Working Methodology of Plate Load Test

1. Introduction

The Plate Load Test (PLT), also known as the Plate Bearing Test, is an in-situ field investigation carried out to evaluate the load–settlement characteristics of soil at the proposed foundation level. The test is commonly adopted during the geotechnical investigation of bridges, culverts, underpasses, retaining structures and other highway infrastructure to assess the bearing response of the founding strata and verify the suitability of the ground for supporting the proposed foundations.

The primary objective of the test is to establish the relationship between the applied bearing pressure and the resulting settlement of the soil under controlled loading conditions. The observed response provides a basis for assessing the ultimate bearing pressure, where a distinct failure condition can be identified, and for evaluating settlement under the applied load. The results may also be used to verify assumptions adopted during geotechnical investigation and foundation design.

During the test, a rigid steel plate of specified dimensions is placed on a properly prepared soil surface at the designated foundation level. A vertical load is applied in predetermined increments through a hydraulic jack and a suitable reaction arrangement. The corresponding settlements are measured using calibrated dial gauges or equivalent displacement-measuring instruments supported on an independent reference frame. Load and settlement observations are recorded at the prescribed intervals for each loading increment in accordance with the applicable test procedure.

The recorded observations are used to prepare a load–settlement curve, which is interpreted to assess the load-bearing behaviour of the founding soil. Where a distinct bearing failure is established, the corresponding pressure may be interpreted as the ultimate bearing pressure. Where no clear failure point is observed, a recognised interpretation criterion should be adopted rather than assuming that rapid settlement alone represents ultimate failure.

For highway and bridge projects, the Plate Load Test is particularly useful for evaluating soil at the founding level of shallow foundations, including open foundations for bridge abutments and piers, box culverts, minor bridge structures and associated works. Under appropriate test arrangements, it may also be used to assess the load–deformation response of compacted soil and pavement subgrade.

The results shall be interpreted in conjunction with borehole investigations, soil classification, groundwater conditions, foundation geometry, anticipated structural loads and applicable design requirements. The recommended allowable bearing pressure shall satisfy both shear safety and permissible settlement criteria, as applicable.

The test represents the response of a relatively small loaded area and may not directly reproduce the behaviour of a full-scale foundation. Scale effects, soil stratification, the depth of compressible layers and differences between plate and foundation dimensions can influence settlement predictions, particularly in cohesive soils. Accordingly, the test results should form part of the overall geotechnical assessment and should not be treated as the sole basis for final foundation design.

The field procedure shall be carried out in accordance with IS 1888, Method of Load Test on Soils, the applicable project specifications and the approved geotechnical investigation programme. The final report should document the test arrangement, plate dimensions, foundation level, loading sequence, settlement observations, load–settlement curves, interpretation methodology and recommendations for foundation design.

2. Scope of Field Investigation

Field investigations were planned and carried out at the site to evaluate the load-bearing behaviour of the soil at the proposed foundation level and assess its likely settlement under the applied loads. The investigation data were analysed to determine the relevant geotechnical parameters required for foundation design.

The scope of work assigned to the consultant included the following activities:

1. Plate Load Tests: Conducting three Plate Load Tests at the specified foundation levels in accordance with IS 1888, Method of Load Test on Soils.
2. Groundwater Observation: Recording the groundwater level wherever encountered during the field investigation.
3. Test Plate Size: Using a rigid steel test plate of 600 mm diameter, subject to confirmation from the original field records.
4. Load–Settlement Curve: Preparing load–settlement curves from the observed test loads and corresponding settlements.
5. Analysis and Reporting: Preparing a report containing analysis and interpretation of the test results and recommendations regarding bearing pressure at the tested foundation levels.

3. Field Investigation Details

3.1 Location of Plate Load Tests

The Plate Load Tests were reported as having been conducted at three locations along the project alignment on 13 January 2026. The recorded locations, testing depths and stress intensity are presented in Table 3.1.

