# A Comprehensive Review of Open Caisson Modeling Technology: Current Practices and Future Prospects
## 5 Challenges and Future Directions
### Challenges
#### 5.1.1 Soil-Structure Interaction (SSI) Mechanism  
Soil-structure interactions involve the interplay between soils and structures in transmitting forces from one to the another via contact surfaces. The deformation patterns during the caisson excavation and sinking are determined by the interactions between the caisson and the surrounding soil. SSI influences both the soil behavior and the caisson response under loading. During the sinking process into earth, the cutting face located under the bottom of the caisson wall played a key role. This was achieved by exploiting the self-weight of the caisson wall. The cutting face of the cutting edge was critical during the sinking process as it reduced the vertical bearing capacity and enhanced the soil failure mechanism advancement toward the central area of the caisson [15; 126]. Numerous studies in the literature have concentrated on defining the cutting face angle, symbolized as \"\(\beta\)\", within ranges such as 30-90\({}^{\circ}\)[131] or 30-45\({}^{\circ}\)[132], demonstrating numerous methods. During the sinking stage of the caisson construction process, the self-weight of the structure and upwards resistance produced from soil-structure interaction (SSI) should be carefully balanced [2; 133]. Caisson-bearing stresses at the base, \(v\), and frictional stresses on the exterior surface of the caisson walls, \(\tau\), were exerted by the surrounding soil. To minimize design inefficiencies and construction risks, accurate assessment of these contact stresses is critical. Unexpected or excessive caisson movements might result from over-prediction [2]. Additionally, heterogeneity of ground conditions or poor excavation leads to non-uniform resistance, which can cause a loss of verticality or tilting [128]. However, under-estimation of the contact stresses during caisson sinking was the main risk, which might cause the caisson to become wedged and unable to achieve the desired formation level. In this scenario, potential remedial measures can be very expensive, having substantial associated risks, and remain largely unconfirmed. Presently, increasing caisson wall thickness was used to ensure sufficient self-weight W to comfortably overcome the sinking resistances. In terms of both cost and environmental impact, this had obvious inefficiencies, as excessive material utilization exceeds the structural requirements. During construction, particular features were included in the caisson's design to reduce and regulate the contact stresses between the surrounding soil and the structure. As shown in Figure 13, the cross section of the RC caisson wall is tapered at the base to form a cutting face. This was commonly accomplished by means of an angled bearing pad cast from concrete under the wall, which was broken out to start the shaft sinking [134]. Based on the gathered site experience, a footing taper angle, \(\beta\), of 45\({}^{\circ}\) is frequently used in industry [130]. [PERSON] et al. [135] mentioned that \(\beta\) adjustment depended on the ground conditions: for stiff clays, and angle of 30\({}^{\circ}\) was recommended, and for sands and soft clays 45\({}^{\circ}\) was recommended. By cutting into the soil, minimizing the vertical bearing capacity, and moving the soil failure mechanism inside the caisson for excavation, the employment of a tapered cutting face was thought to facilitate the sinking process [130; 136; 137]. In 1945, [PERSON] [138] designed and patented external over-cuts as an additional important construction feature. Usually, the radius of excavation was 30-100 mm larger than the shaft itself [128]. This was typically accomplished by casting a thicker piece of the RC wall, e.g., [126; 128], or, more commonly, by inserting a steel 'cutting shoe' at the front edge of the caisson wall [2; 5; 139]. Identical to pipe-jacked tunnelling, this created a gap among the caisson and the adjacent soil as the caisson lowers, known as the 'annulus'.