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Foundations in Swelling Soil[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaoc(--nospam--at)seaoc.org'" <seaoc(--nospam--at)seaoc.org>
- Subject: Foundations in Swelling Soil
- From: ARW <amirr(--nospam--at)paknet1.ptc.pk>
- Date: Fri, 13 Feb 1998 18:04:05 +-500
THE PROBLEM: I am currently designing a warehouse for a feedmill building, in Pakistan. The soil near the surface (to about a depth of 13 feet) has exhibited a slight swelling potential (generating a pressure of the order of 1.54 K/sq ft, at an existing moisture content of about 20%) in the oedometer test and the geotechnical agency has added a word of caution, to the structural designer, in this respect. It has suggested the use of grade beams, connecting all columns in both directions. The soil is already under an appreciable moisture content and the agency does not foresee a further significant increase in moisture content, particularly as the water table is also not very deep. I have a grid of 50'-0" x 30'-0", with reinforced concrete barrel shells, supported on columns that would rest on spread (individual) foundations, placed at a depth of 6'-6" from the natural grade. The floor is 3'-0" from the natural grade. For a typical interior foundation, the unfactored imposed load is about 120 K. I need a foundation of 9'-0" x 9'-0" at these locations; this foundation size is required, under a permitted 1 inch of settlement (shear does not govern for this foundation size) and including the floor storage load (about 0.672 K/sq ft) and the weight of 3'-0" of soil (about 0.360 K/sq ft) in the column load. For this case, the force generated by swelling tends to equal the column load in my case, with surcharge acting as the reserve margin. On my specific inquiry, the geotechnical engineer has additionally suggested (a) adding a sand cushion (to 90% Modified Proctor), of at least 12", under the foundation and (b) slightly increasing the permitted settlement to achieve a smaller foundation size and hence a higher bearing pressure resulting in greater reserve against the upward force. PRELIMINARY THINKING: I intend to go ahead with the use of grade beams. I might consider using a T-shaped beam to increase its rigidity. On the 50'-0" side I might consider using two intermediate supports for the grade beam, again to increase its rigidity. I would also consider some differential support settlement (say 0.50 inches), for the design of the grade beam. I intend to undertake a spring model for the analysis of the grade beam. QUESTION: What shall be a rational and logical design criterion for the structural design of the grade beam and the foundations? ONE ADDITIONAL QUESTION: I have one additional question: My storage is bagged grain and bagged poultry feed, stacked without shelves. The stacks of the bags are known to sometimes fall to one side and lean against the infill brick masonry walls. How to estimate this loading? In other similar projects, for another client, I have been using an arbitrarily chosen uniform lateral load, to provide at least some degree of partial safety, but I know it is not enough. My previous client knows this too and has been willing to accept partial safety in this case and in fact does not wish to spend large amounts against an unforeseen hazard. On the contrary, he tends to view the infill wall as a sort of structural fuse that protects the rest of the structural system, rather than transfer the full load to it! I would be extremely grateful for any advice that I may get. Rizwan Mirza Consulting Structural Engineer Lahore, Pakistan
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