MASW (Multichannel of Analysis of Surface Waves) Measurements on Very Dense Fill of Freeport Mining

The MASW (Multi-Channel Analysis of Surface Waves) was conducted on fill soil at Tembagapura Freeport Mining. The fill was very dense and is the landfills where people dumped waste disposal like car tire rubber etc. Several attempts were made to make boreholes to collect soil samples and to run SPT (Standard Penetration Test), but they failed. The fill was so dense that the employed drill bits were worn out when penetrating the soil. The MASW team was invited to carry out the measurements and to produce shear wave velocities of the layers, which correspond to SPT Number. The number of measured MASW points was 8 points. They were successfully carried out on the surface of the fill without borings. The offset distance between the hammer blows, and the first geophone was 18 m, and the interval distance between 4.5 Hz geophones was 3 m. The MASW measurements resulted in an average shear velocity of 400 m/s which confirmed the soil was very dense soil. The information regarding the shear wave velocities of the layers was adopted as input of earthquake engineering software to predict the amplification when the earthquake happens.


Background
Freeport Mining Management planned to build a building on a very dense fill in Tembagapura.They tried to drill the fill several times to find it very dense and worn out the drill bit.So, the MASW method was conducted on the site.The MASW explores the soil shear strength by measuring the ground's shear wave velocity profile [1][2] [3].Shear-wave velocity (Vs) is one of the elastic parameters that is very important and corresponds to the soil stiffness, unlike Pressure wave (P wave), which can propagate through water and depends on the water content of the soil [4].One can derive the Young's and shear moduli from the shear wave velocity information.One can utilize the shear wave velocities to derive the load-bearing capacity of the soils and acquire them without needing a borehole [5][6] [7][8] [9] [10].

Methodology
The MASW method utilizes impulse energy (sledgehammer blow, falling weight drop, etc.) (e.g., [11][12][13][14] [16]).The philosophy behind this technique is that impulse signal contains many frequencies, such as a pair of Fourier transformations between time domain t and frequency f as follows: In the field, one generates impulse energy and, according to Fourier theory, propagates many surface and body waves through the soil.About 2/3 of the energy propagates from the loaded point as Rayleigh surface waves.The rest of the energy propagates as the body waves (P and S) [17][18].p ]. Depending on the wavelength, the surface waves penetrate the soil.Small wavelengths will penetrate shallow, and large wavelengths will penetrate deeper.Due to material attenuation and frequency range of signals, the surface waves from the MASW survey usually penetrate the shallow depth (to a few tens meters).One can construct the dispersion curve, the plot between the surface wave velocity and the frequency (wavelength).The small wavelength (high-frequency surface wave) would propagate with the velocity mostly of upper soils.In contrast, the large wavelength (low-frequency surface wave) would propagate with the velocity of deeper soils.
Finally, one employs commercial software to invert the dispersion curve to develop the shear wave velocity profile of soils.This paper employs the Seisimager software to obtain soils' shear wave velocity profile.

Data acquisition
The map of MASW location is presented as follows:  We set up the line of 4.5 Hz geophones (24).The lines are made straight.Then, we generate a hammer blow at a near offset of 18 m (the distance between the blow and the first geophone).And we recorded it a couple of times.At every point, we applied the stacking technique: sum the individual shot recording until a certain number, and finally, we recorded.The coherent signals would add up, whereas the random noise did not.And so we obtain the high signal-to-noise ratio.
The following is the configuration of data acquisition in the field [19]: The distance between the hammer blow the last geophone is called the far offset, whereas the distance between the hammer blow and the first offset is called the near offset.

Results and Discussion
Figure 6 shows the raw data in the time and geophone distance axes are Fast Fourier Transformed 2D to generate frequency and wavelength (wavenumber).The wavelength is converted to phase velocity = v= /k where k is the wave number =2 / , and is the wavelength; is the angle frequency.So, we can pick the dispersion curve from the spectrum 2D image in Figure 7 by picking the large amplitude (blue color or yellowish color) as the arrival of surface wave (Rayleigh wave).
In picking the spectrum of FFT 2D in Figure 6, there are two line of restrictions: 1.The small slope, that is the wavelength = = the near offset.
2. The large slope, that is the wavelength = = the far offset   From the shear wave velocity profiles data, one can compute the average shear wave velocity vs using the relationship: where is the thickness of soil layer and is the shear wave velocity of the layer.Table 1 outlines the SNI's shear wave velocity classification averages, while Table 2 presents the USGS's classifications based on average shear wave velocities.Using these references, Table 3 categorizes the soil layers accordingly.The measurements yielded an average shear velocity of 400 m/s, validating the soil's high density.This data on shear wave velocities within the layers could serve as input for earthquake engineering software, aiding in the prediction of amplification during seismic events.Note: is shear wave velocity; N is Standard Penetration Resistance (ASTM D1586-84), not to exceed 100 blows/ft as directly measured in the field without corrections; is undrained shear strength, not to exceed 250 kPa (ASTM D2166-91 or D2850-87)., , and are average values for the respective parameters for the top 100 feet of the site profile.Refer to FEMA 302 for the procedure to obtain average values for , , and .

Conclusion
The MASW tests were carried out effectively at eight locations in Tembagapura Freeport Mining Indonesia.Unfortunately, seismic downhole data collection wasn't feasible due to the densely packed conditions.Consequently, the MASW technique was used as a substitute for the unavailable downhole data.The measurements indicated an average shear velocity of 400 m/s, affirming the area's soil density.These findings could serve as crucial input for earthquake engineering software to anticipate amplification during seismic events.

Figure 1 .
Figure 1.Pair of Fourier Transform, which explains the frequency content of impulse energy.

Figure 2 .Figure 3 .
Figure 2. The location of MASW points in Tembagapure Freeport Mining

Figure 4 .
Figure 4.The land streamer in case the soil (or the pavement) is too hard to penetrate with a nail.

Figure 5 .
Figure 5.The configuration of data acquisition in the field.

Figure 6 .
Figure 6.Signals from the first to the last geophones at BH-01.The features at BH-01 are representative to all the boreholes.

Figure 7 .
Figure 7. Image of Spectrum FFT 2D of the raw data at BH-01

Table 1 .
Indonesian National Standard (SNI) for site classification.

Table 3 .
Soil classification based on SNI and USGS system.