Study on foundation engineering for Hydro Power Plant in Nepal , using 2- Dimensional Resistivity Imaging Technique
The proposed project Daram Khola Hydroelectric Project ( 10.3 MW ) is located at Righa V.D.C in Baglung Districts, Dhaulagiri Zone, Western Development Region of Nepal. Daram Khola Hydro Energy Company Ltd has acquired the survey license for survey and generation of electricity utilizing the water from Daram Khola. The project is run-off river project. Daram Khola Hydro Energy Company Ltd arranged Geophysical investigation of project site using 2D-Electrical Resistivity Tomography (2D-ERT) survey in the area. This report deals with the results of 2D-electrical resistivity tomography (2D-ERT) that was carried out in headworks, desander, penstock and power house area of the project.
1.2 Location and Accessibility
The proposed project is located at Gulmi District, Western Development Region of Nepal. Most of the study components lie on the right bank of Daram Khola on Righa V.D.C. The Geographical location of the project is from coordinates Easting 437042.42 and Northing 3119897.94 to Easting 433081.77 and Northing 3119196.91. The location Map is shown in Figure 1-1 and 1-2.The project site is accessed by black topped road from Kathmandu to Baglung which is 270 km. From Baglung to Kandebas graveled road of around 50 Km (Mid Hill Highway).
1.3: Scope and Objective of the Study
- To establish 2D Electrical Resistivity sections of the ground profile showing different layers of lithological strata.
- To find out the thickness of the overburden colluvial and alluvial soil materials.
- To find out depths of rock head and rock mass condition up to the depth of investigation.
- To find out weak zones and water bearing zones in the specified survey areas
- To find out plane of weakness for slope stability, deformed zones in bedrock, slopes and slides, sheared zones etc.
- Make suggestion and recommendations on subsurface geological conditions
1.4 Materials and Methodology
The methodology of the study consists of materials collection; literature collection and review, geological observation and geophysical ( 2D ERT ) survey, data computation, processing and interpretation; and finally report preparation.
The Survey was conducted with WDJD-4 Resistivity/IP equipment. Other accessories that were used during the study were District Map, Total Station, Photographic Camera, Brunton Compass etc. Literatures and information related to topography, climate, geology, of the area were collected and reviewed.
Resistivity Meter ( WDJD -4 Resistivity/IP equipment) was used for Two Dimensional Electrical Resistivity Tomography (2D ERT), Total Station was used to locate each electrode position and ERT profiles and other important features like bedrock exposure, river channel, proposed project structures, location of ERT profiles. Photographs were taken to characterize the topographical and morphological character and bedrock exposure of the area. Geological information of the area plays important role for the geophysical data interpretation. So, Geological information was also taken into account during the study phase.
GENERAL GEOLOGY OF THE STUDY AREA
2.1 Regional Geology of the Area:
Geologically, Nepal occupies the central sector of Himalayan arc. Nearly one third of the 2400 km long Himalayan range lies within Nepal. Nepal Himalaya is divided into five major zones, from north to south as shown in Figure 2-1. These zones are
- Tibetan Tethys Himalaya,
- Higher Himalaya,
- Lesser Himalaya,
- Sub-Himalaya (Siwaliks) and
- Indo-Gangetic Plain.
2.2 Geology of the Study Area
Geologically The project area lies in the Lesser Himalayan Zone of Western Nepal where lesser Himalayan rocks of Midland Group Lakharpata Subgroup is exposed. The rock formation of the area is Lakharphata Formation and Syangja Formation of the Lakharpata Subgroup.The predominant lithology of the area is represented by purple phyllites, white to pink quartzites, phyllitic slates, dolomite, limestone, argillaceous and silty slates meta sandstones, colluvial alluvial deposits. In most of the investigation area on headworks area, the rocks are found covered by thick overburden materials composed of alluvial terrace deposits, colluvial deposits metasandstones. Bedrocks are exposed on the Weir Axis area and to the west of the Desander area. Power House area is mostly covered by alluvial deposits and residual soil with larger conglomerate boulders. Bedrocks are exposed on the hill slope at the extreme ends of the surveyed area. Highly weathered meta-sandstones and purple phyllite were observed in the nearby areas.
Bedrock has been exposed on the upstream side of the powerhouse (option – II) while rest of area is covered by alluvial colluvial deposit. Bedrock has been exposed in the left bank of the powerhouse area.
Geophysical methods have wide range of its applicability in locating or tracing an object of interest as suggested by the geophysical response of the object. Detectability of the body depends on the size and distance or depth at which the object occurs. The contrast in physical properties between the body and its surroundings also influences the detectability. There are four main geophysical methods viz. gravity, magnetic, seismic, and electrical (self-potential, electromagnetic, resistivity, induced polarization and well logging) that are mainly applied for subsurface investigation. Electrical Resistivity method has been used in the present study for determining the subsurface information.
3.3 Two Dimensional Electrical Resistivity Tomography Survey ( 2D ERT)
3.3.1 Data Acquisition
Geo-electrical resistivity survey is widely used geophysical method for subsurface investigation for groundwater exploration, environmental application and other engineering application. The benefit of this method is to perform the survey quite fast and in the cost effective manner. Detection of water table, variation of resistivity with depths (distinguishinglayered earth), contaminants plume detection, detection of bedrocks depth, overburden thickness, types of subsurface geology etc. are the objectives geo-electrical resistivity survey. The interpretation of electrical resistivity data is the process of deriving the values of true resistivities (ρ) and thicknesses (t) of various subsurface strata from the values of recorded resistance (R) or apparent resistivity (ρa) at electrode separations (a). There are a number of processing and interpretation techniques for evaluating (ρ) and (t) of each of the stratum as proposed by many investigators. These can be grouped as analytical, numerical, empirical, graphical, computer (software) based etc. and several amongst each category.
Field data were gathered to obtain a continuous coverage of the subsurface along the line of investigation. As mentioned above Wenner electrode configuration was employed in the present study program. Further, Wenner Schlumberger and Dipole-Dipole configuration were also employed in the study for cross reference of the acquired data.
The depth of investigation depends on the length of the profile and spacing between electrodes. To collect information from the depth of about 40 m and deeper, a full length of profile of more than 250 m was used while for information of shallow depth a profile length of about 90 and more was used. The minimum electrode spacing of 3-5 m is used in the survey procedure.
4.3.1 Resistivity Tomograms and Interpretative Cross-sections
The model sections obtained from data inversion are presented as resistivity tomogram sections. These tomogram sections show the variation of modeled electrical resistivity in depth and along the line of investigation. These variations in modeled physical properties have relation with the subsurface geological and hydro-geological set up. Representative resistivity tomograms for each profile are presented in Figure 4 -1A to 4-4A and their interpretations are presented in Figure 4 -1B to 4-4B.
CONCLUSIONS & RECOMMENDATION
Detail information of the outcome of the interpretation is presented in the previous chapters. Geophysical subsurface studies at the site reveal thickness of overburden at different locations to be different. Most of the survey area is colluvial alluvial deposits. Expected depth of the bedrock at different locations can be taken from respective lithological section. It is recommended to consult Geotechnical, Civil and Structural Engineer for the proper design of the structure based on the subsurface geology of the area.