B.E. FIFTH SEMESTER · 2019. 5. 26. · subsoil to allow the Design Consultant to make...

ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY

MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.

DEPARTMENT OF CIVIL ENGINEERING

Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 1

Geotechnical Engineering – II

B.E. FIFTH SEMESTER





BECVE504T Geotechnical Engineering – II

Course Outcomes: At the end of the course, the student will have:

Cos Description Bloom’s

Taxonomy

CO1 Develop different soil exploration techniques to examine the properties of

soil. L3, 4

CO2 Ability to analyze the stability of natural slopes safety and sustainability of

the slopes, design of retaining structures, reinforced earth wall, etc. L4

CO3 Perceive knowledge to practice ground improvement techniques. L5

CO4 Perceive knowledge to design shallow and deep foundation. L5

CO5 Ability to analyze to calculate bearing capacity, earth pressure and

foundation settlement. L4

CO6 Ability to distinguish foundations under loading. L4

Course Objectives:

S.No. Description

1 Provide the students with basic understanding of the essential steps involved in a

geotechnical site investigation.

2 Introduce to the students, the principle types of foundation and factors governing the choice

of most suitable type of foundation for a given solution.

3 Familiarize the students with the procedures used for: a) bearing capacity estimation, b) end

bearing capacity, c) skin friction.

4 Types and function of piles – Factors influencing the selection of pile.





GEOTECHNICAL ENGINEERING – II 3-1-0

Unit – I :

GEOTECHNICAL EXPLORATION:

Importance and objectives of field exploration , principal methods of subsurface exploration , open

pits & shafts , types of boring , number , location and depth of boring for different structures , type of

soil samples & samplers. Principles of design of samplers, collection & shipment of samples, boring

and sampling record. Standard penetration test, corrections to N –values & correlation for obtaining

design soil parameters. (6)

Unit – II:

STABILITY OF SLOPES:

Causes and types of slope failure, stability analysis of infinite slopes and finite slopes, Ǿ center of

critical slip circle, slices method for homogenous c- Ǿ soil slopes with pore pressure consideration.

Taylors stability numbers & stability charts, methods of improving stability of slopes, types,

selection and design of graded filters. (7)

Unit III:

LATERAL EARTH PRESSURE:

Earth pressure at rest , active & passive pressure , General & local states of plastic equilibrium in

soil. Rankines and Coulomb‟s theories for earth pressure. Effects of surcharge, submergence.

Rebhann‟s criteria for active earth pressure. Graphical construction by Poncelet and Culman for

simple cases of wall-soil system for active pressure condition. (8)

Unit – IV:

GROUND IMPROVEMENT:

Method of soil stabilization use of admixtures (lime, cement, flyash) in stabilization. Basic concepts

of reinforced earth, use of geosynthetic materials Salient features, function and applications of

various geosynthetic materials. Vibroflotation , sand drain installation , pre-loading. (5)





Unit –V

SHALLOW FOUNDATIONS:

Bearing capacity of soils : Terzagi‟s theory , its validity and limitations , bearing capacity factors ,

types of shear failure in foundation soil , effect of water table on bearing capacity factors , types of

shear failure in foundation soil , effect of water table on bearing capacity , correction factors for

shape and depth of footings. Bearing capacity estimation from N-value , factors affecting bearing

capacity , presumptive bearing capacity.

Settlement of shallow foundation : causes of settlement , elastic and consolidation settlement ,

differential settlement , control of excessive settlement. Proportioning the footing for equal

settlement . Plate load test : Procedure , interpretation for bearing capacity and settlement prediction.

(8)

Unit – VI

PILE FOUNDATION:

Classification of piles , constructional features of cast – in – situ & pre cast concrete piles. Pile

driving methods , effect of pile driving on ground. Load transfer mechanism of axially loadedpiles.

Pile capacity by static formula & dynamic formula , pile load test and interpretation of data , group

action in piles, spacing of piles in groups , group efficiency , overlapping of stresses. Settlement of

pile group by simple approach , negative skin friction and its effect on pile capacity , general feature

of under reamed piles. (8)

Books :

1) Arora K.R. : Soil Mechanics & Foundation Engineering.

