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Liquefaction Mitigation Using Vertical Composite Drains and Liquefaction Induced Downdrag on Piles: Implications for Deep Foundation Design Liquefaction Mitigation Using Vertical Composite Drains and Liquefaction Induced Downdrag on Piles: Implications for Deep Foundation Design

by Strand, Spencer R

Abstract (Summary)
Deep foundations constructed in liquefiable soils require specialized design. The

design engineer of such foundations must consider the effects of liquefaction on the

foundation and overlying structure, such as excessive settlement, loss of skin friction at

the soil-pile interface, and the development of downdrag on the pile.

Controlled blasting was employed to liquefy a loose, saturated sand in order to

test the liquefaction prevention capabilities of full-scale, vertical composite earthquake

(EQ) drains and to investigate the development of downdrag on full-scale test piles.

Blasting produced liquefaction at a test site without EQ drains which eventually resulted

in 270 mm of settlement. Liquefaction caused the skin friction on the test pile to

decrease to zero immediately following blasting. As pore pressures dissipated and the

sand settled, negative skin friction developed, with a maximum magnitude of about onehalf

of the positive skin friction.

Blasting also produced liquefaction at a site with drains but the settlement was

reduced to 225 mm, a decrease of 17% relative to the untreated site. Nevertheless, the

dissipation rate dramatically increased. Skin friction did not decrease to zero in the

liquefied sand and negative skin friction increased to a value equal to the positive skin

friction in the liquefied layer.

The computer software, FEQDrain, was utilized to develop a calibrated model of

the soil profile using pore pressure and settlement data measured during blast testing.

This model was then used to simulate drainage systems with smaller drain spacing and

larger drain diameter. Results indicated that pore pressures and settlement could be

limited to levels acceptable for many applications. However, development of downdrag

on deep foundations would not likely be prevented.

EQ drains provide an attractive method of liquefaction mitigation. Furthermore,

liquefaction can cause significant amount of downdrag on pile foundations which should

be accounted for in deep foundation design.

Bibliographical Information:

Advisor:

School:Brigham Young University

School Location:USA - Utah

Source Type:Master's Thesis

Keywords:liquefaction eq drain pile deep foundation feqdrain composite vertical downdrag

ISBN:

Date of Publication:08/10/2007

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