Department of Transportation

I-80 Rockfall Mitigation Photo

Existing Geologic Conditions

The highest elevation of Mt. Tammany upslope of the roadway is 1,526 feet above sea level. It appears the highest slope elevation directly adjacent to the roadway is approximately 470 feet above sea level. The elevation of Interstate 80 (I-80) is approximately 320 feet above sea level.

The existing rock cut areas along the westbound direction of I-80 within the Project limits have physical and geological safety hazards. The primary modes of rock instability identified during the data collection and site characterization of the area are planar sliding, wedge sliding, toppling, rock mass failure, and discrete rockfall as evidenced by the large overhangs, steep vertical faces, loose boulders, and rock blocks, which have resulted in rock toppling down and landing on the shoulder and roadway lanes and washouts along the I-80 roadway. A series of large open fissures exists in the area near the steep vertical rock wall, and if not stabilized, there is the potential for a major rockfall event to occur. Rockfall debris has accumulated up to the top of the westbound barrier curb in locations throughout the Project limits.

The Project site has been subdivided from the northern to southern limits into four areas (A, B, C, and D) based on a variation of conditions.

Area A (Milepost 1.04 to 1.15)

Area A consists of moderately wooded, soil and rock debris slopes dipping toward the highway at moderate angles. The existing wooded slope, barrier curb, and stone wall provide some rockfall mitigation in this area. However, additional barriers are recommended to adequately reduce the risk of rockfall reaching the roadway.

Area B (Milepost 1.15 to 1.25)

Area B consists of a series of high, steep rock cuts separated by slopes similar to those in Area A. Area B poses a significant risk for rockfall to reach the roadway.

Area C (Milepost 1.25 to 1.35)

Area C is characterized by a high, vertical rock cut and poses the greatest risk for rockfall events to reach the highway. The western portion of Area C (roughly MP 1.25 to 1.32) currently has no rockfall protection and the eastern portion of Area C (roughly MP 1.32 to 1.35) contains an existing gabion wall that is over 50 years old.

Based on the varying characteristics within Area C, the eastern portion will be considered in “Transition Area C-D,” as noted below.

Transition Area C-D (Milepost 1.32 to 1.38)

Transition Area C-D is a combination of the eastern portion of Area C and the western portion of Area D. Transition Area C-D features a high, vertical rock cut, known as an escarpment, and a gabion wall that was constructed in the 1970s as a slope stabilization project. The existing wall is located within the toe of the talus slope adjacent to the escarpment face. The exposed rock in the escarpment towers more than 200 feet above the gabion wall and contains loose rocks that are up to 10 feet long.

Area D (Milepost 1.35 to 1.45)

Area D is characterized by a high, vertical rock cut (escarpment), which increases in elevation and distance from the roadway from west to east and a long, high talus slope. The existing stone wall does not prevent rockfall from reaching the roadway. The high, vertical rock cut is located in the western portion of Area D (roughly MP 1.35 to 1.38). The long, high talus slope is located in the eastern portion of Area D (roughly MP 1.38 to 1.45) and consists of rock blocks that have previously dislodged and fallen from the escarpment face.

Based on the varying characteristics within Area D, the western portion will be considered in “Transition Area C-D,” as noted above.

Rockfall Events

Rockfall events have significantly impacted traffic conditions along this segment of the I-80 highway. Between 2003 and 2018, within the Project Area, I-80 has been fully closed at least three times due to major rockfall events. A rockfall event is defined as any documented report of rockfall or debris from the rock slopes which directly impacts the highway. This data is compiled from New Jersey State Police crash reports, New Jersey Department of Transportation (NJDOT) Division of Operations and Maintenance records, and NJDOT Department of Geology field reports.

Traffic slowdowns or partial closures have also occurred as a result of the poor conditions associated with the deteriorated rock slopes along this segment of I-80. The slopes and geology in this segment of I-80 are such that trees and shrubbery have grown in the loose, rocky soils. When subjected to heavy rainfall or snowmelt, the fast flow of surface groundwater over such terrain can erode and undermine vegetation causing it to dislodge and fall to the roadway. In addition, this high volume of groundwater often overtops the roadside barrier and can flood the roadway surface causing icy conditions or reduced lane capacity.

Gathering information on rockfall regularity and significance is often observational or anecdotal in character. Maintenance work associated with the clean-up of rockfall debris is not tracked in any formalized fashion at the local maintenance facility. However, interviews with regional maintenance staff have confirmed that they regularly pick up fallen rock material on a weekly basis within the I-80 Project Area. The standard operation consists of a “rolling closure” configuration.

Tracking rockfall events provides valuable data that validates the rating and ranking methodology employed and accepted by professionals nationally. One of the best chronicles of rockfall events directly impacting a highway is obtained from police crash records. Over 600 New Jersey State Police crash records for this section of I-80 were reviewed to identify any crash resulting from rock or natural material entering the highway from the border area. Since the standard alphanumeric coding information in a crash report is sometimes inaccurate or erroneously recorded, the written description in each record was reviewed for specific details.

Between 2001 and 2018, a total of 29 “events” directly related to rockfall activity were identified in the Project Area. These 29 events include the following:

  • 14 occurrences of rocks falling and landing on the highway;
  • six occurrences of a tree falling and landing on the highway;
  • four occurrences of highway flooding; and
  • five occurrences of highway icing.

Based on the review of New Jersey State Police crash records, no crashes resulted from flooding or icing on the highway. One crash resulted from a tree falling on the highway and 13 crashes resulted from rocks falling and landing on the highway. It is important to note that the number of crashes from rocks falling on the highway (13) does not match the number of occurrences of rocks falling as listed above (14) because not all occurrences of fallen rock resulted in crashes while other occurrences of fallen rock resulted in one or two crashes. (There were four occurrences of fallen rock that did not result in any crashes. Seven occurrences of fallen rock resulted in one crash per occurrence and three occurrences of fallen rock resulted in two crashes per occurrence.) In total, there have been 14 crashes that resulted from rockfall activity—one from a fallen tree and 13 from fallen rock. Of this total, there was one crash that resulted in one fatality and one minor injury.

Crash information and records for the year 2019 to present are currently being processed, scanned, verified, and stored in a database by the NJDOT Bureau of Transportation Data and Safety. When available, this data will be obtained and reviewed by the Project Team.

Last updated date: August 27, 2020 10:37 AM