2) Punmia B. C. : Soil Mechanics & Foundations

3) Gopal Ranjan & Rao : Basic & Applied Soil Mechanics

4) P Raj : Geotechnical Engineer





UNIT – 1 GEOTECHNICAL EXPLORATION:

Importance and objectives of field exploration , principal methods of subsurface exploration , open

pits & shafts , types of boring , number , location and depth of boring for different structures , type of

soil samples & samplers. Principles of design of samplers, collection & shipment of samples, boring

and sampling record. Standard penetration test, corrections to N –values & correlation for obtaining

design soil parameters. (6)





1.1 IMPORTANCE AND OBJECTIVES OF FIELD EXPLORATION

The stability of the foundation of a building, a bridge, an embankment or any other structure built on

soil depends on the strength and compressibility characteristics of the subsoil. The field and

laboratory investigations required to obtain the essential information on the subsoil is called Soil

Exploration or Soil Investigation.

Site investigations consist of determining the profile of the natural soil deposits at the site, taking the

soil samples and determining the engineering properties of the soils. It also includes in-situ testing of

the soils. Soil exploration is a must in the present age for the design of foundations of any project.

The extent of the exploration depends upon the magnitude and importance of the project.

Soil exploration involves broadly the following:

1. Information to determine the type of foundation required such as a shallow or deep foundation.

2. Necessary information with regards to the strength and compressibility characteristics of the

subsoil to allow the Design Consultant to make recommendations on the safe bearing pressure or pile

load capacity.

Soil exploration involves broadly the following:

1. Planning of a program for soil exploration.

2. To determine bearing capacity of the soil.

3. To select the type and depth of foundation for a given structure.

4. TO investigate the safety of the existing structures and to suggest the remedial measures.

5. Collection of disturbed and undisturbed soil or rock samples from the holes drilled in the

field. The number and depths of holes depend upon the project.

6. Conducting all the necessary in-situ tests for obtaining the strength and compressibility

characteristics of the soil or rock directly or indirectly.

7. Study of ground-water conditions and collection of water samples for chemical analysis.

8. Geophysical exploration, if required.

9. Conducting all the necessary tests on the samples of soil /rock and water collected.

10. Preparation of drawings, charts, etc.

11. Analysis of the data collected.

12. Preparation of report.

1.2 PRINCIPAL METHODS OF SUBSURFACE EXPLORATION

The various methods of the explorations may be grouped as follows:-

1. Open excavations.

2. Borings.

3. Sub-surface soundings.

4. Geophysical methods.





1. OPEN EXCAVATIONS

In this method of exploration, an open excavation is made to inspect the sub-strata. The methods can

be divided into two categories: (1) Pits and Trenches, (2) Drifts and Shafts.

1. Pits ad Trenches:- Pits and trenches are excavated at the site to inspect the strata. IS:4453-

1967 recommends a clear working space of 1.2m x 1.2m at the bottom of the pit. Shallow pits up to a

depth of 3m can be made without providing any lateral support. For deeper pits, especially below the

ground water table, the lateral support in the form of sheeting and bracing system Fig.1 is required.

Trenches are long shallow pits. As a trench is continuous over a considerable length, it provides

exposure along a line. The trenches are more suitable than pits for exploration on slopes.

2. Drifts and Shafts:- Drifts are horizontal tunnels made in the hill-side to determine the

nature and structure of the geological formation. IS:4453-1980 recommends that a drift should have

the minimum clear dimensions of 1.5m width and 2.0m height in hard rock. In soft rock, an arch roof

is more advantageous than flat roof.

Shafts are large size vertical holes in the geological formation. These may be rectangular or circular

in section. The minimum width of a rectangular shaft is 2.4m and for a circular shaft, the minimum

diameter is 2.4m.

3. BORINGS FOR EXPLORATION

When the depth of exploration is large, borings are used for exploration. The vertical bore hole is

drilled in the ground to get the information about the sub-soil strata. Samples are taken from the bore

hole and tested in a laboratory.

Depending upon the type of soil and the purpose of boring, the following methods are used for

drilling the holes.

