The Parallel Seismic (PS) test consists of impacting exposed foundation substructure either vertically or horizontally with an impulse hammer to generate compression or flexural waves which travel down the foundation and are transmitted into the surrounding soil as shown in Fig. 36. The refracted compression (or shear) wave arrival is tracked at regular intervals by a hydrophone receiver suspended in a water-filled cased borehole (original PS procedure) or by a clamped three-component geophone receiver (new procedure-better for shear wave arrivals) in a cased or uncased borehole (if it stands open without caving). The depth of a foundation is typically indicated by a weaker and slower signal arrival below the tip of the foundation. Diffraction of wave energy from the foundation bottom was also found to be indicative of its depth in PS tests as well. The PS test was found to the most accurate and widely applicable NDE method for determination of unknown bridge foundation depths of all tested NDE methods in the NCHRP research referenced above for the US method (Olson and Aouad, 2000). The PS method is also discussed in ACI 228.2R-98 and was originated by Paquet of CEBTP in Paris, France.

The main objective of Parallel Seismic tests is to determine the depth of the unknown foundations. Based on the NCHRP 21-5 and 21-5(2) research results, several criteria were established for determining the foundation depths based on Parallel Seismic data as follows:

1. Breaks in the slope of the lines in a plot of depth versus recorded time

2. Drop in energy amplitude below the bottom of the foundation, and

3. Diffraction of wave energy at the bottom of the foundation.

Examination of Figure 37 shows an example PS result for the case where subsurface conditions are uniform with depth (this usually means saturated soil conditions where the compression wave velocity is that of water, i.e. about 1500 m/s or 4900 ft/s). This allows one to determine the velocity of the foundation element, and to clearly see the foundation bottom as the point where the wave velocity is slower and the amplitude is weaker in the soil below the bottom of this timber pile. The foundation bottom is then taken as the intersection of the foundation velocity line with the soil velocity line as shown in Fig. 36 y the arrow pointing to a depth of 22 ft. The vertical axis in Figure 36 represent depth below the top of the casing. The horizontal axis represents recorded time in milliseconds (1 mSec = 0.001 seconds).

Şekil 3.1 : Paralel sismik deney yöntemi ve ekipmanı

Şekil 3.2 : PS Result showing pile tip.