Annex L (p. I)
Sample guidelines for groundwater measurements
(1) The groundwater level and the pore water pressure in soil and rock have a great influence on many geotechnical questions: slope stability, bearing capacity, settlements, drainage, erosion and frost actions. Also the interpretation of other site investigation results are influenced by the groundwater situation. Therefore groundwater measurements are important parts of site investigations and can also be a part of a monitoring system in e.g. slopes, dams and groundwater lowering projects.
In some cases it has also been found that the groundwater quality can have an important role for the design of different constructions e.g. durability of steel piles.
(2) Insufficient groundwater measurements may lead to misjudgement of the groundwater conditions and the following consequences:
- an underestimate of the groundwater level or the permeability can give e.g. unstable constructions or increased settlements;
- an overestimate can e.g. give increased construction costs.
(3) There are many factors influencing the groundwater situation: geology, stratigraphy, climate, the atmospheric pressure, and also human activities. In figure L.1 the natural fluctuations in different kinds of ground are presented compared to the actual precipitation. Better correlations may sometimes be obtained by comparing the groundwater fluctuations with the net precipitation or an averaging of the precipitation for a certain period.
(4) Normally, the groundwater pressure is not hydrostatic due to layers of various permeability and groundwater flow which should be considered.
(5) In this annex some additional information about groundwater measurements is presented in order to facilitate the fulfilment of the requirements in section 14.
(6) Guidance for the arrangements of some groundwater measuring systems with respect to different aquifer systems can also be found in the examples mentioned in Annex M.
(7) When locating the piezometers or stand pipes for a certain project it is necessary to know the purpose of the measurements and the soil layer sequence. In complex soil layers and hilly terrain a number of measuring points can be required to model the groundwater conditions. This means that in hilly terrain measuring stations should be located where major changes in slope inclination occur.
(8) In each measuring station the elevation for each piezometer should be chosen with respect to the purpose of the measurements and the permeability variations of the ground.
(9) When the groundwater table should be observed in soils with a high and rather constant permeability, only one pipe penetrating the water table is normally required.
(10) In clays or in soils with a variable permeability with depth a number of (at least three) piezometers are normally required to obtain a groundwater pressure profile.
(11) Installation of the groundwater pipes or piezometers may be performed by pushing or by preboring.
(12) In soft clay it is normally sufficient to push the penetrometer carefully into the ground. However, preboring to the groundwater table is required to keep the piezometer tip or filter water saturated during installation. The remaining annular spacing between the pipe and the soil can be filled with e.g. bentonite to prevent surface water inflow.
(13) When pushing the piezometer into clay, an excess pore water pressure up to about 15 times the undrained shear strength of the clay can be generated. This excess pore pressure will dissipate during the stabilizing period. The stabilizing period might be between 1 day and 20 days depending on the permeability of the soil.
(14) In order to prevent the piezometer from overloading, the penetration rate should be kept sufficiently low or a piezometer with sufficiently high upper pressure limit shall be chosen, provided the precision is satisfactory.
(15) Drilling water additives should be avoided when preboring for groundwater pipes or piezometers.
(16) When installing in prebored holes a filter should be arranged around the filter tip. Above that filter zone a seal should be made with a permeability lower than that of the surrounding ground.