site investigation-for section 3.pdf

Bahir Dar University, Bahir Dar Institute of Technology

Faculty of Civil and Water Resources Engineering

Department of Civil Engineering

Wubete M.

Mar,2016

1

Chapter 1:

Site Exploration

2

Outline

Purpose of Soil Exploration

Different methods

– Test trenches and Pits

– Auger and Wash Boring

– Rotary Drilling

– Soil Sampling (Disturbed and Undisturbed)

– Geophysical Methods

3

Site Exploration

• Investigation of a site for obtaining information about the

subsurface conditions that underlain a proposed structure

• Determination of surface and subsurface soil conditions and

features in an area of proposed construction that may influence

the design and construction and address the expected post

construction problems.

• It consists of determining the profile of the natural soil deposits

at the site, taking the soil samples and determining the

engineering properties of soils. It also includes in-situ testing of

soils.

4

Scope of Exploration

• Simple visual examination of the soil at the surface or from

shallow test pits

• Detailed study of soil and groundwater to a reasonable

depth( influence zone) by sampling from boreholes and in-

situ and laboratory tests

5

Purpose of Exploration

The site exploration provides first hand information for;

• Selection of alternative construction site or choice of

most economical site

• Design of foundations

– to decide the type and depth

– Estimating the load bearing capacity and

probable settlement

• Contractors to quote realistic and competitive

tenders

• Planning construction techniques.

• Groundwater location

6

Purpose of Exploration

• Selection of appropriate construction equipment and

materials (especially for excavation and foundations).

• Study of environmental impacts (EI) of the proposed

construction.

• Selection of the most suitable and economical route for

highways with respect to soil conditions.

• The investigation of the cases where failure has occurred, to

know the causes and design of remedial works

• Need for any suitable soil improvements

7

Common Steps in Site Exploration

Desk Study

Site Reconnaissance

Field Investigations

a) Preliminary Ground Investigation

b) Detailed Ground Investigation

Laboratory Testing

Report Writing

Follow up Investigations during design & construction

Appraisal of performance

Reconnaissance

Its purpose is to estimate the type of soils and rock likely to be

encountered and is accomplished by:

• Collection of data about the project

• Visual inspection of the site condition (topography) and

nearby existing structures

• Geologic study: Geological map, Area Photographs

• Collecting and studying the existing document from

previous experience with similar or adjacent sites.

8

Reconnaissance

Cracks, sticking doors and windows in existing light building

Expansive soils

9

Potential Stability Problems10

What can you detect from photo inspection?

12

Preliminary Site Investigation

A few borings or a test pit to establish the stratification,

types of soil to be expected, and possibly the location of the

groundwater level and its fluctuation.

Depth of bed rock , If the initial borings indicate the upper

soil is loose or highly compressible.

Establish subsurface profile (depth, thickness and

composition of subsurface strata).

Estimate of engineering properties of the soil from index

properties

13

Preliminary Site Investigation

Initial selection of foundation possibilities.

This step may be sufficient to establish the foundation criteria

for small projects

14

Detailed Site Investigation

If the preliminary site investigation get feasible, detailed

investigation is undertaken

Additional borings are required

Determine the specific appropriate engineering properties of

the site soil (strength, compressibility and hydraulic

conductivity)

More sounding field tests undertaken to obtain information

which shall be sufficient for final design

15

16

Methods of Site Exploration

The methods to determine the sequence, thickness and lateral

extent of the soil strata and, where appropriate the level of bedrock.

The common methods include

Test pits /trenches

Boring (and sampling) or drilling

Insitu Tests

Laboratory Tests etc

17

• The excavation of test pits is a simple , cheapest and reliable

method.

• The depth is limited to 4-5m only.

• The in-situ conditions are examined visually

• It is easy to take good undisturbed soil samples from a pit

than a bore hole

• Block samples can be cut by hand tools and tube samples

can be taken from the bottom of the pit.

• Trenches are useful in investigating subsurface conditions

where lateral variations in material conditions are expected

Test Pits and Trenches

** Block Samples

Block samples are cut by hand from material exposed in trial pits

and excavations.

Sample cutting should be done as quickly as possible to prevent

excessive moisture loss, should be protected from rain and direct

sunlight.

Protect the sides with aluminum foil or grease proof paper and

coated with wax and cover with muslin.

18

Sample Labeled, Moisture Control and Transportation

Seal with wax to control

water content of samples

Vertical transportation to prevent

moisture migration

What’s wrong with this sampling?

