Geotech Vane Shear Test

LABORATORY 3

Table of Contents

Introduction.................................................................................................2

Purpose and Objectives...............................................................................2

Theory.........................................................................................................2

Equipment and Apparatuses.......................................................................4

Method and Procedure................................................................................6

Numerical example.....................................................................................6

Lab Data Observation..................................................................................7

Calculation...................................................................................................9

Spring No.2...............................................................................................9

Spring No.3.............................................................................................11

Discussion.................................................................................................14

Question and Answer:............................................................................14

Advantages and Disadvantages:............................................................15

Conclusion.................................................................................................15

References................................................................................................16

Appendix...................................................................................................16

Appendix A:............................................................................................16

SAYED ASADULLAHUNISEL, FACULTY OF ENGINEERING, CIVIL DIVISION

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Introduction

Vane shear test is used to measure the shear strength of a soil. It also estimated and measured the fully saturated clay’s undrained shear strength without derangement in the specimen. This test can be conducted in field and laboratory however, in laboratory can only execute the experiment with low shear strength (0.3 kg/cm2) for which unconfined test cannot be performed. The test apparatus are composed of 3 different diameters of 4-blade stainless vane that is attached in a steel rod that pushed vertically in the soil. The pocket value that can get in small vane should multiply by two however, the value can get in large vane should divide by two and the value that can get in medium vane is as it is. The test is performed by pushing the vane vertically in the soil and rotated it clockwise from the surface to determine the torsional force. The soil will resist the rotation of the vane and its resistance is the force of soil that causes the cylindrical area to be sheared by the vane. When the rotation of the vane is continues it means that the soil fails in shear and it is normal that the rotation is continued after measuring the shear strength.

Purpose and Objectives

The vane test provides a measure of the stress-strain behavior, the undrained shear strength, and the remolded strength of soft saturated cohesive soils.

Theory

Fairly reliable results for the in situ undrained shear strength, cu(ɸ=0 concept) ofsoft plastic cohesive soils may be obtained directly from vane shear tests during the drilling operation (ASTM Test Designatin 2573). The shear vane usually consists of four thin, equal –sized steel plates welded to a steel torque rod. First, the vane is pushed into the soil. Then torque is applied at the top of the torque rod to rotate the vane at a uniform speed. A cylinder of soil of height h and diameter d will resist the torque until the soil fails. The undrained shear strength of the soil can be calculated as follows; if T is the maximum torque applied at the head of the torque rod to cause failure, it should be equal to the sum of the resisting moment of the shear force along the side surface of the soil cylinder (Ms) and the resisting moment of the shear force at each end (Me).

T=M s+M e+M e


Two EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo EndsTwo Ends

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The resisting moment Ms can be given as

M s=(πdh)cu(d2)

The standard rate of torque application is 0.1o/sec . the maximum torque T is applied to cause failure can be given as;

T=f (cu , h ,∧d )

Or

cu=TK

According to ASTM (2010), for rectangular vanes,

K= π d2

2 (h+ d3 )If h/d=2,

K=7 π d3

6

Thus

K= 6T

7 π d3

For tapered vanes,

K= π d2

12 ( dcos iT

+ dcos iB

+6h)

Field vane shear tests are moderately rapid and economical and are used extensively in field soil-exploration programs. The test gives good results in soft and medium stiff clays, and it is also an excellent test to determine the properties of sensitive clays.


Moment ArmSurface Area

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Equipment and Apparatuses

1. Laboratory Vane Apparatus [Figure 1]2. Calibrated springs supplied with the vane apparatus [Figure 2]3. Standard vane, 12.7mm [Figure3]4. Attachment for holding soil sample tubes or glass sampling jars.


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Figure 1; Laboratory Vane apparatus

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1. Hand Knob2. Vertical screw control3. Knurled Knob4. Electrical motor5. Pointer6. Carrier7. Vane deflection scale8. Spring deflection scale9. Vertical shaft10. Rotating socket11. Vane12. Cylinder 13. Both for tighten the cylinder to plate.14. Plate15. Calibrated springs.


