RAJIV KHATRI et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 3, Issue No. 2, 138 - 141
“GEOPHYSICAL - VERTICAL ELECTRICAL SOUNDING - METHOD IN THE EVALUATION OF DIFFICULT TERRAINS” Life Member Indian Society for Earthquake Technology, IIT Roorkey,AMIE (I), Member IGS Jabalpur Chapter,Jabalpur Professor Department of Civil Engineering, Hitkarini College of Engineering & Technology,Dumna Airport Road, Jabalpur,M.P. Pin 482001; Email-khatri_rajiv1@yahoo.co.in;
ABSTRACT :-
Geo-technologist, Chairman IGS Jabalpur Chapter, Professor (Retired) Government Engineering College JabalpurM.P. Email - Prof_vinay@yahoo.co.in
DR. RAJEEV CHANDAK
Reader,JabalpurCollege of Engineering, Jabalpur,M.P.;. Email- rajeevchandak2003@yahoo.com
2. PROBLEM OF SUB-SURFACE INVESTIGATION OF TYPICAL BASALTIC TERRAIN :A very large area of Madhya Pradesh is occupied by lava flows and dykes, giving rise to typical basaltic terrains : contemporary with the formation of the Deccan Plateau. [Part of the Jabalpur district and complete district area of Seoni, Chhindwara and Mandla represent this type of terrain. The topographical features within the terrain give rise to flat high lands or ridge like structures called plateaus, sloping grounds, valleys and low lying areas]. Basaltic terrain consists only of basaltic rocks which have disintegrated and decomposed to different extents giving rise to a large variety of soil and rock materials, the typical of which are mooram ( a thin soil matrix in which boulders and pebbles are embedded), copra, black cotton soil and thick ledge and slabs of the rock itself.
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A complete range of field investigation methods, are available, for detailed field investigations for, civil engineering construction purposes. These are traditional methods and tools, which have been in practice over the last hundred years or so. These methods and tools were considered to be adequate so far as simple moderate structures were being built, based on the field data obtained through a combination of field and laboratory tests. These methods and tools like, all other methods, suffer from limitations of their application to difficult terrains, steep hill slopes, marshy and swampy areas, coastal regions and areas where a frequent variation of soil and rock materials exist in the areas to be investigated. No modification or updating has been done to make these methods suitable for all types of areas and for deeper probes below the ground surface e.g. for obtaining bed rock position, for thick land fill areas, for obtaining the continuity of rock strata and for determining the position of various sub-stratifications.
V.K.SHRIVASTAVA
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RAJIV KHATRI
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Presently a strong need is being felt to develop and put in practice the Geo-physical methods of sub-surface investigation for a more precise and fast assessment of large area characteristics, economically, and where conventional methods cannot be used. It is true that Geo-physical methods require proper interpretation of data which in turn needs a high degree of experience and expertise for making the interpretation. With the availability of computer aided interpreting software, the interpretation of the geo-physical methods data can also be done easily. Now that, we have entered into a phase, where large and big sized structures are required to be built in weak and difficult and sensitive areas, we have to take recourse to the Geo-physical methods and develop them into a popular tool for the enhancement and benefit of the civil engineering activities which require better and more information of every inch of the area. The paper proposes to highlight the effectiveness of Vertical Electrical Sounding technique of geophysical instrumentation.
Key Words : Vertical Electric Sounding (VES), Standard Penetration Test (SPT), Cone Penetration Test (CPT)
1. INTRODUCTION :-
Field investigations are a necessity for collecting important characteristics of the ground up to a certain stipulated depth required prior to the construction of any major Civil Engineering structure. Field Investigation is also needed for collecting samples of various soils and rock materials present in ground up to the desired level; for conducting laboratory test on the samples to ascertain their engineering properties and behaviours. There are few standard and traditional equipments and tools which are invariably always employed for the evaluation of the terrains. These are old and time honored tools although they have their own limitations regarding their suitability in all types of terrains and ground conditions.
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For the engineering evaluation of such terrains, problems arise regarding the type of field and lab tests to be preferred for obtaining engineering data pertaining to the behaviour of different materials vis-à-vis civil engineering construction. 3. CONVENTIONAL TESTS :Various field tests are prescribed in the Codes of Practice which are conventionally adopted in all types of terrains but have limitations of application to the basaltic terrain. The common field tests are – 1. 2. 3. 4. 5. 6.
