NAMIBIA UNIVERSITY OF SCIENCE AND TECHNOLOGY

SCHOOL OF ENGINEERING

BACHELOR OF ENGINEERING: CIVIL ENGINEERING

COURSE: GEOTECHNICAL ENGINEERING CODE: GET710S

LAB REPORT: SHEAR BOX EXPERIMENT SUBMISSION DATE: 27/04/2016

STUDENT NAME: NATANGWE HITIWA 212018280

LECTURER: MR. DAVID P. KATALE

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Table of Content

sINTRODUCTION...........................................................................................................................................1

OBJECTIVES..................................................................................................................................................1

EQUIPMENT.................................................................................................................................................1

PROCEDURES...............................................................................................................................................1

RESULTS.......................................................................................................................................................2

Specimen data.........................................................................................................................................2

Shearing Data..........................................................................................................................................3

DISCUSSION.................................................................................................................................................8

Shear Stress versus Shear Displacement graphs......................................................................................8

Vertical Displacement versus Horizontal Displacement..........................................................................9

Conclusion...................................................................................................................................................9

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INTRODUCTIONMost of the civil works take require soil or ground as a supporting structure to the structure being constructed. This support originates from the friction between the soil particles on the soil. Inadequate friction or shear of the soil grains will result in the failure of the soil therefore failing the structure itself. Shear failure is therefore a major problem in the whole spectrum of civil engineering thus for it to be alleviated or avoided at all a thorough study on the type of shear that could affect the structure has to be done prior to the construction commences. This is usually done by contacting an investigation of the material at the site to be constructed or those to be used in the construction process. A good quality material to better support a building has to have a good cohesion among its particles therefore having a higher tolerance to shearing. Laboratory test for shear on the soil is done using the shear box apparatus that gives data that would be analyzed to give the cohesion of the soil and the failure stress.

OBJECTIVES➢ Determining the shear strength properties of a drained soil sample.

EQUIPMENT➢ Shearing box machine➢ Loads of 2kg, 4kg and 8kg➢ Balance scale➢ Soil sample➢ Vernier Calipers

PROCEDURES➢ The mass of the soil sample was obtained by weighing and recording the mass of an empty

container and then adding in the soil.➢ The dimension of the shear box, shear and porous plates were measured using the Vernier

calipers and recorded.➢ After the base plate was placed in the box, the porous plate, then the shear base plates were

placed into the shear box and the measured soil sample poured in until above the base of the top of the shearing section of the shear box while the box was held in place by screws.

➢ The top of the soil was levelled and the shearing plate and the porous plate were added at the top of the soil.

➢ The cover was added and the complete setup of the shear box was placed in the shearing machine.

➢ The remaining soil in the pan was measured and recorded.➢ The shear box was positioned and fastened on to the machine and the pulley system

straightened and aligned.➢ The horizontal and the shear displacement gauges were put to zero while the vertical

displacement gauge set to 5.2➢ The alignment screws on the shear box were removed before the load was released and the

vertical displacement / consolidation started.

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➢ The mass of 2 kg was gently released on the hanger of the pulley system and a stopwatch started simultaneously.

➢ Vertical displacements were recorded at different time intervals during the process. ➢ After the vertical displacement ceased to change, the machine was then started up to shear the

soil after the initial consolidation. ➢ The vertical, horizontal and shear force readings were recorded at different time intervals during

the test.➢ When there was no more change in the horizontal displacement, the machine was reversed and

the process repeated for the 4 and 8 kg masses.

RESULTSSpecimen dataMass of soil

Specimen Number 1 2 3Initial mass disturbed soil g 717.5 780.0 757.0Mass of soil remaining g 497.5 453.5 490.5Mass of specimen g 220.0 326.5 266.5

Measured dimensions of the shear box

Specimen Number Length1 (mm) Length2 (mm) Depth of specimen (mm) Area (mm2)1 100 100 16 10 000

Specimen Number 1 2 3Top of box to top of baseplate (h1) mm 45 45 45Top of box to top of porous plate (h2) mm 11 10 7Combined thickness of plates (tp) mm 18 18 18Specimen thickness H0 = h1 – (h2 + tp) mm 16 17 20

Volumeof the soil∈the=l x b x w

¿16 x100 x100

¿160000mm3

Therefore Density= massvolume

¿ 220 g160000mm3

2

¿1.375 x1 0−3g /mm3

Shearing Data2 kg

Time (min)

Elapsed Time (min)

Force Reading

Shear Stress kPa

Horizontal Displacement Vertical Displacement

Measured mm

Cumulative mm

Reading mm Cumulative mm

0 0 0 0 0 0 9.322 00.5 15 0.0595 5.95 0.15 0.15 9.322 01 16 0.0635 6.35 0.5 0.65 9.3 0.0221.5 21 0.0833 8.33 0.74 1.39 9.276 0.0462 23 0.0912 9.12 1 2.39 9.228 0.0945 39.55 0.1567 15.67 2.8 5.19 8.814 0.5086 42.06 0.1666 16.66 3.4 8.59 8.81 0.5127 43.5 0.1726 17.26 4 12.59 8.808 0.51410 46 0.1825 18.25 5.8 18.39 8.816 0.50611 46.2 0.1833 18.33 6.4 24.79 8.816 0.50612 45.8 0.1817 18.17 6.95 31.74 8.816 0.50615 45 0.1785 17.85 8.78 40.52 8.79 0.53220 44.9 0.1781 17.81 11.7 52.22 8.738 0.584

0 2 4 6 8 10 12 140

2

4

6

8

10

12

14

16

18

20

Stress VS Horizontal Displacement

Horizontal Displacement (mm)

