Soil arching effect in soil excavation and how to represent that in Plaxis

On a global scale, there should not be much difference in the overall behaviour but at a local scale, the stresses on the H piles would be different because in one case there is no soil present in between the H piles whereas in the other case soil is present in between the H piles and so there is a difference in soil pressures between the two cases. How much difference would there be? This would need to be investigated through numerical modelling. The modelling procedure can vary and depends on how you model the structures and also how accurately you want to determine the stresses on the H piles. Users can model the H piles as beams in PLAXIS 3D and the steel lagging as one continuous plate. This method may not be as accurate in capturing the intricacies involved in modelling the H pile accurately to account for the actual geometry of the beam and its interaction with the soil because beam elements are line elements in 3D and do not account for that. The connection between the beam and the plate is rigid by default but if the user wishes to change the connection type they can do so by adding a custom connection as described in the following article Custom Connections

For more accurate results, users can create the H piles using individual plates instead of the beam option. This would create a full 3D H pile and would require a much small scale model. May be you can create 2 or 3 piles to accurately account for the actual geometry of the pile to interact with the surrounding soils. 
Once you create the numerical model with sufficiently dense mesh, then you can accurately determine the difference in the arching effect of two cases and also determine if the difference is small or large.
 

On a global scale, there should not be much difference in the overall behaviour but at a local scale, the stresses on the H piles would be different because in one case there is no soil present in between the H piles whereas in the other case soil is present in between the H piles and so there is a difference in soil pressures between the two cases. How much difference would there be? This would need to be investigated through numerical modelling. The modelling procedure can vary and depends on how you model the structures and also how accurately you want to determine the stresses on the H piles. Users can model the H piles as beams in PLAXIS 3D and the steel lagging as one continuous plate. This method may not be as accurate in capturing the intricacies involved in modelling the H pile accurately to account for the actual geometry of the beam and its interaction with the soil because beam elements are line elements in 3D and do not account for that. The connection between the beam and the plate is rigid by default but if the user wishes to change the connection type they can do so by adding a custom connection as described in the following article Custom Connections

For more accurate results, users can create the H piles using individual plates instead of the beam option. This would create a full 3D H pile and would require a much small scale model. May be you can create 2 or 3 piles to accurately account for the actual geometry of the pile to interact with the surrounding soils. 
Once you create the numerical model with sufficiently dense mesh, then you can accurately determine the difference in the arching effect of two cases and also determine if the difference is small or large.