improvement, and failure surfaces often extend into the weak clay, reducing overall stability. When the sand layer is sufficiently thick, failure tends to localize within the sand, resulting in higher capacity closer to that of a pure sand deposit. However, the clay’s compressibility still governs settlement behavior, meaning even with a strong sand cover, excessive deformation can occur if clay is soft or highly plastic. These insights underline the importance of considering both bearing capacity and settlement in design, not just increasing sand thickness.
Three-dimensional Bearing Capacity of Sand over Clay
The assessment of bearing capacity in shallow foundations on layered soils has traditionally relied upon empirical models, often assuming the presence of a strip footing.
In their recent research, published in the Australian Geomechanics Society journal, engineers Sean Goodall and Richard Merifield have delved into a comprehensive exploration of the three-dimensional bearing capacity. Specifically, their study examines square and rectangular footings situated on a substrate of sand overlying clay using finite element limit analysis.
Overview of models and problem definitions
The objective was to investigate the three-dimensional bearing capacity of square and rectangular footings resting on sand overlying clay using finite element limit analysis. The problem of a footing resting on sand overlying clay is a rather classical and commonly encountered geotechnical problem, for example, a working platform that is used to support a crane or piling rig, which historically have been assessed using empirical methods that require many simplifying assumptions, and as a consequence have some limitations in practice.
Overview of models and problem definitions
Finite element limit analysis is a particularly advanced method of analysis that can overcome many of the simplifying assumptions and limitations of other methods. One significant benefit, compared to many other methods, is that the finite element limit analysis provides rigorous bounds of the ultimate bearing capacity. There have been a few studies into the sand over clay problem using finite element limit analysis but what makes our study particularly special is the three-dimensional considerations. To the best of our knowledge this appears to be the first study of this scope and size that has applied three-dimensional finite element limit analyses to the sand over clay problem.
Selected results from benchmarking the solutions and analysis
A benchmarking exercise was performed, which indicated the results of our study compared well with published experimental data and the results of other studies. A parametric study was also performed on a number of variables to provide readers with an understanding of the effect of various assumptions, boundary conditions and parameters.
Selected results from benchmarking the solutions and analysis
The results of the modelling have been used to develop dimensionless design charts for a range of commonly encountered geotechnical conditions, working platforms and more generally footings resting on sand overlying clay. It is hoped that the result of this study assists with both safer and more sustainable working platform designs in industry.
Result from the design chart – Limit analysis
Selected results from the analysis showing failure mechanism
Conclusion
When designing shallow foundations on sand over clay, engineers must account for both the apparent strength of sand and the long-term settlement risks associated with clay. While well-compacted sand may provide high allowable bearing capacity, the weaker clay often governs settlement and potential shear failure under load.
By combining traditional field testing methods with advanced three-dimensional analysis, engineers can achieve more accurate predictions of ultimate bearing capacity and better manage the balance between safety and efficiency. The work by Goodall and Merifield demonstrates how applying rigorous finite element modelling can move practice beyond empirical rules of thumb, leading to safer, more sustainable foundation and working platform designs across Australia.
Sean Goodall and Richard Merifield.