Plate Load Testing
Plate Load testing is usually used for the verification of the bearing capacity of the land of the substrate, of the foundation layer and the base layer of the flooring, although it is not uncommon to its use to investigate the bearing capacity of the soil surface or to the state compaction of granular materials and road surfaces.
The load test consists in transmitting to the soil to be tested and the subsequent increasing load increments, detecting simultaneously the sagging and therefore allows to directly determine a form of deformation or compressibility of the soil. The load transmission takes place via the application of a force by means of a hydraulic system of thrust, of a circular plate. The subsidence induced by the ground plate are simultaneously detected by centesimal or digital data acquisition systems, integral with a frame of reference outside the area of influence of the applied load. Of fundamental importance, in any case, is the use of the proper contrast medium whose mass is a function of the expected load.
These tests are generally conducted with metal plates of standard diameter D=300/450/600 millimeters, could be carried out from ground level and inside the wells whose depth is limited only by the size of the jack and related extensions.
SPECIFICATION FOR PLATE LOAD TEST
The plate load test shall be carried out at locations as shown in the drawings or as directed by the Engineer using approved equipment. The test should be carried out in accordance to BS 5930 or ASTM D1194.
2.0 PLATE LOAD TEST EQUIPMENT
The details of equipment including the certificates of instrument and copies of calibration charts from approved agency and setup for conducting the plate load test shall be submitted for the approval by the Engineer before commencement of the test.
For bearing plates, three circular steel bearing plates, not less than 25mm in thickness and varying in diameter from 300mm to 1000mm including the minimum and maximum diameter specified or square steel bearing plates of equivalent area. In rock, plates larger than 1000mm diameter may be used, depending on the jointing frequency. As an alternative, three small concrete footings of the size mentioned or larger can be cast in-situ. Such footing must have a depth of not less than two thirds of their width.
The Hydraulic or Mechanical Jack Assembly should have sufficient capacity to provide and maintain the maximum estimated load but not less than 50 tons in any case.
The applied load shall be measured using calibrated electronic load cell. The force measuring devices should be capable of recording the load with an error not exceeding ± 2 percent of the load increment used or 1% of maximum load whichever is lesser.
The Settlement-Recording Devices, such as dial gauges should be able to measure to an accuracy of at least 0.25mm.
Precise leveling optical survey (to accuracy of 0.01mm) with respect to fixed datum to be carried out to measure plate settlement and movements of the reference beam for correction of dial gauges readings.
The reference beam and measuring devices should be protected from the direct rays of the sun and from wind by means of tarpaulins or other forms of shelter, to minimize errors of measurement which can easily arise from these causes.
Miscellaneous Apparatus required, including loading columns, steel shims, and other construction tools and equipment required for preparation of the test pits and loading apparatus.
3.0 PLATE TEST PROCEDURE
The plate load test shall be carried out in accordance to BS5930 or ASTM D1194 with the following additional requirement:
- The Plate Load Test pit should be at least 4 times as wide as the plate and to the foundation depth to be placed.
- The test shall be carried out at the same level of the proposed foundation level oras directed by the Engineer while the same conditions to which the proposed foundation will be subjected should be prepared if possible.
- At least three (3) test locations are required for calibration on size effect of test plates, and the distance between test locations shall not be less than five (5) times the diameter of the largest plate used in the tests.
- The test surface should be undisturbed, planar and free from any crumbs and loose debris. When the test surface is excavated by machinery, the excavation should be terminated at 200mm to 300mm above the test surface and trimmed the test surface manually.
- To ensure even transference of the test load on to the test surface, the steel plate should be leveled and have full contact with the ground. Sand filling or cement mortar or plaster of paris could level small uneven ground surface.
- If the plate bearing test is carried out below the groundwater level, it is essential to lower the groundwater level by a system of wells or other measures outside and below the test position.
- The preparation of the test surface may cause an unavoidable change in the ground stress which may result in irreversible changes to the subsoil properties. It is essential that the exposure time of the test surface and the delay between setting up and testing should be minimized. The time lag shall be reported with the test result.
- Support the loading platforms or bins by cribbing or other suitable means, at points as far removed from the test area, preferably not less than 2.4m. The total load required for the test shall be available at the site before the test is started.
- The support for the beam with dial gauges or other settlement-recording devices shall not less than 2.4m from the center of the loaded area.
- Mackintosh Probe Test to be carried out at load test location (center of plate) attesting level before the test for calibration purpose.
- Loading shall be applied in 3 cycles as shown in Table 1. The time interval of each stage of loading should not less than 15 minute. Longer time interval is required at certain specified loading stages.
- The settlement at each stage of loading should be taken at the interval of every 15 minutes before and after each load increment. If the required time interval is more than 60 minutes, the reading shall be taken at every 15 minutes interval.
- In the load measurement, the test record sheet should include the targeted load schedule, load cell readings (primary measurement) & pressure gauges readings (secondary measurement).
- The testing contractor shall control the loading using load cell readings to achieve the targeted load in each stage of loading & record the actual readings in the load cell & the pressure gauge simultaneously.
- Continue each test until a peak load is reached or until the ratio of load increment to settlement increment reaches a minimum, steady magnitude. If sufficient load is available, continue the test until the total settlement reaches at least 10 percent of the plate diameter, unless a well-defined failure load is observed.
The plate bearing load test shall be discontinued if any of the following occurs:
- Faulty jack or gauges,
- Instability of the kentledge,
- Improper setting of datum,
- Unstable reference bench mark or reference beam,
- Measuring instruments used are found to have been tempered.
The Contractor shall re-perform the test at location as directed by the Engineer.
The cost and time incurred shall be borne by the Contractor.
4.0 TEST REPORT
In addition to the continuous listing of all time, load, and settlement data for each test, the report shall include at least the followings:
- General information such as date, weather conditions, temperature, location of test, test surface soil description and others.
- Measured data. All data shall be checked for misrecording or miscalculation.
- Notes or abnormal phenomenon during the test shall be described.
- Load settlement relationship shall be plotted and presented in the report.
- Evaluation of the yielding load, elastic modulus, subgrade reaction and allowable bearing pressure.