Use of GPS, RS and GIS in Geotechnical Engineering
Ms. Anamika Sharma
Department of Civil Engineering, Jabalpur Engineering College, Jabalpur.
Civil engineering applications such as transportation, environmental engineering, geotechnical engineering, urban planning, etc. rely on highly accurate reliable, up-to-date data. Precisely if we talk about Geotechnical Engineering the main task about a site is to acquire accurate data about a site and to assess the suitability of the site and then prepare an adequate and economic safe design for the proposed work. Now-a-days, geotechnical considerations are given due recognition in the construction of engineering projects. The traditional methodology alone is no longer followed, as the chances of error are just too much in them. Modern investigation techniques such as GPS, RS and GIS are used as with these technologies not only the missing spatial data could be generated to cope with the existing geographically referenced data and the corresponding attribute information, but modeling and simulation for specific requirement and fore costing can also be carried out. This paper provides an overview of how these technologies can be effectively put to use in geotechnical investigations for acquiring economic and safe design for the proposed work.
Geotechnical investigations are made to determine those geologic, seismologic, and soil conditions that affect the safety, cost effectiveness, and design of a proposed engineering project. Insufficient geotechnical investigations, faulty interpretations of the results, or failure to portray results in a clearly understandable manner have contributed to costly construction changes and post construction remedial work and could be the cause of failure of the structure. The investigations are performed to determine the geologic setting of the project; the geologic, seismologic, and soil conditions that influence selection of the project site; the characteristics of the foundation soils and rocks; all geotechnical conditions which influence project safety, design, and construction; and sources of construction materials.
The strength stability durability and cost effectiveness of major engineering projects such as Dams, Bridges, and Tunnels etc are invariably related to the terrain geomorphology as well as subsurface geological parameters. Past engineering projects in India in which geological considerations at time of construction were taken care off, have shown that geotechnical knowledge has played a very important in their stability and durability. However, there are cases in which the post failure studies have revealed that ignorance of geotechnical conditions that existed in and around these structures, were basically responsible for such failures.
The methods employed for geotechnical investigations and the magnitude of the investigations depend on the size of the project, type of the structure, and the complexity of regional and local geotechnical conditions. The traditional methods of soil investigations include plate load test, standard penetration test, CPT, and analysis of chemical and engineering parameters of soil or rock sample in laboratorary. In major engineering projects such as dams, Bridges and tunnels, high relief roads data collection is very difficult job, due to inhospitable environment in most of the construction sites. Hence taking into account such hazards or any unforeseen circumstances, GPS and RS technologies will provide not only fast and accurate but also reliable geomorphologic and geological data, which can be very successfully analyzed by GIS to give complete picture about the stability and durability of the structure.
Analysis of the following geological parameters of the site is done for geotechnical investigations:
.Topographical and drainage characteristics
Geophysical investigations of site
Geological structures and rock types
Hydrological parameters of the site
2. Use of GPS
Global Positioning System (GPS) is a space navigational system that can pinpoint your position anywhere on the globe, usually within few yards or meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellite transmit data via high frequency radio waves back to earth and , by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe.
The GPS consist of three segments: space, control and user segment. The space segment is the satellite constellation. The user segment includes GPS receiver, hand held or vehicle installed. The control segment consists of unmanned ground station to monitor the satellites. The concept behind GPS is the distance measurement between the receivers and satellites by the method of space resections. Differential GPS is required to obtain the accuracy required by many GIS data capture applications in terms of sub centimeters scale. DGPS requires the use of a base station at a known location to remove systematic error from the GPS signals. Recent GPS/GIS devices have the capability to collect a wide range of GIS attribute data in addition to position of interest such as points, lines and area features.
Taking into consideration the inhospitable terrain, where most of the engineering construction are carried out the use of GPS supplemented by GIS and remote sensing techniques by a geologist is highly recommended. GPs and remote sensing techniques are very useful tools for data captures. GPs in fact is highly useful to cross check the data collected by a satellite imagery analyst and the surveyor
3. Use of Remote Sensing
Remote sensing is the science and art of obtaining useful information about an object, area or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area or phenomenon under investigation. The data is acquired from space craft and satellites that record the interaction between matter and electromagnetic energy. Although through conventional surveying techniques, high quality data can be provided, its cost and turnaround time prohibit its use for acquisition of data in large areas. On the other hand, remote sensing satellite s which increasingly are being used in engineering applications are capable of providing data for large areas, in repeated revisits and at lower cost.
Remote sensing satellite imaging system can be grouped into four categories: conventional, high-resolution, hyper spectral and radar. Concentrating on high resolution satellites , with these satellites, large scale digital maps (1:24000 to 1:2400 ) will be easy and inexpensive to obtain also their images will be available in digital form which will reduce the time and cost of data extraction. However the drawback of high resolution satellite systems is that the swath widths are limited to 4-36 KM i.e. it cannot cover very large areas.
Following procedure is adopted for acquiring data using satellite systems:
· Determine the coverage area and acquire the images (scenes) from a satellite platform that covers that area. After determining the revisit times of various satellites, the geographic coverage and swath width one can determine the spatial domain that contains the required data and dates and time of image capture. Once the specific satellite platform is chosen the required images can be purchased from a vendor of satellite image s in digital form.
Second step is to process the images to extract the required spatial and thematic information.
This is where GIS comes in.
4. Use of GIS
GISs are computer hardware and software systems designed to input, store analyze and present geospatial data. Geospatial data include mostly any kind of information which can be linked to a particular place relative to the surface of the earth. This includes much of the data traditionally represented on paper maps as well as other information.
"GIS is an integrated system of computer hardware, software and trained personnel linking topographic demographic utility, facility, image and other resource data that is geographically referenced". NASA
GIS is used to display and analyze spatial data which are tied to data bases. This connection is what gives GIS its power, maps can be drawn from the database and data can be referenced from the maps. When a database is updated, the associated map can be updated as well. GIS uses layers called "themes", to overlay different types of information, each theme represents a category of information. With internet GIS offers a consistent and cost effective means for the sharing and analysis of geographic data among government agencies
The use of GIS in geotechnical engineering investigation has been rather recent in comparison to its own applications such as water resources, agriculture, geology, metrology etc., as far as geotechnical engineering is concerned inclusion of spatial attributes of the data is itself a significant advance over common practice, where the spatial component of these data is often ignored through the averaging of multiple data values from different location and application of a safety factor to account for variability among other factors.
To manage and analyze the large quantities of geotechnical data, there is a need to develop a customized geographic information system (GIS). Customized geotechnical modules allow engineers and geologists to automate the production of boring logs, cross sections, and parameter plots. Other modules are used to calculate liquefaction potential, settlement, and dredging volumes. The use of such a GIS database allows the engineers to easily and rapidly manage large volumes of data while also being able to examine it in detail. Using the GIS, we can also integrate geotechnical (borehole, CPT etc.), and geophysical data to create-complex three dimensional models of the subsurface conditions.
Apart from the location data collected by GPS, the other attributes, which can be recorded, are: traverse distance, bearing of a station, track history, high precision clock, in built memory for data record, and net displacement, datum option. Besides these advantages the other salient features of GPS, RS and GIS are multifunctional, operational simplicity, portability, compatibility with modern electronic gadgets. It is expected that technologies like Remote Sensing and GIS will allow many existing and new applications to take advantage of the information provided by them, which will result in, easier and faster data acquisition, lower overall cost and the availability of new data that could not be obtained before
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