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 locations for load test shall be based on exploratory borings (For N Value), shall be conducted at an elevation of the proposed foundation level.
  • In case the water table is within the depth equal to the width of the test plate, the test shall be conducted at the water table level.
  • In case the water table is higher than the test level it shall be lowered to the test level and maintained by pumping through a sump, away from the test plate.
  • However, for the soils like cohesionless silt and fine sand, which cannot be drained by pumping from the sump, the test level shall also be water table level.
  • Test Pit –

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 –

  • Generally, the square plate is used except in case of road problem and circular footing.
  • For clayey and silty soils and for loose to medium dense sandy soils with N < 15, a 450 mm square plate or concrete blocks shall be used.
  • In the case of dense sandy or gravelly soils (having 15 < N < 30) three plates of sizes 300 mm to 750 mm shall be used.
  • The side of the plate shall be at least four times the maximum size of the soil particles present at the test location.

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 –

  • Apply the load in cumulative equal increments up to 1 kg/cm2 or one-fifth of the estimated ultimate bearing capacity, whichever is less.

  • The load is applied without impact, fluctuation or eccentricity and in case of hydraulic jack load is measured over the pressure gauge, attached to the pumping unit kept over the pit, away from the testing plate through extending pressure pipes.

Settlement and Observation –

  • Settlements should.be observed for each increment of load after an interval of 1, 2.25, 4, 6.25, 9, 16, and 25 min and thereafter at hourly intervals to the nearest 0.02 mm.
  • In case of clayey soils, the ‘time settlement’ curve shall be plotted at each load stage and load shall be increased to the next stage either when the curve indicates that the settlement has exceeded 70 to 80 per cent of the probable ultimate settlement at that stage or at the end of a 24-hour period

  • For soils other than clayey soils, each load increment shall be kept for not less than one hour or up to a time when the rate of settlement is appreciably reduced to a value of 0.02 mm/min.

  • The next increment of load shall then be applied and the observations repeated.

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

  • From the corrected load-settlement curves, no difficulty should be experienced in arriving at the ultimate bearing capacity in case of dense cohesionless soils or cohesive soils (See Arithmetic Graph) as the failure point is well defined.

  • But in the case others curves, where yield point is not well defined, settlements shall be plotted as abscissa against corresponding load intensities as ordinate, both to logarithmic scales ( see Logarithmic Graph ), which give two straight lines, the intersection of which shall be considered as yield value of soil.

  • From Arithmetic Graph the safe bearing pressure for medium and dense sands could be read, corresponding to a plate settlement ( SP ), which shall be calculated as under [ St taken as permissible settlement of footing, recommended in IS: 1904-1978 (Code of practice for the structural safety of buildings: Shallow foundations, second revision )]

plate load test, settlement of foundation,plate load test formula

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:- 

  • It has a limited depth of influence. It could only give the bearing capacity of soils with depth up to two times the diameter of the test plate.

  • It may not provide information on the potential for long-term consolidation of foundation soils.

  • There is scale effect as the size of a test plate is smaller than the actual foundation.

  • To gain access to the test position, excavation is carried out which causes a significant ground disturbance. The change in ground stress leads to the change of soil properties, which the test is planned to investigate.

  • Since a load test is of short duration, consolidation settlement cannot be predicted. The test gives the value of immediate settlement only. If the underlying soil is sandy in nature, immediate settlement may be taken as the total settlement. If the subsoil contains clay, the settlement of the test plate is only a fraction of the total settlement.

  • A satisfactory load test should, therefore, include adequate soil exploration (see IS: 1892-1979*) with due attention being paid to any weaker stratum below the level of the footing.

  • Another limitation is the concerning of the effect of size of the foundation. For clayey soils, the bearing capacity (from shear consideration) for a larger foundation is almost the same as that for the smaller test plate. But in dense sandy soils, the bearing capacity increases with the size of the foundation. Thus, tests with smaller size plate tend to give conservative values in dense sandy soils. That may, therefore, be necessary to test with plates of at least three sizes and the bearing capacity results extrapolated for the size of the actual foundation (minimum dimensions in the case of rectangular footings).
  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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