OPERATING INSTRUCTIONSFORDIRECT SHEAR APPARATUSMULTIPLE TYPE, MOTORISED 12 SPEEDS
The equipment comprises the following :-
1. One loading unit consisting
of one loading frame, on AC electric motor, one turret gear box, one loading
yoke with lever system of loading and fixtures for proving ring.
2. One shear box assembly
comprises of :- a. Two halves of the shear box , b. Two plain gripper plates,
c. Two perforated gripper plates, d. One base plate, e. One top loading pad and
two porous stones.
3. One shear box housing (water
jacket).
4. Two dial gauges, 0.01mm x
25mm
5. One specimen cutter.
6. Set of weights to give a
normal stress of 3 kgf/cm2 through lever.
0.05 kgf/cm2 4 nos.
0.1 kgf/cm2 1
no.
0.2 kgf/cm2 1
no.
0.5 kgf/cm2 3
nos.
1.0 kgf/cm2 1 no.
(Additional set of weights
to reach a total capacity of normal
stress of 8 kgf/cm2 can be had at an extra cost).
INTRODUCTION : 'HT' Direct
Shear Apparatus, electrically operated meets the essential requirements of IS:
2720 (Part XIII). It has a normal load
capacity of 8 kgf/cm2, when loads are applied through lever and 1.6 kgf/cm2
when loads are applied directly. It has arrangement for applying twelve rates
of strain from .0002mm/min to 1.25mm/min. with such facilities the apparatus
provided means to conduct various types of shear tests on large variety of
soils and study shear strength characteristics.
The following tests can be performed with this
apparatus.
1.
Undrained test, quick test.
2.
Consolidated-undrained, or
consolidated quick test.
3.
Consolidated-drained test or
slow test.
4.
Multiple reversal test to
obtain residual strength.
This Direct Shear Apparatus is designed for
easy operation. Tests on this apparatus provide information
about the ultimate shear resistance and enable to compute angle of internal
friction and cohesion of soil
samples. The apparatus also provides
facility for measuring the residual strength apart from peak shear
strength. The information obtained from
the tests on this apparatus is of importance in design of sub-structure, in determining
bearing capacity of soils, and in stability calculations of earth slopes
etc. The apparatus is suitable for
operation on 220 volts, 50 cycles, single phase AC supply.
DESCRIPTION : 'HT' Direct Shear Apparatus, electrically
operated is of constant strain type in which twelve rates of strain can be
applied. The apparatus has two main
units, shear box with water jacket and
loading unit which are illustrated in General Assembly Drawing Fig 1 and Fig
2. The numbers given against the
components in the description below pertain to these figures.
SHEAR BOX WITH WATER JACKET (FIG 1) :
SHEAR BOX WITH WATER JACKET : The shear box complete with base plate,
porous stones and gripper plates (plain or perforated) along with a water
jacket as normally assembled for a test is shown in Fig 1.
SHEAR BOX: The Shear
Box (3) is made into two halves. The top
half, which is fitted with a loading U-arm (9) slides over the lower half. The two halves are held together by two
easily removable pins (8). Three shear
clearance adjustment screws (4) with lock-nuts are provided to raise the upper
half above the lower half. One the sides
of the lower half of the shear box holes are provided to enable entry of water
below the gripper plates under the specimen.
GRIPPER PLATES : Two
pairs of gripper plates (10) are provided, one pair is plain and the other is
perforated.
POROUS STONES : A pair of porous stones (2) is also provided.
LOADING PAD : The top
loading pad (7) has a recess for steel ball on which the loading yoke rests for
application of normal load. The lifting
screws (5) are provided for lifting the pad from the box or to place it in the
box. These screws are to be removed after
the loading pad is placed over the specimen in the box.
BASE PLATES : The base plate (11) fits at the bottom of the
shear box on four pins on the sides of the lower half of the shear box.
The base plate has cross grooves on its top face.
WATER JACKET : The water jacket (1) is meant to contain the
bottom of the shear box rigidly and to hold water.
LOADING UNIT (FIG 2) : The Loading Unit consists of the loading
frame (1), shear stress application arrangement through lead screw (22) driven
for twelve rates of strain by an electric-motor (29) and a turret gear box
(27), proving ring fixtures for measuring shear stress, loading yoke (5) for
normal stress with lever (4) as well as direct loading system and strain dial
gauge (12) and consolidation dial gauge
(15).
LOADING FRAME : Loading
frame (1) carries the water jacket (17) containing the shear box and all
other components are assembled on this frame .
The water jacket rests on two ball roller strips which are held between
V-grooves and the flat surface on the underside of the water jacket and
matching grooves on strips which are fixed firmly to the channel of the frame
ARRANGEMENT FOR APPLICATION
OF SHEAR LOAD
: The shear load application is
through the advancement of a lead screw
(22) which is secured against rotation by a sliding key (19). The lead screw can be driven with twelve
speeds of travel with the help of turret type reduction gear box (27). The motor is connected to the input shaft of the gear box (27) with the help of a
V-belt . The output of the gear box is
coupled to the worm of the lead screw by
a sprocket and chain drive. The gear box
has six settings and a setting lever is provided. A handle (25) is also provided for setting
the position of change gears either A or B.
