GEII ModuleI PartII.ppt - Government Engineering...

INDIRECT METHODS

SOUNDING OR PENETRATION TESTS

Dr. K. M. Kouzer, Associate Professor in Civil Engineering, GEC Kozhikode

STANDARD PENETRATION TEST (SPT)● Reference can be made to IS 2131 – 1981 for details on SPT.

● It is a field test to estimate the penetration resistance of soil.s a e d es o es a e e pe e a o es s a ce o so

● It consists of a split spoon sampler 50.8 mm OD, 35 mm ID,min 600 mm long and 63 5 kg hammer freely dropped from amin 600 mm long and 63.5 kg hammer freely dropped from aheight of 750 mm.

● Test is performed on a clean hole 50 mm to 150 mm diameter● Test is performed on a clean hole 50 mm to 150 mm diameter.

● Split spoon sampler is placed vertically in the hole, allowed tofreely settle under its own weight or with blows for first 150freely settle under its own weight or with blows for first 150mm which is called seating drive.

Th b f bl i d f th t 300 t ti● The number of blows required for the next 300 mm penetrationinto the ground is the standard penetration number N

● N value is termed as Refusal if 50 blows for any 150 mmpenetration or 100 blows for 300 mm penetration or 10


successive blows produce no advance.

STANDARD PENETRATION TEST (SPT)



CORRELATIONS OF N Denseness φSPT VALUES FOR COHESIONLESS

0 - 4 Very Loose < 28°

4 10 Loose 28° 30° SOILS4 - 10 Loose 28° - 30°

10 - 30 Medium 30° - 36°

30 - 50 Dense 36° - 42°

50 V D 42> 50 Very Dense > 42°


CORRELATIONS OF SPT VALUES FOR COHESIVE SOILSCOHESIVE SOILS

2u m/kN

Nq ≈u /N

075.0q

N Consistency qu (kN/m2)

0 - 2 Very soft < 250 - 2 Very soft < 25

2 – 4 Soft 25 – 50

4 – 8 Medium 50 – 100

8 15 Stiff 100 2008 – 15 Stiff 100 – 200

15 – 30 Very Stiff 200 – 400

> 30 Hard > 400


CORRECTIONS FOR SPT VALUE

● Correction for Overburden PressureFor Cohesionless soilsFor Cohesionless soils,

N2N2000

log77.0N 10 ≤⎥⎦

⎤⎢⎣

⎡=′

where q is Effective overburden pressure in kN/m2.

q ⎥⎦

⎢⎣

● Correction for Dilatancy

For fine sand and silt below water tableFor fine sand and silt below water table,

15Nfor)15N(5.015N >′−′+=′′ )(

15NforNN ≤′′=′′


ADVANTAGES

● Relatively quick & simple to perform

E i t & ti f t t i id l il bl● Equipment & expertise for test is widely available

● Provides representative soil samplep p

● N is correlated with most properties of soil such as

friction angle undrained cohesion density etcfriction angle, undrained cohesion, density etc.

● Provides useful index for relative strength &

compressibility of soil

● Able to penetrate dense & stiff layers● Able to penetrate dense & stiff layers

● Results reflect soil density, stress strain behavior

● Numerous case histories available


DISADVANTAGES

● Requires the preparation of bore hole.

D i ff t i l t d t tl t ti f● Dynamic effort is related to mostly static performance

● Standards regarding energy are not uniformg g gy

● If hard stone is encountered, difficult to obtain reliable

resultresult.

● Test procedure is tedious and requires heavy equipment.

● Not possible to obtain properties continuously with

depthdepth.


PROBLEMS● A SPT is conducted in a sand deposit at a depth of 20 m.

Water table is at ground level. If N value is 40, find the corrected Nvalue. Saturated Unit weight of sand is 19 KN/m3. Take γw = 10 kN/m3.

Solution

Correction for overburden pressure

γ’ = γsat - γw = 19 – 10 = 9 kN/m3

q = Eff. Overburden pressure = γ’ D = 9 x 20 = 180 kN/m2

2000⎤⎡= 0.77 x log10 (2000/180) x 40

= 0.805 x 40 = 32.2 ≅ 32 < 2 N

Nq

2000log77.0N 10 ⎥

⎦

⎤⎢⎣

⎡=′

Correction for dilatancy

= 15 + 0.5 x (32 – 15))15N(5015N −′+=′′ ( )

= 23.5 ≅ 23

Corrected N Value = 23

)15N(5.015N +


● A SPT is conducted in a sand deposit at a depth of 20 m.

Water table is at 8m below ground level. If N value is 40, find the corrected NWater table is at 8m below ground level. If N value is 40, find the corrected N

value. Unit weight of sand above water table is 18 KN/m3 and saturated unit

weight of sand below water table is 19 KN/m3. Take γw = 10 kN/m3.

