Direct Shear Test Procedure

Determination of Liquid Limit and Plastic Limit of Soil

SOIL CONSISTENCY

Soil consistency is the strength with which soil materials are held together or the resistance of soils to deformation and rupture. Soil consistency is measured for wet, moist and dry soil samples. For wet soils, it is expressed as both stickiness and plasticity, as defined below. Soil consistency may be estimated in the field using simple tests or may be measured more accurately in the laboratory.
Note: in each case, indications will be obtained concerning the relative value of soil for fish-pond construction, particularly when the wet-soil consistency is determined.

Determination of wet-soil consistency

Testing is done when the soil is saturated with water, as, for example, immediately after a good rainfall. First, determine stickiness, that is, the ability of soil materials to adhere to other objects. Then, determine plasticity, that is, the ability of soil materials to change shape, but not volume, continuously under the influence of a constant pressure and to retain the impressed shape when the pressure is removed.

Field test for stickiness of wet soil

Press a small amount of wet soil between your thumb and forefinger to see if it will stick to your fingers. Then slowly open your fingers. Rate the stickiness as follows:

0 Non-sticky, if no soil or practically no soil sticks to your fingers;		1 Slightly sticky, if the soil begins to stick to your fingers but comes off one or the other cleanly and does not stretch when the fingers are opened;
**2 Sticky, if the soil sticks to both the thumb and forefinger and tends to stretch a little and pull apart rather than pulling free from your fingers;		**3 Very sticky, if the soil sticks firmly to both thumb and forefinger and stretches when the fingers are opened.

Field test for plasticity of wet soil Roll a small amount of wet soil between the palms of your hands until it forms a long, round strip like a wire about 3 mm thick. Rate the plasticity as follows:
0 Non-plastic, if no wire can be formed;
1 Slightly plastic, if a wire can be formed but can easily be broken and returned to its former state;

**2 Plastic, if a wire can be formed but, when it is broken and returned to its former state, it cannot be formed again;
**3 Very plastic, if a wire can be formed which cannot be broken easily and, when it is broken, it can be rolled between your hands and be reformed several times.

Determination of moist-soil consistency Field test for moist-soil consistency Testing is done when the soil is moist but not wet, as, for example, 24 hours after a good rainfall. Try to crush a small amount of moist soil by pressing it between your thumb and forefinger or by squeezing it in the palm of your hand. Rate moist soil consistency as follows:
0 Loose, if the soil is non-coherent (single-grain structure);		1 Very friable, if the soil crushes easily under very gentle pressure but will stick together if pressed again;
2 Friable, if the soil crushes easily under gentle to moderate pressure;		**3 Firm, if the soil crushes under moderate pressure but resistance is noticeable;
**4 Very firm, if the soil crushes under strong pressure, but this is difficult to do between the thumb and forefinger;		5 Extremely firm, if the soil crushes only under very strong pressure, cannot be crushed between the thumb and forefinger, but must be broken apart bit by bit.

Determination of dry-soil consistency Field test for dry-soil consistency Testing is done when the soil has been air-dried. Try to break a small amount of dry soil by pressing it between your thumb and forefinger or by squeezing it in the palm of your hand. Rate dry soil consistency as follows:
0 Loose, if the soil is non-coherent (single-grain structure):		1 Soft, if the soil is very weakly coherent and friable. breaking to powder or individual grains under very slight pressure;
2 Slightly hard, if the soil resists light pressure, but can be broken easily between thumb and forefinger;		3 Hard, if the soil resists moderate pressure, can barely be broken between the thumb and forefinger, but can be broken in the hands without difficulty;
4 Very hard, if the soil resists great pressure, cannot be broken between the thumb and forefinger but can be broken in the hands with difficulty;		5 Extremely hard, if the soil resists extreme pressure and cannot be broken in the hands.

Source: ftp://ftp.fao.org/fi/cdrom/fao_training/FAO_Training/General/x6706e/x6706e08.htm

Shear strength of soil

 Very good read to understand Shear strength of soil

https://drive.google.com/open?id=0B2Zmh2o7sC1VbWFFZ09rZV8tRVk

https://drive.google.com/open?id=0B2Zmh2o7sC1VZlUtNWJIRlFTeUU

Typical values of shear strength parameters

Typical relationships for estimating the angle of internal friction, f, are as follows:

Empirical values for f, of granular soils based on the standard penetration number, (from Bowels, Foundation Analysis).

