CYCLIC PLATE LOAD TEST

Nithin S M2 GE

Roll no:101222

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AIM This is a field test which determines the

coefficient of elastic uniform compression Cu or modulus of subgrade reaction ks

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THEORY

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WINKLER MODEL

w

k

q

𝑞=𝑘𝑤k = spring constant w = vertical deflection of the contact surface

COEFFICIENT OF ELASTIC UNIFORM COMPRESSION

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“Coefficient of elastic uniform compression is a dynamic property of soil used in the design of machine foundation and other dynamic analysis of foundations, Coefficients of elastic uniform compression (Cu) is the compressive stress causing unit elastic compression for a given area under dynamic loading”

COEFFICIENT OF ELASTIC UNIFORM COMPRESSION

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The subgrade reaction at any point the beam is assumed to be directly proportional to the vertical displacement of the beam at that point. In other words, the soil is assumed to be elastic and obeys Hooke’s Law. Hence, the modulus of subgrade reaction (ks) for the soil is given by:

𝑘𝑠=𝑞𝑤

=𝐶𝑢

q = bearing pressure at a point along the beamw = vertical displacement of the beam at that point.ks = the coefficient of elastic uniform compression, Cu.

CYCLIC PLATE LOAD TEST

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Plate Load Test – IS:1888-1982

CYCLIC PLATE LOAD TEST

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Bearing Plate: Rough mild steel bearing plate in circular or square

shape Dimension: 30 cm, 45 cm, 60 cm, or 75 cm.

Thickness > 25 mm Smaller size for stiff or dense soil. Larger size for soft

or loose soil Bottom of the plate is grooved for increased

roughness. Concrete blocks may be used to replace bearing

plates.

CYCLIC PLATE LOAD TEST

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Test Pit:

Usually to the depth of foundation level. Width equal to five times the test plate Carefully leveled and cleaned bottom. Protected against disturbance or change in

natural formation

CYCLIC PLATE LOAD TEST

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TEST ARRANGEMENT Selection of Location

Based on the exploratory boring. Test is carried out at the level of proposed foundation. If

water table is below the foundation level but the depth is less than width of plate then the test is carried out at the level of water table. If the water table is above the foundation level then the water level is reduced to proposed foundation level by pumping out the water during the test; however, in case of high permeability material perform the test at the level of water table.

In case the soil is expected to have significant capillary action and the water table is within 1 m below the foundation, it becomes necessary to perform the test at the level of water table in order to avoid the effect of higher effective stresses due to capillary action resulting in lower values of interpreted settlements.

Reaction supports should be at least (3.5 x width of plate) away from the test plate location, and loading arrangement should provide sufficient working space.

Test plate should be placed over a 5 mm thick sand layer and it should be centered with the loading arrangement.

CYCLIC PLATE LOAD TEST

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LOADING CRITERION A seating pressure of 7 kPa is applied and then released after

some time before the test. Loads are applied in the increments of approximately 1/5th of

the estimated ultimate safe load. (Or, one may choose to increase the load at an increment of 0.5 kN.)

At each load settlement is recorded at time intervals of 1, 2, 4, 6, 9, 16, 25 min and thereafter at intervals of one hour.

For clayey soil, the load is increased when time settlement curve shows that the settlement has exceeded 70-80% of the probable ultimate settlement or a duration of 24 Hrs.

For the other soils, the load is increased when the settlement rate drops below 0.02 mm/min.

The minimum duration for any load should, however, be at least 60 min.

Record the final readings of the dial gauges Remove quickly but gradually the entire load and the plate is

then allowed to rebound.

CYCLIC PLATE LOAD TEST

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LOADING CRITERION Again note the dial guage readings when no further rebound

occurs or the rate of rebound becomes negligible Increase the load gradually till its magnitude acquires a value

equal to the proposed next higher stage of loading (one-fifth or one-sixth load) and maintain constant. Note the final reading of the dial gauges under the new load as explained earlier

Reduce the entire load to zero and record the final dial guage reading when the rate of rebound becomes negligible.

Continue the cycles of loading, unloading and reloading till the estimated ultimate load has been reached, the final values of dial guage readings being noted each time.

The magnitude of the load increment should be such that the ultimate load is reached in five to six increments. The initial loading and unloading cycles up to the safe bearing capacity of the soil should be with smaller increment in load.

CYCLIC PLATE LOAD TEST

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OBSERVATIONS The bearing

pressure–settlement curve obtained from a typical cyclic plate load test is shown in Figure.

The recoverable part of the settlement (during unloading) represents the elastic part and the non-recoverable part signifies the plastic settlement (permanent set)

CYCLIC PLATE LOAD TEST

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OBSERVATIONS The elastic part of

the settlement is plotted as a function of average contact pressure (bearing pressure) in Figure

The slope of this curve is referred to as the modulus of subgrade reaction, ks, or the coefficient of elastic uniform compression, Cu.

CYCLIC PLATE LOAD TEST

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SPRING CONSTANT (k) Referring to Figure in Slide no 12. Calculate the elastic settlement

(se(1), se(2),………….) for each loading stage

Plot a graph of q versus se as shown in Figure in Slide no 13.

Calculate the spring constant of the plate as

The spring constant for vertical loading for proposed foundation can then be extrapolated as follows (Terzaghi, 1955) Cohesive soil:

Cohesionless soil:

CYCLIC PLATE LOAD TEST

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Shear Modulus, G

We know So Cu = subgrade modulus

E= modulus of elasticity= Poisson’s ratioA= area of the plate

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THANK YOU