By Bruce F. Mandella, Retired WL Port-Land Employee
Q: What are the various steps and design considerations that need to be addressed by the structural engineer to ensure structurally sound, safe, and economical mill foundations?
A: Once the site is selected, the first step would be to choose a qualified geotechnical engineer who is knowledgeable and familiar with the geological conditions of the area. Once that person is selected -- and prior to proceeding with a soils investigation -- the structural engineer assigned to the project will need to provide technical data on the proposed mill structures.
This data should include the approximate size (footprint) and location of the main mill structures, which will generally include the mill tower; grain storage including metal bins, tanks, concrete silos, and/or flat storage; truck and/or rail receiving pits; grinding building; warehouse; boiler building; maintenance shop, and tank farms. The structural loading that these structures place on the underlying soil will also need to be developed. In general, the structural loads will be broken down into dead, live, wind, and seismic.
Once the technical data is developed, the structural engineer, in concert with the geotechnical engineer, will review the possible foundation types that may be utilized for each structure. This discussion will also determine the location, number, and depth of the proposed soil borings that will be required to provide the necessary geotechnical information for design.
At this point, the drilling contractor will be selected to drill the soil borings, determine blow counts, and collect soil samples. It is best to have the geotechnical engineer subcontract with the drilling contractor since the geotechnical engineer will have a technician on-site when this work is performed with the possibility that the scope may be altered once the initial underlying soil is visible.
Upon completion of the soils investigation, the geotechnical engineer will perform laboratory testing and analysis on the soils samples. These tests and analysis will be used by the geotechnical engineer in determining/confirming the proposed foundations for each structure with emphasis on projected settlements (both total and differential), allowable soil bearing, soil improvements (if necessary), frost line, water table for design considerations, passive and active soil pressure, and soil density/characteristics. A consolidated geotechnical report will then be prepared which will address all aspects of the site geology, foundation design data, proposed foundation types to be used along with the field data, and soil classifications.
At this point, the structural engineer is now equipped with the necessary site-specific design information so that a structurally sound, safe, and economical foundation design of the various mill structures can be provided.
To more fully answer your question, our next e-newsletter will address the various types of foundations and any necessary soil improvements that may need to be considered.
