Enhancing Site Characterization with MASW

The Multichannel Analysis of Surface Waves (MASW) method is a geophysical technique widely used for site characterization. It provides crucial insights into the subsurface properties by analyzing the propagation of seismic surface waves. MASW is particularly valued for its efficiency, non-invasive nature, and ability to deliver high-resolution results. Below is an outline of its importance in site characterization and its role in determining primary (P) and secondary (S) wave properties:

1. Site Characterization

MASW plays a critical role in understanding the subsurface conditions of a site by providing detailed information about:

• Shear Wave Velocity (Vs) Profile: Shear wave velocity is a direct indicator of soil stiffness, essential for seismic site response analysis and foundation design. MASW derives the Vs profile by inverting the dispersion curve of surface waves.
• Layer Stratigraphy: The method effectively identifies subsurface layers, including their thickness and velocity contrasts. This helps distinguish between different soil and rock types.
• Soil and Rock Stiffness Parameters: Parameters like Young's modulus and Poisson’s ratio, which are derived from Vs, are used in structural and geotechnical engineering applications.
• Seismic Site Classification: MASW helps classify sites based on Vs30 (average Vs of the top 30 meters), an important parameter in seismic hazard analysis and building codes.

2. Determination of P- and S-Wave Properties

MASW focuses primarily on surface waves to derive S-wave properties, but it indirectly supports the evaluation of P-wave properties through complementary methods.

S-Wave Properties

• Vs Profiling: MASW directly determines the shear wave velocity, a critical parameter in: Evaluating seismic site response. Assessing liquefaction potential. Calculating dynamic soil properties.
• Seismic Hazard Assessment: Shear waves significantly influence ground motion behavior, making their analysis essential in understanding site-specific earthquake response.

P-Wave Properties

• Though MASW does not directly measure P-wave velocity (Vp), it can be inferred using supplementary methods or by combining Vs data with density and Poisson’s ratio.
• Vp provides insights into: Compressibility and fluid saturation of subsurface layers. Aquifer or hydrocarbon exploration.

Advantages of MASW for Site Characterization

1.????? Non-invasive: Minimal disruption to the site.

2.????? Cost-effective: Lower cost compared to invasive techniques like boreholes.

3.????? Depth Versatility: Can investigate shallow layers (few meters) to deeper strata (tens of meters).

4.????? High Resolution: Provides a detailed Vs profile that enhances the understanding of subsurface variability.

MASW is an indispensable tool for geotechnical engineers, seismologists, and environmental scientists. By characterizing the subsurface and elucidating P- and S-wave properties, it lays the foundation for safe, efficient, and effective design in construction and hazard mitigation.

CASE STUDY

The following case study highlights how the MASW test provided more accurate data for site characterization and design parameters in a recently completed project. The client engaged us for a third-party review of the geotechnical report and to conduct FEM analysis for the raft foundation of a 40-story building.

According to the initial geotechnical investigation, the site was characterized by refusal strata and highly weathered rock. To validate the stratification and derive the modulus values—key parameters for evaluating settlement in Plaxis 3D—the MASW test was conducted on-site. This approach significantly enhanced the accuracy of the input data for the analysis.

The test was performed on the foundation level. Based on the velocity profile, the site was classified as “Class C,” indicating a stratigraphy of very dense soil and soft rock. Modulus values were derived from primary shear wave velocities (Vs) and used as input for settlement evaluation in the Plaxis analysis.

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Sarathy Geotech & Engineering Services Pvt Ltd. , Minnu Sanoj

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