1) Auger Boring: - Augers are used in cohesive and other soft soils above water table. Hand

augers are used for depths up to about 6m. They are used in boring about 15 to 20cm in diameter. It

is

Fig. 1





attached to the lower end of a pipe about 18mmdiameter. The pipe is provided with a cross – arm at

its top. The hole is advanced by turning the cross-arm manually and at the same time applying thrust

in the downward direction. When the auger is filled with soil, it is taken out. Mechanically operated

augers are used for greater depths and they can also be used in gravely soils. Fig.2 shows a post hole

auger and a helical (spiral) auger.

Samples recovered from the soil brought up by augers are badly disturbed and are useful for

identification purposes only. Auger boring is fairly satisfactory for highway explorations at shallow

depths and for exploring borrow pits.

2) Wash boring: - Soil exploration below the ground water table is usually very difficult to

perform by means of pits or auger-holes. Wash boring in such cases is a very convenient method

provided the soil is either sand, silt or clay. The method is not suitable if the soil is mixed with gravel

or boulders. Fig.3 shows the assembly for a wash boring. To start with, the hole is advanced a short

depth by auger and then a casing pipe is pushed to prevent the sides from caving in. The hole is then

continued by the use of a chopping bit fixed at the end of a string of hollow drill rods. A stream of

water under pressure is forced through the rod and the bit into the hole, which loosens the soil as the

water flows up around the pipe. The loosened soil in suspension in water is discharged into a tub.

The soil in suspension settles down in the tub and the clean water flows into a sump which is

reused for circulation. The motive power for a wash boring is either mechanical or man power. The

bit which is hollow is screwed to a string of hollow drill rods supported on a tripod by a rope or steel

cable passing over a pulley and operated by a winch fixed on one of the legs of the tripod. The

purpose of wash boring is to drill holes only and not to make use of the disturbed washed materials

for analysis. Whenever an undisturbed sample is required at a particular depth, the boring is stopped,

and the chopping bit is replaced by a sampler. The sampler is pushed into the soil at the bottom of

the hole and the sample is withdrawn.

Fig. 2





Fig.3 Wash boring.

3) Rotary Drilling: - In the rotary drilling method a cutter bit or a core barrel with a coring bit

attached to the end of a string of drill rods is rotated by a power rig. The rotation of the cutting bit

shears or chips the material penetrated and the material is washed out of the hole by a stream of

water just as in the case of a wash boring. Rotary drilling is used primarily for penetrating the

overburden between the levels of which samples are required. Coring bits, on the other hand, cut an

annular hole around an intact core which enters the barrel and is retrieved. Thus the core barrel is

used primarily in rocky strata to get rock samples. As the rods with the attached bit or barrel are

rotated, a downward pressure is applied to the drill string to obtain penetration, and drilling fluid

under pressure is introduced into the bottom of the hole through the hollow drill rods and the

passages in the bit or barrel. The drilling fluid serves the dual function of cooling the bit as it enters

the hole and removing the cuttings from the bottom of the hole as it returns to the surface in the

annular space between the drill rods and the walls of the hole. In an uncased hole, the drilling fluid

also serves to support the walls of the hole. When boring in soil, the drilling bit is removed and

replaced by a sampler when sampling is required, but in rocky strata the coring bit is used to obtain

continuous rock samples. The rotary drilling rig of the type given in Fig. 4 can also be used for wash

boring and auger boring.





Coring Bits: - Three basic categories of bits are in use. They are diamond, carbide insert, and saw

tooth. Diamond coring bits are the most versatile of all the coring bits since they produce high

quality cores in rock materials ranging from soft to extremely hard. Carbide Bits are used to core soft

to medium hard rock. They are less expensive than diamond bits but the rate of drilling is slower

than with diamond bits. In saw-tooth bits, the cutting edge comprises a series of teeth. The teeth are

faced and tipped with a hard metal alloy such as tungsten carbide to provide wear resistance and

thereby increase the life of the bit. These bits are less expensive but normally used to core

overburden soil and very soft rocks only.

