capacity of rubble stone foundation to support concentrated

huangyhg

capacity of rubble stone foundation to support concentrated
question
how does one evaluate capacity of rubble and random ashlar stone foundation to support concentrated gravity loads?
background
i am designing a post and lintel replacement for a bearing wall. new structure will occur in the first story and support second story, attic, and roof loads. total unfactored end reactions are each about 8 kips.
cl of new lintel is over the outside half of the 100 year old 18" thick stone foundation wall. outside face of foundation wall is random ashlar with fair mortar; inside face is graded (12" to 24") rubble, fairly well fitted, with very poor mortar. geometry of foundation wall is good. column loads at the foundation level can be redistributed over about 24 inches of foundation wall with an r/c grade beam.
typical solutions (based on known geometry and material properties) such as lateral distribution of the concentrated load to new footings both sides of the existing foundation wall or replaceing a portion of the foundation wall with an r/c pier are beyond the scope of the project.
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if the wall is in good condition and the mortar has not deteriorated, you should be able to consider it as two low strength concrete block walls representing the inside and outside face of about 6" wythes and a bearing of approximately 50 psi (well distributed load on a sound wall - condition of walls varies dramatically).  a starting point... you may be able to contact some of the parks departments that are involved with restoration.
dik