21

Boring refers to advancing a hole in the ground.

Boring is required for the following:

• To obtain representative soil and rock samples for

laboratory tests.

• To identify the groundwater conditions.

• Performance of in-situ tests to assess appropriate soil

characteristics.

Boring

22

Some of the common types of boring are as follows

Auger boring

Wash boring

Percussion boring

Rotary drilling

Boring

24

Hand Auger

It is the simplest method of boring used for small projects in soft

cohesive soils.

For hard soil and soil containing gravels boring with hand auger

becomes difficult.

• It can be used for depths up to 3 to 5m..

• The length of the auger blade varies from 0.3-0.5m.

• The auger is rotated until it is full of soil, then it is withdrawn to

remove the soil and the soil type present at various depths is noted.

24

Hand Auger Mechanical Auger

Auger Boring

25

• The soil samples collected in this manner are disturbed samples and

can be used for classification test.

• Generally, it is suitable for all types of soils above water table but

suitable only for clays below the water table.

• Auger boring may not be possible in very soft clay or coarse

sand because the hole tends to collapse when auger is removed.

• Soils with boulders and cobbles are difficult to investigate.

25

Auger Boring

26

26

Auger Boring

27

Mechanical Auger means power operated augers. The power

required to rotate the auger depends on the type and size of

auger and the type of soil.

Downwards pressure can be applied hydraulically, mechanically

or by dead weight

a

ab

c d

a. Continuous Flight Auger

b. Hollow-stem auger plugged

during advancing bore

c. Plug removed and sampler

inserted

d. Truck mounted auger boring

machine

Mechanical Auger

28

• The most common method is to use continuous flight augers.

Continuous flight augers can be solid stem or hollow stem with

internal diameter of 75-150mm.

• Hollow stem augers are used when undisturbed samples are

required. Plug is withdrawn and sampler is lowered down and driven

in to the soil below the auger.

• If bed rock is reached drilling can also take place through the hollow

stem.

• As the auger acts as a casing it can be used in sand below water

table. The possibility of rising sand in to the stem by hydrostatic

pressure can be avoided by filling the stem with water up to the

water table

There is a possibility that different soil types may become

mixed as they rise to the surface and it may be difficult to

determine the depths of changes of strata. Experienced

driller can however detect the change of strata by the

change of speed and the sound of drilling.

Disadvantages of auger:

• The soil samples a re highly disturbed

• It becomes difficult to locate the change in strata

29

30

Wash boring Water with high pressure pumped through hollow 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 rock and the borehole sites.

The rods are raised and drop for chopping action of the chisel by

means of winch.

This method best suits in sandy and clayey soils and not in very hard

soil strata (i.e. boulders) and rocks.

Wash boring can be used in most type of soil but the progress is

slow in coarse gravel strata. 30

31

Wash boring

31

32

• Primarily intended for investigation in rock, but also used in

soils.

• 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.

Rotary Boring/ Drilling

33

Advantages

The advantage of rotary drilling in soils is

• The progress is much faster than with other investigation

methods

• Disturbance of the soil below the borehole is slight.

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

34

No “hard-and-fast rule” exists for determining the number

of borings or the depth to which the test borings are to be

advanced.

For most buildings, at least one boring at each corner and

one at the center should provide a start for investigation in

rock, but also used in soils.

Depending on the uniformity of the subsoil, additional test

borings may be made

Site Exploration Range

How Many Borings?

Conventionally, the number (density) of borings will

increase:

• As soil variability increases

• As the loads increase

• For more critical/significant structures

35

36

Numbers of Borings

Guideline for trial pits and boring layout (Teng)

7.5

How many bore holes?

37

120 m

Proposed site for a multi-story shopping complex

Not enough bore holes; soil profile and properties not well defined..Not enough bore holes; soil profile and properties not well defined..

bore hole

120 m

Too many bore holes and blows the budget.Too many bore holes and blows the budget.

How Many Bore Holes?

38

How many bore holes?

39

120 m

About right?About right?

trial pit

40

According to Tomlinson

i. For widely spaced strip of pad foundation, Boring depth depth(D)

1.5B

ii. For Raft Foundations, depth(D) 1.5B, B-width of foundation

iii. For closely spaced strip or pad foundations where there is

overlapping of the zones of pressure, D 1.5 width of building

iv. For group of piled foundation on soil, D 1.5 width of pile group.

