Keywords

1 Introduction

For successful design and construction of civil engineering infrastructures, it is essential to undertake geotechnical investigation to find out the nature, extent, and engineering properties of soils at the site. Such investigations are generally done by digging boreholes at different location at the site extending the same up to the desired depths for collection of soil sample from different depths. The various methods for extending such boreholes are use or trial pits, shafts and headings, percussion boring, mechanical augers, hand portable augers, wash borings, and rotary drilling. Position of the ground water table is also determined during the boring process. The collected soil samples can be divided into two main categories, undisturbed and disturbed. Undisturbed samples are required mainly for shear strength and compressibility characteristics, which are obtained by using techniques preserving the in-situ soil-structures and moisture content of the soil. On the other hand, disturbed sample are used for classification purpose only.

Alternatively in-situ tests for geotechnical investigation have also been devised. Commonly used techniques are as follow:

  • Plate load test

  • Standard penetration test

  • Dynamic cone penetration test

  • Static cone penetration test

  • Pressure meter test

  • Dilatometer test.

Each of the above tests has their merits and demerits. However, standard penetration test is most widely used all over the world. Specific correlations have been developed for each of these methods by several investigators to interpret and use the observed data for classifying in the soil strata and engineering properties. Conducting these test at the site needs lot of planning, time, and money. Some time, it become very large with respective to the total budget of the project.

In order to circumvent such difficulties, development of alternative methods is required.

‘In 1912, Conrad Schlumberger conducted the first geo-electrical resistivity experiment in the fields of Normandy. Around 1915, a similar idea was developed by Frank Wenner in the United State of America [1]. The classical methods of geo-electrical resistivity surveys have undergone significant changes in the last four decades. Data acquisition was more or less carried out manually till the 1980s, which is labor intensive and slow, and the quality of the measured data might be of poor quality. Initially, multi-electrode systems with manual switching [2] were used before the emergence of computer-controlled multi-electrode systems with automatic measurements and data quality control, which has tremendous impact on the quality of the data and the speed with which they are collected. When adequate computers became widely available, the inverse problem of ERT could be solved numerically, and the work of Loke and lane [3] at Birmingham University was among the first such solution, and their approach is still widely used.’

Recently geo-electrical resistivity survey in the area of archeology and geotechnical engineering has been reported, Abu-Zeid [7], Ganiyu [4]. They have done investigation of soil moisture content over a cultivated farmland in Abeokuta Nigeria using electrical resistivity methods and soil analysis. Constantin et al. [5], Zhou et al. [6], Abu-Zeid et al. [7], Sudha et al. [9] used ERT in subsurface soil characterization. They correlated soil resistivity with standard penetration test resistance number. This study opens the avenue to estimate bearing capacity of foundations. Thus, determination of soil strength from ERT results can be very effective.

In India, application of these methods are in initial stage, and not much expertise is available; therefore, an initiative of undertaking research in the area is warranted. The main aim of this paper is to present a case study for characterization of the subsurface soil through application of ERT.

1.1 Brief Review of Available Geophysical Exploration Techniques

Different geophysical exploration methods like seismic reflection and refraction technique, electrical resistivity method, magneto telluric method, etc., are available of geophysical perspective; however, these methods are used in different fields cutting across in various disciplines in science and engineering.

Advantages of these methods are exploration which can be covered in wider area in lesser time and expense, and these techniques are non-invasive.

The imaging of the soil lying below the ground surface is based on the collection of data pertinent to the methodology used. The anomaly like presence of ground water, soil stratification, presence of rock, faults, joints and fissures, and their extent can be interpreted easily from their tomographic imaging. In this respect, geophysical exploration techniques score over the geotechnical techniques in site investigation procedures is described above. But it should be noted that these methods give approximate idea about the subsurface conditions. However, adoption of such procedures help the civil engineers in taking quick decision for site selection and planning the next stage of geotechnical investigation, saving plenty of money, time, and energy.

Seismic methods: These methods are based on the principles that elastic shock waves moves at different velocities in different materials. At the interface of the two different strata of different materials, the elastic stress waves are partly reflected and partly refracted. There are two types of body waves namely the compression waves (P-waves) and shear waves (S-waves). In the compression waves, the direction of the movements of the particles coincides with the direction of propagation. The shear waves can be separated into two components: (1) SV-waves in which the motion of particles is in the plane of propagation and (2) SH-waves where in the motions of the particles is perpendicular to the plane of propagation. If a P-wave impinges on the boundary between two layers, there will be two reflected waves and two refracted waves. These waves follow certain laws. These are available in standard text books (Das and Ramana 2011; Kolsky 1963).

