KEENE VALLEY - A landslide triggered by recent heavy rains has damaged upscale houses in the Adrian's Acres mountainside subdivision, and is still active.
One house, a seasonal residence of the Machold family of Philadelphia, is no longer habitable, Keene town Code Enforcement Officer John Hudson said.
"The whole hillside is out of equilibrium," said Andrew Kozlowski, glacial geologist and acting associate geologist of the New York State Geological Survey. Kozlowski deals with natural disasters from glacial masses that are of concern to communities.
A landslide damaged the driveway of the Machold family’s seasonal residence in the Adrian’s Acres subdivision in Keene Valley.
(Photo — Martha Allen)
A landslide damaged walkways at the Machold family’s seasonal residence in the Adrian’s Acres subdivision in Keene Valley.
(Photo — Martha Allen)
Diagram of a rotational slump
(Graphic — Natural Resources Canada)
The Adrian's Acres slide continues for more than half a mile along Adrian's Way road, Kozlowski said Thursday, with the "toe," or bottom, of the failure between 300 and 400 yards downslope. He called it the largest such slope failure he has seen in New York.
The deck of the damaged Machold house looks twisted, its posts askew. The deck posts have rotated backwards, Kozlowski pointed out, so that the sides that originally faced downslope now face uphill. This is the dynamic of this landslide, which he called a complex rotational slide.
"It's a big, cohesive, soupy mass," Kozlowski said. "It looks like the ground dropped. It dropped, and rotated backward, uphill. The whole (Machold) house is moving this way. ... The foundation is breaking up."
Other houses are also in jeopardy as the situation becomes progressively worse. According to measurements made by the state Geological Survey team, the hillside was still moving Thursday at the rate of one millimeter per hour.
"It's still active. We're hoping it's slowing," Kozlowski said. He plans to return Tuesday to reassess the situation, but more rain is predicted between now and then.
Carol Treadwell, executive director of the Ausable River Association, also visited the site and presented what she termed "a very preliminary interpretation" of the landslide, or "rotational slump," to the Keene town board.
"The recent cracks that opened up are not part of a new process," Treadwell wrote. "There is evidence on the hillside of slump blocks and scarps (steep cliffs formed by erosion or faulting) that have been there for some time. The recent re-initiation of movement along the cracks was probably a result of the recent prolonged snow and ice melt. Sliding like this usually occurs after prolonged or exceptionally heavy rainfall. Sometimes a rise in the water table may be the culprit of sliding."
Kozlowski said the soil on the hillside contains glacial till (sediment of various particle sizes deposited directly by a glacier) as well as sand. When glaciers retreated during the last ice age, he explained, valleys were sometimes dammed and so became lakes. Keene Valley was once such a lake, and the sand found surprisingly high upslope in Adrian's Acres was deposited by the lake over many years.
This year Adrian's Acres accumulated a great deal of snowpack, which migrated through the sand, in addition to heavy rain in April, according to Kozlowski.
"Groundwater can loosen soil," he said.
He compared the movement of the slide to the way a hole dug in the sand at the edge of the water behaves: The sand keeps sliding in and filling the hole. A hole dug away from the water's edge does not fall in on itself, because the drier sand is more cohesive.
The hillside slide demonstrates a lack of balance between the force of friction and the force of gravity. When groundwater reduces the friction between layers of soil, the force of gravity takes over and the slope fails.
"It's a fascinating geological process," Kozlowski observed, "and a heartbreaking one for the people who own homes that are affected."