Fig.4 Rotary drilling rig (After Hvorslev, 1949)

4) Percussion Drilling: - Percussion drilling is another method of drilling holes. Possibly this is

the only method for drilling in river deposits mixed with hard boulders of the quartzitic type. In this

method a heavy drilling bit is alternatively raised and dropped in such a manner that it powders the

underlying materials which form slurry with water and are removed as the boring advances.





1.3 TYPES OF SOIL SAMPLES

Soil samples are obtained during sub-surface exploration to determine the engineering properties

of the soils and rocks. Soil samples are generally classified into two categories :

1) Disturbed samples:- These are the samples in which the natural structure of the soil gets

disturbed during sampling. However, these samples represent the composition and the mineral

content of the soil. Disturbed samples can be used to determine the index properties of the soil,

such as grain size, plasticity characteristics, and specific gravity.

2) Undisturbed samples:- These are the samples in which the natural structure of the soil and the

water content are retained. However, it may be mentioned that it is impossible to get truly

undisturbed sample. Some disturbance is inevitable during sampling, even when the utmost care

is taken. Even the removal of the sample from the ground produces a change in the stresses and

causes disturbances.

Undisturbed samples are used for determining the engineering properties of the soil, such as

compressibility, shear strength, and permeability. Some index properties such as shrinkage limit can

also be determined. The smaller the disturbance, the greater would be the reliability of the results.

1.4 SOIL SAMPLES AND SAMPLES

DESIGN FEATURES AFFECTING THE SAMPLE DISTURBANCE

The soil samples can be of two types: disturbed and undisturbed. A disturbed sample is that

in which the natural structure of soils get partly or fully modified and destroyed, although with

suitable precautions the natural water content may be preserved. Such a sample should, however, be

representative of the natural soil by maintaining the original proportion of the various soil particles

intact. An undisturbed sample is that in which the natural structure and properties remain preserved.

The sample disturbance depends upon the design of the samplers and the method of

sampling. The design features governing the degree of disturbance are (i) cutting edge, (ii) inside

wall friction, and (iii) non-return valve. Fig. 5 shows a cutting edge. The following terms are defined

with respect to the diameters marked in Fig. 5.

The disturbance of the soil depends mainly upon the

following design features:

1) Area ratio:- The area ratio is defined as,

Area ratio

where, D1 = inner diameter of cutting edge.

D2 = outer diameter of cutting edge.

For obtaining good quality undisturbed samples,

the area ratio should be 10 percent or less. Fig. 5





2) Inside clearance :- The inside clearance is defined as

Where, D3 = inner diameter of the sampling tube.

For an undisturbed sample, the inside clearance should be between0.5 and 3 percent.

3) Outside clearance :- The outside clearance is defined as

where, D4 = outer diameter of the sampling tube.

For reducing the driving force, the outside clearance should be as small as possible.

Normally, it lies between zero to 2 percent.

4) Inside wall friction: - The friction on the wall causes disturbance of the sample. The inside

surface of the sampler should be smooth. It is usually smeared with oil before use to reduce

friction.

5) Design of non-return valve: - The non-return valve provided on the sampler should be of proper

design. It should have an orifice of large area to allow air, water or slurry to escape quickly when

the sampler is driven. It should immediately close when the sampler is withdrawn.

6) Method of applying force: - The degree of disturbance depends upon the method of applying

force during sampling and upon the rate of penetration of the sampler. For obtaining undisturbed

samples, the sampler should be pushed and not driven.

1) SPLIT – SPOON SAMPLERS

1. The most commonly used sampler for obtaining a disturbed sample of the soil is the standard

split-spoon sampler. It consists mainly of three parts, (i) Driving shoe, made of tool-steel, about

75 mm long, (ii) steel tube about 450 mm long, split longitudinally in two halves, and (iii)

coupling at the top of the tube about 150 mm long.

2. The inside diameter of the split tube is 38 mm and the outside diameter is 50.0 mm. The coupling

head may be provided with a check valve and 4 venting ports of 10 mm dia. to improve sample

recovery.

3. This sampler is also used in conducting standard penetration test.





(a) (b)

Fig. 6: (a) Standard Split spoon sampler, (b) Spring; Core catcher.

