Load tests are conducted on piles after the completion of 28 days from the time they are cast. These tests come in two types, namely the initial and routine tests. Both types of tests are performed on each type of loading, which includes vertical and horizontal (lateral) pull out. The purpose of these tests is to assess the strength and stability of the piles. The initial tests are performed to gather data on the load-carrying capacity and deformation characteristics of the pile. The routine tests, on the other hand, are conducted to monitor the pile’s performance over time and to check for any signs of deterioration. The results of these tests can help engineers make informed decisions about the design and construction of piles for future projects. Overall, load testing is an important step in ensuring the safety and reliability of piled foundations.
Initial Load Tests on Piles
An important test to verify the accuracy of design load calculations and establish acceptance criteria for regular testing involves assessing the suitability of piling systems. The initial test on piles should be conducted at one or more locations, depending on the number of piles needed. The load applied during the initial (cyclic) load test should be 2.5 times the safe carrying capacity of the pile. These tests should follow the guidelines outlined in Appendix ‘A’ Clause 6.3 of IS-2911 Part IV. By conducting these tests, it is possible to gain insight into the performance of the piling system and ensure that it meets safety standards.
Routine Load Tests on Piles
The selection of piles for the Routine Test is determined based on the number of piles required, with a maximum of ½% of the total number of piles needed. However, the number of tests may be increased up to 2% depending on the nature and type of structure. During the test, a load that is 1½ times the safe carrying capacity of the pile is applied using the Maintained load method, as described in Clause 6.2 of IS-2911 (Part IV) – 1985. The Routine Test serves two purposes: to ensure the safe load capacity of piles and to detect any unusual performance contrary to the findings of the Initial Test.
It is important to note that the test will only be performed at the cut-off level, and a detailed report on the test results will be prepared.
Vertical Load Tests on Piles
The test will be performed according to the guidelines outlined in IS-2911 (Part IV) 1995.
Fig: Vertical load test on piles
The process of preparing a pile head involves chipping off any unsound concrete until reaching a point where the concrete is still structurally sound. Any reinforcement present in the head is then cut and the surface leveled with Plaster of Paris. A bearing plate with a hole is then placed on the head to provide a resting point for the jack.
To ensure that the piles are stable, a suitable reaction Kentledge is designed to achieve the desired reaction. In cases where anchor piles are required, they should be placed at a center-to-center distance of three times the pile diameter, with a minimum distance of 2 meters.
To measure settlement, dial gauges with 0.01mm sensitivity are used. For a single pile, two dial gauges are required, while four dial gauges are required for a group of piles. These gauges are positioned at equal distances around the piles on datum bars resting on immovable supports. The distance between the supports should be at least 3 times the pile diameter, with a minimum distance of 1.5 meters, where D is the diameter of the pile or the circumscribing circle for non-circular piles.
During load application, the load is applied as specified, depending on the type of test being carried out. The load is maintained until the rate of displacement of the pile top is either 0.1mm in the first 30 minutes or 0.2mm in the first hour, or two hours, whichever occurs first. The next increment of load is then applied once the aforementioned criterion is met. The test load is maintained for 24 hours.
For initial tests, the safe load on a single pile is determined by taking the least of the following two values: 2/3 of the final load at which the total displacement attains a value of 12mm, unless otherwise required based on the nature and type of the structure. In such cases, the safe load should correspond to the stated total displacement permissible. Alternatively, the safe load can be taken as 50% of the final load at which the total displacement equals 10% of the pile diameter for uniform diameter piles or 7.5% of the bulb diameter for under reamed piles.
Routine Tests – Acceptance
The limit for settlement at the test load must not go beyond 12 mm. This means that any settlement that occurs when a load is applied for testing should not exceed the specified maximum of 12 mm. It is crucial to adhere to this limit as settling beyond this point can result in structural issues and compromise the safety and stability of the structure. Therefore, it is necessary to closely monitor the settlement during the testing process to ensure it remains within the acceptable range. Any settlement that exceeds 12 mm should be carefully analyzed and addressed promptly to prevent any further damage or safety hazards.
Lateral Load Tests on Piles
Fig: Horizontal load test on piles
To ensure the proper placement of a jack, it must be positioned horizontally between two piles. Additionally, it is important that the load on the jack is distributed evenly between the two piles. In order to avoid any accidents or complications, the load should be increased in increments of 20% of the estimated safe load and then stopped once it reaches the cut off level. At this point, the rate of displacement should be monitored, and only increased once it is below 0.1 mm per 30 minutes.
If the cut-off level is within reach, then a single dial gauge should be used to measure the displacement. However, if the cut-off level is not easily accessible, then two dial gauges, placed 30 cm apart vertically, should be used to calculate the lateral displacement of the cut-off level by using similar triangles.
When determining the safe load on the pile, it is important to consider the following factors. Firstly, the safe load should not exceed 50% of the final load at which the total displacement increases to 12 mm. Secondly, the safe load should not exceed the final load at which the total displacement corresponds to 5 mm. Ultimately, the safe load will be determined by whichever of these two factors results in the smallest load capacity.
Pull out Tests on Piles
To provide an uplift force to the piles, a suitable setup needs to be designed. This setup should ensure that the load increments and consequent displacements follow the same pattern as in a vertical load test. This is important because a vertical load test is used to determine the bearing capacity and performance of a foundation system, and replicating the same conditions during an uplift test can provide accurate results.
The uplift test is a type of load test that is performed on piles to determine their capacity to resist upward forces. In this test, a load is applied in increments to the pile until it reaches a specified maximum load. The resulting displacements are measured, and the data is used to calculate the pile’s uplift capacity.
To ensure that the uplift test accurately reflects the pile’s capacity, the load increments and displacements must be consistent with those of a vertical load test. This means that the setup must be designed to provide a similar loading pattern, with incremental increases in load and corresponding displacements. By replicating the conditions of a vertical load test, the uplift test can provide reliable data on the pile’s performance under upward forces.
Fig: Pull-out test on piles
According to the given context, there are two conditions that determine the safe load of a particular item. The first condition is that the safe load shall be the least of either 2/3rd of the load at which the total displacement is 12 mm or the load corresponding to a specified permissible lift. This means that if the load at which the total displacement is 12 mm is less than the load corresponding to the specified permissible lift, then the safe load would be 2/3rd of that load. However, if the load corresponding to the specified permissible lift is less than 2/3rd of the load at which the total displacement is 12 mm, then the safe load would be the load corresponding to the specified permissible lift.
The second condition that determines the safe load is that it shall be half of the load at which the load displacement curve shows a clear break. In other words, if the load displacement curve for a particular item shows a clear break at a certain load, then the safe load for that item would be half of that load.
Therefore, when determining the safe load for a particular item, both of these conditions must be taken into consideration, and the load that results in the least safe load should be used.