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About Robert Cummings, P.E.
Expertise
I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.
Experience
30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.
Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.
Publications
Mining Engineering, AEG Bulletin.
Education/Credentials
BS and MS Geological Engineering
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You are here: Experts > Education > Votech Education > Construction & Contractors > excavating and backfilling
Expert: Robert Cummings, P.E. - 8/30/2008
Question
I am contemplating putting an addition onto my home using a cargo shipping container (20'X8'X8.5'). I want to bury it down six feet and backfill using a lightweight backfill. I have heard that tire chips make a good backfill (unit weight ~ 35pcf, hydraulic conductivity ~ .03 cm/s). How would this compare with the unit weight of soil and its characteristics? We live in a wet climate in the Pacific Northwest.
Answer
Most wet sand-silt-gravel mixes will weigh in the neighborhood of 135 pcf if compacted and 125 pcf if in the native density state, so there is a significant difference with lightweight backfill, as you no doubt appreciate. Clayey soils may weigh a little less -- a whole lot less if they are dry.
However, that is not the only determinant of the active earth pressures on a buried structure. The other determinants are the soil shear strength and whether the soil is properly drained (so that water pressures do not accumulate). If you backfill with engineered soil fill, and properly compact and drain the backfill, the active earth pressure distribution can usually be approximated by that of a fluid having a density of 35 pcf. If the soil is not properly drained, you can add the unit weight of water to that and the equivalent fluid density becomes around 90 pcf (buoyancy of the soil particles accounts for the difference), and improperly compacted and undrained clayey backfill, due to the weaker shear strength, can have an equivalent active fluid density of nearly 100 pcf, to which any swelling pressures would be additive. Tires don't swell, and I don't know what their shear strength characteristics or their longevity would be in a buried application, but I can tell you that you will not attain the backfill benefits if you don't drain the backfill.
The other thing to consider before burying any ferric metal in the ground is whether the ground is corrosive. You would be surprised how fast buried steel will degrade in aggressive ground. For this you need to have a soil test that measures soluble salts, pH and conductivity (or its reciprocal, resistivity). There are standards for corrosion protection that are indexed to these parameters. The solution if you might have an aggressive soil problem, is to add galvanic protection.
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