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Slump Blocks and Slope Failure on the Navesink Highlands:

A map of the slump block zone has two sheets:

A graphic illustration of slump block stages is here: Slump Block Stages.

Definition

Geological studies, as well as experience in construction, in the courtroom and in the councils of local government, testify to the unique fragility of the slopes of the Navesink Highlands and the risks of tampering with them. Documentation of the risk and its occurrences goes as far back as two centuries and as recently as 1999.

A key problem of slope fragility that is unique to the area has been so clearly established that it has a specific geological name: slump blocking. The main study of the phenomenon was done in 1974 by James P. Minard for the U.S. Geological Survey (USGS) and is the source of geological information in this text. Slump blocking is a fundamental issue for the high elevation area of Atlantic Highlands, Highlands and the Navesink section of Middletown, which together form the topographic zone called "Navesink Highlands."

At the outset, it is important to be clear about what slump blocking is not. It is not erosion -- the wearing away of the earth's surface by natural processes such as water flows. It is not a landslide -- the sliding of a mass of loosened rocks or earth down along the surface of a hillside or slope' In erosion and landslides, materials at higher elevations become. loosened from their setting and tumble or slide or wash down to lower elevations.

In contrast, slump blocking is not mere surface movement. It means that an entire block of land slips downward' The soil in the displaced block remains intact, in one piece, undisturbed. The top of the block ends up being lower than its headlands; the toe of the block now rests at a lower point of the slope, all soil in the block between top and toe has moved down with them; and all parts of the block stay part of the block, in the same formation and sequence of layers as before. There is also rotational movement of the block which tilts its upper surface inward. Slump blocks can be quite large. some in this area measure up to 590 feet wide and 2,950 long and drop vertically as much as 85 feet. A 1986 report on slump blocking at Mt Mitchill compares the phenomenon to the simple image of "a slab of gelatin slipping halfway off a tilted plate", (Asbury Park Press, January 12, 1986).

Thus, while the issues of geology and slump blocking relate to generalized concerns about protecting steep slopes, they are more fundamental and far-reaching than ordinary slope instability. The attached graphic exhibit shows geological and historical stages of actual slump block movement and secondary slumping along Sandy Hook Bay at and below today's Mt. Mitchill Scenic Overlook park. The Monmouth County Park System has mounted this drawing on one of the display panels along the cliff edge in the park.

 

USGS findings

Minard's paper on slump blocking described the geological composition and structure (physiography) of the Navesink Highlands slopes as unlike that of any other coastal area in New Jersey. It includes a base of deposits of minerals and clay, with the remaining layers made up of various types of sand. The base layer, as well as parts of the upper strata, contain abundant amounts of glauconite, whose presence means susceptibility to slumping is higher. This substance, in the presence of water saturation, undergoes physical and chemical changes that reduce the stability of the base of the slope and have been directly associated with slumping. Under normal rain conditions and with no barriers, water filters through the sand layers, hits impervious clay, and moves laterally in search of lower ground to drain. However, during periods of heavy rain, or in the presence of structures that block drainage, water builds up into the sandy layer and alters its cohesiveness as well as increasing the weight of the slope. Construction produces high frequency vibration and undermines and overloads the slope. These activities can activate a slump.

For about 4 miles, the Navesink Highlands ridge is punctuated by 14 slump block lines which Minard mapped. Included are seven locations where Minard says slumping has definitely occurred, and seven where it "probably has occurred." It has taken place mainly at elevations between 100 and 200 feet above mean sea level along the steeper northern side of the ridge. Because this side faces the bay and prized water views, it has been far more disturbed by construction than the inland-facing gentler southern slopes, which in Atlantic Highlands and Middletown contain much of the mature forested slopes known as Lenape Woods. However, Minard made clear that the whole zone is "an area of possible geological hazards." Parts of five geological formations have been identified as being in the slumping movement. From top to bottom, they are Cohansey Sand, Vincentown Formation, Hornerstown Sand, Tinton Sand, and Red Bank Sand, including both Shrewsbury and Sandy Hook. Described as a layer cake of sand, silt and clay, these formations are the geological content of the Navesink Highlands hills.

