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Engineering Letter and Opinion from John B. Thornton, P.E.

Comparison of Factor of Safety between Bell-Bottom Piers and Pressed Piles

Bell Bottom Piers have been thoroughly researched at numerous universities. Reese and O’Neil research report No. 09-11F conducted load tests on instrumented drilled piers at two locations in Houston, Texas. One location was at Hwy. 610 and 225 and the other at Hardy Road. From their research the capacity of drilled piers is calculated as:

Q = fAs + qAp

f = .5C and q = 9C

For a drilled pier with a nine inch diameter shaft and a twenty-two inch diameter bell bottom the capacity in shear strength profile of 1000 psf is 31,370 pounds. The grade beam load for a one story brick veneer house is 900 pounds per linear foot. Using the City of Houston spacing criteria of six feet, the total load per pier is 5,400 pounds. The factor of safety is 5.8.

The pressed pile method uses a six (inch) diameter, 12 inch long cylinder pressed into the ground. The weight of the house is used as the reaction force. The skin friction developed by the cylinders increases until it is equal to the reaction weight of the house. At that point the factor of safety is 1 or incipient failure. Incipient failure is a condition where any increase in load or condition will cause failure. A change in the moisture condition or the surfical soils will reduce it’s (sic) undrained shear strength causing failure.

John B. Thornton, PE
Thornton Engineering and Construction
1814 Redwood
Sugar Land, TX 77478

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