In situ methods vane shear test

1. IN- SITUE METHODS :
VANE SHEAR TEST AND PRESSURE
METERS
Submitted by
Balakrishnan v s
Mtech ocean technology
cusat

2. VANE SHEAR TEST
• Laboratory test
• In-situ test

3. Introduction
• In contrast with other methods Vane Shear Test VST, attempts to measure
undrained shear strength directly.
• Can be conducted in the laboratory as well as in the field
• A selection of vanes available in terms of size, shape, and configuration,
depending upon the consistency and strength characteristics of the soil.

7. AIM
• To determine the undrained shear strength of the clay
Theory
• It is cheaper and quick methods of measuring the shear strength of soft
clay
• Can be conducted in the laboratory as well as in the field
The principle is that an applied torque is resisted by a uniformly distributed
shear stress per unit area acting around the curved surface of a cylinder of
soil.

8. SIGINIFICANCE
• It is suitable for the measurement of shear strength of cohesive soils ,
when soil is of low shear strength(less than 0.5kg/cm^2) for which
triaxial or unconfined tests cannot be performed
• The test gives undrained strength of the soil
• The results obtained from undisturbed and remoulded soils are useful
for evaluating the sensitivity of soil.

9. APPARATUS
1)Vane shear apparatus
• Consist of a steel rod placed vertically and four stainless steel blades placed orthogonally,
which are called vanes
• Arrangement for rotating the vane steadily at a rate of
approx 1/60 rev/min(0.1 degree /s) is provided(length of rod =60mm,d=2.5mm)
• A torque measuring arrangement such as calibrated torsion spring is also attached
2) Specimen container
• 3 nos having dimensions of at least 37.5mm dia and 75mm length(L/D ratio 2 or 3)

11. Procedure
• Clean specimen and fill it with remoulded soil
• Mount the specimen container with vane shear apparatus
• Gently lower the vanes into the specimen ,note the reading of the angle of
twist
• Rotate the vanes at a uniform rate
• Note the final angle of twist repeat the procedure
The difference btw initial and final readings will gives the angle of torque

13. Calculations

14. IN-SITU METHOD
• The vane shear test (VST), or field vane (FV), is used to evaluate the inplace
undrained shear strength (suv) of soft to stiff clays & silts at regular depth intervals
of 1meter (3.28 feet).
• The test consists of inserting a four-bladed vane into the clay and rotating the
device about a vertical axis,
• The standard vane has a rectangular geometry with a blade diameter D = 65 mm,
height H = 130 mm (H/D =2), and blade thickness e = 2 mm.

15. Procedure
• The test is best performed when the vane is pushed beneath the bottom
of an pre-drilled borehole.
• For aborehole of diameter B, the top of the vane should pushed to a
depth of insertion of at least df= 4B.
• Within 5 minutes after insertion, rotation should be made at a constant
rate of 6°/minute (0.1°/s) with measurements of torque taken frequently.

19. • The conventional interpretation for obtaining the undrained shear strength from the
recorded maximum torque (T) assumes a uniform distribution of shear stresses both
top and bottom along the blades and a vane with height-to-width ratio H/D = 2
Undrained Strength and
Sensitivity
• The test is normally reserved for soft to stiff materials with suv < 200 kPa. (2
tsf:

20. • After the peak s,, is obtained, the vane is rotated quickly through
10 complete revolutions and the remolded (or "residual") value
is recorded. The in-situ sensitivity of the soil is defined by
• St =Su(Peak)/su(Remould)

22. ADVANTAGES of VST
• Assessment of undrained strength (Suv)
• Simple test and equipment
• Measure in-situ clay sensitivity (St)
• Long history of use in practice

23. DISADVANTAGES of VST
• Limited application to soft to stiff clays
• Slow and time-consuming
• Raw (Suv) needs (empirical ) correction
• Can be affected by sand lenses and seams

24. PRESSURE METERS

25. • Pressuremeter tests can be carried out both in soils and in rocks.
• The aim of a Pressuremeter test is to obtain information on the
stiffness, and in weaker materials on the strength of the ground, by
measuring the relationship between radial applied pressure and the
resulting deformation

28. The Borehole Pressuremeter:
The instrument is inserted into a performed hole.
The pressuremeter has a slightly smaller outside diameter than the diameter of the hole, and
can therefore be lowered to the test position before being inflated.
The radial expansion of the probe when pressurized is inferred from measurements of
volume made at the ground surface using the control/measuring unit.