4. After the bore hole has been made, the sampler is attached to the drilling rod and lowered into the

hole. The sample is collected by jacking or forcing the sampler into the soil by repeated blows of

a drop hammer. The sampler is then withdrawn. The split tube is separated after removing the

shoe and the coupling and the sample is taken out. It is then placed in a container, sealed, and

transported to the laboratory.

5. If the soil encountered in the bore hole is fine sand and it lies below the water table, the sample

recovery becomes difficult. As the sampler is lifted, the springs close and form a dome and retain

the sample.

6. While taking samples, care shall be taken to ensure that the water level in the hole is maintained

slightly higher than the piezometric level at the bottom of the hole. It is necessary to prevent

quick sand conditions.

7. The split tube may be provided with a thin metal or plastic tube liner to protect the sample and to

hold it together. After the sample has been collected, the liner and the sample it contains are

removed from the tube and the ends are sealed.

2) SCRAPER BUCKET SAMPLER

1. If a sandy deposit contains pebbles, it is not possible to obtain samples by standard split-spoon

sampler or split-spoon sampler fitted with a spring core catcher. The pebbles come in between

the springs and prevent their closure. For such deposits, a scraper bucket sampler can be used.





2. A scraper bucket sampler consists of a driving point which is attached to its bottom end. There is

a vertical slit in the upper portion of the sampler. As the sampler is rotated, the scrapings of the

soil enter the sampler through the slit.

3. When the sampler is filled with the scrapings, it is lifted. Although the sample is quite disturbed,

it is still representative.

4. A scraper bucket sampler can also be used for obtaining the samples of cohesionless soils below

the water table.

Fig. 7: Scraper Bucket Sampler.

3) SHELBY TUBES AND THIN-WALLED SAMPLERS

1. Shelby tubes are thin wall tube samplers made of seamless steel. The outside diameter of the

tube may be between 40 to 125 mm. The commonly used samplers have the outside diameter

of either 50.8 mm or 76.2 mm.

2. The bottom of the tube is sharpened and leveled, which acts as a cutting edge. The area ratio

is less than 15% and the inside clearance is between 0.5 to 3%.

3. Fig. 8 (b) shows a thin-walled sampler (IS : 2132-1972). The length of the tube is 5 to 10

times the diameter for sandy soils and 10 to 15 times the diameter for clayey soils. The

diameter generally varies between 40 and 125mm, and the thickness varies from 1.25 to

3.15mm.





4. The sampler tube is attached to the drilling rod and lowered to the bottom of the bore hole. It

is then pushed into the soil. Care shall be taken to push the tube into the soil by a continuous

rapid motion without impact or twisting.

5. The tube should be pushed to the length provided for the sample. At least 5 minutes after

pushing the tube into its final position, the bottom before it is withdrawn. The tube is taken

out and its ends are sealed before transportation.

6. Shelby tubes are used for obtaining undisturbed samples of clay.

Fig. 8: (a) Shelby tube, (b) Thin-walled Sampler.

4) PISTON SAMPLER

1. A piston sampler consists of a thin-walled tube with a piston inside. The piston keeps the

lower end of the sampling tube closed when the sampler is lowered to the bottom of the hole.

Fig.9.a).

2. After the sampler has been lowered to the desired depth, the piston is prevented from moving

downward by a suitable arrangement, which differs in different types of piston samplers.





3. The thin tube sampler is pushed past the piston to obtain the sample Fig.9.b). The piston

remains in close contact with the top of the sample.

4. The presence of the piston prevents rapid prevents rapid squeezing of the soft soils into the

tube and reduces the disturbance of the sample.

5. A vacuum is created on the top of the sample, which helps in retaining the sample. During the

withdrawal of the sampler, the piston provides protection against the water pressure which

otherwise would have occurred on the top of the sample.

6. Piston samplers are used for getting undisturbed soil samples from soft and sensitive clays.

Fig. 9: Piston Sampler.

5) DENISON SAMPLER

1. The Denison sampler is a double-walled sampler. The outer barrel rotates and cuts into the

soil. The sample is obtained in the inner barrel. The inner barrel is provided with a liner

barrel is provided with a liner. It may also be provided with a basket-type core retainer.