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Figure 2 ; Calibrated Springs

Figure 3 ; Vane

11Ch

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alcu

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LABORATORY 3

Method and Procedure

The vane apparatus is assembled by mounting the vane and spring appropriate for the soil to be tested. Instruction provided with the vane apparatus are to be followed for adjustment of the pointer used in reading the spring and vane deflection.

The soft clay to be tested may include tube samples – 38mm (1 ½ in) or 115mm (4 ¼in) – or soil in glass sampling jars or Proctor molds. The clamping attachment will hold the soil container vertically below the vane shaft.

The soil surface should be trimmed so as to permit the vane to be lowered into the soil to a depth sufficient to ensure that shearing will take place on the horizontal edges of the vane without movement of the soil sample surface.

With the vane in position, apply torque to the vane at a rate that should not exceed 0.1 deg/s. This rate will normally give a time to failure of from 2 to 5 min. In very soft clays the time to failure may be longer. Record the maximum torque with motorized apparatus.

Record values of spring and vane deflection at intervals of 15 s or less as needed to prepare torque or strength curves. Following determination of the vane shear strength, remold the soil by rotating the vane rapidly through a minimum of 10 revolutions.

Immediately repeat the vane test to determine the remolded vane shear strength. After the test select a representative portion of the sample for a water content determination.

Numerical example

Data from the vane test are recorded in the term of a spring deflection and a vane deflection on the data and calutation sheet. The torque is obtained by noting the angular spring deflection and reading the relecant spring calibration chart. Alternatively the torque maybe computed using the relevant spring constant. The vane shear strength is now computed suing the vane constant T as defined at the bottom of the data sheet. The data maybe summaried as illustrated in figure. Water content data included for use in making, comparisons with other vane test data.


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Lab Data Observation

Vane Diameter (D) =12.7mm

Vane Length (L) =12.7mm

Vane Height (H) =12.7mm


Observed Data from Spring No.2

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Observed Data from Spring No.3

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Calculation

Vane Constant

K=π D2L2 (1− D

3 L )=π (12.7 x10−3)2(12.7 x10−3)2 (1− (12.7 x10−3)

3(12.7 x10−3))=2.15×10−6m3

Spring No.2From the plotted Graph

Gradient ¿( y¿¿2− y1)

(x2−x1 )=

(0.270−0.200 )(90−67 )

=3x 10−3 ¿

Spring Data Point 1:

Spring No.2 point 1Time Spring(o) Torque (Nm) Shear strength

(KN/ m2)30 5 0.015 6.9860 10 0.030 13.9590 15 0.045 20.93

120 20 0.060 27.91150 25 0.075 34.88180 30 0.090 41.86210 35 0.105 48.84240 40 0.120 55.81270 45 0.135 62.79300 50 0.150 69.77330 55 0.165 76.74360 55 0.165 76.74

Maximum Degree of spring deflection =55

Maximum Torque, T = Maximum spring x Gradient for spring no 2

Tmax = 55 × 0.003 = 0.165Nm

Su=TK

= 0.165

2.15×10−6=76.74 kN /m2


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Spring No.2 point 2Time Spring(o) Torque (Nm) Shear strength v

(KN/ m2)30 4 0.012 5.5860 8 0.024 11.1690 17 0.051 23.72

120 22 0.066 30.70150 26 0.078 36.28180 31 0.093 43.26210 34 0.102 47.44240 39 0.117 54.42270 46 0.138 64.19300 52 0.156 72.56330 57 0.171 79.53360 63 0.189 87.91



Tmax = 63× 0.003 = 0.189Nm

Su=TK

= 0.189

2.15×10−6=87.91kN /m2

Spring No.3

From the plotted Graph

Gradient ¿( y¿¿2− y1)

(x2−x1)=

(0.280−0.200 )(140−100)

=2x 10−3 ¿

Spring data, point one:

Spring 3 point 1

Time Spring(o) Torque (Nm) Shear strength(KN/ m2)

