11.5.1 Water pressure
Some commonly used piezometers in excavations are the open standpipe piezometer，the pneumatic piezometer，and the electronic piezometer.The piezometer can be installed in the soils within the excavation site，soils outside the excavation site，or on the diaphragm wall.Those installed on a diaphragm wall are usually electronic piezometers.
The basic configuration of an open standpipe piezometer，also called the Casagrande piezometer，is as diagramed in Figure 11.19.As shown in the figure，the main part of the open standpipe piezometer is a permeating pipe made of porous stone.The bottom end of the permeating pipe is a lid and the top end is connected to a vertical pipe（usually a PVC pipe）.When groundwater enters the permeating pipe，it will go up along the vertical pipe.When it goes up to the height which balances the water pressure at the level where the permeating pipe is placed，the height in the vertical pipe reads the water pressure at the center of the permeating pipe installed in soils.
Figure 11.19 Schematic configuration of an open standpipe piezometer
Figure 11.20 Schematic configuration of a pneumatic piezometer
The height in the vertical pipe can be measured by using a water level indicator，which is a coaxial cable of negative and positive poles in which the groundwater can be seen as the electrolyte.Put the indicator into the vertical pipe and let it contact the water，and a low⁃current circuit would then be formed and a signal be generated to reveal the height of water in the vertical pipe.The open standpipe piezometer is simple in its principle and reliable in its result，with easy installation.It is required，however，to wait till the groundwater has fully flowed into the vertical pipe before the height is measured.As a result，measuring the water pressure in clayey soils usually takes a long time.
The pneumatic piezometer，also called the closed piezometer，is as diagramed in Figure 11.20，illustrating its basic configuration.When the air pressure in the air pipe is smaller than the water pressure，p，as shown in the figure，the air keeps flowing into the pipe，and the air pressure will grow.When the air pressure in the pipe is equal to or exceeds the water pressure，p，the membrane moves outward and the air current is facilitated；air will flow out of it and the air pressure will thus be decreased.The largest reading of the air pressures in the pipe is then the water pressure.Nitrogen and CO2 are the most widely used gases.Using a pneumatic piezometer to measure the water pressure does not require water in great quantity to flow into the main part of the piezometer and thus does not take a long time to wait.It is suitable to be used in clayey as well as sandy soils.
The electronic piezometer in its basic design is similar to the earth pressure cell（see Figure 11.16）except it isolates the bearing plate from the influence of earth pressure.Therefore，a permeable stone is placed in front of the bearing plate which is acted on by groundwater flowing through the permeable stone.Thus，the water pressure is measured.The electronic piezometer can be divided into the resistance and vibrating types.Figure 11.21 diagrams the basic configuration and the installation of the electronic piezometer.The strengths and shortcomings of the open standpipe piezometer，the pneumatic piezometer，and the electronic piezometer are listed in Table 11.1.
The installation of a piezometer in soils is elucidated as follows：
①Bore a hole using a drill machine to 50 cm below the designed depth.
②Place the main part of the piezometer（no matter whether it is the open standpipe type，the pneumatic type，or the electronic type）in the bored hole and have the center of the piezometer located at the designed depth.Fill the bored hole with sandy soil.The sandy soil is used to help groundwater permeate into the main part of the piezometer and be measured.
③Seal the top（or both ends）of the sandy soil with bentonite to avoid groundwater from other elevations reaching the permeable pipe and influencing the measurement results.Above the bentonite is again filled with sandy soils to provide sufficient overburden weight to stabilize bentonite and sealing.
Figure 11.21 Schematic configuration of an electronic piezometer
Figure 11.22 Two piezometers installed in the same borehole
In the same bole can be placed more than one piezometer，all of them requiring sealing to avoid water pressure from other elevations，as shown in Figure 11.22.Depending on the monitoring aim，the piezometer can be embedded in clayey or sandy soils.Using one in clayey soils can derive the effective stress by measuring excess pore water pressure induced by soil deformation during excavation.Neither the excess pore water pressure nor the effective stress directly relates to excavation safety.They are，nevertheless，helpful to understand the deformation behaviors and stability characteristics.The piezometer aimed，as above elucidated，should be embedded in the central section of the excavation site.
The piezometer embedded in sandy or permeable soils can monitor the variation of water pressure，which is helpful for the diagnosis of the excavation safety.If the water pressure changes abruptly while the stress on the reinforcement of the retaining wall is abnormal，it must get extraordinary attention and an effective remedial measure should be adopted to safeguard the excavation safety.
Table 11.1 Strengths and shortcomings of various types of piezometers
