United States Department of Transportation
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Field Instrumentation and Testing to Study Set-Up Phenomenon of Piles Driven Into Louisiana Clayey Soils: Tech Summary
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2016-07-01
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Abstract:Piles driven into saturated cohesive soils (clays and silts) usually experience a time-dependent increase in pile resistance (mainly frictional), known as “pile set-up” or “freeze.” Field observations showed that pile set-up is signifi cant and continues to develop for a long time after installation, especially for fi ne-grained soils. The increase of pile resistance over time or set-up is believed to be attributed to three main mechanisms: (1) the increase of eff ective stress due to dissipation of excess pore water pressure generated during pile driving, (2) thixotropy, and (3) stress independent increase or “aging” after the completion of excess pore water pressure dissipation. During pile driving, the soil around the pile (within an infl uence zone) undergoes large lateral deformations and disturbance, resulting in the development of excess pore water pressure around the pile and change in soil’s permeability within the disturbed zone. It is believed that a large contribution to pile set-up is related to the dissipation of excess pore water pressures (or consolidation), and the subsequent remolding and reconsolidation of the soil within the infl uence zone. At early stages, the dissipation of excess pore water pressure can be non-uniform with respect to the log of time depending on soil permeability and extent of soil disturbance. After that, the dissipation becomes uniform. Following that, aging may account for an additional pile set-up. Set-up can occur in all pile types driven in diff erent soil types (organic and inorganic, clayey, silty, and even sandy soils). Several empirical relationships have been proposed to estimate the pile set-up resistance with time. Most of the available developed models did not consider the soil properties in the formulations and that the total resistance (Rt ) was used instead of side resistance (Rs ). Therefore, set-up prediction models that can estimate set-up at diff erent soil conditions are needed. The accurate prediction/ estimation of the increase in pile resistance with time (or set-up) can be incorporated into a rational design through reducing the number of piles, shortening pile lengths, and/or reducing pile crosssectional area (using smaller-diameter piles). Incorporating any or a combination of these benefi ts will result in a cost reduction and savings to pile foundation design in Louisiana. The foundation cost will be reduced by substantial amount if load and resistance factor for set-up (Φset-up) is incorporated successfully into the LRFD framework. Currently the American Association of State Highway and Transportation Offi cials (AASHTO) does not have any recommendation to incorporate set-up into the LRFD framework.
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