9.3 Installation of piezometers
9.3.1 Execution
9.3.1.1 General
9.3.1.1.1 An installation plan shall be drawn up and documented prior to configuring a piezometer.
9.3.1.1.2 If the ground conditions are unknown, an investigation of the soil or rock properties shall be carried out in advance. In stratified grounds, the variation of groundwater potential with depth shall be considered when choosing the installation level and filter length.
9.3.1.1.3 The installation of piezometers shall not permanently affect the groundwater flow and quality and shall be made in such a way that the groundwater conditions can be measured correctly in accordance with the design. Any hydraulic connections opened up between different layers during installation shall be closed again by means of suitable seals immediately and prior to the measurements. A seal at the ground surface shall be made to prevent precipitation, condensation or seepage water entering the system directly. The installation of the piezometer shall be documented in a record (see 12.1.7).
9.3.1.1.4 Uponcompletion, the position and elevation shall be established (e.g. height of upper edge of the open measuring pipe above reference level for the groundwater measurement) and documented on a site plan.
9.3.1.1.5 Protective measures shall be carried out as requested.
9.3.1.2 Open systems
9.3.1.2.1 Open systems can be installed in boreholes or by ramming, pushing or water-jetting the piezometer pipe into the ground.
9.3.1.2.2 When installing piezometer pipes in boreholes, a separate borehole shall in general be drilled for each groundwater layer and for each piezometer pipe. If morethan one piezometer should be installed in one borehole, it shall be proven that the equipment and the procedure allow for correct measurements in all layers. Attention shall be paid to the difficulties involved in sealing groundwater layers so as to prevent connections between them. The threaded joints of piezometer pipes passing through several groundwater layers shall be watertight. Installation of several piezometer pipes in the same borehole should be restrictedto special cases. The diameters selected for the drilled holes shall depend on the intended configuration of the borehole and depth of drilling.
9.3.1.2.3 Open piezometers comprise a filter fitted with filter pipes to hold back soil material and solid- walled pipes placed on top of the filter pipes which extend up to the ground surface.
9.3.1.2.4 The piezometer pipe shall be constructed and dimensioned in such a manner as to fulfil its purpose safely during installation and measurement.
9.3.1.2.5 The types of filter and filter pipe shall be selected according to the ground structure and method of installation.
9.3.1.2.6 If a filter pack is used, the filter material shall enclose the filter pipe completely and extend at least two times the diameter of the borehole over the top of it to allow for possible settlement. The top of filter pack shall not be higher than the top of thelayer of interest. The filter gravel or sand shallbe placed continuously in small quantities to avoid bridging (Dmax less than 15 % of free annular spacing). The filter pipes and solid- walled pipes shall be installedwith centring devices to ensure that the annulusis completely filled. A seal shall be installed above the filter pack to avoid any pressure equalisation via the area outside the pipe.
9.3.1.2.7 In order to ensure the filter stability, the thickness and gradation of the filter pack shall be assessed as a function of the gradation of the surrounding ground and the purpose of the measurements. The construction of the screen shall be chosen as a function of the filter pack. Filter packs are generally required in open systemsto prevent soil particles from entering the filter and clogging it. They shall be filter-stable against the surrounding ground and the sealing.
9.3.1.2.8 The length and depth of the filter pipe depend on their intended purpose and the ground conditions.
9.3.1.2.9 When configuring piezometers in boreholes, all layers which separate groundwater head shall be sealed using swelling clay or by injecting a suspension. The swelling clay used (e.g. pellets, beads, granules) shall have good settling properties to ensure that the entire annulus is filled uniformly over the required height.
9.3.1.2.10 The sealing suspension should be placed by means of injection pipes, inserted in the annular space or an injection device inserted in the casing. The injection process shall proceed without interruption fromthe bottom of the borehole to its top.
9.3.1.2.11 The height ofthe seal and the seal material depend onthe thickness and permeability of the layer to be sealed. It shall be not less than 1 m.
9.3.1.2.12 Penetration of surface water shall be prevented by a seal of swelling clay which should generally be at least 1 m high and installed at least 0,5 m below frost depth. Where shallow piezometers should not allow for this, the seal shall be installed between ground level and the top of the filter pack. Piezometers shall be protected against frost heave by installing frost-proof material between the seal and the ground surface.
9.3.1.2.13 A graphical presentation of the installation with the corresponding soil and rock strata should be prepared for all groundwater measuring stations (see 12.1.7).
9.3.1.2.14 The piezometer pipes shall have cover and be locked, if required.
