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Construction design and analysis for the 100ft soldier pile excavation for the 2nd and Broadway Station in Los Angeles by Pirooz Barar, P.E. of PB&A Inc.

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No earth pressures, please!

In many deep excavations, geotechnical engineers only specify the lateral earth pressures that are to be used by the shoring designer.  An example such specification could go like:

  • Lateral earth pressures are to be calculated with a rectangular earth pressure diagram of 24 H (in psf) above the water table and 12 H (in psf) below the water table.
  • The equivalent fluid pressure for lateral earth pressures should be taken as is 21 pcf on the active side and 185 pcf on the passive side.

 

The need to define lateral earth pressures originates from a need to calculate sensible/conservative brace support reactions. While this approach is common practice in many States, it is the wrong way to design earth retaining structures. In fact, defining only lateral earth pressures on shoring walls is doing a disservice to both the client and our profession. Furthermore, such simplistic assumptions can easily backfire on the geotechnical engineer as they are usually applicable for a limited number of cases. Here is why:

  • Pressure is no property: Soil does not know an apparent earth pressure, it has mechanical and physical properties. Soil has a friction angle, effective cohesion, undrained shear strength, an elastoplastic behavior.
  • Construction staging: Lateral earth pressures are highly dependent on construction staging. How we get to the final excavation makes a difference. Simplified lateral earth pressure diagrams totally ignore construction staging.
  • Wall-to-soil friction? With wall friction acting pressures can be reduced, while passive earth pressures can be increased.
  • Support prestress makes a difference. Are we prestressing a tieback or are we using a passive raker? Lateral earth pressures tend to focus near a support, especially when it is prestressed.
  • Wall deflections? Will you get any realistic wall displacement out of these specifications?
  • Surface profile makes a difference. Most simplistic specifications are made for horizontal ground conditions. What happens when the contractor makes a slopped cut? Is the 185 pcf still valid if we do a passive berm?
  • You think you are safe! In a recent publication with 6 benchmarked excavations it was shown that when you excavate with more than two supports the traditional limit-equilibrium approach can be severely unconservative for wall bending. You would be surprised by how much. 

 

While there is some quick verification value to using a simplistic lateral earth pressure diagram, this approach degrades the value that geotechnical engineers can bring to the table. If we want as geotechnical professionals to stop being treated as commodity then we have to add value. One way to go forward, is to provide more scientific geotechnical specifications for deep excavations. I thus conclude, no earth pressures, please!


Specify this...

Effective soil friction angle

Effective soil cohesion

Undrained shear strength

Soil density, above and below the water table

Soil elasticity parameters (loading/reloading modulus of elasticity, soil model, reference stress, etc)

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