2. The sampler is lowered to the bottom of the drilled hole. A downward force is applied on the

top of the sampler. A fluid under pressure is introduced through the inner barrel to cool the

coring bit when the outer barrel rotates. The fluid returns through the annular space between





the two barrels. The rotation of the outer barrel is continued till the required length of the

sample is obtained.

3. The Denison sampler is mainly used for obtaining samples of stiff to hard cohesive soils and

slightly cohesive sands. However it cannot be used for gravelly soils, loose cohesionless

sands and silts below ground water table and very soft cohesive soils.

DEPTH OF FOUNDATION

The depth of foundation of exploration required at a particular site depends upon the degree

of variation of the subsurface data in the horizontal and vertical directions. It is not possible to fix the

number, disposition and depth of borings without making a few preliminary borings or soundings at

the site. The depth upto which the stress increment due to superimposed loads can produce

significant settlement and shear stresses is known as the significant depth. The significant depth is

generally taken as the depth at which the vertical stress is 20% of the load intensity.

Fig. 10: Depth of exploration. Fig.11: Depth of exploration for closely –spaced footings.

Fig.12: Depth of exploration for friction piles.

1) The depth of exploration should be about 1.5 times width of the square footing.

2) The depth of exploration should be about 3.0 times width of the strip footing.

3) If the footings are closely spaced, the whole of the loaded area acts as a raft foundation. In

that case, the depth of boring should be at least 1.5 times the width of the entire loaded area.

4) In pile foundation, the depth of exploration below the tip of bearing piles is kept at least 1.5

times the width of the pile group.

5) In friction piles, the depth of exploration is taken 1.5 times the width of the pile group

measured from the lower third point.





6) In road fills, the minimum depth of boring is 2m below the ground surface or equal to the

height of the fill, whichever is greater.

7) In gravity dams, the minimum depth of boring is twice the height of the dam.

RECONNAISSANCE

The geotechnical engineer makes a visit to the site for a careful visual inspection in

reconnaissance. The information about the following features is obtained in reconnaissance.

1) The general topography of the site, the existence of drainage, ditches and dumps of debris

and sanitary fills.

2) Existence of settlement cracks in the structure already built near the site.

3) The evidence of landslides, creep of slopes and the shrinkage cracks.

4) The stratification of soils as observed from deep cuts near the site.

5) The location of high flood marks on the nearby building and bridges.

6) The depth of ground water table as observed in the well.

7) Existence of springs, swamps, etc. at the site.

8) The drainage pattern existing at the site.

9) Type of vegetation existing at the site. The type of vegetation gives a clue to the nature of the

soil.

STANDARD PENETRATION TEST

1) The standard penetration test is the most commonly used in-situ test, especially for

cohesionless soils which cannot be easily sampled.

2) The test is extremely useful for determining the relative density and the angle of shearing

resistance of cohesionless soils.

3) It can also be used to determine the unconfined compressive strength of cohesive soils.

The test is conducted in a bore hole using a standard split-spoon sampler. When the

bore hole has been drilled to the desired depth, the drilling tools are removed and the sampler is

lowered to the bottom of the hole. The sampler is driven into the soil by a drop hemmer of 63.5 kg

mass falling through a height of 750mm at the rate of 30 blows per minute (IS: 2131-1963). The

number of hammer blows required to drive 150mm of the sample is counted. The sampler is further

driven by 150mm and the number of blows recorded. Likewise, the sampler is once again further

driven by 150mm and the number of blows recorded. The number of blows recorded for the first

150mm is disregarded. The number of blows recorded for the last two 150mm intervals are added to

give the standard penetration number (N). In other words, the standard penetration number is equal

to the number of blows required for 300mm of penetration beyond a seating drive of 150mm.

If the number of blows for 150mm drive exceeds 50, it is taken as refusal and the test is

discontinued.

The standard penetration number is corrected for dilatancy correction and overburden

correction:-

a) Dilatancy Correction: - Silty fine sands below the water table develop pore pressure which is

not easily dissipated. The pore pressure increases the resistance of the soil and hence the

penetration number (N).





Terzaghi and Peck (1967) recommend the following correction in the case of silty fine

sands when the observed value of N exceeds 15.