The depth being measured from a depth of two-thirds of the

length of piles

v. For piled foundation on rock, D 3.0m inside bedrock.

Depth of Borings

41

According to EBCS 7,1995

Depth of Borings

b.For structures on footing,

B

D

DBB

D

DB

BH BH

3

c.For structures piled foundation,

D pile length from the surface B m

a.For structures on

mat foundation,

Depth of Investigation Based on Pressure Bulb

42

Must explore to the depth to which the soil

will be significantly stressed (perhaps to

10% of the applied stress). This is of the

order of twice the width of the loaded area.

Isolated footing

Pile group

Stress contours

Soil stressed to greater depth due

to interaction between piles and

due to shaft friction

43

Depth of Investigation-Based on Pressure Bulb

44

Depth of Investigation -Based on Pressure Bulb

Closely spaced strip on pad footings

45

Depth of Investigation -Based on Pressure Bulb

Minimum boring depth, D

ASCE(1972)

1. Determine the net increase in the effective stress Ds’ and the

vertical effective stress s0’

2. Determine D1 when Ds’= q/10

3. Determine D2 when Ds’= s0’ /20

4. Find D = min(D1 and D2)

q

46

Sowers and Sowers(1970)

Rough estimate of min. depth of borings, D( unless bedrock is

encountered) is as follows:

1. D = 3S 0.7 …… for light steel or narrow concrete building

2. D = 6S 0.7 …… for heavy steel or wide concrete building

Where D = approximate depth of boring for preliminary

investigation

S = number of stories

47

Two problems that are common to the drilling of holes

i. Caving of the sides of the hole

Caused by the release of the stresses caused by the

removal of overburden material from the hole

Holes drilled above the water table may not cave in even

in sandy soil due to the presence of apparent cohesion

present between particles

Caving in sandy soils cannot be avoided below the water

table.

Holes drilled in cohesive soil may remain stable even up to

considerable depth below the water table.48

Stabilization of bore Holes

Caving in fine sandy and silty soils below water table can be

minimized to certain extent by use of drilling mud

a. Drilling mud … Bentonite( pure form of thixotropic

clay) mixed with water. i.e. Bentonite Slurry, which has

the following properties

Supports the excavation by exerting hydrostatic pressure

on the wall

Provide almost instantaneously a membrane with low

permeability

b. Use of casing pipe49


Caving of the sides of the hole can be prevented by

a. Drilling mud … Bentonite( pure form of thixotropic

clay) mixed with water. i.e. Bentonite Slurry, which has

the following properties

Supports the excavation by exerting hydrostatic pressure

on the wall

Provide almost instantaneously a membrane with low

permeability

b. Use of casing pipe

50


51


Regained strength of a partially thixotropic material

Heaving of the bottom of the hole can be prevented by

a. Keeping the water table in the hole sufficiently above the

water table outside the hole to counteract the inflow

There are 2 main categories of soil samples:

- Undisturbed samples .

The soil structure, and particle size distribution should be preserved

to its original Insitu stratum.

The water content of the samples are also tried to be

preserved as far as possible, to truly represent site conditions.

These samples are required for carrying out tests in lab.: shear

strength, consolidation, permeability, compressibility, Insitu-

density and water content…

52

Soil Sampling & Sample Disturbance

- Disturbed samples.

The particle size distribution should be preserved to its original Insitu

stratum (representative), though the soil structure may be seriously

disturbed

The water content of the samples may have also changed. If it

is permissible, the water content should also be preserved.

Collected as drilling or digging progress for soil identification

and classification(Index Properties).

Tests that can be conducted: Chemical Analysis, Atterberg’s

Limit, Sieve analysis, Sp.Gravity, compaction etc.

53


- Disturbance of samples is due to.

The change in state of stress as the sample is retrieved

from the soil

Resistance against the sides of the sample

During transportation to the laboratory

As the sample is retrieved from sampling tube

Evaporation of moisture from the sample due to improper

sealing

54


To reduce side friction onsample as tube is pushed intothe soil

Sample Disturbance

The degree of disturbance can be expressed in terms of area ratio

55

Sample Disturbance

56

Area ratio is the ratio of the volume

of soil displaced to the volume of

collected sample

57

Good quality samples necessary.

AR≤ 10%

sampling tube

soil(%) 100

..

....2

22

DI

DIDOAR

area ratio

Thicker the wall, greater the disturbance.


