Mt. Tabor Reservoir Complex
As part of a Federal requirement to improve the reliability and safety of drinking water for major metropolitan areas, the Portland Water Bureau began developing designs to replace the open reservoirs at Mt. Tabor with buried reservoirs. The project would involve decommissioning Reservoir No. 1, constructing a rectangular 50 MG concrete reservoir within the existing Reservoir No. 5 footprint, and constructing a rectangular 20 MG concrete reservoir in the northern half of the existing Reservoir No. 6 footprint. Shallow, reflective ponds would be constructed above the reservoirs to maintain the appearance of the original “open” reservoirs. The geotechnical exploration program included 15 mud-rotary borings and 15 geoprobe holes.
Soft compressible silt and clay was encountered beneath Reservoir 5. Several options were provided to minimize potential differential settlement of the proposed 50 MG tank including overexcavation and backfilling with crushed rock, founding the structure on a concrete mat, using deep piles, or improving the strength of the silt and clay using specialized construction stone columns or jet grouting.
The borings near Reservoir 6 encountered loose to medium dense sand overlying stiff clayey silt. Seismic liquefaction analyses indicated that the majority of the sand deposit has adequate factors of safety against liquefaction. Geotechnical foundation recommendations were provided for shallow spread footings founded on a concrete membrane slab. Recommendations were also provided at both reservoir sites for base slabs, subdrain systems, temporary cut slopes, lateral earth pressures, site grading, fill placement, and seismic ground response.