This method can be adopted by using a single geophone and producing a series of detonations or impacts at increasing distance from geophone. Then, arrival time is plotted against the distance between the source and the geophone. From this plot, the velocities of the direct wave and the refracted wave can be estimated. The general types of soil and rock can be determined from knowledge of these velocities. Typical values are as in Table 1.

Table 1 Speed of velocity in different type of rock and soil
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Magneto telluric (MT) Method: It refers to a technique in which electrical resistivity is determined by making measurements of electric and magnetic fields related to naturally occurring currents (“telluric”, caused mostly by lightning strikes) flowing in the ground. The MT theory, along with appropriate data inversion procedures, allows the determination of the resistivity distribution in the subsurface, on depth scale ranging from a few tens of meters to hundreds of kilometers. Magnetic data can also collect aerially using either a light helicopter or aeroplane by magnetometer mounted on it.

Gravimetric Method: It is a potential field technique which measures variations in the earth's gravitational field. These variations are caused by density contrasts in the near surface rock and sediment. Gravimetric surveys are carried out using extremely sensitive instruments capable of measuring tiny variations in the gravitational field. Instruments for measuring gravity are called gravity meters or gravimeters.

Electrical resistivity tomography (ERT): It is one of the most important geophysical tool for imaging the shallow subsurface soil and for characterization of soil for making the soil profile. Electrical resistivity method is also one of the methods mostly used in archeological geophysics, as well as in engineering geological investigations. Its other applications are characterizing subsurface hydrogeology, determining depth of bedrock/overburden thickness, determining depth of groundwater, mapping stratigraphy, mapping clay aquitards, mapping salt-water intrusion, mapping vertical extent of certain types of soil and groundwater contamination, and estimating landfill thickness. With the help of software (like RES2DINV and RES3DINV), we can create 1D, 2D, and 3D image of the anomaly. Among all the methods as described above, electrical resistivity is simplest of all the method as described and easy to perform.

The method depends on differences in electrical resistance of different soils and rock types. The flow of current through soil is primarily due to electrolytic action and therefore depends on concentration of dissolved salts in pore water: The mineral particles of a soil are poor conductors of current. The resistivity of soil therefore decreases with increasing water content and concentration of salt. Dense sands above the water table would show high value of resistivity due to low degree of saturation and lack of the presence of dissolved salts. Saturated clay having high void ratio would exhibit low value of resistivity due to relative abundance of pore water and free ions in the pore water. For ERT, four electrodes are arranged in a manner in which direct current (DC) or low-frequency alternating current (AC) of known magnitude introduce in ground using a pair of current electrode at one point and measure the potential deference at potential electrode in terms of resistivity (ohm). After calculating the apparent resistivity (ohm-m) and comparing the apparent resistivity of soil with the standard resistivity of soil, we can characterize the soil. Different types of soil composition have different resistivity value, so that it is easy to characterize the soil at different location under the ground surface. Typical values of resistivity of different types of soils/rocks are as follows:

Apparent resistivity represents the weighted average of true resistivity in a large volume of soil, the close to the surface being more heavily weighted than the soil at depth. Presence of a stratum of oil of high resistivity lying below a stratum of low resistivity forces the current to flow closer to the surface, resulting in higher values of apparent resistivity. The opposite is true if a stratum of low resistivity lies below a stratum of high resistivity.

Electrical resistivity tomography (ERT) is one of the most commonly applied techniques of geophysical surveying. The purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated [8]. The aim of ERT is to scan the subsurface along the survey line using a selected array.

2 Soil Characterization

2.1 Experimental Procedure

For lateral and vertical profiling of soil using resistivity technique, resistance (ohm) is calculated in the field using any of the Wenner method, Fig. 5, and Schlumberger method, Fig. 6, using instrument SSR MP-1. In these methods, four electrodes are used in which inner electrodes are potential electrodes and outer electrodes are current electrodes. Direct current (DC) is applied at the current electrodes, and potential difference is measured at potential electrodes. Difference between Wenner and Schlumberger method is only about spacing between the electrodes. In Wenner method, the electrodes are equally spaced, but in Schlumberger method, spacing between the current electrodes is three times the spacing between the potential electrodes.

In Fig. 1, a schematic diagram of the arrangement of the model blocks and apparent resistivity data points is shown. The data points are designated as x, and its spatial locations in a two-dimensional Cartesian frame of reference (designated as x and y) are also shown in this figure. Layers are numbered as 0, 1, 2,…..as shown.

Fig. 1
figure 1

Model block and apparent resistivity data point

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For measuring resistance for layer 1, the spacing is maintained as 1 m between all the electrodes taking ‘0’ as the reference. It means that the current electrode is placed at 0 and 3 and putting the potential electrode at 1 and 2. After measuring the resistance at that location, all the four electrodes are moved by 1 m in the y-direction increasing the value of n by 1 unit up to the layer 6. For layer 7 and 8, the spacing between potential electrodes are kept as 2 m along with n value 4 and 5. Following the above procedure, resistance is measured at all the data points.