Solution


γ’ = γsat - γw = 19 – 10 = 9 kN/m3

q = Eff. Overburden pressure = γ1 D1 + γ’ D2 = 18 x 8 + 9 x 12 = 252 kN/m2

= 0.77 x log10 (2000/252) x NN2000

log77.0N 10 ⎥⎤

⎢⎡

=′= 0.693 N < 2 N

= 0.693 x 40 = 27.7 ≅ 27

qg10 ⎥

⎦⎢⎣

Correction for dilatancy

= 15 + 0.5 x (27 – 15))15N(5.015N −′+=′′= 21


)(


● A SPT is conducted in a sand deposit at a depth of 20 m.

Water table is at 25m below ground level If N value is 40 find theWater table is at 25m below ground level. If N value is 40, find the

corrected N value. Unit weight of sand above water table is 18 KN/m3

and saturated unit weight of sand below water table is 19 KN/m3 Takeand saturated unit weight of sand below water table is 19 KN/m . Take

γw = 10 kN/m3.

SolutionSolution


Eff O b d D 18 20 360 kN/ 2q = Eff. Overburden pressure = γ1 D1 = 18 x 20 = 360 kN/m2

2000⎤⎡= 0.77 x log10 (2000/360) x N

= 0.573 N < 2 N

Nq

2000log77.0N 10 ⎥

⎦

⎤⎢⎣

⎡=′

= 0.573 x 40 = 22.9 ≅ 22

There is no correction for dilatancy as the water table is below SPT depthThere is no correction for dilatancy as the water table is below SPT depth



CONE PENETRATION TEST (CPT)


STATIC CONE PENETRATION TEST (SCPT)


CONE PENETROMETERS


Advantages and Disadvantages

● Advantages of SCPT are

C ti i t ith d th i d d– Continuous resistance with depth is recorded.

– Static resistance is more appropriate to determine staticpp p

properties of soil.

C b l t d ith t ti f il– Can be correlated with most properties of soil.

● Disadvantages of SCPT areg

– Not very popular in India.

– If a small rock piece is encountered, resistance shown is

erratic & incorrect.

– Involves handling heavy equipment.


LOCATION OF WATER TABLE● Soil strength is usually reduced

below water table.

● Foundations below the watertable will be uplifted by thewater pressure.

● For soils with high permeability,GWT is generally determinedby directly measuring to thestabili ed ater le el in thestabilized water level in thebore hole after 24 to 48 hours.

● For soils with low permeability● For soils with low permeability,install a piezometer with aporous base and a removableporous base and a removabletop cap in the bore hole. GWTis measured periodically until


the water level stabilizes.

BORE LOGBORE LOG

● The data obtained

from bore holes,

site observationssite observations

and laboratory

results and

presented in a

chart form known

as Bore logsas Bore logs


BORE LOGS


SOIL PROFILE


SOIL INVESTIGATION REPORT

● Introduction gives the scope

● Description of the proposed structure location and● Description of the proposed structure, location andgeological conditions of the site

D t il f th fi ld l ti● Details of the field exploration programme

● Details of the methods of explorations

● General description of the sub soil conditions

● Details of the Lab tests

● Depth of ground water table and changes in water levels

● Discussion of the results● Discussion of the results

● Recommendations about type of foundations, All. Soilil it tpressure or pile capacity, etc.

● Conclusions


OTHER IN-SITU TESTS


VANE SHEAR TEST (VST)


FIELD VANE SHEAR TESTS S


FIELD VANE SHEAR TEST

● Best suited for the determination of shear

strength of saturated cohesive soils,strength of saturated cohesive soils,

especially sensitive clays, susceptible for

sampling disturbances.p g

● Torque required to cause the failure is

measured.measured.

∫ τπ×+×τ×π=2/D

rdrr22D

DHT ∫ τπ×+×τ×π=0

rdrr222

DHT

DD 32

τ×π

+τ×π

=6

DH

2

DT

32

⎟⎠⎞

⎜⎝⎛ +π

=τ

6

D

2

HD

T

2


⎠⎝ 62

PROBLEM

● A vane used to test a deposit of soft clay, required a

torque 75 N m The vane dimensions are D = 100 mmtorque 75 N.m. The vane dimensions are D = 100 mm,

and H = 200 mm. Find the value of undrained shear

strength of soil.

Solution

23

mm/N0204601075T ×

τ22

mm/N02046.0

6100

2200

1006D

2H

D=

⎟⎠⎞

⎜⎝⎛ +×π

=⎟⎠⎞

⎜⎝⎛ +π

=τ

= 20.46 kN/m2

⎠⎝⎠⎝


GEOPHYSICAL METHODS● Technique of determining subsurface materials or stratification of

soils and rocks by measuring some physical property of thematerials, viz. magnetism, density, electrical resistivity orcombination of these properties, and through correlations.

