This is a soil test used to determine the ultimate capacity of the ground or the verification of the bearing capacity of the land/foundation layer and the base layer of the flooring. Site survey considers ground condition at the strength and the depth. This is essentially very important for the operations and designs of working platforms. The plate bearing test is normally used in the operations of temporary structures like pads for crane outriggers and piling rigs.
Bearing test plates
A hydraulic system is involved in thrusting a circular plate. The gradual caving on the ground by the plate is detected by digital data acquisition systems. The plate bearing plate is generally carried out at the foundation level whether in the shallow pit or right on the surface. Plates of 300, 450, 600 and 720mm are available where they are placed on the ground and connected to a load cell to the reaction load.
However, due to the large size of over 720mm plates, the test is suitable for backfills where it requires a bigger reaction load. Moreover, the plate used is larger than that of CBR test hence its suitability for larger collective backfills. A 15 tonne tracked excavator is suitable for this reaction load. The plate diameter differs from matching the anticipated design load and reaction load requirements i.e. the excavator involved. The plate bearing test is carried in harmony with BS 1377 part 9: 1990. It’s basically loading a steel plate of known diameter and recording the settlements that coincide to each load increment.
The soil tests
The soil is tested by increasing the load thus detecting soil deformity and sagging. The test load is increased bit by bit until the plate settles at an increased rate. The aggregate value of the plate load divided by steel plate area gives the final bearing capacity of the soil. Tests could be carried out from either inside the wells with shallow depth and ground level. Its application in shallow wells is limited by the size of the jack and other associated extensions.
Specifications of Plate Load Test
This test is carried in accordance with an Engineer using approved equipment. This test is in harmony with BS 5930 or ASTM D1194.
Plate load test equipment
The plate load test equipment includes the approved certificates of instrument and calibration charts copies. These should be submitted for approval by the engineer before the beginning of the test. It consists of three round steel bearing plates with a thickness of not less than 25mm with varying diameter of 300mm to 1000mm. Plates of more than 1000mm diameter can be used in rocky areas depending on joining frequency. The jack assembly or the hydraulics should have the capacity to sustain a load of not less than 50 tonnes.
The applied load shall be adjusted using an electronic load cell. The recording devices should do it with high accuracy or with an error of not more than ± 2 percent of load increase. The settlement recording devices should, therefore, be able to measure not less than 0.25mm.
An accuracy of 0.01mm optical survey can be carried to measure movements of the reference beam and plate measurement settlement for corrections of dial instruments readings.
Moreover, the measuring devices should be protected from direct sunlight and the wind by providing shelter, which minimises errors of measurement. In addition to the gauges, you should include steel columns, loading columns, and other necessary construction tools for the preparation of loading gadgets and test pits.
The plate test procedure
The plate test procedures shall be carried out in accordance with BS5930 or ASTM D1194. In addition, the following requirements are critical for successful deployment;-
• The plate load test pit should be more than 4 times as wide as the plate and to the foundation depth, while the test should be carried out in the same level of the planned foundation level and as directed by the engineer. Three test are required for adjustment on the size effect of the test plates. The distances between the test locations shall be at least 5 times the diameter of the bigger plate.
• The surface of the test should be intact, free from any obstacles, and debris. Moreover, the test surface should be excavated by an excavator machinery and stopped at 200mm to 300mm above the test surface. The test surface should be trimmed manually. The steel test should have even or full contact with the ground so as to ensure even change of the test load. You should use Mortar or plaster to level small uneven ground surface.
• The ground preparation may cause the inevitable change in the ground stress causing irreversible changes to the subsoil properties. Thus, it’s very important that exposure time of test surface and the delay between set up time be minimised and reported with the test results.
• Always support the loading platforms far from the test area about over 2.4m away. The test load should be at the site before the test is started. In addition, the support platform with dial devices shall not be less than 2.4m from the centre of the loaded area. Before the calibration test, Mackintosh Probe Test should be carried out at the test location (centre of the plate).
• The loading shall be done in 3 cycles. Time intervals for each stage of loading should be more than 15 minutes while longer intermission is needed at particular loading stages.
• Every stage settlement should be in the intervals of 15 minutes before and after each load increment. If by any chance the required time intermission is more than 60 minutes then the reading should be done at every 15 minutes interval.
• In every load measurement, it should include targeted load schedule, pressure devices reading (secondary measurement) and load cell readings (primary measurements).
• The testing engineer shall control load cell readings by loading reading and recording from the pressure gauge so as to attain the targeted load in each stage all simultaneously. In addition, each test should be done until a peak load is reached.
Reasons for disengaging the plate bearing test
Faulty devices, the uncertainty of the kentledge, inappropriate setting of datum, unbalanced reference bench mark, and if the measurement devices are tampered with. The contractor shall always follow the direction of the engineer re-perform the tests if need arises
The report should include overall information on the date, temperature test surface soil narrative, weather condition, settlement data for each test, measured data, any abnormalities during testing, load settlement relationship, and evaluation of the yielding load.