Plate Load Test, Standard Penetration Test, Cone Penetration Test, Auguring, drilling and collection of cores of soils & rocks, Pressure Meter Test, Permeability Test, etc.
All these methods belong to the category of destructive or semi-destructive testing tools. 4. GEOPHYSICAL INSTRUMENTATION METHOD FOR SUB-SURFACE EVALUATION :Since the construction activity has now been taken to all types of ground locations and conditions, these destructive tools no longer serve the purpose. Geophysical instrumentation is gradually being preferred as a viable and more versatile tool which
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RAJIV KHATRI et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 3, Issue No. 2, 138 - 141
can provide all important information of these subsurface areas up to any depth, as an alternative to the destructive methods and tools.
In order to illustrate the effectiveness of the method case histories of two typical basaltic terrain is being given.
In the Middle Amur sedimentary basin (MASB) Vertical Electrical Sounding method has been used for discovery of lacustrine sediments in the southwestern and eastern parts of the MASB. The Correlation of seismic and drilling data confirmed the correctness of the interpretations and showed that boreholes penetrated a thin sequence of deep-water lacustrine sediments.
Case History 1 –
Case History 2 –
Construction activity in the Special Economic Zone (SEZ) near Chhindwara is proposed. The investigation site at Chhindwara is constituted of a raised plateaus of basaltic rocks. The ground surface is highly undulating and the rock has weathered and disintegrated up to a depth of 1.5 m to 2.0 m. At some places moorum type of soil has developed on weathered rock surface and in some other places the boulders are directly exposed on the surface without soil cover.
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An electrical measurement through Geophysical Instrumentation is a non-destructive or non-invasive methodology which is capable of being used in any type of terrain / topographic conditions and it has, practically, no limitations. The instrument used are handy and can be carried any where. The instrumentation can be done in much smaller time frame as compared to any of the destructive equipments; it is economical, dependable and is repeatable.
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Vertical Electrical Sounding Method has also been used in the city of Burdur in southwestern part of Turkey for determining the settlement properties of the soil and for defining the zones vulnerable for liquefaction in the city. The VES data has also provided very useful information on vertical and horizontal extends of geologic units and water content in the subsurface.
Construction activity is taking place within the premises of Pt. Dwarka Prasad Mishra Indian Institute of Information Technology and Design Manufacturing (P.D.M.I.I.I.T.D.M.) near Dumna Airport road in Jabalpur, Madhya Pradesh. The character of the ground is very typical of any terrain belonging to the Central India and the Deccan Plateau where large scale lava flows constituted the terrains. The flows have decomposed and disintegrated into thick top layers of rounded boulders of various sizes covered by a very thin veneer of red or black cotton soil. The weathering has affected the lava flow even up to a depth of 5.0 m to 10.0 m below the ground level. The boulders occur as separated entities or as highly jointed members in the weathered basaltic layer.
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To check for such buried features and boundaries, as well as to validate the geomorphological model assumed for the study area, resistivity survey was conducted. The Schlumberger technique, known as vertical electric sounding (VES), was selected as it is known to be superior over other techniques in distinguishing lateral from vertical variations in resistivity and field operations are faster. The instrument used was DDR-3 IGIS Hydrabad resistivity instrument. It is a compact digital averaging resistivity meter that contains a transmitter and receiver functions packed in one unit. It is designed to measure extremely weak electrical signal. Measurements at each ground level were repeatedly stacked to eliminate ground noise and other electrical interferences that can hamper the survey. This stacking feature automatically cancels spontaneous potential in resistivity measurement. The instrument can transmit up to 200 mA under more or less 200 V, which is sufficient enough for ordinary resistivity surveying. As in usual field procedure, the computed apparent resistivity values were plotted at logarithmic scale paper to gain an initial view of the resulting curve. This is undertaken prior to interpretation or at the sounding site to preclude unwanted curve, which results when errors are committed in readings and in distances set up. The interpretation of the measured values is facilitated through the use of a computer. A resistivity sounding interpretation was made using the database management and sounding interpretation including plotting of sounding curves. A computer programme is designed to automatically fit the theoretical curve to that of the master curves using iterations and test for convergence/divergence processes. The resulting interpretation was then compared with and adjusted based on drilling and test pit data available in the area. 5. CASE HISTORY :-
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The extent of decomposition and disintegration of rock has occurred up to a depth of 2.0 m below which large stone blocks (ledges) are present which changes into bed rock at depth of 3 to 4 m. The problem of finding out the compressive strength of such weathered and disintegrated materials and to decide upon the foundation depth level of the preferred foundation type cannot be solved by any of the conventional field testing methods because – 1.
