  We present the results of our radio observational campaign on GRB 100418a,
for which we used the Australia Telescope Compact Array (ATCA), Very Large
Array (VLA) and the Very Long Baseline Array (VLBA). GRB 100418a was a peculiar
GRB with unusual X-ray and optical afterglow profiles featuring a plateau phase
with a very shallow rise. This observed plateau phase was believed to be due to
a continued energy injection mechanism which powered the forward shock, giving
rise to an unusual and long-lasting afterglow. The radio afterglow of GRB
100418a was detectable several weeks after the prompt emission. We conducted
long-term monitoring observations of the afterglow and attempted to test the
energy injection model advocating that the continuous energy injection is due
to shells of material moving at a wide range of Lorentz factors. We obtained an
upper limit of $\gamma$ $<$ 7 for the expansion rate of the GRB 100418a radio
afterglow, indicating that the range-of-Lorentz factor model could only be
applicable for relatively slow moving ejecta. A preferred explanation could be
that continued activity of the central engine may have powered the long-lasting
afterglow.
