Site Investigation and Exploration

Relevant Course: Geotechnical Engineering

Relevant Department: Civil Engineering 

Relevant Semester: 5th and above

Pre- requisite : Background in Soil Basics is Preferable

Course Description and Outline :

To obtain information on the subsurface soil and rock to design earthworks and foundations for a proposed structure/ excavation. Module will mostly cover various topics on site investigation methodologies, drilling and subsurface exploration techniques, sampling methods and various insitu-field testing.

Schedule for Lecture Delivery

Session 1 : 08-Oct-2015 (2-4 pm)

Session 2  :14-Oct-2015 (2-4 pm)

Session 3 : 15-Oct-2015 (2-4 pm)

Teacher Forum

SITE INVESTIGATION - INTRODUCTION

Introduction With Examples

Site investigations are mostly carried out to obtain information on the physical properties of soil and rock around a site to design earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions. Investigations are also needed to carryout tests on ground or backfill materials for various related applications like underground transmission lines, oil and gas pipelines, radioactive waste disposal etc. The knowledge of the ground plays a key role in the design and choice of construction technique. Sufficient information must be obtained to enable a safe and economic design and to avoid any difficulties during construction. The principal objects of the investigation are,

  • To determine the sequence, thicknesses and lateral extent of the soil strata and, where appropriate, the level of bedrock.
  • To obtain representative samples of the soils (and rock) for identification and classification and for use in laboratory tests to determine relevant soil parameters.
  • To identify the groundwater conditions.

The investigation may also include the performance of in-situ tests to assess appropriate soil characteristics. Additional considerations arise, if it is suspected that the ground may be contaminated. The main components of site investigation are,

  • Desk study - information available on geological maps, contour-topographic maps, photographs, utility records etc.
  • Site reconnaissance - walkover survey.
  • Ground investigation.
  • Geotechnical report.

A study of geological maps and memoirs, if available, should give an indication of the probable soil conditions of the site in question. If the site is extensive and if no existing information is available, the use of aerial photographs can be useful in identifying features of geological significance. Previous experience of conditions in the area may have been obtained by adjacent owners or local authorities. All information obtained in advance enables the most suitable type of investigation to be decided. The actual investigation procedure depends on the nature of the strata and the type of project but will normally involve the excavation of boreholes or trial pits. The number and location of boreholes or pits should be planned to enable the basic geological structure of the site to be determined and significant irregularities in the subsurface conditions to be detected. The greater the degree of variability of the ground conditions, the greater the number of boreholes or pits required.

Preliminary investigation include, location/confirmation of buried services, assessment of structures, particularly historic structures likely to be affected, access restrictions, geology from exposed cut faces etc., any geotechnical problems, cracks , settlements of the existing structures, information on any new construction coming up, any unexpected hazards are very important. Investigation include the following,

       a)  The geological structure of the area - topography and geomorphology, existence of hills, slopes, valleys, plains, plateaus, concurrence of folds, faults and major unconformities, joint system, river lakes springs, drainage system.

    b) The lithology of the area - type of rocks in different areas and their sequence, physico-mechanical properties

   c) Ground water condition- relative position of water table, seasonal variation, whether the project is above the local and regional water table.

     d) Seismicity of the region – seismic attenuation, peak ground acceleration, ground is  liquefiable or not?

MINIMUM DEPTH AND NUMBER OF BORING

There is no hard and fast rule to decide on the boring location and spacing. The number of borings or the spacing between borings for a project is related to the type, size and weight of the structure planned. In general boring should be located to obtain maximum information with minimum number of boreholes. The type of structure often decides the number and the depth of borings. When deep excavations are anticipated, the depth of boring should be at least 1.5 times the depth of excavation. Sometimes, subsoil conditions require the foundation load to be transmitted to bedrock. The minimum depth of core boring into bedrock is about 3m, if the bedrock is irregular or weathered, the core boring may have to be deeper.

