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Geophysics has been used in trenchless projects more and more frequently over the past few years. Major construction projects often use geophysics to image the sub-surface along the proposed route to detect changes in soil and rock stratigraphy, locate buried utilities and map fracture zones. The ability of geophysics to mitigate risks associated with unexpected delays has often resulted in significant cost savings. A variety of geophysical complementary methods are employed on projects of this nature.
Advancement in technology for trenchless projects is taking place at a rapid pace, and more and more sophisticated, advanced and costly machines are available in market. The performance of these machines, and the productivity, largely depends on the subsurface medium encountered, and most important a prior knowledge of subsurface properties, enabling right choice of machines, bits and tools.
Various techniques like Ground Penetrating Radar (for detection of buried utilities and shallow stratigraphy), Seismic Refraction (to map soil-bedrock interface and determine mechanical strength of subsurface), Electrical Tomography (to map water saturated zones, faults, fractures) and seismic reflection (deep investigations for faults, fractures, shears) are available to provide quick information on subsurface conditions. Use of these techniques ensures proper equipment selection, less down time for equipment, and as a result, savings on projects.
The full potential of geophysics in engineering investigations has still to be realized in India. The various techniques can provide important information about the ground, its mass properties, its small-scale variations, and its anomalies of structure or content. The advantage of a geophysical survey is that it enables information to be obtained for large volumes of ground that cannot be investigated by direct methods due to cost. In India, Geophysics is still insufficiently or inappropriately used in engineering and the newer capabilities are not appreciated, so there is a need for up-to-date guidance about how to apply geophysical investigations.
The present paper aims to apply geophysical techniques to route investigations. It explores the roles of geophysical methods and provides the background to geophysics as an investigative tool. The paper also explains the need for a conceptual ground model to enable appropriate investigative methods to be chosen. The resulting report should be of interest to geotechnical and civil engineers, geologists and engineering geologists, specialist geophysics contractors, contractors, consultants and clients.
BENEFITS AND LIMITATIONS OF GEOPHYSICS
Geophysical investigation is an indirect approach for investigation subsurface or structures. Various physical properties of subsurface like conductivity, hardness, presence of anomalies, dielectric properties, moisture content, density, interfaces etc., can be determined using various techniques like seismic refraction, seismic reflection, electrical imaging, ground penetrating radar, EM, VLF, Borehole geophysics etc. It is even possible to determine dynamic modulus like shear modulus, bulk modulus, poisson’s ratio etc., using geophysical techniques. Interpretation of geophysical survey data usually requires some prior knowledge of the underlying geological structure. For optimum interpretation of geophysical data it is important that adequate direct information is available, which can be provided by boreholes or trial pits for example.
Geophysical surveys can offer considerable time and financial savings compared with borehole investigations. At an early stage of site investigation it may be beneficial to undertake a reconnaissance geophysical survey to identify areas of the site which should be further investigated using invasive techniques i.e. those where anomalies have been identified. Geophysics has a unique advantage of providing continuous profile of subsurface rather than discreet information as provided by boreholes. This is critical in areas with complex geology and in projects like tunnels, where a small shear zone can lead to major challenges during execution. Geophysical surveys can be used effectively to determine the geological, hydrogeological and geotechnical properties of the ground mass in which the engineering construction is taking place.
Using geophysical techniques to solve engineering problems has sometimes produced disappointing results, particularly when a method, which lacked the precision required in a particular site investigation has been used, or when a method has been specified that is inappropriate for the problem under consideration. In most of the cases these problems can be avoided by taking services of an experienced geophysicists and access to various techniques available. In other cases the geological conditions at the site have been found to be more complex than anticipated at the planning stage of the geophysical survey and hence interpretation of the geophysical data by the geophysicists has not yielded the information expected by the engineer. It is often advisable to undertake a feasibility study at the field site to assess the suitability of the proposed geophysical techniques for the investigation of the geological problem.
Once the geophysical data has been obtained, it is possible to produce a model of the geological structure, which gives a realistic correlation with the data. The best overall model is obtained by using all the available geological information from boreholes and field mapping. Without this input of precise information, which includes knowledge of the fundamental physical properties of the geological material at the site, the model cannot be constrained in practical terms. There needs to be close collaboration between site geologists, engineers and geophysicists in the interpretation of the geophysical data.(...)
1. What is geophysics?
1.1 Benefits and limitations of geophysics
1.2 Planning and site investigation in tunnelling
1.3 General principles of geophysical investigations
1.4 Challenges of the underground
1.5 Importance of geology
1.6 When to conduct geophysical investigations
1.7 Geophysical scope issues
1.8 Geophysical techniques
2. Ground penetrating radar
2.1 Basic principles
2.2 Application areas
2.3 Advantages
2.4 Limitations
3. Seismic refraction
3.1 Basic principle
3.2 Features
3.3 Applications
3.4 Advantages
3.5 Limitations
4. Electrical resistivity tomography
4.1 Basic principle
4.2 Applications
4.3 Advantages
5. Seismic reflection
Conclusions
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Non-Invasive Subsurface Investigations for Trenchless Projects | 10 pages | | | €5.40 |
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