Table 3.1: Details of Plate Load Test Locations

4. Work Methodology

The Plate Load Test comprises two principal activities:

1. Field investigation.
2. Analysis and interpretation of test results.

4.1 Field Investigation

The test is conducted at the specified foundation level to evaluate the load–settlement behaviour of the soil under vertical loading. The test location is prepared, and a rigid steel plate of the specified dimensions is placed on the exposed and levelled soil surface. Load increments are applied through a suitable loading arrangement.

The load is applied using a hydraulic jack against an appropriate reaction system. Settlement is measured using calibrated dial gauges or other suitable displacement-measuring instruments mounted on an independent reference frame. Readings are recorded at the time intervals prescribed by the applicable test procedure, and loading continues until the specified termination criteria are met.

The recorded load and settlement values are used to prepare a load–settlement curve. The curve is examined for a distinct failure condition or a significant change in slope. Where a clearly defined bearing failure is observed, the corresponding pressure may be interpreted as the ultimate bearing pressure. Where no distinct failure point is established, an appropriate recognised interpretation criterion should be used.

The allowable bearing pressure is assessed by considering shear safety and permissible settlement, as applicable. A factor of safety applied to ultimate bearing pressure does not, by itself, establish the final allowable pressure; settlement criteria and the limitations of extrapolating plate-test results to full-scale foundations must also be considered.

4.1.1 Apparatus Required for Plate Load Test

The following apparatus and accessories are generally required, subject to the approved test arrangement and project specifications:

1. Test plate: A rigid steel plate of specified dimensions and adequate thickness.
2. Hydraulic jack and pump: For applying and controlling the vertical load.
3. Reaction beam, truss or loading platform: To provide the required reaction.
4. Dial gauges or displacement-measuring instruments: For measuring vertical settlement.
5. Pressure gauge or load-measuring device: For determining the applied load, with suitable calibration.
6. Loading column and bearing accessories: For transferring the load centrally to the plate.
7. Dead weights or kentledge: Such as concrete blocks or sandbags, where a gravity-loading arrangement is used.
8. Reference frame and gauge supports: To provide an independent datum for settlement measurements.
9. Levelling and alignment instruments: Including a spirit level, plumb bob and measuring tape.
10. Excavation and safety equipment: For test-pit preparation and safe working.

4.1.2 Test Procedure

1. Preparation of the test pit: Excavate to the specified foundation depth and provide adequate working space for the loading system and settlement instruments.
2. Preparation of the test surface: Expose and level the soil at the proposed foundation level without disturbing its natural condition. Prepare the seating surface in accordance with the test procedure.
3. Placement of the test plate: Position the rigid steel plate centrally on the prepared surface and ensure uniform contact with the soil.
4. Installation of the loading arrangement: Position the hydraulic jack and loading column centrally. Arrange the reaction beam, truss or kentledge to apply the load vertically without eccentricity.
5. Installation of settlement gauges: Install suitable dial gauges at appropriate positions. Support them on an independent reference frame isolated from the loading system and ground likely to be disturbed by it.
6. Application of seating load: Apply and release or adjust any seating load as required by the applicable procedure, then establish the initial gauge readings.
7. Application of test load: Apply load in predetermined increments in accordance with IS 1888 and the approved test programme. Record the applied load using the calibrated measuring system.
8. Settlement observations: Record settlement at the prescribed intervals for each load increment. Continue observations until the applicable settlement-rate or other termination criterion is satisfied.
9. Subsequent loading increments: Apply the next increment after completing the required observations for the preceding increment. Continue to the prescribed maximum test load or termination condition.
10. Unloading and completion: Where required, unload in specified stages and record rebound or recovery. Record final readings, groundwater observations and unusual test behaviour.
11. Field records: Compile load increments, applied pressures, time-dependent settlement readings and relevant site observations for analysis.

4.2 Analysis and Interpretation of Test Results

4.2.1 Determination of Bearing Capacity from the Plate Load Test

After completion of the field test, the recorded load and settlement observations are analysed to establish the load–settlement relationship of the soil at the tested foundation level.

