Increase in Shear Modulus by Soil Mix and Jet Grout Methods-Article

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The study evaluated soil mixing (SM) and jet grouting (JG) as potential methods to reduce liquefaction risk and earthquake-induced deformation at Tuttle Creek Dam in Kansas. The dam requires seismic rehabilitation because loose foundation sands may liquefy during the design earthquake.<br /><br />Researchers conducted cross-hole seismic tests on one SM column, one JG column, and nearby untreated soil. They compared in-situ measurements with laboratory tests on recovered cores, including resonant-column tests. The untreated foundation includes a cohesive surface blanket over loose fine-to-medium sand and deeper dense coarse-to-gravelly sand.<br /><br />Both treatments substantially increased stiffness: the small-strain shear modulus of treated soil was at least about three to four times that of untreated soil. Shear-wave velocities increased roughly two- to fourfold. SM showed higher measured velocities than JG, likely because JG columns contained more untreated soil inclusions; overall, the authors found no appreciable difference in modulus gain between the methods for the tested grout mix. Laboratory resonant-column results showed similar shear-modulus degradation behavior for SM and JG as strain increased.<br /><br />Laboratory strength and stiffness generally increased with soil grain size. The authors provide recommended ranges of unit weight, unconfined compressive strength, and elastic and shear moduli for numerical analyses, assuming a cement-to-water ratio of 1.0 and installation conditions similar to those tested. They recommend using in-situ stiffness values for design because laboratory values were substantially higher, potentially due to additional curing, sample selection, and cross-hole waves passing through untreated soil.<br /><br />Compression-wave measurements were of limited value for untreated saturated soil, where velocities largely reflected water. Although the test program successfully characterized both treatments, neither SM nor JG was ultimately selected for the dam’s seismic retrofit.

Keywords

soil mixing

jet grouting

liquefaction mitigation

seismic rehabilitation

cross-hole seismic tests

small-strain shear modulus

shear-wave velocity

resonant-column tests

Tuttle Creek Dam

earthquake-induced deformation