Bricks, Masonry, Asphalt and Concrete - excavating under piers

Can anyone suggest a way to excavate a crawl space under a 100 year old house that is sitting on brick piers? We plan to close in the entire space by building a block wall in between piers set about 11 feet apart, extending below grade about 5 feet, and the existing piers are about 4 feet above. I'm not sure how deep the footings are underneath, but if all is securely shored up how close can we dig without the piers collapsing? This will be an exterior wall, but then we plan to pave the entire floor inside with concrete, and there are several interior piers as well. Do I need to call in a structural engineer? The back wall has been enclosed to a height of about 7 feet, so we are not digging too deep there. The main concern is the side wall, which has been closed in with concrete blocks between the piers, which are about 18 inches square. We plan to remove those blocks, dig deeper to pour a footing and then rebuild the entire wall, which is about 34 feet long. The front wall and other side are not involved.

You should call in a foundation engineer with experience in underpinning. The details of the work you are interested in depends on the condition of the existing piers and the actual soils conditions of your site.