The plate load test is performed on-site to determine the ultimate bearing capacity of soil at the desired depth for designing shallow foundations. Data from the plate load test is helpful to confirm the design assumptions made from soil tests or can be used as a design parameter.
The 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 shallow foundations.
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 2.5~3) to determine the safe bearing capacity.
Plate Load Test Apparatus / Equipment
The following plate load test apparatus is necessary for performing the test.
- A steel plate is at least 300 mm square and 6 mm thick.
- Hydraulic jack & pump
- A hydraulic jack with a capacity of at least 1.5 times the anticipated test load.
- A set of steel shims, at least 6 mm thick.
- Reaction beam or reaction truss
- A dial gauge, with a range of 0-250 mm and an accuracy of 0.02 mm.
- Pressure gauge
- A loading frame with a capacity of at least 1.5 times the anticipated test load. The frame should be designed so that it can be firmly attached to the ground, and so that the load can be applied to the center of the plate.
- Necessary equipment for the loading platform.
- A steel rule or tape measure is at least 3 m long.
- Tripod, Plumb bob, spirit level, etc.
- A hammer.
- A set of wrenches.
- A clean, dry cloth.
Plate Load Test Procedure
The necessary steps to perform a plate load test is written below-
- Excavate the test pit up to the desired depth. The pit size should be at least 5 times the size of the test plate (Bp).
- At the center of the pit, a small hole or depression is created. The size of the hole is the same as the size of the steel plate. The bottom level of the hole should correspond to the level of the actual foundation. The depth of the hole is created such that the ratio of the depth to the width of the hole is equal to the ratio of the actual depth to the actual width of the foundation.
- A mild steel plate is used as a load-bearing plate whose thickness should be at least 25 mm thickness and size may vary from 300 mm to 750 mm. The plate can be square or circular. Generally, a square plate is used for square footing and a circular plate is used for circular footing.
- A column is placed at the center of the plate. The load is transferred to the plate through the centrally placed column.
- The load can be transferred to the column either by gravity loading method or by truss method.
For the gravity loading method, a platform is constructed over the column, and load is applied to the platform by means of sandbags or any other dead loads. The hydraulic jack is placed in between the column and the loading platform for the application of gradual loading. This type of loading is called reaction loading.
- At least two dial gauges should be placed at diagonal corners of the plate to record the settlement. The gauges are placed on a platform so that it does not settle with the plate.
- At first, a seating load of 7 KN/m2 is applied which releases after some time before the actual loading starts.
- The initial readings are noted.
- The load is then applied through the hydraulic jack and increased gradually. The increment is generally one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity or any other smaller value. The applied load is noted from the pressure gauge.
- The settlement is observed for each increment and from the dial gauge. After increasing the load-settlement should be observed after 1, 4, 10, 20, 40, and 60 minutes and then at hourly intervals until the rate of settlement is less than .02 mm per hour. The readings are noted in tabular form.
- After completing the collection of data for a particular loading, the next load increment is applied and readings are noted under the new load. This increment and data collection are repeated until the maximum load is applied. The maximum load is generally 1.5 times the expected ultimate load or 3 times the expected allowable bearing pressure.
Calculation of Bearing Capacity from Plate Load Test
After the collection of field data, the load-settlement curve is drawn. It is a logarithmic graph where the load applied is plotted on X-axis and settlement on Y-axis. From the graph, the ultimate load for the plate is obtained which is the corresponding load for the settlement of one-fifth of the plate width.
When the points are plotted on the graph, the curve is broken at one point. The corresponding load to that breakpoint is considered to be the ultimate load on the plate. The ultimate bearing capacity can be calculated from the ultimate load from the plate. The ultimate bearing capacity is then divided by a suitable factor of safety to determine the safe bearing capacity of soil from the foundation.
General Equations for Calculation of Bearing Capacity of Soil
Soil Bearing Capacity Calculation for Clayey Soil
Following is the equation to determine soil bearing capacity for clay from the plate load test.
Ultimate Bearing Capacity = Ultimate Load for the Plate
Soil Bearing Capacity Calculation for Sandy Soil
The followings are the equation to determine soil bearing capacity for sand from the plate load test.
Typically, the range for the factor of safety varies from 2 to 3.
Equations for Foundation Settlement Calculation from Plate Load Test
The following equations can be used for foundation settlement calculation.
Foundation Settlement Calculation for Clayey Soil
Following is the equation to determine foundation settlement for clay from the plate load test.
Foundation Settlement Calculation for Sandy Soil
Following is the equation to determine foundation settlement for sand from the plate load test.
Advantages of Plate Load Test
The advantages of the Plate Load Test are discussed below-
- Being able to understand the foundation behavior under loading conditions.
- Evaluation of bearing capacity of soil at a certain depth and prediction of settlement for a certain load.
- A shallow foundation can be calculated considering the allowable bearing capacity, which can be predicted from the plate load test.
- Time and cost-efficient.
- Easy to perform.
Limitations of Plate Load Test
For gathering the necessary information regarding the design of a shallow foundation, the plate load test is very useful, but it has the following limitations.
- The test predicts the behavior of soil located at a depth less than twice the depth of the width of the bearing plate. But in practical conditions, the influence zone of a foundation is up to a much greater depth.
- The plate load test is performed for a short time period, so it cannot predict the settlement for a longer period, especially for cohesive soil.
- The bearing capacity for clayey soil is almost similar to the bearing capacity obtained from the plate load test, but in the case of dense sandy soil, the plate load test provides a conservative value. The actual capacity obtained for dense sandy soil is higher than the results from the plate load test.
- The settlement for losing sandy soil is usually greater than the settlement indicated by the Plate Bearing Test.
- As the width of the bearing plate is very small compared with the actual foundation, it only provides an estimate of the bearing capacity up to a depth of twice the width of the bearing plate.
- The failure load is often not well defined in the load settlement curves obtained from the test. Hence, errors may arise based on personal interpretation.
- The effect of the water table may not be taken into account properly in the test. It is advised to lower the water level by pumping if it is encountered at the testing depth.