Soil Sleuthing: What Geotechnical Investigations Can Reveal
First in a series of blogs on the science and engineering involved in the Malibu Creek Ecosystem Restoration Project is on Geotechnical Surveys
Joe Goldstein, PE, Senior Engineer at Geosyntec in Los Angeles, California provided insight on an important phase in the planning and engineering of removing Rindge Dam.
What is a Geotechnical Investigation?
A geotechnical investigation involves engineers and geologists evaluating the subsurface conditions of a site. This process occurs when planning construction or infrastructure development. It is crucial for comprehending the geological, hydrological, and soil properties of the site. These properties will play a key role in the planning, design, and eventual deconstruction phases in the removal of Rindge Dam.
How are they done?
In a geotechnical investigation, engineers and geologists use different methods to collect information about the underground condition of the site. These methods include drilling boreholes, gathering soil samples, conducting geophysical surveys, and analyzing already available geological data. The collected data is then used to evaluate factors such as soil stability, bearing capacity, groundwater levels, and potential hazards such as landslides.
Why are they necessary?
The results of a geotechnical investigation play a vital role in guiding the design and construction phases. They ensure that necessary actions are taken to reduce any risks associated with the geological conditions of the site. Recommendations may include implementing slope stabilization measures, installing drainage systems, or employing other engineering solutions customized to the site's individual features. Ultimately, a comprehensive geotechnical investigation serves to minimize uncertainties and enhance the safety and efficiency of the project overall.
In 2002, the U.S. Army Corps of Engineers conducted a feasibility study to determine if Rindge Dam could be removed and would be beneficial to the ecosystem. The study included drilling several borings into the impounded sediment as part of geotechnical investigations. (Reference: Malibu Creek Ecosystem Restoration Study: Appendix D - Geotechnical Engineering. U.S. Army Corps of Engineers, November 2020. Pg D3-2.)
What is being investigated for Rindge Dam?
In addition to the removal of the dam itself, the sediment impounded by the dam will also be removed. The sediment deposits in the reservoir are being considered for beneficial reuse at local nearshore locations and/or via direct beach placement for beach nourishment and for enhancement of the resiliency of the nearby coastline. The methods of removal, transport, and the permissible disposal locations for the impounded sediments depend on the physical characteristics of the impounded sediments. These characteristics can be obtained by conducting a geotechnical investigation of the impounded sediments.
What challenges are there in geotechnical investigations?
Investigations can be challenging because of logistics and dam location for acquiring the sediment samples. We also want to minimize environmental impact with the process, studying how best to get the samples with minimal vegetation damage and habitat impacts. Due to the steep terrain along Malibu Canyon, we’ll be using helicopters to air drop the drilling equipment to the sampling sites.
As with the US Army Corp of Engineers with the feasibility studies, helicopters will aid in the upcoming geotechnical surveys, dropping the drill rigs and equipment onto the impounded surface by helicopter, assembled the drill rigs, drilled a hole, then repeated the process at the next drill location. (Reference: Malibu Creek Ecosystem Restoration Study: Appendix D - Geotechnical Engineering. U.S. Army Corps of Engineers, November 2020. Pg D3-2.)