| 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 one-half 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. |
