**8. Application of wave propagation methods for ground monitoring**

In the past 20 years, the use of wave propagation methods (e.g. seismic. etc.) seismic reflection and refraction, seismic down-hole, up-hole and cross-hole etc. for ground monitoring has been promoted intensively because it is non-destructive and is conducted under in situ condition. The wave propagation methods are commonly used to determine stiffness-depth profile. Typically, the wave propagation methods can be classified into subsurface and near-surface methods as schematised in Fig. 21.

The subsurface methods including seismic down-hole, seismic up-hole, and seismic crosshole are normally employed to monitor ground stiffness when the depth of interest is greater than 15 meters (Hooker 1998). In these methods, one or more boreholes are usually required in order that the source and/or the receiver can be installed. While the seismic cone penetration test can provide both stiffness and strength properties of the geomaterials during its penetration. The disadvantage of the subsurface method is that the cost of measurement is relatively high due to the cost of borehole casting. An alternative method is the near-surface method which provides simpler and more cost-effective approaches. The near-surface method is performed on the basis of the ability of wave propagation through the ground strata. When waves propagate through soil layers having different properties, they refract and/or reflect at different time. Once the arrival time is known, wave velocities and stiffness of each layer can be determined.

With the near-surface method, the Spectral Analysis of Surface Waves (SASW) using surface waves is another technique that can monitor both ground stiffness at shallow depth and layer thicknesses of subsurface profiles. The surface wave method utilises the dispersive characteristic of Rayleigh waves, which are elastic waves that propagate along the ground surface. Surface (Rayleigh) wave velocity varied with frequency is measured by utilizing the dispersion characteristics of surface wave and the fact that surface waves propagate to

depths that are proportional to their wavelengths or frequencies in order to determine the stiffness of subsurface profiles. The objective in SASW testing is to make field measurements of surface wave dispersion (i.e., measurements of surface wave velocity at various wavelengths) and to determine the shear wave velocities of the layers in the profile based on the fact that surface waves propagate to depths that are proportional to their wavelengths (Mayne et al. 2001).

Wave Propagation Methods for Determining Stiffness of Geomaterials 187

Power Amplifier

Horizontal (capillary) tube with scale

**Signal Generator** LabVIEW 7 Express®

Personal Computer (PC)

D: High air-entry ceramic disk

B: S-wave receiver A: S-wave transmittter C: Specimen

E: Porous disk

the same specimen without disturbance. Fig. 23 illustrates shear wave time series during

Input signal

Output signal

**Digital Oscilloscope** PICO ADC200

Voltage divider

**Figure 22.** Schematic diagram of Go test using bender elements under the suction and moisture

pressure (uw)

D

E

"Y" tube

(kPa) o-ua = 34.5 kPa CL-1-Std-Opt

**Figure 23.** Shear wave time series during desaturation of lean clay specimen compacted near optimum

1 05 <sup>6</sup>

0 200 400 600 1000 microsecond

1 06 <sup>2</sup>

1 05 <sup>8</sup>

1 05 <sup>1</sup>

800

1 05 <sup>4</sup>

a <sup>0</sup> <sup>b</sup> <sup>0</sup> <sup>c</sup> <sup>0</sup> <sup>d</sup> <sup>0</sup> <sup>e</sup> <sup>0</sup> <sup>f</sup> <sup>0</sup> <sup>g</sup> <sup>0</sup> <sup>h</sup> <sup>0</sup> <sup>i</sup> <sup>0</sup> <sup>j</sup> <sup>0</sup>

0 2

1 05

1.8 1 05

ua-uw = 621.2

1.6 1 05

ua-uw = 414.4

1.4 1 05

ua-uw = 276.5

1.2 1 05

ua-uw = 172.4

1 1 05

ua-uw = 103.4

8 1 04

ua-uw = 56.5

6 1 04

ua-uw = 34.5

ua-uw = 21.4

4 1 04

a 1 b <sup>1</sup> <sup>l</sup> c <sup>1</sup> 2l d <sup>1</sup> 3l e <sup>1</sup> 4l f <sup>1</sup> 5l g <sup>1</sup> 6l h <sup>1</sup> 7l i <sup>1</sup> 8l j <sup>1</sup> 9l

2 1 04

ua-uw = 6.9

ua-uw = 0

Matric suction

2 1 04

0

desaturation of lean clay specimen compacted near optimum.

variation (Sawangsuriya et al. 2009b).

