Evaluation of the Thin Layer Effect on the Ultimate Bearing Capacity of Strip Foundation on Sand

Shallow foundations such as strip foundations are widely used in transmitting loads from the superstructure to the supporting soils. In many cases, the ground materials are not uniform and may have thin layers, which are not usually detected in geotechnical site investigations. In this research, the effects of a thin layer on the ultimate bearing capacity of a strip foundation on the sand bed are investigated by small-scale physical models. Due to very limited research that has been carried out on the thin layer effect on the ultimate bearing capacity, it seems that further studies can understand the effect of this layer. The investigations were carried out by varying the material type, thickness, and depth of the thin layer. The results indicate that the weak thin layer decreases both the ultimate bearing capacity and stiffness of the soil-foundation system and the strong thin layer increases both the ultimate bearing capacity and the soil-foundation system stiffness. The amount of this effect depends on the thickness, depth of deposition, and material type of the thin layer. According to the results, the weak layer for the critical depth of 1.2B led to the most reduction in ultimate bearing capacity by 40%, while no effects were observed at the depth of 3.6B. The strong layer is also for the state where this layer is just below the footing, had the highest increase in ultimate bearing capacity by 76%, but at a depth of about 2.4B, it was ineffective.