2.
3.
Plate Load Test – This test is not ment for testing fragmented and loose rock layers. It is basically designed to find out the safe bearing capacity of cohesive soils only. Since there is little or no soil cover present, this test method cannot be used. Standard Penetration Test - SPT tests require auguring or drilling of a hole in the ground and for conducting the penetration test at different levels below the ground surface. Drilling by any method cannot be conducted in grounds which have thick layer of boulders and SPT is not designed for such ground conditions. Cone Penetration Test - CPT is also out of question to be used under such conditions.
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RAJIV KHATRI et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 3, Issue No. 2, 138 - 141
The non-invasive electrical measurement is done selecting profiles suitable for taking cross-sections in the desired area. The data obtained in the form of true resistivity values at different depths below the ground level can be read directly as the value of the compressive strength of the layer after using a suitable factor of safety. The method does not require any sample of soils / rock to be collected and tested in the laboratory. If done carefully, the log can be obtained showing exact position of soil, Disintegrated Rock (DR), Weathered Rock (WR), Soft Rock (SR) and bed rock identical with the actually prepared drilling core log. There are certain specific property indices (SPI) prepared to designate each type of geo-material present below the ground. The SPI also indicates the value of ultimate bearing capacity in case of soil materials and ultimate compressive strength values in case of DR, WR, SR and bed rock materials under natural confined condition.
Top Gravelly Soil
0.50 m
Soil with Pebbles & Boulders Boulder
1.50 m 2.00 m
Ledge + Boulder
3.00 m
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Rock Slab
4.00 m
Rock + Slab
5.00 m
II. VES Data For Site 1 at PDMIIITDM, Jabalpur M.P. SN
Depth
True Resistivity
Log
Safe Compressive Strength
Mixed Soil + N.A. Boulders HFR 2 2.0 m 251.2 Ω N.A. (WR) 3 3.0 m 518.0 Ω WR 800 kN/m2 Basaltic 4 4.0 m 615.4 Ω Rock 1000 kN/m2 Bed Basaltic > 1000 5 5.0 m 628.0 Ω Rock kN/m2 Bed For Site 2 at PDMIIITDM, Jabalpur M.P. 1
SN
1.0 m
Depth
119.3 Ω
True Resistivity
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Log
1.0 m
104.8 Ω
2
2.0 m
672.0 Ω
3
3.0 m
734.7 Ω
4
4.0 m
609.2 Ω
5
5.0 m
629.0 Ω
Safe Compressive
Strength N.A. N.A.
900 kN/m2 1000 kN/m2 > 1000 kN/m2
(WR-Weathered Rock, HFR-Highly Fisherd Rock)
For Site 1 at Chhindwara M.P. SN
Depth
True Resistivity
1
1.0 m
78.5 Ω
2
2.0 m
100.5 Ω
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I. General Profile Based on VES Logs 0.00 m
1
Top Mixed Soil layer WR HFR (WR) Basaltic Rock Bed Basaltic Rock Bed
Log
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In such cases some alternative method or tool has to be used for probing the ground up to various depths to obtain important characteristics of the ground upon which decision regarding the depth and type of foundation to be provided of different types of structures proposed to be built can be taken.
3 4
3.0 m 4.0 m
117.0 Ω 105.5 Ω
5
5.0 m
125.6 Ω
Top Mixed Layer Top Mixed Layer WR Ledge Bed Rock
Safe Compressive Strength N.A. N.A. 250 kN/m2 400 kN/m2 500 kN/m2
For Site 2 at Chhindwara M.P.
SN
Depth
True Resistivity
1
1.0 m
61.2 Ω
2
2.0 m
64.0 Ω
3
3.0 m
94.3 Ω
4
4.0 m
108.0 Ω
5
5.0 m
184.2 Ω
Log Top Mixed Layer Top Mixed Layer Ledge Bed Rock Bed Rock
Safe Compressive Strength N.A. N.A. 250 kN/m2 400 kN/m2 600 kN/m2
III. Lab Tests a. Gradation of soil which is mostly lateritic in nature Gravel – 86%; Sand 4%; Silt 4-6% and Clay 4-0% b. Bulk density values – between 1.68 to 1.72 gm / c.c. c. Water content w = 6.4 to 21 % d. Cohesion c = 0 e. Angle of internal friction Φ = 28º and above. 6. INTERPRETATION :During the Vertical Electrical Sounding the data obtained for each 1.0m thick layer represented the
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RAJIV KHATRI et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 3, Issue No. 2, 138 - 141
f)
For generating safe compressive strength values for the layer, the true resistivity values were processed considering the confinement conditions of the layer and with the help of the suitable multiplication factor, the values for ultimate compressive strength for the layer were computed and safe compressive strength values were obtained using a factor of safety of 5 or 6.