The disturbance is considered negligible

Sample Disturbance

58

The soil sample can be considered undisturbed If the area ratio is less

than or equal to 10% , the disturbance is considered negligible

However , area ratio have to be much more than 10 % for very stiff

or hard clay soils to prevent the edges of the sampling tube from

getting distorted during sampling

Inside clearance allows for elastic expansion of the soil as it enters

the tube, and thus produces disturbance of soil structure and

reduction of density in dense sand .to minimize the expansion and

disturbance of soil sample , inside clearance should not be more than

1 to 3%

Inside clearance reduces frictional drag on the sample from the wall

of the tube and helps to retain the cores

Sample Disturbance

59

The outside clearance should also not be much greater than inside

clearance. It is usually between 0 to 2%.

Outside clearance facilitates the withdrawal of the sample from the

ground

Common types of samplers

i. Split-Spoon- With these sampler-disturbed samples of

soft rock, cohesive and cohesionless soils are obtained.

This sampler is used for making standard penetration.

ii. Thin –wall tube(shelly tube) –for soils sensitive to

disturbance and suitable to take undisturbed sample only

for cohesive soils( clays/silt)

iii. Piston sampler - For very soft silts and clays and

provides best-undisturbed sample of cohesive soils

60

Sampling Methods

For Standard Split-Spoon Sampler Di=1.38in, Do=2in

• AR(%)=111.5%>10%

• disturbed sample

• used for soil tests to obtain soil index properties

For Thin-Walled Tube (Shelby tube) Sampler Di=1.875in, Do=2in

• AR(%)=13.75%>10%

• Relatively undisturbed sample

• used for soil tests to obtain soil engineering properties

This indicates that thesample collected bySplit-spoon is highlydisturbed

61

These test are used to determine the properties of the soil without

disturbing effect of boring and sampling. The most commonly used field

tests are:

Penetration or sounding Test

Vane Shear Test

Plate Loading test

Pile Loading Test

Dutch Cone Penetration

62

In-situ Tests

63In bore holes

VST

SPT

CPT

63

Penetration tests are conducted mainly to get information on

the relative density of soils with little or no cohesion.

The tests are based on the fact that the relative density of a

soil stratum is directly proportional to the resistance of the soil

against the penetration of the drive point.

From this, correlations between values of penetration

resistance versus angle of internal friction ), bearing

pressure, density have been developed.

64

In-situ Tests

Standard Penetration Test

SPT

65

This is the most commonly used in-situ test, especially for

cohesionless soils which cannot be easily sampled.

The test is extremely useful for determining the relative density

and the angle of internal friction of cohesionless soils.

It can also be used to determine the unconfined compressive

strength of soils..

The sampler is driven into the soil by hammer blows to the top

of the drill rod, the number of blows required for the last two

intervals are added to give the standard penetration number, N

66

Standard Penetration Test(SPT)


The blows required to produce the first 150mm penetration,

termed as seating blows, are usually ignored as it is

assumed that it is disturbed due to drilling process. The test is

refusal and halted if

a. 50 blows are required for any 150-mm increment

b. If a total of 100 blows are obtained to drive the required 300mm

c. 10 successive blows produce no advance

67

Energy Ratio

It is necessary to standardize SPT to some energy ratio(Er),which

is the ratio of the actual hammer energy to the sampler to input

energy.

Energy ratio (Er) x blow count should be constant for any soil and

can be expressed as

r1 1 r2 2E N =E N

This indicates that the larger values of energy ratio

decreases the blow count linearly

69

100 ar

in

Actual Hammer energy to sampler, EE

Input energy, Ex

Corrections to Measured SPT Value

In the field, the energy can vary from 30% to 90%.

The discrepancies appear to be rise from the ff factors:

Difference in some features of SPT equipment, drilling rig,

hammer and skill of operation

Driving hammer configuration and the way hammer load is

applied

Whether liner is used inside the split barrel sampler… Side

friction increases the driving resistance (N)

70


overburden pressure- the bigger the o.b.p the more is N

value for soil of the same density.