1. Preparation of the load–settlement curve: Calculate the representative settlement from the available gauge readings after applying any necessary instrument corrections. Plot applied bearing pressure on the horizontal axis and settlement on the vertical axis, commonly increasing downward. Select a scale that clearly shows changes in curve slope. A logarithmic scale may be used where justified by the interpretation method, but it is not mandatory for every test.
2. Identification of ultimate bearing pressure: Examine the curve for a distinct failure point or a significant change in slope indicating bearing failure. If no clear failure point is present, use a recognised and stated interpretation criterion. Do not automatically define ultimate pressure as the pressure at a settlement equal to one-fifth of the plate width without verifying the applicability of that criterion.
3. Calculation of ultimate bearing pressure: Where the interpreted ultimate load is available, calculate the corresponding pressure using the loaded area of the plate.
4. Assessment of allowable bearing pressure: Calculate a shear-based value using an appropriate factor of safety, then check the permissible settlement criterion. The recommended value should satisfy the applicable shear and settlement requirements.
5. Reporting: Include the load–settlement curve for each test, interpreted bearing pressure, settlement response, calculation basis, groundwater observations and limitations relevant to foundation design.

4.2.1.1 Equations for Ultimate Bearing Pressure

Where:

• q_u(p) = interpreted ultimate bearing pressure of the test plate;
• P_u = load corresponding to the interpreted ultimate condition;
• A_p = plan area of the test plate.

For a circular plate:

For a square plate:

Here, D is the circular plate diameter and B_p is the side length of the square plate. Ensure that the load and area units are consistent.

4.2.1.2 Assessment of Allowable Bearing Pressure

This equation gives a shear-based allowable value where the defined ultimate pressure and selected factor of safety are applicable. The final allowable bearing pressure must also meet the permissible settlement requirement. Confirm whether the pressures used in the calculation are gross or net and apply the definitions consistently.

4.2.2 General Steps for Bearing Capacity and Settlement Assessment

1. Step 1 – Plot the load–settlement curve: Plot applied pressure against the corresponding recorded settlement.
2. Step 2 – Determine ultimate bearing pressure: Interpret the curve using an appropriate failure criterion, where identifiable.
3. Step 3 – Assess allowable bearing pressure: Check shear safety using an appropriate factor of safety and verify settlement acceptability.
4. Step 4 – Estimate foundation settlement: Use a suitable method that considers soil type, footing dimensions, plate dimensions, applied pressure and the limitations of plate-test extrapolation.

4.2.2.1 Settlement Estimation for Sandy Soil

An empirical relationship attributed to Terzaghi and Peck is sometimes used to estimate the settlement of a footing on sandy soil from plate-test settlement. Before applying it, confirm the original reference, the applicable assumptions and that the test-plate settlement corresponds to a suitable pressure for comparison with the proposed footing.

Where:

• S_f = estimated settlement of the proposed footing (mm);
• S_p = settlement of the test plate at the selected test pressure (mm);
• B = width of the proposed footing (m);
• B_p = width or diameter of the test plate (m);
• 0.3 = empirical constant expressed in metres in this relationship.

Illustrative data

Substituting the illustrative values:

5. Summary of Plate Load Test Results

The following table reproduces the supplied interpreted values. The settlement figures should be verified against the original test records and calculation sheets before the report is issued.

Table 5.1: Summary of Plate Load Test Results

Reported factor of safety: 2.5. The net safe bearing pressures in the table equal the listed ultimate pressures divided by 2.5; confirm that the ultimate values and net/gross pressure definitions are consistent before relying on these results for design.

The calculation for the first location is:

6. Conclusion and Recommendations

The Plate Load Tests provide site-specific information on the load–settlement response of the soil at the investigated foundation levels. The reported ultimate and net safe bearing pressures are summarised in Table 5.1. These values should be adopted for foundation design only after verification of the original load–settlement curves, calculation sheets, test plate dimensions, test pressures and the basis used to interpret ultimate bearing pressure.

The final allowable bearing pressure shall be selected after checking both shear safety and permissible settlement for the proposed foundation dimensions and loading conditions. The assessment should also consider the subsurface soil profile, groundwater conditions, potential scale effects and other findings from the geotechnical investigation.

Reference: IS 1888, Method of Load Test on Soils, using the applicable edition and amendments, together with the approved project specifications and the original reference for any empirical settlement correlation adopted.