Xcel Energy Inc. has submitted the following document to the Federal Energy Regulatory Commission: FERC Comment Document HDR Response Summary. Discuss the ability of the geotextile filter fabric to function as intended as a filter in the event that the proposed decommissioning of the dam is not completed within the assumed timeframe. Design Report The Design Report was updated to discuss the ability of the geotextile filter fabric to function as a filter in the long term.
A geotextile was selected as a filter between the Filter Backfill and the Gabion Baskets so that the excavation did not need to be further expanded for secondary or even tertiary filters and further destabilize the concrete structure by removing more buttressing fill. If geotextile begins to degreade prior to decommissioning it will be a gradual process allowing ample time for inspection and repair and will not result in a sudden collapse of the backfilled hole due to the presence of the gabion baskets. Their response does not adequately respond to their Comments Nos.
8 and 9. The plans indicate the onsite soil as ?Type C? that suggests that the soil is unstable; however, the plans show near-vertical excavations. a. Provide plans and sections that clearly show the proposed embankment excavations. b. Submit supporting stability analyses for the proposed embankment excavations.
c. Include descriptions of the existing embankment and foundation materials, Design Report Plans and Specifications Stability, bearing capacity, settlement calculations and updated drawings will not be provided due to the lack of available information. Proposed construction repairs are intended to immediately increase stability at what is currently an open hole and obtaining data for further geotechnical analysis will require a geotechnical investigation and further delay construction. Construction is intended to be performed without personnel entering the excavation but an OSHA Soil Type C was assigned as a precaution in the absence of shear strength data should personnel enter the excavation.
The near-vertical excavations reflect minimal excavation beyond existing conditions to allow for fill placement. Further excavation laying back slopes to OSHA acceptable grades would further destabilize the concrete structure by removing the soil buttress. Fill or gabion baskets are replacing eroded soil and will not overconsolidate soils beyond preconsolidation pressures so settlement and bearing capacity failure likelihood are not considered to be significant enough to impact performance.
The resumes provided in the Quality Control Inspection Program (QCIP) do not include details of previous experience with similar construction activities (e.g., design and installation of gabions and geotextile filter fabric at gravity dams). Revise the QCIP resumes with specific experience. Include the resumes of all persons with stop work authority.
QCIP Resumes were updated to include experience with gabion baskets and geotextile although these items are not considered overly complicated as construction materials. Revise Section 2.0 of the QCIP to define the personnel as shown in the organizational chart. Refer to Chapter 7 of their Engineering Guidelines for information related to the QCIP.
QCIP The QCIP was updated to define the personnel in the organizational chart consistent with Chapter 7. Revise the QCIP to include Xcel?s oversight of installation of temporary shoring. QCIP The QCIP will not be updated to include Xcel's oversight of installation of temporary shoring because temporary shoring is not part of construction. The hole is currently smaller than temporary shoring and would need to be excavated larger to accommodate shoring which would further destabilize the concrete structure by removing buttressing earth fill.
The design was developed so that personnel are not required to enter the erosion hole.