Degree of cementation: The more cemented zone the

higher is N value

Length of the drill rod- the shorter the rod the more is N

value

Bore hole diameter - the smaller the size of the hole the

more is N value

71


So, the standard practice now is to express the N-value to an

average energy ratio of 60% , i.e. N60 (Skempton 1986)

N60 is used to correlate the soil properties of cohesive soil

( e.g. qu, Su, OCR etc. )

60 N C C C Cm E B S RN

where:

CE: Hammer Energy ratio (Er) factor (efficiency)=actual energy

ratio/Standard energy ratio (60%)

CB: correction for borehole diameter

CS: sampler correction

CR: correction for rod length

Nm: measured SPT N value

72

Skempton 1986

73

Correction for Overburden Pressure

This type of correction is applied only to Cohesionless soils. The

Correction suggested by Liao and Whitman (1986) is as follows:

'60 60N C NN

N

v

95.76C =

σ

where:

CN: Adjustment for effective overburden pressure

s’v(kPa): in-situ effect vertical stress

74

Terzaghi and Peck considered dilatancy correction for N>15,which is

an indication of Dense sand.

In such soil, the fast rate of application of shear through the blows

of hammer, is likely to induce negative pore water pressure in

saturated fine sand under undrained loading condition

Correction for Dilatancy


75

Saturated Sands that contain appreciable fine soil particles such as

silty or clayey sands could give abnormally high N values if they

have the a tendency to dilate or abnormally low N Values if they have

tendency to contract during undrained shear conditions associated

with driving SPT.

Silty fine sands and fine sands below the water table develop pore

water pressure which is not easily dissipated. The pore water pressure

increases the resistance of the soil and hence the penetration number

(N).



76

Terzaghi and peck (1967) recommend the following correction

N'' = 15+ 0.5(N'-15)

where:

N’’= corrected value

N’= Value obtained after overburden correction


For N’ >15 …..Terzaghi and Peck

N'' = N' for N' 15.0


77

Terzaghi and Peck(1974)

In current practice the correction for water table is ruled out due to

its conservative nature

Correction for Water Table


1

DwCw= 0.5

D+B

where:

D = Depth of footing base from surface

Dw=Depth of water table from surface

78

79

N60 cu (kPa) consistency visual identification

0-2 0 - 12 very soft Thumb can penetrate > 25mm

2-4 12-25 soft Thumb can penetrate 25mm

4-8 25-50 medium Thumb penetrates withmoderate effort

8-15 50-100 stiff Thumb will indent 8 mm

15-30 100-200 very stiff Can indent with thumbnail; not thumb

>30 >200 hard Cannot indent even withthumb nail

Use with caution; unreliable.

not corrected for overburden

79

80

(N)60 Dr (%) (deg) consistency

0-4 0-15 <28° very loose

4-10 15-35 28-30° loose

10-30 35-65 30-36° medium

30-50 65-85 36-42° dense

>50 85-100 >42 very dense

not corrected for overburden

80

The relative density is useful to describe the consistency of sand. It

may be determined from SPT data using the following empirical

correlations(Kulhawy and Mayne 1990)

0.18

100

t

1 60

A OCR

50

A

OCR

NDr = X100%

CpC C

Cp = 60 + 25logD

C = 1.2 + 0.05log

C = OCR

where:

Dr= relative density

(N1) 60 = SPT N value corrected for field procedures and overburden stress

Cp=grain-size correction factor

CA= aging correction factor

COCR =overconsolidation correction factor

D50 =grain size at which 50% of the soil is finer(mm)

t= age of soil(time since deposition((years)

OCR=overconsolidation ratio

81

Cone Penetration Testing(ASTM D 5778)

Cone Penetration Test is a static in-insitu test used for

subsurface exploration in fine and medium sands, soft silts and

clays without taking soil samples

The test is not well adapted to gravels deposits or to stiff/hard

cohesive deposits.

The corrected total tip resistance can be estimated as:

(1 )t cq = q cu a

where:

qt= corrected total cone resistance

qc = Qc/Ac =cone resistance

Qc=force required to push the cone

Ac= end area of the cone

a=net area ratio

uc =measured pore water pressure immediately behind the cone

83


84


Friction sleeve resistance can be estimated as:

where:

Qs= Force on friction sleeve

As = Area of friction sleeve

Friction Ratio

sf = Qs

As

f = fs

Rqc

85

• The CPT defines the soil profile with much greater resolution than SPT does

• However the CPT has disadvantagesa. No soil sample is recovered . So there is no opportunity to

inspect the soilb. The test is unreliable in soils with significant gravel content

Cone Penetration Testing (ASTM D 5778)

ASTM D 577886

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

qt (MPa)D

ep

th (

m)

0

5

10

15

20

25

30

35

40

0 100 200 300 400

fs (kPa)

