inspection building pit retaining walls

huangyhg

inspection building pit retaining walls
i'm designing an vehicle inspection building to check commercial vehicles.  it is a single-bay facility, 18 feet wide by 80 feet long.  the center of the bay has an inspection pit 6 feet deep by 3 feet wide, running 70 feet long to allow a person to wall underneath a vehicle to inspect the undercarriage.  there are trench drains that run along the entire length of the pit at the floor level, so the concrete floor is not tied to the walls of the inspection pit.  the design vehicle is an hs20-44.  i've designed the walls of the pit as retaining walls to withstand the soil pressure and the surcharge load of the vehicle.  problem is, the base of the pit retaining walls works out to be 15 feet wide, since it's one slab to accomodate both walls of the pit.  i might as well extend the base to the perimeter of the building and make it a raft foundation.
does this sound right?  i've tried to figure out a different way to design the walls of the pit so that they don't need such a huge base slab, but nothing is coming to mind.
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which floor level are the trench drains?  pit floor or ground floor?
since there is typically concrete placed under the trench drain, could you increase the concrete thickness under the trench drain (if needed) and tie the floor slab to the top of the pit walls?  depending on how deep your trench drain is, this "u" shaped section of concrete around the trench drain may provide some good resistance for the wall.
the trench drains are at ground level, thus in the way of tying the floor slab into the pit walls.
after sending in this post, i thought of looking at this differently.  since both walls of the pit will essentially be loaded at the same time and at the same load, it's possible to design this more as a "u" shaped concrete structure resisting the moment at the center of the pit floor instead of having a huge base extending away from the pit.  then, it's just a matter of making the pit floor slab thick enough to carry the double moment of the walls.
i would approach this like a u-shape.  any unbalanced moment will just cause a slight rotation in the opposite wall pushing it into the soil harder and this can easily be absorbed.  (assuming free at the top).  this will be the best design.
aggman,
agreed.  i've already taken that approach.  thanks for the confirmation!