To construct shallow foundations, the load capacity of the soil at the appropriate depth must be known. The plate load test is conducted on-site to assess the soil’s ultimate bearing capability at the specified depth. The data from the plate load test is then used as a design parameter or to validate the design assumptions derived from soil testing.
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Plate load test: What does the term mean?
The plate load test is used to detect soil properties such as ultimate carrying capacity, soil stiffness, and the propensity of soil settlement. Other soil qualities that can be determined by this test include the potential for soil erosion.
The subgrade, base, and surface layers of the test area are all evaluated as part of this process. When using trenchless technology to create subterranean pipelines and when designing temporary working platforms for drill rigs, these considerations are significant aspects of the design process. The plate load test can be performed on cohesion-free soil as well as sandy and clayey ground.
Plate load test: How exactly does one conduct a test?
In most cases, a plate load test will be executed at the level of the foundation that is under a working platform. This test would take place on the ground’s surface itself or in a shallow hole. A circular steel plate is placed on the ground in this form of an in-situ test, and increasing weight is given to the plate until it starts to settle more rapidly.
Plate load test: Methods
Gravity loading platform method
Sandbags are used in this form of plate load testing. First, a column is created over an existing column on the test plate. After that, the link ad is applied by the sandbags, which are then deemed to be a dead load. To ensure that the load is transferred effectively, the hydraulic jack is positioned between the rigid plate and the top surface of the column. Recording the amount that the plate has settled requires the use of the dial gauge.
Reaction truss method
In this technique, a steel truss of the appropriate dimensions is positioned such that it can counteract the effect of the hydraulic jack. The nails, which are driven into the ground with the use of a hammer, are used to either regulate or prevent the truss from becoming unstable. In most cases, the truss will be constructed using a segment of mild steel. The dial gauge will record the settlement once it has occurred. This strategy is gaining more and more traction in today’s world due to the ease with which it can be put into practice.
Plate load test: Factors affecting the safe load capacity of soil
Among the many variables that might have an effect on the load-bearing capability of the soil, the following are the most significant ones to consider:
- Category of soil
- Water table below the surface
- The weight per unit area of the soil
- Void ratio
- Porosity
Plate load test: Equipment and apparatus used
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To successfully complete the test, you will need the following plate load test equipment.
- A steel plate that is 300 mm square in dimension with a thickness of 6 mm.
- A hydraulic jack with a capacity that is at least 1.5 times more than the load that is expected to be applied during the test.
- A set of steel shims with a thickness of at least 6 mm.
- A reaction truss or beam, sometimes called a reaction beam.
- A dial gauge that can measure distances from 0 to 250 millimetres and has an accuracy of 0.02 millimetres.
- Pressure gauge.
- A loading frame with a capacity that is at least 1.5 times more than the load that is expected to be placed on it during the test. The frame must be constructed in such a way that it can be securely fastened to the ground and that the weight can be distributed evenly across the plate’s centre.
- A lint-free and water-free cloth.
- Tripod, Plumb bob, spirit level, etc.
- A hammer.
- Set of wrenches.
- The essential components needed for the loading platform.
- At a minimum, the length of a steel rule or tape measure is 3 metres.
Plate load test: Procedure
Below are the procedures required to conduct a plate load test
- Excavate the test pit to the specified depth. The size of the pit should be at least five times that of the test plate (Bp).
- Create a tiny hole or depression in the middle of the pit. Matching the thickness of the steel plate, the hole is the same size. To avoid damaging the real foundation, the bottom of the hole must be level with it. The hole is dug such that its depth-to-width ratio is the same as the foundation’s depth-to-width ratio.
- Load test plates typically consist of mild steel and range in size from 300 to 750 millimetres in thickness. You have the option of a round or square plate. As a rule of thumb, a square plate is used for square footing, whereas a round plate is used for circular footing.
- In the middle of the plate is a column. The column in the middle transfers the weight to the plate.
- Both the gravity loading technique and the truss method can be used to transmit the weight to the column.
- In the gravity loading technique, a platform is built above the column, and then sandbags or other dead weights are used to press down on the platform, compressing the column.
- To apply gradual loading, the hydraulic jack is situated between the column and the loading platform. This kind of loading is called “response loading.”
- If you want to keep track of the settlement, you need to put up at least 2 dial gauges in the plate’s diagonal corners. To prevent the gauges from sinking with the plate, they are supported by a stand.
- Before the real loading begins, apply a sitting load of 7 T/m2 and then release it.
- Preliminary data is recorded.
- The hydraulic jack is then used to progressively apply the weight.
- It is common practice to set the increment at a value no more than one-fifth of the theoretically safe loading capacity, one-tenth of the final bearing capacity, or any other lower number. A pressure gauge is used to measure the force exerted.
