I would like to build walls around a post building that is currently serving as a snow roof over a double wide mobile home.  There is not a foundation or walls, other than roof skirting 4′ below the eaves, between the posts.  The mobile home support blocks which are setting on the ground and are spaced about every 3′- 4′, are approximately 2′ inset from the treated 6″x6″ roof posts which are approximately 4′ deep and have approximately 1′ of concrete poured around them from gound level to base of the post.  I don’t know if a post footing was poured prior to setting the posts.  Frost line in our area is 24″.  The entire post building is 26′ x 56′ with the posts being about 12′ tall above the ground and approximately 14′ o.c. The roof is constructed of 2″x12″ trusses and 2″x8″ purlins, OSB and metal sheeting, gable ends and side skirting is enclosed with 2″x4″ frame and plywood and extends 4′ below the eave.  We don’t live in a high wind area.

 I would like to pour a 8″ x 24″ footing just below frost line and build a surface bonded block wall four courses high then pour a slab over that.  The footing and blocks will be between the posts and extend to the outside of the posts with the blocks turned 90*.  The footing will be continuous for 12″ on the outside edge of the post and 6″ on the inside edge of the post.  The blocks will be nearly flush with the inside edge of the post.

The plan is to complete the foundation project in phases to retain as much structural integrity of the post building as possible.  I planned to nail 2″x6″ cross ties between the posts for support.  My plan is to dig one side at a time and to dig and pour that in segments, e.g. one corner and end plus one 14′ length bay in one pour.  How close can I safely dig approximtely 32″ deep next to the trailer blocks and/or posts on silt loam soil (ML) without jeopardizing the stability of the trailers or the post building?  How much lineal distance of trench could I safely open & not jeopardize structural integrity of the post building?