The construction of some very large residences in the last couple of years has required extensive dewatering of the land—actually removing the water from the underground water table—to make way for deep, sprawling basements beneath.
The process is not new, though it has rarely been done for residential structures, anywhere, until recently.
On most of the South Fork the water table is 30 to 40 feet below the surface or deeper. In some areas, though, it can be as little as a few feet down to hit water, especially nearer the ocean, where saltwater also intrudes and pushes the water table higher—the freshwater table floating above the more dense saltwater layer.
For most of the 20th century, homes built in such areas were simple beach cottages, usually without foundations, never mind basements. But in the last few years, a new trend in house design has arisen: cavernous underground “lower levels” that often rival, equal or even exceed the total square footage of the visible house itself. These levels can require foundations that extend as much as 20 feet below the surface, and well below where the ground is saturated with water.
So, to make way for what will, by necessity, be submerged foundations, contractors must remove the water from the ground beneath the house, and keep it at bay while the concrete foundation is formed.
“It’s a relatively simple idea, but many times can be hard to implement,” said engineer Paul Grosser, whose firm, P.W. Grosser Consultants, has overseen dewatering projects locally. “It’s a little more art than science sometimes.”
The complicating factors can be many. First, the water, tens of millions of gallons worth, must be drawn from the ground. But because the water table is essentially an underground lake, more will want to rush in to take the place of whatever is removed. Thus, the water removal must be constant until the foundation is completed and can hold up to being inundated.
Also, all that water pumped out of the ground must be put someplace, and environmental controls will rarely allow it to just be spit out somewhere else.
Water removal may be the easiest step. Most projects employ small wells, about 2 inches in diameter and extending slightly deeper than bottom of the foundation. The well heads are placed every 5 feet or so and are connected to a single pipe that directs the water away.
A large system can withdraw hundreds of thousands of gallons a day. In a proposal for a new mansion in Sagaponack recently, engineers said they planned to pull 850,000 gallons a day out of the ground, for two months.
As the water is pumped, the water table near the well-point system lowers gradually. The draw-down creates a basin in the water table—the surrounding water table remains at its natural level but dips downward toward the well pumps as it reaches the edge of the well system’s suction effect, which engineers call the “depression pit.” If the suction of the wells were to stop, the depression pit would immediately begin leveling off as the water table tried to level itself off.
“The biggest issue is keeping it dry while everything is going on,” said Southampton Village Engineer Aldo Andrioli. “It’s a hole, that water wants to pour in. If there’s a power shortage, it will start coming back in very quickly.”
The work can take many weeks and cost somewhere well into six figures, depending on the project. The deep hole must be dug, the foundation laid out in forms and then filled with concrete. Once poured, the foundation can take weeks to cure and harden to watertightness.
In the upcoming Sagaponack project the engineers have said they will have to keep pumping for 60 days while the more than 12,000-square-foot foundation is created. Over that period, the well system will have drawn out more than 51 million gallons of water, enough to fill more than 1,500 standard swimming pools.
All that water has to be put somewhere. Mr. Grosser said that the easiest solution is to simply discharge it into a nearby water body, if one exists. In the case of Sagaponack, one can gaze over the dunes at the Atlantic, where tens of millions gallons of fresh water would be absorbed easily. But the State Department of Environmental Conservation has not typically allowed that easy out, or any other out that would carry the water off the property.
So engineers typically have to put the water back into the ground, within the boundaries of the lot. On two other projects in Sagaponack in the last year, one on Gibson Lane the other on Bridge Lane, this was achieved by creating a large recharging area, essentially a sump, into which the water was pumped and allowed to simply percolate back into the ground not very far, but just far enough, from where it was removed. The trick is to make the recharge area large enough so that the discharge of the dewatering pumps does not overwhelm the rate at which the water was filtering back into the ground, yet not so large that the recharge starts flowing back into the depression pit.
“You have to get an equilibrium,” Mr. Grosser said.
Engineers have said the risks of negative impacts from dewatering are minor.
At the project on Gibson Lane, torrential rains from Hurricane Arthur on July 4 overfilled the property’s recharge basin and caused part of its retaining wall to collapse, sending a flood of water cascading out across the property and across an adjacent roadway.
Sagaponack Village Engineer Drew Bennett said recently that there are minor concerns about causing shortages of water in a neighboring property’s well if the well is very close to the property being dewatered and sufficiently shallow that the depression pit could draw it dry. He suggested that project managers should make sure that neighbors are connected to Suffolk County Water Authority mains.
Southampton Town Environmental Analyst Marty Shea said that the greater concern of such projects may fall to the homeowner, for the soundness of their home’s foundation. In any place that is potentially subject to flooding, as from a hurricane’s storm surge, or simply even prone to rising water tables during intense rainfall, Mr. Shea said he would be concerned about the foundation’s structural integrity.
“What happens in a severe storm event, whether it’s intense rainfall or a coastal storm, the groundwater table rises and falls, and can crack a structure that is built in groundwater,” Mr. Shea said. “I’m sure the engineers are going to town designing these foundations ... but I never recommend building foundations into groundwater, even if there are no FEMA restrictions.”
Beyond such concerns, though, engineers say the dewatering itself is a drop in, or from, the bucket.
“It’s all temporary—when it’s done, you stop,” Mr. Grosser said, “and it will go back to the way it was before in a hurry.”
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