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The geotechnical investigation for any worksite is preceded by the preliminary geological-geotechnical survey. This consists of an on-site superficial survey, sometimes involving small investigation activities, such as the opening of shallow pits and shafts. Usually, the field visit is preceded by a desk study comprising the collection of existing written and drawn information about the site, topographic and geological charts in particular, and geotechnical charts if available.
Currently, the use of two and three-dimensional aerial images obtained from satellite and free web navigation tools is also very useful, particularly for large works outside urban areas, such as roads, dams, and for the stabilization of natural slopes. When dealing with densely populated areas, geotechnical characterization reports for nearby constructions can generally be found, which represent important sources to be collected and checked.
Data analysis of all this information, as well as the preliminary site survey data, is compiled in a report. This will form the basis of the preliminary design or viability studies stage of the construction works. This report will also define the most appropriate geotechnical investigation program for the project. In general, the geotechnical investigation includes geophysical investigations. This article discusses the non-destructive geophysical methods of discussion.
1. Electrical Resistivity Method
The resistivity method is one of the most widely used among the non-destructive geophysical methods. The technique determines the apparent electrical resistivity of the ground. This property in a layered ground is variable in depth with the lithology and microstructure. This method is also particularly very sensitive to the water content.
Figure-1 shows a conventional arrangement with a pair (AB) of current electrodes and a pair (MN) of potential electrodes.
By establishing an electrical field from AB, the measured potential in MN allows calculation of the resistivity of a given soil section encompassed by the electrical field.
There are two main ways of performing the investigations. In the first mode, the electrodes are maintained on the same line but the current electrodes are progressively expanded with reference to the fixed central point. This induces an electrical field deeper with a consequent variation of the measured resistivity. The result is interpreted as a vertical 1-D sounding below the central position, which is called geoelectric sounding.
In the second mode, the relative distance between the electrodes is fixed but the array is laterally moved. This enables the covering of a given rectangular area in plan. The subsurface of the profile of the area is characterized in terms of resistivity to a given depth. Thus, it provides 2-D and 3-D models of the investigated ground, which is called geoelectric profile.
Traditionally, the interpretation of the results of the geoelectric soundings was achieved by using theoretical resistivity charts. At present, the interpretation of the results of any kind of geoelectric investigation is done by applying a specific software.
2. Seismic Refraction Method
The seismic refraction method is a well-established and traditional non-destructive method. This method consists of the generation of elastic waves on a given point of the surface. These waves are detected by receivers (vertically-sensitive geophones) placed along a linear array at the surface. The source of the waves may be a small explosion or a mechanical vertical impact of a hammer on a steel plate placed on the ground surface. The method is shown in Figure-2(a).
If soil-A is closer to the surface and it is underlain by soil-B of higher wave propagation velocity, then some of the waves will refract at the interface. It can be demonstrated that the critical angle of refraction (ic) is related to the ratio of the wave velocities by the following equation:
V1/V2 = sin(ic)
Critical angle of refraction is the ratio of velocity of refracted waves travelling along the interface to the velocity of the lower layer.
Figure-2(b) depicts a typical record of results, relating the distance (d) of each receiver from the source with the time interval between the generation of the elastic waves and the detection of the first wave arrival. The first wave is a compression wave, the velocity of which is greater than that of a shear wave. It can be seen that beyond a given distance from the source, the refracted waves are received prior to direct waves which travel on the surface at a lower velocity. For direct waves, the arrival time is:
d/V1 = t1
Taking h1 as the thickness of soil-A, the arrival time for a refracted wave is:
t2= [2h1/cos(ic) x V1]+[d/V2]-[2h1tan(ic)/V2]
For the point of intersection of the two straight lines:
t1 = t2
With the help of the above equations, the value of thickness h1 of upper layer can be obtained.
3. Spectral Analysis of Surface Waves (SASW) Method
The SASW method involves the generation of Rayleigh waves at a given point on the surface. The induced vertical motion at various distances is recorded by receivers placed along a linear array, as shown in Figure-3.
Rayleigh waves propagate in a layered ground with varying wavelengths and phase velocities, due to the variation of the shear stiffness from layer to layer. The greater the wavelength, the deeper the layer involved by the wave dispersion, as shown by Figure-4.
The interpretation of these records, through numerical codes especially developed for this purpose, leads to a profile with the Vs distribution in depth. Therefore, this method can be considered a non-destructive alternative to seismic field (destructive) tests such as the cross-hole or down-hole tests.
4. Ground Penetrating Radar (GPR) Method
The application of GPR is based on the fact that electromagnetic (EM) waves propagate in the ground at the velocity of light divided by the square root of the dielectric constant of the material.
V = C/ (k)0.5
Where, c is the velocity of light and k is the dielectric constant. Different soil layers will have different values for this constant.
Basically, GPR at the surface transmits EM waves into the ground. When the waves meet an interface between materials, a part of the energy is reflected towards the ground surface. The greater the difference between the values of the dielectric constant of the two materials, the greater the reflected energy.
In general, the transmitter and the receiver are associated in the same apparatus. By moving this apparatus at the surface along a straight line, and maintaining a high emission rate, a continuous record is obtained, as represented in Figure-5. The interpretation of these results is performed with the help of specialized software.
5. Advantages and Disadvantages of Geophysical Methods
Table-1 summarizes the main advantages and disadvantages of the non-destructive methods described and their capabilities.
|Electrical resistivity||1. Inexpensive technique. |
2. Fairly straightforward interpretation.
|1. Multiple measurements using different source–receiver layouts are required. 2. Non-uniqueness of results.||1. Geological mapping |
2.Hydrogeology 3. Water table depth
4. Top of bedrock
5. Cavity detection
|Seismic refraction||1. Inexpensive technique. |
2. Fairly straightforward interpretation.
|1. Relatively large source–receiver offset. 2. Only works if VP increases with depth.||1. Geological mapping |
3. Water table depth
4. Top of bedrock
|SASW||1.Good characterization of shallow material. |
2. Accurate profile of Vs with depth.
|1. Multiple measurements using different source–receiver layouts are required. 2. Interpretation requires high-level expertise.||1. Geological mapping |
2. Profile of Vs with depth
|GPR||1. Portable equipment. |
2. Easy for non-expert to visualize information.
|1. Very limited penetration in clay-rich environments. |
2. Dispersion and scattering higher than in other seismic tests.
|1. Geological mapping |
2.Hydrogeology 3. Cavity detection
The ground-penetrating radar method is based upon the propagation of electromagnetic waves in the ground. When the waves meet an interface between the material, some amount of energy is reflected back to the ground. Thus, a continuous soil profile can be recorded.
In SASW method, the results cannot be interpreted by a non-expert, whereas, in the GPR method, the results are very simple and can be interpreted by a non-expert.
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