14.4 Test procedure
(1)P The location and depth of each individual piezometer or groundwater pipe shall be chosen considering the purpose of the measurements, the topography, the stratigraphy and the soil conditions, especially the permeability of the ground or identified aquifers.
(2)P The number and location of measuring stations shall be chosen in such a way that an acceptable interpolation of the groundwater situation can be made for the actual purpose.
(3)P For monitoring projects e.g. groundwater lowering, excavations, fillings and tunnels, the location shall in addition be chosen with respect to expected changes to be monitored.
(4) The natural fluctuations in the groundwater pressure should also be measured in a reference measuring station outside but close to and in the same layers as the influenced zone also before and after the actual activities.
(5)P In order to obtain measurements reflecting the pore pressure in the intended point, soil or rock layer, provisions shall be made to ensure the required filter and sealing off towards other layers or aquifers and the function of the measuring equipment.
(6)P Installation of piezometers shall be made either by pushing or by preboring.
(7) During the installation the following should be checked, depending on the type of measuring equipment:
- the function of filters before and after installation;
- the saturation of filters and other water filled parts of the piezometer until inserted below water;
- that air bubbles in circuits are flushed away;
- that the generated excess pore pressure does not exceed the capacity of the equipment;
- that anti-freeze medium is added if applicable;
- that cables or pipes extended above ground surface are protected against filling or damage;
- that surface water cannot flow into the borehole;
- that elevation or depth of filter or stand pipe tip has been taken;
- that the screening to separate different aquifers is sufficient.
(1)P The number and frequency of readings and the length of the measuring period shall be settled in advance in a preliminary plan for each commission considering the purpose of the measurements and the stabilizing period.
(2) All measurements should be related to the actual elevation system at the site.
(3) In order to find long term fluctuations regular measurements at reasonable frequent intervals over a long time span should be chosen.
(4) For short term measurements, e.g. for monitoring effects of pile driving, groundwater lowering or excavations, also automatic recording systems should be considered.
(5) In order to check that reliable results have been obtained at a certain occasion, the measurements taken should be recorded and checked on site by e.g. comparisons with previous and neighbouring measurements.
14.5 Interpretation of groundwater measurements
(1)P The results of a groundwater measurement shall be expressed in one of the following ways:
- water pressure, related to end of open pipe or the average filter level, in m. water collumn;
- water table, elevation, in m;
- pore pressure at a certain depth or elevation, in kPa.
(2)P The interpretation of the groundwater measurements shall be made with respect to actual measurements, the design of the piezometer and the atmospheric pressure if applicable.
(3) For observational boreholes and open pipes with or without a filter, the groundwater table is normally related to the upper end of the pipe or the ground surface which should be recorded.
(4) For hydraulic piezometers the interpretation should be based on the measured pressure and the difference in elevation between the measuring unit and the middle of the filter.
(5) For pneumatic or electrical equipment where the membrane is located above the filter the interpretation should be based on the measured pressure and the difference in elevation between the middle of the filter and the membrane.
(6)P For pore pressure transducers measuring the total pressure correction shall be made for the actual atmospheric pressure.
(7) The possible time lag between the changes in atmospheric pressure and corresponding changes in the pore pressure should be considered.
14.6 Reporting of groundwater measurements
(1)P Records from a groundwater measurement project shall in addition to general information required in 2.6 contain the following information whenever applicable. Records shall be filed according to commission and presented in the ground investigation report.
- date of installation;
- type of measurement made, related to a certain elevation or atmospheric pressure;
- type of equipment e.g. by reference to published standard;
- atmospheric pressure while setting the zero reading for closed system transducers;
- ground level at the piezometer location;
- elevation of tip of pipe or filter;
- elevation of upper end of pipe;
- date of each reading;
- interpreted pore pressure or water table elevation;
- signatures at installation and each reading;
- other measures taken to investigate the groundwater flow or quality.
(2) The presentation of the results should consider the purpose of the measurements, the number of piezometers and the length of the measuring period. Some examples are presented in annex L.2.
(3)P The ground investigation report shall contain an evaluation of the groundwater measurements with respect to the accuracy or the necessity of making further installations or measurements.
(4) Inaccuracy in the measurements may occur for many reasons such as:
- gas or air bubbles in the system;
- clogging of the filter;
- insufficient tightening around the pipes;
- temperature fluctuations and freezing;
- changes in calibration factors;
- corrosion due to e.g. a galvanic cell;
- human activities, e.g. pumping in neighbouring wells.
(5) A demand for further installations or measurements may arise where the soil profile is more complicated and unexpected groundwater pressure profiles are obtained.
14.7 Derived values of groundwater or pore pressures
(1)P Values of groundwater pressures shall be derived from groundwater measurements considering the natural fluctuations in ground, the season of the measurements and the actual meteorological conditions.
(2)P If applicable both upper and lower values for extreme and normal circumstances shall be derived. See ENV 1997-1, subclause 2.4.1, paragraph (10)P and (11).
(3) Upper and lower values for extreme circumstances can be derived from the measured values by adding or subtracting part of the expected fluctuations at that typical site considering the ground conditions. Further information can be found in annex M.3.
(4) Upper and lower values for normal circumstances can be derived from the measured values by adding or subtracting part of the expected fluctuations at the actual site. The reduction depends on the design situation, e.g. the time of consolidation for a soil layer.