The movement of the lead screw is reversed with the help of a
forward-reverse switch (26). The lead
screw of the loading jacket abuts on the side of the water jacket and with
specific rate of strain pushes the water jacket horizontally.
PROVING RING FIXTURES : The U-arm of the upper half of the shear box
rests against the loading thrust piece (11).
The other end of the thrust piece is threaded to fit the proving ring
(10). (A proving ring is to be ordered
extra and does not form a part of the outfit).
The other end of the proving ring is held by means of the abutment
screw.
NORMAL LOADING SYSTEM : The loading yoke, after setting the sample,
is moved on to loading pad. The dead
weight of the loading yoke includes and stress of 0.1 kgf/cm2 on the sample.
The test can be conducted at
this level, which in turn forms the lowest possible, level of stress. The next incremented level of stress that can
be mobilized is due to loading lever and weight hanger. The above two levels can be advantageously
used particularly in the case undisturbed clay samples to determine load stress
(cohesion).
The loading lever (4) has a
ratio of 1:5. The fulcrum is on a knife
edge on the capstan (6). The knife edge
rests on a groove provided on the lever.
After counter balancing, for subsequent leveling and for lowering or
raising the yoke, the capstan is used.
The weights are placed on the hanger (2). These weights
have been marked to indicate the normal stress on the specimen when
placed on the hanger during lever loading.
In case of direct loading, these indicated values of stress have to be
divided by 5 to get the actual stress.
DIAL GAUGE : The consolidation dial
gauge (15) rests on the centre of the top cross head of the loading yoke
(5). The strain dial gauge (12) rests
against the water jacket.
SETTING UP : Mount the frame on a firm
level base. Position the water jacket
and fix the adapter to the water jacket.
Place the lower half of the shear box inside the water jacket in contact
with the inside of the thick wall of the jacket.
Place the base plate inside
the shear box with the grooves facing up.
Place one porous stone and one of the gripper plates (either plain or
perforated according to the requirements of the test to be performed) over the
bottom pad with its facing up and at right angles to the axis of the
application of load. Place the top half
of the shear box over the bottom half with the U-arm resting against the thrust
piece. Line it up and push in the
locating pins to hold the two halves of the shear box firmly.
Screw the shear clearance
adjustment screws provided on the top half of the shear box so that the two
halves are sitting flush with each other.
Adjust the position of the water jacket
so that the locating yoke stands vertical and the centre line of the
yoke passes through the centre of the
shear box.
Unscrew the abutment screw
and screw in the thrust piece to one end of the proving ring of appropriate
capacity. An adaptor is screwed on the
other end of the proving ring. (Proving
ring is not a part of standard equipment
and can be supplied at extra cost).
Move the abutment screw towards
the proving ring and fix the adaptor.
Place the specimen in the
shear box as described under specimen preparation. Place the required gripper with grooves
downwards, at right angles to the lead screws
Place the top loading plate on the top of the second porous stone
resting over the gripper. Place the
loading yoke cross bar on the loading ball of the top loading plate. Check that the yoke is central to the shear
box and it is vertical.
Fix the strain dial gauge
and consolidation dial gauge to their respective brackets. Adjust the plunger of the former to rest against water jacket and the
plunger of the latter to rest on the loading yoke cross bar.
The consolidation dial gauge
should be set with its plunger pushed upped about three-fourth of its
travel.
Different levels of
predetermined loading are applied by adding loading weight on the hanger. The desired shear strain can be applied by
engaging the turret to the appropriate position by reference to the speed chart.
APPLICATION OF SHEAR : After setting up the specimen in the shear
box according to the requirements of the type of test undertaken, proceed as
follows for application of shear load :-
1. Since the test is for
determination of residual strength, the whole system is attached to water
jacket thrust piece and proving ring with the help of an abutment screw.
2. Check that the V-belt is in
proper tension.
3. Screw the clearance
adjustment screws down just into contact with the lower half of the shear box
and a half turn further so that the upper half of the shear box is raised above
the lower half. Unscrew the adjustment
screws well above the lower face of the upper half of the shear box and lock
it.
4. Note all dial gauge
readings.
5. Put the forward-reverse
switch (26) on 'forward' position. When
the proving ring dial gauge shows that shear load is being applied start taking
readings.
6. For manual operation set the
strain setting lever handle (25) on 'N'
and operate the lead screws by the hand wheel (21).
SPECIMEN PREPARATION :
UNDISTURBED SPECIMEN : Testing of soil in undisturbed form is
required when the strength of cohesive soil in its natural state is to be
assessed. Trim the larger undisturbed
sample to the required size of the specimen, or take a clean specimen cutter
and force it into the ground. Remove the cutter with sample by under digging. (Accessories for trimming the sample to the
size can be supplied at extra cost).
