## 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  
According to [PERSON] et al. [8] and [PERSON] et al. [12], when the abutment moves into the backfill behind the abutment, it causes soil densification, which increases the lateral earth pressure on the abutment. On the other hand, when the abutment moves away from the backfill, the granular soil flows towards the additional volume created adjacent to the abutment, thus reducing the soil density and lateral earth pressure. [PERSON] et al. [12] reported that a balance between the opposite effects of soil densification and granular flow could be reached when both effects are of comparable magnitude, leading to steady-state cyclic lateral earth pressures. At this moment, residual volumetric strain changes become insignificant with increasing cycles [8], resulting in closed hysteresis loops, which means that the shakedown state has been reached [8,20,77,78]. The abutment type, mode, and range of movements significantly impact the lateral earth pressure development. Most importantly, the design recommendations based on the full-scale tests are still lacking. More monitoring of various types of abutments, with different abutment foundations, and in all possible cases and conditions of abutment movement are thus needed to further clarify and justify the current design approaches. [PERSON] et al. [46] performed strain-controlled stress path triaxial tests on clay samples, pluviated granular samples (Leighton Buzzard sand) and glass ballotini. Samples were tested at different stress paths and cyclic straining experienced by backfill soil located at mid-height behind integral abutments (full height frame abutment and embedded wall abutment). The loading scheme for the laboratory triaxial tests was based on five factors: (a) the location of the representative soil element and size of the idealised integral abutment, (b) the initial stress on the soil element, (c) the total stress path that would be imposed on the representative soil sample because of abutment movements, (d) the amplitude of the strain cycles that would be imposed as a result of temperature change and deck length, and (e) the initial loading direction. Results showed that there is no accumulation of lateral earth pressure in the clay samples and glass ballotini. However, there was a significant increase in lateral earth pressure for almost all cyclic strain levels in the pluviated sand specimens. It should be noted that the total stress path considered was constant, irrespective of the imposed strain cycles and magnitude of strain cycles based on the typical range of temperature cycles and bridge deck length. The results from [PERSON] et al. [46] help develop the guidelines in PD 6694-1 [25]. While their observations and findings are valuable, the cyclic stress paths emanating from soil-structure interactions in an actual integral abutment are more complex; it may not be possible for a triaxial test to capture the SSI comprehensively. Integral bridges founded on piles are the most common practice in the USA, Europe, Australia and other Asian countries. [PERSON] and [PERSON] [20] used three different methods to compute the thermal displacement of piles and abutment: (a) the AASHTHO [23] which computes the movement as the free expansion \(\Delta=\alpha L\Delta T\), where \(\alpha\) is the coefficient of thermal expansion, \(L\) is the expansion length, and \(\Delta T\) is the difference between the extreme temperature and installation temperature, (b) an analytical method, using classical mechanics theory, which only considers the increase in soil pressure on the piles due to thermal movements, and (c) a 3D finite element modelling (FEM), which considers both effects from mechanical loads and the soil pressure due to temperature variations. The comparative results indicate that the piles and abutment displacement using the AASHTO [23] method is higher than the other two methods.