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PLATE LOAD TEST

INTRODUCTION

General –

As part of the test related to the geotechnical investigation, The plate load test was done at the site of Proposed Structure at (Location) to find out the safe bearing capacity of the soil. It is also used to find the modulus of subgrade reaction useful in the design of raft foundation -and in the design of pavements.

The test was conducted as per IS 1888-1982. You can also follow DIN 18134.

A plate load test is a field test, which is performed to determine the ultimate bearing capacity of the soil and the probable settlement under a given load. This test is very popular for the selection and design of a shallow foundation. For performing this test, the plate is placed at the desired depth, then the load is applied gradually and the settlement for each increment of the load is recorded. At one point a settlement occurs at a rapid rate, the total load up to that point is calculated and divided by the area of the plate to determine the ultimate bearing capacity of soil at that depth. The ultimate bearing capacity is then divided by a safety factor (typically 2.0~2.5) to determine the safe bearing capacity.

Plate Load Test Equipment – 

The following apparatus is necessary for performing the plate load test.

PLATE LOAD TEST PROCEDURE

The necessary steps to perform a plate load test is written below:

Selection of Location –

The pits, usually at the foundation level, having in general, normally of width equal to five times the test plate or block, shall have a carefully levelled and cleaned bottom at the foundation level; protected against disturbance or changes in a natural formation.

Dead Load – 

A dead load of all equipment used, such as ball and socket, steel plate, loading column, jack, etc., shall be recorded prior to application of load increment.

Size and Shape of Plate –

Test Arrangement-

  • The loading platform shall be supported by suitable means at least 2.5 m from the test area with a height of 1 m or more above the bottom of the pit to provide sufficient working space. No support of loading platform should be located within a distance of 3.5 times the size of test plate from its centre.

  • The test plate shall be placed over a fine sand layer of maximum thickness of 5 mm so that the centre of plate coincides with the centre of reaction girder/beam, with the help of plumb and bob and horizontally levelled by a spirit level to avoid eccentric loading.

  • The hydraulic jack should be centrally placed over the plate with the loading column in between the jack and reaction beam to transfer the load to the plate.

  • The hydraulic jack should be centrally placed over the plate with the loading column in between the jack and reaction beam to transfer the load to the plate.

  • A ball and socket arrangement shall be inserted to keep the direction of the load vertical throughout the test.

  • A minimum seating pressure of 70 g/cm2 shall be applied and removed before starting the load test.

  • The two supports of the reference beam or datum rod shall be placed over firm ground, fixed with minimum two dial gauges resting at diametrically opposite ends of the plates. The dial gauges shall be so arranged that settlement is measured continuously without any resetting in between.

Load Increments –

Settlement and Observation –

The test shall be continued until,

  • A settlement of 25 mm under normal circumstances or

  • 50 mm in special cases such as dense gravel, gravel and sand mixture, is obtained or

  • Until failure occurs, whichever is earlier.

Alternatively, where settlement does not reach 25 mm, the test should be continued to at least two times the estimated design pressure. If needed, rebound observations may be taken while releasing the load.

DETERMINATION OF ULTIMATE BEARING CAPACITY/ SAFE BEARING PRESSURE/SETTLEMENT

Where

B = the size of footing in m,

BP = size of test plate in m,

SP = settlement of test plate in m, and

St = settlement of footing in m.

THE LIMITATIONS OF PLATE LOAD TEST

Plate load test limitations are given below:- 

  1. Plate Load tests; therefore, do not have much significance in such soil, to determine the allowable bearing pressure based on settlement criteria.
  2. Plate load tests should be used with caution, and the present practice is not to rely much on such tests.
  3. Moreover, plate load tests may also not be proper in sandy soil, when scale effects give very misleading results.

PLATE LOAD TEST REPORT

    1. The continuous listing of all time, load and settlement data, for each test shall be recorded with details of test elevation, natural water table, a profile of test pit, size of bearing plate and irregularity, if any, in a routine procedure

    2. It is necessary to excavate soil below the test plate to a depth equal to twice the dimension of the plate to examine and record the subsoil profile.

      Load vs Settlement Curve (Arithmetic Graph)
      Load vs Settlement (Log -Log Graph)

       

       

 

 

Which one of the following is the standard plate size in plate bearing test?

The plates are made off mild steel. The thickness of the test plate is 25mm (minimum).

The standard size of the test plates are :
Square plate: (300 x 300 mm), (450 x 450 mm), (600 x600 mm) and (750 x 750 mm).
Circular plate: Diameters of circular plates are 300 mm, 450 mm, 600mm and 750 mm.

These sizes are decided as per available N- value, which is obtained from the drilling report (subsurface exploration).

In the plate load test, k value is?

In the plate load test, the k value represents sub grade modulus of reaction.

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