by Mike Sharkey
Staff Writer
Building near the water in Florida — be it the ocean or one of the many rivers — isn’t easy. For the developers of The Shipyards, building on the St. Johns River on land that spent decades as Jacksonville’s shipyards epicenter won’t be, either. Then again, if all $860 million projects were easy, they’d be popping up all over town.
With much of the preliminary demolition over — in the past year the old sheds have been razed and about half of the foundation has been removed — some of One Shipyard Place’s most difficult engineering is set to begin. Before anyone notices that Phase I is coming out of the ground, engineers with Brasfield & Gorrie, LLC must first address what needs to go in the ground.
Recently, demolition crews peeling away the foundation of the old sheds found pine pilings. Finding them wasn’t a surprise, but finding about 1,400 of them caught the developers off guard. The question now is what to do with them.
According to Brasfield & Gorrie engineer Erik Sharpe, that is still being determined.
“That is not an issue, but an unknown,” said Sharpe.
The pilings have been in the ground since just after the turn of the 20th Century. Needing a way to fortify the already water-laden soil, shipyards workers vertically drove hundreds of pilings into the ground and the old shed was built on top. Because the sheds foundation didn’t bear much weight, the pilings were suitable for years. Today, the plans for One Shipyard Place and modern engineering standards dictate that Sharpe and his colleagues analyze the situation thoroughly before deciding how many of the pilings will have to be removed.
“As the slab is demolished, we’ll do an engineering survey of the existing conditions and the wood pilings,” said Sharpe, adding the pilings are not rotting but are remarkably preserved.
However, many will have to be removed, especially at load-bearing points. Once footer locations have been determined, the wood pilings will be extracted and replaced with stone columns poured at least 50 feet into the ground.
Sharpe said his firm had several options available but chose to employ what’s called vibro-replacement, a soil compaction system often used in clays, silts and mixed or stratified soils. In vibro-replacement, stone backfill is compacted in lifts to construct columns that improve and reinforce the soil strata and aid in the dissipation of excess pore water pressures.
“When the pilings are removed, stone will be pushed into the void and compacted on top,” explained Sharpe. “The columns will support the footing of the building. Stone columns are our most viable options. They are the most cost-effective and they meet the needs of the design.”
The water table of the St. Johns will also be a major factor. As the pilings are removed and replaced with stone, excess water will permeate to the top. Sharpe said that water will have have to be removed. Again, the question is how.
“We have two options,” said Sharpe. “We can use JEA’s sanitary sewer lines or we can haul it to another location on site and and allow it to percolate back into the ground.”
Both options will require a great deal of bureaucratic red tape. Obtaining permission from JEA won’t be easy and Sharpe said the Department of Environmental Protection won’t allow him to store the water on site for very long.
“Time is of the essence,” said Sharpe. “We can’t just put it over there and forget about it.”