# Study on the Influence of Groundwater Variation on the Bearing Capacity of Sandy Shallow Foundation
## 1 Introduction  
Buildings in cities along rivers or coastal lines are often accompanied by the problem of foundation submergence during design, construction and use [1, 2]. Rivers or oceans can raise the water table of coastal foundations to a certain extent. The engineering properties of naturally deposited coarse-grained soils (such as gravel layer, pebble, gravelly soil and medium coarse-grained sand) have little change in their natural or saturated state. Fine-grained soils (such as fine sand, silt and clay) are quite different. Their mechanical properties will change if saturated, and the shear strength will be reduced to different degrees, and this is not conducive to the safety and stability of the foundation [3, 4, 5, 6]. At present, the engineering community usually regards \"capillary rise height plus foundation burial depth\" as the standard of the critical depth of submergence of buildings without fully considering the rise of the water table on the physical and mechanical properties of the foundation soil layer [7]. The evaluation of building submergence according to this criterion is imperfect, and sometimes there are certain risks. Therefore, it is necessary to conduct an in-depth study on the influence of the foundation's bearing capacity brought by the water-level change [8, 9]. The bearing capacity of the foundation is an important index to evaluate the strength and stability of the foundation in the field of geotechnical engineering. The theoretical research related to the bearing capacity of foundations under general conditions has been relatively mature. Scientists such as [PERSON], [PERSON] and [PERSON] [10, 11, 12] have proposed various methods to calculate the bearing capacity of soil. [PERSON] first proposed thebearing-capacity equation, considering the general shear failure of soil under a rough strip foundation. Using the superposition principle, he demonstrated the influence of the cohesion of the soil, the angle of internal friction, the overload (soil at the foundation level), the unit weight of the soil and the width of the foundation on the ultimate bearing pressure. Later, [PERSON] introduced two factors, foundation shape and load angle, into the bearing-capacity equation. [PERSON] et al. studied the influence of foundation burial depth and groundwater level on the bearing capacity of rectangular foundations. They found that when the depth of the foundation increased, the bearing capacity increased too. [PERSON] and [PERSON] [13] conducted experimental and numerical studies on the bearing capacity of a model circular foundation which was on reinforced soils. [PERSON] and [PERSON] [14] found that cohesion and internal friction angle are important parameters for determining soil bearing capacity. These scholars have constantly improved the calculation methods, which make it easy to calculate the foundation bearing capacity under general conditions. However, the formulas that they came up with cannot be directly applied to the calculation of foundation bearing capacity under the condition of groundwater-level fluctuation. The problem of fluid-solid interaction is not fully considered. Studies have shown that the coupling effect of water and soil has a certain impact on the bearing performance of soil and can be characterized by the model [15]. Due to the constant ebb and flow of the Yangtze River in China, the foundations of buildings along the coast remain under fluctuating water levels for a long time. The influence of the fluctuating water level on the foundation cannot be ignored, and it often leads to the softening of the foundation and a decrease in the bearing capacity [16; 17]. Therefore, it is meaningful to study the influence of fluctuating water level on the bearing capacity of the foundation. However, the existing calculation methods of foundation bearing capacity cannot be directly applied to the situation of groundwater-level fluctuation, so it is necessary to study a more suitable method [18; 19; 20; 21].