TOPIC:PLATE LOAD TEST IN DETERMINATION

OF BEARING CAPACITY OF SOIL

Presented by:

NIYITEGEKA Simon

INTRODUCTIONPlate load test is a field test to determine the ultimate bearing

capacity of soil, and probable settlement under a given loading.

The Test basically consists of loading a steel plate placed at the

foundation level and recording the settlements corresponding to

each load increment. The test load is gradually increased till the

plate starts to sink at a rapid rate.

The total value of load on the plate in such a stage divided by the

area of the steel plate gives the value of the ultimate bearing

capacity of soil. The ultimate bearing capacity of soil is

divided by suitable factor of safety (which varies from 2 to 3) to

arrive at the value of safe bearing capacity of soil.

Procedure:

A pit is dug at site up to the depth at which the foundation is proposed to

be laid.

The load is applied to the test plate through a centrally placed column.

The loading to the test plate is applied with the help of a hydraulic jack.

The load to the plate is given with the help of sand bags, stones or

concrete bags.

The load is applied in convenient increments say of about one-fifth of the

expected safe bearing capacity.

The settlement of plate is recorded by means of dial gauge. The load is

directly recorded from the pressure gauge of the hydraulic jacks. The

observations are plotted on a graph.

At the points where graph sharply takes a turn is the ultimate

bearing capacity(the plate starts sinking at a rapid rate).

Bearing plate: is circular or square, made of mild steel of not less than 25mm in thickness and varying in size from 300 to 750 mm,

For clayey and silty soils and for loose to medium dense sandy soils with N

Loading arrangement

The loading arrangement may be done with the help of a

hydraulic jack . The reaction of the hydraulic jack may be

borne by either of the following two methods:

Gravity loading platform method

Reaction truss method

Gravity loading method

A platform is constructed over a vertical column resting on the test plate, and the loading is done with help of sand bags,stones, or concrete blocks.

When load is applied to the plate, it sinks or settles.

The settlement of the plate is measured with the help of sensitive dial gauges.

The load is indicated on the load-gauge of the hydraulic jack.

Reaction Truss Method

The truss is held to the ground through soil anchors. These anchors are firmly driven in the soil with the help of hammers.

The reaction truss is usually made of mild steel sections,

Figure 1:Reaction by gravity loading(vertical section)

Figure 2:Reaction by truss( vertical section)

Figure 3: Load-settlement curves

Notes:

A load-settlement curve is plotted out to arithmetic scale,

Curve A (figure 3) is typical for loose to medium cohesionlesssoil :it is straight line in the earlier stage and there is no clear point of failure.

Curve B is for cohesive soil:it may not be straight in the early part and leans towards settlement axis as the settlement increases.

Curve C,for partially cohesive soils, presents the characteristics of both curve A and B,

Curve D is purely for dense cohesionless soils.

For curve B and D: no difficulty is experienced in arising at the ultimate bearing capacity,

For curve A and C: yield point is not well defined; therefore settlements and load intensities are plotted to logarithmic scale: such plot gives 2 straight lines, and intersection between them will be the yield value of soil.

Limitations of plate load test

The test results reflect only the character of the soil

located within the depth less than twice the width of

bearing plate , Since the foundations are generally larger, the

settlement and resistance against shear failure will depend on

the properties of a much thicker stratum.

Results from PLT are not recommended to be used for

design of strip footings, since test is conducted on square

and circular plate.

It is essentially a short duration test, and hence the test does

not give the ultimate settlement, particularly in the case of

cohesive soils.