A system for measuring transient pore water pressure in the ground utilizes a unique probe assembly and two-stage emplacement technique therefor. In the initial stage, a large, robust primary probe member of a probe assembly in the measuring system is pushed into the ground to the selected measurement region. In the final stage, a small secondary probe member which is contained within the primary probe member is ejected into the soil when the primary probe member is emplaced at the desired region. The secondary probe member contains a pressure sensor in the form of a highly miniaturized, fast response piezometer which is capable of achieving a responsive coupling with interstitial fluid in the region of measurement.