An attempt of using a T-bar to predict spudcan penetration resistance in clay has previously been proved feasible. This methodology enables the effects associated with the spudcan large penetration on its resistance to be accounted for. To extend the application of miniature penetrometer to assess spudcan penetration in two-layered soil, additional effects such as normalised spudcan level to the soil interface and soil wedge trapped underneath the spudcan base require in-depth investigation. Based on some preliminary results of spudcan penetration tests performed in different layered soil profiles, this paper suggests a means to incorporate these effects into the design of spudcan bearing capacity.
The design of spudcan foundation is usually considered analogous to that of the conventional preembedded shallow foundation despite its large diameter, conical base geometry and large penetration mechanism. In recent years, research investigating the size effect1 and conical base effect 2,3,4 on the spudcan bearing resistance have been carried out with the aim to enhance the confidence level of using existing theoretical solutions. Nonetheless, the acknowledgement of the continuous change in spudcan foundation failure mechanism with penetration5, 6 unfolds the inherent limitations underlying these solutions, which were established based on a single mode of failure mechanism. Since they share similar penetration mechanism, this leads to the idea of utilising the readings of miniature penetrometers ? such as T-bar, ball and cone ? to shed lights on the spudcan penetration behaviour. To adopt this alternative approach, the understanding of the performance of spudcan and miniature penetrometer in different types of soil profiles and how they differ from each other is to be established.
The concept of using miniature penetrometer to predict spudcan bearing capacity combines the knowledge developed in site investigation and foundation design. This concept has been implemented in pile design and the understanding of the concept has been considerably well developed7, 8. Owing to the very large dimension and uncommon base geometry of the spudcan, compared to those of site investigation penetrometers, more research is needed to apply this concept to spudcan design.
In clay, the use of T-bar is becoming commonly used due to its simplicity in data interpretation and advancement in quantifying the clay properties. Owing to the differences in penetration rate, dimension and geometry, the variations that could possibly exist between T-bar and spudcan penetration resistances are mainly due to the effects of drainage and strain rate on clay characteristic, as well as soil wedge trapped underneath the spudcan9. On the other hand, piezocone and ball are deemed more suitable to penetrate through sand due to structural compatibility. Considering the huge dimension of spudcan and its limited penetration in sand, the stress level induced strength variation can vary the piezocone and spudcan resistance profiles developed in the sand to a considerably large extent.
To extend the penetrometer based design approach to spudcan foundation in layered soil, additional aspects are to be considered, which create more challenges to the whole process. Based on limited studies performed using miniature cone penetrometer10, 11 and spudcan12, 13