The corrected penetration number,

Where NR is the recorded value, and NC is the corrected value.

If

b) Overburden pressure Correction: - In granular soils, the overburden pressure affects the

penetration resistance. Gibbs and Holtz (1957) recommend the use of the following equation

for dry or moist clean sand.

…….. (i)

where , NR = observed N-value, Nc =corrected N-value,

= effective overburden pressure (kN/m2).

Equation (i) is applicable for

Fig.13: Overburden Correction Diagram.

Peck, Hansen and Thornburn (1974) give the chart for correction of N-values to an effective

overburden pressure of 96 kN/m2.

Fig. 13. Shows the correction diagram. At

The value of N/NR is 2.0

The correction given by Bazaraa (1967), and also by Peck and Bazaraa (1969), is one of the

commonly used corrections. According to them,





STATIC CONE PENETRATION TEST

The Dutch cone has an angle of 600 and an overall diameter of 35.7 mm, giving an end area

of 10 cm2. For obtaining the cone resistance, the cone is pushed downward at a steady rate of 10

mm/sec through a depth of 35mm each time. The cone is pushed by applying thrust and not by

driving.

After the cone resistance has been determined, the cone is withdrawn. The sleeve is pushed

on to the cone and both are driven together into the soil and the combined resistance is also

determined.

Fig.14: Dutch Cone.

1) The cone test is very useful in determining the bearing capacity of pits in cohesionless soils,

particularly in fine sands of varying density.

2) The cone resistance is approximately equal to 10 times the penetration resistance N.

IN – SITU VANE SHEAR TEST

1) In – situ vane – shear test is conducted to determine the shear strength of a cohesive soil in its

natural condition.

2) The apparatus is similar to one used in laboratory. It consists of four blades, 100 mm (or 150

mm or 200 mm long), attached at right angles to a steel rod.

3) The steel rod has a torque – measuring device at its top.

4) The height – diameter ratio (H/D) of the apparatus is generally equal to 2.

5) For conducting the test, the shear – vane is pushed into the ground at the bottom of the bore

hole. When a torque is applied through the handle at the top of the rod, the soil is sheared

along a cylindrical surface.

6) The torque required to shear the cylinder of the soil is measured by means of a spring

balance.





7) The undrained shear strength Su of the soil is determined from the equation,

8) The vane – shear test is extremely useful for determining the in – situ shear strength of very

soft and sensitive clays, for which it is difficult to obtain undisturbed samples.

Fig.15: In – situ vane shear test.

GEOPHYSICAL METHODS

A number of geophysical methods are used in preliminary investigations of sub – soil strata.

The methods can be used for the location of different strata and for a rapid evaluation of the subsoil

characteristics. The geophysical methods can be broadly divided into the two categories: Seismic

methods and Electrical resistivity methods.

1) Seismic Method: - The seismic methods are based on the principle that the elastic shock

waves have different velocities in different materials. Seismic methods of subsurface

explorations generally utilize the refracted waves.

i) The shock wave is created by a hammer blow or by a small explosive charge at a point P.

Fig.16. The shock wave travels through the top layer of the soil (or rock) with a velocity V1,

depending upon the type of material in layer – I. The observation of the first arrival of the

waves is recorded by geophones located at various points, such as A, B, C.

ii) The geophones convert the ground vibration into electrical impulses and transmit them to a

recording apparatus.

iii) It is assumed that V3 > V2 > V1 in Fig.16. At geophones located close to the point of impact,

such as point A, the direct waves with velocity V1 reach first.





