Adding fiber reinforcement to a loose sand alters the effective stress of the sand skeleton. The rate of pore water pressure build-up when undrained is reduced, and liquefaction may be delayed or prevented. Here the results of cyclic triaxial tests are presented that show this benefit. They are interpretated so the load carried by the fibers is separated from that carried by the pore water and sand skeleton. A newly defined pore pressure ratio demonstrates when the pore water pressure build-up is insufficient for liquefaction to occur. The rule of mixtures is used, along with separate constitutive laws for the sand skeleton and fibers. Account is given to the anisotropic fiber orientation distribution, by which the fibers affect the sand skeleton differently during compression and extension cycles. It is found that the stress state of the sand skeleton remains bounded by critical state strengths after many cycles, as is the case for unreinforced sands. Not correctly determining the load carried by the sand skeleton and adopting the conventional rather than a new pore pressure ratio makes the fiber reinforcement technology appear less effective at suppressing liquefaction than it actually is. This may, unfairly, hinder its uptake in industry.