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Parallel-seismic tests are non-destructive surface techniques used to test the quality of pile foundations. It is used to determine the length and integrity of pile foundations that are not accessible (too long and slender or buried below a pile cap).
Compared to other non-destructive surface techniques, the parallel seismic method is more versatile for determining foundation depths.
This article discusses the working and features of the parallel seismic test method in construction.
Working of Parallel Seismic (PS) Method
In a parallel seismic system, a borehole is drilled adjacent to the foundation that needs to be tested. The borehole is cased with PVC, filled with water, and an array of hydrophones (receivers).
Image Courtesy: Everest Geophysics
Now, seismic waves are generated by hitting the structure using a hammer. These compressional or shear waves that travel down the foundation are recorded by the hydrophone receiver inside the borehole. As shown in Figure-1, hydrophones are closely spaced at every one meter.
The generated waves travel downwards along the structure at a higher velocity until they reach the foundation base. Beyond the foundation base, i.e., when traveling through the soil, the transit time increases due to the lower velocity of waves in the soil.
The test relies on determining the direct arrival time of these compressional or shear waves at the receiver stations (hydrophone). The point at which the velocity decreases rapidly (travel time ‘t’ increases) is interpreted as the depth of the structure (inflection point in Figure-1). The depth-time graph obtained determines the depth of the foundation tested.
Similarly, if there is any discontinuity or inclusion in the foundation, the signals or waves detour around it, increasing the path length and the transit time. This would show a deviation in the straight line in the depth-time graph obtained from the parallel-seismic test.
Figure-2 shows an example of results from the parallel-seismic test. The change in slope of the line representing the arrival time of stress pulse as a function of depth indicates the depth of the pile shaft.
Features of Parallel Seismic Test Method
- The borehole drilled for the test has a diameter of 150 to 200mm. The borehole is drilled parallel and close to the structure near to foundation or directly near the foundation. It is drilled within 1.5 m of the foundation.
- As the borehole moves away from the foundation, the interpretation of the PS test data becomes more difficult.
- Parallel seismic testing is relatively fast for testing foundations under existing structures.
- PS testing can be applied on foundations placed on damp soil.
- It is applicable for deep foundations like sheet piles, abutment piers, etc.
- The test determines the length of the pile, type of foundation, and scours in the foundation.
FAQs
Parallel-seismic tests are non-destructive surface techniques used to test the quality of pile foundations.
The borehole drilled for the test has a diameter of 150 to 200mm. The borehole is drilled parallel and close to the structure near to foundation or directly near the foundation. It is drilled within 1.5 m of the foundation.
The main advantages of parallel seismic testing are:
1. Parallel seismic testing is relatively fast for testing foundations under existing structures.
2. PS testing can be applied on foundations placed on damp soil.
3. It is applicable for deep foundations like sheet piles, abutment piers, etc.
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