State transportation departments, cities, and counties increasingly face challenges from deteriorating culverts beneath roads and highways. During replacement or rehabilitation of this critical, failing infrastructure, officials must minimize inconvenience to motorists and maximize return to the public, while balancing the immediate cost with the longest design life. Open-cut replacement of a culvert is an option, but more often, trenchless methods — such as lining, sliplining, or standard tunneling methods — are used to replace or rehabilitate the deteriorating line.
The first step in deciding on a solution is determining why a culvert failed. The following questions can help:
- Is the existing culvert sized appropriately, or is more capacity needed?
- Is the culvert invert damaged?
- Has the pipe diameter or shape changed in any way?
- Has the bedding material immediately adjacent to the pipe retained its structural integrity?
- Is there flow outside and around the culvert?
- Is there any settlement or pavement distress apparent at the ground surface above the culvert?
For example, as corrugated metal pipe (CMP) deteriorates, the protective galvanized coating corrodes or erodes, leaving the steel pipe, which rusts away over time. This commonly occurs at the pipe invert. As the invert deteriorates, the surrounding soil pressure forces the pipe to turn inward at the invert and close up or buckle, effectively reducing the pipe circumference.
These conditions negatively impact the pipe bedding material, which loses structural integrity as it erodes under the damaged invert and around the damaged pipe. Hydrostatic pressure at the upstream end of the culvert can also cause groundwater to flow through the bedding. This can wash the bedding material out from around the culvert’s circumference. These conditions can also occur at the pipe seams. In addition, settlement can occur above the culvert as the bedding material erodes.
Some of these problems occurred on a busy highway in Northern Delaware. Along the highway — Route 4 in the area of the Christiana Hospital and the Helen Graham Cancer Center near Wilmington — the Delaware Department of Transportation (DelDOT) awarded a contract to mill and overlay the road’s surface. The work was to begin during 2008. Because of the overlay and mill contract, DelDOT maintenance engineers focused on an existing 48-inch-diameter culvert that carried a stream under the highway. The culvert’s invert was rusting through and the seams had opened up to the extent that bedding material was beginning to migrate into the pipe. The overlay and mill contract was put on hold until the culvert could be replaced.
DelDOT staff considered lining the CMP, but hydrologic conditions required a higher flow capacity, so they specified a 54-inch-diameter reinforced concrete pipe (RCP) culvert. Initially, the designers discarded the option to tunnel and replace the line because of a lack of experience with this construction method. However, there were serious concerns about open-cut construction because of questionable soils in the area, high traffic volume, and proximity to the hospital. In spite of these concerns, the staff decided to proceed with a contract to replace the line with open-cut construction.
The job was bid in the summer of 2008, and Eastern States Construction, Wilmington, Del., was the lowest bidder. Upon award of the contract, Eastern States contacted Tenbusch, Inc., Lewisville, Texas, to explore the cost and availability of equipment to tunnel the job. Based on the information gathered, Eastern States submitted a Value Engineering Proposal to replace the existing CMP with new, 54-inch-diameter RCP, as originally specified, by tunneling instead of open-cut. The cost savings to the state was approximately 30 percent of the original contract amount.
The original contract called for a three-phase plan that included paving in the median and then routing traffic across the median, first in one direction and then in the other. The tunnel plan called for a work pit in the median and tunneling both sections of highway from the single work pit. This plan also satisfied the significant wetlands restrictions in the drainage area served by the culvert.
Normally, when tunneling is mentioned, the expectation is that it will be the most expensive option. This is often not the case because tunneling through an existing pipe requires less material removal and therefore less time. The deteriorating culvert is replaced as the tunnel is excavated. The replacement may be with conventional jacking pipe materials — clay, steel casing, polymer concrete pipe, or concrete pipe — or with tunnel liner plate. In either case, it is possible to replace the existing pipe with the same size culvert or with a larger culvert of greater flow capacity.
When replacing the line with jacking pipe, the new pipe is jacked segment-by-segment as workers within a protective shield remove the existing culvert in pieces and excavate any face material. The shield has steering capability to maintain grade. The jacking unit is substantial and requires a sizeable work pit and a stable backstop to push against.
In some instances the existing culvert may enter the ground at the surface on both the upstream and downstream ends. If there is no place for a jacking pit in the middle, the pipe can be replaced with tunnel liner plate. The shield used with liner plate advances by pushing itself forward off of the assembled liner plate.
Jacking a pipe column requires the use of a lubricant to help reduce the skin friction that the column generates. The longer the column of new pipe, the more important the use of lubricant becomes.
Tenbusch supplied Eastern States Construction with a tunneling shield and a 250-ton hydraulic jacking unit. The tunneling shield protected the men as they removed the existing CMP and excavated as necessary to accommodate the new RCP. As the tunnel was excavated, the hydraulic jacking unit provided the necessary force to jack the 54-inch RCP column under the roadway.
The contract allowed Eastern States Construction 75 days to complete the project. Eastern States began construction Sept. 21, 2008, tunneled at a rate of approximately 8 feet per day, and finished installing the new pipe on Oct. 11 — a total of approximately 15 working days.
Problems associated with products chosen in years past that have reached their design life are beginning to escalate. In many similar cases, tunneling with long-design-life products can provide states, counties, and cities with a less-expensive option to rehabilitate or replace deteriorating culverts under roads or highways with minimal disruption to motorists.
The American Concrete Pipe Association (ACPA) recently awarded the Delaware Department of Transportation (DelDOT) its 2009 Project Achievement Award for the Route 4 culvert replacement project. The Project Achievement Award is based on public involvement and education, use of new materials or large-diameter concrete pipe or precast boxes, use of new technologies, innovation, complexity, cost effectiveness, and environmental benefits. According to the ACPA, the Route 4 project — DelDOT’s first use of tunneling and pipe jacking of a storm sewer — proved to the agency that tunneling and jacking using reinforced concrete pipe is a cost-saving construction alternative to the open-cut method. ACPA honored DelDOT and the Route 4 tunneling project during the American Association of State Highway and Transportation Officials’ Bridge and Structures Subcommittee meeting in Louisiana in July.