SPT Penetration, N-Value (blows/ foot)	f (degrees)
0	25 - 30
4	27 - 32
10	30 - 35
30	35 - 40
50	38 - 43

Relationship between f, and standard penetration number for sands, (from Peck 1974, Foundation Engineering Handbook).

SPT Penetration, N-Value (blows/ foot)	Density of Sand	f (degrees)
<4	Very loose	<29
4 - 10	Loose	29 - 30
10 - 30	Medium	30 - 36
30 - 50	Dense	36 - 41
>50	Very dense	>41

Relationship between f, and standard penetration number for sands, (from Meyerhof 1956, Foundation Engineering Handbook).

SPT Penetration, N-Value (blows/ foot)	Density of Sand	f (degrees)
<4	Very loose	<30
4 - 10	Loose	30 - 35
10 - 30	Medium	35 - 40
30 - 50	Dense	40 - 45
>50	Very dense	>45

Correlation between SPT-N value, friction angle, and relative density

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Correlation between SPT-N value and friction angle and Relative density (Meyerhoff 1956)
SPT N3 [Blows/0.3 m - 1 ft]	Soi packing	Relative Density [%]	Friction angle [°]
< 4	Very loose	< 20	< 30
4 -10	Loose	20 - 40	30 - 35
10 - 30	Compact	40 - 60	35 - 40
30 - 50	Dense	60 - 80	40 - 45
> 50	Very Dense	> 80	> 45

Typical values of shear strength

Undrained shear strength	su (kPa)
Hard soil	su > 150 kPa
Stiff soil	su = 75 ~ 150 kPa
Firm soil	su = 40 ~ 75 kPa
Soft soil	su = 20 ~ 40kPa
Very soft soil	su < 20 kPa
Drained shear strength	c´ (kPa)	f´ (deg)
Compact sands	0	35° - 45°
Loose sands	0	30° - 35°
Unweathered overconsolidated clay
critical state	0	18° ~ 25°
peak state	10 ~ 25 kPa	20° ~ 28°
residual	0 ~ 5 kPa	8° ~ 15°

Direct shear test

A direct shear test is a laboratory test to measure the shear strength properties of soil. The value internal friction angle and cohesion of the soil are required for design of many engineering problems such as foundations, retaining walls, bridges, sheet piling. Direct shear test can predict these parameters quickly.

The test is performed on three or four specimens from a relatively undisturbed soil sample.
A specimen is placed in a shear box which has two stacked rings to hold the sample; the contact between the two rings is at approximately the mid-height of the sample.

A confining stress is applied vertically to the specimen, and the upper ring is pulled laterally through a specified strain until the sample fails.

The load applied and the strain induced is recorded at frequent intervals to determine a stress–strain curve for each confining stress. As a vertical normal load is applied to the sample, shear stress is gradually applied horizontally, by causing the two halves of the box to move relative to each other. The shear load is measured together with the corresponding shear displacement.

Several specimens are tested at varying confining stresses to determine the shear strength parameters, the soil cohesion (c) and the angle of internal friction, commonly known as friction angle.

The results of the tests on each specimen are plotted on a graph on the y-axis and the confining stress on the x-axis. The y-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle.

Direct shear tests can be performed under several conditions. The sample is normally saturated before the test is run, but can be run at the in-situ moisture content.

The rate of strain can be varied to undrained or drained conditions, depending whether the strain is applied slowly enough for water in the sample to prevent pore-water pressure.

The test has several advantages:
•  It is easy to test sands and gravels.
•  Large samples can be tested in large shear boxes, as small samples can give misleading results due to imperfections such as fractures and fissures, or may not be truly representative.
•  Samples can be sheared along predetermined planes.

The disadvantages of the test include:
•  The failure plane is always horizontal in the test, and this may not be the weakest plane in the sample. Failure of the soil occurs progressively from the edges towards the centre of the sample.
•  There is no provision for measuring pore water pressure in the shear box and so it is not possible to determine effective stresses from undrained tests.
•  The shear box apparatus cannot give reliable undrained strengths because it is impossible to prevent localised drainage away from the shear plane.