M h i l W M t● Mechanical Wave Measurements– Crosshole Tests (CHT)

D h l T t (DHT)– Downhole Tests (DHT)

– Spectral Analysis of Surface Waves

S i i R f i– Seismic Refraction

– Suspension Logging

● Electromagnetic Wave Techniques– Ground Penetrating Radar

– Electromagnetic Conductivity

– Electrical Resistivity


– Magnetometer Surveys

Advantages and Disadvantages

● Advantages

N d t ti– Non destructive

– Fast and economical testingg

– Theoretical basis for interpretation

A li bl il d k– Applicable to soils and rocks

● Disadvantagesg

– No samples or direct physical penetration

– Models assumed for interpretation

– Affected by cemented layers or inclusionsy y

– Results influenced by water, clay and depth


SEISMIC REFRACTION METHOD

● General soil types & depth of different strata are judged.

● Shock or seismic waves are created by detonating small● Shock or seismic waves are created by detonating smallcharges of explosives or by mechanical blows on a plate

th fnear the surface.

● Four basic wave types: Compression (P), Shear (S),Rayleigh (R), and Love (L).

S RTime

Amplitude

P

Time

Source Receiver (Geophone)

Oscilloscope


Receiver (Geophone)

EQUIPMENT

Seismograph Spectrum Analyzer


Portable Analyzer Velocity Recorder


● Seismic waves havedifferent velocities indifferent velocities indifferent types of soil orrockrock.

● The waves are refractedwhen they cross theboundary betweendifferent types of soil.

● Radiating waves aregpicked up and time oftravel from sourcetravel from sourcerecorded by geophonesor seismometers


or seismometers.



DETERMINATION OF SOIL LAYERS


Limitations of Seismic Refraction Test

● Cannot be used where a hard layer overlies a soft layer

C t b d i d b t h lt● Cannot be used in area covered by concrete or asphalt

pavement which represent hard layer

● Frozen surface layer also may give results similar to the

situation of a hard layer over a soft layersituation of a hard layer over a soft layer.

● Discontinuities such as a rock fault or earth cuts, dipping

or irregular under ground rock surface and existence of

thin layers of varying materials may cause mis-

interpretation of test data.


ELECTRICAL RESISTIVITY METHOD

● Based on the measurement and

recording of changes in the meanrecording of changes in the mean

resistivity or apparent specific

resistance of various soils.

● Significant variations in resistivity

can be detected between differentcan be detected between different

types of soils, above and below

water table, between unfissured

● Four metal spikes are driving in ground along a straight line at equal

,

rocks and soils.

distances to serve as electrodes

● Current I from a battery, flows through the soil between the two outery, g

electrodes, producing an electrical field within the soil.

● Potential difference E between the two inner electrodes is then


Potential difference E between the two inner electrodes is then

measured.

ELECTRICAL RESISTIVITY METHOD EQUIPMENTS


ELECTRICAL RESISTIVITY METHOD


ELECTRICAL RESISTIVITY MEASUREMENTS


UNIVERSITY QUESTIONS

● Briefly explain (i) Objectives of soil exploration;

(ii) Reconnaissance survey in soil exploration(ii) Reconnaissance survey in soil exploration.

● Explain in detail Auger and Shell boring methods of site

exploration. What are its advantages and limitations?

● Explain (i) Wash Boring; (ii) Rotary Drilling What are its● Explain (i) Wash Boring; (ii) Rotary Drilling. What are its

advantages and limitations?

● List out the factors, which affect the spacing between the bore holes.

● What is meant by significant depth of exploration? Give guidelines, which enable the determination of the depth of exploration.


UNIVERSITY QUESTIONS● What are the various types of samples?

● Why undisturbed samples are necessary?y p y

● Briefly explain the methods of obtaining undisturbed samples.p

● Bring out the role of preservation and handling of samples in sub-surface exploration.in sub surface exploration.

● What is a bore hole? Distinguish between thin-wall and thick-wall samplersthick-wall samplers.

● Explain the working of a piston sampler with sketches.

D fi I id Cl d O t id Cl● Define Inside Clearance and Outside Clearance as applied to sampler. What are its significance?


UNIVERSITY QUESTIONS

● What are penetration and sounding tests? Under what conditions do you recommend these tests? y

● What is Standard Penetration Test? Explain the test setup and the procedure of conducting the test. What are the p gcorrections to the observed SPT (N) value?

● Discuss the merits and demerits of SPT in the sub-surface investigation.

● Briefly discuss any two methods of determining the ground water table.

● Prepare a typical soil exploration report for the construction of a multi-storied commercial building.

● Explain Seismic Refraction Method of exploration. What are its limitations?

● Explain Electrical Resistivity method of exploration. What are it li it ti ?


its limitations?

PLATE LOAD TEST


GEII ModuleI PartII.ppt - Government Engineering...

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