Confining pressure ()

Pore air

pressure (ua) Pore water

A B <sup>C</sup>

Isotropic stress

Triaxial cell

Air supply

(Sawangsuriya et al. 2009b).

**Figure 21.** Wave propagation methods in ground monitoring (Sawangsuriya et al. 2008c).

The seismic methods have been employed to monitor ground improvement such as dynamic compaction and vibrofloatation methods (Hooker 1998). The measurement was made at the site before and after the ground improvement to establish stiffness profile in order that the performance of ground improvement can be monitored. Moreover, in some ground treatments e.g. grouted backfill and stone column, the near-surface wave propagating method can be employed to monitor their performance (Cuellar 1997).

Since ground moisture is sensitive to rise in water table, infiltration, and evaporation. Changes in ground moisture and hence in its modulus can occur over the service life of an engineered structure irrespective of the initial moisture conditions imposed during construction. The variation in ground modulus with moisture should be monitored systematically to reflect mechanical behavior of compacted engineered structures after construction (i.e., during postcompaction state). The influences of ground moisture should be also considered in performance assessment in such a way that the anticipated in-service conditions are taken into account. Sawangsuriya et al. (2009b) developed a laboratory method for identifying and examining the variation of modulus with moisture and suction in order to improve the ground performance assessment. Their study evaluated the Go of compacted specimens using bender elements under the suction and moisture variation in the laboratory as shown in Fig. 22. Such method allows monitoring the modulus change with respect to the suction and moisture on the same specimen without disturbance. Fig. 23 illustrates shear wave time series during desaturation of lean clay specimen compacted near optimum.

186 Wave Processes in Classical and New Solids

(Mayne et al. 2001).

depths that are proportional to their wavelengths or frequencies in order to determine the stiffness of subsurface profiles. The objective in SASW testing is to make field measurements of surface wave dispersion (i.e., measurements of surface wave velocity at various wavelengths) and to determine the shear wave velocities of the layers in the profile based on the fact that surface waves propagate to depths that are proportional to their wavelengths

**Figure 21.** Wave propagation methods in ground monitoring (Sawangsuriya et al. 2008c).

propagating method can be employed to monitor their performance (Cuellar 1997).

The seismic methods have been employed to monitor ground improvement such as dynamic compaction and vibrofloatation methods (Hooker 1998). The measurement was made at the site before and after the ground improvement to establish stiffness profile in order that the performance of ground improvement can be monitored. Moreover, in some ground treatments e.g. grouted backfill and stone column, the near-surface wave

Since ground moisture is sensitive to rise in water table, infiltration, and evaporation. Changes in ground moisture and hence in its modulus can occur over the service life of an engineered structure irrespective of the initial moisture conditions imposed during construction. The variation in ground modulus with moisture should be monitored systematically to reflect mechanical behavior of compacted engineered structures after construction (i.e., during postcompaction state). The influences of ground moisture should be also considered in performance assessment in such a way that the anticipated in-service conditions are taken into account. Sawangsuriya et al. (2009b) developed a laboratory method for identifying and examining the variation of modulus with moisture and suction in order to improve the ground performance assessment. Their study evaluated the Go of compacted specimens using bender elements under the suction and moisture variation in the laboratory as shown in Fig. 22. Such method allows monitoring the modulus change with respect to the suction and moisture on

**Figure 22.** Schematic diagram of Go test using bender elements under the suction and moisture variation (Sawangsuriya et al. 2009b).

**Figure 23.** Shear wave time series during desaturation of lean clay specimen compacted near optimum (Sawangsuriya et al. 2009b).