Usually for a building project, boreholes are made at the building corners, at the centre of the site and location where heavily loaded columns or machinery pads are proposed. At least one borehole should be taken to hard rock, after borings to the depth of significant stress level or significant depth (figure 1). The significant depth is usually two times the width of the foundation. But the borehole shouldn’t be terminated at soft/organic soil deposit. 

Sowers and Sowers(1970) given a general guidelines to determine the boring depth for different structures.  For light steel or narrow concrete buildings,

Where, Db=depth of boring, S=  number of storey  and a=3 if Db in meters For heavy steel or wide concrete buildings,

Where, b=6 if Db in meters

The engineer should also take into account the ultimate cost of the structure while making decisions regarding the extent of field exploration. The exploration cost generally should be 0.1 to 0.5% of the cost of the structure

METHODS OF SOIL BORINGS

Soil borings can be made by following methods

  1. Auger boring
  2. Wash boring
  3. Percussion drilling, and
  4. Rotary drilling.

Site Investigation and methods of soil borings

1. Auger boring

Boring by an auger is carried out by holding it vertically and pressing it down while the auger is rotated. The turning action cuts the soil which fills the annular space. Once the annular space is filled, the auger is withdrawn and cleaned.. Auger boring is convenient in the case of partially saturated sands, silts and medium stiff cohesive soils. It is mostly used for highways or small structures and can’t be used for advancing holes to a depth exceeding 3-5m. Portable power driven helical augers (76-305mm in dia) are available for making deeper boreholes. Soil samples through auger is highly disturbed and are useful for identification purpose only.

 

2. Wash boring

Water with high pressure pumped through hallow boring rods is released from narrow holes in a chisel attach to the lower end of the rods. The soil is loosened and broken by the water jet and the up-down moment of the chisel. The soil particles are carried in suspension to the surface between the soil and the borehole sites. The rods are raised and drop for chopping action of the chisel by means of winch. Wash boring can be used in most type of soil but the progress is slow in coarse gravel strata. The accurate identification of soil strata is difficult due to mixing of the material has they are carried to the surface. The method is unacceptable for obtaining soil samples. It is only used for advancing the borehole to enable tube sample to be taken or field test to be carried at the hole bottom. The advantage is that the soil immediately below the hole remains relatively un-disturbed.

Fig. 3: Schematic view of wash boring (Das, 2010)

3. Rotary drilling

The rig consists of a derrick, power unit, winch, pump and a drill head to apply high-speed rotary drive and downward thrust to the drilling rods. The drilling tool, (cutting bit or a coring bit) is attached to the lower end of hollow drilling rods. The coring bit is fixed to the lower end of a core. Water or drilling fluid is pumped down the hollow rods and passes under pressure through narrow holes in the bit or barrel. The drilling fluid cools and lubricates the drilling tool and carries the loose debris to the surface between the rods and the side of the hole. The fluid (bentonite slurry) also provides some support to the sides of the hole if no casing is used There are two forms of rotary drilling, open-hole drilling and core drilling. Open- hole drilling is generally used in soils and weak rock, just for advancing the hole. The drilling rods can be removed to allow tube samples to be taken or in-situ tests to be carried out. Core drilling, is used in rocks and hard clays where the diamond or tungsten carbide bit cuts an annular hole in the  material and an intact core enters the barrel, to be removed as a sample. Typical core diameters are 41, 54 and 76mm, but can range up to 165 mm. 

The advantage of rotary drilling in soils is that progress is much faster than with other investigation methods and disturbance of the soil below the borehole is low.

Limitations,

  • The method is not suitable if the soil contains a high percentage of gravel/cobbles, as they tend to rotate beneath the bit and are not broken up.
  • The natural water content of the material is liable to be increased due to contact with the drilling fluid

4. Percussion drilling

Percussion drilling is an alternative method of advancing a borehole, particularly through hard soil and rock. A heavy drilling bit is raised and lowered to chop the hard soil. The chopped soil particles are brought up by the circulation of water. Percussion drilling may require casing.

