Various ground tests are required before any construction to measure certain elements is carried out. CBR and plate bearing testing are two of the crucial processes required when building roads, pavements and airstrips, among others. This guide explains what each process is and when it is necessary to use.
The California Bearing Ratio test was developed to measure the load bearing capacity of subbases and subgrades. Developed in the 1930s by the California Division of Highway, CBR testing was a means to classify soil subgrades with great ease. It is a penetration test that evaluates the strength of the ground. Over time, this method of testing was adapted beyond California and is now used internationally in civil engineering. Note that the CBR test is arbitrary and, therefore, is not a true representation of the elements.
The test calculates the pressure that it would take a standard plunger to penetrate soil or aggregate. This figure is then divided by the pressure required to achieve the same level of penetration through crushed rock. The resulting ratio is the CBR and is expressed as a percentage. A standard piston with a diameter of 76mm is used on a soil sample to penetrate at a 1.25 mm/minute rate. CBR is considered the ratio achieved at 2.5mm penetration. However, some tests have a higher ratio at a depth of 5.0mm than at 2.5 and in such instances, the former acts as the CBR.
In a majority of cases, the depth of penetration is inversely proportional to the CBR, meaning the deeper the plunger goes, the lower the rating. This process occurs under controlled moisture and density. A vehicle can serve as the reaction load that provides the necessary force to drive the plunger. 20-30 minute intervals are required between each test, and the ground has to be approximately 500 to 1,000 mm. For the best results, engineers have to do at least three tests on a single site. Depending on the site, engineers can carry out 6 tests in a day. The results of the different tests are then tabulated and graded on a curve.
This test has to be conducted on soils with particles of not more than 20mm. Any surfaces with particles larger than this need plate bearing testing. Dynamic cone penetrometer is a type of CBR test that uses handheld devices to get the required values. Ground preparation is necessary when carrying out CBR testing.
Before beginning the construction of a road or airstrip, engineers must find out how much load the ground is capable of handling. They have to factor in all the weight that a particular area will have to bear in its lifetime. CBR testing applies when trying to determine this element. With the results from a CBR test, engineers can then calculate how thick the construction should be to accommodate the expected load during use. CBR testing comes in handy when deciding if a certain area is suitable for the intended use.
Hard surfaces tend to have a higher CBR rating. It is why sand will have a higher rating than clay. Grounds that are considered good quality achieve up to 100% rating. Such a figure means that a particular region is strong and compact enough for heavy loads.
Engineers conduct the plate load test to determine the safe loading capacity of a specific site. The data derived from this test is useful in developing the right designs for the structures that will be set up in that area. It makes it possible for engineers to confirm various design assumptions. Plate load testing also gives data about the potential settlement of a site under a certain load.
During this type of field test, a plate is positioned on the ground at the desired depth then loaded gradually. With every addition of the load, the settlement of the ground is recorded with every increment. This process continues until the point when the plate begins to sink at a fast rate. The engineer then measures the load required for this reaction and divides that figure with the area of the steel plate to arrive at the ultimate bearing capacity. To calculate the safe bearing capacity, the ultimate bearing capacity is divided by a safety factor.
Besides the plate, the test requires a hydraulic jack & pump, pressure gauge, loading columns, reaction beam/truss and dial gauges. The ground has to be excavated to the intended depth before setting up the plate. A good pit has to be at least five times the test plate. It must also have a hole where the plate will fit and, therefore, should have the same diameter. The thickness of the plate used for this test should be at least 25 mm and measure between 300 mm to 750 mm. A column is set in the middle of the steel plate to facilitate load transfer. At least two gauges on the sides of the plate are required to help record the settlement.
The load is transferred onto the plate using two methods, which define the different types of plate bearing testing. In gravity loading, the weight is placed on a platform built on top of the column. The jack goes between the platform and column so that it can adjust accordingly with each load increment. In the reaction truss method, a truss is positioned on the jack with nail anchors holding the sides to prevent undesirable movements. The truss is responsible for generating the reaction required to move the plate.
Plate load testing is suitable when looking to build shallow foundations. The test allows engineers to understand how much weight an area can hold without settling. PLT is critical when constructing temporary structures such as landing pads for crane outriggers. The procedure is part of the ground testing proceed when building on sand or clayey soil. Plate bearing testing makes it possible to gauge the type of foundation that would be appropriate in a particular site. As with CBR, this testing method is invaluable when evaluating and designing highway and airport pavements. However, the big size of the plates used on PLT makes it ideal when dealing with large aggregate backfills, which also means a larger reaction load is required compared to CBR.