# Bearing Capacity of Ring Foundations on Sand Overlying Clay
## 1 Introduction  
The bearing capacity of shallow foundations resting on homogeneous soil deposits has been investigated by [PERSON] [1], [PERSON] [2], [PERSON] [3], and [PERSON] [4]. In reality, however, naturally formed soil profiles beneath foundations are not homogeneous and are usually deposited in layers. The bearing capacity of footings placed on layered sand-clay media is an important consideration that arises frequently in foundation engineering [5]. Several studies have been conducted to investigate this problem. [PERSON] and [PERSON] [6] proposed a load spread model in which the sand layer merely spreads the vertical load to the clayey stratum, and the bearing capacity of strip footing was determined. [PERSON] [7] and [PERSON] and [PERSON] [8] used the limit equilibrium approach to estimate the bearing capacity of footings on sand overlying clay. They assumed that a punching shear failure occurs in the sand layer. [PERSON] et al. [9] proposed a limit-equilibrium-based model for calculating the bearing capacity of a sand layer on clay. The sand layer is assumed to be in a state of passive failure, and the shear force acting on the sides of the sand block is taken into consideration. [PERSON] et al. [10] developed a simplified conceptual model for predicting the bearing capacity of circular foundations placed on sand overlying clay soils. Expect for the theoretical solutions mentioned above, extensive studies have also been performed on the subject, by using upper bound limit analysis [11; 12; 13], finite element method [14; 15; 16], finite element limit analysis (FELA) [5; 17], and model tests [18; 19; 20]. As summarized, previous studies have focused on the bearing capacity of strip footings, circular footings, and spudcan footings. More recently, studies have been conducted to investigate the bearing capacity of ring foundations, which are widely used to support offshore and onshore axisymmetrical structures (e.g., water towers, storage tanks, annular platforms, transmission towers, silos, and chimneys). [PERSON] et al. [21] performed model tests to investigate the behavior of ring footings in sand. [PERSON] and [PERSON] [22] presented results of laboratory tests for ring foundations on sand. [PERSON] and [PERSON] [23] used the stress characteristics method to compute the bearing capacity factor \(N_{y}\) for ring foundations on sand. By using the same approach, [PERSON] and [PERSON] [24]determined the bearing capacity factors \(N_{c}\), \(N_{q}\), and \(N_{y}\) for both smooth and rough ring footings. On the basis of the numerical code FLAC, [PERSON] and [PERSON] [25] calculated the bearing capacity factor \(N_{y}\) for ring foundations on cohesionless soil. [PERSON] et al. [26] also applied FLAC to estimate \(N_{y}\) for both smooth and rough ring footings, and the effect of the dilation angle was considered. [PERSON] et al. [27] evaluated the bearing capacity and settlement of ring footings by using the finite element code Plaxis. In addition, the bearing capacity of ring footings on reinforced sand has also been investigated experimentally and numerically [28; 29]. Recently, [PERSON] and [PERSON] [30] obtained the bearing capacity factors for ring foundations by using FELA. Under undrained condition, the undrained bearing capacity factor \(N_{c}\) for ring footings was calculated by using FLAC [31] and Plaxis packages [32; 33; 34]. By using the finite difference code FLAC3D, the influence of eccentric load on the bearing capacity of ring foundations was studied by [PERSON] and [PERSON] [35]. A large number of studies, as reviewed above, have concentrated on the bearing capacity of ring foundations on many kinds of soil, yet particular attention has not been given to the complicated issue for sand overlying clay caused by interlayer condition. In this study, a series of numerical simulations using the FELA software OptumG2 [36] are applied to calculate upper and lower bounds on the bearing capacity for ring footings on sand overlying clay.