The Parallel Seismic (PS) method is a borehole test method primarily used to determine depths of foundations. However, it can also be used to measure the thickness of the scour zone when it has been filled with mud or soft sand after the flood surge has passed.

Parallel Seismic tests can be performed on concrete, wood, masonry, and steel foundations. Some portion of the structure that is connected to the foundation must be exposed for the hammer impacts. A borehole is also required. Typically, a 5- to 10-cm-diameter hole is drilled as close as possible to the foundation (within 150 cm preferred). The borehole should extend at least 3 to 4 m below the expected bottom of the foundation. In case of hydrophone use, the hole must be cased, capped at the bottom, and the casing and hole filled with water. For geophone use, the hole must usually be cased and grouted to prevent the soil from caving in during testing.

Basic Concept: The PS method involves striking any part of the exposed structure that is connected to the foundation (or hitting the foundation itself, if accessible) and using a hydrophone or a three-component geophone to record compressional and/or shear waves traveling down the foundation. Analysis of the PS data is performed in the time domain. In PS tests, reliance is placed on identifying direct arrival times of compressional and shear waves at the receiver locations, as well as the wave amplitudes. The PS tests are performed at 30- to 60-cm vertical receiver intervals in a nearby borehole. Figure 50 shows the shaft and borehole configuration used for the test. Plotting the first arrival times as a function of depth and observing the depth where a change of slope occurs shows the depth to the bottom of the shaft and the scour depth. In addition, the foundation depth can be obtained by observing the depth where the signal amplitude of the first arrival energy is significantly reduced.

Figure 50. Parallel Seismic setup.

Data Acquisition: In a Parallel Seismic test, a hammer strikes the structure, and the response of the foundation is monitored by a hydrophone or a geophone receiver placed in the borehole. The hammer input and the receiver output are recorded by a signal analyzer and stored for further analysis. The receiver is first lowered to the bottom of the hole, and a measurement is taken. Then, the receiver is moved up 0.5 or 1 m, and the second measurement is made. This process is continued until the receiver has reached the top of the boring.

Figure 51. No scour case. Data not filtered. Note the uniform data amplitudes across the water-sediment interface. The linear refraction first-arrival pattern A-B changes to the hyperbolic first-break pattern C-D at C, which occurs at the base of the pier. (Mercado, E.J. MacDonald Geophysics 2002)

The following is an experimental example of the use of the parallel seismic method applied to a replica of a bridge with two cylindrical piers in a water-filled pond and shows the usefulness of the method along with some of the recorded data.

Two 60-cm-diameter, 5-m-long model piers were constructed. The depth of the pit before excavating the scour zone was 1.5 m. Adjacent to Pier 1 (60 cm away) a 10-cm-diameter, 8-m-deep hole was drilled and lined with PVC pipe. A refraction wave was generated in the pier by striking the top of the pier with a hammer. The refraction wave was recorded at 30-cm intervals in the PVC pipe. The pit was emptied, and a 1.2-m-deep scour zone was hand-dug around the pier and filled with soft mud created by liquefaction of the trench sides that slumped into the trench. This placed the bottom of the scour zone at a depth of 2.7 m. The refraction experiment was repeated, and the data were analyzed.

Figure 52. Scour case. Data not filtered. Note strong attenuation of data amplitudes where energy traverses the mud-filled scour zone. (Mercado, E.J. MacDonald Geophysics 2002)

Data Processing and Interpretation: Figure 51 shows the observed data from a Pier 1 test where the soft mud had been removed from the trench; thus, the water-soil interface is water over stiff competent clay. The seismic data show strong, continuous energy transmission across this interface, and the first breaks fall along the straight line A-B, corresponding to a refraction velocity of 4,730 m/s, which represents the P-wave velocity in the concrete pier. The acoustic characteristics of the mud are its strong energy attenuation as a function of frequency. The high frequencies are more strongly attenuated than the low frequencies, and much stronger than the attenuation characteristics of either water or competent soil.

Figure 52 shows the seismic data under the condition that the scour zone has been partially filled with soft mud that has slumped in from the sides of the trench. The anomalously low amplitude of the seismic data at recording depths 1.8 through 2.7 m is a consequence of the strong energy-attenuating nature of the mud compared to water above and competent soil below the mud-filled scour hole.

To take further advantage of this characteristic, the data were digitally filtered with a strong low-cut filter. The filtered data are shown in figure 53, the no-scour case, and figure 54, the scour case. The no-scour case, figure 53, shows uniform data amplitudes across the water-bottom interface. Figure 54, the scour case, shows virtually no high frequency in the mud-filled depth range 1.8 to 2.7 m.

Figure 53. No scour case. Seismic data after digital filtering with a strong low-cut filter. Note the uniform amplitudes across the water-sediment interface. (Mercado, E.J. MacDonald Geophysics 2002)

Figure 54. Scour case: Seismic data after digital filtering with a strong low-cut filter. Note the severe energy attenuation of data transmitted through the mud filled scour zone. (Mercado, E.J. MacDonald Geophysics 2002)

Advantages: The method can be used when the foundation tops are not available or when the piles are too long and slender (such as H piles or driven piles) to be tested by sonic echo techniques.

Limitations: Probably the biggest limitation is that a drill hole is needed close to the pier to measure the seismic travel times along the structure