Full-scale, controlled blasting field tests on driven displacement pile-improved ground were conducted to study the response of densified and reinforced ground to blast-induced excess pore pressures. In order to make appropriate comparisons to the baseline response of the native, unimproved ground, explosive charges sufficient to induce liquefaction were detonated in a control zone and the resulting postliquefaction settlements were measured. Excess pore pressures generated in the improved ground were observed to be significantly smaller than that in the unimproved ground, and resulted in settlements that were generally one-sixth to one-third of that measured in the unimproved ground. Piles tipped into a dense bearing layer settled significantly less than the surrounding soil and piles that were floated above the bearing layer. Importantly, measured excess pore pressures pointed to a change in soil response from contractive to dilative during blasting, indicating that the improved ground mobilized significant strength during blasting, similar to the response expected from cyclic mobility of dense soils. The energy of scaled ground motions developed from velocity measurements are used to relate the observed soil response to blasting to that expected from earthquake-induced ground motions. The paper concludes with a comparison of shear strains expected from shear strain compatibility (SSC) between the improved ground and the displacement piles to those implied by the measured pore pressures. The comparison indicated that some portions of the improved ground responded in an incompatible manner during the blast-induced ground motions and that the assumption of SSC may not be appropriate for design of some reinforcement-type ground improvements.
The installation sequence and spacing of displacement piles can influence the driving resistance (i.e., blow count) and associated capacity, however, engineers can rarely account for this effect in a direct manner. Increasing the availability of high-quality measurements of the effects of driving sequence and spacing to designers could help improve the understanding of production data and inform decisions midproject. This paper presents some observations of pile driving resistance and pile capacity measured following an experimental program on the use of timber displacement piles in a well-characterized test site. Significant increases in pile capacity were observed as a function of decreased pile spacing depending on the order of installation. After correcting for the estimated residual loads, a fourfold difference in shaft capacity was observed between free-field and a pile in-filled at five pile head diameters.
Changes in soil fabric following liquefaction have been studied using various in-situ methods, and often return inconclusive or conflicting observations. The time-rate variation of stiffness, when observed, is usually not evaluated over significant periods of time, limiting investigations about aging in post-liquefaction regain of stiffness. Even more uncommon is the application of geophysical techniques to evaluate changes in shear wave velocity (VS) as a proxy for small-strain stiffness. This study uses controlled blasting to examine long-term post-liquefaction regain of stiffness following multiple blast events. The Multichannel Analysis of Surface Waves (MASW) technique was used to observe changes in VS of aged deposits at a test site in South Carolina. Blast-induced liquefaction of the target liquefiable layer resulted in significant reduction to its initial small-strain stiffness owing to the destruction of the aged soil fabric. The time-rate variation in VS indicated that the initial small-strain stiffness was not re-established over many months following liquefaction. Following a second blast event, the small-strain stiffness reduced again, but recovered more quickly, similar to previously reported observations of young sand deposits. This study provides a significant basis for interpreting in-situ body and surface wave measurements of aged and young sand deposits densified using blast liquefaction.
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