## 3 Geotechnical Guidelines and Practices for the Design of Integral Bridges
### Finnish Practices
#### 3.9.1.1 SSI Analysis Based on PD 6694-1  
PD 6694-1 [25] provides a method for determining the earth pressures behind embedded wall abutments and full height frame abutment using SSI analysis. The SSI analysis needs to be carried out using the numerical model. The method has been calibrated against a number of experimental results reported by [PERSON] et al. [12], [PERSON] et al. [14], [PERSON] and [PERSON] [47], and [PERSON] et al. [74]. Furthermore, it is advised that the SSI analysis method may be used as an alternative design option to the limit equilibrium method for frame abutments on spread footings. For the calculation of the lateral earth pressure profile along the integral abutment at various depths below the ground level, PD 6694-1 [25] advises that the following points should be considered for the SSI analysis:  
* the nonlinear response of the backfill to deck expansion and contraction;
* the effect of cyclic movement of an abutment on soil properties, which may be based on 120 cycles with a magnitude of characteristic thermal movement (\(d_{K}\));
* variation of soil properties at different depths and at different strains;
* the degree of compaction of soil;
* the superimposed thermal effects of daily and seasonal temperature changes;
* the rotational and axial stiffness of the deck;
* horizontal soil arching between piles;
* in certain conditions, minimum earth pressure can be more vulnerable than maximum earth pressure. Therefore, the pressure envelope design covering all the possible conditions of expansion and contraction is needed. This should incorporate the possible combinations of minimum earth pressures with maximum expansion and maximum earth pressures with minimum expansion. PD 6694-1 [25] advises that an appropriate numerical model capable of addressing all the factors to be considered as described above is required for numerical modelling. It further advises that the analysis approach needs to be calibrated against comparable experience, experimental results, field monitoring data, laboratory modeling, and/or case histories. There are few studies examining the SSI effects of integral abutments with different modes and magnitude of abutment movements on abutment foundation and backfill. The small scale (1 g) tests conducted by [PERSON] et al. [12] were for a stiff abutment subjected to different magnitudes of cyclic rotational movements. [PERSON] et al. [14] investigated embedded and spread foundation abutments retaining Leighton Buzzard sand under a range of cyclic movements in the centrifuge. [PERSON] and [PERSON] [47] also studied an abutment pinned at the base, retaining sand in the centrifuge under cyclic amplitudes of 0.10%, 0.40% and 1.26% d/H. [PERSON] et al. [12] reported that a significant lateral stress increase occurred in the first ten cycles, and the rate of increase slowed afterwards and then continued towards a steady-state condition. In contrast, [PERSON] et al. [14] found that the increase in lateral stress was insignificant under small number of cycles but significantly higher under a large number of cycles. However, the rate of lateral stress increase was much less after the first twenty cycles. With more than a thousand cycles, [PERSON] and [PERSON] [47] showed that lateral stress kept increasing until the passive pressure limit was reached. Therefore, it is evident that the test findings are not always consistent and conclusive. For the consideration of the rotational and axial stiffness of the deck, it is advised that the SSI model may be done explicitly or by using elastic lateral and rotational springs. Furthermore, to model the soil response using the springs, it is advised to convert the elastic modulus to equivalent spring stiffness, which varies with the depth and the applied soil strain. An approximate expression developed by [PERSON] et al.