PROCEDURES FOR SAMPLING SOIL

Two types of soil samples can be obtained during subsurface exploration: disturbed and undisturbed. In disturbed samples, soil structures gets modified or distroyed during the samplingg. With suitable precautions, the natural moisture content and the proportion of mineral constituents can be preserved, called representative samples useful for identification tests. When the original soil structure changes and also soil from the other layers get mixed-up or the mineral constituent get altered, the samples are called non-representative samples, vrtually of no use. In undisturbed samples original soil structure is preserved an the material properties have not undergone any alteration or modification. Disturbed but representative samples can generally be used for the following types of laboratory test:

  • Grain size analysis
  • Determination of liquid and plasic limits
  • Specific gravity of soil solids
  • Determination of organic content
  • Classification of soil

Disturbed soils however cannot be used for consolidation, hydraulic conductivity or shear strength tests. Undisturbed samples may be obtained for these types of laboratory tests.

Split Spoon Sampling

Split spoon samplers can be used in the field to obtain soil samples that are generally disturbed, but still representative. The tool consists of a steel driving shoe, a steel tube that is split longitudinally.

Fig. 4: (a) Un-assembled Split spoon sampler (b) after sampling

Degree of disturbance

The degree of disturbance for a soil sample is usually expressed with area ratio, as

 

Where

AR= Area ratio (ratio of disturbed area to total area of soil)

Do= Outside diameter of the sampling tube

Di= Inside diameter of the sampling tube

When area ratio is 10% or less, the sample is generally considered to be undisturbed.

Another parameter which can be used as an index of sample disturbance, is the recovery ratio Lr.

Lr= (Recovery length of the sample) / (Penetration of the sampler)

Lr =1 good recovery

Lr <1 = Soil is compressed

Lr >1 = soil has swelled

To reduce friction, sampler wall should be smooth and should be properly oiled before use.


 


SOIL EXPLORATION- Various Field Tests

1)      Standard penetration test (SPT)

2)      Dynamic cone penetration test (DCPT)

3)      Static cone penetration test (CPT)

4)      Seismic cone penetration test (SCPT)

5)      Pressuremeter test (PMT)

6)      Field vane shear test (FVT)

7)      Plate load test

8)      Geophysical tests

Standard Penetration test (SPT)

 In SPT, a borehole is extended to pre-determined depth, drill tools are removed and sampler is lowered to the bottom of the hole. Sampler is driven into the soil by hammer blows with weight of the hammer of 65Kg falling vertically and freely from a height of 750mm. Number of blows required to penetrate every 150mm is recorded while driving the sampler. Blows required to penetrate 300mm is added leaving first 150mm (as seating drive) , known as SPT "N".

SPT carried out at every 0.75m vertical intervals in a borehole. If depth is more, 1.5m may be considered. If boulder is present, 450mm would be difficult to penetrate, such cases first 300mm itself can be considered for SPT "N".

For refusal,

   >50 blows are required for any 150mm penetration

   > 100 blows are required for 300mm penetration.

   -  10 successive blows produce no advance.

Correction applied in "SPT N"

The standard penetration number is a useful guideline in soil exploration and the assessment of subsoil conditions, provided that the results are interpreted correctly. Because soil non- homogeneous and stress dependent, a wide variation in the N value may be obtained in the field. For soil deposits that contain large boulders and gravel, the standard penetration numbers may be erratic. There is need for corrections with SPT 'N'value. More ever, there is some loss of hammer energy during the test at the machine part. The SPT hammer energy efficiency can be expressed as,

Er(%) = [(Actual hammer energy to the sampler) / (Input energy) ] X 100

The theoretical input energy is W.h , Where, W - weight of the hammer and h - height of drop.

In the field, the magnitude of Er can vary from 30 to 90%. Thus usual practice is to express the 

In the field, the magnitude of Er can vary from 30 to 90%. Thus usual practice is to express the N-value to an average energy ratio of 60% (N60). 

