CFGS LLC Speeds Hazardous Dam Rehab with Drone Photos and GPS Equipment
The use of drones, advanced photogrammetry, and GPS equipment slashed nearly two-thirds of the time estimated for earthmoving for the rehabilitation of a potentially dangerous dam in North Attleboro, Massachusetts.
Site contractor CFGS LLC of Griswold, Connecticut, excavated and placed 14,000 tons of seven different types of select, engineered materials at Hoppin Hill Dam, an earthen dam undergoing $1 million in critical repairs by general contractor Aqua Line Utility Inc. of Weymouth, Massachusetts. Materials had to be placed precisely in specific patterns and compacted in layers to reconstruct and reduce the steep pitch of the downstream side slope of the dam as part of the overall rehabilitation.
Concerns Over Dam Safety
According to the Massachusetts Office of Dam Safety (ODS), Hoppin Hill Reservoir Dam is a High Hazard Potential earth embankment dam, owned by the City of Attleboro and located in the Town of North Attleboro. ODS maintains records of dams located throughout the state, ensures compliance with acceptable practices pertaining to dam inspection, maintenance, operation and repair of dams. High Hazard Potential refers not to the condition of a dam but rather to the consequences of its failure (a breach) or mis-operation (unscheduled release of water), which will probably cause loss of life and serious damage to homes, industrial or commercial facilities, important public utilities, main highways or railroads.
The 22-acre Hoppin Hill Reservoir is an important component of the City's water supply system. In addition, the dam and area surrounding the reservoir are accessible by the public for passive recreation. Roughly trapezoidal in cross-section, the dam is an 1,100-foot long curved earthen embankment about 23 feet tall, with a 12-foot-wide crest and a hydraulic height of 20 feet. Originally constructed around 1910, the dam underwent major rehabilitation in 1987, which involved replacing the upper sluice gate system and the valve for the dam's aerator basin bypass, removing trees in the embankment area, and replacing and repairing deteriorated concrete facing on the upstream slope.
But the dam has deteriorated significantly over the years since, and was rated in poor condition based on a 2008 dam safety inspection. This resulted in the Massachusetts Department of Conservation and Recreation (DCR) issuing a Certificate of Non-Compliance and Dam Safety Order in 2009, requiring rehab measures to be taken by the City of Attleboro to bring the dam into compliance with current dam safety regulations and practices.
Specifically, DCR wanted improvements to be made in five areas: spillway, upstream face and slope, downstream slope, crest, and instrumentation. In line with this, an analysis by consulting engineers Weston & Sampson in 2010 had noted that some sections of the dam have large cracks that must be repaired or replaced with new material.
These issues are being addressed in Aqua Line's $1 million construction contract with the City. Steve Trudeau, Project Manager, indicated the project includes the demolition of the existing concrete spillway, constructing a new cast-in-place spillway and a new precast concrete discharge channel, repairing and modifying an existing concrete valve vault, and repairing concrete paving on the upstream face of the dam.
All told, about $1.5 million is needed for the entire project to cover design, permitting, bidding, oversight, construction and contingencies. The state announced the City is eligible for a $160,000 grant and an $840,000 low-interest loan, under the Dam and Seawall Repair or Removal Program created by the Massachusetts General Court in 2013.
That legislation promotes public health, public safety, and ecological restoration, and was passed to address the significant infrastructure issues related to dams and seawalls. A 2011 State Auditor's report identified 100 relatively large dams in unsafe or poor condition with an estimated $60 million in remediation costs needed to reduce the likelihood of dam failures.
Hoppin Hill Dam is one of about 1500 Massachusetts dams that are large enough to pose risk to human life and property. The 2013 Report Card of the American Society of Civil Engineers (ASCE), which the group publishes every four years using infrastructure facts from the states themselves, pointed out that many dams in Massachusetts were built during the industrial revolution and are now between 100 and 200 years old. Less than 10 percent are still serving their original intended purpose and more than 56 percent of all dams in the Commonwealth are privately owned.
High-Tech Equipment Needed
Earthen embankment dams rely on the performance of their aggregate materials for the facilities to maintain structural integrity. For this reason, specifications for designing and placing these materials are very stringent, and sophisticated equipment needs to be employed.
CFGS, LLC was hired by general contractor Aqua Line Construction to perform the earth moving and fine grading aspect of the Hoppin Hill Dam project. Owned and operated by Josh Poitras and Robert Quint, CFGS is a high-technology-based site work and earth moving company, as explained by Quint.
"We take the engineer's design to the next level by creating a 3-D model that is loaded into our company's GPS-guided earth moving equipment," Quint said.
"This enables the equipment operators to perform cut, fill and grade functions to within a few hundredths of a foot, without the clutter of numerous grade-stakes. The speed and accuracy of these systems enabled us to perform the site work for a dam rehab estimated to take 14 weeks, and complete the project including placing 14,000 tons of select material placement and a 230-foot-long precast discharge spillway, in under five weeks," Quint said.
The site contractor's fleet of equipment for the project included a John Deere 290G Excavator equipped with Topcon GX60 Full Positioning System, a John Deere 700K Dozer also equipped with Topcon GX60 Full Positioning System, plus a Komatsu PC400 Excavator, Komatsu 500 Front End Loader, two John Deere 300D Off-Highway Trucks, and a HAMM 89-inch Steel Drum Roller (used in static for filter and vibration elsewhere).
SM Lorusso Corp. supplied the 14,000 tons of aggregate materials required for the project, with the majority consisting of 1-1/2-inch dense graded crushed gravel (about 11,705 tons). Other materials included 1,192 tons of 3/8-inch stone, 923 tons of C-33 coarse washed sand, 250 tons of 6-inch to 8-inch modified rock fill, and 4-inch to 6-inch stone.
US Construction fabric provided some 1,100 square yards of Mirafi filter fabric.
Drones Speed Accuracy Measurements
Strict oversight is necessary during construction to ensure that elevations, dimensions and other details correspond exactly to the design. On an 1100-foot long earthen structure, this can be tedious and time-consuming if done on foot using traditional instruments. An emerging technology for checking existing conditions during and after construction is the use of unmanned aerial vehicles (UAV), also known as unmanned aerial systems (UAS) or simply drones.
In this case, Aqua Line Utility is employing the services of Air-Terra 3-D of Meredith, New Hampshire. The company is using a Lockheed Martin Indago UAS, a collapsible drone with four helicopter rotors that weighs less than 5 pounds. The UAS measures 32 inches by 32 inches by 7 inches, but can be folded to a 12-inch by 9-inch by 6-inch configuration. Air Terra's COO Tom Range II said they typically fly the UAS approximately 300 feet above the dam and take very high-resolution video and still images.
"Photos are then processed in a photogrammetry software called Pix4D that "˜stitches' all the photos together in one complete photo of the whole project," Range said.
"The pixels are generated in 3-D points with an accuracy of 1 to 2 inches horizontal and 1 to 3 inches vertical. We then use the 3-D data to create digital surface files to check existing conditions and verify that the project was built to plan," he said.
Construction began in earnest in October 2015, and was essentially complete by the end of June 2016, according to Aqua Line Project Manager Trudeau.
The 1,100-foot Hoppin Hill Dam is a critical part of Attleboro's infrastructure, and its reconstruction will maintain reservoir integrity while protecting the City's water supply and ensuring public safety downstream.