|TO:||All Design Section Staff|
|DATE:||August 17, 1995|
|SUBJECT:||Drilled Shaft Foundations|
Reference is made to my memo of January 23, 1995 regarding drilled shafts (copy attached). There is no change in this current policy. If the soils report lists options other than drilled shafts that are structurally feasible and more economical, another option should be used.
To perform a reasonable economic comparison, the cost of the other options (based on BDM cost data) should be compared to 175% of the cost of the shaft option (based on BDM cost data). The 175% provides a conservative cost overrun estimate (unexpected obstructions) for comparison of options, based on recent construction experience. If the cost of the other options is more than 10% above the adjusted cost of the shaft option, then shafts should be used. If the cost of the other options is less than 10% above the adjusted cost of the shaft option, the other (lowest cost) option should be used. I am requesting that documentation be provided in the project job file that supports any decision to use drilled shafts. The three valid reasons for specifying drilled shafts are economy (based on the above comparison),. constructability/access, and structural feasibility.
If you have any questions regarding this policy, please let me know. WSDOT is involved in several regional and national efforts to improve shaft design criteria, construction specifications, inspection methods. Hopefully, this effort will result in shaft foundations being more economical in the future. In the meantime, this policy should be adhered to.
cc: J. Weigel, 7354
T. Allen, 7365
F. Higgins, 7340