(1)P The type of equipment to be used for groundwater measurements shall be chosen in advance with respect to the actual type of ground, the purpose of the measurements and the response time of the equipment and the soil system.
(2) At the planning of groundwater measurements for determining the groundwater table or the pore pressure, the soil and rock conditions ought to be considered especially where there are great variations in the permeability. At the selection of the measuring system it is also important to consider the purpose of the measurements:
- to have a single observation of the groundwater table or the pore pressure profile;
- to determine the fluctuations in the groundwater level or pore pressure for a certain period;
- to have a monitoring system.
(3) There are two main methods for measuring the groundwater pressure, open systems and closed systems:
- open systems in the form of observation boreholes and open perforated pipes can only be used in homogeneous soils and rock with a high permeability, e.g. sand, gravel or very fissured rock, where there is no risk for soil particles to enter the borehole or pipe;
- open systems as open pipes with filter tips and an inner plastic hose can also be used in soils with low permeability e.g. glacial deposits, silts and clay if the response time is short enough for the purpose of the measurements;
- closed systems should be used in low permeable soils and when continuous recording is required or short term variations should be monitored e.g. for flow-net derivations. Also in case of high artesian water pressure closed systems are recommended.
(4) Guidance for the choice of measuring system depending on the purpose of the measurements and the type of ground at the measuring level can be found in table 14.1.
|Ground conditions||Gravel, coarse sand||Fine sand, coarse silt||Fine silt, glacial deposits, clay|
|Purpose of measurements|
|measurements of groundwater levels or pore pressure profiles and their fluctuations||observation borehole open pipe filter tip||open pipe filter tip piezometer (hydraulic, pneumatic, electric)||filter tip piezometer (hydraulic, pneumatic, electric)|
|measurements of variations in pore pressures due to fluctuations, pumping, excavations, loading or unloading, effects of pile driving or for monitoring of e.g. slopes etc.||Filter tip piezometer (hydraulic, pneumatic, electric)||filter tip piezometer (hydraulic, pneumatic, electric)||piezometer (hydraulic, pneumatic, electric)|
(5)P In cases where lakes, rivers or creeks are situated within or close to the actual investigation area the water level in those shall be measured as references to other measurements. Also the water level in wells, the occurrence of springs and artesian water shall be noted.
14.3.2 Basic requirements
(1)P The equipment shall measure the pore pressure in relation to the actual atmospheric pressure or shall be capable to measure both the total pressure and the atmospheric pressure.
(2)P Metallic parts of the equipment in electric connection with each other shall be manufactured from the same type of alloy in order to avoid galvanic cells in the ground.
(3) In case of risk of freezing the measuring system should be filled with an antifreeze medium with a density equal to that of water or replaced by a closed system if applicable.
(4)P In cases where the pore pressure is to be measured at a certain level or in a certain layer, special equipment with packers shall be used, or special installation measures shall be taken to ensure that connections with other layers are blocked.
(5)P The equipment selected shall be adequate to provide reliable data during the whole observation period.
(6)P Open pipes with filter tips provided with an inner plastic hose shall allow gas bubbles to pass through the hose.
(7)P Closed hydraulic piezometers shall allow flushing of deaired water through the system in order to release possible air bubbles.
(8) Piezometers of the closed type should be calibrated before each installation or reinstallation.
(9) Closed systems should preferably allow for so called zero-checks to be performed during the measuring period especially for long term measurements.
(10) Pneumatic piezometers should be calibrated with the actual length of circuit and the actual measuring unit.
(11)P The required precision of the measurements for a certain project, shall be decided in advance so that a proper equipment for the project can be chosen.
(12) Taking into account all possible sources of error and the compensation for the atmospheric pressure, the precision of the measurements should normally not be worse than 1 kPa in the range 1 kPa to 100 kPa and 2 kPa for values greater than 100 kPa.