A location was chosen near the Gymkhana ground, IIT (BHU), and its coordinates were obtained with the help of a GPS. Remotely sensed photograph of the site with the marked point is shown in Fig. 2. Figure 3 shows the soil profile with the apparent resistivity inversion obtained using RES2DINV software. It extends up to 15 m in y-direction and 4.88 m in z-direction. From the diagram, it is seen that the measured resistivity of soil at the site lies in the range of 50–210 Ω-m, comparing the values with the standard resistivity of soil (Table 2), and the soil extending over the zone is identified as clayey sand. But for the soil lying between 3.96 and 4.88 m depth, they have high resistivity values ranging from 490–750 Ω-m, and comparing these values with standard values, it is inferred that soil type should be silty sand with the possibility of the presence of some gravel packet within the strata.

Fig. 2
figure 2

Location of resistivity data point (coordinate: 25.259581, 82.988533)

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Fig. 3
figure 3

Soil profile apparent resistivity inversion

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Table 2 Resistivity of different type of soil and rock
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2.2 Observation Made and Interpretation of Data

To verify whether the inferred soil profile from the measured resistivity data is correct or not, grain size classification was made conducting sieve analysis following Indian Bureau of Standards in geotechnical engineering laboratory at IIT BHU, Varanasi.

Soil samples from an arbitrary depth 1 m were collected, and the test was conducted taking 1 kg of soil. The sieve test results are presented in Table 3.

Table 3 Sieve test results
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Grain size classification of the collected soil sample as specified by Indian standard soil classification system (ISSCS) as presented in Table 4 has been made, and the type of soil as obtained from the test is included under the remark column of Table 3. Thus, the different constituents of the soil at the depth of 1 m are:

Table 4 Classification of soil as per Indian Standard (ISSCS)
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Sand: Coarse: 22.2%, Medium: 54.1%, Fine: 18.3%

Silt and clay: 4%

From the curve (Fig. 4), D10, D30, and D60 can be calculated; these values are:

Fig. 4
figure 4

Grain size distribution curve

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Fig. 5
figure 5

Wenner array arrangement

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Fig. 6
figure 6

Schlumberger array arrangement

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D10 = 0.16954, D30 = 0.6411, D60 = 1.4457.

\({C}_{u}\)=\(\frac{{D}_{60}}{{D}_{10}}\), and the value is 8.5271.

\({C}_{C}= \frac{{D}_{30}^{2}}{{D}_{60*}{D}_{10}}\), and the value is 1.6768.

From the result analysis, it can be seen that the soil is SW (well graded sand) according to Indian standard soil classification system, and for SW, \({C}_{u}\) should be greater than 6 and \({C}_{C}\) should be between 1 and 6.

The soil as classified at that depth from electrical resistivity technique (ERT} is clayey sand. As per ISSCS classification, the soil passing through 75 \(\upmu \) sieve is fine grained and is classified either as silt and clay depending on its plasticity value, which is not possible from ERT test. Thus, the prediction as made by ERT is fairly close to the classification based on direct measurement.

Thus, it can be concluded that the predictions are close to the nature of soil at the ground. Similarly, soils from different depths are collected and categorized, and the predicted soil variation with depth is found to be fairly accurate.

Following the above procedure, the following cases were also taken up: determination of the extent of the foundation of a wall, foundation depth of the Rajputana hostel (which is about 100 years old), extent of root zone of a tree, and finally, determination of the depth of the ground water table. All these are significance from civil engineering profession. As investigating foundations of ancient structures involves time, money, and elaborate procedure for getting permission to do so, even though it was the initial objective of the paper, instead of a very ancient structure investigation were carried out to explore the extent of a structure more than 100 years old with remarkable success. The method can be easily extended to ancient structure. However, these studies have not been reported here for reasons of space and brevity.

3 Conclusion

The characterization of the subsurface soil has been carried out in IIT BHU campus, India with application of electrical resistivity method. The resistivity value is correlated with the sieve analysis and grain size distribution result obtained in geotechnical laboratory, IIT BHU. The calculated value of Cu = 8.5271 and Cc = 1.6768 of the soil from the study area in the laboratory is similar to the field result through ERT. It is also concluded that the Schlumberger array arrangement gives good result for deep soil body as compared to the Wenner array arrangement. For getting the high accuracy in ERT, it is required to take minimum electrode spacing. In other problems like the extent of foundation of an wall and 100-year-old structure, determination of the extent of root zone of a tree having direct bearing on studying the foundations of ancient structure has also been applied successfully. For reasons of space and brevity, these are not presented here.