## 3 Geotechnical Guidelines and Practices for the Design of Integral Bridges
### Finnish Practices
#### 3.9.1.2 Comments on SSI Research Studies and Current Published Guidelines
Since the analytical method only considers the soil pressure on the piles and excluded the soil pressure on the abutments, the calculated displacement from the analytical method is also significantly higher than that from the FEM method. Consequently, the effect on the abutment-soil interaction due to the presence of pile foundations and the combined effect of the abutment on the pile-soil interaction requires more investigation. The stress profile during the cyclic movement of the abutment from the neutral position could be different to when the cyclic movements start from a different position of the abutment wall. In addition, it was observed by [PERSON] et al. [46] that the increase in lateral stresses occurred only if the active state was reached at the end of each cycle. They postulated that the readjustment of the soil fabric due to rotations of non-spherical sand grains at the active state contributed to the build-up of maximum horizontal stress. However, it is not associated with the densification during cyclic movement, which was confirmed by subjecting spherical glass ballotini to the same stress path when no build-up of horizontal stress occurred. Hence, further tests at various magnitudes of cyclic abutment movement and at the different direction of the initial loading are needed to confirm the current design approaches and postulate further recommendations for future design methods depending on the starting point of cyclic loading movement. [PERSON] et al. [7] performed a comprehensive SSI study on a 140 m long integral bridge and compared the obtained earth pressure distribution behind the abutment with the earth resistance calculated using the Limit Equilibrium (LE) approach specified in PD 6694-1 [25]. For the SSI approach, a soil model that satisfied clause 9.2.2 in PD 6694-1 was developed, and soil stiffness parameters for the SSI analysis were taken from the published literature. The stress profiles obtained from the LE approach and SSI approach were found to be significantly different. It was reported that the SSI stress profile at the top of the abutment follows the passive limit \(\left(K_{pt}\right)\). In addition, the stress profile below the peak resistance from the SSI approach was significantly less than that of the LE approach. This being the case, the authors recommended further studies on the range of abutment types and stiffness to correlate the effects of stiffness of the structure and soil on the performance of integral bridges. An experimental study conducted by [PERSON] et al. [17] to examine the combined effects of abutment-soil interaction and pile-soil interaction under cyclic horizontal displacements has also confirmed the stress ratcheting effects behind the abutment. From the experimental results, it was found that the earth pressure coefficient was approximately equal to the passive earth pressure coefficient suggested by the [PERSON] et al. [12], NCHRP [79], [PERSON] and [PERSON] [80], [PERSON] [81], [PERSON] [82] and [PERSON] [83] when \(\frac{\Delta}{H}\) is lower than 0.004. Moreover, the earth pressure coefficient was approximately equal to two-thirds of the earth pressure coefficient of Rankine theory but significantly less than that from Coulomb theory. When \(\frac{\Delta}{H}\) is higher than 0.004 and less than or equal to 0.006, the passive earth pressure coefficient \(K_{P}\) from [PERSON] et al. [12], NCHRP [79], [PERSON] and [PERSON] [80], [PERSON] [81] and [PERSON] [83] are less than that of experimental results. Moreover, \(K_{P}\) from Rankine theory is close to the test result, but still far less than the Coulomb theory when \(\frac{\Delta}{H}\) reaches 0.006. The testing earth pressure coefficient is larger than all of those except Coulomb theory when \(\frac{\Delta}{H}\) is between 0.006 and 0.012. However, it exceeds the Coulomb theory when \(\frac{\Delta}{H}\) is larger than 0.012.