Brandon Cole, P.E., City of Issaquah, Washington

The City of Issaquah planned the replacement of the 56th Street Bridge with a new bridge facility 92-feet wide by 160-feet long including 1000-feet of associated approach roadways, storm drain, utilities, and two signals in 1995. The bridge was to include six traffic lanes two bike lanes and two sidewalks. The existing bridge structure was 24-foot by 110-foot structure with no sidewalks, presenting a chock point in an otherwise wide arterial. The bases of its beams were within the 10-year flood level with a large commercial area immediately upstream. Flood conditions caused a two to three foot water backup behind the structure contributing dramatically to the local flooding.

Originally the plan was for a 3.2 million-dollar structure with a one-year construction window. As construction costs escalated and the time frame extended, we ordered the consultant to reduce the cost and time frame. The estimate had escalated from 3.2 to 4.5 million dollars and an impossible one-year construction schedule. Our design consultant stated one million dollars could not be cut from the project without reducing the scope of work and they firmly said it could not be done in one year. They said they would try but it would cost us a lot more design money and add another construction season. Initially they refuted our claims that the design was less than perfect. However the evidence was overwhelming and in the end we were refunded a large portion of the design costs. We rejected the consultant, the design and schedule. The trouble was we actually needed to increase the scope of work by adding bike lanes and sidewalks to the final 800-feet of the project. This was required to comply with the City's part of the development agreement. We were going to be at least 30% over the LID preliminary assessment roll, a lawsuit ripe for the picking. Had we lost control of the project, or were we being fleeced? Perhaps some of each happened, it is really hard to say.

We decided to redesign the entire project in-house from the ground up, since we now had the means. The decision to redesign in-house had many doubters and was what you would call a career decision. In any case the tight time frame did not allow slack time for consultant re-selection. We had less than four months to do the entire redesign and get it out to bid or miss the construction window for that year. Waiting till the next year would have been another career decision. First we identified every major cost to try to reduce or eliminate them. The design had to be simplified while increasing the scope of work. We were now the boss and could change any aspect without debate. Having designed over 20 bridges myself I knew it could be done but I did not know how I was to proceed. The Cities assigned only one draftsman and me to the task, and 75 plan sheets needed to be drawn. The major costs identified to be reduced or eliminated included gravel fill, sand drains, surcharge, pilings, retaining wall, substructure, and temporary retaining wall. We also had to cut the construction time to one year or less, which required the elimination of the surcharge.

The original predicted fill settlement was 12 - 18 inches, which could destroy a deep gravity sewer. When planning the approach fills of the bridge we looked at tires, wood chips, expanded shale, and then foam as a last resort. We could have no more than 400-psf increase in the original ground dead load for an ultimate settlement of one inch or less. All alternatives except foam appeared too heavy for this demand. We checked with the Washington State DOT to see if they had used foam on an approach fill before and they had not. They had only used it for some shoulder work in minor quantities. In fact when we talked to the local manufacturer (Western Insulfoam) and told them of the quantity we needed, the salesman thought we were talking of board feet not cubic yards. Others told us foam fill would not work and it would never save money. It was joked that it would float away in the next flood. This was actually a sobering thought when a fill depth of up to 12 feet was required, consuming 2,500 cubic yards of the 1.5pcf product.

We also discovered the foams compressive strength (2,000psf) to be the same as structural fill. The next problem was the utilities which were so dense that they formed an almost continuous layer 44 -feet wide. There was sewer, power, two phone systems, water, and two gas lines. With all the utilities and the risk of floatation real a soil cover of five feet was required. Now could you send cranes for girder and placement on the foam without severe damage? The Gas Company needed to do a lot of welding on top also. We found that in Switzerland they used a 12-inch flat concrete cap and a 4-foot soil layer on top to solve utility and floatation problems. The utilities could install using conventional burial techniques, and the girders posed no problems. The utility companies were quite happy to lay conduits in with the fill placement rather than dig through the original 15-foot surcharge and temporary retaining walls for placement.

The cost of the foam although relatively high, simplified the project, saved the City over one year in construction time, and some money. We actually saved over 1.2 million dollars in costs by eliminating 200,000-feet of sand drain, a temporary retaining wall and a surcharge of up to 15-feet in a tight construction area. We used a deck bulb-tee beam instead of a pour in-place deck beam to save time and allow construction outside the fisheries window. With no down-drag forces and less lateral forces we downsized the piling without sacrificing strength. We switched from a 24-inch diameter driven pile to an 18-inch auger cast system to reduce stream vibrations and allow for construction outside the fisheries window also.

The retaining walls were also redesigned since foam exerts little lateral static or dynamic force. We integrated the retaining wall system into the foam fill with a special KeystoneÔ Wall assembly procedure. The foam eliminated the predicted settlement damage to a deep gravity sewer, which ran under the fill area. The foam worked structurally as expected with very precise dimensional sizing allowing quick assembly. The original plans called for a settlement of 12 - 18 inches. The foam fill reduced that too less than ½ inch after 180 days. The proof is in the pudding. The foam performed exactly as planned. I would recommend this system to anyone in a surcharge situation with utilities and a short construction time frame.