I notice that quite a number of deep excavation projects adopt double I-section walers to form a box beam since standard rectangular hollow sections don't come with such large sizes. The I-sections are usually factory butt-welded together at the flanges.
How are such members spliced together? Unlike a single I-section, the inner flanges of the double I-section would not be accessible. There would also be problem connecting the webs due to the same accessibility issue.
From the pictures I found on the internet, it isn't clear how this can be done to achieve effective moment transfer.