This week contractors began field work in the reservoir upsteam of Matilija Dam. A barge-mounted drilling rig is being used to sample the sediments trapped by the dam to update the work that was completed as part of the Corps of Engineers Feasibility Study. Additional core samples will be collected from the dam itself, in order to assess the current state of the concrete and provide data for the structural analysis of the dam removal project. These field studies are the first tasks in the Matilija Dam Removal 65% Planning Design Project funded by the $3.3M California Department of Fish and Wildlife Proposition 1 Grant received in May 2017.
excerpts from the contracted scope of work:
Subtask 1.1 - Geotechnical Field Investigations to Characterize Fine Sediment and Organics
...obtain and log sediment samples at six locations to an estimated depth of 60-90 feet, and perform
laboratory testing. Sampling and testing shall be performed to screen for the presence of contaminants such as heavy metals and pesticides, and the results shall be compared to earlier test findings by the US Army Corps of Engineers.
Previous investigations by the United States Army Corps of Engineers (USACE) identified that the characteristics of the organic materials in the fine sediment upstream of the dam could affect water quality during and following dam removal. Several borings were abandoned after methane gas was detected and as a result, the full depth of the sediment was not penetrated at those locations. The six borings included in the this scope of work, as indicted above, will be used to characterize the limits of the organic materials as well as collect other geotechnical information (SPT blowcount, grain size distribution and relative quantities, plasticity, shear strength, etc.) related to the sediment to confirm the transport of fine sediment from the reservoir during initial and subsequent flushing events.Subtask 1.2 - Field Investigations to Characterize Dam Concrete
...perform concrete coring and testing to determine the appropriate material properties for use in the structural analyses
...obtain concrete cores from the downstream dam face near the two proposed orifice locations and at 4 to 6 other locations along the upstream face of the dam. The downstream cores shall be obtained from a barge platform in the plunge pool or other means of access. The upstream cores shall be obtained from the same barge platform used for the geotechnical investigations under Task 1.1.
Selected samples shall be tested for bulk specific gravity, unconfined compressive strength, splitting tensile strength, and elastic modulus properties. Petrographic analysis and gel fluorescence testing shall also be conducted to assess the presence of Alkali Silica Reaction (ASR) in the concrete.More info:
County Approves Matilija Dam Contract
