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The reuse of existing piles is an efficient way to reduce pile input-resources, construction costs, and the overall environmental impact by minimizing the project carbon footprint. The old piles can be used either completely, partially, or in combination. This is an effective practice towards structural sustainability.
The existing piles are completely removed when they obstruct the construction of a new foundation and structure. When they are pulled out entirely, the soil around them loosens, which lowers the bearing capacity of the adjacent piles. Therefore, the reuse or removal of existing piles is performed following detailed study and observation.
This article explains the cases when piles are reused and their respective properties.
Challenges in the Reuse of Existing Piles
The major concerns that come up while deciding to reuse an existing pile are:
- Whether the existing pile meets the requirements of the new project.
- Consequences of removing the existing piles.
The reuse of existing piles could be a cost-effective, sustainable, and efficient solution. But all the existing piles and foundations must be checked for their reliability before deciding to reuse them.
Some important considerations while reusing existing piles are:
- The existing piles must be checked for sufficient reliability. This is done by pile integrity test, pile capacity testing, measuring the pile length, etc.
- The location of the applied load and the existing piles of the old structure must be compatible with the new structure.
Possibilities for Reusing Existing Piles
Figure-1 below illustrates the different ways in which an existing pile can be reused.
1. Complete Reuse of Existing Piles
As shown in Figure-1, the first option implies the complete use of the existing pile. This can be adopted when the load of the new building is approximately the same as that of the existing building. The option saves costs in terms of design and new foundation construction.
2. Partial Reuse of Existing Piles
In this case, the existing pile can carry only a part of the new load. Here, the designer decides the load distribution for the new and existing piles. The load-carrying role is determined based on the quality of the existing piles.
3. Replacement of Existing Piles
In this option, the existing piles are re-drilled and re-concreted to form new piles. This option is technically difficult and demands a huge capital.
4. Remove and Replace
This is the easiest of all possibilities, where the old pile is completely replaced by a new pile. It is undertaken when there is a collision between the location of the new and the old piles, or when the old pile is inferior in quality.
Reuse of Piles Using NDT
Generally, an existing pile or foundation is covered either using a cap or the superstructure. In some situations, the foundation is covered within the water for the underpass structure. Both the cases make the inspection and the evaluation process very challenging. Hence, certain expert solutions must be followed to determine the length of the pile and its integrity.
Non-destructive testing (NDT) is one such solution to evaluate the length and the integrity of the existing piles. However, a single NDT method cannot determine all the required parameters, therefore a combination of NDT methods must be adopted in this evaluation. The test methods are selected based on the following criteria:
- Material and the type of foundation
- Structural system of the foundation
- Condition of the pile foundation
- Surrounding area of the pile
- Pile configuration as per the layout
- Groundwater level
- Geometry of the foundation
Investigation of Existing Piles
To guarantee the quality of the existing pile foundation, several kinds of tests are performed, which includes:
- Sonic integrity test
- Carbonation test of concrete
- Compressive test of concrete
- Tensile test of reinforced bars
The sonic integrity testing is a low-strain method used to investigate the soundness and the length of the piles. This test determines the defects present in the concrete piles.
In the concrete carbonation test, a mixture of 1% phenolphthalein and 99% ethyl alcohol is sprayed on the pile's top surface. The portions that turn red in color are free from carbonation and can be reused. If it is possible to treat the area subjected to carbonation, then the pile can be reused.
Core specimens are taken to determine the compressive strength of the existing concrete piles. Tensile strength test is conducted on reinforcement bars cut from the top part of the piles.
The reuse of existing piles reduces the volume of newly constructed piles, thus decreasing the occurrence of sludge from the construction site.
There are mainly four possibilities to reuse an existing pile foundation. They are:
1. Complete reuse of existing pile
2. Partial reuse of existing pile
3. Replacement of existing piles
4. Remove and replace of existing piles
An existing pile foundation can be investigated for reuse by the following tests:
1. Sonic integrity test
2. Carbonation test on concrete
3. Compressive test on concrete
4. Tensile test on concrete