Over burden and dilatancy correction in granular soil

  • For overburden correction,

Corrected SPT value N’ = CN * N60

 Where, CN (correction factor for O/B)         

                               

Po - effective over burden pressure in kN/m2  

  • Dilatancy correction

If after overburden correction, N' exceeds 15 in saturated fine sand and silts, N value is corrected for dilatancy as, N' > 15, dense sand , shear due to hammer blows likely to induce negative pore water pressure in a saturated fine sand under undrained condition of loading.

Final corrected value, N" = 15 0.5 (N'-15).

Correlation between angle of friction and standard penetration number

The peak friction angle, of granular soil has also been correlated with  by several investigators. Some of these correlations are as follows:

  • Peck, Hanson and Thornburn (1974) give correlation between  and    in a graphical form which can be approximated as ( Wolff, 1989)     

  • Schmertmann (1975) provided the correlation between .Mathematically the correlation can be approximated as (Kulhway and Mayne, 1990)

Where,

 

Correlation between Modulus of Elasticity and Standard Penetration Number

The modulus of elasticity of granular soils (Es) is an important parameter in estimating the elastic settlement of foundations. A first order estimation for Es was given by Kulhawy and Mayne (1990) as 

Where,  Pa = atmospheric pressure ( same unit as Es)

 for sands with fine, 10 for clean normally consolidated sand and 15 for clean over-consolidated sand.

CONE PENETRATION TEST

The standardized cone-penetrometer test (CPT) involves pushing a 1.41-inch diameter 55o to 60o cone through the underlying ground at a rate of 1 to 2 cm/sec.  CPT soundings can be very effective in site characterization, especially sites with discrete stratigraphic horizons or discontinuous lenses.  Cone penetrometer testing, or CPT (ASTM D-3441, 1974) is a valuable method of assessing subsurface stratigraphy associated with soft materials, discontinuous lenses, organic materials (peat), potentially liquefiable materials (silt, sands and granule gravel) and landslides.  Cone rigs can usually penetrate normally consolidated soils and colluviums, but have also been employed to characterize the weathered strata

Tip resistance and friction

The tip resistance is measured by load cells located just behind the tapered cone. The tip resistance is theoretically related to undrained shear strength of a saturated cohesive material, while the sleeve friction is theoretically related to the friction of the horizon being penetrated. The local friction is measured by tension load cells embedded in the sleeve for a distance of 4 inches behind the tip. They measure the average skin friction as the probe is advanced through the soil.  It is the ratio of skin friction divided by the tip resistance. It is used to classify the soil, by its behavior, or reaction to the cone being forced through the soil. High ratios generally indicate clayey materials while lower ratios are typical of sandy materials (or dry desiccated clays).  Typical skin friction to tip friction ratios are 1 % to 10%. Sands are generally identified by exhibiting a ratio < 1%. Figure 12 shows the typical cone penetrometer test results correlated with the soil profile, and formula for calculating  from cone.

Fig. 8: Typical cone penetrometer test results correlated with the soil profile

Pore pressure measurement (CPTU)

CPT with piezometers terms as piezocones measure insitu pore pressure in either dynamic (while advancing the cone) or static (holding the cone stationary) modes. The Differential Pore Pressure Ratio is used to aid in soil classification. When the cone penetrates dense materials like sand, the sand dilates and the pore pressure drops. In clayey materials high pore pressures may be induced by the driving of the cone head.  If transient pore pressures are being recorded that seem non-hydrostatic,  usually penetration be halted and allowed at least 5 minutes to equilibrate, so a quasi-static pore pressure reading can be recorded. Sometimes equilibration can take 10 to 30 minutes, depending on the soil.  

CPT can also be fitted with the temperature sensor. This has been found to be very useful in assessing the precise position of the zone, or zones, of saturation, which is of great import in slope stability and consolidation studies. A temperature shift of about 6oF is common at the groundwater interface, even perched horizons within landslides.

