Plate Load Test as per IS 1888

Plate Load Test According to IS 1888-1982

Loading a plate in the field is one of the simplest ways to see how soil carries load and settles. At QAQC-Civil Engineering we use this test to confirm the bearing capacity of the ground under foundations, to check the assumptions made in the geotechnical investigation, and to verify that the subgrade or foundation stratum behaves as the designer intended. This page sets out the practical procedure from IS 1888-1982, Method of Load Test on Soils, together with the field checks that matter most while you are working in the test pit.

Purpose and Scope

The plate load test measures how far a rigid steel plate of known size settles when a known load is applied at foundation level. From the load–settlement curve you can estimate the safe bearing capacity of the soil and, for granular soils, the likely settlement of the actual foundation. IS 1888-1982 covers the test method for both shallow and deep conditions and is the reference usually cited in specifications alongside the geotechnical report.

The test is most reliable in granular soils, where the plate and the foundation respond in a similar way. In cohesive soils the results need careful interpretation, because the size of the loaded area strongly affects settlement and time–settlement behaviour.

Apparatus and Test Setup

  • Test plate: a rigid mild steel or cast iron plate, square or circular, with a known area. Common sizes are 300 mm, 450 mm, 600 mm and 750 mm; the plate must be thick enough to stay rigid under the maximum test load.
  • Loading arrangement: a kentledge or reaction truss with a hydraulic jack of adequate capacity, a calibrated pressure gauge or proving ring, and a spherical seating to ensure axial load transfer.
  • Settlement measurement: at least two dial gauges with 0.01 mm least count, mounted on a datum bar supported independently of the loading system so that kentledge movement does not affect readings.
  • Excavation and preparation tools: a test pit of suitable size, a level base, and a thin layer of sand for bedding the plate.

Every gauge, proving ring and pressure gauge should have a current calibration certificate. Before loading starts, record the plate dimensions, gauge serial numbers and calibration dates on the test data sheet.

Preparing the Test Pit and Seating the Plate

Dig the test pit down to the level where the foundation will bear. Make the pit large enough for the plate, the loading column and the datum bar, with a clear margin around the plate so the soil at the edge stays undisturbed. Keep the pit base level and untouched; do not loosen or compact the soil you are about to test.

Clean the base, spread a thin, even layer of fine sand, and bed the plate on it so it sits on the soil surface with good contact over its full area. Any rocking, gap or high spot shows up as an erratic settlement reading and invalidates the test. Stand the loading column vertically over the centre of the plate, apply a small seating load to bed the plate, then release it before taking the zero reading. Mount the dial gauges on the plate or on a reference bar fixed to the plate, and set the datum bar clear of the loading arrangement.

Loading Procedure and Settlement Readings

Apply the load in steps of roughly one-fifth of the estimated ultimate load, or as the contract specifies. For each step:

  • Apply the load steadily and without shock.
  • Hold the load until the rate of settlement falls to a stable value — commonly taken as not more than 0.02 mm per minute, or as agreed in the specification.
  • Record the settlement on all dial gauges at the start and end of each load stage, and at intermediate intervals if settlement is still moving.
  • Continue loading until the soil fails, until settlement reaches the limiting value agreed for the test, or until the maximum load the setup can safely apply is reached.

In cohesive soils, hold each load stage until the rate of settlement is essentially constant; this can take considerably longer than in granular soils. Note the time of every reading so that time–settlement curves can be plotted alongside the load–settlement curve.

Interpreting the Results

Plot the load–settlement curve from the recorded data. The safe bearing capacity is usually taken from the load that corresponds to a specified settlement, or from the ultimate load divided by a suitable factor of safety, depending on the governing specification. For granular soils, the settlement of the actual foundation can be estimated from the plate settlement using the standard relationship based on the ratio of foundation width to plate width.

Always read the results alongside the borehole log, the groundwater level and the foundation geometry. A plate load test shows how the soil behaved under one plate at one location; it does not replace a proper geotechnical assessment of the whole site.

Practical Checks on Site

  • Keep the test pit dry; groundwater or rain will change the soil condition and the result.
  • Protect the pit sides from collapse, especially in loose or cohesionless soil.
  • Check that the reaction load is at least 1.5 times the maximum test load, and that the kentledge is stable.
  • Make sure the datum bar is independent of the loading frame and is not disturbed by site traffic.
  • Record everything — plate size, depth of test, soil description, gauge readings, time, weather and any site disturbance — on a signed data sheet.

Frequently Asked Questions

What size of plate should be used?
IS 1888-1982 permits square or circular plates, with 300 mm, 450 mm, 600 mm and 750 mm being common sizes. The plate should be large enough to be representative of the soil and small enough to be handled and loaded safely on site. The size used must be recorded, because the interpretation depends on it.
How long does a plate load test take?
In granular soils, each load stage may stabilise within a relatively short time, so the test can often be completed in a few hours. In cohesive soils, each stage may take much longer, and the test can run over a day or more. Plan the site programme accordingly.
Can the test be done below the water table?
It is difficult and the results are less reliable. Where the test level is below the water table, the pit must be dewatered and the soil condition at the base carefully maintained. Record the groundwater level and any dewatering on the data sheet.
Does the plate load test give the safe bearing capacity directly?
No. The test gives a load–settlement curve. The safe bearing capacity is derived from that curve using the criteria in the governing specification, and should be reviewed together with the geotechnical investigation and the foundation design.

Getting a valid plate load test comes down to discipline on site: a level, undisturbed pit base, a properly bedded plate, independently supported datum bars, and a complete record of every reading and site condition. Pair the load–settlement curve with the borehole log and groundwater data, and the test becomes a dependable check on the bearing capacity assumed in the foundation design.