Valley of Fire, Nevada (07/2019)
My research interests lie at the intersection of geotechnical engineering, subsurface imaging, and geostatistics. At the core of my research work is bringing innovative and practical subsurface imaging tools to applications in natural hazards, and advancing these tools to the broad benefit of a more resilient and globally connected society.
Multidimensional Site Response Using a Large-Scale, Site-Specific Subsurface Model
Over the past decade, numerous studies have examined ground motions recorded at borehole array sites and found that, on average, more than 50% of sites are poorly modeled using 1D ground response analyses (GRAs). While 3D GRAs could be used to overcome limitations associated with 1D GRAs, their daunting computational costs and the lack of site-specific 3D subsurface models are impediments to improved modeling of site effects in engineering practice.
In this project, we address both of these challenges by utilizing: (1) a new framework called the ‘H/V geostatistical approach’ to develop large-scale, site-specific, 3D Vs models, and (2) a new, open-source, finite element software called ‘Seismo-VLAB’ (SVL) to perform large-scale 2D and 3D finite element analyses. The investigations are performed at the Treasure Island Downhole Array (TIDA).
By combining a time- and cost-effective 3D site characterization framework with an open-source 3D numerical analysis software, we aim to provide more insights on the influence of complex subsurface conditions on site effects and to enable robust 3D seismic GRAs to be performed by a broad community of academic researchers and industry design engineers.
State-of-the-Art in Modeling Aleatory Variability in 1D Site Response
A significant amount of seismic site response research over the past decade has focused on our abilities to replicate recorded ground motions at vertical seismometer arrays. By compiling results from a total of 662 vertical seismometer array sites in the U.S., Japan, and elsewhere globally, we found that 50% are poorly modeled using 1D ground response analyses (GRAs).
While there is no doubt that some sites are indeed too variable to be modeled using 1D GRAs, it is possible that this simple approach could still be effectively used at many sites if spatial variability is incorporated via a rational, site-specific approach. In this study, we investigate 5 alternative approaches that can be used to account for spatial variability in 1D GRAs:
(1) Vs randomization, (2) shear wave travel time randomization, (3) utilization of Vs suites obtained from surface wave testing, (4) incorporation of a pseudo-3D Vs model derived from a horizontal-to-vertical spectral ratio geostatistical approach, and (5) damping modifications.
These approaches are investigated at the Treasure Island and Delaney Park Downhole Arrays. Through qualitative and quantitative comparisons, we assess the effectiveness of these approaches, highlight their limitations, and propose practical improvements to help overcome these limitations in practice.
An H/V Geostatistical Approach for Building Site-Specific 3D Subsurface Models
A growing body of literature has suggested that 3D site response provides a more satisfactory fit to recorded ground motions at several downhole array sites. However, there is rarely a 3D subsurface model available for these analyses. The lack of affordable and reliable 3D site characterization methods is an impediment to future progress in accurate site response modeling. Indeed, even at our most valued downhole array sites in the United States, Japan, and elsewhere globally, we are generally limited to 1D representations of Vs.
This project describes a geostatistical approach that can be used for building pseudo-3D Vs models as a means to rationally account for spatial variability in site response, increase model accuracy, and reduce uncertainty. Importantly, it requires only a single measured Vs profile and a number of simple, cost-effective, horizontal-to-vertical spectral ratio (H/V) noise measurements.
This approach is demonstrated at the Treasure Island and Delaney Park Downhole Array sites. While the pseudo-3D Vs models can be used to perform 1D, 2D, or 3D site response, we only investigate their implementation in 1D analyses in this study. This 1D application is shown to produce superior fits to the recorded site response at both case study sites. Using the proposed approach, we also investigate the lateral area that is likely influencing site response and show that it could extend to significant distances (as much as 1 km) from the boreholes.
Improved Implementation of Travel Time Randomization for Modeling Vs Uncertainty
In-Situ Vs and Damping Inversions at Downhole Arrays
Estimating Unbiased Statistics for f0 Using Spatially Distributed HVSR Measurements
PUBLICATIONS AND PRESENTATIONS
[T10] Hallal, M.M. “A data-driven geostatistical approach to account for subsurface spatial variability in seismic site response,” presented to the Geo-Institute Graduate Student Organization at North Carolina State University, July 22, 2022.
[T9] Hallal, M.M., Ayoubi, P., Asimaki, D., Cox, B.R. “Modeling two-dimensional site effects at the Treasure Island Downhole Array,” presented at the 4th International Conference on Performance-Based Design in Earthquake Geotechnical Engineering (PBD-IV), Beijing, China, July 15-17, 2022.
[T8] Hallal, M.M., Cox, B.R. “Modeling 2D and 3D site effects at the Treasure Island Downhole Array using Seismo-VLAB and a site-specific 3D Vs model,” presented at the 12th National Conference on Earthquake Engineering (12NCEE), Salt Lake City, UT, June 27-July 1, 2022.
[T6] Hallal, M.M., Cox, B.R. “Comparison of different methods used to account for shear wave velocity variability in 1D ground response analyses,” presented at the International Foundation Congress and Equipment Expo (IFCEE), Dallas, TX, May 10-14, 2021.
[T2] Hallal, M.M., Cox, B.R. “Theoretical evaluation of the interval method commonly used for downhole seismic testing,” presented at the ASCE Geo-Congress 2019: Eighth International Conference on Case Histories in Geotechnical Engineering, Philadelphia, PA, March 24-27, 2019.
[T1] Hallal, M.M., Ramadan, R., Itani, A., Jammal, N., Zein, S. “Sustainable earth structures for displaced populations,” presented at the Sixteenth FEA Student and Alumni Conference (FEASAC), Beirut, Lebanon, April 19-21, 2017.
[R1] Cox, B.R., Stolte, A.C., Hallal, M.M. (2017). “Seismic wave velocity profiling using direct push crosshole measurements: Metro Sports Centre Rammed Aggregate Pier Trial Christchurch, New Zealand,” Geotechnical Engineering Report GR17-17, University of Texas at Austin, October 2017.