Caution while using CPT

  • Some cautions are needed when using CPT method to evaluate discrete low-strength horizons or partings, such as landslide slip surfaces. Tip resistance reported as "undrained shear strength" is actually an average value, taken over the zone within few cm of the cone tip. If the tip penetrates low strength horizons, such as a landslide slip surface, the tip resistance reported on the CPT log may be much higher than actually exists on the discrete plane of slippage.
  • The desiccated clay will often be interpreted as sand or silt mixtures because of recorded sleeve friction.

 

Seismic Cone Penetration Testing (SCPT)

Seismic Cone Penetration Testing (SCPT) provides a rapid and cost-effective method for directly measuring shear wave velocity of soils in situ. Shear wave velocity is used as an index of liquefaction resistance. SCPT shear wave velocity measurements are used in these evaluations,

  • Liquefaction Risk
  • Earthquake generated ground-surface movements
  • Foundations for vibrating equipment
  • Behavior of offshore structures due to wave loading

The seismic CPT equipment is used as an add-on to traditional CPT system. CPT data is taken in the same push that seismic information is generated. The SCPT probe consists of a seismic adapter threaded to a standard CPT probe. The CPT probe records tip resistance, sleeve friction, pore pressure, and tilt angle, and the seismic adapter is used to measure the shear wave velocities. The amount of data that is measured is too great for cordless transmission so the SCPT probe is run with a data cable through the rods.

 


Reference

  • All Relevant IS codes related to soil exploration and field tastings.
  • American association of state highway and transportation officials (1967). Manual of Foundation Investigations, National Press Building, Washington, D.C.
  • American society of civil engineers (1972). Subsurface Investigation for Design and Construction of Foundations of Buildings, Part I, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 98, No. SM5, 481–490.
  • Das, B. M. (2010). Principles of Geotechnical Engineering, Cengage Learning, New Delhi.
  • Das, B. M. (2011). Principles of Foundation Engineering, Cengage Learning, New Delhi.
  • Kulhawy, F. H., and Mayne, P. W. (1990). Manual on Estimating Soil Properties for Foundation Design, Final Report (EL-6800) submitted to Electric Power Research Institute (EPRI), Palo Alto, California.
  • Peck, R. B., Hanson, W. E., and Thornburn, T. H. (1974). Foundation Engineering, 2nd ed., Wiley, New York.
  • Ranjan, G. and Rao, A. S. R. (2000). Basic and Applied Soil Mechanics, New Age International (P) Ltd., New Delhi.
  • Schmertmann, J. H. (1970). Static Cone to Compute Static Settlement Over Sand, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 96, No. SM3, 1011–1043.
  • Seed, H. B., Tokimatsu, K., Harder, L. F., and Chung, R. M. (1985). Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations, Journal of Geotechnical Engineering, ASCE, Vol. 111, No. 12, 1426–1445.
  • Skempton, A. W. (1986). Standard Penetration Test Procedures and the Effect in Sands of Overburden Pressure, Relative Density, Particle Size, Aging and Overconsolidation, Geotechnique, Vol. 36, No. 3, 425–447.
  • Sowers, G. B., and Sowers, G. F. (1970). Introductory Soil Mechanics and Foundations, Macmillan, New York.
  • Wolff, T. F. (1989). Pile Capacity Prediction Using Parameter Functions, in Predicted and Observed Axial Behavior of Piles, Results of a Pile Prediction Symposium, sponsored by Geotechnical Engineering Division, ASCE, Evanston, Ill., June 1989, ASCE Geotechnical Special Publication No. 23, 96–106..


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PRESSUREMETER TESTS

A pressure meter is a field test to measure the “at-rest horizontal earth pressure” and soil modulus. Louis Menard from  France in 1955 was the first brought the pressuremeter to the forefront.  This test is very useful for many geotechnical applications namely,

  • Bearing capacity of shallow and deep foundations
  • Settlement of all types of foundations
  • Deformation of laterally loaded piles and sheet piles
  • Resistance of anchors.