Fig.16: Seismic method.

iv) At points which are located away from the point of impact, such as point B, the refracted

waves reach earlier than the direct waves. These waves start from point P, travel with

velocity V1 in the upper layer, get refracted at the interface, move with much higher velocity

V2 in the second layer, emerge again at the interface and travel back to the ground surface at

a lower velocity V1 in the upper layer.

v) At points further away from the point of impact, such as point C, the waves which are

refracted twice, once at the interface of the layers I and II, and once at the interface of the

layers II and III, reach earlier.

vi) The time (t) of arrival of the first impulse at various geophones is taken as ordinate and the

distance (X) of the geophones from the point of impact P is taken as abscissa.

vii) The velocity in any layer is equal to the reciprocal of the slope of the corresponding velocities

computed.

viii) Upto a certain distance X1, the direct waves in the layer I each first. At this point, the first

two lines in graph intersect, which indicates that the direct wave traveling a distance X1

with a velocity V1 and the refracted wave traveling with a velocity V1 in distance 2H1 and

with a velocity of V2 in distance X1 reach simultaneously, where H1 is the thickness of the

layer I. Thus,

……………….i)

From equation i)

Gives reliable results. The following empirical equation gives more reliable results for impact shock.

Likewise, the thickness of the second layer (H2) is obtained from the distance X2 corresponding to

the point of intersection of the second and the third line in Fig.16.

The procedure is continued if there are more than three layers.





Limitation of the seismic methods: -

1) The methods cannot be used if a hard layer with a greater seismic velocity overlies a softer

layer with a smaller seismic velocity.

2) The methods cannot be used for the areas covered by concrete, asphalt pavements or any

other artificial hard crust, having a high seismic velocity.

3) If the area contains some underground features, such as buried conduits, irregularly dipping

strata, and irregular water table, the interpretation of the results becomes very difficult.

4) If the surface layer is frozen, the method cannot be successfully used, as it corresponds to a

case of harder layer overlying a softer layer.

5) The methods require sophisticated and costly equipment.

6) For proper interpretations of the seismic survey results, the services of an expert are required.

2) Electrical resistivity methods:- The electrical resistivity (ρ) of a conductor is expressed as

where R = electrical resistance (ohms), A = area of cross-section of the conductor (cm2),

L = length of conductor (cm), ρ = electrical resistivity (ohm-centimeter).

The resistivity of a material depends upon the type of material, its water content and the

concentration of dissolved ions and many other factors. Rocks and dry soils have a greater resistivity

than saturated clays.

The method is also known as the resistivity mapping method. Four electrodes are used at a

constant spacing a. To conduct the test, four electrodes, which are usually in the form of metal

spikes, are driven into the ground. The two outer electrodes are known as current electrodes. The two

inner electrodes are called potential electrodes. The mean resistivity of the strata is determined by

applying a D.C. current to the outer electrodes and by measuring the voltage drop between the inner

electrodes. A current of 50 to 100 milliamp is usually supplied.

The mean resistivity (ρ) is given by the formula

Fig.17: Electrical resistivity method.

Limitation of the electrical resistivity methods: -

1) The methods are capable of detecting only the strata having different electrical resistivity.

2) The results are considerably influenced by surface irregularities, wetness of the strata and

electrolyte concentration of the ground water.

3) As the resistivity of different strata at the interface changes gradually and not abruptly as

assumed, the interpretation becomes difficult.





4) The services of an expert in the field are needed

SUB – SOIL INVESTIGATION REPORT

A report is the final document of the whole exercise of soil exploration. A report should be

comprehensive, clear and to the point. Many can write reports, but only a very few can produce a

good report. A report writer should be knowledgeable, practical and pragmatic. No theory, books or

codes of practice provide all the materials required to produce a good report. It is the experience of a

number of years of dedicated service in the field which helps a geotechnical consultant makes report

writing an art. A good report should normally comprise the following:

1. A general description of the nature of the project and its importance.

2. A general description of the topographical features and hydraulic conditions of the site.

3. A brief description of the various field and laboratory tests carried out.

4. Analysis and discussion of the test results

5. Recommendations

6. Calculations for determining safe bearing pressures, pile loads, etc.

7. Tables containing borelogs, and other field and laboratory test results

8. Drawings which include an index plan, a site-plan, test results plotted in the form of charts

and graphs, soil profiles, etc.

Fig.18: A typical bore-hole log.





The following graphical presentations should be attached to the report:-

1) A site location map.

2) A plan view of the location of the boring with respect to the proposed structures and those

nearby.

3) Boring logs.

4) Laboratory test results.

5) Other special graphical representations.

The exploration report should be well planned and documented, as they will help in

answering questions and solving foundation problems that may rise later during design and

construction.

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