Long Term Behavior of Piles-PDF

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The paper argues that pile foundations must be designed for both tolerable settlement and adequate resistance to failure, including changes in capacity over time. The authors recommend combining static bearing-capacity calculations with field load tests. Their approach includes defining subsurface conditions and soil properties, selecting pile types and installation methods, testing piles at different embedments, and using compression and tension tests to set production-pile lengths and design loads.<br /><br />The authors attribute many differences between short- and long-term pile performance to changes in pore-water pressure as soil shifts from undrained to drained behavior. Piles in clay or dense sand may lose capacity as excess negative pore pressures dissipate; piles in loose to medium sand may gain capacity as positive excess pressures dissipate.<br /><br />A case history from two coal-terminal sites in Newport News, Virginia, illustrates the importance of this effect. At one site, low Standard Penetration Test (SPT) values in the Yorktown Formation were interpreted as indicating very loose soil, leading to long piles. At the other, the authors believed the low values reflected positive pore pressures generated during sampling, rather than weak soil. They used static calculations and pile load tests to establish suitable embedment.<br /><br />At the second site, test piles embedded about 40 feet into the Yorktown Formation supported the required loads. Compression testing confirmed the calculated capacity; tension testing indicated a pullout safety factor of at least 1.5, and possibly 1.8 or more. Production piles installed to this embedment performed satisfactorily.<br /><br />The authors conclude that driving-induced pore pressures can distort SPT results and affect pile capacity over time. Static bearing-capacity theory, supported by load testing—and pore-pressure measurements when short-term capacity matters—can reliably guide pile design.
Keywords
pile foundations
bearing capacity
load testing
pore-water pressure
settlement
static calculations
pile design
SPT values
Yorktown Formation
compression and tension tests