Pressuremeter test is performed by applying pressure to the sidewalls of the borehole. It consists of two units, one readout unit that rests on the ground surface and a probe that is inserted into the borehole. Once the probe is at the test depth, the guard cells are inflated to brace the probe in place. Then the measuring cell is pressurized with water, inflating its flexible rubber bladder, which exerts a pressure on the borehole wall. As the pressure in the measuring cell increases, the borehole walls deform. The pressure within the measuring cell is held constant for approximately 60 seconds and the increase in volume required to maintain the constant pressure is recorded.

The pressuremeter modulus, Ep, of the soil is determined with the use of the theory of expansion of an infinitely thick cylinder. Thus,

PLATE LOAD TEST

The plate load test is a semi-direct method to measure the allowable pressure of soil to induce a given amount of settlement. Plates, round or square, varying in sizes, from 30 to 60 cm and thickness of about 2.5 cm are employed for the test. The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a cross beam or a steel truss anchored suitably at both the ends. The settlement of the plate is measured by a set of three dial gauges of sensitivity 0.02mm placed at 1200 apart. The dial gauges are fixed to independent supports which do not get disturbed during the test. Figure 3 shows the arrangement for the plate load test

Detailed Procedure

The method of performing the test is being discussed here. A pit of size not less than 5 times the size of the plate is excavated. The bottom of the pit coincides with level of the foundation. If water table is above the level of foundation, pumping is required to take out the water carefully and it should be kept just at the level of the foundation. A suitable size of the plate is selected for the test. Normally a plate of size 30cm is used in sandy soils and bigger size in clay soils. The ground should be leveled and the plate is seated over the ground.  A seating load of about 70 kg/cm2 is first placed and released after sometime. A higher load is next placed on the plate and settlements are recorded by means of the dial gauges. Observations on every load increment shall be taken until the rate of settlement is less than 0.25mm per hour. Load increments shall be approximately one-fifth of the estimated safe bearing capacity of the soil.  The average of the settlements recorded by 2 or 3 dial gauges taken as the settlements of the plate for each of the load increment. The test should continue until a total settlement of 2.5cm or the settlement at which the soil fails, whichever is earlier, is obtained. After the load is increased, the elastic rebound of the soil should be recorded.

Interpretation from test results:

 From the test results, a load-settlement curve should be plotted as shown in the fig. The allowable pressure of the prototype foundation for an assumed settlement may be found and by making use of the following equations as suggested by Terzaghi and Peck.

For granular soils,

Limitation

  • If the soil layer is not homogenous, misleading results may be produced. More ever, not much suitable for clayey soil.
  • If a poor stratum is encountered, its load capacity will not be reflected which leads to overestimation of results
  • The settlement result is based on small time frame while the actual loading will be for a large time frame 

VANE SHEAR TEST

Vane shear test can be used as a reliable in-situ test for determining the shear strength of soft-sensitive clays. It is in beds of such material that the vane shear test is the most valuable, for the simple reason that, there is at present no method known by which the shear strength of clays can be measured. The vane shear test should be regarded as a method to be used under the following conditions,

  • the clay is deep, normally consolidated and sensitive.
  • only the undrained shear strength is required.

It has been found that the vane shear test gives similar results to that as obtained from unconfined compression tests on undisturbed samples. It is necessary that the soil mass should be in saturated conditions if the vane test is to be applied. Vane shear test cannot be applied for partially saturated soils for which the angle of shearing resistance is not zero.

 

The vane consists of a steel rod having at one end with four projecting blades or vanes parallel to the axis, and situated at 900 intervals around the rod. A posthole borer is first employed to bore a hole up to a point just above the required depth. The rod is pushed or driven carefully until the vanes are embedded at the required depth. At the other end of the rod above the surface of the ground a torsion head is used to apply a horizontal torque and this is applied at a uniform speed of about 0.1º/sec until the soil fails, thus generating a cylinder of soil. The area consists of the peripheral surface of the cylinder and the two round ends. The first moment of these areas divided by the applied moment gives the unit shear value of the soil. In India the diameter used is 50mm and height of the blade is 100mm.

