DWLR soil safer types usage lengths diameter stability functions basic data conditions behaviours sub-surface appurtenant Piezometers foundations More Details applications water  level construction pore pressure simplest form Casagrande tip extension pipe allegianceindia accurate results earth/rock fills economical design Proper evaluation groundwater level piezometric level design improvement working principles higher reliability geotechnical domain concrete structures pressure transducers geotechnical sensors piezometer definition vibrating wire piezometer The  Casagrande piezometer Allegiance Water Management important geotechnical monitoring instruments