30. Test methods for Bore-hole pressuremeters:
• Borehole pressure meter consists of two main elements;
a radially-expanding cylindrical probe which is suspended inside the borehole at the
required test level, and a monitoring unit (known as pressure-volumeter) which is
deployed at ground level.
the probe consists of three cells. The outer two cells are known as ̳guard cells‘ and
are normally filled with pressurized gas.
• The central measuring cell is filled with water and is connected to a sight tube
which records volume change in the pressure-volumeter.
• Pressure is provided by means of a CO2bottle.
• Resulting changes in measuring-cell volume are recorded at 15 s, 30s, 60s and
120 s after each pressure increment is applied.

31. The self-boring Pressuremeter:
The instrument is self-bored into the ground with the purpose of
minimizing the soil disturbance caused by insertion.
A self-boring pressuremeter incorporates an internal cutting mechanism at
its base
Factors affecting the amount of disturbance caused by insertion are:
• Soil type
• Distance of the cutter back from the lower edge of the cutting shoe
• Diameter of cutting shoe relative to the un-inflated outside diameter of
the pressure meter membrane;
• The downward force applied during drilling
• The amount of vibration during drilling.

32. The Cambridge self-boring pressuremeter

33. Test methods for Self-boring pressuremeter test
Drilling
• A cutter at the foot of the instrument rotates inside an internally tapered shoe.
• As the instrument is pressed steadily against the bottom of the hole, a plug of soil
is extruded into the taper.
• The top of this plug of soil is sliced off by the cutter positioned inside the shoe such that
the pressure needed to drive the soil up the taper is made equal to the in-situ vertical
stress.
• The soil cuttings resulting are carried away up the inside of the instrument by a flow of
flushing fluid, normally water, supplied from the surface.

35. Procedure:
• The testis carried out by applying gas pressure to the inside of this
sleeve and measuring the resulting changes in radius of the elastic
sleeve as a cavity is formed in the soil.
• The pressure at which the sleeve lifts from the rigid body of the
instrument gives the in-situ total stress.
• Two pressure cells mounted through and moving with the
sleeve as it expands, give continuous readings of the pore water
pressureThus, the stress-strain response of the soil can be
obtained

36. Readings:
• All the measurements made by the instruments are transmitted to the
surface by a protected cable passing up inside the gas supply line.
• After application of corrections, self-boring pressuremeter test results are
plotted as a curve of corrected pressure (p) as a function of cavity strain (εc).
Cavity strain is the radial strain of the cavity
Where d0= original diameter of the pressuremeter just before the start of
inflation, under (ideally) the in situ horizontal total stress, and d = current
diameter of the cavity, after expansion under pressure p.Self-boring
pressuremeter results are plotted as applied pressure as a function of cavity strain.

37. Displacement Pressuremeter:
• The instrument is pushed into the ground from the base of a
borehole.
• The soil displaced by the probe during insertion enters the body
of instrument, reducing the disturbance to the surrounding.

38. Advantages of Pressure testing:
For the Self-boring pressure meter
1. The tests are performed on virtually undisturbed soil.
2. A large number of fundamental soil properties are obtained from a
single test.
3. To derive these properties, no empirical correcting factors whatever
are needed.
4. The test is controlled by a semi-automatic system and is
largely independent of operator influence.
5. Results can be obtained quickly.
6. Commercial operation has shown that the instruments, though
more complex than conventional site investigation equipment, are
reliable and have enough redundancy to permit useful readings even
if a single fault appears

39. Disadvantages of Pressuretesting:
1. The instrument will not penetrate gravels, clay stones or the like.
2. Operating in sands usually demands a cased borehole to a level one or
two metres above the desired test locations.
3. Failure planes and deformation modes are not usually appropriate to those
occurring in the final design
4. In practice, only two stress paths can be followed-undrained and fully drained.
5. Undrained tests must usually be performed at high rates of strain so as
to prevent introducing errors
6. The instruments and their associated equipment are complex by
conventional site investigation standards.
7. Results obtained are ,sometimes surprising and in several cases have
challenged conventional assumptions of soil mechanics

40. References
• Soil properties through laboratory observations _ Dr . K.S.Beena ,CUSAT
• Site charaterization and instrumentation_Dr. P.Anbazhagan , IISC BANGALORE

41. Thank
You…………………………………