Beyond mapping these zones, Minard's USGS report recommended actions to avoid triggering slump blocking. The report calls for "careful thought, planning, investigations, tests, and analyses" before construction in this entire "area of geological hazards." Precautions should include "avoidance of the removal of material from the toes of possibly critical slopes, prevention of excessive water infiltration in the ground in critical areas, and avoidance. of excessive loading on upper surfaces in these areas" (Minard 1974, page 23).

 

The case of Mount Mitchill in 1782 (1)

The earliest recorded slump block event in the Navesink Highlands was in April l782. A newspaper report about it in a New York City newspaper read as follows:

On the ridge of the mountains, commonly called Navisink hills, in Monmouth County, East Jersey, a considerable quantity of land, some say 40 acres, gave way...and sunk directly down a considerable depth; forming a cavity equal in circumference, at bottom, to the void space above. The tops of the trees, that sunk with the soil, and which were mostly of considerable bulk are now nearly level with the edges of the remaining ground. Round this again the earth opens, in one continuous fissure, a foot or more in breadth, for a

considerable distance; and as is conjectured, from its present appearance, will shortly go down also....

A later report noted that "the noise was heard for a distance of several miles" and that the location, known then as Greenland Bank, was "the highest point of the highlands," which was named Mt. Mitchill in 18 16. The block that slumped measured about 400-feet wide and 2,500 feet long, according to Minard.

 

The case of Mount Mitchill in the l970s - l980s

Far-reaching measures for protection of Mount Mitchill have been taken at taxpayer expense since the 1970s -- at the top, the bottom and the middle of its steep northern slopes. These add up to a powerful lesson about the fragility of this land. In chronological order:

* First was the creation of a Monmouth County park called Mount Mitchill Scenic Outlook at the far eastern end of Atlantic Highlands next to the Highlands border. Begun with five acres in 1974, the park now occupies 12 acres of relatively level top land above the cliffs looking over Sandy Hook the Bay and the Ocean toward Manhattan. At the same time, there were plans to build a 15-story high-rise tower known as Eastpointe on immediately adjacent top land in the neighboring Borough of Highlands. Construction of the tower in the mid-l 970s, despite public warnings from Minard of USGS, triggered faults in soil blocks and produced actual movement on the cliffside. In 1975, the US Environmental Protection Agency studied the bluff area and concluded in a letter dated November 24, 1975 that "the question of geological instability is extremely significant." In 1979, the Monmouth County Engineer Charles B. Van Benschoten warned:

Any vibratory energy such as pile-driving or use of heavy earth-moving equipment should be a mater of concern. Developers and local authorities should be guided by the recommendations in the Minard study. The possibility of a slump is ever-present and erosion left unchecked can become a monster.

The faults that occurred and the further risk required a major change in the County route known as Ocean Boulevard, which then ran in front of Eastpointe. It was forever closed to through traffic, and what had been a 40-foot wide road was reduced to a l6-foot access road used only for access to Eastpointe. In 1982, a new Boulevard section was constructed to swing farther south, away from Eastpointe; the cost was an estimated $280,000, paid by County taxpayers.

* Second, efforts began in 1986 to protect a l3-acre triangle of beach, dune, wetland and cliff along the bayshore coast directly below Mount Mitchill, at the Atlantic Highlands / Highlands border. The cliff is the toe of a slumped block of soils from the slopes of Mount Mitchill that gave way in 1782 (see above). In 1986, a developer's plan to build 400 Town houses beside the toe of the slumped land raised the specter of additional slump blocking, as well as obliteration of a scenic and recreation area. The history and continuing risk of slumping were among the counts in legal actions aimed at stopping the project, which New Jersey Superior Court decided to do. Finally, after long efforts to acquire and protect this beachfront, it became a County park in 1997. In the year 2000, it was named Popamora Point -- a site on the Bayshore Trail which is being developed between the Atlantic Highlands harbor and the border of the Highlands. (Popamora was the chief of the Lenape people in 1664 when he signed a land sale deed with English settlers for the entire

peninsula between the Ocean and Keyport.)

* Third, between the beachfront park at the foot of Mount Mitchill and the Scenic Outlook park at its top are steep slopes which eventually reach 266 feet above mean sea level. Steps have also been taken to ensure public protection of that slice of these fragile land's-end slopes, which have a long history of slumping. The County bought and demolished four houses -- one at the top, one in the middle and two nearer the bottom of the slope leading down from the Scenic Overlook park. Clearing away these structures allowed the County to institute uniform measures to protect soils and vegetation, eliminate impervious cover, and close off public access on the steep slope there.

