No one really knows exactly what conditions lie below the wavy floor of the old building at the site proposed for a new Piazza Library (shown above). Already, a costly “auger-cast column” system has been defined, which will require drilling down 20 to 30 feet in an eight foot grid across the breadth and width of a new building on this site and pouring piers to support the new floor, and this cost was not included in the original 9.3 million dollar estimate. In addition, there could be contamination which must be abated, or archeological features which need to be preserved– no one knows until we dig.
Some supporters of the Piazza library site, including the Renton Reporter, have repeated the idea that in terms of construction costs, remodeling the Cedar River Library carries “unknown” costs, while building at the Piazza site is highly predictable in cost. (You will find this assertion in this article for example.)
I don’t agree with this opinion however, as there can be many additional construction impacts hidden in the soil conditions, and these generally won’t be known until we start digging. These unknown underground conditions are an issue for the Piazza site, but not the Cedar River Site.
Some hidden conditions that have added to costs in Renton projects before are soils that are inadequate to support the building or the floors without reinforcement, contamination from previous uses, and the discovery of Native American artifacts on the site.
Already, a Geotechnical report provided to KCLS dated November 2nd 2011, done by Geo Engineers, has determined that the Piazza Library will need a more robust foundation system than the typical spread footings and slab-on-grade that were assumed when the 9.3 million dollar estimate was produced. The engineering firm has determined that the soil is so compressible and subject to liquefaction at the Piazza Library Site that the builders should install “lean auger-cast columns” (poured in-place concrete piers made with a lean concrete mix) up to thirty feet deep in a grid approximately every five to eight feet across the entire floor just to support the loads. The site is underlayed with multiple layers of silt and clay and alluvial sand, which makes it subject to liquefaction in an earthquake.
Of course contamination is always a concern when digging up old building sites, especially on locations where commercial use dates back before World War 2. While I am not aware of any surface evidence of fuel oil tanks, it is always possible that there are tanks buried in the site (either from heating oil or automobile fuel). It is also possible that before the sporting goods store moved in years ago, someone used the site in a way that allowed contamination to flow into the soils. Old fuel tanks and contaminated soil conditions are common discoveries in older parts of town, and they add cost to building projects. If these conditions exist, even deep under the surface, they will easily be discovered in the process of perforating the site with the newly required “lean auger-cast columns.” Here is a Seattle Times article which says that even the removal of a relatively small, basic home heating tank can cost up to $100,000 if there has been leakage. This cost would be many times greater for an underground gasoline tank. Click here to learn more about Renton sites where this work has been undertaken at great expense, such as the Texaco site that now hosts Big Foot Java, and the recently closed Jet City Espresso, which was formerly Charley’s Automotive.
Native American artifacts were discovered during the expansion of Renton High School in 2001, and during the building of the Henry Moses Aquatic Center in 2004. Both of these discoveries halted the projects and added costs, and could have led to costly redesigns if satisfactory resolution had not been reached with the tribes and the state regarding how to preserve the discoveries. Click here to read more (see page 25 of the report). The discovery report refers to the sites as “floodplain”, in reference to the Lake Washington and Black River conditions that existed before 20th century river and lake manipulation. No doubt the Piazza Library site is part of the floodplain, and was a historic area of activity for the Duwamish tribes.
While none of these risks should disqualify a site from construction, it is simply wrong to imply that there are no “unknowns” prior to excavating a site like the Piazza library location.
This photo, which is hosted by the UW Library, shows a 1930′s gas station at 3rd and Logan, in what appears to be the parking lot of the former Big 5 Sporting Goods store. The fuel tanks were single-walled back then, and they lacked monitoring for detecting leaks. There is no word yet on whether records exist which show the removal of these fuel tanks.