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Plate Load Test for Soil

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The plate load test is a common field test used in soil engineering to determine the bearing capacity and settlement behavior of soil. This test is usually carried out at construction sites before foundation design, especially for shallow foundations such as isolated footings, strip footings, and mat foundations.

The main purpose of the test is to understand how the soil responds when a load is applied. In this test, a steel plate is placed on the ground at the required foundation level, and load is applied gradually through a hydraulic jack. By measuring the settlement of the plate under each load increment, the engineer can estimate the safe bearing capacity of the soil and predict the expected settlement under the foundation load.

Purpose of Plate Load Test

The plate load test is used to determine:

1. The ultimate bearing capacity of soil.

2. The safe bearing capacity of soil.

3. The settlement of soil under loading.

4. The modulus of subgrade reaction.

5. The suitability of soil for supporting foundations or pavements.

This test is useful because it provides direct information about soil behavior under actual field conditions.

Instruments Used in the Test

The main instruments used in the plate load test are:

1. Circular or square steel plate.

2. Hydraulic jack for applying the load.

3. Reaction frame or counterweight, such as a loaded truck.

4. Dial gauges, settlement gauges, or LVDTs to measure settlement.

5. Bearing plate and loading arrangement.

The steel plate is placed at the foundation level where the structural loads are expected to be transferred to the soil.

Procedure of the Test

First, the soil surface is excavated and leveled at the required foundation depth. Sometimes, a thin layer of sand is placed between the steel plate and the soil to ensure proper contact.

After that, the steel plate is placed on the prepared soil surface. The hydraulic jack is then positioned above the plate, and settlement gauges are fixed to measure the vertical movement of the plate.

Next, the reaction frame or counterweight is arranged so that the hydraulic jack can apply load on the plate. The load is applied gradually in increments, and the settlement is recorded for each loading stage.

Each load increment is maintained for a certain period until the settlement becomes nearly stable. This process continues until soil failure occurs or until the maximum required load is reached.

After completing the test, the load and settlement readings are plotted in the form of a load-settlement curve.

Load-Settlement Curve

The load-settlement curve is the main result obtained from the plate load test. At the beginning of the test, settlement increases slowly as the load increases. As the load becomes higher, the settlement starts to increase at a faster rate. When the soil approaches failure, even a small increase in load may cause a large settlement.

From this curve, the engineer can determine the ultimate bearing capacity of the soil. The safe bearing capacity is then obtained by applying a suitable factor of safety.

The ultimate soil bearing capacity is determined from the curve as follows:

1- if the curve has clear failure point such sudden bend or sharp increase in settlement, then the load is determined at the point of sudden bend and divided by the plate area to determine ultimate bearing capacity. 

2.if there is no clear failure point: then a tangent is drawn at the initial stage of the curve and another tangent is drawn at the steep final part of the curve and the load determined at the point of intersection. The ultimate bearing capacity is the load at intersection point divided by the plate area. 

The safe bearing capacity can be calculated as:

Safe bearing capacity = Ultimate bearing capacity / Factor of safety

The factor of safety depends on the type of structure, soil condition, importance of the project, and local code requirements. In many foundation designs, the factor of safety usually ranges from 2.5 to 3. However, the final value should always be selected according to the project specifications and relevant codes.

Safe bearing capacity is the maximum pressure that can be safely transferred to the soil without causing shear failure or excessive settlement.

Modulus of Subgrade Reaction

The plate load test can also be used to determine the modulus of subgrade reaction. This parameter is important in pavement design and in the analysis of mat foundations. It represents the relationship between soil pressure and settlement.

It can be calculated as:

K = Pressure / Settlement

Where:

K = modulus of subgrade reaction

Pressure = load / plate area

Settlement = vertical movement of the steel plate

A higher value of K means that the soil is stiffer. A lower value of K means that the soil is softer and more compressible.

Advantages of Plate Load Test

The main advantages of the plate load test are:

1. It is conducted directly in the field.

2. It gives practical information about soil behavior.

3. It helps in estimating soil settlement.

4. It is useful for shallow foundation design.

5. It is useful for pavement design.

6. It gives direct load-settlement data.

Limitations of Plate Load Test

Although the plate load test has many advantages, it also has some limitations. The main limitation is that the size of the test plate is much smaller than the actual foundation. Therefore, the test mainly represents the behavior of soil near the surface and may not fully represent deeper soil layers.

Also, the result may be affected by soil layering, groundwater conditions, and the difference between the plate size and actual foundation size. For clayey soils, long-term settlement may not be fully predicted from a short-duration plate load test.

Therefore, the plate load test should not be used alone. It should be used together with other soil investigation methods such as boreholes, laboratory tests, standard penetration tests, and engineering judgment.

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