A Plate Load Testing Guide
A plate loading test is used to establish the full bearing capacity of the ground, and the possible settlement when subjected to a load. The findings of this test are applied in the designing temporary working stations for piling pads or rigs for crane outriggers. Also, a fairly accurate CBR value can be derived from the sub grade reaction in this test.
How to Conduct a Plate Load Test
A. Test Setup
A pit is dug at the testing site which is as deep as the proposed foundation. The pit should be five times wider than the width of the testing plate. In the middle of the hole a square depression is made which is equal to the size of the testing plate, and its bottom level should also correspond to the level proposed foundation.
B. Testing Procedure
The load should be placed on the test plate via a centrally placed column. The test load can be transmitted to the column through one of the following methods:
(i) Gravity loading method
In this case, the loading platform is built over the column that is on the test plate. A testing load is then applied using dead weight such as pig iron, concrete blocks, sand bags, lead bars etc. on top of the platform.
(ii) Reaction Truss Method
In this method, instead of building a loading platform, the effect of the hydraulic jack is supported by a steel truss. The truss is then anchored to the ground using steel anchors and strong ropes are used to ensure its lateral firmness. When the load exerted, the test plate starts to sink slowly. The settlement is then recorded using sensitive dial gauges.
C. Interpretation of Results
The intensity of the load and the settlement observations are plotted as load settlement curves.
The value the bearing capacity which is the SBC in this test, can be recorded from the load settlement curves by matching the value of load intensity that corresponds to the desired test plate settlement.
D. Limitations of plate load testing
The plate load test is beneficial in obtaining essential information about soil in designing of a foundation. However, it can be faced with the following limitations:
(i) The results from the plate test can be disingenuous if the type and characteristics of the soil changes at low depths.
(ii) The Plate Load Test is conducted in short durations and does not give the exact settlements especially when carried out in cohesive soils.
(iii) The bearing capacity of clayey soils (from sheer consideration) in a large foundation is approximately the same as in smaller test plates. However, the bearing capacity of dense sandy soils is directly proportional to the size of the foundation. This means that if the test is done with smaller test plate, the results are conservative in dense sandy soils.
Considering the above limitations, the plate load testing technique of determining SBC of soils can be regarded as tolerable for light structures under standard conditions. However, for an unusual type of soil, massive and important structures, appropriate laboratory or field tests should be used to ascertain the SBC of soil.