Insitu Testing Services

 

 

Plate Bearing Tests

iGeo Ltd are able to carry out both Constant Rate and Incremental Plate Bearing tests (BS1377:1990 Part 9, Method 4.1), as well as the previously mentioned CBR plate bearing test.

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Plate bearing tests are often carried out to validate that placed piling or crane mats are able to take the proposed imposed pressures (typically incremental tests), although sometimes plate bearing tests are requested by consultants in an attempt to assess the cohesion value, or angle of friction of the formation soils, pavement sub-base or piling mat materials (typically constant rate tests).

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Incremental Plate Bearing Tests

The Incremental Loading Plate Bearing test (BS1377:1990 Part 9, Method 4.1) is used to simulate the reaction of the soils during the application of the proposed pressure including a suitable factor of safety, to assess if the bearing soils are suitable.  Often a reload cycle is included to assess any reversible/elastic settlement. This type of test is typically used on placed stone to assess the material before a piling rig or crane is placed on the mat.

 

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The test uses restraint normally from an excavator, JCB, dumper, tele-handler or roller to apply five or more incremental pressures using a 200kN jack, measuring the displacement at regular intervals at each pressure until there is less than 0.02mm of movement per minute, then the pressure increased to the next increment.  The test continues until the increments and re-cycles are complete, until the reaction is exceeded, or until penetration exceeds 40-50mm, whichever happens soonest.  The pressure and penetration is measured with three electronic displacement measuring devices and one electronic load measuring device connected to a laptop computer. 

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Due to the heavy equipment and the electronic sensors being connected to the data-loggers and a laptop device, we would require access with our support vehicle to within 5m of the test locations.

 

As the test requires the settlement to reduce to less than 0.02mm per minute before increasing to the next pressure increment, the test time can be highly variable, from less than 30 minutes on a tarmac surface, to around an hour on a decent thickness of well compacted crushed concrete, to in excess of 3 hours on unrolled sub-bases, loose or inadequate thickness or soft soil. These times exclude moving, setting up and dismantling time. 

 

The plate size used is dependent upon the maximum proposed load/pressure, the size of the piling rig, track, crane mat, foundation/raft, formwork mat or scaffolding mat with an appropriate factor of safety.  It is recommended the plate diameter should be over five times the maximum grain size being tested.

 

In order to simulate the proposed pressures accurately, the plate size used should be similar in size where possible, however this is often not possible due to the maximum plate diameter available, the 20 tonne (200kN) limit of our equipment, or due to limited kentledge available. 

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It should be noted that the zone of influence of a plate bearing tests is in the order of 1.5 to 2.0 times the plate diameter.  The same applies for the imposed load/foundation size.  Where the plate size is not similar to the imposed load size, in non-homogenous soils, this may result in misleading calculations/interpretations.  A test plate that is too small, may result in a soft zone beneath the plate’s bulb of pressure not being picked up by the test, leading to an unsafe assessment.  Conversely, if the test does not pick a firm zone beneath the plate’s bulb of pressure, it may lead to a conservative design.

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Constant Rate Plate Bearing Tests

The Constant Rate test (BS1377:1990 Part 9, Method 4.1) results may be used by others, in an attempt to assess the cohesion value, or angle of friction of the formation soils, pavement sub-base or piling mat materials.  In addition, an empirical Californian Bearing Ration (CBR) value could be calculated.

 

For cohesive and granular soils, the cohesion and angle of friction values respectively could be assessed from any yielding observed on the graphs.  If yielding was not observed, failure could be taken at the pressure required for the 15% the plate diameter penetration (if achieved).  An empirical CBR value is calculated using the modulus of sub-grade reaction ‘k762’ value at 1.25mm penetration (if achieved) and using a the CBR value calculated from the conversion formula set out in the Design Manual for Roads and Bridges (DMRB: IAN 73/06 Rev 1(2009): Clause 7.14).

 

The test uses restraint normally from an excavator, JCB, dumper, tele-handler or roller to push a plate into the material to be tested at a constant rate of 1mm/minute using a 200kN jack.  The penetration and pressure is measured with three electronic displacement measuring devices and an electronic load measuring device connected to a laptop computer.

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Due to the heavy equipment and the electronic sensors being connected to the data-loggers and a laptop device, we would require access with our support vehicle to within 5m of the test locations.

 

This testing is normally taken to the full extent of the displacement measuring device (40-50mm), or until the restraint offered by the kentledge is exceeded, whichever the greater. For each test, if the restraint is sufficient, the test time may be up to 50 minutes (this excludes moving, setting up and dismantling time).

 

The plate size used is dependent upon the maximum grain size of the material to be tested, also the thickness of the material to be tested:

• It is recommended in BS1377:1990 Part 9 Paragraph 4.1.2 that the plate diameter should be over five times the maximum grain size or fissure spacing.  Typically, a 300mm diameter plate is suitable.

• A larger plate diameter maybe required where the thickness to be tested exceeds 1.5 to 2.0 times the plate diameter, as long as the available kentledge sufficient.  The plate sizes available are 160mm, 300mm, 450mm, 600mm and 760mm. 

• Conversely, the plate diameter may need to be restricted where the layer in question is thin.  E.g. a 160mm diameter plate may be required for a Type 1 layer 250mm to 300mm thick.

 

It should be noted that the zone of influence of a plate bearing tests is in the order of 1.5 to 2.0 times the plate diameter.  The same applies for the imposed load/foundation size.  Where the plate size is not similar to the imposed load size, in non-homogenous soils, this may result in misleading calculations/interpretations.  A test plate that is too small, may result in a soft zone beneath the plate’s bulb of pressure not being picked up by the test, leading to an unsafe assessment.  Conversely, if the test does not pick a firm zone beneath the plate’s bulb of pressure, it may lead to a conservative design.​​​​​​​​​​​​​

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