We present an experimental apparatus specifically designed to investigate the
precursors of failure in granular materials. A sample of granular material is
placed between a latex membrane and a glass plate. A confining effective
pressure is applied by applying vacuum to the sample. Displacement-controlled
compression is applied in the vertical direction, while the specimen deforms in
plane strain. A Diffusing Wave Spectroscopy visualization setup gives access to
the measurement of deformations near the glass plate. After describing the
different parts of this experimental setup, we present a demonstration
experiment where extremely small (of order 10−5) heterogeneous strains are
measured during the loading process