30 6 0.012 5.5860 11 0.022 10.23


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90 13 0.026 12.09120 18 0.036 16.74150 25 0.050 23.26180 27 0.054 25.12210 30 0.060 27.90240 34 0.068 31.63270 41 0.082 38.14300 46 0.092 42.79330 53 0.106 49.30360 58 0.116 53.95390 58 0.116 53.95420 58 0.116 53.95



Tmax = 58× 0.002 = 0.116 Nm

Su=TK

= 0.122

2.15×10−6=53.95 kN/m2


Spring 3 point 2Time Spring () Torque (Nm) Shear strength

(KN/ m2)30 4 0.008 3.7260 8 0.016 7.4490 15 0.030 13.95

120 21 0.042 19.53150 27 0.054 25.12180 33 0.066 30.70210 38 0.076 35.35240 45 0.090 41.86270 49 0.098 45.58300 56 0.112 52.09330 60 0.120 55.81360 62 0.124 57.67390 68 0.136 63.26420 68 0.136 63.26



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Tmax = 68× 0.002 = 0.136 Nm

Su=TK

= 0.136

2.15×10−6=63.26 kN/m2


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Discussion

Question and Answer:

I. For the Tapered Vane shown in [Figure 6] develop an expression for the constant K needed in computation of Su=T/K, where T is the torque required to rotate the vane.

A=π r2

Answer:

K= π d2

12 ( dcos iT

+ dcos iB

+6h)

II. For the same vane, develop an expression for evaluation of the vertical (Suv) and horizontal (SUH) undrained shear strengths.

Answer:

Su=2T

xπ d3(HD + 1(n+3)

×SuhSuv )

Where:

T is the Maximum Torque measured


iB

iT

Figure 4 ; Geometry of field vane “Tapered Vanes”

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H/D is the aspect ratio of the vane

D is the Diameter of the vane

Suh/Suv is the ratio of the undrained strength in both vertical and horizontal planes.

X is the factor describing the location of the failure surface with respect to diameter of the vane .

n is the power law describing the shear stress distribution on the horizontal planes.

Advantages and Disadvantages:Advantages:

The test is simple and quick.

It is ideally suited for the determination of the undrained shear strength of non-fissured fully saturated clay.

The test can be conveniently used to determine the sensitivity of the soil.

The test can be conducted in soft clays situated at a great depth, samples of which are difficult to obtain.

Disadvantages:

The test cannot be conducted on the clay containing sand or silt laminations or the fissured clay.

The test does not give accurate results when the failure envelope is not horizontal.

Conclusion

Vane shear test is used to measure the shear strength of a soil. It also estimated and measured the fully saturated clay’s undrained shear strength without derangement in the specimen. This test can be conducted in field and laboratory however, in laboratory can only execute the experiment with low shear strength (0.3 kg/cm2) for which unconfined test cannot be performed. The test apparatus are composed of 3 different diameters of 4-blade stainless vane that is attached in a steel rod that pushed vertically in the soil. The pocket value that can get in small vane should multiply by two however, the value can get in large vane should divide by two and the value that can get in medium vane is as it is. The test is performed by pushing the vane vertically in the soil and rotated it clockwise from the surface to determine the torsional force. The soil will resist the rotation of the vane and its resistance is the force of soil that causes the cylindrical area to be sheared by the vane.


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LABORATORY 3

When the rotation of the vane is continues it means that the soil fails in shear and it is normal that the rotation is continued after measuring the shear strength.

References

1. Mr. Khatta Marwah, Laboratory Sheet, 2014, UNISEL, Civil Engineering Department.2. Braja M Das, Fundamentals of Geotechnical Engineering.3. ASTM Standards, 2002, copyright ASTM International, 100 Barr Hrbor Drive.4. Roy Whitlow, Basic Soil Mechanics.

Appendix

Appendix A:


Figure 5; Vane Apparatus Figure 6; Top View of the Vane & Spring Deflection Scale

Figure 7 ; Calibrated Spring Supplied

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LABORATORY 3


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Geotech Vane Shear Test

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