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It appears you are using the standard triangular shaped soil pressure on the
walls.  I suggest you use the trapezoidal loading recommended by Terzaghi &
Peck for cofferdam type construction.  It will give a more uniform type
loading for designing the full height.  At a four foot side wall I would
think that shear is what will govern the design.

Any design aid/documentation on sheeting and shoring will illustrate this

seaint(--nospam--at),Internet writes:
    I am seeking some comments with regard to the design of a 
          deep wet well which will contain a submersible pump to 
          discharge storm water.  The structure will be a square box 
          (4' x 4' interior dimensions) that extends from the ground 
          surface to a depth of 32 ft. below grade.  The structure 
          will be reinforced concrete and is subject to an equivalent 
          lateral pressure of 100 lbs/ft^3 (i.e. lateral pressure = 
          100 * depth).  Utilizing PCA's "Rectangular Concrete Tanks 
          Book", and with ACI 350 in mind, I'm getting a wall 
          thickness at the bottom of the structure in the neighborhood 
          of 22" thick (shear design controlling).  The client thinks 
          this is ridiculous and we're over-designing.  They also say 
          they have never seen anything over 18" thick and usually 
          it's 12" thick.
          What typical wall thickness have others arrived at for these 
          types of buried structures ?
          We are not utilizing shear reinforcing in the corner of the 
          tank to handle shear (only the concrete's ability to take 
          the shear).  We are thinking it will be easier to construct 
          with minimal rebar and more concrete.  Any different 
          philosophies out there ?
          Thanks in advance for any help.


Richard Lewis, P.E.
Missionary TECH Team

The service mission like-minded Christian organizations
may turn to for technical assistance and know-how.