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Abstract Jet grouting is one of the ground improvement techniques that are currently more widely used for upgrading gravity quays, especially to increase their berth depth. Irregular columns are formed because of a rotating high‐pressure injection of a cementitious fluid in the ground that mixes with the natural soil. Despite its popularity, there is hardly any field experimentation on the stiffness gain of the mixture, especially under the base of port caissons. As fresh columns behave like a dense fluid with an excess of pore pressure, the evolution of mixture stiffness, especially at early ages after injection, is a key aspect when programming working windows for ground improvement, drill locations, sequencing and timing. Otherwise, crane rails, pavement and other services are at risk of undergoing undesirable local movements. One of the upgrading programs of the Port Authority of Valencia, Spain, is the increase of the berth depth of its quays, many engineered as concrete caissons. The concern about the temporary local loss of the foundation stiffness during the execution of each jet grouting column prompted research to find an in situ procedure capable of measuring the stiffness evolution of the jet grouting columns at early ages. This paper presents a successful procedure using PS suspension logging, expanding its scope of geotechnical engineering applicability in an innovative way. The use of the technique for the intended purpose and the execution process itself stands out as a novelty, with the insertion of the borehole casing pipe through the axis of the recently executed column. The key outcomes of this research are: (1) the novelty of studying the evolution of the stiffness in the early ages of the jet grouting columns executed under port caissons, where it is not possible to execute test columns; and (2) to propose an adequate and reliable method at the beginning of the works to plan the injection sequence of the next columns of jet grouting.