Liquefaction Mitigation of Silty Sands Using Rammed Aggregate Piers Based on Blast-Induced Liquefaction Testing

To investigate the liquefaction mitigation capability of rammed aggregate piers (RAP) in silty sand, blast liquefaction testing was performed at a soil profile treated with a full-scale RAP group relative to an untreated soil profile. The RAP group consisted of 16 piers in a $4\times 4$ arrangement at $2\mathrm{m}$ center-to-center spacing extending to a depth of $9.5\mathrm{m}$. Blasting around the untreated area induced liquefaction ($r_u\approx 1.0$) from a 3 to $11\mathrm{m}$ depth, producing several large sand boils and causing a settlement of $10\mathrm{cm}$. In contrast, the installation of the RAP group reduced excess pore water pressure ($r_u\approx 0.75$), eliminated sand ejecta, and reduced the average settlement to between 2 and $5\mathrm{cm}$ when subjected to the same blast charges. Although the liquefaction-induced settlement in the untreated area could be accurately estimated using an integrated cone penetration test (CPT)-based settlement approach, settlement in the RAP treated area was significantly overestimated with the same approach, even after considering RAP treatment-induced densification. Analyses indicate that settlement after RAP treatment could be successfully estimated from liquefaction-induced compression of the sand and RAP acting as a composite material. This test program identifies a mechanism that explains how the settlement was reduced for the RAP group despite the elevated $r_u$ values in the silty sands that are often difficult to improve with vibratory methods.