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Geotechnical testing scheduled to start next week in Washington Park will aid the design of a new underground reservoir that will replace the park’s existing open air reservoirs.

Testing work for the Portland Water Bureau‘s proposed Washington Park Reservoir Improvements Project is scheduled to start during the week of April 20 and be complete by mid-May. Crews will drill geotechnical test anchors along Southwest Sacajawea Boulevard on the east side of the upper reservoir and along the promenade on its north side.

Information gathered will support design work for a plan to build a 15-million-gallon underground reservoir with a reflecting pool on top in the footprint of the existing upper reservoir. Washington Park’s other reservoir would be decommissioned, and a lowland habitat area and reflecting pool would replace it.

Geotechnical testing will assist Water Bureau engineers in evaluating soil properties and behaviors, and understanding how the soil will perform under certain conditions and how it may impact the construction of the new reservoir. Information gleaned will be used to evaluate and minimize potential risks associated with issues such as load-bearing capacity, slope instability and water infiltration.

The testing, which will be handled by Condon-Johnson & Associates, will reduce a degree of uncertainty in the design work by helping engineers accurately anticipate factors that may come up during construction that could potentially reduce the overall project cost, Water Bureau spokeswoman Jaymee Cutisaid.

The information will help engineers design the shoring wall that will hold up the soil during construction, she said. Testing will also tell engineers the strength of the soil to accurately account for the number of anchors needed for the project.

City officials announced last year that they plan to proceed with the project after receiving a mandate from the Oregon Health Authority to follow a federal regulation that outlaws the use of uncovered reservoirs to store finished drinking water. The rule is intended to reduce the risk of exposure to contaminants.

Along with geotechnical testing, Water Bureau officials are seeking Portland City Council approval to demolish the historic Weir Building and remove portions of two reservoir basins to make way for the project. A City Council hearing to review these plans is scheduled to take place April 23 at 2 p.m. at Portland City Hall.

Hoffman Construction crews are scheduled to start work on the project on July 1, 2016 and wrap up in late 2020.