- There is a settlement that is measured in increments using a dial gauge. It is recommended to monitor settlement every hour after increasing the load for the first hour, then every four minutes for the next hour, and so on, until the rate of settlement drops below 0.2 mm per hour. The data is tabulated for easy reference.
- When data collection for a certain loading is complete, the next load increment is applied, and fresh measurements are taken. Once an increment has been applied, data gathering will continue until the maximum load has been imposed. Maximum load is typically 1.5 times ultimate load or 3 times bearing pressure.
Plate load test: How the results of the test should be interpreted
After calculating the overall amount of the load that must be present to bring about a settlement, we are then in a position to establish the final bearing capacity of the ground (the maximum vertical pressure that can be applied before shear failure occurs). To do this, divide the entire value of the load by the plate’s surface area.
Ultimate ground bearing capacity = total load value required to induce settlement/area of the steel plate
The safe bearing capacity of the ground can be calculated from this by dividing the ultimate bearing capacity by a safety factor (usually 3).
Plate load test: What size of plate is often utilised
Plate load tests typically use plates with a diameter of either 0.3 metres or 0.6 metres, with the size of the pressure bulb created (and, consequently, the depth of ground being tested) being directly related to the size of the plate – typically twice its diameter. Plate load tests are performed to determine how deep the ground is (so about 0.6m depth for a 0.3m diameter plate).
As a direct consequence of this, the dimensions of the plate have an effect on the outcomes of the plate-bearing test. As a result of testing with a bigger plate that goes deeper into the earth, the characteristics of the soil being evaluated will be different. If you use a larger plate, the bearing capacity value will be lower than it would be if you used a smaller plate since the subgrade soil at higher depths is often weaker than the compacted material directly under the surface.
The size of the plate has an effect on the calculation of the ground’s bearing capacity, so you should use a plate that has a diameter that is comparable to the structure that is being supported by the working platform. For example, if the ground tracks of a piling rig have a diameter of 0.6 metres, we should use a plate that has a diameter that is relatively close to 0.6 metres so that we can accurately determine how the ground will settle when it is subjected to the load.
Plate load test: Testing of working platforms
Let’s take a look at how this works in the context of plate load testing a working platform, which consists of two unique layers: a layer of stronger, more compacted granular material that sits above a layer of weaker subgrade.
If the platform is thicker than 0.6 metres, then the bearing capacity and settlement characteristics of the granular layer will be the only ones that can be determined using a plate load test with a 0.3-metre diameter. The characteristics of the weaker ground beneath the platform will not be determined.
Although these findings can be used for quality control purposes, such as verifying that the platform has been compacted appropriately and that it complies with the specification, they are not sufficient for determining the platform’s safe load-bearing capability.
Plate load test: Advantages
When trying to measure the soil’s bearing capacity, employing a plate-bearing test can be quite beneficial for many reasons. The following are some of the most significant benefits:
- The plate load test can be completed in a reasonable amount of time and is not very difficult.
- They can be carried out at the site.
- This testing procedure applies to a diverse variety of soil types when used appropriately.
- These tests provide reliable findings.
- They are economical in terms of both money and time.
- The apparatus used for testing plate bearings is portable and simple to set up.
Plate load test: Limitations
A plate load test is an excellent tool for collecting data for shallow foundation design, although it does have certain drawbacks.
- The soil below a depth equal to or more than double the bearing plate’s breadth can be predicted using this test. On the other hand, in real life, a foundation’s zone of impact may extend far deeper.
- Settlement for extended periods, particularly in cohesive soil, cannot be predicted using the plate load test because of how briefly the test is conducted.
- Plate load tests offer a conservative estimate of soil carrying capacity for thick sand, but a more accurate picture emerges when testing on clayey soil. The plate load test results for thick sand are lower than the actual capacity attained.
- In most cases, the settlement for shifting sandier soil is higher than the settlement indicated by the plate load test.
FAQs
Why is a plate load test needed?
In geotechnical engineering, plate load tests are often performed to determine the physical attributes of a ground surface, including its true strength and ground-bearing capacity.
What size plate is used for plate load testing?
Size can range between 300 and 750 mm. hence, the maximum size allowed is 750 millimetres. For clayey and silty soils, as well as loose to medium-dense sandy soils with N 15, a 450 mm square plate is used.
What distinguishes the plate bearing test from the CBR test?
CBR testing is conducted on fine materials, whereas plate load tests are conducted on coarse materials. The outcome of the plate load test corresponds to a CBR value.
How long does the plate bearing test last?
When conducting plate load tests following BS1377: Part 9:1990 or Eurocode 7, the average duration per test should be between 1.5 and 2 hours, based on site circumstances and needs. Typically, four to five tests can be conducted every shift.