Commonly Used Monitoring Devices - Instrumentation for Foundations Engineering.

Some of the more common monitoring devices are as follows:

Inclinometers. The horizontal movement preceding or during the movement of slopes can be investigated by successive surveys of the shape and position of flexible vertical casings installed in the ground (Terzaghi and Peck, 1967). The surveys are performed by lowering an inclinometer probe into the flexible vertical casing. The inclinometer probe is capable of measuring its deviation from the vertical. An initial survey (base reading) is performed and then successive surveys are compared to the base reading to obtain the horizontal movement of the slope.

Figure 17.7 shows a sketch of the inclinometer probe in the casing and the calculations used to obtain the lateral deformation. Inclinometers are often installed to monitor the performance of earth dams and during the excavation and grading of slopes where lateral movement might affect off-site structures. Inclinometers are also routinely installed to monitor the lateral ground movement due to the excavation of building basements and underground tunnels.

Inclinometer probe in a casing.
FIGURE 17.7 Inclinometer probe in a casing.

Piezometers. Piezometers are installed in order to monitor pore water pressures in the ground.
Several different types are commercially available, including borehole, embankment, or push-in piezometers. Figure 17.8 shows an example of a borehole piezometer.

Pneumatic piezometer installed in a bore-hole.
FIGURE 17.8 Pneumatic piezometer installed in a bore-hole.
In their simplest form, piezometers can consist of a standpipe that can be used to monitor ground-water levels and obtain groundwater samples. Figure 17.9 shows an example of a standpipe piezometer. It is standard procedure to install piezometers when an urban project requires dewatering in order to make excavations below the groundwater table. Piezometers are also used to monitor the performance of earth dams and dissipation of excess pore water pressure associated with the consolidation of soft clay deposits.

Standpipe (Casagrande) piezometer.
FIGURE 17.9 Standpipe (Casagrande) piezometer.
Settlement Monuments or Cells. Settlement monuments or settlement cells can be used to monitor settlement or heave. Figure 17.10 shows a diagram of the installation of a pneumatic settlement cell and plate. More advanced equipment include settlement systems installed in borings that can not only measure total settlement, but also the incremental settlement at different depths.

Pneumatic settlement cell installation.
FIGURE 17.10 Pneumatic settlement cell installation.

Settlement monuments or cells are often installed to measure the deformation of the foundation or embankments during construction or to monitor the movement of existing structures that are located adjacent to the area of construction.

Pressure and Load Cells. A total pressure cell measures the sum of the effective stress and pore water pressure. The total pressure cell can be manufactured from two circular plates of stainless steel. The edges of the plates are welded together to form a sealed cavity that is filled with fluid.

Then a pressure transducer is attached to the cell. The total pressure acting on the sensitive surface
is transmitted to the fluid inside the cell and measured by the pressure transducer (Slope Indicator
Company, 1998).

Total pressure cells are often used to monitor total pressure exerted on a structure to verify design assumptions and to monitor the magnitude, distribution, and orientation of stresses. For example, load cells are commonly installed during the construction of earth dams to monitor the stresses within the dam core. During construction of the earth dam, the total pressure cells are often installed in arrays with each cell being placed in a different orientation and then covered with compacted fill.

For the monitoring of earth pressure on retaining walls, the total pressure cell is typically placed into a recess so that the sensitive side is flush with the retaining wall surface.

Load cells are similar in principle to total pressure cells. They can be used for many different types of geotechnical engineering projects. For example, Fig. 17.11 shows a center-hole load cell that is designed to measure loads in tiebacks. This center-hole load cell can also be used to measure loads in rock bolts and cables. As shown in Fig. 17.11, for best results, the load cell is centered on the tieback bar and bearing plates are placed above and below the cell. The bearing plates must be able to distribute the load without bending or yielding.

Center-hole load cell.
FIGURE 17.11 Center-hole load cell.
Crack Monitoring Devices. For construction in congested urban areas, it is essential to monitor the performance of adjacent buildings, especially if they already have existing cracking. This can often be the case in historic districts of cities, where old buildings may be in a weakened or cracked state.

Monitoring of existing cracks in adjacent buildings should be performed where there is pile driving or blasting at the construction site. The blasting of rock could be for the construction of an under- ground basement or for the construction of road cuts. People are often upset by the noise and vibrations from pile driving and blasting and will claim damage due to the vibrations from these construction activities. By monitoring the width of existing cracks, the geotechnical engineer will be able to evaluate these claims of damage.

A simple method to measure the widening of cracks in concrete or brickwork is to install crack pins on both sides of the crack. By periodically measuring the distance between the pins, the amount of opening or closing of the crack can be determined.

Other crack monitoring devices are commercially available. For example, Fig. 17.12 shows two types of crack monitoring devices. For the Avongard crack monitoring device, there are two installation procedures: (1) the ends of the device are anchored by the use of bolts or screws, or (2) the ends of the
device are anchored with epoxy adhesive. The center of the Avongard crack monitoring device is held
together with clear tape that is cut once the ends of the monitoring device have been securely fastened
with bolts, screws, or epoxy adhesive.

Other Monitoring Devices. There are many other types of monitoring devices that can be used by the geotechnical engineer. Some commercially available devices include borehole and tape exten- someters, soil strainmeters, beam sensors and tiltmeters, and strain gauges. See  Geotechnical Instrumentation for Monitoring Field Performance (Dunnicliff, 1993) for a further discussion of monitoring devices.

Crack monitoring devices. The upper photograph shows the Avongard crack monitoring device. The lower diagram shows the VW Crackmeter
FIGURE 17.12 Crack monitoring devices. The upper photograph shows the Avongard crack monitoring
device. The lower diagram shows the VW Crackmeter

1 comentarios:

Khalid Rehman said...

very valuable geotech information.
Research Officer
Wapda Mangla Dam Pakistan

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