Describe Plate Load Test as per IS Code, Discuss the limitations, Plate Load Test Equipment, Procedure for Plate Load Test, and the effects of the size of the plate on bearing capacity and settlement?
Plate Load Test
This test was originally designed to determine the modulus of subgrade reaction which is used in the design of rigid pavements.
Plate load test for soil is used to determine
- Ultimate bearing capacity of soil based on shear criteria.
- Allowable bearing pressure based on settlement criteria.
- Settlement of foundation.
The test is performed in the following steps
- Loading on the rigid plate which is placed at the level of the foundation.
- Determining settlement corresponding to the load applied.
- Plotting load settlement curve.
- Determining ultimate bearing capacity or allowable bearing pressure using load settlement curve.
Plate Load Test Equipment / Apparatus
A list of plate load test apparatus is necessary for performing the test.
|Plate Load Test – Image from IS code|
- Test plate
- Hydraulic jack & pump
- Reaction beam or reaction truss
- Dial gauges
- Pressure gauge
- Loading columns
- Necessary equipment for the loading platform.
- Plumb bob
- Spirit level, etc.
Plate Load Test Procedure
👉 Plate load test procedure starts from performed at the level of foundation and if the water table is at or above the test level, then it must be lowered by pumping before conducting the test.
👉 The Plate load test procedure is performed in a test pit dug up to the base level of the footing having general width equal to five times the width of the test plate.
👉 The size of the rigid plate varies from 300 mm to 750 mm (300 mm, 450 mm, 600 mm and 750 mm) and thickness not less than 25 mm which may be circular or square. Smaller size plates are used in dense or stiff soils whereas larger ones are being used in loose or soft soils.
👉 The test plate is seated at the centre of the pit over a thin layer of sand and load is applied with a hydraulic jack or through truss loading during performing during Plate load test procedure.
👉 Initially, minimum seating pressure of 7 kPa is first applied to the plate and removed after seating the load test. Now three-dial gauges are attached at 120° to each other to record the settlement.
👉 In the Plate load test procedure, the test load is applied in increments not more than 100 kPa or not more than 20% of the expected ultimate bearing capacity.
👉 The settlement is recorded by taking an average of the three dial gauges and the load settlement curve is plotted either on the arithmetic scale or log scale during Plate load test procedure.
👉 The load is continued until the following stage is attained:
- Settlement becomes progressive indicating shear failure.
- Loading pressure exceeds three times allowable pressure.
- The total settlement of plate becomes more than 10% of the size of plate
- Loose sands and soft clays
- Dense sand and stiff clay
Determination of Ultimate Bearing Capacity from Plate Load Test
Case-I: For clays:
In clays, bearing capacity is independent of the size of the footing
quf = qup
In sandy soils, bearing capacity is proportional to the size of footing.
quf = qup × (Bf/Bp)
Where, Bf = Width of footing
Bp = Width of plate
Using FOS = 2.5 – 3.0, safe bearing capacity can be determined.
Allowable bearing pressure based on settlement criteria using plate load test:
- The permissible settlement of footing is prescribed by codal provisions.
- Using following empirical relations, permissible settlement of plate can be computed, say Sp
For Dense sands
(Sf/Sp) = [Bf (Bp + 0.3) / Bp (Bf +0.3)]2
(Sf/Sp) = (Bf/Bp)
For Silts; n = 0.5
(Sf/Sp) = (Bf/Bp)n+1
Where, Bf and Bp are width of foundation and plate respectively in ‘m’.
- Using above relations, ﬁnd permissible settlement of plate and using load settlement curve, determine safe settlement pressure or allowable pressure on plate; qap
- If soil is clay, then allowable pressure on the foundation will be equal to safe or allowable pressure on the plate. Whereas in sand it depends upon the ratio of the width of footing and plate.
qaf = qap
qaf = qap × (Bf/Bp)
Limitations of Plate Load Test
The limitations of plate load test is described below:-
👉 In no case shall a test plate smaller in width than 30 cm be used, because experimental evidence has indicated that the load settlement behaviour of the soil is qualitatively different for smaller widths of the test plates compared to that of larger widths so it’s considered as limitations of plate load test.
👉 Settlement of a foundation cannot exceed about four times the settlement of a plate of 30 cm width. Howsoever large its width may be.
👉 If the soil plate load test may lead to misleading results. Consider, for example, the subsoil conditions illustrated. Where the upper stratum is a strong soil like dense sand and the lower stratum is a weak soil like soft clay. In such a situation, the load test reflects the load settlement characteristics of the stronger stratum but does not give any indication of the settlement behaviour of the poorer soil below.
The foundation being of a much larger width its bulb of pressure of the same load intensity as that on the test plate will extend into the weaker stratum. Hence, the extrapolated settlement of the foundation will be much smaller than the actual settlement, leading to unsafe design so that we considered as limitations of plate load test.
👉 The effect of capillarity in a sand bed is to increase its effective vertical stress or its stiffness. A test plate resting on a capillary sand bed undergoes a smaller settlement than a plate resting on dry or submerged sand bed.
👉 A plate load test is of short duration. The settlement measured is only the immediate settlement. In granular soils, immediate settlement can be taken as the total settlement, while in cohesive soils, consolidation settlement, which constitutes most part of the total settlement, cannot be predicted through this test. Hence the plate load test is not of much relevance in clayey soils for which the settlement criterion is very important in the determination of the allowable bearing pressure of a foundation.
Plate Load Test IS Code
Plate Load Test IS Code is “IS Code 1888” which give a detailed guideline for Plate Load Test is code mainly use for Method of load test on soil
Identify the is code which describes the procedure of plate load soil and Download Plate Load Test IS Code 1888 pdf from below