LECTURE 10. Module 3 : Field Tests in Rock 3.6 GEOPHYSICAL INVESTIGATION

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1 LECTURE GEOPHYSICAL INVESTIGATION In geophysical methods of site investigation, the application of the principles of physics are used to the study of the ground. The soil/rock have different characteristics and comprised of materials that have different physical properties. Using some geophysical instruments, it is possible to map the ground characteristics together with their spatial variations. In this brief notes, an overview of the commonly adopted geophysical techniques used in geotechnical site investigation are discussed. The geophysical techniques which are discussed are, electrical resistivity method, seismic refraction technique, cross hole technique and ground penetrating radar (GPR) Electric resistivity method Electrical resistivity is the resistance of a volume of material to the flow of electrical current. current is introduced into the ground through a pair of current electrodes resulting potential difference is measured between another pair of potential electrodes. Apparent resistivity is then calculated as, V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes). Figure 3.36: Electrical resistivity arrangement and cumulative resisvity plot 81

2 There are two different arrangements are possible with the four electrodes to be used. In Wenner aarrangements, the electrodes are kept at equal distances where as, in case of Schumberger arrangements, distances are different. Using Wenner arrangement arrangement the resistivity is given by Using Schumberger arrangement the resistivity is given by, Figure 3.37: Wenner aarrangement Figure 3.38: Schumberger arrangement 82

3 Figure 3.39: A typical circuit for resistivity determination and electrical field for a homogeneous sub surface stratum Table 3.1: Resistivity of Different strata Material Resistivity (ohm.m) Sand Clay, saturated silt Clayey sand Gravel Weathered rock Sound rock >

4 Advantages and limitations, Method can be used to determine the depth and thickness of subsurface layers, depth to the water table, and bedrock. Profiling can be used to detect and locate contaminant plumes. Resistivity values can be used to estimate geological formations. The resistivity data are sometimes ambiguous and proper interpretation is required. Method may be better supplemented with other investigation methods like boreholes etc. Electrical resistivity is slow because electrodes must be driven into the ground between measurements. Alignment with buried electrical power lines, utilities and fences must be avoided as the current injected into the ground will flow more easily through the metal feature. Data are influenced by near surface conductive layers. The current will always travel most easily along highly conductive layers. If the surface is highly conductive it may not be possible to collect data below the top layer Seismic refraction method Seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source (hammer, weight drop or small explosive charge) located on the surface. The seismic waves travel through the subsurface at a velocity dependent on the density of the soil/rock. When the seismic wave front encounters an interface where seismic velocity drastically increases, a portion of the wave critically refracts at the interface, travelling laterally along higher velocity layers. Due to compression stresses along the interface boundary, a portion of the wave front returns to the surface. A series of seismic receivers, geophones (right) are laid out along the survey line at regular intervals and receive the reflected wave energy. The test involves the measurement of travel times of P-and S-waves from an impulse source to a linear array of points along the ground surface at different distances from the source. The output of all of the receivers recorded when the impulse load is triggered. 84

5 Figure 3.40: Seismic Refraction Testing The arrival times of the first waves to reach each receiver are determined and plotted as a function of source-receiver distance. Used for determination of wave velocity and thickness of each layer, and the dip angle. Effective for sites at which layers velocities increase with depth. Figure 3.41: Seimic Refration testing for layered soil 85

6 Figure 3.42: Geophical test setup and geophone alighnment and corresponding arrival time of elastic waves Figure 3.43: Time distance graph of seismic refraction testing 86

7 Determine the thickness of the top layer The value can be obtained from the plot shown. Thickness of second layer can be obtained as Here is the time intercept of the line cd in figure, extended backwards. 87

8 Figure 3.44: Seismic velocities of some geologic material a) Unsaturated b) Saturated 88

9 Table 3.2: Wave velocity for different soil and rock types Type of soil or rock P-wave velocity (m/sec) Soil Sand, dry silt and fine grained top soil Alluvium Compacted clays, clayey gravel and dense clayey sand Loess Rock Slate and shale Sandstone Granite Sound Limestone Travel time of waves depend on media (greatest in igneous, i.e. consolidated rocks, and least in unconsolidated rocks) Seismic velocity increases markedly from unsaturated to saturated zone. The acoustic velocity of a medium saturated with water is greatly increased in comparison with velocities in the vadose zone. Thus, the refraction method is applicable in determining the depth to the water table in unconsolidated sediments. Limitations:- If the Upper strata is denser than the lower - the method may not be very successful. Velocity of contrast should be high. Surface terrain and the interfaces of the layers are steeply sloping method may not be successful 89

10 3.6.3 Cross hole test These test methods are limited to the determination of the velocity of two types of horizontally travelling seismic waves in soil materials; primary compression (P-wave) and secondary shear (S-wave) waves. It is assumes that, the method used to analyze the data obtained is based on first arrival times or interval arrival times over a measured distance. The Crosshole Seismic Test makes direct measurements of P-wave velocities, or S-wave velocities, in boreholes advanced primarily through soil. At selected depths down the borehole, a borehole seismic source is used to generate a seismic wave train. Downhole receivers are used to detect the arrival of the seismic wave train in offset borings at a recommended spacing of 3 to 6 m. The distance between boreholes at the test depths is measured using a borehole deviation survey. The borehole seismic source is connected to and triggers a data recording system that records the response of the downhole receivers, thus measuring the travel time of the wave train between the source and receivers. Figure 3.45: Typical sectional view of Cross hole test 90

11 The P-wave or S-wave velocity is calculated from the measured distance and travel time for the respective wave train. The seismic cross hole method provides a designer with information pertinent to the seismic wave velocities of the materials in question. This data may be used as follows: For input into static/dynamic analyses, For computing shear modulus, Young s modulus, and Poisson s ratio. For determining Seismic Site Class using the appropriate Building Code; and For assessing liquefaction potential. Assumptions inherent in the test methods are, Horizontal layering is assumed. Snell s law of refraction applies to P-waves and S- waves and to the velocities derived from crosshole tests. If Snell s law of refraction is not considered in the analysis of Crosshole seismic testing data, the report shall so state, and the P-wave and S-wave velocities obtained may be unreliable for certain depth intervals near changes in strata. 91