Question

Outline a program of hydrogeological investigations for this site designed to identify potential groundwater flow paths and to assess the likely rate of flow of water into the proposed excavation. In your answer, you should also consider the potential for seasonal variability in groundwater level and explain how this could be evaluated. In your discussion, assume that the groundwater system is unconfined.

Answer 1

Piezometers

Groundwater piezometers are accurate, reliable, and inex pensive tools for determining hydrostatic pres-sure at particular depths or in selected layers of soil or rock. Piezometers typically consist of a filter head at-tached to a pipe. The materials vary, but the filter heads typically porous plastic, and the pipe may be plastic or metal. Measurement of the pressure at the head is by pressure transducer or by inference from the depth of water measured in the tube. A simple piezometer may also consist of a pipe driven vertically into the ground to a definite elevation or stratum. The pipe is driven so that no leakage occurs, and groundwater can enter only at the bottom. Measurement of the water in the tube reveals the water pressure at the bottom of the tube.

Because groundwater moves from points of high hydrostatic pressure to points of lower pressure, it is possible by measuring pressure at a number of points to determine the movement of water. Results may be plotted in both plan and section and contours or equipotential lines drawn on the pressure surface in-dicated by the piezometer readings. Flow lines drawn perpendicular to the equipotential lines show direction of flow, hydraulic gradient, and areas of concentrated flow.

Piezometers, as well as being a principal tool in drain-age investiga tions, are useful in planning development of confined groundwater, in analyzing effect of engi-neering structures on local groundwater condi tions, determining need for and location of relief wells, and in mea suring pore pressure in the foundation of struc-tures.

Effective positioning of piezometers horizontally and vertically requires knowledge of underlying aquifers, preferably based on carefully logged borings. Piezom-eters may also be located in a grid pattern with a num ber of pipes of different length at each location depending on depths to aquifers. Piezometer loca-tions should be referenced horizontally and vertically. Multiple piezometers may be set in a single bore hole, if desired, by carefully sealing each piezometer at its prescribed depth.