REMOULDED SPECIMEN : When the soil is to be disturbed and
compacted as for construction of compacted embankment, the strength is
determined by conducting tests on remoulded specimens.
a) COHESIVE SOILS : For preparation of the moulded specimens
compact the disturbed soil sample into a larger mould to the required density
at the required moisture content. Press
the specimen cutter into the mould and remove the cutter with the sample by
under-digging, or extract the sample from the compaction mould and trim it to
the required size. Alternatively, mix
required water into the soil sample, keep it for some time for uniform
distribution of moisture stratically, compact it straight into the specimen
cutter. Do not compact the soil in the
shear box, as the cross grooved base plate rests on pins which are likely to get damaged.
b) NON-COHESIVE SOIL : Tamp the sample in the shear box itself with
the base plate, porous stone and gripper plates at the bottom of the box. Weigh the specimen after being cut to size
and use the trimming for estimation of
moisture content. Calculate dry bulk
density of the specimen.
TEST PROCEDURE :
i.
UNDRAINED TEST : Set up the apparatus as described previously. Place the base plate, porous stone and plain
gripper in the shear box. Transfer the
specimen over the plain gripper in the shear box with a gentle push. Over the specimen, place the top plain
gripper, porous stone pad one after another.
The serration of the gripper plates should be at right angles to the
direction of the shear and provide water
to the water jacket so that the specimen does not get dried during the
test. Reset the loading yoke on the
loading pad. Apply required normal
stress. Raise the upper part of the box
release the lower part slightly by unscrewing on the shear clearance screws and
then withdraw the screws completely.
(For most of the soils a spacing of approximately one millimeter between
the two halves would be satisfactory).
Remove two pins holding the two halves together before applying the
shear load. Apply the shear load at
a constant rate of deformation of
1.25mm/min. For any strain controlled
test take sets of readings every 15 seconds for the first 2 min and then sets
of readings at different suitable time-intervals as normally done for
consolidation test. Continue the test
till the specimen fails or till 20 percent longitudinal displacement takes
place. A the time of failure the dial
gauge reading starts to decrease after reading
a maximum. Note down the reading
of the proving ring dial gauge at the time of failure or when horizontal
displacement of approximately 20% of the
length of the specimen takes place.
Using the calibration chart of the proving ring find the load against
maximum reading. At the end of the test
remove the specimen from the shear box
and determine final moisture content.
Repeat the test on at least three (preferably four) separate specimens
of the same solid density at different normal stresses.
ii.
CONSOLIDATION UNDRAINED TEST : Assemble the apparatus ;and place the specimen in
the same manner as described above except
that instead of the plain gripper plates, perforated gripper plates and
saturated porous stone are used at the bottom and top of the specimen and
testing procedure in general is also the same.
Apply the normal load and allow the specimen to consolidate under the
normal load. If the normal load is of
large intensity, the load should be increased gradually and number of readings
should be taken at suitable intervals of time.
This gives compression verses time relationship, when the sample is
completely consolidated under the applied normal load, the shear stress is
applied such that there is no volume change or change in moisture content. Repeat the test on at least three (preferably
four ) separate specimens of same soil and density at different normal
stresses.
iii.
CONSOLIDATED DRAINED TEST : Assemble
the apparatus and fit the specimen, perforated gripper plates and porous stone into the shear box as
described above. The testing technique
in general is also the same. Apply the
normal load and allow the specimen to consolidate. During the
test note the dial gauge readings at different suitable intervals of
time as in a consolidated test. This
gives compression versus time relationship.
When the sample is completely consolidated, the shear stress is applied
at a strain rate to ensure at least 95%
of the pore pressure generated is
dissipated (or no scope for development
of pore pressure at all). Calculate the
rate of strain to be used for the specimens described subsequently under separate heading. Continue the shear stress till the specimen
fails or 20% displacement takes place.
Repeat the test on 3 to 4 specimens with different normal stresses.
CALCULATING RATE OF SHEAR
FOR CONSOLIDATED DRAINED TEST : For sandy soils
a rate of strain of 0.2mm/min may be suitable.
For clayey soils, a rate of strain of 0,01mm/min or lower may be
used. Rate of strain suitable for the
soil under test may be ascertained as described below.
For the consolidation data
collected during the application of normal load, the compression dial reading
should be plotted against the log of the time and from this curve, the value of
co-efficient of consolidation Cv should
be calculated from the formula :-
0.197 h2
Cv =
t 50
Where - 2 h
= initial thickness of the specimen, and
t 50 = time corresponding to 50
percent consolidation.
The requisite time to
failure when theoretically 95% dissipation is ensured may be obtained from the
following equation :-
h2 20 h
2
tf =
n Cv (1-Uo) 3 Cv
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