0

5

10

15

20

25

30

35

40

0 1000 2000 3000

u2 (kPa)

Clean

Sand

Clayey Silt

Clay

Sand

87

Cone Penetration Testing

88

The undrained shear strength Friction sleeve resistance can be

estimated as:

Where: po=overburden pressure above the tip of the

cone and it is in the same unit of qc and

same type of pressure

Nk= cone factor(constant for soil)

With adjustment

CPT Correlations for Cohesive soils

cu

qs =

k

Po

N

u

qs = t

kt

Po

N

For normally consolidated clay oflow sensitivity (S<4) and PI <30, avalue of Nk=18 may be used

89

For normally consolidated clay oflow sensitivity (S<4) and PI <30, avalue of Nkt=14 may be used

5.513 2

50 kTN = PI Bowles (1996)

CPT Correlations for Cohesive soils

90

CPT Correlations for Non Cohesive soils

The design friction angle can be estimated as follows

5 c= 29 + q - for silty sand

+ for gravel

91

Bowles (1996)

CPT Correlations

92

Comparisons of SPT and CPT

93

– Core recovery percentage

– Rock Quality Designation (RQD)

Defines the fraction of solid core recovered greater

than 4 inches in length

Calculated as the ratio of the sum of length of core

fragments greater than 4 inches to the total drilled

footage per run, expressed as a percentage

Rock Sampling

94

Rock core drilling is commonly used to advance the borehole and

provide core samples for examination and testing

Core Recovery ,Lr is a measure of determining sample disturbance

and can be expressed as :

Rock Quality Designation(RQD) is an index or measure of the quality

of a rock mass [Stagg and Zienkiewicz(1698)].

Rock Coring

Length of recovered sampleCore recovery(%)= x100

Length of rock cored

Lengths of intact pieces of core>100mmRQD(%)= x100

length of core advance

95

Recovery ratio, Lr is a measure of determining sample

disturbance and can be expressed as :

Lr = 1.0 … a good recovery

Lr > 1.0 indicates swelling in the sample

Lr < 1.0 means the sample is compressed

96

9cm

97

Fairly reliable results for the undrained shear strength, cu (=0

concept), of very soft to medium cohesive soils may be obtained

directly from vane shear tests

Shear vane apparatus.

3/10/201698

Vane Shear Test

(a) resisting moment of shear force ; (b) Assumed variation in shear

strength mobilization3/10/2016

99

Vane Shear Test

Calculation of Undrained Shear strength for Rectangular-Shaped ends Vane

3/10/2016100

Vane Shear Test

dA=rddr

Su

SuSuo

ab

c

d

T = Ms + Me + Me

For calculation of Me, let the distribution of mobilized shear strength

follows triangular and then we can develop an equation for others.

3/10/2016101

Vane Shear Test

Two ends

• The resisting moments can be given as:

21(2 )

2sM RH Su R D HSu

2

00

R

e uo

r

M rS dA

where and

3/10/2016102

Vane Shear Test

dA rd dr uo u

rS S

R

32 2

0 00 0

23

00

42

00

42

0

42

0

=

=4

=4

=4

=

R R

e u u

r r

R

u

r

R

u

r

u

u

r rM rS rd dr S d dr

R R

Sr d dr

R

S rd

R

S Rd

R

S R

R

3

u

πD×S

16

2

uo u

rS S

R

From above equation

3/10/2016103

Vane Shear Test

2

2

2

1 2

2

1

2 8

1

2 4

s eT M M

D HSu

S D H

a

u

3

u

3

3

2 3 2 3

2

TSu

πD×S

16

πD

T T

⇒Su= =D H D D H D

π + π +

=D H D

π

2 8

⇒

+2 4

2

2

2

1 , 2 ,vh

2

s eT M M

D HSu vv3

u

πD×S

16

Anisotropic with respect to shear strength

=1/2 for triangular end shear

= 2/3 for uniform end shear

= 3/5 for parabolic end shear

3/10/2016104

Vane Shear Test

2 3

TSu=

D H Dπ +a

2 4

where a=constant

⇒ For rectangular end vane

2 3

2 6

Su=

T

D H DFor rectangular shaped end

vane and uniform end shear,

both ends of vane are fully

submerged

105

105

The Undrained Shear strength obtained from Vane shear test may

give results that are unsafe( overestimated) for foundation design.

