For an accurate response to change in ground water pressure, the volume of water required to operate the device must be readily available from the water bearing strata without a significant pressure change. The relative response characteristics of various types of piezometers are illustrated by the hydrostatic time lag, which is the time taken for the piezometer to reach equilibrium with the in-situ ground water pressure. The significance of the time lag depends on the anticipated pore water pressure fluctuations of the water bearing strata in natural deposits.
Some types of works, such as dams and other significant embankments, require a piezometer that behaves relatively at low hydrostatic time lag or almost instantaneous response time. However, in some open pit mines, there is an adequate period of time available and the piezometer can be kept in its position for several years before mine development. The pore water pressure study is able to be carried out for quite a long period of time, therefore the piezometer type that behaves relatively high hydrostatic time lag is considered to be appropriate and acceptable for application in the Hongsa open pit mine.
Since the Hongsa coal deposit is dominantly contains fine grained sediments with coefficient of permeability of less than 10-7 m/sec, a particular design and assembly of the Standpipe Piezometers should be installed in the boreholes in order to measure the ground water levels, its behavior and hydrostatic pressure studies and analyses.