1. lectures Notes on: Soil Mechanics

2. 3- Shear Strength 3-1 Types of Triaxial Test:
The triaxial test will be classified according to the conditions of drainage obtaining during each stage.
Unconsolidated – Undrained Test:
In this test to drainage and no dissipation of pore pressure is permitted during the application of the all-round stress which indicates that the sample will not have any volume change due to dissipation of water and no drainage is allowed during the application of the deviator stress.
consolidated – Undrained Test:
In this test drainage will be permitted during the application of all-round pressure (σ3) and the sample will considered under this pressure and no drainage is allowed during the application of deviator stress and this test can be carried out on undisturbed or compacted samples of earth-fill, in particular when the degree of saturation is not enough to result in a sufficient range of strengths in the undrained test to define a satisfactory failure envelope. It may also be used to examine the effect on “C” and “Φ” of flooding foundation strata and earth-fill materials, and indicates the magnitude of the accompanying volume change.

3. 3-1-3 Consolidated Drained Test:
In this test drainage is permitted throughout the test so that full consolidation occurs under the all-round stress and no excess pore pressure is set up during the application of the deviator stress. When the consolidation takes place the sample is then sheared by increasing the axial load at a sufficiently slow rate to prevent any build-up of excess pore pressure. The minor principal stress “σ3” at failure is this equal to “P”, the consolidation pressure, the major principal stress “” in the axial stress, since the pore pressure is zero, the effective stresses are equal to the applied stresses, and the strength envelope in terms of effective stress is obtained directly from the stress circles.
The values of “C” and “Φ” are denoted by (Cd), (Φd). The drained test also provides information on the volume changes which accompany the application of the all-round pressure and the deviator stress, and on the stress-strain characteristics of the soil.

4. Total and effective stress failure envelope
The triaxial test gives the strength in terms of the principal stresses, whereas the shear box gives the stresses on the failure plane directly. To relate the strengths from the two tests we need to use some results from the Mohr circle transformation of stress.
Total and effective stress failure envelope

5. Shear failure pure shear failure
Unconfined Compression Test on Saturated Clay
Typical value of the cohesion according to soil consistency
Unconfined Compression Test
a-pure shear b- buckling c- partial shear failuer
Shear failure pure shear failure

6. Significance of unconfined compression test
• A quick test to obtain the shear strength parameters of cohesive (fine grained) soils either in undisturbed or remolded state .
• The test is not applicable to cohesion less or coarse grained soils
• The test is strain controlled and when the soil sample is loaded rapidly, the pore pressures (water within the soil) undergo changes that do not have enough time to dissipate Hence the test is representative of soils in construction sites where the rate of construction is very fast and the pore waters do not have enough time to dissipate

7. Vane Shear Test :
This test used to find shear strength of a given soil specimen.
The structural strength of soil is basically a problem of shear strength. Vane shear test is a useful method of measuring the shear strength of clay. It is a cheaper and quicker method. The test can also be conducted in the laboratory. The laboratory vane shear test for the measurement of shear strength of cohesive soils, is useful for soils of low shear strength (less than 0.3 kg/cm2) for which triaxial or unconfined tests can not be performed. The test gives the undrained strength of the soil. The undisturbed and remoulded strength obtained are useful for evaluating the sensitivity of soil.

8. EXPERIMENTAL PROCEDURE
1.Prepare two or three specimens of the soil sample of dimensions of at least 37.5 mm diameter and 75 mm length in specimen.(L/D ratio 2 or 3).
2.Mount the specimen container with the specimen on the base of the vane shear apparatus. If the specimen container is closed at one end, it should be provided with a hole of about 1 mm diameter at the bottom.
3.Gently lower the shear vanes into the specimen to their full length without disturbing the soil specimen. The top of the vanes should be at least 10 mm below the top of the specimen. Note the readings of the angle of twist. see figure
4.Rotate the vanes at an uniform rate say 0.1o/s by suitable operating the torque application handle until the specimen fails.
5. Note the final reading of the angle of twist.
6. Find the value of blade height in cm.
7. Find the value of blade width
The shear strength or cohesion can be determined by the following equation :

9. The shear strength or cohesion can be determined by the following equation
Diagram of' vane shear

10. 3-2 Soil Penetrometer, Pocket Style
The Pocket Penetrometer is designed as a lightweight instrument for use by field personnel to check visual classification of soils. It can be used to verify whether excavation side walls require shoring, based on cohesive soils classifications.
It indicates consistency, shear strength, and approximate unconfined shear strength. Direct-reading scale—in tons/sq ft, or kg/sq cm—corresponds to equivalent unconfined compressive strength. The readings obtained from the Penetrometer do not replace laboratory test results due to the fact that a small area of penetration test could give misleading results. The instrument should not be used for obtaining foundation design data.
Select test spots with a smooth surface. Hold the pocket penetrometer at right angles to the surface being tested. To minimize errors, take several readings near each other, discard those readings that may vary significantly from the majority and average the readings.
Pocket pentrometer