Therefore reduction factor should be used

=1.7-0.54log(PI) ……Bjerrum(1974)

=1.05-0.045(PI)0.5 ……Chandler(1998)

Vane Shear Test

(design) (Vane shear)Su = Su

107

General Expression

108

108

Plate Load Test

109

109

It is the most reliable method to determine the bearing

capacity and the settlement at site

Plate Load Test

2

( 0.3)............f

( 0.3)

.....

p

p

b bp

bp b

b

bp

S= or footing rest on sand

S

S= for footing rest on clay

S

where :

S= Settlement of proposed foundation

Sp= settlement of test plate

bp= width(diameter) of plate

b=width of proposed footing(least dimension)

110

110

For a given maximum permissible settlement, Sp can be

calculated using the eqn.

From the load-settlement curve the load intensity under the

plate could be read

The ultimate bearing capacity is determined as follows:

Plate Load Test

,

,

,

,

............

1.0.....

= for footing rest on sand soils

for footing rest on clay soils

u F

u p

u F

u p

q b

q bp

q

q

111

111

Limitation of Plate Load Test

112

112

Seismic Refraction Method

When a shock or impact is made at a point on or in the earth,

the resulting seismic (shock or sound) waves travel through

the surrounding soil at speeds related to their elastic

characteristics

The radiating waves are picked up by detector called

geophones placed at increasing distance from the origin of

shock

Geophysical Method

115

115

Seismic Method

116

116

Geophysical Method

117

117

Geophysical Method

118

118

Limitations

The basic eqns. Made is based on assumption that P-wave velocity

V1<V2<V3

The methods cannot be used when hard layer overlies a soft layer

The method cannot be used in an area covered by concrete of

pavement

When soil is saturated , the p-wave velocity is deceptive

This methods are used as preliminary or supplementary to others

Geophysical Methods

119

119

120

Establishing the highest and the lowest possible levels of

water during the life of the project is necessary.

In soils with high coefficient of permeability, the level of

water in bore hole will stabilize in a bout 24 hrs after

completion of the bore hole drilling.

The depth of the water table can then be measured

using a chalk coated steel tape.

Groundwater Table Location

121

121

In soils with low K-values, this process may take a

week.

The depth of the water table can then be measured

by installing piezometer

If the seasonal ground water table variation is to be

measured, piezometer may be installed in to bore hole

and the variation is recorded for longer time.

Groundwater Table Location

122

122

Laboratory tests are useful in providing reliable data for

calculating ultimate bearing capacity of soils, stability and

settlement behavior of foundation, and for determining physical

characteristics of soils. Results of laboratory tests should be

used in conjunction with borehole records and results of field

test.

The common laboratory tests that concern the foundation

engineers are

Grain size analysis –ASTM D422

Classification-ASTM D2487

Atterberg limits –ASTM D4318

Natural moisture content –ASTM D2216

Laboratory Tests

123

123

Unit weight

Specific gravity-ASTM D854

Unconfined compression test –ASTM D2166

Direct shear test –ASTM D3080

Triaxial compression test

Unconsolidated Undrained--ASTM D2850

Consolidated Drained--ASTM D7181

Consolidated Undrained--ASTM D4767

Consolidation test—ASTM D2435

Compaction

Standard---ASTM D698

Modified--ASTM D1557

Laboratory Tests

124

124

1. Introduction: - purpose of investigation, type of

investigation carried out.

2. General description of the site: - general configuration and

surface features of the site.

3. General geology of the area:- from available geology

records a summary of the geology and other relevant

conditions of the site are described since these could affect

the scope of the work

4. Details of the field exploration program, indicating the

number of borings, their location and depth.

5. Details of the methods of exploration.

Soil Exploration Report

125

125

6. Description of soil conditions found in bore holes (and test

pits):- A brief summary of the sequence of deposits their

nature, thickness and variability.

7. Discussion of laboratory test results.

8. Discussion of results of investigation in relation to foundation

design and constructions.

9. Recommendations on the type and depth of foundations,

allowable bearing pressure and methods of construction.

10. Conclusion: - The main findings of investigations should be

clearly stated. It should be brief but should mention the salient

points.


127

127

Why is a geotechnical subsurface exploration conducted?

What are the drilling methods that are commonly used in the

geotechnical subsurface exploration?

Under what soil conditions is mud rotary drilling used?

What are the types of samplers that are commonly used in

sampling soils?

What factors affect the SPT blow count (Nvalue)?


site investigation-for section 3.pdf

Documents