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On Site Plate Load Test

Testing the strength of the soil is extremely important in case of any kind of construction before the structure even gets designed. Without knowing the soil's load bearing capacity, you cannot put load on it on a guess - it may cause devastating structural failures. So, today we will talk about one of the best ways to determine the soil strength on site, namely the plate load test.

What is the Plate Load Test

The method of conducting a test on site to estimate the ultimate load bearing capacity of the soil on site by determining the settlement under a given load is called a Plate Load Test. This is performed on soil that is assumed or tested to be having an uniform kind of strata down to a reasonable depth in regard to the influence of stress. That can be verified by a boring or sounding test.

Components of a Plate Load Test

The following equipments and materials are needed to perform a plate load test:

  1. Two hard metal plates, of square shape. Dimensions of the plates are generally 75 cm by 75 cm, and 45 cm by 45 cm respectively.
  2. A hydraulic jack.
  3. A steel joist.
  4. Two dial gauges, indicated here by G1 and G2.
  5. A method of loading the plates with the expected amount of load. We are using 60 tonnes of load by means of 1900 sand bags each of 35 kilograms.

Plate Load Test Procedure

  • A. Test Setup:
    1. First, dig a pit on site at the depth in which the foundation is expected to form. This can be predicted by initial soil analysis. For common buildings, generally the test is performed at 3 meters depth.
    2. At the center of the pit, make a depression to fit the large plate. Accordingly, it will be sized 75 cm by 75 cm. The bottom of the depression should match with the expected footing bottom.
    3. The smaller plate (45 cm x 45 cm) should be placed right on top of the larger plate and they both should be machined on the sides.
  • B. Loading Platform:
  • A loading platform is made and fixed on top of the column that is made above the steel plates. This platform is meant to hold the test load. We will place the load we mean to test with on this platform. That is, now place the 1900 bags of sand each weighing 35 kg, totaling to 60 tonnes, on top of this platform, incrementally.

  • C. The test is prepared and loose material so that 75 cm x 75 cm plate resists horizontally in full contact with the soil subgrade.
  • D. A hydraulic jack is utilized that applies the load to the test plate. A central column takes the load and applies that on the soil through the plates at the bottom. As the pressure is applied, the whole assembly starts sinking down slowly in response to the load.
  • E. The dial gauges G1 and G2 are placed on two diagonally opposite sides of the test system, with a sensitivity of 0.02 cm at the very least. They will measure how much the plates have sunk down.
  • F. As the loads are increased in multiples of 5 tonnes (that is, place 5 tons, 10 tons, 15 tons, 20 tons, etc.), the plates are decreased and are measured.

Interpreting of Plate Load Test Results

LOAD TIME (min) G1(mm) G2(mm)
5 tones 1 5.28 3.7
  2.15 5.3 3.73
  4 5.1 3.73
  6.25 5.31 3.74
  9 5.31 3.75
  16 5.23 3.75
  25 5.36 3.79
LOAD TIME (min) G1(mm) G2(mm)
10 tones 1 4.12 2.5
  2.15 4.21 2.52
  4 4.24 2.55
  6.25 4.3 2.55
  9 4.37 2.567
  16 4.37 2.57
  25 4.38 2.56

This sort of results should go on for all the way up to 60 tonnes, we have only provided a couple of steps for example. When analyzed, we can figure out that the soil in this case has net allowable bearing capacity for design of shallow foundations may be considered as 25t/? with minimum depth of foundation at 3.0 m with respect to the existing ground levels. So the gross bearing capacity of the may be considered 30 t/?.

Why should you use Plate Load Test

Plate load test is great for figuring out the soil's capability of taking the weight of the structure! It provides data that enlightens the engineers about not only the shear failure of the soil on site, but also the settlement rate of the soil which are both extremely important about constructing a building there.

It is also good in the sense that the plate load test doesn't require any soil sample extraction, so it is a mostly clean process. It is also a relatively quick test. The loading test is done under conditions that are mostly similar to what would happen in reality, so the test results show a pretty clear idea of what would happen when the actual structure is erected in the scene.

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