Appeal 14011

OSSC Section 1803.5.12

OSSC Section 1803.5.12 consequences of liquefaction due to differential settlement, soil strength loss due to lateral soil movement and lateral soil loads on foundations based on site peak ground acceleration for the maximum considered earthquake ground motions.

The project scope includes the design of a New Access Ramp/Walkway at the T-4 Wharf - Berth 411 located at 11040 N Lombard St in Portland, Oregon. The structure consist of (1) Steel framed upper landing on the shore, (1) Steel frame lower landing suspended from the Berth 411 structure above, (1) steel framed access ramp connecting the upper and lower landings and (1) additional steel framed walkway connecting the lower level landing to the existing Fire System Maintenance Walkway beneath the dock at the lower level. The upper landing is founded on shallow foundations using a bearing pressure of 2500 psf under static loading in accordance with the recommendations of the geotechnical report.

In accordance with section 1803 of the OSSC Geo Engineers has conducted a geotechnical investigation and prepared a site specific seismic hazard geotechnical report dated June 10. The report states that the design of the project is feasible for static conditions, but may not be practical to design the ramp and platform for seismic conditions.

Under seismic loads, liquefaction induced settlements of about 4 to 5 inches with a corresponding lateral spread of about 24 inches may be expected at the location of the proposed ramp abutment, located 35 feet back from the top of the river bank. The geotechnical report shows the river bank on which the foundations for the upper landing are located has a static slope stability factor of safety (FS) is 1.5 and seismic FS of 0.8. This implies that under Maximum Considered Earthquake, failure of the riverbank is likely. Per OSSC the required FS under seismic conditions is 1.1.

It is not practical and cost prohibitive to design for the anticipated slope failure during a Maximum Considered Earthquake. The structure has not been designed to accommodate expected movements during a Maximum Considered Earthquake. Instead, the structure has been designed to allow limited rotation to accommodate some vertical movement between the upper level foundation and the lower level support beneath the pier. In addition, PTFE slide bearings have been provided at the top foundation to allow horizontal movement in line with the ramp. This will allow for some soil settlement over time and adequate performance during smaller seismic events. In case of a Maximum Considered Earthquake, in the event of the river bank failure, it is likely that the upper landing will lose support, and these movements would most likely overcome the ability of the walkway's structural system to adjust to movement.

The purpose of the new access ramp is to provide OSHA compliant access to the existing maintenance walkway beneath the pier at Berth 411. We are attempting to provide safe access for the limited times maintenance staff are required to work from the existing catwalk.
The soils report referenced above notes that the factor of safety of the bank during a design seismic event is 0.8. This means the liquefied "Soil will move toward the water in the maximum considered earthquake ground motions and we could see a significant drop of the soil at the bank causing the foundations at the upper landing to fail. Deep foundations would be required to support the upper landing. The soils suitable for supporting a deep foundation system are 80' below the ground surface. This means that a deep foundation would be very expensive, if one could be designed, to support the shore end of the ramp making the project economically infeasible. We believe that these costs are not justified given the nature and use of the walkway. The walkway is a portion of an Industrial facility, is very seldom occupied and has a restricted limited use. There is no public access to the facility The entire facility has security control with a single guarded gate. Access is for scheduled routine maintenance and inspections by ship's crew members. The under dock catwalk is estimated to be accessed not more than 40 times annually. The water access is limited by a steel sheet pile wall a ways out from the shore. The top of sheet pile wall elevation is 5'-0" above the mean low water elevation at this location. The probability is very small that there would be anyone on the new ramp or existing catwalk during a design earthquake.

In addition, there is an existing pathway to access the shore and stair up to the wharf at the far end that will remain. This provides an alternate means of egress in the improbable likelihood someone is using the under dock catwalk during a design seismic event.

Engineering will also be provided to show that the Berth 411 structure will not be impacted should the access ramp lose it's foundation support at the upper landing due lateral spreading.

As noted above, we have designed an at-grade foundation at the upper landing of our proposed ramp to allow for some soil settlement over time and adequate performance during smaller seismic
events.