g)
7. CONCLUSION :-
i)
j)
k)
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On the basis of the generalized VES log it is inferred that the top soil cover is very thin and is underlain by layers of pebbles and boulders of variable thickness which merges imperceptibly into thick layer of boulders and ledges resting on thick slabs of basalt. With this type of arrangement and the data analysis from the laboratory test sufficient information regarding the type of foundation which can be provided to any structure being planned on such terrains and of course the foundation depth of the structure can also be decided depending upon the details of the structure.
h)
Khalil, M.A., Hafez, M.A., Santos, F.M., Ramalho, E.C., Mesbah, H.S.A., El-Qady, G.M. (2010), “An approach to estimate porosity and groundwater salinity by combined application of GPR and VES: A case study in the Nubian sandstone aquifer”, Near Surface Geophysics 8 (3), pp. 223-233 Kate Dr. J.M. IITD (1984) “Comparison of True Resistivity Values with SPT Generated N Values”, International Seminar, Tokyo, Japan, 1984. Shrivastava V.K. and Khare D.K. (1999) “Umar Aqueduct :Success Story of a Geotechnically Difficult and Forbidden Project.” IGS and ISSMFE 1999 – International Seminar held at Seoul, Korea Shrivastava V.K. and Khare D.K. (2000) “Difficulties in Assessing the Bearing Capacity of Soils.” IGC 2000 Millennium Seminar held at IIT Mumbai, Shrivastava V.K. (2002) “Use of Electrical Resistivity in Geotechnical Explorations.” National Seminar on Recent Trends in Civil Engineering at MBM Engineering College, JNV University, Jodhpur, 2002. Mościcki, W.J., Sokołowski, T. (2010), “Electric resistivity and compactness of sediments in the vicinity of boreholes drilled in the years 2007-2008 in the area of Starunia palaeontological site (Carpathian region, Ukraine)”, Annales Societatis Geologorum Poloniae 79 (3), pp. 343-355 C. Subbarao, and N. V. Subbarao, “Delineation of effluent contaminated zones by electrical surveys at two industrial sites in Visakhapatnam, India” (1994), Environmental Geology, Volume 24, Number 4 / December, 1994, 281-286
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apparent resistivity values for all subsequent layers except the top 1.0 m layer. The values had to be converted into true resistivity values for each layer and also from the values the identity of the geotechnical character of the material was also interpreted and is given in the log.
l)
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From the above study of the terrain and instrumentation it is clear that Vertical Electrical Sounding data if carefully obtained, processed and interpreted in the light of the terrain characteristics, it is possible to generate numerical values for safe bearing capacity or safe compressive strength, as the case may be, without using the conventional field equipments and the laboratory support needed by such equipments. This also is evidently clear that such terrains are not friendly to the use of conventional tools and methods, yet the terrain has to be characterized for the engineering behaviour of the materials present. 8. REFERENCE :a)
b) c)
d)
e)
M.Desai (India) (1994), “Geophysical Instrumentation for Engineering Field Tests”. Proceedings of the XII International Conference on Soil Mechanics and Foundation Engineering, Vol.6, page 103 RJ Whitley Australia (1994), Proceedings of the XII International Conference on Soil Mechanics and Foundation Engineering, Vol. 5 page 195 Y Iwaskai, Japan (1994), Proceedings of the XII International Conference on Soil Mechanics and Foundation Engineering,Vol.5, page 199 A.K. Dhawan (1994), “Geophysical Investigation of Tehri Dam”, Proceedings of the XII International Conference on Soil Mechanics and Foundation Engineering, Vol.4, page 1345. Yadav, G.S., Dasgupta, A.S., Sinha, R., Lal, T., Srivastava, K.M., Singh, S.K. (2010), “Shallow subsurface stratigraphy of interfluves inferred from vertical electric soundings in western Ganga plains, India”, Quaternary International
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