Ground Improvement Assessment Using SCPTu and Crosshole Data

A case study is presented where preloading was applied to reduce tank settlements at a liquid natural gas (LNG) facility in Puerto Rico. Stone columns were installed to further reduce settlements and provide an adequate factor of safety against soil liquefaction in the case of a seismic event. Seismic piezocone penetration tests (SCPTu) were used to quantitatively assess the effects of soil improvement. Soil properties estimated from SCPTu data taken before and after both improvement methods are compared to assess the degree of soil improvement. Preconsolidation stresses were estimated using methods based solely on net cone tip resistance, pore pressure difference, shear wave velocity, and a multiple regression based on cone tip resistance and shear wave velocity. Preloading provides additional vertical stress, which is directly related to an increase in preconsolidation stress if time rate consolidation properties are considered. Stone columns increase the lateral stress locally within the zone of influence of the column. Soils initially appear to be normally consolidated to slightly overconsolidated, and a significant increase in preconsolidation stress was noticed in clay layers after preloading. The high clay content of the soils likely led to a reduced zone of influence for the stone columns, and little improvement was noticed immediately after installation. Crosshole test (CHT) shear wave velocities taken across stone columns showed little to no increase in stiffness when compared to downhole shear wave velocities of the soil.