Considering a cylinder of soil generated by the blades of the vane when they are inserted into the undisturbed soil in-situ and gradually turned and rotated about the axis of the shaft or vane axis. The turning moment applied at the torsion head above the ground is equal to the force multiplied by the eccentricity. 

Cohesive strength can be measured as, 

The test is not suitable for fibrous peats, sands or gravels, or in clays containing laminations of  silt or sand, or stones.

The results of a vane shear test may be influenced by many factors, namely:

  1.  type of soil, especially when permeable fabric exists;
  2.  strength anisotropy;
  3.  disturbance due to insertion of the vane;
  4.  rate of rotation or strain rate;
  5.  time lapse between insertion of the vane and the beginning of the test; and
  6.  progressive/instantaneous failure of the soil around the vane.  

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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 geophisical 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)

Fig 1: Electrical resistivity arrangement and cumulative resisvity plot

 

There are two different arrangements are possible with the four electrodes to be used. In Wenner arrangements, 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, 

Fig 2: Wenner aarrangement

Fig 3: Schumberger arrangement

Fig. 4: A typical circuit for resistivity determination and electrical field for a homogeneous sub surface                                                                                              stratum

 

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, traveling laterally along higher velocity layers. Due to compressional 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.  

Fig 5: Seismic Refraction Testing

  • 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.
  • 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.

Fig. 6: Geophical test setup and geophone alighnment and corresponding arrival                                                            time of elastic waves.

Fig 7: Seimic Refration testing for layered soil

Fig 8: Time distance graph of seismic refraction testing

Determine the thickness of the top layer

The value  Xcan be obtained from the plot shown. Thickness of second layer can be  obtained as

Here Ti2 is the time intercept of the line cd in figure, extended backwards

Table 2: Wave velocity for different soil and rock types

  Figure 9: Seismic velocities of some geologic material a) Unsaturated b) Saturated

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 

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 8: Typical sectional view of Cross hole test

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 stratigraphy.

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Assignment

1. List the different stages of soil investigation and briefly discuss at least four observations to be made during site reconnaissance?

2. Discuss in brief the principle and methodology of the seismic refraction method together with neat sketches. Is it possible to detect a soft soil layer lying between two hard layers using seismic refraction test. Write your answer with clear reasoning.

3. A standard penetration test (SPT) was carried out at 10 m depth in fines and layer having saturated unit weight of 20 kN/m3. The water table is at ground level. A number of blows recorded during the test are given as:  0 - 15cm = 6 blows;  15 - 30cm = 16 blows;  30 - 45cm = 32 blows.  Find the standard penetration value N'' after required corrections.

4. An in situ vane shear test was conducted in clay soil at the bottom of borehole. A torque of 165Nm was required to shear the soil. What was the undrained strength of clay?  The vane was 100 mm in diameter and 150mm long

5.a) What is a disturbed, undisturbed and  a representative sample?  How disturbance in sample is characterized?

    b) Determine the area ratio for the following samplers and comment on the nature of the samples obtained in each of the samplers.

Give your comments on the disturbance of each sampler. Which of sampler would be suitable to carryout shear strength test on samples.

6.How Static Cone penetration test (CPT) is different than SPT. State the limitations of CPT method. The cone penetration resistance obtained in a clay soil during CPT was 50kg/cm2. Determine the undrained strength of the clay. The total overburden pressure at the depth was 100kN/m2.  

7. The standard penetration numbers determined from a sandy soil in the field are given. Using the expression given below, determine the variation of the peak soil friction angle, . Estimate an average value of  for the design of a shallow foundation.

8. Following is the variation of the field standard penetration number (N60) in a sand deposit. The groundwater table is located at a depth of 6 m. Given, the dry unit weight of sand  from 0 to a depth of 6 m is 18 kN/m3 and the saturated unit weight of sand for depth 6 to 12 m is 20.2 kN/m3. Using the relationship of Skempton, calculate the corrected penetration numbers.

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