SOIL INVESTIGATION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 39

local density (sands). The ground-probe contains two sets of electrodes, one for the
current and one for measurement. In an EM-investigation, electromagnetic
induction is used to measure the electrical conductivity of the subsoil from the
surface. A transmission coil on one side of the instrument sends an alternating
current with a fixed frequency into the soil, which creates a primary magnetic field
in the subsoil. This induces small currents in the subsoil that in turn create a
secondary magnetic field. A receiving coil records the secondary magnetic field
and the primary field. From these data the electrical conductive capacity of the
subsoil can be found, which represents an average value of the electrical
conductive capacity of the layered subsoil.
The groundwater-sampling probe has an externally located filter element.
During penetration of the probe pressurised nitrogen gas is applied to the filter,
keeping it free of incoming water and/or soil particles. At the measuring location,
groundwater is let entering the sampling chamber, where the electric resistance, the
pH and the temperature are being measured. Next the nitrogen gas expels the water
and the probe can advance to the following location. As an addition/alternative, the
groundwater can be fed through a set of anaerobic flow cells, where several
physical/chemical parameters can be measured.
The determination of hydrocarbon pollution in soils (DNAPLs) is performed by
lightening the soil through a sapphire glass window with UV light, mounted in a
standard CPT probe. The fluorescent hydrocarbon reflects the light, which is
detected by a photo-multiplication tube and registered via an electric cable to a
data-acquisition system.
Monitoring
The goal of geotechnical monitoring is the development and maintenance of
sensor network and remote observation (e.g. InSAR) technologies for early
warning systems, which detect technical and environmental factors that affect
safety and health, and which make it possible to forecast failures. A detection
system for failure mechanisms might be cheaper and safer than the alternative:
over-dimensioning. Monitoring will increase the understanding of mechanical soil
behaviour.
Developments in communication and sensor technology have advanced so far
that it seems possible to utilise it to effectively support the management and
monitoring of construction works and their functioning in an economically efficient
manner. Acoustic sensors may detect the noise that (sandy) soils produce when
deforming. However, most of the recently developed sensor and communication
technology (e.g. optic fibres, wireless LAN), needs to be tested under field
circumstances, to prove their robustness, suitability and efficiency.
Determining real failure processes is still a research field in development.
Strength depends on a number of parameters, which are hard to determine.
Numerical calculation methods for strengths and performance suffer from
significant uncertainty between calculated and actual strength. Monitoring supports
such models, fed by real time data from sensors and remote observation, when
calculating the short and long time future of the structure and reporting if
immediate safety issues are at stake.