 

The case of Cameron Circle in the 1970s

Minard's 1974 USGS report mapped one slump block in the area below Cameron Circle / Eyrie Road in Atlantic Highlands and noted problems arising from excavation and construction work for a sewer outfall pipeline along the bayshore. Between 1972 and 1976, homeowners there experienced slump blocking, property loss and structural damage, attributed to the construction. Eleven homes and backyards were affected; one homeowner's porch collapsed and others had to jack up sagging floors. The residents blamed installation of the sewerage pipe at the cliff base for damming underground water and starting the slumping. Three lawsuits by homeowners were entered in New Jersey Superior Court seeking $500,000 in damages from the builder. The Monmouth county Bayshore outfall Authority.

In April 1976, Judge Andrew A. Salvest decided for the homeowners. He concluded that the pipe blocked natural water drainage on the lower slope and backed up water into soil blocks, which then slumped of their own increased weight. The Judge's decision said that "the damming effect" created by the pipeline was "a proximate cause" for the "sliding and damage to the property owners' buildings"

(Russell et al, Tosi et al, and Day et al. Dockets no. L-30624-72, L36710-72 and L41330-73, p.6).

 

The case of the "Sudden City" construction plan in the 1990s

In the late 1990s, a developer proposed a large construction project on 24 acres in Middletown just below the far eastern end of Atlantic Highlands next to the Highlands boundary. The plan called for five towers of 6 and 7 stories with 467 dwelling units along the toe of the southern slopes of Mount Mitchill, paralleling Highway 36 west of the Eastpoint shopping center. Its location was to be between roughly 100 and 200 feet of elevation in the same geological formations where most slumping has occurred on northern slopes. The project's size and urban mass led neighborhood groups in the community associations known as the Lenape Woods Coalition to label the project "Sudden City."

Following several years of lawsuits and State agency hearings aimed at stopping the project, the project was not approved by the Middletown Planning Board in Marsh 1999, based partly on a geologist's testimony about the risks of slump blocking at the site. The developer's plan would have taken two of the risks that Minard warned against. It would have cut away 1.35 acres of steep slope at the foot of Mt. Mitchill's southern slopes, eliminating the stable base which has held up the slope since time immemorial; and built walls 600 feet long and 10 feet high, creating a barrier to back up rain water into the slope and increase its weight and the downward force of gravity.

A second developer acquired the property in 1998 and by October 2000 reduced the project to 180 units and its acreage (including parking and a stormwater structure) to less than a third of the tract. The remaining two-thirds, including most of the steep slope area, was preserved as open space -- eight acres acquired by Atlantic Highlands as an extension of the Lenape Woods Nature Preserve, and eight acres acquired by Middletown to construct ballfields for shared use with Atlantic Highlands.

 

The case of South Linden Avenue/Shore Drive in 1999

On January 3, 1999, residents around South Linden Avenue and Shore Drive in Highlands heard a loud noise as the bluff at and below South Linden Avenue in Highlands gave way. Within seconds, a "wall of mud, rocks and vegetation" on the hill slammed down into the Highland Shores condominium complex on Shore Drive 100 feet below the hilltop. On South Linden Avenue, which runs along the top of the slope, a section 20 feet long and 5 feet wide collapsed. Down below, the residents of eight condo town houses on Shore Drive had to be evacuated because the hillside was found to be "unstable and at risk of another collapse," according to Borough Engineer Walter Hopkin of Schoor DePalma.

Hopkin was later quoted in the Asbury Park Press as stating that "This is not an erosion issue. It was a slope failure....Anyone can see that that area has subsided." After geological and engineering studies, the remedy was to build a wall 300 feet long and 1l feet high and to re-grade the hillside to a more gradual slope. Highlands Borough bonded $500,000 for the cost of this construction.

From Paul D. Boyd

Historian, Atlantic Highlands Historical Society

Chair, Environmental Commission of Atlantic Highlands

(1) Quoted material in the above section is from Barber and Howe's Historical Collections of the State of New Jersey, 1844, page 357 .

 


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Borough of Atlantic Highlands | 100 First Avenue, Atlantic Highlands, NJ 07716 | Phone: 732-291-1444 Fax: 732-291-9725




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