For projects of some importance, or where the realization may be critical, monitoring is an important part of the geotechnical investigation program. Observations and measurements are essential elements in these investigations, and form the final part of the geotechnical investigation.
A geotechnical design is, after all, to a certain extent hypothetical. The geotechnical investigation, as extensive as it may be, gives input to a geotechnical model which can by no means cover all facets of soil behaviour. The designer must therefore make certain (simplified) assumptions, based on a long time experience and available geotechnical data.
Observations and measurements on site are very valuable for the geotechnical engineer to overcome the uncertainties of geotechnical design and to achieve sustainable and economical designs.
Although monitoring is primarily associated with measurements during the execution phase of projects, it can also be a useful tool to optimize design. Also after realization of a project, we continue monitoring to better understand soil behaviour and calibrate our design parameters.
Inclinometer measurements
Horizontal deformation of structures and soil masses can be determined by inclinometer measurements. To do so an inclinometer casing is installed in a near-vertical borehole. An inclinometer probe is used to survey the casing and to establish horizontal deformation of the casing.
Settlement measurements
Vertical deformation can also be measured over a certain length (e.g. under embankments) by means of a settlement casing or horizontal inclinometer casings. These casings are installed under the embankment and the deformation of the casings are measured. In this way a continuous settlement profile can be obtained. Different measurement systems are possible: open hydrostatic, closed hydrostatic and horizontal inclinometers.
Water pressure measurements using closed piezometers
Closed piezometers are used in cohesive, low-permeability soil layers (loam/clay/peat) to accurately measure the water pressure and any decrease or increase in the water pressure. The water pressure is measured by a water pressure cell that can be manually read off or connected to a logger for continuous measurement.
Soil pressure measurements
Ground pressure cells can be placed in boreholes to measure the natural soil pressures and the influence of certain earthworks on the soil pressures. Soil pressure cells can also be applied on contact surfaces of ground and constructions such as under embankments, against earth-retaining constructions or around tunnels.
Water level measurements using open piezometers
In sand layers water pressure is measured by means of open piezometer tubes.
With a rise or fall of the water pressure in the soil at the measurement position, a certain amount of water will flow into or out of the tube until the water level becomes balanced with the new water pressure.
The observed level in the piezometer tube is measured with a water level gauge.