Stre

ss (k

Pa)

3

0 2 4 6 8 10 12 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Verstical Displacement VS Horizontal Displacement

Horizontal Displacement (mm)

Verti

cal D

ispal

cem

ent (

mm

)

Shearing results for the 4kg weight

Time (min)

Elapsed Time (min)

Force Reading

Shear Stress kPa

Horizontal Displacement Vertical DisplacementMeasured mm

Cumulative mm

Reading Cumulative mm

0 0 0 0 0 0 6.712 0.0000.5 12 0.0476 4.76 0.2 0.2 6.718 -0.0061 26 0.1031 10.31 0.5 0.7 6.65 0.0621.5 34 0.1349 13.49 0.85 1.55 6.59 0.1222 40 0.1587 15.87 1.1 2.65 6.336 0.3765 65 0.2579 25.79 2.9 5.55 5.996 0.7166 68.5 0.2717 27.17 3.48 9.03 5.984 0.7287 77 0.3055 30.55 4.1 13.13 5.962 0.7510 77 0.3055 30.55 5.9 19.03 5.948 0.76411 78 0.3094 30.94 6.45 25.48 5.936 0.77612 78.5 0.3114 31.14 7.1 32.58 5.93 0.78215 78 0.3094 30.94 8.9 41.48 5.9 0.81220 74 0.2936 29.36 11.9 53.38 5.842 0.870

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0 2 4 6 8 10 12 140

5

10

15

20

25

30

35

Shear Stress vs Horizontal Displacement for 4 kg

Horizontal displacement mm

Shea

r Str

ess k

Pa

0 2 4 6 8 10 12 14

-0.2

0

0.2

0.4

0.6

0.8

1

Vertical Displacement VS Horizontal Displacement

Horizontal Displacement (mm)

Verti

cal D

ispla

cem

ent (

mm

)

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For the 8 kg mass

Time (min)

Elapsed Time (min)

Force Reading

Shear Stress kPa

Horizontal Displacement Vertical Displacement

Measured mm Cumulative mm Reading mm

Cumulative mm

0 0 0 0.00 0 0 8.660 0

0.5 35 0.1388 13.88 0.25 0.2 8.640 0.02

1 53 0.2103 21.03 0.3 0.5 8.570 0.09

1.5 67 0.2658 26.58 0.85 1.35 8.330 0.33

2 79 0.3134 31.34 1.15 2.5 8.300 0.36

5 120 0.4760 47.60 2.95 5.45 8.020 0.64

6 130 0.5157 51.57 3.5 8.95 8.020 0.64

7 134 0.5316 53.16 4.1 13.05 8.028 0.632

10 139.5 0.5534 55.34 5.9 18.95 8.022 0.638

11 140 0.5554 55.54 6.52 25.47 8.010 0.65

12 141 0.5593 55.93 7.15 32.62 7.996 0.664

15 149 0.5911 59.11 8.9 41.52 7.950 0.71

20 154 0.6109 61.09 11.9 53.42 7.666 0.994

25 149 0.5911 59.11 13.6 67.02 7.610 1.05

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0 2 4 6 8 10 12 14 160.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

Shear stress kPa vs Horizontal displacement mm for the 8 kg mass

Horizontal Displacement mm

Shea

r Str

ess k

Pa

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

Vertical Displacement VS Horizontal Displacement

Horizontal Displacement (mm)

Verti

cal D

ispla

cem

ent (

mm

)

The normal stress exerted on a sample can be calculated using the Coulomb Envelope equation:

Normal stress= Normal forceCross sectional areaof the specimen

∗Lever factor

Using a lever factor of 10, the normal stress from the 2 kg mass: 2 X9.81

0.01X 10=19.67 kN /m2

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For the 4Kg mass: 4 X9.81

0.01X 10=39.24 kN /m2

For the 8Kg mass: 8 X 9.81

0.01X 10=78.48 kN /m2

Mass (kg) Maximum Shear Stress (kPa) Normal Stress (kPa)2 18.33 19.674 31.14 39.248 61.09 78.48

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

f(x) = 0.766120052419307 x + 1.32569149952787

Shear Stress at failure VS Normal stress

Normal Stress (kPa)

Shea

r Str

ess a

t fai

lure

(kPa

)

∅

Therefore, from the graph, friction angle for the failure envelope: ∅=tan−1(0.7661)

∅=37.5 ⁰

DISCUSSIONShear Stress versus Shear Displacement graphsFrom the shear (horizontal) displacement versus shear stress graphs, it is evident that the soil sample is of the dense sand type. This can be seen from the pattern of the graphs were by the shear stress increases to a maximum value and then falls again as the shear displacement increases. This condition

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will be expected in dense sandy soils as the sand grains would slide over each other therefore resulting in the type of graph.

Vertical Displacement versus Horizontal DisplacementThe change in the height of the sample to the change in the horizontal displacement implies a soil sample of a dense sandy type too. This can be seen from the initial dip in the graphs at the beginning of the graph and the later increase in the graph. This implies that there was an initial compression during the experiment by which the soil grains fell in the spacing in-between the grains therefore decreasing the volume.

The internal friction angle of 37.5⁰ also falls within the range for dense sandy soils of 35⁰ – 38⁰.

ConclusionAlthough the direct shear test is a simple test to perform, one of its short comings is that the soil sample is sheared along a predetermined surface and not in the natural weakest surface the soil would have failed normally. This distorts the results of the experiment into presumed results. Results could also have been affected by the fact that the shear force distribution has not been uniformly distributed all over the soil sample as the soil is being sheared.

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