The piezometer
What is a piezometer?
The piezometer is an instrument for measuring the level of the water table and/or the interstitial pressures. This instrumentation is installed in the probing hole and performed with continuous core drilling or core destruction.
Types of piezometer
There are different types of piezometers adopted in low to the hydrogeological complexity present in the study site. Below are some of them.
Open Tube Piezometer
In simple hydrogeological conditions, characterized by a single free water table, the most suitable and economical choice for the measurement of the groundwater level is the open tube piezometer (Fig.1A). It consists of a cracked or microslotted PVC pipe (Fig.1 B and C) in the pitched sections and installed inside the probing hole.
Particular care must be taken when placing the siliceous gravel calibrated in the gap between the piezometric pipe and then drilled. Generally, it is advisable to isolate the first 2–3 meters of the surface by injecting bentonite into the cavity in order to avoid the runoff water reaching the water table.
In more complex hydrogeological conditions, characterized by pressurized strata or artesian strata in multifalda contexts, it is advisable to use Casagrande or Vibrating Chord piezometers (explained in the following paragraphs) by inserting groundwater insulation methods in pressure with respect to the free flow.
Impact of soil permeability on the time taken to restore hydrogeological conditions for first reading
Once the installation phase of the piezometric pipe has been completed, it will be necessary to wait a certain period of time for the first reading. This time depends heavily on the permeability of soils, as it is necessary to restore the conditions of hydrogeological equilibrium. In particular, the use of circulation fluids during the drilling phase alters the static level of the aquifer. In the case of inconsistent soils or fractured lithotypes, the first reading can also be performed the day after the completion of the survey, while for cohesive soils (silts and clays) it will be necessary to wait at least 7–10 days.
Figure 1 – Open tube piezometer with photographic details of blind and microslotted PVC tubes
The measurement of the water level inside open tube piezometers is commonly done by the electric frequency meter or by a pressure transducer (Fig.2).
Figure 2 – A) Diagram of use of the freatimeter, B) Instrumentation of the freatimeter and protection cover of the piezometric piping.
What ground is it suitable for?
It is mainly suitable for sandy-gravelly coarse soils.
Piezometer of Casagrande
Unlike the open-tube piezometer, the Casagrande one allows the measurement of neutral pressures.
The principle of operation is based on the change in the volume of a vessel (in this case the piezometer) following immersion in a water table. The pressure of the water acting on the bottom of the piezometer exerts pressure on the moving piston, causing a change in the volume of the piezometer itself.
The pressure exerted on the moving piston is transmitted to a pressure transducer. The latter provides a reading of the pressure exerted by groundwater, which can be used to determine the water pressure at that point.
In addition, the change in the volume of the piezometer can be used to determine the flow rate of aquifers.
Figure 3 – Casa Grande filters and piezometric system (from Sisgeo srl mod.)
The instrument consists of a cylindrical tube (steel or PVC) with one end open and the other closed by a movable piston (Fig.3). The tube connects the porous element (at the base of the piezometer) to the surface of the hole (on the ground floor).
The reading of the piezometric level or pressure can take place through pressure transducers or through reading with an electric probe or a frequency meter.
What ground is it suitable for?
It is mainly suitable for cohesive soils (clays and limi).
Piezometer with Vibrating Rope
A vibrating string piezometer is a type of instrument used to measure interstitial water pressure or water level in environments no longer accessible after installation.
The device consists of a vibrating string (usually steel or carbon fiber), a control unit to vibrate the string, a sensor to detect the propagation speed of sound waves and a recording system to record data (Fig.4).
Fig.4 – Vibrating wire piezometer, da Soil instruments
The string is passed inside a probing hole and the pressure of the interstitial water is determined by measuring the propagation velocities of the sound waves along the string. The propagation speed of sound waves depends on the pressure of groundwater. Generally, due to their strength and durability, they are used for large works such as dams, tunnels, bridges etc.
What ground is it suitable for?
It can be used in a variety of soils, from coarser to finer.
Conclusion
Each type of piezometer has its pros and cons, advantages and disadvantages (such as cost, installation, adaptability to environmental conditions, maintenance) and there is no single type of piezometer that is the perfect solution for every situation, so it is important to choose the most suitable one for the specific needs of the project.
